Mirage of Fusion Power

July 30, 2023 by Dr Rajiv Desai

Mirage of Fusion Power:

“We say that we will put the Sun into a box. The idea is pretty. The problem is, we don’t know how to make the box”.

Pierre-Gilles de Gennes

Figure above shows ITER fusion reactor tokamak. A tokamak is a device used in nuclear fusion research for magnetic confinement of plasma. It consists of a complex system of magnetic fields that confine the plasma of reactive charged particles in a hollow, doughnut-shaped container. The tokamak (an acronym from the Russian words for toroidal magnetic confinement) was developed in the mid-1960s by Soviet plasma physicists. It produces the highest plasma temperatures, densities, and confinement durations of any confinement device. Fusion energy scientists believe that tokamaks are the leading plasma confinement concept for future fusion power plants.

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Section-1

Prologue:

The demand for energy is ever growing, as more people around the world experience liberation from poverty. Energy consumption is an indicator of quality of life. As such, it is very important that we continue to meet the ever increasing energy demands of the world as more countries aim to lift their standards of living, and consume more energy along the way. However, this creates a unique problem for policy makers in light of global warming. Although increasing fossil fuel based plants is an option, it is not a good one as it contributes quite heavily to global warming. So it is crucial that we supply the said increase in energy, in the cleanest, and carbon free/neutral way possible. Renewable energy technologies like solar or wind have come a long way since their introduction, but they still suffer from intermittency and unreliable energy production. The key is energy density, which determines the amount of energy a source can produce given an area of land. Although solar and other renewable technology are excellent sources, they suffer from being a low energy density source of energy. Nuclear fission reactors on the other hand, offers very high energy density, but is generally held in low esteem by the general public. The 1986 Chernobyl and 2011 Fukushima accidents have heightened our fears about nuclear technology’s ability to provide a safe way of generating clean power. Fission reactors produce heaps of radioactive waste, which we have bury in the ground. Therefore, we need a source that has high energy density, is carbon free/neutral, is safe, and does not have a negative connotation attached to it. This is where fusion enters the fray. Although the public’s perception of fusion is ever so slightly tainted due to its “nuclear” nature, it does not suffer from it as badly as nuclear fission. Fusion reactors are inherently safer posing very low risk to populations in the vicinity, generating no long-lasting waste and poses little or no meltdown risk. Thus, it satisfies all the requirements that we are looking for in an energy source except for one, which is that it does not exist in a commercial capacity yet. Civilization development constantly demands more efficient sources of energy, sources which would simultaneously pose minimal threat to the environment. Nuclear fusion power plants (FPP), also referred to as thermonuclear reactors, may be the best answer to the problem.

Heat, density, time, and a small amount of seawater. In just 20 years, this simple recipe could power entire cities and help slow the rising tide of climate change. That means if scientists can figure out a way to reliably produce and sustain nuclear fusion on Earth using elements commonly found in ocean water, virtually unlimited energy could be available at the push of a button — all without the risk of harmful carbon emissions from burning fossil fuels, the variability of wind and solar power, and the potential of meltdowns and radioactive waste from nuclear fission. The only downside is that it’s all theoretical. Researchers have toiled over fusion power since the 1950s and have yet to build a reactor that can produce more energy than it consumes. Achieving controlled fusion energy for power production is vastly more difficult. It is certainly far more difficult than the controlled fission reactors that for 40 years have provided about 20% of the electricity on the US grid. Consider: it took only about four years between the scientific discovery of nuclear fission—the breaking up of uranium nuclei by neutron bombardment—and the operation of the first self-sustaining controlled fission reactor. By contrast, seventy years and the prolonged efforts of thousands of scientists have yet to achieve self-sustaining controlled fusion. It remains, nevertheless, a grand challenge in science and engineering that continues to inspire the imaginations and efforts of succeeding generations of students and researchers, not least because it holds out the possibility of an important, clean, and sustainable energy source.

Fusion reactions power the stars. It is nuclear fusion—a process in which two lighter nuclei, typically isotopes of hydrogen, combine together under conditions of extreme pressure and temperature to form a heavier nucleus. Our Sun is a gigantic fusion device. In the core of the Sun, hydrogen atoms move at incredible speed. Hydrogen atoms combine to form a helium atom through a series of reactions. The reaction releases lots of energy in the form of light and heat. The fusion reaction is induced by bringing two or more light atoms close enough together so that the residual strong force (nuclear force) in their nuclei will pull them together into one larger atom. The protons are moving so fast that—aided by properties of quantum physics—they sometimes slam into each other with enough force that they overcome the Coulomb Barrier. Fusion energy is produced by nuclear fusion of two lighter atomic nuclei to form a heavier nucleus. The latter weighs slightly less than the total of the two nuclei. The small difference in mass m is transformed into energy E according to Albert Einstein’s famous formula E = mc^2, where c is the speed of light approximating 3 × 10^8 m/s. Every second our Sun coverts 600 million tonnes of hydrogen into helium. To replicate the fusion reaction on earth, we need two kinds of hydrogen: deuterium and tritium. But because they are both positively charged, they tend to repel one another. Enough external source of energy must be supplied to overcome this electrostatic force or “Coulomb barrier.” The easiest way to do this is to heat the atoms, which has the side effect of stripping the electrons from the atoms and leaving them as bare nuclei. In practice, the nuclei and electrons are left in a high-temperature plasma state so that the nuclei have enough kinetic energy to overcome their repulsion. Hydrogen has the smallest nuclear charge, therefore, reacts at the lowest temperature. Most fusion reactions combine isotopes of hydrogen; protium, deuterium, or tritium to form isotopes of helium (3He or 4He i.e., 3/2He or 4/2He where numerator is atomic mass A and denominator is atomic number Z. Atomic mass is associated with the number of neutrons and protons while atomic number is the number of protons in the nucleus of an element.) When a nucleus of deuterium fuses with a nucleus of tritium (D-T reaction), an α-particle (helium nucleus) is produced and a neutron released. The nuclear reaction results in a reduction in total mass and a consequent release of 17.6 MeV energy per reaction in the form of the kinetic energy of the reaction products. About 80% of the energy (14.1 MeV) becomes kinetic energy of the neutron traveling at 1⁄6 the speed of light. On the Sun, due to the strong gravity, hydrogen atoms fuse at 15 million° C. On Earth, however, because of the weaker gravitational forces, they need to be heated at temperatures as high as 150 million °C in order to collide. At 150 million °C, hydrogen isotopes atoms crush and end up forming an ‘electrically-charged gas’ known as plasma. Scientists came up with the idea of a Tokamak: a chamber using a powerful magnetic field to contain the hot plasma. Deuterium can be found in sea water. We have enough supplies to last millions of years. Tritium can be generated from lithium, extracted from the crust of the earth. For decades scientists have been trying to figure out how to produce this energy through various experiments. The current leading designs of confining plasma in the fusion reactor are mainly two types, the magnetic confinement such as in tokamak and inertial confinement by laser. The International Thermonuclear Experimental Reactor (ITER) tokamak in France and the National Ignition Facility (NIF) laser in the United States are two major programs in these technologies. Compared with fission energy, fusion energy has important benefits. These benefits include abundant supply of fuel (hydrogen isotopes), less radioactivity on the products, and inherently safe as there is no critical mass.

The joke about fusion power is that it’s 30 years away and always will be. But despite its complexity, scientists working on the technology say it’s worth the trouble. That’s because the nuclear fusion reaction has a higher energy potential than all other energy sources we know. It can release nearly 4 million times more energy than chemical reactions like burning coal, oil or gas, and four times more than nuclear fission, the process currently used in all nuclear power plants around the world. Discovered in the early 20th century, fusion is seen as the future of energy by many policymakers, especially in Europe. Recreating the energy production process of the Sun — nuclear fusion — on Earth in a controlled fashion is one of the greatest challenges of 21st century. If achieved at affordable costs, energy security would be greatly enhanced and environmental degradation from fossil fuels greatly diminished. Considering the harsh deadline imposed on humanity by global warming, and the ever-growing need for energy, we require a continuous source of energy that is also energy dense.

Let’s pretend, for a moment, that the climate doesn’t matter. That we’re completely ignoring the connection between carbon dioxide, the Earth’s atmosphere, the greenhouse effect, global temperatures, ocean acidification, and sea-level rise. From a long-term point of view, we’d still need to plan for our energy future. Fossil fuels, which make up by far the majority of world-wide power today, are an abundant but fundamentally limited resource. We cannot simply generate more coal, oil, or natural gas when our present supplies run out. Renewable sources like wind, solar, and hydroelectric power have different limitations: they’re inconsistent. There is a long-term solution, though, that overcomes all of these problems: nuclear fusion as it is cleaner and safer than nuclear fission — the latter being the process we use in current nuclear power stations. Nuclear fusion is the only primary energy source left in the Universe that we have yet to exploit. Ever since the process that powers the stars was harnessed in the 1950s for hydrogen bombs, technologists have dreamt of unlocking it in a more controlled manner for energy generation.

Fusion, which powers the sun and stars, has long been considered the Holy Grail of energy production. Fusion can provide a nearly limitless energy source without producing harmful carbon emissions or long-lived waste. The world’s first commercial facility would mark the dawn of a new era of energy. But the claim that we’re close to achieving abundant, clean energy made with almost nothing but seawater, is indeed, quite frankly, hype. We are not there yet. Not by a longshot. The devil is always in the data. Remember CERN’s claim in 2011 that they had observed neutrinos travelling faster than the speed of light? Neutrinos play a role in many fundamental aspects of our lives. They provide dramatic confirmation of fundamental theories concerning stellar interiors. And speed of light is considered a fundamental constant of nature. Its significance is far broader than its role in describing a property of electromagnetic waves. It serves as the single limiting velocity in the universe, being an upper bound to the propagation speed of signals and to the speeds of all material particles. So, CERN’s experiment would have upended everything, if it had been true. But remember how CERN also had to retract the claim when they discovered that a loose fiber optic cable may have fudged their facts? That’s unchecked enthusiasm for you. You could argue that societies need hype, just like we need hope — if it’s positive hype, it can paint constructive visions of the future and push people to innovate and find new solutions. Nuclear fusion experts say the technology will one day be part of a sustainable energy mix. Although the science underpinning fusion research is solid, the amount of energy these reactions produce has yet to exceed the amount needed to instigate them. Breaking through this barrier and achieving a self-sustaining reaction is the holy grail of fusion research. Can we ever generate electricity on commercial basis from fusion power plant? My endeavour is to study whether fusion power is a viable energy source for mankind or a black hole for money, time, energy and efforts.

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Abbreviations and synonyms:

D = deuterium

T = tritium

p = proton

n = neutron

n = particle density of a plasma

e = electron

e⁺ = positron

v_e = electron neutrino

Q value for a reaction = the amount of energy absorbed or released during the nuclear reaction.

Q as energy gain factor = the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state.

MCF = magnetic confinement fusion

ICF = inertial confinement fusion

MFE = magnetic fusion energy

IFE = inertial fusion energy

FPP = fusion power plant

NPP = nuclear power plant (fission)

BEN = binding energy per nucleon

ITER = International Thermonuclear Experimental Reactor

JET = Joint European Torus

NIF = National Ignition Facility

FRC = field-reverse configuration

DEMO = Demonstration power plant

CFS = Commonwealth Fusion Systems

MHD = magnetohydrodynamic

PFM = plasma-facing material

PFC = plasma-facing component

LCOE = levelized cost of electricity

CANDU = CANada Deuterium Uranium [uses heavy water (deuterium oxide) for moderator and coolant]

HTS = high-temperature superconductor

Be = beryllium

Li = lithium

eV = electron volt = 1.602 × 10⁻¹⁹ joule

MeV = 10⁶ (1,000,000) electron volts

K = Kelvin = C + 273

C = Celsius

MWth =megawatt thermal (thermal power)

MWe = megawatt electric (electric power)

Note:

Symbols n and Q are used in two different contexts, so please understand the context before ascribing meaning to symbols n and Q.

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Glossary:

Stellar refers to stars. Interstellar space is the part of space that exists between stars.

nuclear fusion = fusion = thermonuclear fusion unless specified otherwise.

nuclear fission = fission

fusion energy gain factor (Q) = the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state.

break-even = when fusion power produced equals the heating power input i.e., Q = 1

ignition = when a fusion reaction produces enough energy to be self-sustaining after external energy input is cut off = infinite Q

inertial confinement = a technique that aims multiple lasers at tiny fuel pellets and achieves fusion conditions by rapidly compressing and heating a small quantity of fusion fuel

magnetic confinement = a technique in which charged particles (deuterium–tritium plasma) are trapped in a small region as magnetic field prevents the particles from coming into contact with the reactor walls, which will dissipate the heat of the nuclei and slow down its movement.

nuclear binding energy = minimum energy required to split a nucleus of an atom into its component parts i.e., individual protons and neutrons.

nucleon = one of the subatomic particles of the atomic nucleus, i.e., a proton or a neutron.

nucleosynthesis = any of several processes that lead to the synthesis of heavier atomic nuclei.

nuclear force = the force that acts between nucleons and binds protons and neutrons into atomic nuclei; the residual strong force or strong nuclear force

weak nuclear force = the force that acts inside of individual nucleons, which means that it is even shorter ranged than the strong force. It is the force that allows protons to turn into neutrons and vice versa through beta decay.

nuclear fusion = reaction in which two or more nuclei combine, forming a new element with a higher atomic number (more protons in the nucleus).

tokamak = acronym for a torus-shaped vacuum chamber surrounded by magnetic coils

For temperature in keV to Kelvin: 1 keV corresponds to 11.6 million degrees Kelvin.

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Isotopes:

Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. Some common isotopes in fusion are:

Protium is a hydrogen isotope with one proton and no neutrons. It is the most common form of hydrogen and the most common element in the universe.
Deuterium is a hydrogen isotope with one proton and one neutron. It is not radioactive and can be extracted from seawater.
Tritium is a hydrogen isotope with one proton and two neutrons. It is radioactive, with a half-life of about 12.33 years. Tritium does not occur naturally but can be made by bombarding lithium with neutrons.
Helium-3 is a helium isotope with two protons and one neutron.
Helium-4 is the most common, naturally occurring form of helium, with two protons and two neutrons.

Helium isotopes = helium-3 (or ³He) with two protons and one neutron, and helium-4 (or ⁴He) with two protons and two neutrons.

³He = 3He =He-3 = 3/2He

⁴He = 4He =He-4 = 4/2He

Hydrogen isotopes = protium (¹H) with one proton, deuterium (²H) with one proton and one neutron, tritium (³H) with one proton and two neutrons.

¹H = 1H = H-1 =1/1H = protium = hydrogen = H

²H = 2H = H-2 = 2/1H = deuterium = D

³H = 3H = H-3 = 3/1H = tritium = T

DT = deuterium and tritium = D + T

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Neutrons speed: slow and fast neutrons:

Neutrons are classified by their energies (expressed in electron-volts, eV), which are directly related to their velocities (in meters or kilometers per second) and temperatures (in kelvins). For a particle the size of a neutron (1.675 × 10^–27 kg), 1 eV of energy corresponds to a velocity of 13.8 km/s; keep in mind that the energy of a neutron depends on the square of the velocity (remember, K = ½ mv² ). As such, classification can imply a neutron’s velocity or its temperature. Fast neutrons have an energy of 0.1–1 MeV (megaelectron-volt), or a velocity of 4000–14,000 km/s. Slow neutrons have an energy of 100 eV or less, corresponding to a velocity of 138 km/s. (Remember, references can differ greatly about the energy and velocity cutoffs. It should be clear, however, that fast neutrons are, well, faster and more energetic than slow neutrons.)

Thermal neutrons have an average temperature of room temperature, or about 295 K. This corresponds to an energy of 0.025 eV and a velocity of 2.2 km/s. Neutrons with an energy/velocity/temperature higher than this are called hot neutrons, and neutrons with an energy/velocity/temperature lower than this are called cold neutrons. Even within cold neutrons, there are other classifications, going down to ultracold neutrons, which have energies in the range of nanoelectron-volts and velocities on the order of meters per second.

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Section-2

Nuclear Binding Energy:

It is important to note that the total mass of a nucleus when the nucleons are together is smaller when we compare it to the total of the particles separated. It is invariable for all the atoms. Nuclei are made up of protons and neutrons, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. This binding energy can be calculated from Einstein formula:

Nuclear binding energy E = Δmc^2

where E is the nuclear binding energy, c is the speed of light, and Δm is the difference in mass. This ‘missing mass’ is known as the mass defect, and represents the energy that was released when the nucleus was formed.

Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable nuclei is always a positive number, as the nucleus must gain energy for the nucleons to move apart from each other. Nucleons are attracted to each other by the strong nuclear force.

Nuclear binding energy is the energy required to separate an atomic nucleus completely into its constituent protons and neutrons, or, equivalently, the energy that would be liberated by combining individual protons and neutrons into a single nucleus. The hydrogen-2 nucleus, for example, composed of one proton and one neutron, can be separated completely by supplying 2.23 million electron volts (MeV) of energy. Conversely, when a slowly moving neutron and proton combine to form a hydrogen-2 nucleus, 2.23 MeV are liberated in the form of gamma radiation. The total mass of the bound particles is less than the sum of the masses of the separate particles by an amount equivalent (as expressed in Einstein’s mass–energy equation) to the binding energy. The nucleons are held together through forces which we refer to as the strong nuclear force. The greater the nucleus components are bound, the greater will be the binding energy which it requires in order to separate them. The binding energy of a magnesium nucleus (12 protons plus 12 neutrons), for example, is approximately two times greater than for the carbon nucleus (6 protons plus 6 neutrons). The nuclear binding energy for hydrogen nucleus (H-1), a proton, is zero because there is only one particle in the nucleus.

The enormity of the nuclear binding energy can perhaps be better appreciated by comparing it to the binding energy of an electron in an atom. The nuclear binding energies are on the order of a million times greater than the electron binding energies of atoms. The forces that bind nucleons together in an atomic nucleus are much greater than those that bind an electron to an atom through electrostatic attraction. This is evident by the relative sizes of the atomic nucleus and the atom (10^−15 and 10^−10m, respectively). The energy required to pry a nucleon from the nucleus is therefore much larger than that required to remove (or ionize) an electron in an atom. In general, all nuclear changes involve large amounts of energy per particle undergoing the reaction. This has numerous practical applications.

The term “nuclear binding energy” may also refer to the energy balance in processes in which the nucleus splits into fragments composed of more than one nucleon. If new binding energy is available when light nuclei fuse (nuclear fusion), or when heavy nuclei split (nuclear fission), either process can result in release of this binding energy. This energy may be made available as nuclear energy and can be used to produce electricity as in nuclear power, or in a nuclear weapon. When a large nucleus splits into pieces, excess energy is emitted as gamma rays and the kinetic energy of various ejected particles (nuclear fission products).

Binding Energy per Nucleon:

In nuclear physics, one of the most important experimental quantities is the binding energy per nucleon (BEN). This quantity is the average energy required to remove an individual nucleon from a nucleus—analogous to the ionization energy of an electron in an atom. If the BEN is relatively large, the nucleus is relatively stable. BEN values are estimated from nuclear scattering experiments.

A graph of binding energy per nucleon (BEN) versus atomic mass A is given in figure below. This graph is considered by many physicists to be one of the most important graphs in physics. The binding energy per nucleon curve is obtained by dividing the total nuclear binding energy by the number of nucleons.

In above graph of binding energy per nucleon for stable nuclei, the BEN is greatest for nuclei with a mass near 56Fe. Therefore, fusion of nuclei with mass numbers much less than that of Fe, and fission of nuclei with mass numbers greater than that of Fe, are exothermic processes.

Energy in above graph is reported in units of MeV or mega-electron volts. A MeV is equal to 1.602 x 10^-13 joules.

BEN-versus-A graph implies that nuclei divided or combined release an enormous amount of energy. This is the basis for a wide range of phenomena, from the production of electricity at a nuclear power plant to sunlight.

It really matters what atoms you start with. In the graph above you see the nuclear binding energy per nucleon as a function of atomic mass (the number of proton and neutrons). The atom with the maximum binding energy per nucleon is Iron-56. Fusion occurs when small atoms combine to produce larger atoms (move to the right), and fission is when large atoms break up to form smaller atoms (move to the left). As you can see, the slope at the “fusion side” is steeper than that of the “fission side”, and so typically the former will release more energy. However, the “fusion side” slope quickly flattens, so that larger atoms release significantly less energy, and if you try to fuse atoms heavier than Iron they will absorb energy. This is a key mechanism behind core collapse supernovae.

But how come both fission and fusion generate energy?

Nuclear fusion is defined as the combining, or fusing, of two nuclei and, the combining of nuclei also results in an emission of energy. For many, the concept is counterintuitive. After all, if energy is released when a nucleus is split, how can it also be released when nucleons are combined together?

Remember that the structure of a nucleus is based on the interplay of the compressive nuclear strong force and the repulsive electromagnetic force. For nuclei that are less massive than iron, the nuclear force is actually stronger than that of the Coulomb force. As a result, when a low-mass nucleus absorbs nucleons, the added neutrons and protons bind the nucleus more tightly. The increased nuclear strong force does work on the nucleus, and energy is released.

Once the size of the created nucleus exceeds that of iron, the short-ranging nuclear force does not have the ability to bind a nucleus more tightly, and the emission of energy ceases. In fact, for fusion to occur for elements of greater mass than iron, energy must be added to the system!

Fusion of light nuclei to form medium-mass nuclei converts mass to energy, because binding energy per nucleon is greater for the product nuclei. The larger BEN is, the less mass per nucleon, and so mass is converted to energy and released in such fusion reactions.

Just as it is not possible for the elements to the left of iron in the graph above to naturally fission, it is not possible for elements to the right of iron to naturally undergo fusion, as that process would require the addition of energy to occur. Furthermore, notice that elements commonly discussed in fission and fusion are elements that can provide the greatest change in binding energy, such as uranium and hydrogen.

Iron’s location on the energy-mass curve is important, and explains a number of its characteristics, including its role as an elemental endpoint in fusion reactions in stars.

Light elements, such as hydrogen and helium, have small nuclei that release lots of energy when they fuse together. Moving to heavier atoms, less energy is released in each fusion event; until, at iron (26 protons and 30 neutrons), no more energy is released by fusion. Any bigger, it takes energy to make fusion happen. Atoms with really huge nuclei, such as uranium and plutonium do the opposite of fusion: they release energy when they break apart.

The iron limit:

The buildup of heavier elements in the nuclear fusion processes in stars is limited to elements below iron, since the fusion of iron would subtract energy rather than provide it. Iron-56 is abundant in stellar processes, and with a binding energy per nucleon of 8.8 MeV, it is the third most tightly bound of the nuclides. Its average binding energy per nucleon is exceeded only by 58Fe and 62Ni, the nickel isotope being the most tightly bound of the nuclides.

Two points to note in the graph above:

First, typical BEN values range from 6–10 MeV, with an average value of about 8 MeV. In other words, it takes several million electron volts to pry a nucleon from a typical nucleus, as compared to just 13.6 eV to ionize an electron in the ground state of hydrogen. This is why nuclear force is referred to as the “strong” nuclear force.

Second, the graph rises at low A, peaks very near iron (Fe, A=56), and then tapers off at high A. The peak value suggests that the iron nucleus is the most stable nucleus in nature (it is also why nuclear fusion in the cores of stars ends with Fe). The reason the graph rises and tapers off has to do with competing forces in the nucleus. At low values of A, attractive nuclear forces between nucleons dominate over repulsive electrostatic forces between protons. But at high values of A, repulsive electrostatic forces between forces begin to dominate, and these forces tend to break apart the nucleus rather than hold it together.

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Overcoming coulomb barrier for fusion:

The Sun’s energy is produced by nuclear fusion. Thermonuclear power is the name given to the use of controlled nuclear fusion as an energy source. While research in the area of thermonuclear power is progressing, high temperatures and containment difficulties remain. The cold fusion controversy centered around unsubstantiated claims of practical fusion power at room temperatures. Nuclear fusion is a reaction in which two nuclei are combined, or fused, to form a larger nucleus. We know that all nuclei have less mass than the sum of the masses of the protons and neutrons that form them. The missing mass times c^2 equals the binding energy of the nucleus—the greater the binding energy, the greater the missing mass. We also know that the binding energy per nucleon, is greater for medium-mass nuclei and has a maximum at Fe (iron). This means that if two low-mass nuclei can be fused together to form a larger nucleus, energy can be released. The larger nucleus has a greater binding energy and less mass per nucleon than the two that combined. Thus mass is destroyed in the fusion reaction, and energy is released. On average, fusion of low-mass nuclei releases energy, but the details depend on the actual nuclides involved.

Theoretically, you could fuse any atom – if you had enough energy, confinement and time. Stars fuse atoms by squashing them together, under high pressures and temperatures. Mankind did fusion for the first time when we set off the Ivy Mike atomic bomb on November 1rst 1952. But, the first controlled fusion was done by mankind, at Los Alamos National Labs in 1958. The machine that did with named SCYLLA 1. Since then, we have studied many fusion reactions, by shooting beams of atoms at other atoms. “Fusibility” is measure in units of Barns. A barn (symbol: b) is a metric unit of area equal to 10^−28 m^2 (100 fm^2). Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is approximately the cross-sectional area of a uranium nucleus.

Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei heavier than iron-56, the reaction is endothermic, requiring an input of energy. The heavy nuclei bigger than iron have many more protons resulting in a greater repulsive force. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy when they fuse. Since hydrogen has a single proton in its nucleus, it requires the least effort to attain fusion, and yields the most net energy output. Also since it has one electron, hydrogen is the easiest fuel to fully ionize.

The repulsive electrostatic interaction between nuclei operates across larger distances than the strong force, which has a range of roughly one femtometer—the diameter of a proton or neutron. The fuel atoms must be supplied enough kinetic energy to approach one another closely enough for the strong force to overcome the electrostatic repulsion in order to initiate fusion. The “Coulomb barrier” is the quantity of kinetic energy required to move the fuel atoms near enough. Atoms can be heated to extremely high temperatures or accelerated in a particle accelerator to produce this energy.

An atom loses its electrons once it is heated past its ionization energy. An ion is the name for the resultant bare nucleus. The result of this ionization is plasma, which is a heated cloud of ions and free electrons that were formerly bound to them. Plasmas are electrically conducting and magnetically controlled because the charges are separated. This is used by several fusion devices to confine the hot particles. The major obstruction to fusion is the Coulomb repulsion force between nuclei. Since the attractive nuclear force that can fuse nuclei together is short ranged, the repulsion of like positive charges must be overcome in order to get nuclei close enough to induce fusion.

The major obstruction to fusion is the Coulomb repulsion between nuclei. Since the attractive nuclear force that can fuse nuclei together is short ranged, the repulsion of like positive charges must be overcome to get nuclei close enough to induce fusion. Figure below shows an approximate graph of the potential energy between two nuclei as a function of the distance between their centers. The graph resembles a hill with a well in its center. A ball rolled to the left must have enough kinetic energy to get over the hump before it falls into the deeper well with a net gain in energy. So it is with fusion. If the nuclei are given enough kinetic energy to overcome the electric potential energy due to repulsion, then they can combine, release energy, and fall into a deep well. One way to accomplish that end is to heat fusion fuel to high temperatures so that the kinetic energy of thermal motion is sufficient to overcome coulomb barrier to get the nuclei together.

Figure above shows potential energy between two light nuclei graphed as a function of distance between them. If the nuclei have enough kinetic energy to get over the Coulomb repulsion hump, they combine, release energy, and drop into a deep attractive well. Tunneling through the barrier is important in practice. The greater the kinetic energy and the higher the particles get up the barrier (or the lower the barrier), the more likely the tunneling.

You might think that, in our Sun, nuclei are constantly coming into contact and fusing. However, this is only partially true. Only at the Sun’s core are the particles close enough and the temperature high enough for fusion to occur! Nuclear fusion involves the collision of two nuclei at high speeds. In order for them to fuse, they need to overcome their coulombic barrier. This would be energetically impossible – if not for a quantum tunneling effect which lowers the energy needed for fusion. The tunneling only happens when the atoms are distances on the order of 1 femtometers. The nuclear fusion process itself is also quite remarkable because one would initially assume that two protons would stridently repel each other due to Coulomb repulsion. It is understood, however, that despite the tendency for strong repulsion of the positively charged protons, quantum mechanical tunneling (of the wave functions of the interacting protons) through the Coulomb barrier can occur. Quantum mechanical tunneling is what makes fusion in the Sun possible, and tunneling is an important process in most other practical applications of fusion, too. Since the probability of tunneling is extremely sensitive to barrier height and width, increasing the temperature greatly increases the rate of fusion. The closer reactants get to one another, the more likely they are to fuse. Thus most fusion in the Sun and other stars takes place at their centers, where temperatures are highest. Moreover, high temperature is needed for thermonuclear power to be a practical source of energy.

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Section-3

Nuclear fission versus fusion:

The foundation of nuclear energy is harnessing the power of atoms. Both fission and fusion are nuclear processes by which atoms are altered to create energy, but what is the difference between the two? Simply put, fission is the division of one atom into two, and fusion is the combination of two lighter atoms into a larger one. They are opposing processes, and therefore very different.

Nuclear Fission:

The word fission means “a splitting or breaking up into parts”. Nuclear fission releases heat energy by splitting atoms. The surprising discovery that it was possible to make a nucleus divide was based on Albert Einstein’s prediction that mass could be changed into energy. In 1939, scientist began experiments, and one year later Enrico Fermi built the first nuclear reactor.

Nuclear fission takes place when a large, somewhat unstable isotope (atoms with the same number of protons but different number of neutrons) is bombarded by high-speed particles, usually neutrons. These neutrons are accelerated and then slammed into the unstable isotope, causing it to fission, or break into smaller particles. During the process, a neutron is accelerated and strikes the target nucleus, which in the majority of nuclear power reactors today is Uranium-235. This splits the target nucleus and breaks it down into two smaller isotopes (the fission products), three high-speed neutrons, and a large amount of energy.

This resulting energy is then used to heat water in nuclear reactors and ultimately produces electricity. The high-speed neutrons that are ejected become projectiles that initiate other fission reactions, or chain reactions.

Atoms are held together by two fundamental natural forces: weak and strong nuclear bonds. The binding energy is the total amount of energy held within the atomic bonds. The greater the amount of binding energy held within the bonds per nucleon, the more stable the atom. Furthermore, atoms attempt to become more stable by increasing their binding energy per nucleon. The graph of binding energy per nucleon suggests that nuclides with a mass larger than about 130 amu should spontaneously split apart to form lighter, more stable, nuclides. Spontaneous fission generally occurs in atoms with atomic numbers above 90. Spontaneous fission is a relatively slow process except for the heaviest isotopes. Experimentally, we find that spontaneous fission reactions occur for only the very heaviest nuclides those with mass numbers of 230 or more. Even when they do occur, these reactions are often very slow. The half-life for the spontaneous fission of 238U, for example, is 4468 billion years.

We don’t have to wait, however, for slow spontaneous fission reactions to occur. By irradiating samples of heavy nuclides with slow-moving thermal neutrons it is possible to induce fission reactions. When 235U absorbs a thermal neutron, for example, it splits into two particles of uneven mass and releases an average of 2.5 neutrons. More than 370 daughter nuclides with atomic masses between 72 and 161 amu are formed in the thermal-neutron-induced fission of 235U. Several isotopes of uranium undergo induced fission. But the only naturally occurring isotope in which we can induce fission with thermal neutrons is 235U, which is present at an abundance of only 0.72%. The induced fission of this isotope releases an average of 200 MeV per atom, or 80 million kilojoules per gram of 235U. The attraction of nuclear fission as a source of power can be understood by comparing this value with the 50 kJ/g released when natural gas is burned.

The first artificial nuclear reactor was built by Enrico Fermi and co-workers beneath the University of Chicago’s football stadium and brought on line on December 2, 1942. This reactor, which produced several kilowatts of power, consisted of a pile of graphite blocks weighing 385 tons stacked in layers around a cubical array of 40 tons of uranium metal and uranium oxide. Spontaneous fission of 238U or 235U in this reactor produced a very small number of neutrons. But enough uranium was present so that one of these neutrons induced the fission of a 235U nucleus, thereby releasing an average of 2.5 neutrons, which catalyzed the fission of additional 235U nuclei in a chain reaction. The amount of fissionable material necessary for the chain reaction to sustain itself is called the critical mass.

Fission reaction does not normally occur in nature
Fission produces many highly radioactive particles
The energy released by fission is a million times greater than that released in chemical reactions; but lower than the energy released by nuclear fusion
One class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb
Fission is the splitting of a large atom into two or more smaller ones
Critical mass of the substance and high-speed neutrons are required
Takes little energy to split two atoms in a fission reaction
Nuclear fission is the splitting of a massive nucleus into photons in the form of gamma rays, free neutrons, and other subatomic particles.

In a typical nuclear reaction involving ²³⁵U and a neutron:

²³⁵₉₂U + n = ²³⁶₉₂U

followed by

²³⁶₉₂U = ¹⁴⁴₅₆Ba + ⁸⁹ ₃₆Kr + 3n + 177 MeV

There are, however, many possible fission products. For any fission reaction, the sum of all neutrons and protons in the products is the same as the reactants and the total number of protons in the products is the same as the reactants. A small amount of mass is lost and this mass is converted into energy.

The complete fission of one kilogram of Uranium-235 — the fissile component of nuclear fuel — can generate about 77 terajoules. But we cannot convert all of that energy into useful forms like heat and electric power. Instead, we have to engineer a complex system that can control the nuclear fission chain reaction and convert the generated energy into more useful forms. This is what nuclear power plants do — they harness the heat generated during nuclear fission reactions to make steam. This steam drives a turbine connected to an electric power generator, which produces electricity. The overall efficiency of the cycle is less than 40 per cent. In addition, not all of the uranium in the fuel is burned. Used fuel still contains about 96 per cent of its total uranium, and about a fifth of its fissile Uranium-235 content. The huge energy potential of nuclear fuel is currently mitigated by the engineering challenges of converting that energy into a useful form.

The Fermi reactor at Chicago served as a prototype for larger reactors constructed in 1943 at Oak Ridge, Tennessee, and Hanford, Washington, to produce 239Pu for one of the atomic bombs dropped on Japan at the end of World War II. As we have seen, some of the neutrons released in the chain reaction are absorbed by 238U to form 239U, which undergoes decay by the successive loss of two particles to form 239Pu. 238U is an example of a fertile nuclide. It doesn’t undergo fission with thermal neutrons, but it can be converted to 239Pu, which does undergo thermal-neutron-induced fission.

Uranium 238: 92 protons 146 neutrons
Uranium 239: 92 protons 147 neutrons
Neptunium 239: 93 protons 146 neutrons
Plutonium 239: 94 protons 145 neutrons

When you mine uranium in nature, near to 99.3% of it is uranium-238, and 0.7% is the fissile uranium-235. However, that amount of uranium-235 is not enough to carry out a nuclear chain reaction. So what we do is we enrich uranium. Meaning we make uranium go through a process that removes a bit of U-238 so that the concentration of U-235 increases (much like evaporating water will make it saltier, because removing water increases concentration of salt). When it gets to like 4~5% uranium-235, a chain reaction is now possible to occur. So we then stop removing U-238, and now we have a mixture of 5% U-235 and 95% U-238. The U-238 goes into the reactor simply because it is still there – we just don’t need to go through the trouble and cost of removing it anymore, because we already have the concentration of U-235 we need for the reactor to work. Heat generated in the reactor core is transferred to a cooling agent in a closed system. The cooling agent is then passed through a series of heat exchangers in which water is heated to steam. The steam produced in these exchangers then drives a turbine that generates electrical power. There are two ways of specifying the power of such a plant: the thermal energy produced by the reactor or the electrical energy generated by the turbines. The electrical capacity of the plant is usually about one-third of the thermal power.

It takes 10^11 fissions per second to produce one watt of electrical power. As a result, about one gram of fuel is consumed per day per megawatt of electrical energy produced. This means that one gram of waste products is produced per megawatt per day, which includes 0.5 grams of 239Pu. These waste products must be either reprocessed to generate more fuel or stored for the tens of thousands of years it takes for the level of radiation to reach a safe limit.

If we could design a reactor in which the ratio of the 239Pu or 233U produced to the 235U consumed was greater than 1, the reactor would generate more fuel than it consumed. Such reactors are known as breeders, and commercial breeder reactors are now operating in France.

The key to an efficient breeder reactor is a fuel that gives the largest possible number of neutrons released per neutron absorbed. The breeder reactors being built today use a mixture of PuO2 and UO2 as the fuel and fast neutrons to activate fission. Fast neutrons carry an energy of at least several KeV and therefore travel 10,000 or more times faster than thermal neutrons. 239Pu in the fuel assembly absorbs one of these fast neutrons and undergoes fission with the release of three neutrons. 238U in the fuel then captures one of these neutrons to produce additional 239Pu.

The advantage of breeder reactors is obvious they mean a limitless supply of fuel for nuclear reactors. There are significant disadvantages, however. Breeder reactors are more expensive to build. They are also useless without a subsidiary industry to collect the fuel, process it, and ship the 239Pu to new reactors.

It is the reprocessing of 239Pu that concerns most of the critics of breeder reactors. 239Pu is so dangerous as a carcinogen that the nuclear industry places a limit on exposure to this material that assumes workers inhale no more than 0.2 micrograms of plutonium over their lifetimes. There is also concern that the 239Pu produced by these reactors might be stolen and assembled into bombs by terrorist organizations.

The fate of breeder reactors in the United States is linked to economic considerations. Because of the costs of building these reactors and safely reprocessing the 239Pu produced, the breeder reactor becomes economical only when the scarcity of uranium drives its price so high that the breeder reactor becomes cost effective by comparison. If nuclear energy is to play a dominant role in the generation of electrical energy in the 21st century, breeder reactors eventually may be essential.

Although the “pile” Fermi constructed at the University of Chicago in 1942 was the first artificial nuclear reactor, it was not the first fission reactor to exist on Earth. In 1972, a group of French scientists discovered that uranium ore from a deposit in the Oklo mine in Gabon, West Africa, contained 0.4% 235U instead of the 0.72% abundance found in all other sources of this ore. Analysis of the trace elements in the ore suggested that the amount of 235U in this ore was unusually small because natural fission reactors operated in this deposit for a period of 600,000 to 800,000 years about 2 billion years ago.

Nuclear fission has proved that it can produce greenhouse-gas-free energy: the roughly 440 nuclear plants operating in 31 countries around the world collectively have the capacity to generate some 370 gigawatts of electrical power, or about 15% of the global total. But fission power also produces a stream of radioactive nuclear waste, laced with potentially bomb-grade plutonium — some 10,000 tonnes of waste per year, worldwide. No doubt, highly radioactive waste is produced, but the volume is rather low: only about 1m^3 i.e. about 28 tons of irradiated fuel per GWyr. In addition, about 27 tons of the irradiated fuel can in principle be reprocessed and reused in other reactors as it consists of a mixture of about 224 kg 235U, 26400 kg 238U and 170 kg of fissile Pu isotopes, the rest – fission products and non-fissile elements – must be disposed of. Hence, in the strict sense only 1 ton or about 50 dm^3 of highly active waste is produced per GWyr (the total volume after packaging for disposal becomes about 4m^3). Moreover, the danger of this waste is known and new methods are being developed to store it in a safe way, or even to eliminate it by transmutation thereby producing energy. This is in sharp contrast with the large amount of waste produced by burning fossil fuels: gigatons of CO2 spread around the world and nobody knows what will be the precise consequences!

While nuclear fission may be less damaging to the environment than burning oil or coal, this energy source has faced its own crises in the form of pollution from radioactive waste and deadly meltdowns of aging power plants like Chernobyl and Fukushima. As a result, public opinion on nuclear energy in the U.S. remains lukewarm even today, according to a 2022 Pew Research Survey.

However, it may be too soon to count nuclear fission out quite yet. In recent years there have been advances in both materials (e.g. molten salt instead of water coolant) and machine learning software incorporated into these plants that make them safer than their predecessors. Additionally, dedicating large complexes to nuclear power plants may become less popular as small modular reactors (SMRs) and microreactors come on the scene. Ranging between the size of a shipping container and a jet engine, these smaller scale reactors are designed to be more nimble than traditional nuclear power plants. For this reason it may be easier in the future to run an SMR in a remote community to create sustainable power or to power a spacecraft using a microreactor. Companies like NuScale, TerraPower and X-Energy are already hard at work to bring these possibilities to life.

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Nuclear Fusion:

The word fusion means “a merging of separate elements into a unified whole”. Nuclear fusion refers to the “union of atomic nuclei to form heavier nuclei resulting in the release of enormous amounts of energy”. In the core of the Sun hydrogen is being converted into helium. This is called nuclear fusion. It takes four hydrogen atoms to fuse into each helium atom. During the process some of the mass is converted into energy. However, achieving and controlling fusion has been a lot more difficult for scientists to crack than fission.

One problem facing fusion technology is that in order to create self-sustaining power (a point called “fusion ignition”) it needs to be sparked by a massive amount of energy. In theory, after this initial power push the fusion reactor should then be able to create and sustain even more power than was initially fed into it. However, actually achieving this is easier said than done.

That said, labs like the U.S.’s National Ignition Facility (NIF) and France’s International Thermonuclear Experimental Reactor (ITER) have made progress in recent years with NIF reporting that their reactor was able to generate up to 70 percent of its input energy. Start-ups like Helion Energy’s plasma accelerator raises fusion fuel to 100 million degrees Celsius and directly extracts electricity with a high-efficiency pulsed approach.

Scientists have claimed to be on the brink of cracking nuclear fusion for decades, but hopefully with any luck that promise may finally be coming true. Scientists continue to work on controlling nuclear fusion in an effort to make a fusion reactor to produce electricity. Some scientists believe there are opportunities with such a power source since fusion creates less radioactive material than fission and has a nearly unlimited fuel supply. However, progress is slow due to challenges with understanding how to control the reaction in a contained space.

Both fission and fusion are nuclear reactions that produce energy, but the applications are not the same. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy. Fission is used in nuclear power reactors since it can be controlled, while fusion is not utilized to produce power since the reaction is not easily controlled and is expensive to create the needed conditions for a fusion reaction. Research continues into ways to better harness the power of fusion, but research is in experimental stages. While different, the two processes have an important role in the past, present and future of energy creation.

Fusion occurs in stars, such as the sun
Few radioactive particles are produced by fusion reaction, but if a fission “trigger” is used, radioactive particles will result from that
The energy released by fusion is three to four times greater than the energy released by fission
One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to “trigger” a fusion reaction
Fusion is the fusing of two or more lighter atoms into a larger one
High density, high temperature environment is required
Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion
Nuclear fusion is the reaction in which two or more nuclei combine together to form a new element with higher atomic number (more protons in the nucleus). The energy released in fusion is related to E = mc^2 (Einstein’s famous energy-mass equation). On earth, the most likely fusion reaction is Deuterium–Tritium reaction. Deuterium and Tritium are both isotopes of hydrogen. Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing 17.6 MeV of energy.

² ₁Deuterium + ³ ₁Tritium = ⁴₂He + ¹₀n + 17.6 MeV

On the other hand, nuclear fission is the splitting of a massive nucleus into photons in the form of gamma rays, free neutrons, and other subatomic particles.

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Comparison chart:

Nuclear Fission versus Nuclear Fusion comparison chart
	Nuclear Fission	Nuclear Fusion
Definition	Fission is the splitting of a large atom into two or more smaller ones.	Fusion is the fusing of two or more lighter atoms into a larger one.
Natural occurrence of the process	Fission reaction does not normally occur in nature.	Fusion occurs in stars, such as the sun.
Byproducts of the reaction	Fission produces many highly radioactive particles.	Few radioactive particles are produced by fusion reaction, but if a fission “trigger” is used, radioactive particles will result from that.
Conditions	Critical mass of the substance and high-speed neutrons are required.	High density, high temperature environment is required.
Energy Requirement	Takes little energy to split two atoms in a fission reaction.	Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion.
Energy Released	The energy released by fission is a million times greater than that released in chemical reactions, but lower than the energy released by nuclear fusion.	The energy released by fusion is three to four times greater than the energy released by fission.
Nuclear weapon	One class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb.	One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to “trigger” a fusion reaction.
Energy production	Fission is used in nuclear power plants.	Fusion is an experimental technology for producing power.
Fuel	Uranium is the primary fuel used in power plants.	Hydrogen isotopes (Deuterium and Tritium) are the primary fuel used in experimental fusion power plants.

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What are the benefits of pursuing fusion as compared to next-generation nuclear fission reactors?

Fusion and fission are totally different scientific and technological concepts, although both involve nuclear reactions. The fuel assemblies in the core of a fission reactor contain several tons of radioactive fuel which generates energy by the splitting (“fissioning”) of atomic nuclei in a chain reaction. Fusion is not a chain reaction. The entire system contains a few kilograms of the radioactive fuel component (tritium) with only a few grams reacting at any given time in the reaction chamber.

Three very unique safety features make fusion technology an attractive option to pursue for future large-scale electricity production.

First, fusion presents no risk of nuclear proliferation. Unlike the fissile materials such as uranium and plutonium used in fission reactors, tritium is neither a fissile nor a fissionable material. There are no enriched materials in a fusion reactor like ITER that could be exploited to make nuclear weapons.

Second, nuclear fusion reactors would produce no high activity/long-life nuclear waste. The “burnt” fuel is helium, a non-radioactive gas. Radioactive substances in the system are the fuel (tritium) and materials activated while the machine is running. The goal of the ongoing R&D program is for fusion reactor material to be recyclable in less than 100 years.

Third, fusion reactions are intrinsically safe. A “runaway” reaction and the resulting uncontrolled production of energy is impossible with fusion. Fusion reactions cannot be maintained spontaneously: any disturbance or failure stops the reaction. This is why it is said that fusion has inherent safety aspects. Moreover, the loss of the cooling function due to an earthquake or flood would not affect the confinement barrier at all. Even in the case of the total failure of the water cooling system, ITER’s confinement barriers will remain intact. The temperatures of the vacuum vessel that provides the confinement barrier would under no circumstances reach the melting temperatures of the materials.

Nuclear risks associated with fusion relate to the use of tritium, which is a radioactive form (isotope) of hydrogen. However, the amount used is limited to a few grams of tritium for the reaction and a few kilograms on site. During operation, the radiological impact of the use of tritium on the most exposed population is much smaller than that due to natural background radiation. For ITER, no accident scenario has been identified that would imply the need to take countermeasures to protect the surrounding population.

Advantage of Nuclear Fusion over Nuclear Fission:

In case of fusion reactions, fusion reactors cannot sustain a chain reaction so they can never melt down like fission reactors
Fusion reaction produces very less or, if the right atoms are chosen, no radioactive waste
In case of nuclear fission large radioactive waste is produced and disposal of radioactive waste is a complicated problem
The energy released by fusion is three to four times greater than the energy released by fission. This is because the amount of mass transformed into energy is that much greater in a fusion reaction than in a fission reaction
Fusion is essentially inexhaustible, low-cost fuel, available worldwide
High energy-density of fuel in fusion, allowing straightforward base-load power production without major transportation cost
In fusion there is no production of greenhouse gas, soot or acid rain, and no possibility of runaway reaction or meltdown that could pose a risk to public safety with minimal proliferation risk
Fusion has only short-lived radioactive wastes

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In fission, energy is gained by splitting heavy atoms, for example uranium, into smaller atoms such as iodine, caesium, strontium, xenon and barium, to name just a few. However, fusion is combining light atoms, for example two hydrogen isotopes, deuterium and tritium, to form the heavier helium. Both reactions release energy which, in a power plant, would be used to boil water to drive a steam generator, thus producing electricity.

Fission and chain reactions:

Fission is the nuclear process that is currently run in nuclear power plants. It is triggered by uranium absorbing a neutron, which renders the nucleus unstable. The result of the instability is the nucleus breaking up, in any one of many different ways, and producing more neutrons, which in turn hit more uranium atoms and make them unstable and so on. This chain reaction is the key to fission reactions, but it can lead to a runaway process resulting in nuclear accidents. In conventional nuclear power stations today, there are systems in place to moderate the chain reactions to prevent accident scenarios and stringent security measures to deal with proliferation issues.

Fusion: inherently safe but challenging:

Unlike nuclear fission, the nuclear fusion reaction in a tokamak is an inherently safe reaction. The reasons that have made fusion so difficult to achieve to date are the same ones that make it safe: it is a finely balanced reaction which is very sensitive to the conditions – the reaction will die if the plasma is too cold or too hot, or if there is too much fuel or not enough, or too many contaminants, or if the magnetic fields are not set up just right to control the turbulence of the hot plasma. This is why fusion is still in the research and development phase – and fission is already making electricity.

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Dr. Gerald Kulcinski provided the following graphic comparing the societal impacts of nuclear fission and various forms of nuclear fusion technology.

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Fission and Fusion Yields are depicted in the figure below:

Deuterium-tritium fusion and uranium-235 fission are compared in terms of energy yield. Both the single event energy and the energy per kilogram of fuel are compared. Then they are expressed in terms of a nominal per capita U.S. energy use: 5 x 10^11 joules. This figure is dated and probably high, but it gives a basis for comparison. The values above are the total energy yield, not the energy delivered to a consumer.

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Does nuclear fusion release more energy than nuclear fission?

It doesn’t, not atom per atom. A typical fission reaction releases about 200 MeV of energy. A typical fusion reaction releases about 18 MeV. That’s less than 10% of the energy released by fission!

However, if you have equal weights of fuel, then the energy released by fusion is greater. That’s because one atom of U-235 has a mass of 235 nuclei, whereas the mass of D + T has the mass of 5 nuclei. That’s a factor of 47 less weight. So for equal weights of fuel, fusion releases four times more energy.

In most US thermonuclear weapons, about half of the energy comes from fission! The fusion core emits a large number of fast neutrons, and these neutrons are capable of causing fission in ordinary U-238. So the fusion core in such a bomb is surrounded by U-238, and the induced fission gives about half of the energy. It also gives almost all of the fallout. But in the neutron bomb, this layer is omitted, resulting in much less fallout. The emitted neutrons, which can kill anyone nearby, give the neutron bomb its name.

Note:

U238 is more stable than U235 and when you blast the thermal neutron into it, instead of fissioning into barium and krypton it will absorb the neutron, then release a beta particle and be converted into plutonium. However, 238U can produce energy via “fast” fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of 238U to split, but Uranium-238 cannot maintain a self-sustaining fission reaction.

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Section-4

Nuclear fusion in stars:

Around 1920, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper The Internal Constitution of the Stars. At that time, the source of stellar energy was unknown; Eddington correctly speculated that the source was fusion of hydrogen into helium, liberating enormous energy according to Einstein’s equation E = mc^2. This was a particularly remarkable development since at that time fusion and thermonuclear energy had not yet been discovered, nor even that stars are largely composed of hydrogen. Eddington’s paper reasoned that:

-1. The leading theory of stellar energy, the contraction hypothesis, should cause the rotation of a star to visibly speed up due to conservation of angular momentum. But observations of Cepheid variable stars showed this was not happening.

-2. The only other known plausible source of energy was conversion of matter to energy; Einstein had shown some years earlier that a small amount of matter was equivalent to a large amount of energy.

-3. Francis Aston had also recently shown that the mass of a helium atom was about 0.8% less than the mass of the four hydrogen atoms which would, combined, form a helium atom (according to the then-prevailing theory of atomic structure which held atomic weight to be the distinguishing property between elements; work by Henry Moseley and Antonius van den Broek would later show that nucleic charge was the distinguishing property and that a helium nucleus, therefore, consisted of two hydrogen nuclei plus additional mass). This suggested that if such a combination could happen, it would release considerable energy as a byproduct.

-4. If a star contained just 5% of fusible hydrogen, it would suffice to explain how stars got their energy. (it is now known that most ‘ordinary’ stars contain far more than 5% hydrogen.)

-5. Further elements might also be fused, and other scientists had speculated that stars were the “crucible” in which light elements combined to create heavy elements, but without more accurate measurements of their atomic masses nothing more could be said at the time.

All of these speculations were proven correct in the following decades.

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Nuclear fusion and nucleosynthesis:

Before we begin studying the nuclear reactions in stars, let us understand the basic elementary structure of the Universe. The Universe is made up of two major elements: hydrogen and helium. Stars form when huge clouds of dust and gas collapse under their own gravity. These clouds are also made up of hydrogen and helium.

In Astrophysics, in contrast to conventions in chemistry, every element except hydrogen and helium is termed as metal. So in Astrophysics, non metals such as carbon, nitrogen, oxygen etc are all called metals. This is just a convention due to the relative abundance of the first two elements. Now, stars begin their life with fusion of hydrogen.

Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright.

Nuclear fusion is an atomic reaction that fuels stars. In fusion, many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc^2). Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium. The production of new elements via nuclear reactions is called nucleosynthesis. An important fusion process is the stellar nucleosynthesis that powers stars, including the Sun. In the 20th century, it was recognized that the energy released from nuclear fusion reactions accounts for the longevity of stellar heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei. A star’s mass determines what other type of nucleosynthesis occurs in its core (or during explosive changes in its life cycle). Different reaction chains are involved, depending on the mass of the star (and therefore the pressure and temperature in its core).

Small stars:

The smallest stars only convert hydrogen into helium.

Medium-sized stars (like our Sun):

Late in their lives, when the hydrogen becomes depleted, stars like our Sun can convert helium into oxygen and carbon.

Massive stars (greater than five times the mass of the Sun):

When their hydrogen becomes depleted, high mass stars convert helium atoms into carbon and oxygen, followed by the fusion of carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Later reactions transform these elements into calcium, iron, nickel, chromium, copper and others. When these old, large stars with depleted cores supernova, they create heavy elements (all the natural elements heavier than iron) and spew them into space, forming the basis for life. Each of us is made from atoms that were produced in stars and went through a supernova. A supernova is an explosion of a massive supergiant star. Supernovas are the source of elements heavier than iron. Spectroscopic analysis of the ring of material ejected by Supernova 1987A observable in the southern hemisphere, shows evidence of heavy elements.

The table below shows temperature (Celsius) for various nuclear fusion reactions in stars:

Scientist now believe that many heavy elements found on Earth and throughout the universe were originally synthesized by fusion within the hot cores of the stars. This process is known as nucleosynthesis. For example, in lighter stars, hydrogen combines to form helium through the proton-proton chain. Once the hydrogen fuel is exhausted, the star enters the next stage of its life and fuses helium. An example of a nuclear reaction chain that can occur is:

4/2He+4/2He→8/4Be+γ,

8/4Be+4/2He→12/6C+γ,

12/6C+4/2He→16/8O+γ⋅

Carbon and oxygen nuclei produced in such processes eventually reach the star’s surface by convection. Near the end of its lifetime, the star loses its outer layers into space, thus enriching the interstellar medium with the nuclei of heavier elements. A planetary nebula is produced at the end of the life of a star. The greenish color of this planetary nebula comes from oxygen ions.

Stars similar in mass to the Sun do not become hot enough to fuse nuclei as heavy (or heavier) than oxygen nuclei. However, in massive stars whose cores become much hotter (T>6×10^8K), even more complex nuclei are produced. Some representative reactions are

12/6C+12/6C→23/11Na+1/1H,

12/6C+12/6C→24/12Mg+γ,

12/6C+16/8O→28/14Si+γ⋅

Nucleosynthesis continues until the core is primarily iron-nickel metal. Now, iron has the peculiar property that any fusion or fission reaction involving the iron nucleus is endothermic, meaning that energy is absorbed rather than produced. Hence, nuclear energy cannot be generated in an iron-rich core. Lacking an outward pressure from fusion reactions, the star begins to contract due to gravity. This process heats the core to a temperature on the order of 5×10^9K. Expanding shock waves generated within the star due to the collapse cause the star to quickly explode. The luminosity of the star can increase temporarily to nearly that of an entire galaxy. During this event, the flood of energetic neutrons reacts with iron and the other nuclei to produce elements heavier than iron. These elements, along with much of the star, are ejected into space by the explosion. Supernovae and the formation of planetary nebulas together play a major role in the dispersal of chemical elements into space.

Eventually, much of the material lost by stars is pulled together through the gravitational force, and it condenses into a new generation of stars and accompanying planets. The new generation of stars begins the nucleosynthesis process anew, with a higher percentage of heavier elements. Thus, stars are “factories” for the chemical elements, and many of the atoms in our bodies were once a part of stars.

In massive stars there is an ‘onion skin’ of fusion shells with the outer layers dropping fuel to lower layers and heavier and heavier nuclei being cooked up as you move towards the center of the star.

Hydrogen Fusion:

Hydrogen fusion is the fundamental nuclear reaction in stars. Any star that is fusing hydrogen in its core is known as a main sequence star. Our Sun is a main sequence star. The two most prominent reactions that fuse hydrogen into helium are: PP Chain and CNO Cycle.

The PP Chain:

PP Chain stands for Proton-Proton chain. In this reaction, 4 hydrogen nuclei combine to form 1 helium nucleus. The PP Chain is one of the most important nuclear reactions in stars. Two protons come together and form a deuterium nucleus (one proton and one neutron). This is a two step process. First two protons combine to form a diproton. Then one of the two protons changes into a neutron by releasing a positron and a neutrino (beta plus decay). Now, on this deuterium, another proton attacks and forms helium-3. This helium-3 combines with another helium-3 produced parallel to it and forms a helium-4 thereby releasing 2 hydrogen atoms as shown. Note that the total mass number (number of nucleons) is always conserved. This nuclear reaction is the reason behind the existence of every life form on Earth. This is how the Sun is producing its energy. A single reaction produces 26.7 MeV of energy. In a single second, the Sun produces more energy than produced by the mankind so far. The PP chain initiates at about 15 million K. So, when the temperature of the collapsing cloud of gas reaches this mark, stars are formed. This reaction is slow. For a Sun like star, it will take 10 billion years to convert hydrogen into helium in its core.

The CNO Cycle:

CNO stands for Carbon-Nitrogen-Oxygen. The CNO cycle is yet another nuclear reaction by which stars produce helium from hydrogen using carbon, nitrogen and oxygen as catalysts. The CNO cycle is a dominant source of energy for stars that are about 1.3 times more massive than the Sun. This reaction becomes dominant at about 17 million K. The core temperature of Sun is 15 million K and thus PP chain is the dominant reaction on Sun. In heavier stars, the CNO cycle and other processes are more important.

The CNO cycle dominates in stars heavier than the Sun as seen in the figure above.

As a star uses up a substantial fraction of its hydrogen, it begins to synthesize heavier elements.

Helium Fusion: Triple Alpha Process:

Once all the hydrogen has been converted into helium in the core, it is time for the next nuclear reaction. After helium, carbon forms via the triple alpha process. This reaction is simple. Two helium-4 nuclei come together and form beryllium-8. This beryllium-8 nucleus is further attacked by a helium-4 and forms a stable carbon-12. The net release of energy is about 7.275 MeV and the reaction requires a temperature of 100 million K. Once a star starts burning helium to carbon, end of the star is near.

Production of Heavier Elements:

The reaction sequence does not stop at carbon. However, it should be noted that only massive stars can host full scale nuclear reactions beyond this point. Let us glance over some key nuclear reactions in stars beyond helium.

Carbon Fusion:

Carbon fusion begins at a whooping 500 million K. The common products of this reaction are neon, oxygen, sodium and magnesium. Stars below 8 solar masses cannot host a carbon fusion. Stars between 8-11 solar masses begin carbon fusion with a flash but this disrupts the star. The ones with mass above 11 solar masses go on to fuse even heavier elements.

Neon Burning:

Neon burning begins at temperature of around 1.2 billion K. During neon burning, oxygen and magnesium accumulate in the central core while neon is consumed. After a few years the star consumes all its neon and the core ceases producing fusion energy and contracts.

Oxygen Burning:

The oxygen core that forms due to previous nuclear reactions requires very high temperatures to fuse further elements. At about 2 billion K, oxygen core transforms into a silicon, phosphorus and sulfur core. This reaction takes place in a few years and the amount of energy released is tremendous.

Alpha Ladder:

Once silicon forms in the core, a ladder of reaction begins. Silicon has a mass number of 28. The reaction sequence stops at Ni-56. The next element in the chain is Zn-60 but conversion from Ni to Zn is thermodynamically unfavorable. This is because the reaction is endothermic (absorbs energy). Silicon fusion begins at about 3 billion K. The intensity of this reaction can be realized from the fact that while PP chain took 10 billion years to finish, silicon burning ends in a single day. So nickel and iron are the last major fusion products in the core. This progression of nuclear fusions ends even for the most massive stars when iron dominates the stellar core. This is because iron is an extremely stable element and stars aren’t massive enough to trigger its fusion.

When all nuclear fusion ceases, the star undergoes a final and catastrophic gravitational collapse. This triggers a supernova that flings the elements the star has forged during its lifetime out into the universe. This material from these dead stars becomes the building blocks of the next generation of stars, the planets, and everything around us, including our own human bodies. The heaviest elements are synthesized by fusion that occurs when a more massive star undergoes a violent supernova at the end of its life, a process known as supernova nucleosynthesis. After a supernova explosion, the star’s core is left. If the core is less dense, it becomes a neutron star. A neutron star is made almost all of neutrons. If the core is more dense, it becomes a black hole.

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Energy generation in stars:

(i) Nuclear fusion occurs is the core of the stars, since its temperature is of the order of 10^7K.

(ii) Most of the stars including our Sun fuse hydrogen into helium and some stars even fuse helium into heavier elements.

(iii) The early stage of a star is in the form of cloud and dust.

(iv) Due to their own gravitational pull, these clouds fall inward.

(v) As a result, its gravitational potential energy is converted to kinetic energy and finally into heat.

(vi) When the temperature is high enough to initiate the thermonuclear fusion, they start to release enormous energy which tends to stabilize the star and prevents it from further collapse.

(vii) The sun’s interior temperature is around 1.5×10^7K.

(viii) The sun is converting 6×10^11 kg hydrogen into helium every second and it has enough hydrogen such that these fusion lasts for another 5 billion years.

(ix) When the hydrogen is burnt out, the sun will enter into new phase called red giant where helium will fuse to become carbon. During this stage, sun will expand greatly in size and all its planets will be engulfed in it.

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Nuclear fusion in Sun:

According to NASA, the Sun can be divided into several layers. From the center of the Sun outward, the layers are: the core, radiative zone, convective zone, photosphere, chromosphere, and the corona. The chromosphere and corona are often referred to as the Sun’s atmosphere. Nuclear fusion occurs in the core of the Sun. It is there where there is so much pressure and heat that atoms of hydrogen fuse and become helium. The radiative zone and convection zone are areas through which energy is moved outward from the core.

The Sun is a hot star. Really hot star. But all of the heat and light coming from the Sun comes from the fusion reactions happening inside the core of the Sun. Inside the Sun, the pressure is millions of times more than the surface of the Earth, and the temperature reaches more than 15 million Kelvin. Massive gravitational forces create these conditions for nuclear fusion. On Earth, it is impossible to achieve such conditions.

The Sun burns hydrogen atoms, which fuse to form helium nuclei, and a small amount of matter is converted into energy. At its core, the Sun consumes approximately 600 million metric tons of hydrogen each second. Hydrogen, heated to very high temperatures, changes from a gaseous state to a plasma state. Normally, fusion is not possible because the strongly repulsive electrostatic forces between the positively charged nuclei prevent them from getting close enough together to collide and for fusion to occur. The mechanism to overcome the coulomb barrier is by the temperature and by the pressure. Temperature gives kinetic energy to particles and pressure confines particles. At close distances, the attractive nuclear force allows the nuclei to fuse. The Sun’s gravitational force confines the positively-charged hydrogen nuclei and the high temperatures cause the nuclei to move around furiously. As a result they collide at high speeds overcoming the natural electrostatic repulsion that exists between the positive charges and subsequently fuse to form the heavier helium. During most of the Sun’s life, energy has been produced by nuclear fusion in the core region through the proton-proton chain. This process converts hydrogen into helium.

In general, proton-proton fusion can occur only if the kinetic energy of the protons is high enough to overcome their mutual electrostatic repulsion. The first step in all the branches is the fusion of two protons into a deuteron. As the protons fuse, one of them undergoes beta plus decay, converting into a neutron by emitting a positron and an electron neutrino. It must be noted a deuteron-producing event is very rare due to it being initiated by a weak nuclear force. Diprotons are the much more common result of proton-proton reactions, but diprotons almost immediately decay back into two protons. The cross-section of the deuteron-producing reaction is so small that it has not been possible to measure it experimentally. This is very important because it significantly limits the reaction rate. The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton. The Q value for the deuteron-producing reaction is plus 1.442 megaelectron volts. The Q value for a reaction is the amount of energy absorbed or released during the nuclear reaction. The deuteron produced in the first stage can then fuse with another proton to produce the isotope of helium 3. The Q value is 5.5 megaelectron volts. This second process is mediated by the strong nuclear force rather than the weak force. Therefore, it is extremely fast in comparison to the first step. It is estimated that each newly created deuterium nucleus exists for only about one second before it is converted into helium-3. Helium-3 is then in various branches converted into helium-4.

The primary source of solar energy, and that of similar size stars, is the fusion of hydrogen to form helium (the proton–proton chain reaction), which occurs at a solar-core temperature of 15 million kelvin. The net result is the fusion of four protons into one alpha particle, with the release of two positrons and two neutrinos (which changes two of the protons into neutrons), and energy.

The Sun produces energy by fusing protons or hydrogen nuclei 1H (by far the Sun’s most abundant nuclide) into helium nuclei 4He. The principal sequence of fusion reactions forms what is called the proton-proton cycle:

The combined reactions are as follows:

¹H + ¹H → ²H + e⁺+ v_e, (1.44 MeV)

¹H + ²H → ³He + γ (5.49 MeV)

And

³He + ³He → ⁴He + ¹H + ¹H (12.86 MeV)

where e⁺ stands for a positron and is v_e an electron neutrino. (The energy in parentheses is released by the reaction.) Note that the first two reactions must occur twice for the third to be possible, so that the cycle consumes six protons (1H) but gives back two. Furthermore, the two positrons produced will find two electrons and annihilate to form four more γ rays, for a total of six.

The overall effect of the cycle is thus

4 ¹H⁺ + 2 e^– → ⁴He²⁺ + 2 ν_e+ 6γ (26.7 MeV)

where the 26.7 MeV includes the annihilation energy of the positrons and electrons and is distributed among all the reaction products.

The atomic mass of one hydrogen atom is roughly 1.008 amu. When four hydrogen atoms combine, the total mass of resultant products should add up to 4.032 amu due to mass conservation. However, the resultant helium nucleus weighs less than 4.032 amu (4.003 amu) which means the lost mass has converted into energy.

1 amu = 1.67377 x 10^-27 kilogram

The mass difference between the helium nucleus and the hydrogen nuclei is 4.8 x 10^−29 kg. So, the energy released is:

E = (4.8 x 10^−29 kg) x (3.0 x 10^8 m/sec)^2 = 4.3 x 10^−12 Joules each time the proton-proton chain creates one helium nucleus.

Of all of the mass that undergoes this fusion process, only about 0.7% of it is turned into energy. By this process our Sun converts 600 million tons of hydrogen into 596 million tons of helium every second. Using the mass-energy equivalence, we find that this 4 million tonnes of matter is equal to about 3.6 x 10^26 joules of energy released per second!

The solar interior is dense, and the reactions occur deep in the Sun where temperatures are highest. It takes about 32,000 years for the energy to diffuse to the surface and radiate away. However, the neutrinos escape the Sun in less than two seconds, carrying their energy with them, because they interact so weakly that the Sun is transparent to them. Negative feedback in the Sun acts as a thermostat to regulate the overall energy output. For instance, if the interior of the Sun becomes hotter than normal, the reaction rate increases, producing energy that expands the interior. This cools it and lowers the reaction rate. Conversely, if the interior becomes too cool, it contracts, increasing the temperature and reaction rate. Stars like the Sun are stable for billions of years, until a significant fraction of their hydrogen has been depleted.

The release of fusion energy produces an outward thermal gas pressure that prevents the Sun from gravitational collapse. Astrophysicists find that hydrogen fusion supplies the energy stars require to maintain energy balance over most of a star’s life span.

The figure shows the Sun as a circle and the Sun’s core as a smaller concentric circle within it. Arrows labeled fusion radiate outwards from the core. Arrows labeled gravity radiate inwards from the surface.

The Sun produces energy by fusing hydrogen into helium at the Sun’s core. The red arrows show outward pressure due to thermal gas, which tends to make the Sun expand. The blue arrows show inward pressure due to gravity, which tends to make the Sun contract. These two influences balance each other.

The stages of stellar evolution are the result of compositional changes over very long periods. The size of a star, on the other hand, is determined by a balance between the pressure exerted by the hot plasma and the gravitational force of the star’s mass. The energy of the burning core is transported toward the surface of the star, where it is radiated at an effective temperature. The effective temperature of the Sun’s surface is about 6,000 K, and significant amounts of radiation in the visible and infrared wavelength ranges are emitted.

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So, now that we know how energy is generated inside a star via nuclear fusion, we can answer the following question:

Why does the onset of nuclear fusion signal the transition of a protostar into a true star?

The answer is that the nuclear fusion generates energy, and this energy provides enough radiation pressure to finally balance the inward pull of gravity, stopping the contraction that began when the clump of gas began to collapse in on itself. The energy generated in the star is being radiated outwards as photons of light. As the photons pass through the star, they created a net outward push (radiation pressure), which along with the thermal pressure of the material in the star, resists gravity. When the force of gravity is exactly balanced by the total pressure, we say that the star is in hydrostatic equilibrium.

There are several final points that should be mentioned. The temperature that the core of a protostar reaches depends on its mass. The more massive the protostar, the hotter it gets. If the core reaches a high enough temperature (more than 20 million kelvin), a different set of fusion reactions proceed more efficiently than the proton-proton chain. This process, called the CNO (carbon-nitrogen-oxygen) cycle, occurs in stars more massive than the Sun. The CNO cycle still requires hydrogen to proceed, so even in these stars the main fuel for the fusion reaction is hydrogen. In both the proton-proton chain and the CNO cycle, one element is being converted into another via nuclear fusion. This process of creating new elements is called nucleosynthesis.

Finally, if there is no way for us to directly observe the core of a star, how do we know that nuclear fusion is indeed its power source? The answer is in the first step of the proton-proton chain—the process also generates neutrinos. Neutrinos can pass through large quantities of matter (e.g., the entire Sun) without interacting in any way, so the neutrinos that are generated leave the Sun and travel through space. They are very difficult to detect, but on Earth, several experiments have detected solar neutrinos, verifying the Sun’s core is generating energy via the proton-proton chain.

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Section-5

Science of nuclear fusion reactions:

Types of fusion based on process employed:

-1. Thermonuclear fusion:

Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. Such temperatures cause the matter to become a plasma and, if confined, fusion reactions may occur due to collisions with extreme thermal kinetic energies of the particles. There are two forms of thermonuclear fusion: uncontrolled, in which the resulting energy is released in an uncontrolled manner, as it is in thermonuclear weapons (“hydrogen bombs”) and in most stars; and controlled, where the fusion reactions take place in an environment allowing some or all of the energy released to be harnessed for constructive purposes.

Temperature is a measure of the average kinetic energy of particles, so by heating the material it will gain energy. After reaching sufficient temperature, given by the Lawson criterion, the energy of accidental collisions within the plasma is high enough to overcome the Coulomb barrier and the particles may fuse together.

In a deuterium–tritium fusion reaction, for example, the energy necessary to overcome the Coulomb barrier is 0.1 MeV. Converting between energy and temperature shows that the 0.1 MeV barrier would be overcome at a temperature in excess of 1.2 billion kelvin.

There are two effects that are needed to lower the actual temperature. One is the fact that temperature is the average kinetic energy, implying that some nuclei at this temperature would actually have much higher energy than 0.1 MeV, while others would be much lower. It is the nuclei in the high-energy tail of the velocity distribution that account for most of the fusion reactions. The other effect is quantum tunnelling. The nuclei do not actually have to have enough energy to overcome the Coulomb barrier completely. If they have nearly enough energy, they can tunnel through the remaining barrier. For these reasons fuel at lower temperatures will still undergo fusion events, at a lower rate.

Thermonuclear fusion is one of the methods being researched in the attempts to produce fusion power. If thermonuclear fusion becomes favorable to use, it would significantly reduce the world’s carbon footprint.

-2. Beam–beam or beam–target fusion:

Accelerator-based light-ion fusion is a technique using particle accelerators to achieve particle kinetic energies sufficient to induce light-ion fusion reactions. Accelerating light ions is relatively easy, and can be done in an efficient manner—requiring only a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kV between the electrodes. The system can be arranged to accelerate ions into a static fuel-infused target, known as beam–target fusion, or by accelerating two streams of ions towards each other, beam–beam fusion.

The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb interaction cross-sections. Therefore, the vast majority of ions expend their energy emitting bremsstrahlung radiation and the ionization of atoms of the target. Devices referred to as sealed-tube neutron generators are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement that allows ions of those nuclei to be accelerated against hydride targets, also containing deuterium and tritium, where fusion takes place, releasing a flux of neutrons. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves.

A number of attempts to recirculate the ions that “miss” collisions have been made over the years. One of the better-known attempts in the 1970s was Migma, which used a unique particle storage ring to capture ions into circular orbits and return them to the reaction area. Theoretical calculations made during funding reviews pointed out that the system would have significant difficulty scaling up to contain enough fusion fuel to be relevant as a power source. In the 1990s, a new arrangement using a field-reverse configuration (FRC) as the storage system was proposed by Norman Rostoker and continues to be studied by TAE Technologies as of 2021. A closely related approach is to merge two FRC’s rotating in opposite directions, which is being actively studied by Helion Energy. Because these approaches all have ion energies well beyond the Coulomb barrier, they often suggest the use of alternative fuel cycles like p-11B that are too difficult to attempt using conventional approaches.

-3. Muon-catalyzed fusion:

Muon-catalyzed fusion is a fusion process that occurs at ordinary temperatures. It was studied in detail by Steven Jones in the early 1980s. Net energy production from this reaction has been unsuccessful because of the high energy required to create muons, their short 2.2 µs half-life, and the high chance that a muon will bind to the new alpha particle and thus stop catalyzing fusion.

-4. Other principles:

Some other confinement principles have been investigated.

Antimatter-initialized fusion uses small amounts of antimatter to trigger a tiny fusion explosion. This has been studied primarily in the context of making nuclear pulse propulsion, and pure fusion bombs feasible. This is not near becoming a practical power source, due to the cost of manufacturing antimatter alone.
Pyroelectric fusion was reported in April 2005 by a team at UCLA. The scientists used a pyroelectric crystal heated from −34 to 7 °C (−29 to 45 °F), combined with a tungsten needle to produce an electric field of about 25 gigavolts per meter to ionize and accelerate deuterium nuclei into an erbium deuteride target. At the estimated energy levels, the D–D fusion reaction may occur, producing helium-3 and a 2.45 MeV neutron. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces. D–T fusion reactions have been observed with a tritiated erbium target.
Nuclear fusion–fission hybrid (hybrid nuclear power) is a proposed means of generating power by use of a combination of nuclear fusion and fission processes. The concept dates to the 1950s, and was briefly advocated by Hans Bethe during the 1970s, but largely remained unexplored until a revival of interest in 2009, due to the delays in the realization of pure fusion.
Project PACER, carried out at Los Alamos National Laboratory (LANL) in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small hydrogen bombs (fusion bombs) inside an underground cavity. As an energy source, the system is the only fusion power system that could be demonstrated to work using existing technology. However it would also require a large, continuous supply of nuclear bombs, making the economics of such a system rather questionable.
Bubble fusion also called sonofusion was a proposed mechanism for achieving fusion via sonic cavitation which rose to prominence in the early 2000s. Subsequent attempts at replication failed and the principal investigator, Rusi Taleyarkhan, was judged guilty of research misconduct in 2008.

Note:

In this article, fusion means thermonuclear fusion unless specified otherwise.

Nuclear fusion is the process by which two or more atomic nuclei join together, or “fuse,” to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to energy, which is released. Fusion is the process that powers active stars, releasing large quantities of energy.

The origin of the energy released in fusion of light elements is due to an interplay of two opposing forces: the nuclear force that draws together protons and neutrons, and the Coulomb force that causes protons to repel each other. The protons are positively charged and repel each other, but they nonetheless stick together, demonstrating the existence of another force referred to as nuclear attraction. This force, called the strong nuclear force, overcomes electric repulsion in a very close range.

The effect of nuclear force is not observed outside the nucleus, hence the force has a strong dependence on distance; it a short-range force. The same force also pulls the nucleons, or neutrons and protons, together. The nuclear force is stronger than the Coulomb force for atomic nuclei smaller than iron, so building up these nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of these particles. For larger nuclei, no energy is released, since the nuclear force is short-range and cannot continue to act across an even larger atomic nuclei. Therefore, energy is no longer released when such nuclei are made by fusion; instead, energy is absorbed.

Fusion reactions of light elements power the stars and produce virtually all elements in a process called nucleosynthesis. The fusion of lighter elements in stars releases energy, as well as the mass that always accompanies it. For example, in the fusion of two hydrogen nuclei to form helium, seven-tenths of one percent of the mass is carried away from the system in the form of kinetic energy or other forms of energy, like electromagnetic radiation.

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Requirements for nuclear fusion:

A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances, two nuclei repel one another because of the repulsive electrostatic force between their positively charged protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the attractive nuclear force, which is stronger at close distances.

The electrostatic force between the positively charged nuclei is repulsive, but when the separation is small enough, the quantum effect will tunnel through the wall. Therefore, the prerequisite for fusion is that the two nuclei be brought close enough together for a long enough time for quantum tunneling to act. At nucleus radii distances, the attractive nuclear force is stronger than the repulsive electrostatic force. Therefore, the main technical difficulty for fusion is getting the nuclei close enough to fuse.

When a nucleon such as a proton or neutron is added to a nucleus, the nuclear force attracts it to all the other nucleons of the nucleus (if the atom is small enough), but primarily to its immediate neighbors due to the short range of the force. The nucleons in the interior of a nucleus have more neighboring nucleons than those on the surface. Since smaller nuclei have a larger surface-area-to-volume ratio, the binding energy per nucleon due to the nuclear force generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons. It is important to keep in mind that nucleons are quantum objects. So, for example, since two neutrons in a nucleus are identical to each other, the goal of distinguishing one from the other, such as which one is in the interior and which is on the surface, is in fact meaningless, and the inclusion of quantum mechanics is therefore necessary for proper calculations.

The electrostatic force, on the other hand, is an inverse-square force, so a proton added to a nucleus will feel an electrostatic repulsion from all the other protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei atomic number grows.

The net result of the opposing electrostatic and strong nuclear forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei. So very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable. The four most tightly bound nuclei, in decreasing order of binding energy per nucleon, are 62Ni, 58Fe, 56Fe, and 60Ni. Even though the nickel isotope, 62Ni, is more stable, the iron isotope 56Fe is an order of magnitude more common. This is due to the fact that there is no easy way for stars to create 62Ni through the alpha process.

An exception to this general trend is the helium-4 nucleus, whose binding energy per nucleon is higher than that of lithium, the next heavier element. This is because protons and neutrons are fermions, which according to the Pauli exclusion principle cannot exist in the same nucleus in exactly the same state. Each proton or neutron’s energy state in a nucleus can accommodate both a spin up particle and a spin down particle. Helium-4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons (it is a doubly magic nucleus), so all four of its nucleons can be in the ground state. Any additional nucleons would have to go into higher energy states. Indeed, the helium-4 nucleus is so tightly bound that it is commonly treated as a single quantum mechanical particle in nuclear physics, namely, the alpha particle.

The situation is similar if two nuclei are brought together as seen in the figure below. As they approach each other, all the protons in one nucleus repel all the protons in the other. Not until the two nuclei actually come close enough for long enough so the strong nuclear force can overcome (by way of tunneling) the repulsive electrostatic force. Consequently, even when the final energy state is lower, there is a large energy barrier that must first be overcome. It is called the Coulomb barrier.

The Coulomb barrier is smallest for isotopes of hydrogen, as their nuclei contain only a single positive charge. A diproton is not stable, so neutrons must also be involved, ideally in such a way that a helium nucleus, with its extremely tight binding, is one of the products. Using deuterium–tritium fuel, the resulting energy barrier is about 0.1 MeV. In comparison, the energy needed to remove an electron from hydrogen is 13.6 eV. The (intermediate) result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV. The recoil energy of the remaining 4He nucleus is 3.5 MeV, so the total energy liberated is 17.6 MeV. This is many times more than what was needed to overcome the energy barrier.

D-T fusion reaction:

The most convenient, though not necessarily most efficient fusion reaction to carry out in present day magnetic fusion devices like tokamaks is that between deuterium (D) and tritium (T), both isotopes of hydrogen, one with one neutron and the other with two neutrons in their nucleus respectively (normal hydrogen has only one proton and no neutron in its nucleus). The D-T fusion is most conveniently achievable because the collision cross-section of the D-T fusion reactions is the highest and occurs at the lowest temperature.

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Cross section:

Cross section, in nuclear physics, is probability that a given atomic nucleus or subatomic particle will exhibit a specific reaction (for example, absorption, scattering, or fission) in relation to a particular species of incident particle. The probability that a reaction will occur is analogous to the probability of a random bullet hitting its target. Unfortunately, a nuclear cross section are a little more complicated than that due to quantum mechanical effects. At atomic scales, the wave properties of matter become important and strongly effect the interactions between particles. This means for a specific target, the target’s size varies depends on the energy of the bullet, type of bullet and target, and the kind of interaction between them. Using the shooting range analogy, the target would be larger for bullets with certain speeds, but smaller at others.

Cross section is expressed in terms of area, and its numerical value is chosen so that, if the bombarding particle hits a circular area of this size perpendicular to its path and centred at the target nucleus or particle, the given reaction occurs; and, if it misses the area, the reaction does not occur. The reaction cross section is usually not the same as the geometric cross-sectional area of the target nucleus or particle. The concept of a nuclear cross section can be quantified physically in terms of “characteristic area” where a larger area means a larger probability of interaction. The standard unit for measuring a nuclear cross section (denoted as σ) is the barn, which is equal to 10⁻²⁸ m², 10⁻²⁴ cm² or 100 fm². Typical nuclear radii are of the order 10⁻¹⁴ m. Assuming spherical shape, we therefore expect the cross sections for nuclear reactions to be of the order of πr² or 10⁻²⁸ m² (i.e., 1 barn). In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process. Values of cross sections depend on the energy of the bombarding particle and the kind of reaction. Boron, for example, when bombarded by neutrons traveling 1,000,000 cm per second (22,500 miles per hour), has a cross section for the neutron-capture reaction of about 120 barns, and boron’s cross section increases to about 1,200 barns for neutrons traveling at 100,000 cm per second. Because of its large cross sections, boron is a good absorber of neutrons. In contrast, neutrinos emitted in the nuclear reactions that fuel the Sun have cross sections as small as 10 −21 barn, which accounts for their very low rates of interaction.

The reaction cross section (σ) is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution, then it is useful to perform an average over the distributions of the product of cross-section and velocity. This average is called the ‘reactivity’, denoted ⟨σv⟩. The reaction rate (fusions per volume per time) is ⟨σv⟩ times the product of the reactant number densities:

F = n1n2⟨σv⟩

If a species of nuclei is reacting with a nucleus like itself, such as the DD reaction, then the product n1n2 must be replaced by n square/2

⟨σv⟩ increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of 10–100 keV. At these temperatures, well above typical ionization energies (13.6 eV in the hydrogen case), the fusion reactants exist in a plasma state.

The significance of ⟨σv⟩ as a function of temperature in a device with a particular energy confinement time is found by considering the Lawson criterion. This is an extremely challenging barrier to overcome on Earth, which explains why fusion research has taken many years to reach the current advanced technical state.

Figure above shows that the fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The deuterium-tritium fusion rate peaks at a lower temperature (about 70 keV, or 800 million kelvin) and at a higher value than other reactions commonly considered for fusion energy.

In a plasma, particle velocity can be characterized using a probability distribution. If the plasma is thermalized, the distribution looks like a Gaussian curve, or Maxwell–Boltzmann distribution. In this case, it is useful to use the average particle cross section over the velocity distribution.

Note:

The formula for temperature in keV to Kelvin:

In nuclear physics, energy is often expressed in electronvolt and multiples thereof (kiloelectronvolt or keV). As a result of popular misuse of language, temperature is sometimes also indicated in keV. The conversion factor which converts from electronvolt to Kelvin is 11606, in the sense that 1eV corresponds to 11606 K and 1 keV corresponds to 11.6 million degrees Kelvin.

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Process of nuclear fusion:

The release of energy with the fusion of light elements is due to the interplay of two opposing forces: the nuclear force, a manifestation of the strong interaction, which holds protons and neutrons tightly together in the atomic nucleus; and the Coulomb force, which causes positively charged protons in the nucleus to repel each other. Lighter nuclei (nuclei smaller than iron and nickel) are sufficiently small and proton-poor to allow the nuclear force to overcome the Coulomb force. This is because the nucleus is sufficiently small that all nucleons feel the short-range attractive force at least as strongly as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles. For larger nuclei, however, no energy is released, because the nuclear force is short-range and cannot act across larger nuclei.

Fusion powers stars and produces virtually all elements in a process called nucleosynthesis. The Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 600 million metric tons of hydrogen and makes 596 million metric tons of helium each second. The fusion of lighter elements in stars releases energy and the mass that always accompanies it. For example, in the fusion of four hydrogen nuclei to form helium, 0.645% of the mass is carried away in the form of kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation.

It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen. When accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and be brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force. The strong force grows rapidly once the nuclei are close enough, and the fusing nucleons can essentially “fall” into each other and the result is fusion and net energy produced. The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together; this is an exothermic process that can produce self-sustaining reactions.

Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17.6 MeV released in the deuterium–tritium (D–T) reaction. Fusion reactions have an energy density many times greater than nuclear fission; the reactions produce far greater energy per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones, which are themselves millions of times more energetic than chemical reactions. Only direct conversion of mass into energy, such as that caused by the annihilatory collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion. (The complete conversion of one gram of matter would release 9×10^13 joules of energy.)

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Fusion reactions between light elements, like fission reactions that split heavy elements, release energy because of a key feature of nuclear matter called the binding energy, which can be released through fusion or fission. The binding energy of the nucleus is a measure of the efficiency with which its constituent nucleons are bound together. Take, for example, an element with Z protons and N neutrons in its nucleus. The element’s atomic weight A is Z + N, and its atomic number is Z. The binding energy B is the energy associated with the mass difference between the Z protons and N neutrons considered separately and the nucleons bound together (Z + N) in a nucleus of mass M. The formula is

B = (Zm_p + Nm_n − M)c²,

where m_p and m_n are the proton and neutron masses and c is the speed of light. It has been determined experimentally that the binding energy per nucleon is a maximum of about 1.4 X 10^−12 joule at an atomic mass number of approximately 60—that is, approximately the atomic mass number of iron. Accordingly, the fusion of elements lighter than iron or the splitting of heavier ones generally leads to a net release of energy.

The three keys to practical fusion energy generation are to achieve the temperatures necessary to make the reactions likely, to raise the density of the fuel, and to confine it long enough to produce large amounts of energy. These three factors—temperature, density, and time—complement one another, and so a deficiency in one can be compensated for by the others.

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Criteria for terrestrial fusion reactions:

At the temperatures and densities in stellar cores, the rates of fusion reactions are notoriously slow. For example, at solar core temperature (T ≈ 15 MK) and density (160 g/cm3), the energy release rate is only 276 μW/cm3—about a quarter of the volumetric rate at which a resting human body generates heat. Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates depend on density as well as temperature and most fusion schemes operate at relatively low densities, those methods are strongly dependent on higher temperatures. The fusion rate as a function of temperature, leads to the need to achieve temperatures in terrestrial reactors 10–100 times higher than in stellar interiors: T ≈ (0.1–1.0)×10^9 K.

In artificial fusion, the primary fuel is not constrained to be protons and higher temperatures can be used, so reactions with larger cross-sections are chosen. Another concern is the production of neutrons, which activate the reactor structure radiologically, but also have the advantages of allowing volumetric extraction of the fusion energy and tritium breeding. Reactions that release no neutrons are referred to as aneutronic.

To be a useful energy source, a fusion reaction must satisfy several criteria. It must:

-1. Be exothermic

This limits the reactants to the low Z (number of protons) side of the curve of binding energy. It also makes helium 4He the most common product because of its extraordinarily tight binding, although 3He and 3H also show up.

-2. Involve low atomic number (Z) nuclei

This is because the electrostatic repulsion that must be overcome before the nuclei are close enough to fuse is directly related to the number of protons it contains – its atomic number.

-3. Have two reactants

At anything less than stellar densities, three-body collisions are too improbable. In inertial confinement, both stellar densities and temperatures are exceeded to compensate for the shortcomings of the third parameter of the Lawson criterion, ICF’s very short confinement time.

-4. Have two or more products

This allows simultaneous conservation of energy and momentum without relying on the electromagnetic force.

-5. Conserve both protons and neutrons

The cross sections for the weak interaction are too small.

Few reactions meet these above-mentioned criteria.

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Two types of fusion reactions:

Fusion reactions are of two basic types: (1) those that preserve the number of protons and neutrons and (2) those that involve a conversion between protons and neutrons. Reactions of the first type are most important for practical fusion energy production, whereas those of the second type are crucial to the initiation of star burning. An important fusion reaction for practical energy generation is that between deuterium and tritium (the D-T fusion reaction). It produces helium (He) and a neutron (n) and is written

D + T → He + n.

To the left of the arrow (before the reaction) there are two protons and three neutrons. The same is true on the right.

The other reaction, that which initiates star burning, involves the fusion of two hydrogen nuclei to form deuterium (the H-H fusion reaction):

¹H + ¹H → ²H + e⁺+ v_e,

where e⁺ stands for a positron and is v_e an electron neutrino. Before the reaction there are two hydrogen nuclei (that is, two protons). Afterward there are one proton and one neutron (bound together as the nucleus of deuterium) plus a positron and a neutrino (produced as a consequence of the conversion of one proton to a neutron).

Both of these fusion reactions are exoergic and so yield energy. The German-born physicist Hans Bethe proposed in the 1930s that the H-H fusion reaction could occur with a net release of energy and provide, along with subsequent reactions, the fundamental energy source sustaining the stars. However, practical energy generation requires the D-T reaction for two reasons: first, the rate of reactions between deuterium and tritium is much higher than that between protons; second, the net energy release from the D-T reaction is 12 times greater than that from the H-H reaction. The proton-proton cycle is not a practical source of energy on Earth, in spite of the great abundance of hydrogen (¹H). The reaction ¹H + ¹H → ²H + e⁺+ v_e has a very low probability of occurring.

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Fusion reactions for controlled power generation:

Reactions between deuterium and tritium are the most important fusion reactions for controlled power generation because the cross sections for their occurrence are high, the practical plasma temperatures required for net energy release are moderate, and the energy yield of the reactions are high—17.58 MeV for the basic D-T fusion reaction.

It should be noted that any plasma containing deuterium automatically produces some tritium and helium-3 from reactions of deuterium with other deuterium ions. Other fusion reactions involving elements with an atomic number above 2 can be used, but only with much greater difficulty. This is because the Coulomb barrier increases with increasing charge of the nuclei, leading to the requirement that the plasma temperature exceed 1,000,000,000 K if a significant rate is to be achieved. Some of the more interesting reactions are:

-1. H + ¹¹B → 3(⁴He); Q = 8.68 MeV;

-2. H + ⁶Li → ³He + ⁴He; Q = 4.023 MeV;

-3. ³He + ⁶Li → H + 2(⁴He); Q = 16.88 MeV; and

-4. ³He + ⁶Li → D + ⁷Be; Q = 0.113 MeV.

Reaction (2) converts lithium-6 to helium-3 and ordinary helium. Interestingly, if reaction (2) is followed by reaction (3), then a proton will again be produced and be available to induce reaction (2), thereby propagating the process. Unfortunately, it appears that reaction (4) is 10 times more likely to occur than reaction (3).

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Calculating Energy and Power from Fusion:

Let us calculate the energy released by the fusion of a 1.00-kg mixture of deuterium and tritium, which produces helium. There are equal numbers of deuterium and tritium nuclei in the mixture. If this takes place continuously over a period of a year, what is the average power output?

According to ²H + ³H → ⁴He + n, the energy per reaction is 17.59 MeV. To find the total energy released, we must find the number of deuterium and tritium atoms in a kilogram. The power output is best expressed in watts, and so the energy output needs to be calculated in joules and then divided by the number of seconds in a year.

The atomic mass of deuterium (²H) is 2.014102 u, while that of tritium (³H) is 3.016049 u, for a total of 5.030151 u per reaction. So a mole of reactants has a mass of 5.03 g, and in 1.00 kg there are (1000 g)/(5.03 g/mol)=198.8 mol of reactants. The number of reactions that take place is therefore

(198.8 mol)(6.02 × 10²³mol⁻¹) = 1.20 × 10²⁶reactions.

The total energy output is the number of reactions times the energy per reaction:

E = (1.20×10^26 reactions)(17.59 MeV/reaction)(1.602×10^−13 J/MeV) = 3.38×10^14 J

Power is energy per unit time. One year has 3.16 × 10⁷s, so

P = E/t = (3.38×10^14 J)/(3.16×10^7 s) =1.07×10^7 W=10.7 MW.

The energy output of 3.38 × 10^14 from fusing 1.00 kg of deuterium and tritium is equivalent to 2.6 million gallons of gasoline and about eight times the energy output of the bomb that destroyed Hiroshima. Yet the average backyard swimming pool has about 6 kg of deuterium in it, so that fuel is plentiful if it can be utilized in a controlled manner. The average power output over a year is more than 10 MW, impressive but a bit small for a commercial power plant. About 32 times this power output would allow generation of 100 MW of electricity, assuming an efficiency of one-third in converting the fusion energy to electrical energy.

Note:

Attempts to utilize controlled fusion as an energy source on Earth are related to deuterium and tritium fusion.
Ignition is the condition under which controlled fusion is self-sustaining; it has not yet been achieved. Break-even, in which the fusion energy output is as great as the external energy input, has nearly been achieved.
Magnetic confinement and inertial confinement are the two methods being developed for heating fuel to sufficiently high temperatures, at sufficient density, and for sufficiently long times to achieve ignition. The first method uses magnetic fields and the second method uses the momentum of impinging laser beams for confinement.

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Fusion fuels:

The D-T reaction is not necessarily the most desirable fusion reaction as one of its major disadvantages is the production of a 14.1 MeV neutron in each reaction. The neutrons get absorbed in the surrounding fusion chamber (for example in tokamaks in the surrounding mostly metallic structures such as blankets and vessels). The effect of neutron activation on metals is roughly that they become hardened, brittle, as well as radioactive. This has two important effects: first, it becomes necessary to use low activation materials inside the vacuum chambers of fusion devices, and second, the lifespan of the machine components surrounding the fusion plasma gets seriously limited. Thus, even though the first generation of fusion devices will depend on D-T reactions because of the ease of achieving them, future devices may use more efficient reactions resulting in less neutron activation.

Traditionally, D-T reaction has been used to achieve fusion because they reach the highest reaction rate at a lower temperature than other fuels. However, tritium is radioactive and does not occur naturally in any significant quantities. Therefore, it has to be ‘bred’ in a nuclear reaction between the fusion-generated neutrons and lithium surrounding the reactor wall. To bypass the challenges caused by the use of tritium, there are now experiments using alternative or advanced fusion fuels, like D-3He or p-11B. Boron-11 is non-radioactive and comprises around 80 percent of all boron found in nature, so it is readily available. However, the challenge with p-11B fusion is that it would require the plasma to be a hundred times hotter than plasma containing deuterium and tritium.

Table below contains a list of fusion reactions that are most favourable.

Any of the above reactions can in principle be the basis of future fusion reactors for power production. However, the level of difficulty in achieving them depends on several factors. In the first generation tokamak based fusion reactors like ITER, a mixture of D-T particles is confined in a magnetic trap and heated to high energies (about 10–20 keV mean), where the particles have a distribution of kinetic energy (called a Maxwellian distribution), with a small fraction of them attaining 40 keV and fusing.

In addition to the temperature and cross-section of the reactions, one must also consider the total energy of the fusion products E fus, the energy carried by the resultant charged particles Ech and the atomic number Z of the non-hydrogenic reactant. One interesting case is the D-D reaction, which has two branches, one resulting in a T and a proton, the other in a 3He and a neutron, both branches having a 50/50 probability. The T can further undergo a D-T reaction and the 3He can undergo a D–3He reaction. However, while the T in fact gets completely burnt up in a deuterium plasma and adds its energy further to the reaction chain, the optimum temperature for D–3He reactions is much higher than that of D-D reactions and is not expected to contribute to the overall fusion energy. Thus one can calculate the overall D-D fusion energy as E fus =12.5 MeV and the energy in charged particles as Ech = 4.2 MeV.

Another important parameter in the fusion reactions is the neutronicity of the reaction, which is measured as (E fus – Ech)/ E fus, the fraction of the fusion energy released through neutrons. Neutronicity is an important indicator of the magnitude of the problems associated with neutrons, such as radiation damage, biological shielding, remote handling and safety. The higher the neutronicity, the more complex are the problems associated with neutron generation. A fusion reaction is considered aneutronic if the overall neutron energy from all possible chains is less than 1% of the fusion energy.

Direct conversion to electricity:

Since many of the fusion reactions give energy to charged particles, it is possible to convert fusion energy directly to electricity through various methods such as induction effects or through electrostatic effects of decelerating charged particles in an electric field. It is true that in the first generation of D-T reaction based fusion reactors, as 80% of the fusion energy goes into the neutrons, electricity generation will have to be mostly through conventional steam turbines using the heat generated by the neutron absorption. However, in future fusion devices based mainly on aneutronic fusion like p-11B, it would be possible to convert about 80% of the charged particle energy directly into electricity. The fraction of fusion energy which goes into the charged particles also gets released in the form of microwave radiation through synchrotron radiation and x rays through bremsstrahlung radiation. The microwaves can be used either to drive a current in the plasma itself (as required in the case of tokamaks) or suitable technology can be evolved to capture the microwave energy and convert it into electricity. The x ray energy can also in principle be converted into electricity through photoelectric effects by making the rays pass through arrays of conducting foils. However, due to deep penetration of the x rays, this might need many layers of metallic foils to absorb all the energy and would require clever reactor designs.

The controlled fusion reaction:

Large amounts of energy are released in the union or fusion of light nuclei. Nuclear fusion requires that two charged nuclei approach one another closely. They must approach with high enough kinetic energy to overcome their mutual electrostatic repulsion. High kinetic energy can be achieved through high temperature. The sun, for example, fuses hydrogen nuclei into helium at interior temperatures of about 15 million degrees Celsius. For the most promising earth-bound possibility—fusing deuterium and tritium into helium—temperatures about 10 times that of the sun’s core must be achieved and maintained long enough to allow a significant fraction of the fuel to react. At the high temperatures of fusion reactions, matter has decomposed into atoms whose electrons are stripped away. The result is an ionized gas, or plasma, that conducts electricity and responds readily to magnetic and electric forces. The practical use of fusion as a source of energy, then, depends on the simultaneous achievement of high temperatures and effective containment of the plasma. Heating and containing the plasma, and operating the ancillary equipment that may be used to drive the reaction, represent a large investment in energy that the net production of energy from fusion must pay back with interest.

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The Conditions for Fusion:

There exists a repulsive force between protons that has to be overcome for hydrogen fusion to happen

Hydrogen ions are protons, and their positive charge makes them repel one another. In order to overcome this repulsion, the protons must have very high kinetic energy in order to be travelling towards each other at very high speeds. In order to make the molecules of a gas travel at such speeds, the gas has to be heated to millions of degrees Celsius – a temperature that is usually only reached at the centre of a star.

Lawsons’s three criteria:

In 1955 a young engineer working on nuclear fusion decided to work out exactly how enormous the task of achieving fusion is. Although his colleagues were optimistic about their prospects, he wanted to prove it to himself. His name was John Lawson, and his findings – that the conditions for fusion power relied on three vital quantities – became the landmark Lawson Criteria. The genesis of Lawson’s Criteria is simple enough – he calculated the requirements for more energy to be created than is put in, and came up with a dependence on three quantities: temperature (T), density (n) and confinement time (τ). With only small evolution thanks to some subtle changes of definition, this is basically the same figure of merit used by today’s fusion scientists – the triple product, nτT.

The constituents of a 100 million-degree plasma are moving about really fast, and, if left alone would soon be so far apart as to render collisions extremely unlikely. To keep the density of the plasma high enough to ensure collisions do actually occur, the plasma vessel is surrounded by huge electromagnets. These create magnetic fields 10,000 times stronger than the Earth’s magnetic field and confine the plasma to perpetually circulate within the ring-shaped vessel. The amount of energy created relies on particles colliding and fusing – the number of collisions is related to the number of particles in a certain region – thus n, the number density (not mass density) is Lawson’s first criterion. This would seem encouraging for the prospective experiment, as creating high pressure is relatively easy. However there is a catch. At higher densities a process known as bremsstrahlung rears its ugly head, in which collisions between nuclei and electrons generate radiation. Bremsstrahlung can become so dominant that all the power in the plasma is radiated away – the optimum density conditions are surprisingly low, around a million times less dense than air.

Nonetheless the fusion collisions – between the nuclei – have to be at high speed. This allows the nuclei to overcome their electrostatic repulsion, and get close enough for the strong force – that governs fusion – to take over and stick the particles together. The speed of a gas or plasma particle is equivalent to its temperature: the second of Lawson’s criteria. Again there is a limit – if the two particles are moving really fast then the time they are in close enough proximity for fusion to occur decreases. The bremsstrahlung also increases at higher temperatures, due to faster moving electrons. The Goldilocks temperature turns out to be in the vicinity of 100 – 200 million degrees K, a seemingly huge task in the fifties that has become a standard condition today.

With the first two criteria satisfied fusion reactions can occur, but to get a substantial amount of power generated you need time to allow the reactions to happen – on this basis Lawson’s third criteria comes into play, the energy confinement time. This is the time that energy remains in the plasma before escaping, and it is here that the most remarkable gains have been made during the course of fusion research. From only microseconds in Lawson’s time, the confinement time has improved by a factor of a million to reach about one second in JET and is planned to hit around 5 seconds in ITER. The length of time for which particles are confined within the plasma is denoted by τ (the Greek letter tau).

Lawson argues that a machine holding a thermalized and quasi-neutral plasma has to generate enough energy to overcome its energy losses. Lawson assumed an energy balance shown below.

Plasma clouds lose energy through conduction and radiation. Conduction occurs when ions, electrons, or neutrals impact other substances, typically a surface of the device, and transfer a portion of their kinetic energy to the other atoms. Radiation is energy that leaves the cloud as light. Radiation increases with temperature. Fusion power technologies must overcome these losses. The Lawson criterion is a figure of merit used in nuclear fusion research. It compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is higher than the rate of loss, the system will produce net energy. If enough of that energy is captured by the fuel, the system will become self-sustaining and is said to be ignited.

Triple product: density, temperature, time:

The amount of energy released in a given volume is a function of the temperature, and thus the reaction rate on a per-particle basis, the density of particles within that volume, and finally the confinement time, the length of time that energy stays within the volume. This is known as the “triple product”: the plasma density, temperature, and confinement time.

In magnetic confinement, the density is low, on the order of a “good vacuum”. For instance, in the ITER device the fuel density is about 1.0 × 10²⁰ m⁻³, which is about 250,000 times thinner than the earth’s air mantle. This means that the temperature and/or confinement time must increase. Fusion-relevant temperatures have been achieved using a variety of heating methods that were developed in the early 1970s. In modern machines, as of 2019, the major remaining issue was the confinement time. Plasmas in strong magnetic fields are subject to a number of inherent instabilities, which must be suppressed to reach useful durations. One way to do this is to simply make the reactor volume larger, which reduces the rate of leakage due to classical diffusion. This is why ITER is so large.

In contrast, inertial confinement systems approach useful triple product values via higher density, and have short confinement intervals. In NIF, the initial frozen hydrogen fuel load has a density less than water that is increased to about 100 times the density of lead. In these conditions, the rate of fusion is so high that the fuel fuses in the microseconds it takes for the heat generated by the reactions to blow the fuel apart. Although NIF is also large, this is a function of its “driver” design, not inherent to the fusion process.

Plasma physicists combine three parameters – temperature, density and time – by multiplying them together to form what is known as the fusion product or triple product. The triple product is a figure of merit used for fusion plasmas, closely related to the Lawson Criteria. It specifies that successful fusion will be achieved when the product of the three quantities – n, the particle density of a plasma, the confinement time, τ and the temperature, T – reaches a certain value. Above this value of the triple product, the fusion energy released exceeds the energy required to produce and confine the plasma. At this value of the fusion product, called ignition, the reaction becomes self-sustaining: the heat generated by the reaction is enough to keep the plasma hot and so the external heating systems can be turned off. This value is a minimum requirement for ignition.

For deuterium-tritium fusion this value is about:

nτT ≥ 5×10²¹ m^-3 s KeV. JET has reached values of nτT of over 10²¹ m^-3 s KeV.

To achieve fusion, very high temperature is a necessity, but as can be seen from the high value of the triple product, one or both of the other two parameters—density and confinement time—must also be large. Tokamaks are at one extreme of the various ways of satisfying the triple product. Although tokamaks achieve high temperature and long confinement time, the density of gas in the reactor is low, lower than the air we breathe. A related device, known as a stellarator, also confines plasma using magnets, but in a different arrangement. These too achieve long confinement times but again at low density. At another extreme—high density and short confinement time—reside the approaches known as inertial confinement fusion (ICF).

Sibylle Günter, Scientific Director of IPP, highlighted the most recent results from the first high-performance plasma operation of W7-X, which has recently achieved the highest stellarator fusion triple product: the density, confinement time and plasma temperature used by researchers to measure the performance of a fusion plasma.

The fusion triple product has seen an increase of a factor of 100,000 in the last fifty years of fusion experimentation; another factor of five is needed to arrive at the level of performance required for a power plant. Some of the improvements in this product were the result of experimental fusion reactors becoming larger. Plasma takes longer to diffuse from the centre to the walls in a bigger reactor, and this extends the confinement time. Size matters in terms of heat insulation.

Figure below shows how tokamaks have improved their values of triple product over the past 50 years compared with several recent entrants to the field.

Figure above shows evolution of fusion triple product. Tokamak researchers have worked long and hard to gradually improve performance to the point where devices are approaching energy gain. Alternative approaches have a long way to go but proponents believe they can accelerate development. (Points shown for Tri Alpha used deuterium as fuel, not the proton-boron fuel it hopes to use.)

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Conditions for practical fusion yield:

Two conditions must be met to achieve practical energy yields from fusion. First, the plasma temperature must be high enough that fusion reactions occur at a sufficient rate. Second, the plasma must be confined so that the energy released by fusion reactions, when deposited in the plasma, maintains its temperature against loss of energy by such phenomena as conduction, convection, and radiation. When these conditions are achieved, the plasma is said to be ignited. In the case of stars, or some approaches to fusion by magnetic confinement, a steady state can be achieved, and no energy beyond what is supplied from fusion reactions is needed to sustain the system. In other cases, such as the ICF approach, there is a large temperature excursion once fuel ignition is achieved. The energy yield can far exceed the energy required to attain plasma ignition conditions, but this energy is released in a burst, and the process has to be repeated roughly once every second for practical power to be produced.

The conditions for plasma ignition are readily derived. When fusion reactions occur in a plasma, the power released is proportional to the square of plasma ion density, n^2. The plasma loses energy when electrons scatter from positively charged ions, accelerating and radiating in the process. Such radiation is called bremsstrahlung and is proportional to n²T^1/2, where T is the plasma temperature. Other mechanisms by which heat can escape the plasma lead to a characteristic energy-loss time denoted by τ. The energy content of the plasma at temperature T is 3nkT, where k is the Boltzmann constant. The rate of energy loss by mechanisms other than bremsstrahlung is thus simply 3nkT/τ. The energy balance of the plasma is the balance between the fusion energy heating the plasma and the energy-loss rate, which is the sum of 3nkT/τ and the bremsstrahlung. The condition satisfying this balance is called the ignition condition. An equation relates the product of density and energy confinement time, denoted nτ, to a function that depends only on the plasma temperature and the type of fusion reaction. For example, when the plasma is composed of deuterium and tritium, the smallest value of nτ required to achieve ignition is about 2 × 10^20 particles per cubic metre times seconds, and the required temperature corresponds to an energy of about 25,000 eV. If the only energy losses are due to bremsstrahlung escaping from the plasma (meaning τ is infinite), the ignition temperature decreases to an energy level of 4,400 eV. Hence, the keys to generating usable amounts of fusion energy are to attain a sufficient plasma temperature and a sufficient confinement quality, as measured by the product nτ. At a temperature equivalent to 10,000 eV, the nτ product must be about 3 × 10^20 particles per cubic metre times seconds. Magnetic fusion energy generally creates plasmas with a density of about 3 × 10^20 particles per cubic metre, which is about 10^−8 of normal density. Hence, the characteristic time for heat to escape must be greater than about one second. This is a measure of the required degree of magnetic insulation for the heat content. Under these conditions the plasma remains in energy balance and can operate continuously if the ash of the nuclear fusion, namely helium, is removed (otherwise it will quench the plasma) and fuel is replenished.

ICF creates plasmas of much higher density, generally between 10^31 and 10^32 particles per cubic metre, or 1,000 to 10,000 times the normal density. As such, the confinement time, or minimum burn time, can be as short as 20 × 10^−12 second. The objective in ICF is to achieve a temperature equivalent of 4,400 eV at the centre of the highly compressed fuel mass, while still having sufficient mass left around the centre so that the disassembly time will exceed the minimum burn time.

The Heating:

The temperature it takes for a DT fusion reaction to reliably take place on Earth is 150 million degrees, 10 times hotter than the core of the Sun. In order for Deuterium and Tritium to collide, a lot of energy must be supplied to the particles. An efficient method of imparting this energy is by ohmic heating. By letting an electric current flow through the resistive plasma, in the same principle as the metallic heating element in an electric toaster, it is possible to heat the plasma up to about 20 – 30 million degrees centigrade.

Once the plasma has formed, thanks to the heating produced by the electric current, it can be heated further with microwaves or with the injection of neutral energetic particles, using a special accelerator.

To maintain the heat of the plasma it is always necessary to apply a flow of energy. Once we have arrived at the temperature in which the fusion reactions take place, known as “ignition”, the reactions themselves produce most of the energy necessary to keep the plasma at the desired temperature. With the reactor started, therefore, only 10-30% of the energy needed to maintain the plasma temperature will have to be supplied.

Confinement Time:

The plasma also needs to conserve the energy that is supplied to it by the methods described above. To understand how good the insulation is, in plasma physics, the energy confinement time parameter is used. If our reactor has a very long confinement time it will take little power to heat it, if the confinement time is a low value it will take a lot of energy to maintain the temperature.

The Magnetic Field:

In the reactor the goodness of the insulation depends on the magnetic field. Plasma is a gas composed of charged particles, the magnetic field therefore allows it to be isolated from the external environment by containing it. Since the energy contained in the plasma is essentially linked to the energy of movement of its components, if the magnetic field is strong enough, it also allows the energy, i.e. heat, to be retained inside the plasma. To do this, very intense magnetic fields are needed, but there is a limit to the intensity that can be applied. Technically, fields up to about 13 Tesla can be produced, about 200,000 times the Earth’s magnetic field.

Given a certain magnetic field, the volume of the plasma can be increased to improve the confinement time.

The plasma must also be kept in balance in a certain position. This occurs thanks to auxiliary magnetic fields that prevent the plasma from coming into contact with the reactor walls, which would lead to a sudden drop in plasma temperature and its dissipation.

Reactor Dimensions:

It is estimated that in a future fusion reactor a toroidal volume of plasma with a minor radius of about 1 meter will be needed. The reactor will be in the tokamak magnetic configuration, with a shape similar to that of a doughnut. A very large machine equipped with all the systems to produce the magnetic field will therefore be needed. The reactor will be about 20 meters in diameter and 10 meters high, although some suggest that a system of multiple smaller reactors could have advantages, one of which is the possibility of alternating routine maintenance periods.

The reactor will have a capacity of approximately 1000 m^3. A tiny amount of fuel, about half a gram, will be needed to run the reactor. For comparison there is about 10 times more matter inside an inflated balloon.

Among the most significant:

The density limit.

For a given value of the plasma current, the particle density cannot exceed a certain limit, which increases as the current increases. Once the density limit is exceeded, the plasma tends to develop instabilities which cause it to shut down.

Current limit.

For a given value of the toroidal magnetic field, the plasma current that can be induced has an upper limit that cannot be exceeded and that increases with increasing magnetic field. Once this limit is exceeded, the plasma develops very fast instabilities which cause it to shut down. It is interesting to note that this limit does not exist in the Reversed Field Pinch configuration.

Pressure limit.

This is the value of the maximum pressure of the plasma for a given value of the magnetic field. Once the limit is exceeded, the plasma column tends to deform macroscopically and suddenly extinguish the plasma.

Beyond these above mentioned limits the plasma ceases to exist.

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Fusion gain, ignition and breakeven:

Another milestone, called break-even, occurs when the fusion power produced equals the heating power input. Break-even has nearly been reached and gives hope that ignition and commercial plants may become a reality in a few decades.

In nature, stars reach ignition at temperatures similar to that of the Sun, around 15 million kelvins (27 million degrees F). Stars are so large that the fusion products will almost always interact with the plasma before their energy can be lost to the environment at the outside of the star. In comparison, man-made reactors are far less dense and much smaller, allowing the fusion products to easily escape the fuel. To offset this, much higher rates of fusion are required, and thus much higher temperatures; most man-made fusion reactors are designed to work at temperatures over 100 million kelvins (180 million degrees F).

Terms in the fusion lexicon:

Over time, several terms have entered the fusion lexicon.

A fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1, when the power being released by the fusion reactions is equal to the required heating power, is referred to as breakeven, or in some sources, scientific breakeven.

The energy given off by the fusion reactions may be captured within the fuel, leading to self-heating. Most fusion reactions release at least some of their energy in a form that cannot be captured within the plasma, so a system at Q = 1 will cool without external heating. With typical fuels, self-heating in fusion reactors is not expected to match the external sources until at least Q ≈ 5. If Q increases past this point, increasing self-heating eventually removes the need for external heating. At this point the reaction becomes self-sustaining, a condition called ignition, and is generally regarded as highly desirable for practical reactor designs. Ignition corresponds to infinite Q, in which case no energy input is required to start self sustaining fusion reactions in the plasma.

Energy that is not captured within the fuel can be captured externally to produce electricity. That electricity can be used to heat the plasma to operational temperatures. A system that is self-powered in this way is referred to as running at engineering breakeven.

Operating above engineering breakeven, a machine would produce more electricity than it uses and could sell that excess. One that sells enough electricity to cover its operating costs is sometimes known as economic breakeven.

Additionally, fusion fuels, especially tritium, are very expensive, so many experiments run on various test gasses like hydrogen or deuterium. A reactor running on these fuels that reaches the conditions for breakeven if tritium was introduced is said to be at extrapolated breakeven.

For over two decades since 1997, the record for Q was held by JET at Q = 0.67. The record for Qext was held by JT-60 (Japan Torus-60) with Qext = 1.25, slightly besting JET’s earlier Qext = 1.14. In December 2022, the National Ignition Facility reached Q = 1.54 with a 3.15 MJ output from a 2.05 MJ laser heating, which remains the record as of 2023. The hydrogen bomb is the only device currently able to achieve fusion energy gain factor significantly larger than 1.

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Fusion power gain Q:

Now let us examine the conditions which a reactor successfully giving fusion energy output must satisfy. We first note that we have to invest energy in raising the temperature of the D-T mixture to about 100 million °C. This converts the gas mixture into a plasma, which radiates mainly through bremsstrahlung radiation because of electron–ion and electron–electron encounters. Secondly, if the D-T plasma is diluted by some impurities which are not fully ionized, there is some radiative power loss associated with impurity radiations which cools the electrons and through them the D-T mixture because the electrons and ions are in near thermal equilibrium. In addition to this, if the plasma is confined by magnetic fields, it radiates by synchrotron radiation. The power required to maintain the plasma at 100 million °C is thus related to the power required to sustain the plasma temperature against thermal conduction/convection losses plus the radiative power losses of the above three varieties.

The net power output of a fusion power reactor can be measured in terms of the steady state fusion power gain or the Q factor defined as the ratio of the fusion power output to the input power, i.e. the auxiliary power supplied from outside to sustain the reaction:

Q = P output / P input = P fusion / P aux. Thus, for fusion power to be successful, the minimum criterion for a fusion power plant is Q > 1. The state Q = 1 is known as the break-even condition, when fusion output power just equals the auxiliary input power. On the other hand, the thermonuclear fusion plasma can also be confined in an ignited state when P aux reaches 0 which happens for the D-T fusion reaction, for example, when the output alpha particles from the fusion reaction lose all of their energy in keeping the thermonuclear plasma hot and alpha power also accounts to sustain the plasma temperature against thermal conduction/convection losses plus the radiative power losses. In such a scenario, the fusion reaction is completely self-sustained by the alpha power and no external heating power is required.

The gain is the ratio of output to input. Although ITER is due to begin operating with deuterium plasmas in 2025, experiments with tritium, necessary to produce a burning plasma, aren’t due to commence until around 2035. ITER is expected to produce a gain of 10 for up to 10 minutes; Commonwealth Fusion Systems (CFS) will achieve a gain of at least 2. Tokamak Energy (TE), which expects its device to need little external energy to sustain reactions, anticipates a gain of 30.

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ITER’s Q value of ≥10 makes it a first-of-kind machine and a unique scientific device:

How ITER will create the conditions for fusion inside its vacuum chamber.

Fuel is injected in gaseous form into the vacuum vessel (the gas weighs only a few grams and fills the entire volume of the tokamak);
Electricity flowing through the electromagnets, particularly the central solenoid, produces a voltage across the gas;
This voltage rips electrons from the fuel atoms, turning them into charged particles (ions). This new state of matter is called a plasma;
The changing magnetic fields that are used to control the plasma produce a heating effect. But in order to obtain the temperatures approaching 150,000,000 °C that are needed for deuterium-tritium fusion, three sources of external heating must be applied from outside of the tokamak;
The megawatts of heating power injected by these external systems is part of the ratio measured by Q, which compares input heating power to output fusion power.

In ITER, the programmatic goal, Q≥10, signifies delivering ten times more thermal power (500 MW) than that which is delivered by the heating systems (50 MW). Breakeven, which corresponds to Q=1, is the moment when the total fusion power produced during a plasma pulse equals the power injected into the systems that heat the plasma.

How did the ITER designers choose the specific value of Q ≥ 10?

Accounting for the size of ITER’s vacuum vessel and the strength of the confining magnetic field (5.3 Tesla), the ITER plasma (830 cubic metres) can carry a current of up to 15 megaamperes.

Under these conditions, an input thermal power of 50 megawatts is needed to bring the hydrogen plasma in the vessel to about 150 million degrees Celsius. This temperature in turn translates to a high enough velocity, among a sufficient population of hydrogen nuclei, to induce fusion at a rate that will produce at least 500 megawatts of thermal power output.

Why not design ITER for a Q of 30, or 50?

The answer is clear: expense. For tokamaks, size matters: if all other parameters are equal, larger size means greater Q. In simple terms, increasing Q would require an increase in the major radius or in the magnetic field strength. Either approach would have increased the cost of the device unnecessarily, whereas the achievement of Q ≥ 10 is sufficient to allow the primary scientific and technology goals of the project to be satisfied.

The most important practical applications of fusion lie in the future, largely in the field of power production. The major method of generating electric power has been to use heat sources to convert water to steam, which drives turbogenerators. Such heat sources depend on the combustion of fossil fuels, such as coal, oil, and natural gas, and fission processes in nuclear reactors. A potential source of heat might be supplied by a fusion reactor, with a basic element of deuterium-tritium plasma; nuclear fusion collisions between those isotopes of hydrogen would release large amounts of energy to the kinetic energy of the reaction products (the neutrons and the nuclei of hydrogen and helium atoms). By absorbing those products in a surrounding medium, a powerful heat source could be created. To realize a net power output from such a generating station—allowing for plasma radiation and particle losses and for the somewhat inefficient conversion of heat to electricity—plasma temperatures of about 150,000,000 K (12.9 keV) and a product of particle density times containment time of about 3×10^20 particles per cubic metre times seconds are necessary. For example, at a density of 3×10^20 particles per cubic meter, the containment time must be one second. Such figures are yet to be reached, although there has been much progress.

Another possibility in power production is the elimination of the heat–steam–mechanical energy chain. One suggestion depends on the dynamo effect. If a plasma moves perpendicular to a magnetic field, an electromotive force, according to Faraday’s law, is generated in a direction perpendicular to both the direction of flow of the plasma and the magnetic field. This dynamo effect can drive a current in an external circuit connected to electrodes in the plasma, and thus electric power may be produced without the need for steam-driven rotating machinery. This process is referred to as magnetohydrodynamic (MHD) power generation and has been proposed as a method of extracting power from certain types of fission reactors. Such a generator powers the auroras as the Earth’s magnetic field lines tap electrical current from the MHD generator in the solar wind.

The inverse of the dynamo effect, called the motor effect, may be used to accelerate plasma. By pulsing cusp-shaped magnetic fields in a plasma, for example, it is possible to achieve thrusts proportional to the square of the magnetic field. Motors based on such a technique have been proposed for the propulsion of craft in deep space. They have the advantage of being capable of achieving large exhaust velocities, thus minimizing the amount of fuel carried.

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Plasma:

Plasma is one of the four common states of matter – solid, liquid, gas, and plasma. With increasing temperature, all materials are transformed successively from the solid, to liquid and then gaseous state. If the temperature is increased even more, a plasma is formed. Plasma is thus also described as the “fourth aggregate state of matter”: the gas atoms split into their constituent components – electrons and nuclei. Everyday examples of plasmas include plasma columns in neon tubes, electric sparks and the plasma filament in a lightning flash.

A plasma is created when one or more electrons are torn free from an atom of gas. Atoms that have lost some or all of their negatively charged electrons are called ions. An ionized atom has a positive charge because it is missing electrons, but still contains positively charged protons and neutrons (with no charge) in its atomic nucleus. A plasma is generally a mix of these positively charged ions and negatively charged electrons. Most plasmas are created when extra energy is added to a gas, which can occur when gases are heated to high temperatures. Atoms in a hot gas are moving so fast that electrons can be knocked loose when they collide with each other. High energy photons from the Sun, including gamma rays, X-rays, or ultraviolet radiation, can create plasma by knocking electrons away from their atoms. High-voltage electricity, such as from lightning strikes, can also create plasmas.

A plasma may be produced in the laboratory by heating a gas to an extremely high temperature, which causes such vigorous collisions between its atoms and molecules that electrons are ripped free, yielding the requisite electrons and ions. A similar process occurs inside stars. In space the dominant plasma formation process is photoionization, wherein photons from sunlight or starlight are absorbed by an existing gas, causing electrons to be emitted. Since the Sun and stars shine continuously, virtually all the matter becomes ionized in such cases, and the plasma is said to be fully ionized. This need not be the case, however, for a plasma may be only partially ionized. A completely ionized hydrogen plasma, consisting solely of electrons and protons (hydrogen nuclei), is the most elementary plasma.

Matter in the plasma state is far more abundant than matter in the liquid, solid, or gaseous states. 99 percent of all matter, other than the mysterious “dark matter” that astronomers have been puzzling over, is plasma. Most of the matter in the Sun and other stars exists in a plasma state. Though common in the universe, plasma is less abundant on Earth. Regions of Earth’s atmosphere called the ionosphere contain some plasma that is created through ultraviolet radiation from the Sun. The upper layers of Earth’s atmosphere, the thermosphere and exosphere (and to a lesser extent the mesosphere), also contain plasma mixed in with atoms and molecules of different gases. Above the atmosphere, Earth is surrounded by a magnetic field called the magnetosphere. Most of the particles in the magnetosphere are ionized plasma.

Almost all of the observable matter in the universe is in the plasma state. Formed at high temperatures, plasmas consist of freely moving ions and free electrons. They are often called the “fourth state of matter” because their unique physical properties distinguish them from solids, liquids and gases. Plasma is an electrically charged gas. Because plasma particles have an electrical charge, they are affected by electrical and magnetic fields. This is the main difference between a gas and a plasma.

Plasma densities and temperatures vary widely, from the relatively cold gases of interstellar space to the extraordinarily hot, dense cores of stars and inside a detonating nuclear weapon. Plasma densities range from those in a high vacuum with only a few particles inside a volume of 1 cubic centimeter to 1,000 times the density of a solid.

Figure above shows characteristics of typical plasmas.

Fusion researchers have established that creating a plasma by heating a mixture of deuterium and tritium is the easiest way to achieve an environment to fuse and yield energy. Fusion reaction requires immense temperatures of tens of million degrees Celsius to provide atomic nuclei with enough energy to overcome the natural repulsion that exists between them in order for them to fuse. At these temperatures, matter becomes a plasma, in which atomic components, such as nuclei and electrons are unbound. It is impossible for any known material to withstand direct contact with a substance as hot as these plasmas. Therefore, existing fusion reactors use magnetic fields to suspend them. The properties of plasma are very different to those of a normal gas. For example, a plasma is electrically conductive; its motion can be influenced by electric and magnetic fields. The fusion devices exploit this particular property of plasma: they confine the hot plasma in a “magnetic field cage”. At ITER, a device called the tokamak uses a strong magnetic field to confine the plasma used for fusion experiments. However, achieving the precise spatial configuration and strength of this magnetic field, as well as heating the plasma to the required temperature, poses significant challenges. The ITER international fusion energy experiment will be scientists’ first attempt at creating a self-sustained fusion reaction for long durations. “Burning plasmas” in ITER will be heated by the fusion reactions occurring in the plasma itself.

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Burning plasma:

A burning plasma is one in which the fusion reactions themselves are the primary source of heating in the plasma, which is necessary to sustain and propagate the burn, enabling high energy gain. The Sun’s plasma temperature is maintained solely by energy released from hydrogen fusion. The Sun has been burning hydrogen for 4.5 billion years and is about halfway through its life cycle. The fusion between the nuclei of the hydrogen isotopes deuterium (D) and tritium (T) produces one helium nucleus, also called an “alpha particle,” and one neutron. The helium nucleus, which carries 20 percent of the energy produced by the fusion reaction, is electrically charged and remains confined by the magnetic fields of the tokamak (whereas the neutron escapes). The heating provided by these alpha particles contributes to maintaining the temperature of the plasma and decreases the need for external heating. When heating by the helium nuclei (“alpha heating”) is dominant (over 50 percent) the plasma is said to be a “burning plasma.” This is a state of matter that has never been produced in a controlled manner on Earth.

Will ITER be the first burning plasma device in the world?

Yes, it will, and there is a large worldwide consensus around the necessity of building such a device. Achieving a burning plasma in which at least 50 percent of the energy to drive the fusion reaction is generated internally through the alpha particles is an essential last step in the 70-year quest to control fusion reactions in a magnetic fusion device.

At Q = 5, approximately 50 percent of the plasma heating is contributed by the alpha particles. At Q = 10 (ITER), this percentage rises to 66 percent. At Q=20 alpha heating represents 80 percent.

The primary motivation behind the design of ITER is to provide scientists with the opportunity to study, and better understand, a burning plasma. The knowledge acquired in ITER will help scientists and engineers design the commercial fusion-generated electricity plants of the future. As a research device, ITER will be equipped with far more diagnostics and other research components than the commercial facilities that will follow.

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Section-6

Introduction to nuclear fusion (on earth):

Energy requirement of the world:

In the past 100 years world population grew 4 times, whilst energy consumption grew 10 times. Thus energy consumption grew faster than population growth; in other words energy consumption per head also grew more than 2 times. This trend of per capita energy consumption growth is bound to continue as the rest of the world marches relentlessly in an attempt to catch up with the best standard of living in the world. As is well known, per capita consumption of energy is closely correlated with standard of living. There is a close fundamental correlation between the stage of development of a country and its energy consumption. Developed countries have the highest per capita consumption of energy. Poorest, least developed countries have the lowest per capita energy consumption. This can be seen from figure below which plots GDP per capita against energy consumption per capita. There is generally good correlation between GDP per capita and energy consumption per capita.

Figure above showing the close correlation between economic wealth and energy consumption [expressed as equivalent power] per capita.

Thus, as less developed countries move up in their development, energy consumption per capita rises. The world energy consumption per capita will continue to rise in the foreseeable future, driven by the rapid development of China, India, and Latin America.

Rate of population growth was about 1.1% per year in 2017 and population is projected to grow to 10 billion by 2030 and can be expected to continue growing beyond that time given no drastic limitations. If the needs of the growing population cannot be met, there will be economic, political and environmental upheavals. The key is the availability of energy. If energy supplies prove inadequate, then world population will stagnate. Even if world population growth slows down to zero: world energy consumption will continue to rise due to the continuing development of the less developed nations and the corresponding need for a higher energy per capita consumption. Consider the following popular scenario: The world population growth slows down to zero and world population stabilizes at 10 billion at around 2030. However due to continued improvement in the standard of living of the world on average, the demand of energy continues to rise, until world consumption exceeds the supply of conventional energy sources. Then for energy consumption per head to rise on average to ¾ of US 1985 per head consumption (which was equivalent to 10kW per head power consumption, a comfortable level by any reckoning), alternative sources of energy have to make up the shortfall. This is depicted in figure below.

Figure above shows energy consumption based on a stabilized population of 10 billion with average energy consumption per head taken as ¾ of 1985 US per capita consumption. If the shortfall is not made up then obviously the less developed country will unfortunately continue to remain less developed and the average per head consumption will not rise to ¾ US 1985 consumption.

Estimates of energy supply versus consumption indicate the middle of this century as the critical point when world energy supply will no longer keep pace with the demand. The demand grows inexorably because of both the world population growth as well as the growth of average per capita energy consumption. Technological and economic progress are closely correlated with per capita energy consumption. Hence the inadequacy of energy supplies will limit the progress of human civilization, stifling its soaring spirit. The world is already near the critical point when supply of energy barely meets the demand. How is humankind going to produce the vast amount of energy it needs?

In the absence of any new developments, the workhorse is likely to be based on fossil fuels. Currently, 80 per cent of the world’s primary energy is generated by burning fossil fuels—a resource that is rapidly dwindling. And it is well established that burning fossil fuels has adverse effects on climate and the environment. The fossil fuel reserves have started depleting and at the current consumption rate, we will run out of fossil fuels within the next century. More specifically, according to research from Stanford University’s Millennium Alliance for Humanity and the Biosphere, the Earth only has 30 years of oil, 40 years of gas, and 70 years of coal left. So political and military conflicts for control of oil and gas have already dominated the world energy scenario over the last decade. Deployment of conventional fission based nuclear power, on the other hand, has faced serious public opposition due to concerns of proliferation, radioactive hazardous wastes, and the potential for catastrophic Chernobyl like disasters. Alternative clean energy resources such as solar and wind, though having the potential to become a major source of energy, yet have to significantly address issues of energy density (which makes them unsuitable for large urban industrial complexes), efficiency and cost of production before they can become a viable alternative. Thus the human race is at a critical juncture today, when we need to quickly develop a viable alternative source of clean energy with easy global accessibility which can lead to sustainable development.

The Dawning of the Fusion Age:

What is needed to safeguard Mankind’s unimpeded progress is not incremental moves; but one giant bold step – the development of a new limitless source of energy, clean non-polluting energy which will not further aggravate the environment. The technology is already nearly proven. Decades of scientific and technological work have already shown that the technology is feasible. Moreover the last final push is set to begin with an international consortium comprising the major economic and scientific communities of the world. The project is ITER- the International Thermonuclear Experimental Reactor which is currently being built in France at Cadarache. The process involves nuclear fusion which is the same process occurring in the stars causing their glow and powering all the energetics of the universe, including all life on earth. In 50-100 years time, with human control of this limitless clean non-polluting energy, Man’s scientific and technological progress can continue to accelerate.

Nuclear fusion technology of fusing deuterium with tritium, when perfected, will burn only deuterium, will have a fuel supply lasting millions of years, even with continuing energy consumption growth as in the past. In nature, nuclear fusion energy powers all the stars and consequently all life in the Universe.

Fusion energy has been demonstrated on earth in the hydrogen bomb. A fission bomb, called the primary, produces a flood of radiation including a large number of neutrons. This radiation impinges on the thermonuclear portion of the bomb, known as the secondary. The secondary consists largely of lithium deuteride. The neutrons react with the lithium in this chemical compound, producing tritium and helium.

6/3Li + n = 4/2He + 3/1H

This reaction produces the tritium on the spot, so there is no need to include tritium in the bomb itself. In the extreme heat which exists in the bomb, the tritium fuses with the deuterium in the lithium deuteride.

Thermonuclear weapons are the only really successful implementation of large-scale fusion energy release that humans have mastered. The massive, immediate, multi-megaton TNT energy release and resulting catastrophic damage and radioactive fallout precludes any use in generating energy (or really anything useful for that matter), but understanding the physics is fundamental to understanding the enormous energies and conditions necessary to induce large-scale fusion.

In the Teller-Ulam, two-stage, thermonuclear weapon, a spherical plutonium fission primary (with deuterium-tritium fusion fuel at the core to boost it) is imploded by conventional explosives, the plutonium is crushed far beyond critical density, fissions and detonates. The initial radiation released is contained and focused onto the fusion secondary to implode it, ignite fusion, and detonate it. I am mentioning this to illustrate the enormous energy needed to really reach the Lawson criteria on a large scale.

Intensive efforts have been made to harness fusion as an energy source. Decades of Tokamak research has identified a D-T fusion product (density-confinement time-temperature nτT) of ≥ 5×10²¹ m^-3 s KeV as a minimum requirement for ignition. In these decades, the research has pushed the fusion product up by 107 times, to the point when breakeven is only a step away. Immense efforts in science and technology have already been expended. A glimpse of the scientific basis and technological achievements is provided by the Joint European Torus JET, the biggest fusion experiment yet, which reached Q=0.6 in 2000 (In this context Q=1 is breakeven and Q=10 is a good target). The next step necessarily involves international collaboration on an unprecedented scale, to solve the greatest technological challenge we have yet faced. This takes the form of the International Thermonuclear Experimental Reactor, on which work has started in Cadarache France. ITER aims to reach Q=10 to demonstrate feasibility of nuclear fusion as an energy source. Beyond that lies DEMO which will deliver power into the grid. The scientific and engineering plans are well laid and with the support of the best scientific, engineering and political will of all the major world powers and the stakes being the very survival of human civilization, success is anticipated. If energy supplies were to become unlimited then there is no need to restrict the growth of energy consumption for better standard of living.

Meeting Earth’s energy demands:

Our current energy landscape is heavily dependent on the fast-depleting fossil fuels, with 80% of the global energy consumption being based on fossil fuels, and changing this dependence is critical to cut down on the greenhouse gas emissions. Once harnessed, fusion has the potential to be a nearly unlimited, safe and CO2-free energy source.

On the Sun, the process of fusion is driven by the Sun’s immense gravitational force and high temperatures. But the Earth does not have the immense gravitational force required to confine the hydrogen nuclei. So a different approach is needed to achieve fusion reactions on Earth.

In order to achieve fusion on Earth, gases need to be heated to extremely high temperatures of about 150 million degrees Celsius. That is 10 times more than the temperatures in the Sun’s core. Fusion researchers have established that the easiest to accomplish fusion reaction on earth is that between two hydrogen isotopes: deuterium, which is extracted from water, and tritium, which is produced in the wall of the fusion reactor from lithium. When deuterium and tritium nuclei fuse, they form a helium nucleus and a neutron with a lot of energy.

A magnetic cage:

Fusion scientists have developed methods that are able to heat plasma to temperatures of 150 million degrees Celsius. There exists no material however that can contain plasmas at such unimaginable temperatures. So, different plasma confinement methods are used by fusion scientists. One of them is the magnetic confinement wherein the hot plasma particles are contained in a magnetic “cage” made by strong magnetic fields which prevent the particles from escaping. For energy production the plasma has to be confined for a sufficiently long period for fusion to occur. In ITER, 10000 tonnes of superconducting magnets will produce the magnetic fields to initiate, confine, shape and control the plasma.

The tokamak:

The most advanced and best investigated fusion device design available today is the tokamak. Tokamak is a Russian acronym for the term Тороидальная Камера с Магнитными Катушками meaning a torus-shaped vacuum chamber surrounded by magnetic coils, which create a toroidal magnetic field. The largest operating tokamak today is EUROfusion’s flagship device the Joint European Torus or JET. It is located in the Culham Centre for Fusion Energy. ITER, the biggest fusion experiment, being built in Cadarache, France, is also a tokamak. Once operational, ITER will demonstrate the feasibility of fusion as an energy source.

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Three factors put fusion at the forefront of conversations about the planet’s clean energy future:

Fusion Science:

Experts in the fusion field have more powerful tools today. They are increasingly able to model and understand the fundamental physics of fusion and, as a result, more properly control the behaviour of fusion plasma.

Enabling Technologies:

Supercomputing, artificial intelligence, advanced composite materials, high-speed digital control systems, and additive manufacturing (3D printing) offer a wide range of new ways to resolve historical barriers to practical fusion energy.

Private Industry:

After decades of government-led research programs, business leaders have taken initiative to bring entrepreneurial, business-oriented approaches to commercializing fusion energy. In just a few years, investors have injected more than $4.3 billion into private fusion companies.

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Overview of fusion:

Nuclear fusion is the process by which two light atomic nuclei combine to form a single heavier one while releasing massive amounts of energy. Fusion reactions take place in a state of matter called plasma – a hot, charged gas made of positive ions and free-moving electrons with unique properties distinct from solids, liquids or gases. The sun, along with all other stars, is powered by this reaction. To fuse in our sun, nuclei need to collide with each other at extremely high temperatures, around ten million degrees Celsius. The high temperature provides them with enough energy to overcome their mutual electrical repulsion. Once the nuclei come within a very close range of each other, the attractive nuclear force between them will outweigh the electrical repulsion and allow them to fuse. For this to happen, the nuclei need to be confined within a small space to increase the chances of collision. In the sun, the extreme pressure produced by its immense gravity creates the conditions for fusion.

Usually, fusion is not achievable because the strongly repulsive electrostatic forces between the positively charged ions prevent them from getting dense enough to collide and for fusion to happen. However, suppose the states are such that the nuclei can overcome the electrostatic forces; they can reach within a very close distance. In that position, the attractive nuclear power (force binding protons and neutrons together in atomic nuclei) between the cores will exceed the repulsive (electrostatic) energy, allowing the ions to fuse.

These conditions can occur when the temperature rises, letting the ions move with more energy and faster and finally reach speeds high enough to tie nuclei together. The ion can then fuse, generating lots of energy.

In the sun, bulky gravitational forces make the right requirements for fusion, but they are much harder to achieve on Earth. Overcoming that innate repulsion happens in the Sun’s core because it is under immense pressure of gravity as well as heat – around 15 million degrees Celsius and 265 billion bar of pressure. On Earth atmospheric pressure is roughly 1 bar, the interior of a fusion reactor will need to reach 150 million degrees Celsius – 10 times hotter than the Sun’s core. While the sun’s massive gravitational force naturally induces fusion, without that force a temperature even higher than in the sun is needed for the reaction to take place. So on Earth, we need temperatures of around 150 million degrees Celsius to make deuterium and tritium fuse, while regulating pressure and magnetic forces at the same time, for a stable confinement of the plasma and to maintain the fusion reaction long enough to produce more energy than what was required to start the reaction.

Fusion fuel – distinct isotopes of hydrogen – must be heated to enormous temperatures and kept stable under extreme pressure, hence compact enough and confined for long enough to enable the nuclei to fuse. The controlled fusion research program aims to achieve ‘ignition’, which happens when enough fusion reactions occur to become self-sustaining, with fresh fuel then being added to continue it. Once the ignition is started, there is a net energy yield for about four times as much as nuclear fission. Based on the Massachusetts Institute of Technology (MIT), the reactor’s power increases with the pressure’s square, so multiplying the pressure leads to a large increase in the amount of energy production.

With modern technology, the most readily feasible reaction is between the nuclei of the two heavy isotopes of hydrogen, which are deuterium (D) and tritium (T). Each D-T fusion reaction releases 17.6 MeV compared with 3-4 MeV for D-D fusion and 200 MeV for U-235 fission. Based on the mass, the D-T fusion reaction releases about four times as much energy as the uranium fission reaction. Deuterium is available naturally in seawater, and the amount is 30 grams per cubic meter, which makes it very sufficient relative to other energy resources. Tritium is available in small quantities provided by cosmic rays and is radioactive, with a 12 years half-life. Usable amounts can be made in a traditional nuclear reactor or produced in a fusion system from lithium. Lithium can be found in considerable quantities and at weaker concentrations in the seas.

In the fusion reactors, the D-T fusion reaction generates neutrons, and all are absorbed in a blanket containing lithium that surrounds the core. By the absorption of the neutrons, lithium is transformed into tritium and helium. For slowing down the high-energy (14 MeV) neutrons, a one-meter blanket must be used, which is thick enough. The blanket is heated by the absorption of neutrons’ kinetic energy. The heat energy is removed by the coolant (helium, water, or Li-Pb eutectic), and in a fusion power plant, the extracted energy produces electricity by employing traditional methods. If tritium is not produced enough, some additional sources must be applied, such as using fission reactors to irradiate heavy water or lithium with neutrons. Extraneous tritium has difficulties with storage, handling, and transport.

The challenge has been to promote a device that can heat the D-T fuel to a high temperature and keep it long enough to release more energy through fusion reactions than is used to get the reaction to continue. As the D-T reaction is the primary focus of attention, there are long-term hopes for a D-D reaction, but that needs higher temperatures. In all cases, the challenge is to utilize the heat to satisfy human needs, essentially generating electricity. The energy concentration of fusion reactions in gas is less than for fission reactions in solid fuel, and as mentioned, the heat yield per each reaction is 11 times less. Therefore nuclear fission will always have a much larger power density (w/m3) than thermonuclear fusion, which means that we need a more extensive and thus more costly fusion reactor than a fission reactor with the same power output. Also, nuclear fission reactors employ solid fuel, which is denser, and the energy released is richer.

On the other hand, the neutron energy in the fusion is enormous compared to the fission (14.1 MeV rather than about 2 MeV), which has significant structural materials challenges. Up to now, two main experimental procedures have been studied: inertial confinement and magnetic confinement. The first method involves compressing a small pellet holding fusion fuel to too high densities using intense lasers or particle beams. The second includes strong magnetic fields to make the hot plasma.

Is the energy stored in a 150-million-degree plasma dangerously large?

Although 150 million degrees Celsius is an extremely high temperature, the density of the plasma (atoms per cubic metre) is very low—about one million times less than air—and the total energy in the plasma is not very great. The very rapid release of the energy could cause superficial damage to some plasma-facing components (i.e., surface melting) but would not be sufficient to produce structural damage.

A fusion energy source has many attractive features. It would not emit greenhouse gases and fusion fuel is widely available, making it a sustainable energy source. In contrast to nuclear fission, the reaction underpinning current nuclear power plants where nuclei of heavy elements are split, fusion would produce little radioactive waste and catastrophic accidents such as Fukushima and Chornobyl would be impossible.

The downside is that creating a fusion energy source is technically challenging and requires two basic tasks. First, the fuel must be made hotter than the Sun’s core (150 million degrees Celsius). This is because fusing nuclei have positive electric charges that cause them to be repelled from each other. High temperatures force nuclei to overcome this repulsion and fuse. In contrast, nuclear fission does not require high temperatures since splitting of heavy nuclei is induced by neutrons, which have no electric charge.

Secondly, fuel must be sufficiently confined. Fuel heated to high temperatures will tend, like anything that is overheated, to blow itself apart. Therefore, the fuel must be held in place, or “confined”, such that reactions can occur. The Sun solves this confinement problem by simply being so massive that its gravity keeps the hot fuel in place, but we need to be more creative on Earth.

There are two main approaches to the confinement problem. Magnetic Confinement Fusion (MCF) uses magnetic fields to confine the hot fuel for long periods of time. Because the fusing nuclei are electrically charged, their motion can be controlled by magnetic fields. This approach can be seen in ITER, currently under construction in southern France.

The second approach to the confinement problem is Inertial Confinement Fusion (ICF). Here there is a trick in that hot fuel is not actively confined. Instead the fuel is compressed to extremely high densities such that large numbers of fusion reactions happen before it blows apart. The primary example of this is located at Lawrence Livermore National Laboratory in California where the world’s biggest laser is used to compress small pellets of fuel. The NIF achieved “fusion ignition” in December 2022, meaning that the fusion energy emitted from the fuel was greater than the energy delivered by the laser to do the heating and compression. This is a significant breakthrough for fusion energy as it is a proof-or-principle demonstration that more energy can be emitted by the fuel than needs to be put in.

However, many more challenges remain to convert this result into an electricity source. The total energy produced was only enough to boil a kettle several times. It would need to be scaled up and happen several times a second in order to function as a power plant. Building a suitable laser is a vast, as yet unfunded, undertaking.

In 2019, National Geographic described nuclear fusion as the “holy grail for the future of nuclear power.” Not only would it produce more energy more safely, it would also produce far less harmful radioactive waste than fission, from which weapons-grade material in spent fuel rods taking millions of years to decay requires extremely careful and expensive storage. (Furthermore, most fusion reactors emit less radiation than the background emissions in the natural world.) At the other end of the process, fusion requires much less fuel than fission and the fuel is much easier to obtain. Fission requires uranium, a rare substance that must be mined and enriched; fusion requires deuterium, readily extractable from seawater, and tritium – which can be made in the reactor itself from lithium. Lithium is much more easily available than uranium, including from salt flats; the International Thermonuclear Experimental Reactor (ITER) has estimated that “terrestrial reserves of lithium would permit the operation of fusion power plants for more than 1,000 years, while sea-based reserves of lithium would fulfil needs for millions of years.”

Lithium’s increased usage over the past few years to produce raw material for electric batteries has raised concerns about the effects of large-scale mining. That said, the quantities of lithium required by nuclear fusion power stations would be relatively small and would of course lead to the production of more energy.

And unlike fossil-fuel power generators, fusion reactors don’t emit toxins such as carbon dioxide or other greenhouse gases. The main byproduct is helium; the inert, non-toxic gas has several uses in industry, which has suffered several shortages in recent years.

One huge worry with nuclear fission is the capacity for a meltdown, as at Chernobyl or Fukushima. However, that type of uncontrolled chain reaction simply doesn’t happen with nuclear fusion. “It is absolutely impossible for a Fukushima-type accident to happen at ITER,” an official documentation insisted. “The reaction relies on a continuous input of fuel; if there is any perturbation in this process, the reaction ceases immediately.”

The disadvantage of nuclear fusion is obvious: it’s horrendously difficult to achieve while nuclear fission power plants have been online since the 1950s.

Advantages of Nuclear Fusion:

-Abundant energy: Fusing atoms together in a controlled way releases nearly four million times more energy than a chemical reaction such as the burning of coal, oil or gas and four times as much as nuclear fission reactions (at equal mass).

-Fusion has the potential to provide the kind of baseload energy needed to provide electricity to the cities and the industries.

-Sustainability: Fusion fuels are widely available and nearly inexhaustible. Deuterium can be distilled from all forms of water, while tritium will be produced during the fusion reaction as fusion neutrons interact with lithium.

-No CO₂: Fusion doesn’t emit harmful toxins like carbon dioxide or other greenhouse gases into the atmosphere. Its major by-product is helium: an inert, non-toxic gas.

-No long-lived radioactive waste: Nuclear fusion reactors produce no high activity, long-lived nuclear waste.

-Limited risk of proliferation: Fusion doesn’t employ fissile materials like uranium and plutonium (Radioactive tritium is neither a fissile nor a fissionable material).

-No risk of meltdown: It is difficult enough to reach and maintain the precise conditions necessary for fusion—if any disturbance occurs, the plasma cools within seconds and the reaction stops.

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Nuclear fusion basics:

Fusion is a nuclear reaction in which two or more atomic nuclei combine to form one or more different atomic nuclei and often subatomic particles as well. The fusion of light elements releases energy due to the interplay of two opposing forces: the “strong” nuclear force, which holds protons and neutrons together in the nucleus, and the Coulomb force, which causes positively charged protons to repel each other.

At the very short distance scales of an atomic nucleus, the strong force overcomes Coulomb repulsion to bind the protons and neutrons together. When small nuclei are fused together, this increases the strong force nuclear binding, releasing energy in the form of radiation and/or emission of subatomic particles. (Larger nuclei—those above iron—do not release energy when they fuse, as the higher number of protons increases Coulomb repulsion.)

These nuclear binding forces are far stronger than the forces that hold electrons in orbit around a nucleus and influence the energy of chemical processes. This is why fusion fuels offer vastly higher energy density than chemical methods—about a million times denser than fossil fuels.

Two major natural powers are important for fusion, the electromagnetic force and the strong (nuclear) force. Nuclei under normal conditions are quite distant objects with much space between them because of the repellent electrostatic force which acts on the protons. Only when two nuclei are brought together to a distance of approximately 10^-15 m or less, fusion can be initiated. Under these conditions, the repellent force of the positive charged protons is overpowered – by the approximately 100 times stronger nuclear force – as it is in any nucleus. For nuclei lighter than iron or nickel this process releases more energy than was needed to initiate the action.

The deuterium-tritium reaction is the most useful for fusion energy because it most easily overcomes the Coulomb repulsion, and it has the highest energy release among laboratory-feasible reactions. However, the challenges involved in handling radioactive tritium and dealing with the copious amounts of energy and fusion products produced, means most present fusion experiments study the fusion of deuterium to deuterium (which has a lower fusion reaction rate) and then extrapolate the results to DT.

DT fusion produces an extremely energetic (14.1-MeV) neutron, which is potentially useful for breeding more tritium but also creates challenges for the surrounding materials in the tokamak. The reaction also generates a helium nucleus that carries away about one-fifth of the reaction energy (3.5 MeV). In a fusion reactor, this nucleus shares its energy with the surrounding ions, keeping them hot and sustaining the fusion process.

Technical processes for fusion:

At temperatures needed for fusion reactors with positive energy output, matter is not in one of the three well known states solid, liquid, gaseous but in the fourth state known to science as plasma. Matter in the plasma state has special properties. Atoms in the plasma state are partly or totally ionised, which means that the nuclei and the electrons of the core of the atoms are completely separated. The atoms in a plasma with high temperature – like in the sun – are totally ionised. A whole branch of science – plasma physics – deals with the properties of matter in the plasma state.

It is obvious that there is no material on earth which is able to contain matter at 100 million Kelvin. Any wall would transit to an evaporated state within the blink of an eye and furthermore the plasma would instantaneously loose the necessary energy to keep the process of fusion going through heat transfer. Fortunately, it is not necessary to build walls to confine the plasma. Due to the fact that matter in the plasma state does not contain neutral atoms but only ionised nuclei it is in principle possible to confine the plasma with the help of magnetic fields.

Confinement:

Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion.

General principles:

Equilibrium: The forces acting on the plasma must be balanced. One exception is inertial confinement, where the fusion must occur faster than the dispersal time.

Stability: The plasma must be constructed so that disturbances will not lead to the plasma dispersing.

Transport or conduction: The loss of material must be sufficiently slow. The plasma carries energy off with it, so rapid loss of material will disrupt fusion. Material can be lost by transport into different regions or conduction through a solid or liquid.

To produce self-sustaining fusion, part of the energy released by the reaction must be used to heat new reactants and maintain the conditions for fusion.

In a fusion reactor the magnetic field has to fulfil several functions. First of all, it has to confine the plasma somewhere in mid-air within the fusion reactor. Secondly the magnetic field has to have such properties that the plasma zone can be provided with fresh matter for the fusion process without interfering with the fusion and without destabilising the plasma. Last but not least it should be able to remove the fusion products while the reactor is running. Further properties would be advantageous for a future power generating fusion reactor, like the possibility to breed the fusion-fuel within the same process.

The fuel for the fusion process might be in principle any element lighter than iron. For the ITER (International Thermonuclear Experimental Reactor) device the fuel is planned to be deuterium and tritium. ITER will according to the fusion community be the last step prior to a prototype nuclear fusion power plant, the so called DEMO. Deuterium can be extracted from water since it exists as a contingent of 0.015% of all present hydrogen while tritium can be derived from lithium. Furthermore, the heavy isotopes of hydrogen only contain one proton in the nuclei. So the repelling force is relatively low.

Some fusion processes and their energy-output, contain the lightest elements hydrogen and helium:

2H + 3H –> 4He + n + 17.6 MeV

2H + 3He –> 4He + p + 18.3 MeV

2H + 2H –> 3He + n + 3.3 MeV

2H + 2H –> 3H + p + 4.0 MeV

The reaction of tritium and deuterium produces one neutron which carries most of the energy of the reaction. Neutrons are uncharged particles, and as such they are able to escape the fields confining the plasma. As a consequence, the neutrons are suitable to carry the energy out of the plasma to the wall of the reactor where the kinetic energy of the neutrons can be transformed to heat and used for energy production through a turbine as seen in the figure below.

Furthermore, the neutrons can be indirectly used for the breeding of tritium from lithium. The energy which remains with the helium nucleus cannot escape the magnetic confinement and so it contributes to the necessary – as the plasma continuously loses energy through radiation – heating of the plasma.

Creating and maintaining the confining magnetic fields in a tokamak requires three arrays of magnets. External coils around the ring of the tokamak produce the toroidal magnetic field, parallel to the circumference of the torus. The central solenoid uses a powerful pulse of energy that generates a toroidal current within the plasma. The movement of ions with this current in turn creates a second poloidal (parallel to the poles) magnetic field. Finally, poloidal coils around the circumference of the torus are used to control the position and shape of the plasma.

Rather than a single magnetic field, the arrangement results in an array of nested flux surfaces that confine the ionized particles in a variety of orbits in and around the tokamak. Because the particles in the plasma are tied to the magnetic fields, this field structure keeps the hottest parts of the plasma away from the walls, creating an insulating effect that allows very high temperatures to be achieved.

The induced plasma current provides a critical element of the magnetic confinement as well as a certain amount of heating, but this alone is not enough to induce fusion. Additional heating is typically provided by microwaves and particle beams.

There are many combinations of isotopes we can use to achieve fusion. By far the easiest way to achieve fusion is to use mixture of deuterium and tritium nuclei be heated to a temperature of the order of ~10 keV (i.e. ~100 million °C); the resulting process is known as thermonuclear fusion.

² ₁Deuterium + ³ ₁Tritium = ⁴₂He + ¹₀n + 17.6 MeV

But there are many challenges we face in achieving that goal. One of those challenges is to do with turbulent heat and particle transport, which sets the energy confinement time in a reactor and is driven by pressure and density gradients. Therefore it is critical that we have the ability to predict turbulent transport levels in a reactor. Turbulence causes the transport of particles, momentum, and energy. This is believed to originate from plasma microinstabilities. It is important to note that even if the plasma is stable to magnetohydrodynamic (MHD) instabilities, drift waves can be destabilised by high density and temperature gradients (Tang, 1978; Horton, 1999). This is characterised by what is known as the critical gradient (threshold). It is a point in the normalised density (or temperature gradient) beyond which these instabilities grow. Stellarator turbulence growth rates display a different behaviour to that of tokamaks. We notice that while tokamaks have a higher critical gradient, while also having a higher growth rate. Conversely stellarators have a lower threshold, but also a lower growth rate.

Several dozen tokamaks are now in operation around the world. The first to demonstrate fusion at significant scale (10 MW) was the Tokamak Fusion Test Reactor (TFTR) device at the Princeton Plasma Physics Laboratory, though it has since shut down. The largest current device is the Joint European Torus (JET) in the UK, which has a radius of about 3 meters (m) and has made 16 MW of fusion power. A similar device in Japan is being upgraded to study techniques for future facilities and plasma sustainment. Smaller devices, like the DIII-D National Fusion Facility, which is operated for the Department of Energy by General Atomics in San Diego, are used to explore the physics for future facilities and develop techniques to raise fusion performance.

Some fusion research employs a device similar to a tokamak known as a stellarator. Rather than having a large plasma current, a stellarator uses a twisted array of helical windings around the torus to create the poloidal field externally. As a result, it does not need to generate a plasma current, which means it is capable of steady-state operation rather than needing a pulse of energy from the central solenoid. However, stellarators have more complicated geometry and are more difficult to build because of these additional windings. The approach is thought to offer considerable promise, and though past stellarators have encountered more problems with plasma confinement than tokamaks, the technology continues to draw research attention. The Wendelstein 7-X, an advanced stellarator that went online in Germany in 2015 (also the largest so far) is studying how well stellarators can contain energy and reach fusion conditions.

In 1957, Lawson showed that magnetically confined fusion plasmas can indeed give net energy, if it meets the triple product, n_iT_iτ_E> 5×10²¹keV m⁻³s, (n_iis the ion density, T_iis the ion temperature, and τ_Eis the energy confinement time) for a Deuterium-Tritium based plasma (Lawson, 1957). Tokamaks showed great promise when it came to confinement properties, but a second method to achieve magnetic confinement fusion, the stellarator was not far behind. Currently, stellarators are widely considered to be a generation behind the tokamak.

Although tokamaks and stellarators both use helical magnetic fields to confine the plasma, the magnetic geometry of the two concepts is fundamentally different. In a tokamak, the magnetic field strength is constant in the toroidal direction giving us an axisymmetric device. Stellarators lack this toroidal symmetry. While both types of reactors have magnetic minima where particles can be trapped, the tokamak has only one such region while the stellarator could have multiple regions that act as magnetic mirrors. But seeing as tokamaks require a current in the plasma to produce the helical field, stellarators have many features that are advantageous, compared to tokamaks such as disruption free performance, and the absence of a current drive. Although the fusion roadmap (Romanelli et al., 2013) mainly deals with tokamaks, and the time-line relevant to the development of tokamaks as fusion power plants. However, they do also include stellarators and predict that the first burning plasma stellarator will begin operations, or at least be built in the 2040s.

Figure above shows comparison between a typical tokamak geometry, and the geometry of Wendelstein 7-X stellarator.

Inertial confinement:

In inertial confinement schemes, small pellets of deuterium-tritium fuel are dropped one at a time into a vacuum and irradiated by high-energy beams that cause the outer layers to evaporate explosively. The resulting forces compress and heat the remaining pellet core, generating fusion reactions. The pellets used in experiments are 1 mm or less in diameter, and the energy release is controllable. The principal difficulty is that of achieving the necessary compression—100 billion atm—while preserving the stability of the pellet’s core and preventing its premature heating. The first source of energy trained on the pellets was the laser (inertial confinement is thus often labeled “laser fusion”). More recently, energetic electron or ion beams that are focused and pulsed have been proposed to drive the reaction. Development of inertial confinement reactors depends on the beam system: In about one billionth of a second or less, enough energy must be delivered to the pellet to compress and heat the core to a state sufficient for fusion reactions. The performance of available laser systems is far below that required for reactor purposes, although lasers adequate for proposed scientific breakeven experiments (creation of conditions that, in a reactor, could lead to net energy output) are nearing completion.

No current device has been able to generate more fusion power than the heating energy required to start the reaction. Scientists measure this assessment with a value known as fusion gain (expressed as the symbol Q), which is the ratio of fusion power to the input power required to maintain the reaction. Q = 1 represents the breakeven point, but because of heat losses, burning plasmas are not reached until about Q = 5. JET tokamak has achieved around Q = 0.6 with DT reactions. Fusion power plants will need to achieve Q values well above 10 to be economic.

Fusion reactions have been duplicated in man-made devices. The enormous destructive power of the 235U-fueled atomic bomb dropped on Hiroshima on August 6, 1945, which killed 75,000 people, and the 239Pu-fueled bomb dropped on Nagasaki three days later touched off a violent debate after World War II about the building of the next superweapon a fusion, or “hydrogen,” bomb. Alumni of the Manhattan project, who had developed the atomic bomb, were divided on the issue. Ernest Lawrence and Edward Teller fought for the construction of the fusion device. J. Robert Oppenheimer and Enrico Fermi argued against it. The decision was made to develop the weapon, and the first artificial fusion reaction occurred when the hydrogen bomb was tested in November 1952.

The history of fusion research is therefore the opposite of fission research. With fission, the reactor came first, and then the bomb was built. With fusion, the bomb was built long before any progress was made toward the construction of a controlled fusion reactor. More than 70 years after the first hydrogen bomb was exploded, the feasibility of controlled fusion reactions is still open to debate. The reaction that is most likely to fuel the first fusion reactor is the thermonuclear D-T, or deuterium-tritium, reaction. This reaction fuses two isotopes of hydrogen, deuterium (2H) and tritium (3H), to form helium and a neutron.

Why using deuterium and tritium as fusion fuel?

In a commercial fusion power station the fuel will consist of a 50-50 mixture of deuterium and tritium (D-T), since this mixture fuses at the lowest temperature and its energy yield is the largest compared with other fusion reactions. Deuterium can easily be extracted from seawater, where 1 in 6700 hydrogen atoms is deuterium. Tritium can be produced from lithium, which is widely distributed in the Earth’s crust. Thus, the primary fuels for D-T fusion reactors are so abundant in nature that, practically speaking, D-T fusion is an inexhaustible source of energy for global energy requirements. For comparison, if the deuterium in 50 cups of seawater were used in a D-T fusion reactor, the energy produced would be equal to that gained from the burning of 2 tonnes of coal. In addition, the primary fuels (deuterium, lithium) and the direct end product (helium) of fusion are neither toxic nor radioactive, and they do not produce atmospheric pollution nor do they contribute to the greenhouse effect.

How can the plasma be heated to 150 million degrees?

This is a challenging task. Possible heating methods include: (1) compressing the fuel — like air in a piston, (2) forcing an internal electric current through it — like a toaster, (3) bombarding the fuel with high energy neutral particles, and (4) supplying power from microwaves or lasers.

How can the hot plasma be confined?

Three methods for plasma confinement exist: (1) gravitational (as occurs in stars), (2) inertial and (3) magnetic. The most successful method for containing plasmas thus far is a magnetic bottle, which is toroidal — or doughnut — shaped and in which the plasma forms a continuous circuit. The most highly developed magnetic bottle is the tokamak, which was invented by Russian scientists. The tokamak uses strong externally applied magnetic fields to confine the plasma and maintain separation of the plasma from the walls of the containing vessel, which could not withstand the 150 million degree temperature of the plasma.

Developing Fusion:

The tokamak:

In the 1950s, we witnessed a breakthrough at the Kurchatov Institute (Russia). The concept of a “Tokamak” was born which stands for toroidal doughnut-shaped vessel with magnetic coils confining the electrically-charged gas, known as plasma, keeping it away from the walls of the chamber housing the fusion reaction. Europe has a long trajectory in fusion research. In the 1970s, European laboratories joined forces to build JET (Joint European Torus) located at Culham (UK). More knowledge has been accumulated through other tokamak devices such as Tore Supra (France), ASDEX (Germany), RFX (Italy), MAST (UK), TCV (Switzerland). The same technology will be used in ITER, which will be the biggest fusion machine ever.

ITER, which in Latin means “the way”, is the largest international experiment paving the way to fusion energy. Europe is the host of the project which is currently under construction in Cadarache, south of France. It will allow scientists to study a “burning plasma” that will produce a greater thermal output (500 MW) that the one used (50 MW) for about 7 minutes.

DEMO:

The Demonstration power plant (DEMO) will follow ITER, paving a transition from a scientific experiment to a power plant. It will help scientists and engineers to improve key systems and will be connected to the grid.

How much power would a fusion reactor be able to deliver and at what cost? Would it be competitive?

The power output of the kind of fusion power reactor that is envisaged for the second half of this century will be similar to that of a fission reactor, i.e., between 1 and 1.7 gigawatts. In theory, the larger the reactor, the more efficient it would be to operate and the more power it would produce, so it may be advantageous to go larger in the future. For the moment, it is envisaged that future fusion power plants would occupy buildings no bigger than those that presently house fission or coal-fired power stations.

The main goal of ITER and future fusion reactor-based power plants is to develop a new source of clean and sustainable energy. The average cost per kilowatt of electricity cannot yet be extrapolated, however, as this would require the operational experience which will only be available after ITER has been operated for some years. As with many new technologies, costs will be more expensive at first, when the technology is new, and gradually less expensive as economies of scale bring the costs down.

In order to have a rapid market penetration, fusion will have to demonstrate the potential for competitive cost of electricity. Although this is not a primary goal for DEMO, the perspective of competitively priced electricity production from fusion has to be set as a target. One way to do this is to minimize DEMO capital costs (and that of fusion power plants). The ITER Tokamak is a first-of-a-kind experimental machine, built with a vast array of diagnostic systems (over 50!) to learn as much as possible about what is happening in the plasma. A fusion power plant on the other hand would be conceived in quite a different way.

Fusion as an energy source:

The Max Planck Institute for Plasma Physics (IPP) uses fusion devices to investigate the principles of a fusion power plant that would harness energy from the fusion of light atomic nuclei – just like the sun. The aim here is to add to the range of efficient energy sources that could replace coal, oil and gas in the future: nuclear fusion is a third option along with nuclear fission in nuclear power plants and renewable energies, like wind and solar power.

The fusion of the hydrogen isotopes deuterium and tritium is the easiest form of fusion that can be achieved on Earth. The process generates a helium nucleus and a neutron is also released along with large volumes of usable energy. With fusion, one gram of combustible fuel could generate 90,000 kilowatt hours of energy in a power plant, which is equivalent to the combustion heat of eleven tonnes of coal.

Fusion fuels are inexpensive and evenly distributed throughout the world. Almost inexhaustible quantities of deuterium are available in seawater. Tritium – a radioactive gas with a short half-life of 12.3 years – only occurs in trace quantities in nature. However, it can be produced within the power plant from lithium, which is also available in abundance. Given that a fusion power plant could also offer favourable environmental and safety features, fusion could make a sustainable contribution to the world’s future energy supply.

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Difference between fusion on Sun and fusion on Earth:

Nuclear fusion is the same process that powers the sun. The dominant driver for stars like the sun with core temperatures under 27 million F (15 million C), proton-proton fusion begins with two protons and ultimately yields high energy particles such as positrons, neutrinos, and gamma rays. In order to achieve fusion as such low temperatures, stars rely on crushing pressures of more than 200 billion times those of atmospheric pressure on Earth. It’s basically gravity powered—the sun is so massive that gravity is strong enough to trigger fusion. The sun weighs about 333,000 times more than Earth does. That mass creates powerful gravitational forces that produce extreme pressures. This pressure, combined with temperatures up to 27 million degrees Fahrenheit, gets atoms to fuse together. The amount of energy it produces is frankly inconceivable at 384.6 yottawatts (3.846×10^26 W), or 9.192×10^10 megatons of TNT—per second.

A fusion reactor fuses hydrogen atoms into helium atoms, but on a much, much smaller scale. That fusion produces heat, which is used to produce steam to turn a turbine, producing electricity. But it’s very difficult to start a sustained fusion reaction on earth. We don’t have the tremendous amounts of mass and gravity to work with, so scientists use other methods to heat hydrogen up to tremendous temperatures, over 100 million degrees Celsius. That’s about 10 times as hot as the sun. Of course, no material we know of could withstand such high temperatures, so fusion reactors use magnetic fields to contain the super-heated hydrogen. Right now, it takes far more energy to start fusion than you get out of the reactor.

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process called fusion. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and produce one helium nucleus and two positrons. This is a net reaction of a more complicated series of events:

4 H⟶4/2He+2 e+

A helium nucleus has a mass that is 0.7% less than that of four hydrogen nuclei; this lost mass is converted into energy during the fusion. This reaction produces about 3.6 × 10^11 kJ of energy per mole of 4/2He produced. This is somewhat larger than the energy produced by the nuclear fission of one mole of U-235 (1.8 × 10^10 kJ), and over 3 million times larger than the energy produced by the (chemical) combustion of one mole of octane (5471 kJ).

It has been determined that the nuclei of the heavy isotopes of hydrogen, a Deuterium 2/1H and a tritium 3/1H, undergo fusion at extremely high temperatures (thermonuclear fusion). They form a helium nucleus and a neutron:

2/1H+3/1H⟶4/2He+n

This change proceeds with a mass loss of 0.0188 amu, corresponding to the release of 1.69 × 10^9 kilojoules per mole of 4/2He formed. The very high temperature is necessary to give the nuclei enough kinetic energy to overcome the very strong repulsive forces resulting from the positive charges on their nuclei so they can collide.

The main fuels used in nuclear fusion devises are deuterium and tritium, both heavy isotopes of hydrogen. The Deuterium (D) – Tritium (T) reaction has the largest cross section (in other words, the probability of a reaction to take place) and also the largest Q-value (the released energy of a reaction) of all varieties of fusion reactions. It produces an alpha particle (or Helium-4 nucleus) and a neutron, and releases 17.6 megaelectron volt (MeV) of energy in the form of kinetic energy of the products (3.5 MeV to alpha particle and 14.1 MeV to neutron).

Three main conditions are necessary for a controlled thermonuclear fusion:

-1. The temperature must be hot enough to allow the ions of deuterium and tritium to have enough kinetic energy to overcome the Coulomb barrier and fuse together.

-2. The ions must be confined with a high ion density to achieve a suitable fusion reaction rate.

-3. The ions must be held together in close proximity at high temperature with a confinement time long enough to avoid cooling.

Nowadays, there are two main approaches for fusion energy research:

Magnetic confinement fusion

Magnetic confinement fusion is based on the fact that ions and electrons cannot easily travel across a magnetic field. Therefore, hot plasma can be confined by strong magnetic fields.

Inertial confinement fusion

This approach is based on maximizing density by the rapid compression and heating of a small solid DT pellet through the use of lasers or particle beams.

Because of extremely high temperatures (T ~ 10 kiloelectron volt [keV]), matter transition to plasma state occurs. Plasma is in fact called “the fourth state of matter” along with solids, liquids and gases. It consists of a fully ionized or partially ionized gas, containing ions, electrons and neutral atoms. At present, thermonuclear fusion is the main area of research in plasma physics.

On earth, the potential advantages of energy by controlled nuclear fusion are manifold:

Limitless energy production, available all over the world, not subject to local or seasonal variations;
No greenhouse gas emissions;
No accidents such as melt-downs or explosions, due to an intrinsically safe physical process;
No production of radioactive waste with long half-lives; and
No or only a modest proliferation risk.

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Why is it so hard to do nuclear fusion on earth?

Nuclear fusion happens in the deep interior of the Sun because the Sun is colossally massive (more than 300,000 times more massive than the Earth), so its gravity compresses its interior to the temperatures and pressures that are needed for fusion to take place. But very, very slowly and inefficiently. How slowly? It produces less than 300 watts of heat per cubic meter. That’s comparable to… a well-maintained compost pile. Its less than the power density of our human metabolism. That’s all. That’s what you get when you lump enough mass to make more than 300,000 Earths. Obviously, we cannot use gravitational confinement. We are trying to use other means to confine the fusion fuel for sustained fusion. And we aren’t simply trying to mimic the Sun. We want to do better. After all, for a practical fusion power generating station, we want to be able to produce a lot more power, perhaps up to a few hundred megawatts of raw thermal power per cubic meter inside the reactor. So we are trying to best the Sun by a factor of a million, give or take. That is hard.

Fusion processes aren’t easy to replicate here on Earth partially because massive forces of gravity within stars are needed to overcome the repulsion between positively charged atomic nuclei of hydrogen. That incredible gravitational pressure can’t be reproduced here on Earth, so instead the designers of tokamaks must generate fusion in plasmas at incredibly hot temperatures, vastly greater than those at the heart of the sun, to drive nuclei close enough to fuse. The target temperature for plasmas at tokamaks is around 270 million degrees Fahrenheit (about 150 million degrees Celsius). That’s about 10 times the temperature at the core of the sun, about 27 million degrees Fahrenheit (15 million degrees Celsius). The current temperature record for tokamak is held by China’s EAST tokamak which in late 2021 was able to generate plasma at a temperature of about 216 million degrees Fahrenheit (120 million degrees Celsius) for 101 seconds. During this time the plasma briefly reached a peak temperature of about 288 million degrees Fahrenheit (160 million degrees Celsius). Extreme temperatures aren’t the only thing tokamaks have to generate to replicate the gravitational influence of the sun, however. Superheated plasma has to be contained, and to do this tokamaks use incredibly powerful magnetic fields. Currently, it takes more energy to generate these fields than scientists can get out of fusion. The record for fusion energy generation here on Earth was set by the Joint European Torus (JET) lab in Oxfordshire, England in February 2022. The tokamak was able to generate 59 megajoules of energy using a deuterium-tritium fuel mix in an experiment that lasted just over five seconds. Any tokamak looking to meet actual energy demands will have to sustain superheated plasma for much longer periods than this, with the main aim being to create a self-sustaining plasma. If all goes according to plan, ITER will be the first fusion reactor to produce net energy, which means producing more energy than it takes to generate superheated plasma and keep it contained in a powerful magnetic field.

Working with plasma is really tricky. Because plasmas aren’t that common on Earth, scientists had very little experience with them until they started studying fusion. Plasma is difficult to hold: The plasma used in fusion-energy research is hundreds of millions of degrees Fahrenheit. You can’t hold it using a solid container, because the container would just melt. Instead, physicists have to corral it using electromagnetic fields or work with it so quickly (in less than a billionth of a second) that holding it isn’t an issue.

Plasma is difficult to compress: If you don’t compress plasma from all sides perfectly evenly, it will squish out wherever it can. Scientific American explained this well: “Imagine holding a large, squishy balloon. Now squeeze it down to as small as it will go. No matter how evenly you apply pressure, the balloon will always squirt out through a space between your fingers. The same problem applies to plasmas. Anytime scientists tried to clench them down into a tight enough ball to induce fusion, the plasma would find a way to squirt out the sides.”

Theoretically, nuclear fusion could generate huge amounts of carbon-neutral energy – nearly four million times more energy than using coal, oil or gas. But a working commercial plant will have to overcome many logistical hurdles – not least, heating large amounts of gas to 150 million degrees Celsius. Building a commercial reactor will be a “challenging and costly” process, the International Energy Foundation warns. Thirty-five countries are cooperating to build a giant plant in the south of France. The ITER project will not produce electricity for people’s homes – it is instead an experiment to see if nuclear fusion can produce more energy than it uses. It is running over-budget and over schedule, with its projected launch date in 2040s. “One of the reasons that ITER is late is that it is really, really hard,” Professor Ian Chapman, chief executive of the UK Atomic Energy Authority said. “What we are doing is fundamentally pushing the barriers of what’s known in the technology world.”

There’s more to solving nuclear fusion than simply finding the right fuel combination – but that’s the most of it. The requirements for controlled nuclear fusion are much more difficult to obtain than, say, having an exploding warhead with it. However, this is more of a scientific and infrastructure issue than a safety issue. Proponents of fusion power say one reason the technology is safe is that the fuel needs to be constantly fed into the reactor to keep fusion happening, making a runaway reaction impossible. The main issue is that it must be performed perfectly in order to provide enough electricity to be useful. Of necessity, it must be kept under regulation so that it does not output too much. When you think about fusion on a one-to-one nuclei ratio, it’s easy to achieve. However, even today’s supercomputers have difficulty simulating fusion at broad enough scales to be effective. Currently, classical supercomputers are used to run simulations of plasma physics and fusion energy scenarios, but to address the many design and operating challenges that still remain, more powerful computers are a necessity. Quantum computers’ exponentially faster computing speeds have offered plasma and fusion scientists the tantalizing possibility of vastly accelerated fusion device development. Quantum computers could reconcile a fusion device’s many design parameters—for example, vessel shape, magnet spacing, and component placement—at a greater level of detail, while also completing the tasks faster. However, upgrading to a quantum computer is no simple task.

The idea that bombarding neutrons produced by combining tritium and deuterium would make the inner facility very radioactive adds to the complexity. Humans won’t be able to go inside for at least 18 months because it’s so radioactive. That means it must work correctly the first time. For the time being, fusion technology remains a very mysterious source of energy.

Due to all above discussed reasons, nearly 70 years after the first uncontrolled fusion explosion (aka. hydrogen bomb) here on the Earth, we still do not have working fusion power stations.

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FAQ on fusion:

What are the effects of fusion on the environment?

Fusion is among the most environmentally friendly sources of energy. There are no CO2 or other harmful atmospheric emissions from the fusion process, which means that fusion does not contribute to greenhouse gas emissions or global warming. Its two sources of fuel, hydrogen and lithium, are widely available in many parts of the Earth.

Can fusion cause a nuclear accident?

No, because fusion energy production is not based on a chain reaction, as is fission. Plasma must be kept at very high temperatures with the support of external heating systems and confined by an external magnetic field. Every shift or change of the working configuration in the reactor causes the cooling of plasma or the loss of its containment; in such a case, the reactor would automatically come to a halt within a few seconds, since the process of energy production is arrested, with no effects taking place on the outside. For this reason fusion reactors are considered to be inherently safe.

When is electricity generated through fusion expected to be available?

At present, fusion devices produce more than ten megawatts of fusion power. ITER will be capable of producing 500 megawatts of fusion power. Although this will be on the scale needed for a power station, there are still some technological issues to address before a commercial power plant can operate. A prototype of a fusion reactor (DEMO) is expected to be built by 2040. Electricity generation and exploitation is also expected to take place in the second half of the century, depending on funding and technical advancement.

How does the pressure relate to fusion energy?

In order for the fusion reaction to take place, the constituent nuclei of the fuel (isotopes of hydrogen) must be very hot, and they must collide with each other frequently enough. Fusion thus requires high temperature and density simultaneously, which when multiplied together is defined as pressure. Note that density is directly proportional to pressure and indirectly proportional to temperature. As pressure increases, with temperature constant, density increases. In fusion’s case the pressure is in a plasma unlike air where it is in a gas, but the principle holds the same way. Thus, high pressures (>2atm) and temperatures (>100 million degrees, several times hotter than the center of the sun) are required to obtain fusion reactions in a magnetic confinement device. For the fusion reactions to make more power than it requires to sustain the temperature the plasma must also contain the heat. Thus, in much the same way a house has insulation, the plasma has insulation from the magnetic field.

Why does the plasma need to be hotter than the center of the sun?

The sun operates at temperatures significantly lower than what is required for a fusion reactor but overcomes this by having very high pressures and by being very large. Even at the core of the sun, where the plasma is hottest and densest the fusion energy production per volume is similar to energy production from microbes in a compost pile. To make fusion a reality on Earth the reactor must therefore be several times hotter than the center of the sun while obtaining high enough pressures.

Why is pressure important for fusion reactors?

In addition to being required for obtaining fusion, the pressure determines the rate of fusion reactions taking place inside the fusion reactor once the desired temperature is reached. The reaction rate, and thus the fusion power of the device, goes approximately as the pressure squared, so a doubling of pressure leads to a quadrupling of the fusion power. Therefore, techniques that enable increased pressure or that obtain high pressure using cost-effective technologies, improve the overall economics of a fusion reactor. The economics require that magnetic fusion reactors achieve pressures of 3 to 10 atmospheres.

In the temperature range 100–200 million degrees Celsius, the fusion power per unit volume from a D–T plasma is given roughly by formula:

Fusion power per unit volume ≈ 0.08P²(MW/m³)

Where P is the plasma pressure in atmospheres. It is immediately clear from equation that the D–T reaction produces commercially viable power densities (megawatts per metre cubed) at practically sustainable pressures (a few atmospheres). The remarkably large D–T cross section puts fusion power within reach!

What fusion devices achieve the highest performance?

To date the highest performing devices, meaning the ones with highest pressure multiplied by confinement time, is a configuration called a tokamak.

What are the obstacles to obtaining high plasma pressures in fusion devices?

As the plasma pressure is increased inside the magnetic bottle the plasma becomes more and more prone to slowly leaking heat, thus taking more input power to sustain the pressure and temperature. Eventually the pressure becomes large enough for the plasma to suddenly become unstable and the plasma pressure is lost, like popping a balloon. The worst form of this is termed a disruption and can do damage to the internal components of the machine, but does not affect the safety of the device. Additionally, as pressure is increased the plasma can develop turbulence that transports energy from the center of the plasma, cooling it.

How are these obstacles overcome?

Generally, the threshold for these problems arising can be increased by changing the shape of the magnetic bottle or by heating the plasma at the correct locations. However, the most fundamental method to raise the threshold of these phenomenon is to increase the stabilizing magnetic field that winds the long-way around the tokamak. This magnetic field is provided by large electromagnets or “coils”. The higher the field the coils provide the more stable the plasma becomes.

What is a tokamak?

A tokamak is a type of magnetic bottle designed to confine a superheated fusion-relevant plasma. The tokamak is in the shape of a donut, with strong electromagnets used to hold the plasma away from the walls of the chamber containing it. Several good descriptions exist for a tokamak.

How many tokamaks are there?

Tokamaks are the most common type of fusion device. Since their invention in the USSR in the late 1960s over 170 have been built around the world. The current generation, including Alcator C-Mod, are the highest performing type of fusion reactor and are extensively researched. There are approximately a dozen large tokamaks at major fusion research centers around the world. The United States operates three major tokamaks, Alcator C-Mod at MIT in Cambridge, MA; NSTX-U at the Princeton Plasma Physics Laboratory in Princeton, NJ; and DIII-D at General Atomics in San Diego, CA. Other major tokamaks around the world include JET in the UK, EAST in China, KSTAR in South Korea, ASDEX-Upgrade in Germany, SST-1 in India, WEST in France and many others. Various notable tokamaks are shown to scale below.

What other methods are there for containing fusion plasmas?

There are a variety of methods to confine a fusion relevant plasma using magnetic fields. The Stellarator is similar to a tokamak as it uses strong magnets but in a more complex shape and has demonstrated the next highest fusion performance. Wendelstein W7-X is an example recently commissioned in Germany. Other geometries include systems that generate their magnetic fields via the plasma itself. Additionally, lasers can be used to compress small pellets of fuel in what is termed inertial confinement fusion of which the NIF (National Ignition Facility) at Lawrence Livermore National Laboratory is an example.

Who carries out nuclear fusion research?

There are several major (national and international) and many smaller fusion programmes carried out worldwide. Because of the complexity and cost of developing fusion as an energy source, a high degree of international collaboration is required, as is being realized, for example, at JET-EFDA (Joint European Torus — European Fusion Development Agreement), Culham, Abingdon, United Kingdom. Private industry and governmental agencies are also co-operating in the fusion endeavour.

Is nuclear fusion safe?

Everybody knows what happens when fission reactors go bad. We have historical events such as Chernobyl, Three Mile Island, and Fukushima to look to, as well as a plethora of nuclear disaster tropes in fiction that have followed in their wake. But these very disasters have helped the nuclear industry make nuclear power even safer. Safety has become the primary concern of matters involving the use, transportation, and storage of radioactive material. Fusion is safer than fission. Unlike nuclear fission, fusion is not a chain reaction, removing the concern of runaway reactivity and the need for passive negative feedback in a reactor to limit the reaction. There is nothing that can grow out of control like a snowball rolling down a hill. On top of that, a full-scale fusion power plant’s reactor deals with much lighter and much more benign materials than a fission reactor does. Fission uses and produces radioactive materials that can remain radioactive and dangerous for tens of thousands of years or more. However, the proposed fusion fuel sources use and produce materials that are less radioactive and decay much more quickly.

Is solar and wind energy better than nuclear fusion energy?

Solar and wind energy are useful and important renewable energy sources, but unlike nuclear energy, they are limited by their environment. Solar panels can only generate electricity when sunlight is available, and wind turbines can only generate electricity when there is sufficient wind; these sources must store electricity in batteries to make up for the inherent downtime in their production methods. Likewise, hydroelectric energy can only be gathered from sites with significant water flow. The advantage nuclear energy has over these renewable energy sources is that it can produce electricity just about anywhere and at any time, wherever you can build a reactor. Like renewable energy, nuclear fusion energy does not directly produce greenhouse gases. Fusion also produces only low-level nuclear waste, if any. This makes it perfect for an environmentally conscious energy solution. Like renewable energy, fusion also uses a plentiful fuel source. Solar panels have the sun, wind turbines have the air, hydroelectric dams have rivers; likewise, fusion has the sea. One front-runner candidate for fusion reactor fuel is the combination of deuterium and tritium, two isotopes of hydrogen. Deuterium is extremely plentiful in our oceans. In fact, a single spoonful of seawater has so much deuterium in it that it can provide as much energy as a barrel of oil—it’s simply a matter of properly extracting that energy. Other proposed forms of fuel for fusion power include hydrogen and boron and other enablers of aneutronic fusion.

Will nuclear fusion create too much waste?

Nuclear energy in general has a reputation for creating waste in the form of dangerous radioactive byproducts. Fission waste consists of a wide variety of radioactive materials with half-lives on the scale of up to hundreds of thousands of years. These materials, considered high-level waste, must be carefully handled and stored until they decay. However, nuclear fusion, unlike fission, produces no highly radioactive byproducts. Rather, proposed fusion reactor designs produce low-level radioactive material (if any at all).

Nuclear fission power plants have the disadvantage of generating unstable nuclei; some of these are radioactive for millions of years. Fusion on the other hand does not create any long-lived radioactive nuclear waste. A fusion reactor produces helium, which is an inert gas. It also produces and consumes tritium within the plant in a closed circuit. Tritium is radioactive (a beta emitter) but its half life is short. It is only used in low amounts so, unlike long-lived radioactive nuclei, it cannot produce any serious danger. The activation of the reactor’s structural material by intense neutron fluxes is another issue. This strongly depends on what solution for blanket and other structures has been adopted, and its reduction is an important challenge for future fusion experiments.

In fact, it might surprise you to find out that fission energy alone produces surprisingly little waste—if all the United States’ electricity came from nuclear fission, for example, we would generate only 39.5 grams of nuclear waste per person per year, less than the weight of two AA batteries, which is nothing compared to the pollution output of coal and oil—pollution which unlike nuclear waste does not decay over time. The reason why this number is so low is because far more nuclear waste is recycled in some way or another—in France, for example, radioactive residue from uranium and plutonium extraction is made into harmless glass in a process called vitrification—and nuclear fusion technology not only produces even less waste, but can be used to remove highly radioactive waste produced by fission from the world.

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Hybrid fusion:

Ever since fusion has been around there have been proposals for using the neutrons from fusion to start fission, in say, a uranium fuel. A fusion reactor, whose energy is amplified by a fission fuel. Fusion can also be combined with fission in what is referred to as hybrid nuclear fusion where the blanket surrounding the core is a subcritical fission reactor. The fusion reaction acts as a source of neutrons for the surrounding blanket, where these neutrons are captured, resulting in fission reactions taking place. These fission reactions would also produce more neutrons, thereby assisting further fission reactions in the blanket.

The concept of hybrid fusion can be compared with an accelerator-driven system (ADS), where an accelerator is the source of neutrons for the blanket assembly, rather than nuclear fusion reactions. The blanket of a hybrid fusion system can therefore contain the same fuel as an ADS – for example, the abundant element thorium or the long-lived heavy isotopes present in used nuclear fuel (from a conventional reactor) could be used as fuel.

The blanket containing fission fuel in a hybrid fusion system would not require the development of new materials capable of withstanding constant neutron bombardment, whereas such materials would be needed in the blanket of a ‘conventional’ fusion system. A further advantage of a hybrid system is that the fusion part would not need to produce as many neutrons as a (non-hybrid) fusion reactor would in order to generate more power than is consumed – so a commercial-scale fusion reactor in a hybrid system does not need to be as large as a fusion-only reactor.

Hybrid fusion-fission fuel cycles, in which the neutrons emitted in D-T or D-D reactions are used to induce fissions or to create fissile isotopes (uranium-233 or plutonium-239) from fertile thorium-232 or uranium-238 in a blanket surrounding a fusion core, are possible. The fissile material produced can be fissioned in place or removed to fuel nonbreeder fission reactors elsewhere. The result, in effect, is to multiply the energy release per fusion reaction by about an order of magnitude. This makes the conditions that must be achieved in the fusion core less demanding than those for a pure fusion reactor, but the additional engineering complexity of combining fusion and fission technologies in a single device will at least partly offset this advantage and may overwhelm it. The environmental and safety characteristics of hybrid devices would be substantially those of fission reactors, compounded by the addition of fusion’s tritium and activation products. The chances of some kinds of accidents might be reduced; the chances of others increased. Most proponents of hybrids argue that these fuel cycles could best be applied in the hybrid device optimized to produce fuel for fission reactors located elsewhere. The safety or antiproliferation characteristics (or both) of such a fusion-fission nuclear energy system, they argue, might be superior to those of a system consisting primarily of fission breeder reactors.

The hybrid possibility calls attention to a link between any neutron-producing fusion energy system and the potential for proliferation of fission weapons; excess fusion neutrons can be “diverted” by a reactor’s operators to producing fissile materials for bombs. The practical importance of this link may be small, however, given the difficulties of fusion energy technology. Any group or country capable in fusion could acquire fissile materials by a number of easier means. More troublesome, perhaps, is the possibility that knowledge derived from certain aspects of research on inertial confinement approaches to fusion could be applied to the development of fusion weapons. Firm conclusions on the exact nature of this link, or its importance, cannot be reached without access to classified information.

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Section-7

History and experiments of nuclear fusion:

1920-1930: Understanding the stars and the atom:

Following Eddington’s paper, Robert d’Escourt Atkinson and Fritz Houtermans provided the first calculations of the rate of nuclear fusion in stars. And at the same time, Ernest Rutherford was exploring the structure of the atom. With his famous 1934 experiment, Rutherford showed the fusion of deuterium into helium, and observed that “an enormous effect was produced” during the process. His student Mark Oliphant used an updated version of the equipment, firing deuterium rather than hydrogen and discovered helium-3 and tritium, showing that heavy hydrogen nuclei could be made to react with each other. This was the first direct demonstration of fusion in the lab. This understanding of nuclear fusion was tied together by Hans Bethe’s work on stellar nucleosynthesis where he described that it is through proton-proton chain reactions that the Sun and stars release energy.

1950s: Enter the fusion machines:

Magnetic confinement:

By the 1950s, researchers started looking at possibilities of replicating the process of nuclear fusion on Earth. And in 1950 soviet scientists Andrei Sakharov and Igor Tamm proposed the design for a type of magnetic confinement fusion device, the tokamak. This was followed, in 1951, by Lyman Spitzer’s concept for the stellarator. The stellarator concept dominated fusion research throughout the 1950s but lost its sway when the experimental research on tokamak systems by Soviet scientist Lev Artsimovich showed that the tokamak was a more efficient concept.

Inertial confinement:

Laser fusion was suggested in 1962 by scientists at Lawrence Livermore National Laboratory (LLNL), shortly after the invention of the laser in 1960. Inertial confinement fusion (using lasers) research began as early as 1965. Several laser systems were built at LLNL. These included the Argus, the Cyclops, the Janus, the long path, the Shiva laser, and the Nova. Laser advances included frequency-tripling crystals that transformed infrared laser beams into ultraviolet beams and “chirping”, which changed a single wavelength into a full spectrum that could be amplified and then reconstituted into one frequency. Laser research ate money as well, consuming over one billion dollars in the 1980s.

Evolution:

Over time the “advanced tokamak” concept emerged, which included non-circular plasma, internal diverters and limiters, superconducting magnets, operation in the so-called “H-mode” island of increased stability, and the compact tokamak, with the magnets on the inside of the vacuum chamber.

1970-1980: Designs on JET and beginnings for ITER:

By the 1970s it was clear that attaining fusion energy would be one of science’s greatest challenges and collaboration might be key to meeting the challenge. European countries came together and began design work on the Joint European Torus, JET, in 1973. In 1977, the European commission gave the green signal for the project and Culham in Oxford, UK, was selected as the site for JET. The construction of JET, which would become the largest operational magnetic confinement plasma physics experiment, was completed on time and on budget in 1983 and the first plasmas were achieved.

The 80’s also saw the iron curtain being lifted slightly when ITER was set in motion at the Geneva Superpower Summit in November 1985. The idea of a collaborative international project to develop fusion energy for peaceful purposes was proposed by General Secretary Gorbachev of the former Soviet Union to US President Reagan.

1980-2000s: JET record:

The first experiments using tritium was carried out in JET, making it the first reactor in the world to run on the fuel of a 50-50 mix of tritium and deuterium. In 1997, using this fuel, JET set a world record for fusion output at 16 MW from an input of 24 MW of heating. This is also the world record for Q, at 0.67. It should be stated that the world record was achieved in a very short instant of about 1 second. A Q of 1 is breakeven, and to achieve fusion energy the Q value must be greater than 1. The aim of ITER is to achieve a Q of 10.

2000-present: A home for ITER:

In 2005, the ITER Members unanimously agreed that ITER would be built in Cadarache in France. In December 2022, the ITER project passed the 77.7% milestone of work scope completed to first plasma. In 2015 the Wendelstein 7-X stellarator, which is a long-term back up strategy for the tokamak in the European Fusion Roadmap came into operation, and its first operational campaigns exceeded all expectations. In 2021 a new fusion energy world record of 59 MJ was achieved in JET in a 5 second long pulse, while burning only 170 micrograms of deuterium and tritium.

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JET 2021 experiment:

On 21 December 2021, using deuterium-tritium fuel, JET produced 59 megajoules during a five second pulse, beating its previous 1997 record of 21.7 megajoules, with Q = 0.33. This discovery is one of several major developments over the past year or so that is shaping nuclear fusion technology into a stronger potential candidate for fossil fuel-free energy. This reaction, carried out by researchers at the EUROfusion consortium, more than doubled the last record for fusion energy, which topped at 21.7 megajoules in 1997. They used the Joint European Torus (JET) device to create the massive bolt of power.

For the five seconds that the experiment ran, the power output was equivalent to what four on-shore wind turbines would produce. That is to say, that the experiment was able to generate commercial-scale power. Also, significantly, the short duration of five seconds was not because of any systemic failure or fuel disruptions (events during which the fusion energy drops very rapidly) that stop the fusion reactions in the reactor core and the production of energy, as had been the case in earlier experiments, but because the equipment of this laboratory set-up got too hot to keep the experiment running. According to scientists, in the next-generation reactors, which will have suitable cryogenic equipment and cooling apparatus, the fusion process should be sustainable for much longer periods.

However, so far, in none of the fusion experiments around the world, including the latest runs at JET, has the output fusion energy exceeded the energy put in to initiate fusion among the swirling gas of nuclei in the reactor core. But these new results are seen as bringing researchers closer than ever before to that goal of “scientific break-even” (or, in technical language, attaining a Q-value equal to 1) when the output energy at least equals the input energy. Scientists also believe that results at JET are a pointer to electricity from nuclear fusion becoming a reality in 20-25 years from now.

“The record, and more importantly the things we’ve learned about fusion under these conditions, and how it fully confirms our predictions, show that we are on the right path to a future world of fusion energy,” said Tony Donné, CEO of the EUROfusion programme. “If we can maintain fusion for five seconds, we can do it for five minutes and then five hours as we scale up our operations in future machines,” he said.

More significantly, the present achievement makes the success of the ambitious $30-billion International Thermonuclear Energy Reactor (ITER, pronounced “eater”) look more promising because JET uses the same technology and fuel mix (of D and T nuclei) as ITER. An international project aimed at building a commercial-scale experimental fusion reactor, ITER is set to begin operations by 2025. Although, as a mock-up, the six metre-sized JET is only a tenth of the ITER reactor’s volume, it has been used as the test bed to validate many of the ideas and physical scenarios that form the basis of ITER. “For the ITER project,” said Bernard Bigot, director general of ITER, “the JET results are a strong confidence builder that we are on the right track as we move forward toward demonstrating full fusion power.”

The JET is the largest and most powerful operational “tokamak” in the world—and essentially looks just like a giant metallic donut. Tokamak devices confine plasma in the donut shape, with magnetic fields containing the massive amounts of heat needed to perform the process. Inside the JET, temperatures can reach 150 million degrees Celsius—that’s 10 times hotter than the center of the sun.

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Super-X system to eliminate excess heat: 2021:

A key challenge in getting tokamaks on the electricity grid is removing excess heat produced during fusion reactions. Without an exhaust system to handle the intense heat, materials will have to be regularly replaced, significantly affecting the amount of time a power plant could operate for. But a recent experiment suggests the new system, known as a Super-X divertor, would allow components in commercial tokamaks to last much longer. This would greatly increase the power plant’s availability, improving its economic viability and reducing the cost of fusion electricity, researchers say.

Tests at MAST (Mega Ampere Spherical Tokamak) Upgrade, at Culham, near Oxford, which began operating in October 2020, have shown at least a 10-fold reduction in heat on materials with the Super-X system. According to experts this could be a game-changer for creating fusion power plants that can deliver affordable, efficient electricity.

UKAEA is planning to build a prototype – known as STEP (Spherical Tokamak for Energy Production) – by the early 2040s, using a compact machine called the ‘spherical tokamak’. The success of the Super-X divertor is a boost for engineers designing the STEP device, as it is particularly suited to the spherical tokamak.

Researchers built MAST Upgrade to solve the exhaust problem for compact fusion power plants, and the signs are that they have succeeded. Super-X reduces the heat on the exhaust system from a blowtorch level down to more like you’d find in a car engine. This could mean it would only have to be replaced once during the lifetime of a power plant. Initial results from the UK Atomic Energy Authority’s (UKAEA) MAST Upgrade experiment indicate the effectiveness of an innovative exhaust system designed to make compact fusion power plants commercially viable.

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NIF 2022 experiment:

Matter in the interior of giant planets and some relatively cool stars is highly compressed by the weight of the layers above. The extreme pressures generated are strong enough to charge of atoms and generate free electrons, in a process known as ionisation. The material properties of such matter are mostly determined by the degree of ionisation of the atoms. While ionisation in burning stars is primarily determined by temperature, pressure-driven ionization dominates in cooler stellar objects. However, this process is not well understood, and the extreme states of matter required are very difficult to create in the laboratory limiting the predictive power required to model celestial objects.

The only way to study this complex process in the laboratory is to dynamically compress matter to extreme densities which requires very large energy inputs in a very short time. In a new experiment published in Nature, scientists have done just that using the largest and most energetic laser in the world, the National Ignition Facility (NIF). Through their research at the Lawrence Livermore National Laboratory (LLNL), US, the team provide new insights on the complex process of pressure-driven ionisation in giant planets and stars. They investigated the properties and behaviour of matter under extreme compression, offering important implications for astrophysics and nuclear fusion research.

The international research team used NIF to generate the extreme conditions necessary for pressure-driven ionisation. In a NIF ignition experiment, a tiny capsule containing two forms of hydrogen, deuterium (D) and tritium (T), is suspended inside a cylindrical x-ray “oven” called a hohlraum. When the hohlraum is heated by NIF’s powerful lasers to temperatures of more than 3 million degrees Celsius, the resulting x rays heat and blow off, or ablate, the surface of the target capsule, called the ablator. This causes a rocket-like implosion that compresses and heats the DT fuel to extreme temperatures and densities until the hydrogen atoms fuse, creating helium nuclei (alpha particles) and releasing high-energy neutrons and other forms of energy, similar to dwarf stars, for just a few nanoseconds.

The highly compressed metal shell (made of beryllium) was then analysed using X-rays to reveal its density, temperature, and electron structure. The findings revealed that, following strong heating and compression, at least three out of four electrons in beryllium transitioned into conducting states, that is, they can move independent from the nuclear cores of the atoms. Additionally, the study uncovered unexpectedly weak elastic X-ray scattering, indicating reduced localization of the remaining electron, that is a new stage shortly before all electrons become free and thus revealing the pathways to a fully ionised state.

LLNL physicist Tilo Döppner, who led the project, said: “By recreating extreme conditions similar to those inside giant planets and stars, we were able to observe changes in material properties and electron structure that are not captured by current models. Our work opens new avenues for studying and modeling the behavior of matter under extreme compression. The ionization in dense plasmas is a key parameter as it affects the equation of state, thermodynamic properties, and radiation transport through opacity.”

In December 2022, researchers at the Lawrence Livermore National Laboratory in California for the first time produced more energy in a fusion reaction than was used to ignite it, something called net energy gain as seen in the figure below.

Inertial confinement – In the Livermore lab, a 192-beam laser fire pulses at a small capsule filled with deuterium-tritium atoms inside a cylinder called a hohlraum.
The latter heats up and releases X-rays, which heat the nuclei to millions of degrees centigrade and compress them to billions of Earth-atmospheres.
It is called inertial confinement because the nuclei’s inertia creates a short window between implosion and explosion in which the strong nuclear force dominates, fusing the nuclei.
It is relatively easier to attain break-even energy levels through inertial fusion compared to magnetic fusion.

NIF experiment crossed a critical threshold for fusion where the energy that the fusion reaction generated — 3.15 million joules — exceeded the 2.05 megajoules the lasers pumped out to trigger the reaction. Fusion researchers denote the ratio of output energy to input energy with the letter Q, and this is the first time a fusion reaction surpassed Q = 1.

The achievement does not mean that fusion is now a viable power source. While NIF’s reaction produced more energy than the reactor used to heat up the atomic nuclei, it didn’t generate more than the reactor’s total energy use. According to Kim Budil, director of Lawrence Livermore National Laboratory, the lasers required 300 megajoules of energy to produce about 2 megajoules’ worth of beam energy. A commercial laser-fusion power plant would need to generate 100 times more energy from each target than was input, and its lasers would need to fire around 10 times per second. This means designing a system that can accurately focus and fire the lasers on hundreds of thousands of targets each day.

NIF experiment does not mean green power. For one thing, most commercial fusion energy projects are using various forms of magnetic confinement, not NIF’s laser-based approach, so the engineering challenges are different. For another, NIF is a gargantuan, $3.5 billion national lab project funded to research nuclear weapons, not a project designed to produce reliable energy for the grid at the most competitive cost. Huge inefficiencies in NIF’s lasers and in the conversion of fusion heat to electrical power mean its design is inherently impractical. In comparison, magnetic confinement fusion holds some real promise.

What issues lie ahead for laser fusion?

Effectiveness – Some of the input energy is devoted to compressing the capsule instead of raising the temperature.
This fraction will increase as the amount of fuel increases, creating another barrier to high gain.
Gain – Future research will need to focus on reaching the next major milestone – a target gain of G > 100, which is required to run a power plant efficiently.
Rate of firing – The rate at which any reactor fires its lasers at the hydrogen capsules needs to be orders of magnitude faster.
Conversion to usable form – Even if the inertial fusion process is more efficient, the produced energy still needs to be converted into usable electricity.
Cost – The cost to run and maintain an inertial fusion reactor needs to decrease dramatically.
Commercial viability – It is still not obvious whether inertial fusion can be commercially competitive.
Fusion is still far from reality.

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Chinese and German milestones in fusion research:

On 12 April, 2023, China’s experimental advanced superconducting tokamak (EAST), known as the ‘artificial sun’, set a new world record and successfully achieved a steady-state high-constraint mode plasma operation for 403 seconds. The previous record was 101 seconds, set by EAST in 2017. The breakthrough was achieved after more than 120,000 shots in EAST, located at the Institute of Plasma Physics under the Chinese Academy of Sciences (ASIPP) in Hefei, Anhui Province. The temperature and density of particles have been greatly increased during high confinement plasma operation, which will lay a solid foundation for improving the power generation efficiency of future fusion power plants and reducing costs.

Since building its first tokamak ‘Aditya’ in the 1980s, India has made remarkable progress in fusion research and operates an advanced Steady State Superconducting Tokamak (SST) which overcomes the ‘on-off’ nature of conventional tokamaks in heating plasma. Only a few countries have developed these next-generation SSTs. The EAST, for instance, is a tokamak designed for steady-state operation and the Chinese engineers who built it were all nurtured by the ITER programme. India should perhaps take a leaf out of China’s notebook and use its participation in the ITER to get a leg-up on building an indigenous fusion reactor on Indian soil in the next few decades.

Meanwhile, researchers at Germany’s Max Planck Institute for Plasma Physics (IPP) have found a way to significantly reduce the distance needed between the hot plasmas in nuclear fusion devices and the vessel wall. The ASDEX (Axially Symmetric Divertor Experiment) Upgrade tokamak experiment at IPP in Garching near Munich serves as a blueprint for International Thermonuclear Experimental Reactor (ITER) and later fusion power plants. Important elements for ITER were developed there. Plasma operating conditions and components for later power plants are already being tested. A central element of ASDEX Upgrade and all modern magnetic fusion facilities is the divertor. This is a part of the vessel wall that is particularly heat-resistant and requires an elaborate design. In order to handle the high temperatures, the divertor tiles of ASDEX Upgrade and also of ITER are made of tungsten, the chemical element with the highest melting temperature (3422°C).

Without countermeasures, 20% of the fusion power of the plasma would reach the divertor surfaces. For this reason, small amounts of impurities (often nitrogen) are added to the plasma. These extract most of its thermal energy by converting it into ultraviolet light. Nevertheless, the plasma edge (the separatrix) must be kept at a distance from the divertor to protect it. In ASDEX Upgrade until now, this has been at least 25 centimetres (measured from the lower plasma tip – the X-point – to the edges of the divertor). Researchers at IPP have now succeeded in reducing this distance to less than 5 centimetres without damaging the wall. “We accidentally moved the plasma edge much closer to the divertor than we had intended,” IPP physicist Tilmann Lunt said. “We were very surprised that ASDEX Upgrade coped with this without any problems.” “Because the plasma moves closer to the divertor, the vacuum vessel volume can be better utilised,” IPP said. “Initial calculations show that if the vessel were optimally shaped, it would be possible to almost double the plasma volume while maintaining the same dimensions. This would also increase the achievable fusion power.”

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Helion research:

Deuterium and helium-3 fuel is heated to plasma conditions. Magnets confine this plasma in a Field Reversed Configuration (FRC). Magnets accelerate two FRCs to 1 million mph from opposite ends of the 40-foot accelerator. They collide in the center. When the FRCs collide in the center of the system, they are further compressed by a powerful magnetic field until they reach fusion temperatures of 100 million degrees Celsius. At this temperature, the deuterium and helium-3 ions are moving fast enough to overcome the forces that would otherwise keep them apart and they fuse. This releases more energy than is consumed by the fusion process. As new fusion energy is created, the plasma expands. As the plasma expands, it pushes back on the magnetic field. By Faraday’s law, the change in field induces current, which is directly recaptured as electricity. This clean fusion electricity is used to power homes and communities, efficiently and affordably.

In Helion’s novel system, the energy released in the fusion reactions continuously pushes out against its magnetic containment field, which pushes back — causing oscillations (like a piston) that generate an electric current, which Helion captures directly from the reactor. The biggest attraction of Helion’s direct electricity generation method is its simplicity. Other fusion approaches aim to generate heat, in order to boil water and power steam turbines, which make electricity — like at traditional nuclear power plants. Helion can do it with no steam turbines or cooling towers. Helion envisions manufacturing fusion generators in a factory. A 50 mw scale system, packaged into three shipping container-sized units would power 40,000 homes. Helion predicts that in 10 years we will have commercial electricity for sale, for sure.

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Commonwealth fusion research:

Until now, the only way to achieve the colossally powerful magnetic fields needed to create a magnetic “bottle” capable of containing plasma heated up to hundreds of millions of degrees was to make them larger and larger. But the new high-temperature superconductor material, made in the form of a flat, ribbon-like tape, makes it possible to achieve a higher magnetic field in a smaller device, equaling the performance that would be achieved in an apparatus 40 times larger in volume using conventional low-temperature superconducting magnets. That leap in power versus size is the key element in SPARC’s revolutionary design. SPARC is a tokamak under development by Commonwealth Fusion Systems (CFS) in collaboration with the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). SPARC means Smallest Possible ARC. ARC means affordable, robust, compact.

There are roughly 150 tokamaks around the world; the biggest one is under construction in France for $30 billion by an international consortium called ITER. The 20,000-ton machine, the size of a basketball arena, is slated to be complete by 2035. But Commonwealth Fusion intends to make ITER obsolete before it’s even completed. Its edge is in the application of “high temperature” superconductors made with rare earth barium copper oxide (ReBCO).

Superconductors move electrical current with virtually zero loss (far more efficiently than copper, for example). And they are key to making powerful electromagnets. Commonwealth has found that by making its magnets using a special barium copper oxide tape (like the tape found in a VHS cassette) it can achieve magnetic fields more powerful than the ones anticipated at ITER, but at 1/20th the scale.

Whereas ITER’s primary magnets (called solenoids) will weigh some 1000 tons and achieve fields stronger than 12 tesla, Commonwealth is eyeing 15-ton magnets, each using 300 km of ReBCO thin-film tape, that will generate 12 tesla (for comparison, a magnetic resonance imaging machine does 1.5 tesla).

Commonwealth fusion predicts that by 2030 we will see fusion on the grid.

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ST-40 compact fusion reactor: 2023:

ST40 is a high field spherical tokamak with a major radius of 40cm at Tokamak Energy. The device utilizes a 4m tall stainless steel vessel with copper toroidal field coils which produce magnetic fields up to 3T. ST40 uses a novel merging-compression method for plasma start-up. The objective of ST40 is to demonstrate burning plasma conditions in a high-field, low aspect ratio spherical tokamak.

Researchers at Tokamak Energy in the UK alongside colleagues from the Princeton Plasma Physics Laboratory, the Oak Ridge National Laboratory in the US, and the Institute for Energy and Climate Research in Germany have developed a prototype for a compact and relatively simple fusion reactor called the ST40, which is much smaller and cheaper than other currently existing reactors.

“Equivalent temperatures have only previously been obtained on devices with plasma volumes greater than fifteen times larger than ST40 and with substantially more plasma heating power,” the scientists wrote in the article published in Nuclear Fusion. An increase in the plasma volume leads to an increase in the complexity and cost of the entire facility. The ST40 is a tokamak, a type of fusion reactor in which the plasma is suspended and shaped like a donut. Many different prototypes of this type of reactor have been built and widely studied. However, they are typically large to accommodate for the plasma volume, expensive, and time-consuming to build. In contrast, the ST40 stands out for its affordability and compactness. The radius of the plasma donut inside is much smaller, resembling a sphere with a pillar connecting its poles, leading the researchers to call their device a “spherical tokamak”.

“Spherical tokamaks have several beneficial features which make them an attractive option for commercial fusion power production,” explained the scientists in their paper. “[They] exhibit enhanced stability properties, as well as displaying more favorable transport and confinement properties.”

In their study, the physicists used a plasma consisting of nuclei of deuterium, an isotope of hydrogen. Deuterium mixtures are commonly used because their energy requirements for fusion are lower compared to other elements and they release more energy than other fusion reactions. The temperature inside the reactor reached 100 million degrees Celsius and was maintained for about 150 milliseconds. This is not much less than the most advanced tokamak prototypes in existence, such as JET and ITER, which can maintain approximately the same temperature for about 1 second, despite being dozens of times larger and more expensive than ST40, making this result groundbreaking.

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Disputed fusion experiments:

-1. Cold fusion:

Cold fusion is a hypothesized type of nuclear reaction that would occur at, or near, room temperature. It would contrast starkly with the “hot” fusion that is known to take place naturally within stars and artificially in hydrogen bombs and prototype fusion reactors under immense pressure and at temperatures of millions of degrees, and be distinguished from muon-catalyzed fusion. There is currently no accepted theoretical model that would allow cold fusion to occur.

In 1989, two electrochemists, Martin Fleischmann and Stanley Pons, reported that their apparatus had produced anomalous heat (“excess heat”) of a magnitude they asserted would defy explanation except in terms of nuclear processes. They further reported measuring small amounts of nuclear reaction byproducts, including neutrons and tritium. The small tabletop experiment involved electrolysis of heavy water on the surface of a palladium (Pd) electrode. The reported results received wide media attention and raised hopes of a cheap and abundant source of energy.

Many scientists tried to replicate the experiment with the few details available. Hopes faded with the large number of negative replications, the withdrawal of many reported positive replications, the discovery of flaws and sources of experimental error in the original experiment, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts. By late 1989, most scientists considered cold fusion claims dead, and cold fusion subsequently gained a reputation as pathological science. In 1989 the United States Department of Energy (DOE) concluded that the reported results of excess heat did not present convincing evidence of a useful source of energy and decided against allocating funding specifically for cold fusion. A second DOE review in 2004, which looked at new research, reached similar conclusions and did not result in DOE funding of cold fusion. Presently, since articles about cold fusion are rarely published in peer-reviewed mainstream scientific journals, they do not attract the level of scrutiny expected for mainstream scientific publications. Over 2015-2019 Google funded 30 researchers on three projects and found no evidence that cold fusion is possible, but they made some advances in measurement and materials science techniques.

According to physics, fusion can’t happen at temperatures lower than a few millions of degrees Fahrenheit. That is because protons are positively charged and repel each other. Bringing them close together in order to fuse them makes the repulsion forces stronger. This is known as the “Coulomb barrier.” Overcoming it requires a great deal of energy and stars can do it because they have so much mass that gravity’s brute force smashes protons together. The only way earthly scientists can do it is with a particle accelerator or with hot plasma containment facility.

-2. Bubble fusion:

In 2002 Rusi Taleyarkhan and colleagues at Purdue University in Lafayette, Ind., claimed to have observed a statistically significant increase in nuclear emissions of products of fusion reactions (neutrons and tritium) during acoustic cavitation experiments with chilled deuterated (bombarded with deuterium) acetone. Their experimental setup was based on the known phenomenon of sonoluminescence. In sonoluminescence a gas bubble is imploded with high-pressure sound waves. At the end of the implosion process, and for a short time afterward, conditions of high density and temperature are achieved that lead to light emission. By starting with larger, millimetre-sized cavitations (bubbles) that had been deuterated in the acetone liquid, the researchers claimed to have produced densities and temperatures sufficient to induce fusion reactions just before the bubbles broke up. As with cold fusion, most attempts to replicate their results have failed.

-3. Low-energy nuclear reactions (LENR):

LENR are class of nuclear reactions that occur at or near room temperature that are based on non-fusion reactions, for example, neutron-based reactions. LENRs are based on Standard Model physics and can occur in condensed matter under mild macrophysical conditions. Key steps in LENR processes, unlike nuclear fusion or fission, are based primarily on electroweak interactions rather than strong-force interactions. Unlike fission reactions, low-energy nuclear reactions do not produce nuclear chain reactions.

Research at the nanotechnology level is continuing on low-energy nuclear reactions (LENR) which apparently use weak nuclear interactions (rather than strong force as in nuclear fission or fusion) to create low-energy neutrons, followed by neutron capture processes resulting in isotopic change or transmutation, without the emission of strong prompt radiation. The word “low” refers to the input energies that go into the reactions; the output energies may be low or high. The term was chosen by its researchers to distinguish it from the field of high-energy physics.

LENR experiments involve hydrogen or deuterium permeation through a catalytic layer and reaction with a metal. Researchers report that energy is released, though on any reproducible basis, very little more than is input. The main practical example is hydrogen plus nickel powder evidently giving more heat than can be explained on any chemical basis.

The Japanese government is sponsoring LENR research – notably a nano-metal hydrogen energy project (MHE) – through its New Energy and Industrial Technology Development Organization (NEDO), and Mitsubishi is also active in research. There is obvious difference between cold fusion and LENR. Cold fusion entails strong nuclear force while LENR entails weak nuclear force.

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Section-8

Technology of nuclear fusion:

Fusion approaches are grouped together by families/types as seen in the figure below.

Figure above shows approaches to fusion, in color coded families: Pinch Family (orange), Mirror Family (red), Cusp Systems (violet), Tokamaks & Stellarators (light yellow, green), Plasma Structures (gray), Inertial Electrostatic Confinement (dark yellow), Inertial Confinement Fusion (ICF, blue), Plasma Jet Magneto Inertial Fusion (PJMIF, dark pink).

Here is a rough list of fusion approaches:

Magnetic Confinement Fusion (MCF):

-1. Tokamak

-2. Spherical Tokamaks

-3. Stellarators

-4. Levitated Dipole Experiment (LDX)

-5. Magnetic mirrors

-6. Cusped Geometries

-7. Reversed field pinch

Quasi-Stable Structures:

-8. Field-reversed configuration

Inertial Confinement Fusion (ICF):

-9. Direct drive ICF

-10. Fast ignition ICF

-11. Indirect ICF

-12. Heavy Ion Beams ICF

Pinches:

-13. Z-Pinch

-14. Theta-Pinch

Inertial Electrostatic Confinement (IEC):

-15. Fusors

-16. POPS

-17. Penning Traps

-18. Beams

Hybrids:

-19. Magnetized target fusion (Field Reverse Configuration and ICF)

-20. Magnetized Liner Inertial Fusion (Theta Pinch and ICF)

-21. Magneto-inertial fusion (Short Lived Magnetic Fields and ICF)

-22. Polywell (Cusped Geometries and IEC)

-23. Dynomak

-24. Screw Pinch (Theta Pinch and Z Pinch)

In terms of fusion power, some of these ideas are not going to work. One problem is that fusion as a whole, is that it has also been subject to wild claims. Since we do not know what a fusion power plant looks like, people have often claimed that they had the solution, here are some examples:

Bad/Junk/Fruitless Approaches:

-A. Uncontrolled Fusion

-B. Migma Machines

-C. The Hemual Project

-D. Bubble fusion/Sonofusion

-E. Cold fusion/LENR

-F. Muon-catalyzed fusion

-G. Pyroelectric fusion

-H. Ball Lighting

-I. Cross Fire Fusion

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Basic approaches to fusion:

The development of fusion as a viable energy source is based on reactions between high temperature deuterium and tritium plasma ions resulting in the formation of 3.5 MeV α-particles and neutrons of 14.1 MeV kinetic energies, respectively. To achieve fusion reactions with a sufficiently high rate, the burning temperature of the D-T plasma has to be in the range 100 to 300 million K (8 to 25 keV). Since tritium is unstable (beta-decay with 12.26 years half-life), it has to be produced by neutron induced reactions in a lithium containing blanket around the plasma. There, in particular, neutron capture reactions on stable Li-6 and Li-7 isotopes (natural abundances of 7.4 % and 92.6 %, respectively) take place.

To achieve fusion on Earth, one has to create a plasma of the fusion reactants of sufficiently high temperature and density, and also hold it confined for a sufficiently long time away from any surrounding material walls. There are two main approaches for achieving this: magnetic confinement and inertial confinement. Besides these two approaches, other methods are described in some detail below.

-1. Magnetic confinement fusion (MCF):

Magnetic confinement fusion is presently the most promising path to developing future fusion reactors based on this concept, especially as they can confine the plasma in a steady state for long durations. The ITER device, presently under construction at Cadarache in France, will be the first fusion reactor producing 500 MW of fusion power, 10 times more than the input auxiliary heating power. It would also have plasma discharges of 3000 s duration in the non-inductive phase of operations, a first step to steady state operations.

In magnetic confinement the particles and energy of a hot plasma are held in place using magnetic fields. A charged particle in a magnetic field experiences a Lorentz force that is proportional to the product of the particle’s velocity and the magnetic field. This force causes electrons and ions to spiral about the direction of the magnetic line of force, thereby confining the particles. When the topology of the magnetic field yields an effective magnetic well and the pressure balance between the plasma and the field is stable, the plasma can be confined away from material boundaries. Heat and particles are transported both along and across the field, but energy losses can be prevented in two ways. The first is to increase the strength of the magnetic field at two locations along the field line. Charged particles contained between these points can be made to reflect back and forth, an effect called magnetic mirroring. In a basically straight system with a region of intensified magnetic field at each end, particles can still escape through the ends due to scattering between particles as they approach the mirroring points. Such end losses can be avoided altogether by creating a magnetic field in the topology of a torus (i.e., configuration of a doughnut or inner tube).

External magnets can be arranged to create a magnetic field topology for stable plasma confinement, or they can be used in conjunction with magnetic fields generated by currents induced to flow in the plasma itself. The late 1960s witnessed a major advance by the Soviet Union in harnessing fusion reactions for practical energy production. Soviet scientists achieved a high plasma temperature (about 3,000,000 K), along with other physical parameters, in a machine referred to as a tokamak. A tokamak is a toroidal magnetic confinement system in which the plasma is kept stable both by an externally generated, doughnut-shaped magnetic field and by electric currents flowing within the plasma. Since the late 1960s the tokamak has been the major focus of magnetic fusion research worldwide, though other approaches such as the stellarator, the compact torus, and the reversed field pinch (RFP) have also been pursued. In these approaches, the magnetic field lines follow a helical, or screwlike, path as the lines of magnetic force proceed around the torus. In the tokamak the pitch of the helix is weak, so the field lines wind loosely around the poloidal direction of the torus. In contrast, RFP field lines wind much tighter, wrapping many times in the poloidal direction before completing one loop in the toroidal direction.

Figure above shows torus with Poloidal direction (red arrow) and Toroidal direction (blue arrow).

Tokamak:

A tokamak is a machine that confines a plasma using magnetic fields in a donut shape that scientists call a torus. Fusion energy scientists believe that tokamaks are the leading plasma confinement concept for future fusion power plants. In a tokamak, magnetic field coils confine plasma particles to allow the plasma to achieve the conditions necessary for fusion. One set of magnetic coils generates an intense “toroidal” field, directed the long way around the torus. A central solenoid (a magnet that carries electric current) creates a second magnetic field directed along the “poloidal” direction, the short way around the torus. The two field components result in a twisted magnetic field that confines the particles in the plasma. A third set of field coils generates an outer poloidal field that shapes and positions the plasma.

Figure above shows basic tokamak components include the toroidal field coils (in blue), the central solenoid (in green), and poloidal field coils (in grey). The total magnetic field (in black) around the torus confines the path of travel of the charged plasma particles.

To construct the magnetic field cage a tokamak requires three superposed magnetic fields: Firstly, a ring-shaped field produced by plane external coils; secondly, the field of a current flowing in the plasma. The field lines of the combined field are then helical. This is what produces twisting of the field lines and formation of magnetic surfaces, which are necessary for confining the plasma. A third, vertical field fixes the position of the current in the plasma. The plasma current is normally induced by a transformer coil (central solenoid). This is why a tokamak does not work in continuous, but in pulsed mode: In a transformer it is only for a limited time that an increasing current can be generated in the primary winding so that a current can be driven in the plasma. The transformer must, then be “discharged” and the current started up afresh. In order to achieve steady state operation in a future tokamak power plant, investigations are being conducted on methods of generating current in continuous mode, e.g. by means of high-frequency waves.

The first tokamak, T-1, began operation in Russia in 1958. Subsequent advances led to the construction of the Tokamak Fusion Test Reactor at Princeton Plasma Physics Laboratory and Joint European Torus in England, both of which achieved record fusion power in the 1990s. These successes motivated 35 nations to collaborate on the superconducting ITER tokamak, which aims to explore the physics of burning plasmas.

Magnetically confined plasma must be heated to temperatures at which nuclear fusion is vigorous, typically greater than 75,000,000 K (equivalent to an energy of 4,400 eV). This can be achieved by coupling radio-frequency waves or microwaves to the plasma particles, by injecting energetic beams of neutral atoms that become ionized and heat the plasma, by magnetically compressing the plasma, or by the ohmic heating (also known as Joule heating) that occurs when an electric current passes through the plasma.

Employing the tokamak concept, scientists and engineers in the United States, Europe, and Japan began in the mid-1980s to use large experimental tokamak devices to attain conditions of temperature, density, and energy confinement that now match those necessary for practical fusion power generation. The machines employed to achieve these results include the Joint European Torus (JET) of the European Union, the Japanese Tokamak-60 (JT-60), and, until 1997, the Tokamak Fusion Test Reactor (TFTR) in the United States. Indeed, in both the TFTR and the JET devices, experiments using deuterium and tritium produced more than 10 megawatts of fusion power and essentially energy breakeven conditions in the plasma itself. Plasma conditions approaching those achieved in tokamaks were also achieved in large stellarator machines in Germany and Japan during the 1990s.

Magnetic confinement fusion, or MCF, is more usually referred to as magnetic fusion energy (MFE). It aims to contain an incredibly hot gaseous plasma, roughly 100 million degrees Celsius, by the force of a carefully crafted magnetic field configuration. Fusion plasma confinement must be good enough that plasma heat and particles escape more slowly than they react. Otherwise, like a pile of damp sticks to which the flame of a match is held, the reaction will fizzle out when the match—in this case, externally supplied plasma heating—is removed, because the reaction heat cannot overcome losses.

The tokamak magnetic configuration has, since the 1960s, been the most successful at minimizing plasma loss and maximizing confinement. Over the succeeding years, scientific understanding of the loss processes has developed enormously. It is now known within a factor of two or so what it will take to achieve ignition or the burning plasma state quite close to it. The largest tokamaks to have operated—the Tokamak Fusion Test Reactor and JT-60U, which have both closed down, and the Joint European Torus (JET), which is still operating—fall short of the required magnetic configuration parameters. That requirement, simplified drastically is that the product of magnetic field strength, B, and the major radius of the plasma torus, R, must exceed roughly 20 tesla-meters. JET has a BR product of approximately 10. The Tokamak Fusion Test Reactor and JT-60 had similar values. None are sufficient to enter the burning plasma regime. And since no experiment with greater BR has been built since, the maximum confinement performance achievements of MCF have been stagnant since the 1990s, even if scientific understanding and many other aspects of performance have not. JET’s recent deuterium-tritium (D-T) campaign achieved comparable plasma performance to its 1997 experiments, but did so with vital changes to plasma-facing materials and components, as necessary for the success of future D-T experiments and reactors. That achievement deserves to be regarded as major progress.

The International Thermonuclear Experimental Reactor (ITER), a giant tokamak under construction in France by a collaboration of 35 nations, is designed to achieve and study burning plasmas. It will have approximately B = 5.7 tesla and R = 6 meters, for a BR of 34.2. [a magnetic field of 13 tesla on the coil, i.e. 5.7 tesla on the magnetic field axis]. But ITER’s great size and complex organization have made it expensive and painfully slow to construct. It will not operate until 2026 at the earliest and, because of complexities surrounding radioactivity, will not use the tritium fuel needed for burning plasma until probably the mid-2030s. On that timescale, “when will we have fusion” question is answered with “not on the grid in my lifetime.” if ITER is the only option.

In the past year, a team from MIT and Commonwealth Fusion Systems (CFS) has demonstrated that, by using high-temperature superconductors, much higher magnetic field strength in the tokamak configuration can be generated without significant dissipation. This increase makes the proposed SPARC experiment feasible, which is a design with roughly B = 10 and R = 2.2, so BR = 22.4

Using magnetic fields that are twice as strong as those from the 12-tesla magnets planned for ITER, CFS say they can create a sustainable fusion reaction in a machine as small as 1/70th the size of ITER. “We are increasing the amount of fusion power per volume [from ITER’s] by more than a factor of 10,” says CFS cofounder Dennis Whyte, who notes that “fusion power per volume is the closest thing you can come up with to indicate the amount of economic output versus cost.” On paper, SPARC can demonstrate burning plasma performance in a machine whose volume and mass, and hence cost, are roughly 25 times smaller than those of ITER. SPARC can also be built much faster. Already its estimated construction cost of about US$2 billion has been raised from venture capital, and its construction is proceeding. It is aimed—optimistically—to operate in 2025 and to achieve a burning plasma years earlier than ITER.

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-2. Inertial confinement fusion (ICF):

The inertial confinement fusion (ICF) approach, on the other hand, works primarily in a pulsed fashion, achieving thermonuclear fusion through microexplosions of reactant targets induced by high power laser or particle beams at a high repetition rate. The two largest ICF experiments in the world presently are the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in the USA, and the Laser Megajoule (LMJ) facility in France. Both these facilities are designed to achieve ignition conditions. However, given the rather limited driver conversion efficiency (~10%) of the lasers and the conversion efficiency of ~35–40% of fusion to electric power needed for power production, ICF facilities would need a fusion gain of about 100–200 for net power production. Hence, ICF reactor systems for power generation seem to be more challenging.

In this approach, a fuel mass is compressed rapidly to densities 1,000 to 10,000 times greater than normal by generating a pressure as high as 10^17 pascals (10^12 atmospheres) for periods as short as a nanosecond (10^−9 second). Near the end of this time period, the implosion speed exceeds about 3 × 10^5 metres per second. At maximum compression of the fuel, which is now in a cool plasma state, the energy in converging shock waves is sufficient to heat the very centre of the fuel to temperatures high enough to induce fusion reactions (greater than an equivalent energy of about 4,400 eV). If the mass of this highly compressed fuel material is large enough, energy will be generated through fusion reactions before this hot plasma ball disassembles. Under proper conditions, much more energy can be released than is required to compress and shock heat the fuel to thermonuclear burning conditions.

ICF was first developed at Lawrence Livermore National Laboratory in California in the 1960s and 70s, and the laboratory remains at the forefront of the field. Its National Ignition Facility (NIF) is home to the most energetic laser in the world and the 192 beams it produces are converged on a fusion fuel target that is imploded by the 2-megajoule laser pulse to reach high temperature and high density conditions.

The implosion process is complex and involves several stages. The fuel capsule, a plastic sphere containing the deuterium-tritium fuel frozen onto its inner surface, is about the size of a peppercorn and is positioned at the centre of a gold cylinder about a centimetre long called a hohlraum. The laser beams are shone in through two holes, one at either end of the hohlraum, so that the light falls on the inner wall of the cylinder, not on the capsule. The beams heat the hohlraum material to high temperature so that it emits x-rays and these shine on the surface of the capsule causing some of the plastic to explosively fly off the capsule and so driving the rest of the capsule wall and fuel inwards towards its centre at several hundred kilometres per second a seen in the figure below.

Figure above shows schematics of indirect- and direct-drive ICF. Typical targets used in laser-driven ICF are indirectly driven (upper left) or directly driven (upper right). In either case, a spherical capsule is prepared at t = 0 with a layer of DT fuel on its inside surface. As the capsule surface absorbs energy and ablates, pressure accelerates the shell of remaining ablator and DT fuel inwards—an implosion. By the time the shell is at approximately one-fifth of its initial radius it is travelling at a speed of many hundreds of kilometres per second. By the time the implosion reaches minimum radius, a hotspot of DT has formed, surrounded by colder and denser DT fuel.

If symmetry is maintained, the fuel will be compressed into a ball with a density one hundred times that of lead and a temperature of 50 million degrees. According to theory, under these conditions, a fusion burn should begin in a hotspot at the centre of this ball and then propagate outwards consuming all the fuel. This is aided by a process known as alpha-heating, where the helium nuclei—or alpha particles—that are the product of the fusion reaction fly off with high energy and, through collisions, heat the fuel to maintain the burn.

The physical processes in ICF bear a relationship to those in thermonuclear weapons and in star formation—namely, collapse, compression heating, and the onset of nuclear fusion. The situation in star formation differs in one respect: gravity is the cause of the collapse, and a collapsed star begins to expand again due to heat from exoergic nuclear fusion reactions. The expansion is ultimately arrested by the gravitational force associated with the enormous mass of the star, at which point a state of equilibrium in both size and temperature is achieved. In contrast, the fuel in a thermonuclear weapon or ICF completely disassembles. In the ideal ICF case, however, this does not occur until about 30 percent of the fusion fuel has burned.

Over the decades, very significant progress has been made in developing the technology and systems for high-energy, short-time-pulse drivers that are necessary to implode the fusion fuel. The most common driver is a high-power laser, though particle accelerators capable of producing beams of high-energy ions are also used. Two lasers capable of delivering up to 5,000,000 joules in equally short bursts of one tenth of a nanosecond, generating a power level on the fusion targets in excess of 5 × 10^14 watts, are operational.

Inertial confinement fusion is how hydrogen bombs work. It relies on reactions at densities roughly a trillion times higher than magnetic fusion and taking place so quickly that most of the fuel reacts before the high-pressure assembly explodes apart. To produce useful energy, as opposed to devastation, the idea of inertial fusion energy (IFE) is that each explosion’s size should be scaled down to something that is manageable and repeated once per second or faster to provide a source of useful average power. That might sound a bit crazy, but currently almost all the power for ground transport works on the principle of repetitive explosions: the internal combustion engine. It is debatable how small the energy yield of each inertial fusion explosion must be to make it manageable, but optimistic estimates usually take it to be a billion joules. That is the same energy yield as approximately 20 kilograms of TNT—a big explosion. A smaller upper limit is more plausible. IFE also must demonstrate a method of causing fusion micro-explosions without the use of a fission bomb trigger. Delivering the required extreme triggering power is most easily done with high-power lasers—hence the colloquial name “laser fusion.”

In ICF experiments, intense radiation in the form of laser light, X-rays or energetic particles bombard a spherical capsule of deuterium and tritium fuel encased in a beryllium or plastic “ablater”. Rapid heating and expansion of the ablater surface has an effect equivalent to covering the pellet in multiple miniature rockets. The fuel responds by imploding, compressing by a factor of 1000-10,000, with temperatures reaching 100 million degrees. Fusion then initiates, igniting and burning up the fuel before it has a chance to expand. In order to achieve these high levels of compression, the irradiation of the target has to be incredibly uniform.

A crucial aspect of the physics challenges is to produce net energy, ICF must have confinement performance considerably greater than what is needed for ignition. It must be sufficient to ensure that almost all the fuel is burned in each pulse. Fusion confinement performance is technically best evaluated by the Lawson product of density and confinement time. In ICF, it is conventionally expressed as the product of mass-density ρ and assembly radius R, because the confinement time can be considered to be the time taken to travel a distance R at a known speed. The ρR required for D-T ignition is approximately 0.3 grams per square centimeter, but the value required for high gain, to burn at least 50% of the fuel, is approximately ten times larger than ignition: 3 grams per square centimeter. By contrast, in MFE, all that is required is ignition or just a fraction less. Accepting that NIF in 2022 achieved ignition, it is still a factor of 10 short, in Lawson parameter ρR, of what is needed to generate net energy. JET’s 1997 Lawson parameter was only a factor of 5 short of what is needed in MFE. It was arguably closer, even then, than IFE is today.

The amount of tritium consumed in producing one joule of fusion energy is exactly the same for MCF as for ICF. The modest amount of tritium in each small ICF capsule makes it practical to use tritium on every shot but NIF currently executes, at best, one experimental blast per day while JET can run 30 shots per day. Also, the pulse length of JET is approximately a trillion times longer than that of ICF. Huge inefficiencies in NIF’s lasers and in the conversion of fusion heat to electrical power mean its design is inherently impractical. A commercial laser-fusion power plant would need to generate 100 times more energy from each target than was input, and its lasers would need to fire around 10 times per second. This means designing a system that can accurately focus and fire the lasers on hundreds of thousands of targets each day. The big scaling advantage in favour of MCF is that the geometry remains substantially unchanged as you increase the size, where as in ICF it does not. As you scale up the size of the pellet, the size and complexity of the required laser system and optics would almost certainly increase disproportionately. The optics of ICF systems are necessarily very complex to produce the smoothest most even illumination, even slight changes in the geometry would require extensive redesign. So MCF is more likely to succeed than ICF for fusion power. Magnetic fusion not only produces scientific successes, but also seems very attractive, at least to private investors: a total of around 30 private companies are now working on small fusion reactors – most of them in the field of magnetic fusion – which have already raised almost five billion dollars in capital.

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The two varieties of fusion examined so far, magnetic and inertial confinement, exist at extreme ends of the parameters permitted by the triple product, extremes that are complex and expensive to achieve. Other approaches explore the middle ground between these extremes at more moderate levels of confinement and density. Many groups are investigating this middle ground in the hope of finding approaches that are faster, simpler, and cheaper than the multi-billion-dollar facilities like ITER and NIF.

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-3. Magneto-inertial fusion:

One midway approach is known as magneto-inertial fusion and a prototypical example is that being developed at Sandia National Laboratory in Albuquerque, New Mexico. Dubbed magnetized liner inertial fusion, or MagLIF, the Sandia approach uses a device called the Z-machine which stores up a large amount of electric charge and can discharge current pulses in the range of tens of mega-amps. The fusion target is a small cylindrical metal can about 1 cm tall called a liner which is placed in the centre of the Z-machine. This is filled with fusion fuel and a magnetic field is applied along its length to hold the ionized fuel in the centre of the liner. Researchers then use a laser shone in through a window at one end to heat the plasma. Finally, the Z-machine discharges a current of 20 mega-amps along the walls of the liner and this generates an intense magnetic field encircling the liner which rapidly crushes it and its contents. In addition, the implosion boosts the applied magnetic field a thousand-fold as seen in the figure below.

Figure above shows schematic of magnetized liner inertial fusion (MagLIF). Deuterium-tritium fusion fuel is contained inside a cylindrical metal ‘liner’ and a magnetic field is applied to confine the plasma. A laser is used to heat the fuel. An intense electric current is passed along the wall of the liner, creating an encircling magnetic field that crushes the liner and its contents, also amplifying the applied magnetic field.

MagLIF is a relatively new technique and the researchers at Sandia are working to understand the physics of the processes involved. But since beginning work in 2013 they have achieved a 2.5-fold improvement in yield. A power plant based on this approach, they calculate, would require one shot every 10 s. That slow rate is feasible because a MagLIF liner contains more fuel than a typical ICF capsule so produces more energy per shot.

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-4. Field reversed configuration (FRC):

Other approaches employ a plasma phenomenon called a compact toroid, in particular one known as a field-reversed configuration (FRC). This phenomenon, first observed in the 1950s, involves forming plasma into something akin to a rotating smoke ring. Plasma moving in this way generates its own magnetic field which acts to confine it, slowing down the dispersion of the plasma (figure below). Although early FRCs lasted only a few millionths of a second, researchers were intrigued by them because they held the promise of providing a simple way to confine plasma. Although there has been much research on FRCs and other variations of compact toroid, none could be made to survive long enough to be useful for fusion energy. Now a new breed of start-up companies aiming to achieve faster /simpler /cheaper fusion has adopted FRCs for their approaches.

Figure above shows Field-reversed configuration. A ring of plasma (blue) which rotates (red arrow) creates a magnetic field (green) which helps to confine it.

An FRC reactor contains plasma in its own magnetic field by inducing a toroidal electric current inside a cylindrical plasma. Compared to the direction of an externally applied magnetic field, the axial field inside the reactor is reversed by eddy currents in the plasma. TAE Technologies’ reactor (pictured above) uses plasma guns to accelerate two plasmas into each other and then heats them with particle beams using particle accelerators. TAE’s technology seeks to fuse hydrogen and boron. Although FRC machines are less prone to instabilities than are some other magnetic-confinement methods, no lab has yet demonstrated a working FRC reactor that can create a sufficiently dense and stable plasma.

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-5. Magnetized target fusion (Field Reverse Configuration and ICF):

This hybrid approach uses magnetic fields to confine a lower-density plasma (as in magnetic-confinement fusion), which is then heated and compressed using an inertial-confinement method such as lasers or pistons (as in inertial-confinement fusion). Scientists have yet to increase the plasma density to a working level and keep it there long enough for a significant fraction of the fuel mass to fuse.

One example is General Fusion in Vancouver, Canada. The company, founded in 2002, has experimented to prove the feasibility of their approach and are now developing prototype-scale components in preparation for building a demonstration plant.

The first part is a plasma injector that forms FRCs and fires them out at speed into the centre of the second part, a spherical reaction chamber as seen in the figure above. The chamber contains some liquid lithium which is spun up to coat the inner wall. This serves several purposes: it absorbs the high-energy neutrons produced in fusion reactions, preventing them from damaging the reactor structure; it absorbs the energy of the neutrons and carries it out to be harvested and converted into electricity; and the neutrons convert some of the lithium into tritium, which can be collected and used as fuel for the reaction.

The third part of the system provides the compression which implodes the FRC once it reaches the centre of the chamber. It is comprised of up to 400 pneumatic pistons positioned all round the reaction chamber like spines on a sea urchin. These slam down in unison to produce a converging shock wave in the chamber centred on the FRC. The company is currently working to integrate these three components together into a demonstration reactor. This will be able to perform around one shot per day but a working power plant would need to perform one per second.

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-6. Muon-catalyzed fusion:

The need in traditional schemes of nuclear fusion to confine very high-temperature plasmas has led some researchers to explore alternatives that would permit fusion reactants to approach each other more closely at much lower temperatures. One method involves substituting muons (μ) for the electrons that ordinarily surround the nucleus of a fuel atom. Muons are negatively charged subatomic particles similar to electrons, except that their mass is a little more than 200 times the electron mass and they are unstable, having a half-life of about 2.2 × 10^−6 second. In fact, fusion has been observed in liquid and gas mixtures of deuterium and tritium at cryogenic temperatures when muons were injected into the mixture.

Muon-catalyzed fusion is the name given to the process of achieving fusion reactions by causing a deuteron (deuterium nucleus, D+), a triton (tritium nucleus, T+), and a muon to form what is called a muonic molecule. Once a muonic molecule is formed, the rate of fusion reactions is approximately 3 × 10^−8 second. However, the formation of a muonic molecule is complex, involving a series of atomic, molecular, and nuclear processes.

In schematic terms, when a muon enters a mixture of deuterium and tritium, the muon is first captured by one of the two hydrogen isotopes in the mixture, forming either atomic D+-μ or T+-μ, with the atom now in an excited state. The excited atom relaxes to the ground state through a cascade collision process, in which the muon may be transferred from a deuteron to a triton or vice versa. More important, it is also possible that a muonic molecule (D+-μ-T+) will be formed. Although a much rarer reaction, once a muonic molecule does form, fusion takes place almost immediately, releasing the muon in the mixture to be captured again by a deuterium or tritium nucleus and allowing the process to continue. In this sense the muon acts as a catalyst for fusion reactions within the mixture. The key to practical energy production is to generate enough fusion reactions before the muon decays.

The complexities of muon-catalyzed fusion are many and include generating the muons (at an energy expenditure of about five billion electron volts per muon) and immediately injecting them into the deuterium-tritium mixture. In order to produce more energy than what is required to initiate the process, about 300 D-T fusion reactions must take place within the half-life of a muon.

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-7. Lattice Confinement Fusion:

A team of NASA researchers seeking a new energy source for deep-space exploration missions, recently revealed a method for triggering nuclear fusion in the space between the atoms of a metal solid. Called Lattice Confinement Fusion, the method NASA revealed accomplishes fusion reactions with the fuel (deuterium, a widely available non-radioactive hydrogen isotope composed of a proton, neutron, and electron, and denoted “D”) confined in the space between the atoms of a metal solid. In previous fusion research such as inertial confinement fusion, fuel (such as deuterium/tritium) is compressed to extremely high levels but for only a short, nano-second period of time, when fusion can occur. In magnetic confinement fusion, the fuel is heated in a plasma to temperatures much higher than those at the center of the Sun. In the new method, conditions sufficient for fusion are created in the confines of the metal lattice that is held at ambient temperature. While the metal lattice, loaded with deuterium fuel, may initially appear to be at room temperature, the new method creates an energetic environment inside the lattice where individual atoms achieve equivalent fusion-level kinetic energies.

A metal such as erbium is “deuterated” or loaded with deuterium atoms, “deuterons,” packing the fuel a billion times denser than in magnetic confinement (tokamak) fusion reactors. In the new method, a neutron source “heats” or accelerates deuterons sufficiently such that when colliding with a neighboring deuteron it causes D-D fusion reactions. In the current experiments, the neutrons were created through photodissociation of deuterons via exposure to 2.9+MeV gamma (energetic X-ray) beam. Upon irradiation, some of the fuel deuterons dissociate resulting in both the needed energetic neutrons and protons. In addition to measuring fusion reaction neutrons, researchers also observed the production of even more energetic neutrons which is evidence of boosted fusion reactions or screened Oppenheimer-Phillips (O-P) nuclear stripping reactions with the metal lattice atoms. Either reaction opens a path to process scaling.

Figure above shows main elements of the lattice confinement fusion process. In Part (A), a lattice of erbium is loaded with deuterium atoms (i.e., erbium deuteride), which exist here as deuterons. Upon irradiation with a photon beam, a deuteron dissociates, and the neutron and proton are ejected. The ejected neutron collides with another deuteron, accelerating it as an energetic “d*” as seen in (B) and (D). The “d*” induces either screened fusion (C) or screened Oppenheimer-Phillips (O-P) stripping reactions (E). In (C), the energetic “d*” collides with a static deuteron “d” in the lattice, and they fuse together. This fusion reaction releases either a neutron and helium-3 (shown) or a proton and tritium. These fusion products may also react in subsequent nuclear reactions, releasing more energy. In (E), a proton is stripped from an energetic “d*” and is captured by an erbium (Er) atom, which is then converted to a different element, thulium (Tm). If the neutron instead is captured by Er, a new isotope of Er is formed (not shown).

A novel feature of the new process is the critical role played by metal lattice electrons whose negative charges help “screen” the positively charged deuterons. Such screening allows adjacent fuel nuclei to approach one another more closely, reducing the chance they simply scatter off one another, and increasing the likelihood that they tunnel through the electrostatic barrier promoting fusion. This is according to the theory developed by the project’s theoretical physicist, Vladimir Pines. The current findings open a new path for initiating fusion reactions for further study within the scientific community. However, the reaction rates need to be increased substantially to achieve appreciable power levels, which may be possible utilizing various reaction multiplication methods under consideration.

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-8. Electrostatic Confinement Fusion (ECF):

The most interesting, tantalizing, and simplest fusion reactor to make is an inertial electrostatic confinement fusion reactor, or Farnsworth-Hirsch fusor. Basically you need a vacuum container the size of a basketball and a pump to empty it. You put a small spherical wire cage the size of a golf ball in the center, and a larger wire cage the size of a bowling ball around it. You then pump out all the air, and apply a very large (10,000V) electrical potential between the two cages, with the small negative cathode at the center, and the positive anode at the periphery. Then you open a valve and let in a very small amount of Deuterium gas (commercially available) into the vacuum chamber, and it ionizes in the large voltage potential, the D+ ions in the periphery experience a very strong electric field (100K V/m), and are rapidly accelerated towards the center of the reactor, with enough energy to collide and fuse the deuterium nuclei (no tritium required). This fusion can be measured with an external neutron counter.

Theoretically, power generation would be simple, as the fusion products consisting of highly energetic He3 and (few) H3 ions could be decelerated in an electric field and the current from that used to generate electricity directly.

Although they seem like a obvious winners due to their simplicity and effectiveness, ECF reactors have several things limiting scaling:

(a) There is a limit to the deuterium gas density before performance degrades, so generation capacity is limited.

(b) Deuterium ions tend to collide with the wire cage cathode in the center, which absorbs them, heating the cage, and causing energy loss and contamination from metal atoms being dislodged.

(c) As the deuterium ions oscillate back and forth through the center, each time they change direction, they emit bremsstrahlung radiation, or photons of light, losing energy.

(d) Scattering tends to reduce the nice, radial trajectories of the ions into a Maxwellian distribution over time, reducing the fusion that occurs.

There has been work in using electromagnets at the periphery to try and hold a ball of electrons in the center, eliminating the need for a central cage… aka Bussard’s Polywell system. A company called EC3 was trying to continue his work, but no commercial success ever resulted, mostly due to a), c), and d) still limiting it. Maxwellian re-distribution of ion trajectories is the bane of much of plasma fusion physics, more so for ECF researchers.

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-9. Dense plasma focus (DPF):

The final approach to be considered is the dense plasma focus (DPF) which has been studied since the 1960s. The company Lawrenceville Plasma Physics in Middlesex, New Jersey, has developed a DPF device comprised of two concentric cylindrical electrodes, with an outer diameter of 17 cm and around 30 cm long, all enclosed in a vacuum vessel filled with a gas of fusion fuel. When researchers put a pulse of electricity across the electrodes, the gas is ionized between the electrodes in a thin sheath of plasma made up of many tiny filaments. The sheath moves along to the end of the inner electrode where magnetic fields created by the current passing through the plasma twist the filaments into a tiny ball called a plasmoid. The magnetic fields collapse, and this generates two beams flowing from the plasmoid, electrons in one direction and ions in the other. The electrons heat the plasmoid to extreme temperatures, enough to spark fusion reactions. The plasmoid only lasts for 10 billionths of a second but achieves a plasma density close to that of a solid. Again, the energy of the ion beam produced can be harnessed directly into electricity. The company recently reported having achieved an ion temperature of nearly three billion degrees so it is aiming for hydrogen-boron fusion.

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Tokamak Fusion Reactor Overview:

Fusion reactor, also called fusion power plant or thermonuclear reactor is a device to produce electrical power from the energy released in a nuclear fusion reaction. In order for fusion to occur, hydrogen isotopes of deuterium and tritium must be acquired. While deuterium can easily be gathered from ocean water, tritium is slightly more difficult to come by, though it can be manufactured from Earth’s abundant lithium. Once acquired, the hydrogen isotopes are injected into an empty vessel and subjected to temperature and pressure great enough to mimic the conditions at the core of our Sun. Using carefully controlled high-frequency radio waves, the hydrogen isotopes are broken into plasma and further controlled through an electromagnetic field. As the electromagnetic field continues to exert pressure on the hydrogen plasma, enough energy is supplied to cause the hydrogen plasma to fuse into helium.

Figure above shows Tokamak confinement of nuclear fusion plasma. The magnetic field lines are used to confine the high-temperature plasma (purple). Research is currently being done to increase the efficiency of the tokamak confinement model.

Once the plasma fuses, high-velocity neutrons are ejected from the newly formed helium atoms. Those high velocity neutrons, carrying the excess energy stored within nuclei of the original hydrogen, are able to travel unaffected by the applied magnetic field. In doing so, they strike a barrier around the nuclear reactor, transforming their excess energy to heat. The heat is then harvested to make steam that drives turbines.

The historical concern with nuclear fusion reactors is that the energy required to control the electromagnetic field is greater than the energy harvested from the hydrogen atoms. However, recent research by both Lockheed Martin engineers and scientists at the Lawrence Livermore National Laboratory has yielded exciting theoretical improvements in efficiency. A test facility called ITER (International Thermonuclear Experimental Reactor) is being constructed in southern France. A joint venture of the European Union, the United States, Japan, Russia, China, South Korea, and India, ITER is designed for further study into the future of nuclear fusion energy production.

When deuterium and tritium fuse, two nuclei come together to form a helium nucleus (an alpha particle), and a high-energy neutron.

2/1H + 3/1H → 4/2He + n, the energy per reaction is 17.59 MeV.

While nearly all stable isotopes lighter on the periodic table than iron-56 and nickel-62, which have the highest binding energy per nucleon, will fuse with some other isotope and release energy, deuterium and tritium are by far the most attractive for energy generation as they require the lowest activation energy (thus lowest temperature) to do so, while producing among the most energy per unit weight.

All proto- and mid-life stars radiate enormous amounts of energy generated by fusion processes. Mass for mass, the deuterium–tritium fusion process releases roughly four times as much energy as uranium-235 fission, and millions of times more energy than a chemical reaction such as the burning of coal. It is the goal of a fusion power station to harness this energy to produce electricity.

Activation energies (in most fusion systems this is the temperature required to initiate the reaction) for fusion reactions are generally high because the protons in each nucleus will tend to strongly repel one another, as they each have the same positive charge. A heuristic for estimating reaction rates is that nuclei must be able to get within 1 femtometer (1 × 10^−15 meter) of each other, where the nuclei are increasingly likely to undergo quantum tunneling past the electrostatic barrier and the turning point where the strong nuclear force and the electrostatic force are equally balanced, allowing them to fuse. In ITER, this distance of approach is made possible by high temperatures and magnetic confinement. ITER uses cooling equipment like a cryopump to cool the magnets to close to absolute zero. High temperatures give the nuclei enough energy to overcome their electrostatic repulsion. For deuterium and tritium, the optimal reaction rates occur at temperatures higher than 100 million °C. At ITER, the plasma will be heated to 150 million °C (about ten times the temperature at the core of the Sun) by coupling radio-frequency waves or microwaves to the plasma particles, by injecting energetic beams of neutral atoms that become ionized and heat the plasma, by magnetically compressing the plasma, or by the ohmic heating (also known as Joule heating) that occurs when an electric current passes through the plasma.

At such high temperatures, particles have a large kinetic energy, and hence velocity. If unconfined, the particles will rapidly escape, taking the energy with them, cooling the plasma to the point where net energy is no longer produced. A successful reactor would need to contain the particles in a small enough volume for a long enough time for much of the plasma to fuse. In ITER and many other magnetic confinement reactors, the plasma, a gas of charged particles, is confined using magnetic fields. A charged particle moving through a magnetic field experiences a force perpendicular to the direction of travel, resulting in centripetal acceleration, thereby confining it to move in a circle or helix around the lines of magnetic flux. ITER will use four types of magnets to contain the plasma: a central solenoid magnet, poloidal magnets around the edges of the tokamak, 18 D-shaped toroidal-field coils, and correction coils.

A solid confinement vessel is also needed, both to shield the magnets and other equipment from high temperatures and energetic photons and particles, and to maintain a near-vacuum for the plasma to populate. The containment vessel is subjected to a barrage of very energetic particles, where electrons, ions, photons, alpha particles, and neutrons constantly bombard it and degrade the structure. The material must be designed to endure this environment so that a power station would be economical. Tests of such materials will be carried out both at ITER and at IFMIF (International Fusion Materials Irradiation Facility).

Once fusion has begun, high-energy neutrons will radiate from the reactive regions of the plasma, crossing magnetic field lines easily due to charge neutrality. Since it is the neutrons that receive the majority of the energy, they will be ITER’s primary source of energy output. Ideally, alpha particles will expend their energy in the plasma, further heating it.

The inner wall of the containment vessel will have 440 blanket modules that are designed to slow and absorb neutrons in a reliable and efficient manner and therefore protect the steel structure and the superconducting toroidal field magnets. Energy absorbed from the fast neutrons is extracted and passed into the primary coolant. This heat energy would then be used to power an electricity-generating turbine in a real power station; in ITER this electricity generating system is not of scientific interest, so instead the heat will be extracted and disposed of.

At later stages of the ITER project, experimental blanket modules will be used to test breeding tritium for fuel from lithium-bearing ceramic pebbles contained within the blanket module following the following reactions:

n + 6/3Li → 3/1T + 4/2He + 4.8 MeV

n + 7/3Li → 3/1T + 4/2He + n (slow) + (-2.47 MeV)

where the reactant neutron is supplied by the D-T fusion reaction.

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Magnetic confinement fusion (MCF) uses microwaves, electricity, and neutral particle beams from accelerators to heat a stream of hydrogen gas, turning it into plasma (150-300 million degrees Celsius hot) and then compressing it using super-conducting magnets, resulting in fusion. A toroid shape is the most efficient for holding such plasma; reactors with this shape are called tokamak.

Technical design of ITER tokamak fusion reactor is depicted in the figure below:

The main parts of the ITER tokamak reactor are:

Vacuum vessel – holds the plasma and keeps the reaction chamber in a vacuum
Neutral beam injector – injects particle beams from the accelerator into the plasma to help heat the plasma to critical temperature
Magnetic field coils (poloidal, toroidal) – super-conducting magnets that confine, shape and contain the plasma using magnetic fields
Transformers/Central solenoid – supply electricity to the magnetic field coils
Cooling equipment (crostat, cryopump) – cool the magnets
Blanket modules – made of lithium; absorb heat and high-energy neutrons from the fusion reaction
Divertors – exhaust the helium products of the fusion reaction

-1. Vacuum vessel:

The vacuum vessel is the central part of the ITER machine: a double-walled steel container in which the plasma is contained by means of magnetic fields. The ITER vacuum vessel will be twice as large and 16 times as heavy as any previously manufactured fusion vessel: each of the nine torus-shaped sectors will weigh approximately 450 tonnes. When all the shielding and port structures are included, this adds up to a total of 5,116 tonnes. Its external diameter will measure 19.4 metres (64 ft), the internal 6.5 metres (21 ft). Once assembled, the whole structure will be 11.3 metres (37 ft) high.

The primary function of the vacuum vessel is to provide a hermetically sealed plasma container. Its main components are the main vessel, the port structures and the supporting system. The main vessel is a double-walled structure with poloidal and toroidal stiffening ribs between 60-millimetre-thick (2.4 in) shells to reinforce the vessel structure. These ribs also form the flow passages for the cooling water. The space between the double walls will be filled with shield structures made of stainless steel. The inner surfaces of the vessel will act as the interface with breeder modules containing the breeder blanket component. These modules will provide shielding from the high-energy neutrons produced by the fusion reactions and some will also be used for tritium breeding concepts.

The vacuum vessel has a total of 44 openings that are known as ports – 18 upper, 17 equatorial, and 9 lower ports – that will be used for remote handling operations, diagnostic systems, neutral beam injections and vacuum pumping. Remote handling is made necessary by the radioactive interior of the reactor following a shutdown, which is caused by neutron bombardment during operation.

Vacuum pumping will be done before the start of fusion reactions to create the necessary low density environment, which is about one million times lower than the density of air.

-2. Breeder blanket:

ITER will use a deuterium-tritium fuel, and while deuterium is abundant in nature, tritium is much rarer because it is a hydrogen isotope with a half-life of just 12.3 years and there is only approximately 3.5 kilograms of natural tritium on earth. Owing to this limited terrestrial supply of tritium, a key component of the ITER reactor design is the breeding blanket. This component, located adjacent to the vacuum vessel, serves to produce tritium through reaction with neutrons from the plasma. There are several reactions that produce tritium within the blanket. Lithium-6 produces tritium with moderated neutrons, while Lithium-7 produces tritium via interactions with higher energy neutrons.

Concepts for the breeder blanket include helium-cooled lithium lead (HCLL), helium-cooled pebble bed (HCPB), and water-cooled lithium lead (WCLL) methods. Six different tritium breeding systems, known as Test Blanket Modules (TBM), will be tested in ITER and will share a common box geometry. Materials for use as breeder pebbles in the HCPB concept include lithium metatitanate and lithium orthosilicate. Requirements of breeder materials include good tritium production and extraction, mechanical stability and low levels of radioactive activation.

-3. Magnet system:

Most fusion devices nowadays feature normally conducting magnet coils made of copper. The 2.5 metres high main-field coils of ASDEX Upgrade, for example, are composed of copper rails the width of a hand. For cooling purposes they are provided with bores to allow the passage of cooling water. The windings are insulated with fibreglass bands and moulded in synthetic resin, to connect them together and give the coil the necessary mechanical strength. The coil can thus withstand the strong forces exerted between the coils after the coil current is switched on. A future fusion power plant will operate with superconducting coils. Unlike copper coils, superconducting coils — cooled to low temperatures — use no energy after being switched on; the coil current flows with almost no losses.

The niobium-titanium superconductor envisaged for the ITER test reactor is to generate a magnetic field of 13 tesla on the coil, i.e. 5.7 tesla on the magnetic field axis. The superconducting strands are embedded in copper wires and enclosed in a blanket of high-grade steel. Inside the blanket liquid helium circulates and cools the coils to 4.5 Kelvin, close to absolute zero. Model coils have been used to test this superconductor and demonstrate the essential production sequence, from the individual superconducting strands to the complete coil. In the meantime, production is underway; the first main-field coils were completed by companies in Japan and Europe.

ITER is based on magnetic confinement fusion that uses magnetic fields to contain the fusion fuel in plasma form. The magnet system used in the ITER tokamak will be the largest superconducting magnet system ever built. The system will use four types of magnets to achieve plasma confinement: a central solenoid magnet, poloidal magnets, toroidal-field coils, and correction coils. The central solenoid coil will be 18 meters tall, 4.3 meters wide, and weigh 1000 tonnes. It will use superconducting niobium-tin to carry 45 kA and produce a peak field of more than 13 Tesla. The 18 toroidal field coils will also use niobium-tin. They are the most powerful superconductive magnets ever designed with a nominal peak field strength of 11.8 Tesla and a stored magnetic energy of 41 gigajoules. Other lower field ITER magnets (poloidal field and correction coils) will use niobium-titanium for their superconducting elements.

-4. Additional heating:

The temperatures inside the ITER Tokamak must reach 150 million degrees Celsius—or ten times the temperature at the core of the Sun—in order for the gas in the vacuum chamber to reach the plasma state and for the fusion reaction to occur. The hot plasma must then be sustained at these extreme temperatures in a controlled way in order to extract energy. To achieve these extreme temperatures multiple heating methods must be used. This 150-million-degree temperatures is achieved using energy from various sources, including Ohmic heating (from electric currents induced in the plasma), microwaves, ion beams, or neutral beam injection. The ITER Tokamak will rely on three sources of external heating that work in concert to provide the input heating power of 50 MW required to bring the plasma to the temperature necessary for fusion. These are as follows.

Two one-million volt heating neutral beam injectors (HNB) that will each provide about 16.5MW to the burning plasma, with the possibility to add a third injector. The beams generate electrically charged deuterium ions that are accelerated through five grids to reach the required energy of 1MV and the beams can operate for the entire plasma pulse duration, a total of up to 3600 seconds. The prototype is being built at the Neutral Beam Test Facility (NBTF), which was constructed in Padua, Italy. There is also a smaller neutral beam that will be used for diagnostics to help detect the amount of helium ash inside the tokamak.
An ion cyclotron resonance heating (ICRH) system that will inject 20 MW of electromagnetic power into the plasma by using antennas to generate radio waves that have the same rate of oscillation as the ions in the plasma.
An electron cyclotron resonance heating (ECRH) system that will heat electrons in the plasma using a high-intensity beam of electromagnetic radiation.

-5. Cryostat:

The ITER cryostat is a large 3,850-tonne stainless steel structure surrounding the vacuum vessel and the superconducting magnets, with the purpose of providing a super-cool vacuum environment. Its thickness (ranging from 50 to 250 millimetres (2.0 to 9.8 in)) will allow it to withstand the stresses induced by atmospheric pressure acting on the enclosed volume of 8,500 cubic meters. On 9 June 2020, Larsen & Toubro completed the delivery and installation of the cryostat module. The cryostat is the major component of the tokamak complex, which sits on a seismically isolated base.

-6. Divertor:

The divertor is a device within the tokamak that allows for removal of waste and impurities from the plasma while the reactor is operating. At ITER, the divertor will extract heat and ash that are created by the fusion process, while also protecting the surrounding walls and reducing plasma contamination.

The ITER divertor, which has been compared to a massive ashtray, is made of 54 pieces of stainless-steel parts that are known as cassettes. Each cassette weighs roughly eight tonnes and measures 0.8 meters x 2.3 meters by 3.5 meters. The divertor design and construction is being overseen by the Fusion For Energy agency.

When the ITER tokamak is in operation, the plasma-facing units endure heat spikes as high as 20 megawatts per square metre, which is more than four times higher than what is experienced by a spacecraft entering Earth’s atmosphere.

The testing of the divertor is being done at the ITER Divertor Test Facility (IDTF) in Russia. This facility was created at the Efremov Institute in Saint Petersburg as part of the ITER Procurement Arrangement that spreads design and manufacturing across the project’s member countries.

-7. Cooling systems:

The ITER tokamak will use interconnected cooling systems to manage the heat generated during operation. Most of the heat will be removed by a primary water cooling loop, itself cooled by water from a secondary loop through a heat exchanger within the tokamak building’s secondary confinement. The secondary cooling loop will be cooled by a larger complex, comprising a cooling tower, a 5 km (3.1 mi) pipeline supplying water from the Canal de Provence, and basins that allow cooling water to be cooled and tested for chemical contamination and tritium before being released into the river Durance. This system will need to dissipate an average power of 450 MW during the tokamak’s operation. A liquid nitrogen system will provide a further 1300 kW of cooling to 80 K (−193.2 °C; −315.7 °F), and a liquid helium system will provide 75 kW of cooling to 4.5 K (−268.65 °C; −451.57 °F). The liquid helium system will be designed, manufactured, installed and commissioned by Air Liquide in France.

Here’s how the magnetic confinement fusion process will work:

-1. The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. It will squeeze the plasma so that fusion can take place. The power needed to start the fusion reaction will be about 50 megawatts, but the power yield from the reaction will be about 500 megawatts. The fusion reaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.)

-2. The lithium blankets outside the plasma reaction chamber will absorb high-energy neutrons from the fusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons.

-3. The heat will be transferred by a water-cooling loop to a heat exchanger to make steam.

-4. The steam will drive electrical turbines to produce electricity.

-5. The steam will be condensed back into water to absorb more heat from the reactor in the heat exchanger.

Initially, the ITER tokamak will test the feasibility of a sustained fusion reactor and eventually will become a test fusion power plant.

ITER will have a plasma volume of 800 m^3 and a power output of 500 MWth, 30 times that of JET. Fusion energy generation is a matter of size, and at this size it is hoped that ITER will have a gain of 10, producing 500 MWth from an input of 50 MWth.

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Common tools:

Many approaches, equipment, and mechanisms are employed across multiple projects to address fusion heating, measurement, and power production.

-1. Neural networks

A deep reinforcement learning system has been used to control a tokamak-based reactor. The AI was able to manipulate the magnetic coils to manage the plasma. The system was able to continuously adjust to maintain appropriate behavior (more complex than step-based systems). In 2014, Google began working with California-based fusion company TAE Technologies to control the Joint European Torus (JET) to predict plasma behavior. DeepMind has also developed a control scheme with TCV.

-2. Heating

Electrostatic heating: an electric field can do work on charged ions or electrons, heating them.

Neutral beam injection: hydrogen is ionized and accelerated by an electric field to form a charged beam that is shone through a source of neutral hydrogen gas towards the plasma which itself is ionized and contained by a magnetic field. Some of the intermediate hydrogen gas is accelerated towards the plasma by collisions with the charged beam while remaining neutral: this neutral beam is thus unaffected by the magnetic field and so reaches the plasma. Once inside the plasma the neutral beam transmits energy to the plasma by collisions which ionize it and allow it to be contained by the magnetic field, thereby both heating and refueling the reactor in one operation. The remainder of the charged beam is diverted by magnetic fields onto cooled beam dumps.

Radio frequency heating: a radio wave causes the plasma to oscillate (i.e., microwave oven). This is also known as electron cyclotron resonance heating, using for example gyrotrons, or dielectric heating.

Magnetic reconnection: when plasma gets dense, its electromagnetic properties can change, which can lead to magnetic reconnection. Reconnection helps fusion because it instantly dumps energy into a plasma, heating it quickly. Up to 45% of the magnetic field energy can heat the ions.

Magnetic oscillations: varying electrical currents can be supplied to magnetic coils that heat plasma confined within a magnetic wall.

Antiproton annihilation: antiprotons injected into a mass of fusion fuel can induce thermonuclear reactions. This possibility as a method of spacecraft propulsion, known as antimatter-catalyzed nuclear pulse propulsion, was investigated at Pennsylvania State University in connection with the proposed AIMStar project.

-3. Measurement

The diagnostics of a fusion scientific reactor are extremely complex and varied. The diagnostics required for a fusion power reactor will be various but less complicated than those of a scientific reactor as by the time of commercialization, many real-time feedback and control diagnostics will have been perfected. However, the operating environment of a commercial fusion reactor will be harsher for diagnostic systems than in a scientific reactor because continuous operations may involve higher plasma temperatures and higher levels of neutron irradiation. In many proposed approaches, commercialization will require the additional ability to measure and separate diverter gases, for example helium and impurities, and to monitor fuel breeding, for instance the state of a tritium breeding liquid lithium liner.

The following are some basic techniques.

Flux loop: a loop of wire is inserted into the magnetic field. As the field passes through the loop, a current is made. The current measures the total magnetic flux through that loop. This has been used on the National Compact Stellarator Experiment, the polywell, and the LDX machines. A Langmuir probe, a metal object placed in a plasma, can be employed. A potential is applied to it, giving it a voltage against the surrounding plasma. The metal collects charged particles, drawing a current. As the voltage changes, the current changes. This makes an IV Curve. The IV-curve can be used to determine the local plasma density, potential and temperature.

Thomson scattering: “Light scatters” from plasma can be used to reconstruct plasma behavior, including density and temperature. It is common in Inertial confinement fusion, Tokamaks, and fusors. In ICF systems, firing a second beam into a gold foil adjacent to the target makes x-rays that traverse the plasma. In tokamaks, this can be done using mirrors and detectors to reflect light.

Neutron detectors: Several types of neutron detectors can record the rate at which neutrons are produced.

X-ray detectors: Visible, IR, UV, and X-rays are emitted anytime a particle changes velocity. If the reason is deflection by a magnetic field, the radiation is cyclotron radiation at low speeds and synchrotron radiation at high speeds. If the reason is deflection by another particle, plasma radiates X-rays, known as Bremsstrahlung radiation.

-4. Power production

Neutron blankets: Neutron blankets absorb neutrons, which heats the blanket. Power can be extracted from the blanket in various ways. Steam turbines can be driven by heat transferred into a working fluid that turns into steam, driving electric generators.

These neutrons can regenerate spent fission fuel. Tritium can be produced using a breeder blanket of liquid lithium or a helium cooled pebble bed made of lithium-bearing ceramic pebbles.

Direct conversion: The kinetic energy of a particle can be converted into voltage. It was first suggested by Richard F. Post in conjunction with magnetic mirrors, in the late 1960s. It has been proposed for Field-Reversed Configurations as well as Dense Plasma Focus devices. The process converts a large fraction of the random energy of the fusion products into directed motion. The particles are then collected on electrodes at various large electrical potentials. This method has demonstrated an experimental efficiency of 48 percent.

Traveling-wave tubes pass charged helium atoms at several megavolts and these are just coming off the fusion reaction through a tube with a coil of wire around the outside. This passing charge at high voltage pulls electricity through the wire.

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Tokamaks worldwide:

Fusion research is a joint international effort. World-wide most devices today are of the tokamak type, which has been the most thoroughly investigated and has come closest to the ignition conditions. They investigate a wide range of topics, from basic plasma physics to power plants.

ADITYA (India)

Alcator C-Mod (USA)

ASDEX Upgrade (Germany)

COMPASS (Czech Republic)

DIII-D (USA)

EAST (HT-7U) (China)

FT-2 (Russia)

FTU (Italy)

Globus-M (Russia)

GOLEM (Czech Republic)

HBT-EP (USA)

HL-2A (China)

HT-7 (China)

ITER (international experimental reactor; France; under construction)

ISTTOK (Portugal)

JET (European joint experiment; Great Britain)

JT-60SA (Japan; preparations for operation)

KSTAR (Korea)

MAST (Great Britain)

NOVA (Brasil)

NSTX (USA)

SST-1 (India; under construction)

STOR-M (Canada)

T-10 (Russia)

T-15 (Russia)

TCA-Br (Brasil)

TCV (Switzerland)

TEXTOR (Germany) (until 2013)

WEST (formerly: TORE SUPRA; France)

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DEMOnstration Power Plant (DEMO):

In proposals of a roadmap towards the development of commercial magnetic fusion energy (MFE), a demonstration power plant (DEMO) is defined along the next step after the ITER tokamak project. Proposals and extensive safety analyses for commercial fusion reactors have been made in the framework of the European Power Plant Conceptual Study.

DEMO refers to a proposed class of nuclear fusion experimental reactors that are intended to demonstrate the net production of electric power from nuclear fusion. Most of the ITER partners have plans for their own DEMO-class reactors. With the possible exception of the EU and Japan, there are no plans for international collaboration as there was with ITER. Plans for DEMO-class reactors are intended to build upon the ITER experimental nuclear fusion reactor.

The most well-known and documented DEMO-class reactor design is that of the European Union (EU). The following parameters have been used as a baseline for design studies: the EU DEMO should produce at least 2000 megawatts (2 gigawatts) of fusion power on a continuous basis, and it should produce 25 times as much power as required for scientific breakeven, which does not include the power required to operate the reactor. The EU DEMO design of 2 to 4 gigawatts of thermal output will be on the scale of a modern electric power station.

Project	Injected Thermal Input	Gross Thermal Output	Q plasma value
JET	24 MW	16 MW	0.67
ITER	50 MW	500 MW	10
EU DEMO	80 MW	2000 MW	25

Fusion power means thermal power output. Electric power output will be 1/3 of thermal power output.

Different researchers have calculated different quantum of deuterium and tritium for DEMO.

-1. For a fusion reactor with 1 GW fusion power, about 37.1 kg deuterium and 55.6 kg tritium would be burnt per full power operational year.

-2. The fusion power in the range 2.5 to about 5 GW correspond to tritium consumption rates in the range of 0.38 to 0.76 kg T per day.

-3. In a 1 GW-electric (or ~3 GW fusion power) fusion power plant, about 0.5 g/second of tritium is needed to maintain the fusion fuel cycle assuming ~1% fusion burn efficiency.

To achieve its goals, if utilizing a conventional tokamak design, a DEMO reactor must have linear dimensions about 15% larger than ITER, and a plasma density about 30% greater than ITER. According to timeline from EUROfusion, operation is planned to begin in 2051. It is estimated that subsequent commercial fusion reactors could be built for about a quarter of the cost of DEMO. However, the ITER experience suggests that development of a multi-billion US dollar tokamak-based technology to develop fusion power stations that can compete with non-fusion energy technologies is likely to encounter the “valley of death” problem in venture capital, i.e., insufficient investment to go beyond prototypes as DEMO tokamaks will need to develop new supply chains and are labor intensive.

Technical considerations:

When deuterium and tritium fuse, the two nuclei come together to form a resonant state which splits to form in turn a helium nucleus (an alpha particle) and a high-energy neutron.

DEMO will be constructed once designs which solve the many problems of current fusion reactors are engineered. These problems include: containing the plasma fuel at high temperatures, maintaining a great enough density of reacting ions, and capturing high-energy neutrons from the reaction without melting the walls of the reactor.

The activation energy for fusion is very large because the protons in each nucleus strongly repel one another; they are both positively charged. In order to fuse, the nuclei must be within 1 femtometre (1 × 10^−15 metres) of each other, where quantum-tunnelling effects permit the parent nuclei to fuse together into the resonant state. The principle is to form a quasi-Maxwellian distribution for the deuterons and the tritons, at very high temperatures, where the nuclei in the tail of the Maxwellian undergo fusion, while the continuous elastic collisions among the other nuclei will not alter the state of the plasma.
DEMO, a Tokamak reactor, requires both dense plasma and high temperatures for the fusion reaction to be sustained.
High temperatures give the nuclei enough energy to overcome their electrostatic repulsion. This requires temperatures in the region of 100MK, and is achieved using energy from various sources, including Ohmic heating (from electric currents induced in the plasma), microwaves, ion beams, or neutral beam injection.
Containment vessels melt at these temperatures, so the plasma is to be kept away from the walls using magnetic confinement.

Once fusion has begun, high-energy neutrons at about 160GK will flood out of the plasma along with X-rays, neither being affected by the strong magnetic fields. Since neutrons receive the majority of the energy from the fusion, they will be the reactor’s main source of thermal energy output. The ultra-hot helium product at roughly 40GK will remain behind (temporarily) to heat the plasma, and must make up for all the loss mechanisms (mostly bremsstrahlung X-rays from electron deceleration) which tend to cool the plasma rather quickly.

The Tokamak containment vessel will have a lining composed of ceramic or composite tiles containing tubes in which warm liquid lithium metal will flow, cooling the lining.
Lithium readily absorbs high-speed neutrons to form helium and tritium, becoming hot in the process.
This increase in temperature is passed on to another (intermediate) coolant, possibly (pressurized) liquid water in a sealed, pressurized pipe.
Heat from the intermediate coolant will be used to boil water in a heat exchanger.
Steam from the heat exchanger will be used to drive turbines and generators, to create electric current.
Waste heat energy in excess of the generated electrical energy is dumped into the environment.
Helium byproduct is the “ash” of this fusion, and will not be allowed to accumulate too much in the plasma.
Carefully measured amounts of deuterium and tritium are added back into the plasma and heated.
The lithium is processed to remove the helium and tritium, with the balance recycled to collect more heat and neutrons. Only a tiny amount of lithium is consumed.

The DEMO project is planned to build upon and improve the concepts of ITER. Since it is only proposed at this time, many of the details, including heating methods and the method for the capture of high-energy neutrons, are still undetermined.

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Figure below illustrates the basic components and systems of a generic fusion power plant (FPP).

Figure above shows schematic view of main components and systems in a FPP.

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Tokamak versus Stellarator:

Magnetic confinement concept is based on the presence of closed field lines in a magnetised hot plasma which keep the hot plasma away from material walls. The most obvious approach is to bend a cylindrical magnetic field into a torus by a toroidal arrangement of the magnetic field coils. Poloidal field components have to be added to create nested magnetic flux surfaces and thus to avoid plasma energy losses due to drifts in the inhomogeneous toroidal magnetic field. The most important devices among the different magnetic confinement configurations are tokamaks and stellarators.

In a tokamak, a toroidal plasma current is induced by an Ohmic Heating (OH) transformer generating the required poloidal field. Additional poloidal magnetic field coils provide vertical fields by which the radial position and the shape of the equilibrium magnetic surfaces are controlled. Since the plasma current is driven only during the discharge of the transformer, the plasma is confined in a pulsed mode. In an MFE power plant, steady state plasma operation has to be achieved by additional current drive methods. A considerable part of the total plasma current may be produced by the plasma itself due to the bootstrap current effect. The conditions for reaching a high bootstrap current fraction constitute a challenge for the control of magneto-hydrodynamic (MHD) instability modes. The plasma current provides a self-organized plasma equilibrium susceptible to a number of instabilities including kink modes, sawtooth oscillations, conventional and neoclassical tearing modes (NTM), resistive wall modes, and current disruptions. In addition, vertical position instabilities may occur in tokamaks, which lead to Halo currents into plasma facing components when the plasma comes into contact with them. The electromagnetic forces associated with disruptions and Halo currents may lead to severe damages. At present, a good physics understanding of these instabilities has been achieved and stable operation scenarios have been developed. A significant level of active feedback control, however, is necessary in a tokamak to avoid such instabilities or mitigate their consequences.

Despite still existing challenges the tokamak is presently the most mature candidate for a fusion power plant. In particular, the JET tokamak is presently the largest MFE experiment in operation with great relevance for the planned ITER operation. In particular, in JET 16 MW of fusion power have been achieved for about 1 second. The main objectives of ITER are to demonstrate a burning D-T plasma (for a duration of 300-500 seconds) with a fusion energy gain of Q~10, where Q is the ratio between total fusion power and external heating power. This means that 2/3 of the total plasma heating power consists of α-particle heating. Other objectives include the production of steady state plasmas with Q = 5 and the development and tests of fusion technology for a commercial fusion power plant.

For a toroidal plasma confinement system, the plasmas are confined by a magnetic field. In order to have an equilibrium between the plasma pressure and the magnetic forces it is necessary to have a rotational transform of the toroidal magnetic field. Such a rotational transform may prevent the curvature drift of the guiding center of plasma particles towards the wall. As proposed by Spitzer and Mercier, there are three different ways to twist the magnetic field: (i) creating a poloidal field by a toroidal electric current; (ii) rotating the poloidal cross-section of stretched flux surfaces around the torus; (iii) making the magnetic axis non-planar. While tokamaks use the first approach, stellarators usually rely on the latter two methods, namely, in tokamaks the twisting is produced by a toroidal plasma current and in stellarators by external non-axisymmetric coils. These two types of configuration are depicted in figure below. This brings clear difference for the two systems. For example, tokamaks are axisymmetric and can confine all collisionless particles and have relatively good plasma confinement. But the toroidal current is normally generated by a transformer, which makes the device vulnerable to current-driven instabilities and difficult to operate in a steady state. The stellarators, on the other hand, are inherently current free, and thus, able to operate the plasma in a steady state. But more unconfined particle orbits in stellarators can lead to high neoclassical transport of energetic and thermal particles.

Figure above shows schematics of magnetically confined plasmas in (a) tokamaks; (b) stellarator configurations. In the tokamak, the rotational transform of a helical magnetic field is formed by a toroidal field generated by external coils together with a poloidal field generated by the plasma current. In the stellarator, the twisting field is produced entirely by external non-axisymmetric coils.

The geometrical parameters also differ much for tokamaks and stellarators. In tokamaks the aspect ratio R/a (R and a represent the major and minor radii, respectively) is usually in a range of 2.5–4, and the value is even smaller for spherical tokamaks. In stellarators, to avoid resonances between the field lines and harmonics of the symmetry of the configuration, the device is designed to have small rotational transforms per period, which results in much larger aspect ratios (R/a = 5–12) in the present-day stellarators. As a consequence, the effective plasma volume in tokamaks is much larger than in stellarators.

The profiles of the safety factor (q) and magnetic shear (s = r∂q/q∂r) are also very different between the two systems. Tokamaks normally operate with positive magnetic shear throughout the entire plasmas whereas in stellarators the shear is negative (except for non-planar types, which may have a zero magnetic shear). A further difference lies in the shape of the plasma cross-section. In tokamaks the plasma cross-section is toroidally symmetric, while in a three-dimensional stellarator the shape varies as a function of the toroidal angle, as illustrated in figure above.

Whereas the magnetic configurations of the existing large tokamaks and of ITER are similar, stellarators including so-called torsatrons/heliotrons and similar devices feature a variety of different configurations. In all cases, the poloidal field component is generated by external coils. Therefore, such systems can be operated without any externally driven plasma current. Hence, stellarators have an inherent potential for stationary operation. In addition, current driven instabilities, in particular disruptions, do not exist in stellarators. The external coils provide a set of nested magnetic surfaces which entail, to a large extent, passive control of the plasma. The existence of an external confining magnetic field keeps the plasma always centred in the plasma vessel. In classical stellarators such as Wendelstein 7-A (Max-Planck Institute for Plasma Physics (IPP), 1975-1985) toroidal field coils in combination with additional helical windings on top of the vacuum vessel are used to generate the stellarator field. With the development of advanced stellarator configurations such as Wendelstein 7-AS (IPP, 1987-2002) and the optimized Wendelstein 7-X stellarator (IPP, start in 2014) sets of non-planar modular field coils were introduced. They provide the full required three-dimensional (3-d) stellarator field. At present, the Large Helical Device (LHD) in the National Institute for Fusion Science in Japan is the largest operating heliotron-type stellarator.

The main characteristics of tokamak and stellarator systems are summarized in Table below.

Main Characteristics of tokamaks and stellarators (W7-X type):

	Tokamak	Stellarator
Magnetic Field	toroidal field coils (planar), poloidal field coils (planar), vertical field coils (plasma position and shaping)	modular non-planar coils (combined toroidal and poloidal field)
Plasma Current	inductive, current drive (CD) systems (field line twist / rotational transform)	no current, rotational transform by external field
Aspect Ratio	small, ~ 3	generally larger, up to 10
Symmetry (field, vessel)	axisymmetric	non-axisymmetric
Divertor	“Single Null”, axis-symmetric	“Island Divertor”, 3-d shape
Stability Limits	current driven instabilities (tearing & kink modes, disruptions), vertical instabilities	pressure driven modes (interchange-like), passively stable by magnetic well
Density Limit	Greenwald (current) limit, degradation of confinement, ultimately disruptive	heating/radiation power limit (slow thermal decay)
Steady State Operation	requires steady state CD	inherent steady state capability

In above paragraphs general comparison between tokamak and stellarator plasmas is made by reviewing the similarities and differences in their magnetic configuration, MHD behaviors and operational limits, plasmas transport and confinement, plasma rotation and edge/divertor transport. In these two devices, the advantages and disadvantages are as follows: for tokamaks, the advantages include technical simplicity, much lower neoclassical transport (especially at high temperature), stronger toroidal rotations and associated flow-shear, and weaker damping on zonal flows. For stellarators, intrinsically steady-state operation, less MHD activities and nearly disruption-free are great advantages; the stochastic magnetic boundary is also beneficial for impurity retention in the divertor. Turbulence and turbulent transport are comparable in these two systems. Some drift-wave modes are more stable in stellarators. In the energy confinement scaling, an isotope effect appears in tokamaks but not in stellarators. As the number of degrees of the freedom is more for non-axisymmetric systems than axisymmetric ones, the possible configurations in stellarators are much more than in tokamaks. For further optimizing stellarator configuration, the quasi-symmetric stellarator has been proposed. If the neoclassical confinement can be substantially improved, the stellarator could be more attractive for a fusion power reactor in the near future.

Stellarators have several advantages over tokamaks, the other main technology that scientists are exploring for fusion power. Stellarators require less injected power to sustain the plasma, have greater design flexibility, and allow for simplification of some aspects of plasma control. However, these benefits come at the cost of increased complexity, especially for the magnetic field coils.

To advance stellarator design, scientists have turned to high performance computing and state-of-the-art plasma theory. These tools have helped researchers optimize the Helically Symmetric Experiment (HSX) stellarator in Wisconsin and the Wendelstein 7-X stellarator in Germany.

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Presently, a number of physics and technological challenges still exist for the realization of a DEMO. In particular, a list of five DEMO physics issues is identified by the present EU fusion program:

-1. Steady state tokamak operation

-2. High density operation

-3. Power exhaust

-4. Disruptions

-5. Reliable control with minimum sensors and actuators

Stellarators would offer solutions to at least some of these key issues. Steady state operation is inherently possible. High density operation is a favoured scenario in stellarators due to the lack of a Greenwald-like density limit. Disruptions and accompanying effects by excessive forces and runaway electrons do not exist in stellarators. Also, the number of actively controlled parameters and related sensors is smaller in a stellarator due to the predominantly passive control by the external static magnetic field. The power exhaust, however, is a challenge for both concepts. The larger aspect ratio in stellarators may help to keep the particle and energy flux densities at lower values, but the divertors in a stellarator are not yet sufficiently explored and the 3-d shape of divertor, first wall and blanket requires more elaborate solutions. A particular concern in stellarators is the density and impurity control. The High Density H-mode regime (HDH) in W7-AS is very promising in this respect.

In a nutshell, due to various advantages of stellarator, alternative devices including the Wendelstein 7-X stellarator and DEMO versions on the basis of the HELIAS (W7-X like configurations) are considered in the roadmap towards an economical fusion power plant.

Although W7-X will not produce energy, its designers hope to prove that stellarators are also suitable for application in power plants and to demonstrate their capability to operate continuously. Such continuous mode will be essential for commercial operation of a fusion reactor.

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IAEA’s Fusion Device Information System (FusDIS) – provides information on all fusion devices public or private with experimental and demonstration designs, which are currently in operation, under construction or being planned, as well as technical data of these devices. An overview is given in figure below.

Figure above shows that over 130 experimental, public and private, fusion devices are operating, under construction or being planned, while a number of organizations are considering designs for demonstration fusion power plants.

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Figure below compares D-T fusion power plants with other fusion reactant power plants.

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Fusion power:

Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2023, only one design, an inertial confinement laser-driven fusion machine at the US National Ignition Facility, has in any sense produced a positive fusion energy gain factor, i.e. more power output than input.

Fusion processes require fuel and a confined environment with sufficient temperature, pressure, and confinement time to create a plasma in which fusion can occur. The combination of these figures that results in a power-producing system is known as the Lawson criterion. In stars, the most common fuel is hydrogen, and gravity provides extremely long confinement times that reach the conditions needed for fusion energy production. Proposed fusion reactors generally use heavy hydrogen isotopes such as deuterium and tritium (and especially a mixture of the two), which react more easily than protium (the most common hydrogen isotope), to allow them to reach the Lawson criterion requirements with less extreme conditions. Most designs aim to heat their fuel to around 100 million degrees, which presents a major challenge in producing a successful design.

As a source of power, nuclear fusion is expected to have many advantages over fission. These include reduced radioactivity in operation and little high-level nuclear waste, ample fuel supplies, and increased safety. However, the necessary combination of temperature, pressure, and duration has proven to be difficult to produce in a practical and economical manner. A second issue that affects common reactions is managing neutrons that are released during the reaction, which over time degrade many common materials used within the reaction chamber.

Fusion researchers have investigated various confinement concepts. The early emphasis was on three main systems: z-pinch, stellarator, and magnetic mirror. The current leading designs are the tokamak and inertial confinement (ICF) by laser. Both designs are under research at very large scales, most notably the ITER tokamak in France, and the National Ignition Facility (NIF) laser in the United States. Researchers are also studying other designs that may offer cheaper approaches. Among these alternatives, there is increasing interest in magnetized target fusion and inertial electrostatic confinement, and new variations of the stellarator.

Desirable Characteristics of Fusion Power:

The materials that would be used to fuel D–T and eventually D–D reactors are abundant, widespread and easily extracted at modest cost. The cost of deuterium extracted from water is only about $0.02 to $0.03 per GJ (278 kW·h) of electricity when used in a D–D fusion reactor, assuming a net plant electrical efficiency of 33 %. For a D–T reactor, which produces more energy per reaction, the cost of deuterium per GJ of electricity would be about $0.003 to $0.005, and the cost of lithium to produce tritium would be about $0.001 to $0.002 per GJ. The fuel costs are thus negligible and would not be expected to increase due to depletion for a very long time. The amount of deuterium in the earth’s water would allow the production of about 10^22 GJ of electricity if used in D–T reactors, an amount which is more than 10^11 times the entire world annual electricity production, or in D–D reactors, more than 10^10 times the present annual world electricity production. For D–T reactors, the more relevant fuel constraints are set by the availability of lithium to breed tritium. Lithium is most cheaply available from dry salt lakes and saline lakes, of which there are many in such areas as the western United States, where cheap surface salt reserves are estimated to contain enough lithium to produce about 3×10^14 GJ of electricity, an amount equivalent to roughly 500 to 600 times the primary annual energy consumption of the world. Since the United States comprises only 6 % of the world’s surface, and there are other arid regions with surface salt deposits, the sum of such surface deposits is probably adequate to run the world for several to many centuries. A great deal more lithium of the order 10^3 or more is dissolved in the oceans and could be extracted at somewhat higher prices using techniques similar to those presently used for concentrating sea salt through evaporation.

A fusion reactor producing a GW of electricity for a year would require less than a metric tonne of fuel, and produce helium as waste, whereas a coal burning plant of the same capacity would require two million times this much carbon, and even more weight in coal, depending upon the composition, and produce a large amount of waste cinders. The fact that fuel and waste transport requirements for fusion reactors are negligible compared to those of fossil fuels means that fusion would put negligible stress upon transportation infrastructure and result in the savings of the fossil fuel used to move coal.

A fusion reactor does not directly emit CO2 or other greenhouse gases, nor any combustion products that contribute to acid rain, and furthermore, the indirect emissions due to factors like fuel gathering and transport, plant construction and maintenance and activated parts storage would be small. Thus, fusion power would not have appreciable adverse effects upon global warming, atmospheric quality or acidification of the oceans, lakes and streams.

Unlike fission reactors, fusion reactors do not operate through a chain reaction, since the reaction products in fusion reactions do not themselves then initiate further fusion reactions. Thus, there is no danger of a runaway chain reaction causing a fusion reactor to melt down. Moreover, since the energy confinement time in even a large fusion reactor would be short (a few seconds), the total energy stored in the reactor medium would be small, and the afterheat in the blanket would also be much smaller than in a fission reactor. Thus, the worst possible accidents in a fusion reactor should be of significance only to the reactor, not to the society.

Like fission reactors, fusion reactors will produce radioactive waste. The salient difference, however, is that the fission waste consists largely of the fission products from the fuel, over which the plant designer has little control, whereas the fusion waste consists of structural components which have been activated by neutrons. Through the proper choice of fusion reactor materials, the amount of long-term waste can be greatly reduced relative to a fission reactor. If vanadium alloys are used, for instance, then the radioactivity level of the reactor parts can decline to levels below that of coal ash within a quarter century, requiring no geological storage. Silicon carbide composite is even better radiologically, being below coal ash a year after shutdown, but it is more difficult to fabricate than are vanadium alloys.

The weapons proliferation risk posed by fusion reactors should be much less than with fission. Fusion plants will not need to contain fissile materials. Introducing materials that could be bred into fissionable weapons grade material would require modifications to the breeding blanket and should be easily detectable by the emission of characteristic gamma rays which should not otherwise be present in a fusion power plant. At least the first few generations of fusion plants will contain substantial amounts of tritium, which can be used to increase the efficiency of nuclear weapons. However, the tritium is of no use in a weapon unless combined with weapons grade fissionable material, so if fusion plants eventually displace fission facilities entirely, the weapons potential of the tritium would be slight.

Unlike options such as solar energy or biomass, fusion is a high energy density power source, so the amount of land it requires for a plant and for fuel gathering is minor. Since it produces no CO2 or other undesirable gases, it does not need to be located near a geological formation which might be suitable for emission gas sequestration. It is likely that a plant could be designed so that the worst possible accident would not require any significant evacuation. Because little fuel is required to produce a lot of energy, the fuel stockpiled at the plant would require negligible storage area and transportation access. Thus, fusion plants would be steady power sources which could be located close to the markets they served.

Fusion power plants will be built when investors and public utility commissions begin viewing them as worthwhile investments. Exactly when that point will be reached is difficult to say. It is likely that electricity from the first fusion plants will be expensive compared to other options, though the same was once true about large-scale renewable generation. Fusion generation is certainly amenable to economies of scale, but the U.S. market has been trending away from very large (GW-scale) generation projects. The proposed approach of developing a compact fusion pilot plant thus represents a strategic way to develop the technology before scaling up once the investment community has gained confidence in the economics of larger plants.

Another important factor is public acceptance and the degree to which fusion will need to contend with perceptions and misconceptions about fission plants. The general public’s understanding of fusion energy is quite low, and confusion between fission and fusion is common. Both the fusion community and prospective plant owners will need to be proactive in providing effective communication about the technology long before any actual construction begins.

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Section-9

Nuclear fusion fuels and materials:

Fusion fuel supply:

The fuels considered for fusion power have all been light elements like the isotopes of hydrogen—protium, deuterium, and tritium. Fusion energy has the potential to supply safe, clean, and nearly limitless power. Although fusion reactions can occur for light nuclei weighting less than iron, most elements will not fuse unless they are in the interior of a star. To create burning plasmas in experimental fusion power reactors such as tokamaks and stellarators, scientists seek a fuel that is relatively easy to produce, store, and bring to fusion. The current best bet for fusion reactors is deuterium-tritium fuel. This fuel reaches fusion conditions at lower temperatures compared to other elements and releases more energy than other fusion reactions.

Deuterium and tritium are isotopes of hydrogen, the most abundant element in the universe. Whereas all isotopes of hydrogen have one proton, deuterium also has one neutron and tritium has two neutrons, so their ion masses are heavier than protium, the isotope of hydrogen with no neutrons. When deuterium and tritium fuse, they create a helium nucleus, which has two protons and two neutrons. The reaction releases an energetic neutron. Fusion power plants would convert energy released from fusion reactions into electricity to power our homes, businesses, and other needs.

Water made from deuterium is about 10 percent heavier than ordinary water. That’s why it is sometimes referred to as “heavy water.” It will actually sink to the bottom of a glass of ordinary water. Fusion power would provide more energy for a given weight of fuel than any fuel-consuming energy source currently in use, and the fuel itself (primarily deuterium) exists abundantly in the Earth’s ocean: about 1 in 6700 hydrogen (H) atoms in seawater (H2O) is deuterium in the form of (semi-heavy water). Although this may seem a low proportion (about 0.015%), because nuclear fusion reactions are so much more energetic than chemical combustion and seawater is easier to access and more plentiful than fossil fuels, fusion could potentially supply the world’s energy needs for millions of years. In the deuterium + lithium fusion fuel cycle, 60 million years is the estimated supply lifespan of this fusion power, if it is possible to extract all the lithium from seawater, assuming current (2004) world energy consumption. The fusion energy released from just 1 gram of deuterium-tritium fuel equals the energy from about 2400 gallons of oil. While in the second easiest fusion power fuel cycle, the deuterium + deuterium burn, assuming all of the deuterium in seawater was extracted and used, there is an estimated 150 billion years of fuel, with this again, assuming current (2004) world energy consumption.

To avoid certain R&D challenges including structural material damage from energetic neutrons, fusion scientists are interested also in aneutronic fusion reactions (such as deuterium-helium-3 and proton-boron fusion) even though these fusion reactions occur at higher ion temperatures than for deuterium and tritium.

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-1. Deuterium, tritium:

D+T reaction:

The easiest nuclear reaction, at the lowest energy, is D+T:

2/1D + 3/1T → 4/2He (3.5 MeV) + n (14.1 MeV)

This reaction is common in research, industrial and military applications, usually as a neutron source. Deuterium is a naturally occurring isotope of hydrogen and is commonly available. The large mass ratio of the hydrogen isotopes makes their separation easy compared to the uranium enrichment process. Tritium is a natural isotope of hydrogen, but because it has a short half-life of 12.32 years, it is hard to find, store, produce, and is expensive. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium using one of the following reactions:

n + 6/3Li → 3/1T + 4/2He + 4.8 MeV

n + 7/3Li → 3/1T + 4/2He + n (slow) + (-2.47 MeV)

Deuterium can be distilled from all forms of water. It is a widely available, harmless, and virtually inexhaustible resource. It sells for about $13 per gram. In every cubic metre of seawater, for example, there are 33 grams of deuterium. Deuterium is routinely produced for scientific and industrial applications.

Tritium is a fast-decaying radioelement of hydrogen which occurs only in trace quantities in nature. It can be produced during the fusion reaction through contact with lithium, however: tritium is produced, or “bred,” when neutrons escaping the plasma interact with lithium contained in the blanket wall of the tokamak. Lithium from proven, easily extractable land-based resources would provide a stock sufficient to operate fusion power plants for more than 1,000 years. What’s more, lithium can be extracted from ocean water, where reserves are practically unlimited (enough to fulfill the world’s energy needs for ~ 60 million years). Global inventory for tritium is presently around twenty kilos, which ITER will draw upon during its operational phase. The concept of “breeding” tritium within the fusion reaction is an important one for the future needs of a large-scale fusion power plant. Tritium is an exceptionally pricey substance: a single gram is currently worth around $30,000. Should nuclear fusion take off, demand will go through the roof, presenting the world’s fusion masters with yet another challenge.

Only small quantities of fuel:

A fusion reaction is about four million times more energetic than a chemical reaction such as the burning of coal, oil or gas. While a 1000 MW coal-fired power plant requires 2.7 million tonnes of coal per year, a fusion plant of the kind envisioned for the second half of this century will only require about 37.1 kg deuterium and 55.6 kg tritium per full power operational year.

Only a few grams of fuel are present in the plasma at any given moment. This makes a fusion reactor incredibly economical in its fuel consumption and also confers important safety benefits to the installation.

Tritium Breeding:

Deuterium-Tritium fusion is the most promising of the hydrogen fusion reactions, but no tritium occurs in nature since it has a 12 year half-life. The most promising source of tritium seems to be the breeding of tritium from lithium-6 by neutron bombardment with the reaction which can be achieved by slow neutrons. This would occur if lithium were used as the coolant and heat transfer medium around the reaction chamber of a fusion reactor. Lithium-6 makes up 7.4% of natural lithium. While this constitutes a sizable supply, it is the limiting resource for the D-T process since the supply of deuterium fuel is virtually unlimited. With fast neutrons, tritium can be bred from the more abundant Li-7.

The reactant neutron is supplied by the D-T fusion reaction. The reaction with 6Li is exothermic, providing a small energy gain for the reactor. The reaction with 7Li is endothermic, but does not consume the neutron. Neutron multiplication reactions are required to replace the neutrons lost to absorption by other elements. Leading candidate neutron multiplication materials are beryllium and lead, but the 7Li reaction helps to keep the neutron population high. Natural lithium is mainly 7Li, which has a low tritium production cross section compared to 6Li so most reactor designs use breeder blankets with enriched 6Li.

The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of fission power reactors, posing problems for material design. After a series of D-T tests at JET, the vacuum vessel was sufficiently radioactive that it required remote handling for the year following the tests. In a production setting, the neutrons would react with lithium in the breeder blanket composed of lithium ceramic pebbles or liquid lithium, yielding tritium.

How do we handle tritium?

Tritium behaves in a similar way to hydrogen: it easily permeates metals (especially if hot!) and reacts explosively with oxygen. However, tritium decays, as opposed to hydrogen. Although the decay of tritium produces only a low-energy β-radiation, it is difficult to contain tritium. In addition, tritium will readily swap with hydrogen if both meet on the way (the so-called isotopic exchange). This is of particular concern since tritium poses a serious threat to human beings if it replaces hydrogen in our biological systems. Being an isotope of hydrogen, tritium can become part of the hydrocarbons that compose our bodies. Therefore, it is very important to confine tritium within carefully chosen materials and conditions. Polymers, for example, which are present in commercial pumps or used as sealant materials, are not an option. The hydrogen-tritium replacement would considerably alter the properties of the material. Therefore, a rule of thumb among tritium fellows is: no use of polymers, just inorganic/metal substances. However, as mentioned above, an additional challenge arises when metallic ducts contain tritium, as it can readily permeate them. A good trick is to constantly maintain low tritium partial pressures at low temperatures (for instance, around 20°C) in order to control permeation. If this is not possible, it is necessary to implement tritium permeation barriers. If this is still not sufficient, a glovebox surrounding the tubes containing tritium must be considered.

Tritium has a rate of decay of about 5% per year. It radiates rather weakly externally, however it is hazardous if ingested, inhaled, or absorbed through the skin. It can react in order to create dangerous radioactive “tritiated” water, which must be controlled to prevent it from contaminating local groundwater.

Tritium will have its own plant:

In a fusion reactor, most of the systems processing the fusion fuels will be hosted in the so-called Tritium Plant (TP). Here, the different isotopes can be isolated by detritiation of gas streams, so that deuterium and tritium can again be fuelled into the reactor. ITER, now under construction in France, will have a 35 m tall x 80 m long x 25 m wide TP building. These dimensions are necessary to house the systems responsible for tritium recovery, isotope separation, deuterium-tritium fuel storage and delivery.

However, it should be noted that ITER will only test small mock-ups of tritium breeding elements, with an estimated daily production less than 0.4 g. In contrast, the European DEMO, designed to demonstrate tritium self-sufficiency at a reactor scale, may reach a production as high as 250 g/day. Thus, in view of DEMO development, many European labs are studying and designing different tritium breeding, processing and extraction systems. All in all, these collaborative efforts are necessary to ensure that tritium, along with deuterium, will be the fuel of commercial fusion power plants.

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-2. Deuterium:

D+D reaction:

Fusing two deuterium nuclei is the second easiest fusion reaction. The reaction has two branches that occur with nearly equal probability:

D + D → T + 1H

D + D → 3He + n

This reaction is also common in research. The optimum energy to initiate this reaction is 15 keV, only slightly higher than that for the D-T reaction. The first branch produces tritium, so that a D-D reactor is not tritium-free, even though it does not require an input of tritium or lithium. Unless the tritons are quickly removed, most of the tritium produced is burned in the reactor, which reduces the handling of tritium, with the disadvantage of producing more, and higher-energy, neutrons. The neutron from the second branch of the D-D reaction has an energy of only 2.45 MeV (0.393 pJ), while the neutron from the D-T reaction has an energy of 14.1 MeV (2.26 pJ), resulting in greater isotope production and material damage. When the tritons are removed quickly while allowing the 3He to react, the fuel cycle is called “tritium suppressed fusion”. The removed tritium decays to 3He with a 12.3 year half life. By recycling the 3He decay into the reactor, the fusion reactor does not require materials resistant to fast neutrons.

Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons would be only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from lithium resources and a somewhat softer neutron spectrum. The disadvantage of D-D compared to D-T is that the energy confinement time (at a given pressure) must be 30 times longer and the power produced (at a given pressure and volume) is 68 times less.

Assuming complete removal of tritium and 3He recycling, only 6% of the fusion energy is carried by neutrons. The tritium-suppressed D-D fusion requires an energy confinement that is 10 times longer compared to D-T and double the plasma temperature.

Would a sustainable Deuterium-Deuterium (D-D) fusion reaction require much more energy compared to Deuterium-Tritium (D-T) fusion?

JET, so far the only operational fusion experiment capable of producing fusion energy, is routinely operated with Deuterium only, for a number of reasons. This minimises activation (from D-T neutrons and from Tritium retention in walls etc.), enabling to upgrade JET easily and minimising decommissioning issues at the end of JETs operational life. They operate in Deuterium only to investigate the feasibility of D-D fusion – very much concentrating on D-T fusion in ITER and the first true power plants.

Why then not get D-D fusion occurring at high levels in JET?

Because D-D needs much higher temperatures of 400 – 500 million degrees C than can normally be achieved. Already – plasma temperatures of 150 – 200 million degrees C will enable lots of D-T fusion – but not very much D-D fusion.

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-3. Deuterium, helium-3:

D + He reaction:

A second-generation approach to controlled fusion power involves combining helium-3 (3He) and deuterium (2H):

D + 3He → 4He + 1H

This reaction produces 4He and a high-energy proton. As with the p-11B aneutronic fusion fuel cycle, most of the reaction energy is released as charged particles, reducing activation of the reactor housing and potentially allowing more efficient energy harvesting (via any of several pathways). In practice, D-D side reactions produce a significant number of neutrons, leaving p-11B as the preferred cycle for aneutronic fusion.

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-4. Proton, boron-11:

P+B reaction:

Both material science problems and non-proliferation concerns are greatly diminished by aneutronic fusion. Theoretically, the most reactive aneutronic fuel is 3He. However, obtaining reasonable quantities of 3He implies large scale extraterrestrial mining on the moon or in the atmosphere of Uranus or Saturn. Therefore, the most promising candidate fuel for such fusion is fusing the readily available protium (i.e. a proton) and boron. Their fusion releases no neutrons, but produces energetic charged alpha (helium) particles whose energy can directly be converted to electrical power:

p + 11B → 3He + 8.7 MeV

Side reactions are likely to yield neutrons that carry only about 0.1% of the power, which means that neutron scattering is not used for energy transfer and material activation is reduced several thousand-fold. The optimum temperature for this reaction of 123 keV is nearly ten times higher than that for pure hydrogen reactions, and energy confinement must be 500 times better than that required for the D-T reaction. In addition, the power density (w/m3) is 2500 times lower than for D-T, although per unit mass of fuel, this is still considerably higher than for fission reactors.

Because the confinement properties of the tokamak and laser pellet fusion are marginal, most proposals for aneutronic fusion are based on radically different confinement concepts, such as the Polywell and the Dense Plasma Focus. In 2013, a research team led by Christine Labaune at École Polytechnique, reported a new fusion rate record for proton-boron fusion, with an estimated 80 million fusion reactions during a 1.5 nanosecond laser fire, 100 times greater than reported in previous experiments.

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Will fusion run out of fuel?

ITER and future fusion devices will use the hydrogen isotopes deuterium and tritium to fuel the fusion reaction. Deuterium can be distilled from all forms of water. It is a widely available, harmless, and virtually inexhaustible resource. In every cubic metre of seawater, for example, there are 33 grams of deuterium. Deuterium is routinely produced for scientific and industrial applications.

Tritium, however, is only present in nature in trace amounts. The only source of readily available tritium comes from heavy-water fission reactors such as the CANDU type (developed by Canada in the 1950-60s, and adopted since in Argentina, China, India, Pakistan, Romania, and South Korea). However, the tritium generated by these reactors is just a by-product and quantities remain relatively small. The accumulated stock of tritium produced from CANDU reactors worldwide does not exceed 20 kilos in any given year—just enough to fuel ITER for the planned fifteen years of its deuterium-tritium operation phase.

Operating an industrial electricity-producing fusion plant, by contrast, will require an average of 70 kilos of tritium per gigawatt of thermal power (per year at full power). And if all goes well, there could be hundreds, if not thousands, of fusion plants operating in the early decades of the 22nd century. How then, will these reactors be fuelled?

Nature offers a solution that combines elegance and efficiency—if, successful, the fusion reaction itself will produce the tritium that, in turn, will continue to fuel the reaction. What’s more, the process will take place within the vacuum vessel in a safe, continuous, closed cycle. The key to this process is isotope 6 of lithium (Li-6) which, when impacted by neutrons, generates tritium. ITER will test different concepts of “tritium breeding modules,” each one with a unique architecture and composition. Whether liquid or solid, compounds will consist of enriched lithium with a proportion of Li-6 in the 50 percent range (compared to the natural isotopic fraction of 7.5%).

Will there be enough lithium to sustain tritium production for fusion?

Yes, enough for at least several thousand years. Let’s look at the numbers. There are approximately 50 million tonnes of proven lithium reserves in the world (half in brine deposits, half in rocks), which means about 3 million tonnes of Li-6. Like most minerals, lithium is also present in seawater. At a concentration of 0.1 part per million, the mass of lithium contained in the oceans of the planet is estimated at 250 billion tonnes. However, a cost-effective method of recovering lithium from seawater does not yet exist.

It takes 140 kilos of Li-6 to obtain the 70 kilos of tritium necessary to producing one gigawatt of thermal power for one year. Assuming an availability of 80 percent and a conversion efficiency from thermal to electrical power of 30 percent, then the production of one gigawatt of electrical power (the estimated size of an average fusion reactor) will require approximately 500 kilos of Li-6 per year. That brings the total requirement for 10,000 reactors to 5,000 tonnes of Li-6 annually.

Fusion will not be the only avid consumer of lithium. The ever-growing lithium-ion battery market for laptops, mobile phones, cordless power tools (and of course electrical vehicles) will claim its share. However, lithium-ion batteries will not necessarily be in “competition” with fusion. At the scale of the global economy, one could imagine that the “waste” product of the lithium enrichment plants for fusion, namely Li-7, could well be used to produce lithium-ion batteries, thus maximizing the efficiency (and cost) of the overall lithium cycle.

Fusion specialists generally consider that, in a world where all energy would be produced by fusion, the quantity of lithium ore present in landmass would be sufficient to provide the required tritium for several thousand years. And as for the lithium present in oceans, it could last millions of years. As for the immediate needs of the tritium breeding module testing at ITER, Li-6 enriched lithium will be supplied from existing lithium enrichment plants. The next fusion reactors such as DEMO will likely require new dedicated facilities to produce Li-6 enriched lithium in sufficient amounts.

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Material selection:

Corrosion issues in thermonuclear fusion reactors and facilities:

The fusion reaction that requires the lowest energy and, hence, the most readily attainable fusion process on Earth, is the combination of a deuterium nucleus with one of tritium (isotopes of hydrogen). The products of such a fusion reaction are a 3.5 MeV helium ion (α particle) and a 14.1 MeV neutron, referred to as a fusion neutron. On Earth, to produce net power, fusion reactions must take place at very high temperatures of at least 100 million degrees, which is some seven times hotter than the centre of the Sun. At these very high temperatures the fusion fuel turns into a plasma. In the future fusion power reactors the helium ions will stay inside the plasma, so contributing to its internal heating, while the fusion neutrons will leave the plasma and penetrate the components of the reactor located all around the plasma, where their kinetic energy will be transformed into heat that will be recovered by one or several coolants. The vapour produced by a steam generator will be used to run a turbine that will generate electricity. During their slowing down process inside the various components surrounding the plasma, the fusion neutrons will produce nuclear transmutation reactions and atomic displacement cascades inside the various encountered, and therefore irradiated, materials, yielding a degradation of their physical and mechanical properties and enhancing eventually corrosion effects.

Structural material stability is a critical issue. Materials that can survive the high temperatures and neutron bombardment experienced in a fusion reactor are considered key to success. The principal issues are the conditions generated by the plasma, neutron degradation of wall surfaces, and the related issue of plasma-wall surface conditions. Reducing hydrogen permeability is seen as crucial to hydrogen recycling and control of the tritium inventory. Materials with the lowest bulk hydrogen solubility and diffusivity provide the optimal candidates for stable barriers. A few pure metals, including tungsten and beryllium, and compounds such as carbides, dense oxides, and nitrides have been investigated. Research has highlighted that coating techniques for preparing well-adhered and perfect barriers are of equivalent importance. The most attractive techniques are those in which an ad-layer is formed by oxidation alone. Alternative methods utilize specific gas environments with strong magnetic and electric fields. Assessment of barrier performance represents an additional challenge. Classical coated membranes gas permeation continues to be the most reliable method to determine hydrogen permeation barrier (HPB) efficiency.

In nuclear fusion power research, the plasma-facing material (or materials) (PFM) is any material used to construct the plasma-facing components (PFC), those components exposed to the plasma within which nuclear fusion occurs, and particularly the material used for the lining the first wall or divertor region of the reactor vessel.

Plasma-facing materials for fusion reactor designs must support the overall steps for energy generation, these include:

-1. Generating heat through fusion,

-2. Capturing heat in the first wall,

-3. Transferring heat at a faster rate than capturing heat.

-4. Generating electricity.

In addition PFMs have to operate over the lifetime of a fusion reactor vessel by handling the harsh environmental conditions, such as:

-1. Ion bombardment causing physical and chemical sputtering and therefore erosion.

-2. Ion implantation causing displacement damage and chemical composition changes

-3. High-heat fluxes (e.g. 10 MW/m^2) due to ELMs and other transients.

-4. Limited tritium codeposition and sequestration.

-5. Stable thermomechanical properties under operation.

-6. Limited number of negative nuclear transmutation effects

Materials currently in use or under consideration include:

Silicon carbide
Boron carbide
Graphite
Carbon fibre composite (CFC)
Beryllium
Tungsten
Molybdenum
Lithium

Multi-layer tiles of several of these materials are also being considered and used, for example:

A thin molybdenum layer on graphite tiles.
A thin tungsten layer on graphite tiles.
A tungsten layer on top of a molybdenum layer on graphite tiles.
A boron carbide layer on top of CFC tiles.
A liquid lithium layer on graphite tiles.
A liquid lithium layer on top of a boron layer on graphite tiles.
A liquid lithium layer on tungsten-based solid PFC surfaces or divertors.

Graphite was used for the first wall material of the Joint European Torus (JET) at its startup (1983), in Tokamak à configuration variable (1992) and in National Spherical Torus Experiment (NSTX, first plasma 1999). Beryllium was used to reline JET in 2009 in anticipation of its proposed use in ITER. Tungsten is used for the divertor in JET, and will be used for the divertor in ITER. It is also used for the first wall in ASDEX Upgrade. Graphite tiles plasma sprayed with tungsten were used for the ASDEX Upgrade divertor. Studies of tungsten in the divertor have been conducted at the DIII-D facility. These experiments utilized two rings of tungsten isotopes embedded in the lower divertor to characterize erosion tungsten during operation. Molybdenum is used for the first wall material in Alcator C-Mod (1991). Liquid lithium (LL) was used to coat the PFC of the Tokamak Fusion Test Reactor in the Lithium Tokamak Experiment (TFTR, 1996).

In 2021, in response to increasing numbers of designs for fusion power reactors for 2040, the United Kingdom Atomic Energy Authority published the UK Fusion Materials Roadmap 2021–2040, focusing on five priority areas, with a focus on tokamak family reactors:

-1. Novel materials to minimize the amount of activation in the structure of the fusion power plant;

-2. Compounds that can be used within the power plant to optimise breeding of tritium fuel to sustain the fusion process;

-3. Magnets and insulators that are resistant to irradiation from fusion reactions—especially under cryogenic conditions;

-4. Structural materials able to retain their strength under neutron bombardment at high operating temperatures (over 550 degrees C);

-5. Engineering assurance for fusion materials—providing irradiated sample data and modelled predictions such that plant designers, operators and regulators have confidence that materials are suitable for use in future commercial power stations.

Superconducting materials:

In a plasma that is embedded in a magnetic field (known as a magnetized plasma) the fusion rate scales as the magnetic field strength to the 4th power. For this reason, many fusion companies that rely on magnetic fields to control their plasma are trying to develop high temperature superconducting devices. In 2021, SuperOx, a Russian and Japanese company, developed a new manufacturing process for making superconducting YBCO wire for fusion reactors. This new wire was shown to conduct between 700 and 2000 Amps per square millimeter. The company was able to produce 186 miles of wire in nine months.

Containment considerations:

Even on smaller production scales, the containment apparatus is blasted with matter and energy. Designs for plasma containment must consider:

-A heating and cooling cycle, up to a 10 MW/m2 thermal load.

-Neutron radiation, which over time leads to neutron activation and embrittlement.

-High energy ions leaving at tens to hundreds of electronvolts.

-Alpha particles leaving at millions of electronvolts.

-Electrons leaving at high energy.

-Light radiation (IR, visible, UV, X-ray).

Depending on the approach, these effects may be higher or lower than fission reactors. One estimate put the radiation at 100 times that of a typical pressurized water reactor. Depending on the approach, other considerations such as electrical conductivity, magnetic permeability, and mechanical strength matter. Materials must also not end up as long-lived radioactive waste.

Plasma-wall surface conditions:

The fusion reactor lining that face plasma is known as the first wall. Exposure of first wall materials to fusion plasmas and their ensuing property degradation has long been recognised as one of the most important challenges facing fusion energy. For long term use, each atom in the wall is expected to be hit by a neutron and displaced about 100 times before the material is replaced. High-energy neutrons produce hydrogen and helium via nuclear reactions that tend to form bubbles at grain boundaries and result in swelling, blistering or embrittlement.

Selection of materials:

Low-Z materials, such as graphite or beryllium are generally preferred to high-Z materials, usually tungsten with molybdenum as a second choice. Liquid metals (lithium, gallium, tin) have been proposed, e.g., by injection of 1–5 mm thick streams flowing at 10 m/s on solid substrates.

Graphite features a gross erosion rate due to physical and chemical sputtering amounting to many meters per year, requiring redeposition of the sputtered material. The redeposition site generally does not exactly match the sputter site, allowing net erosion that may be prohibitive. An even larger problem is that tritium is redeposited with the redeposited graphite. The tritium inventory in the wall and dust could build up to many kilograms, representing a waste of resources and a radiological hazard in case of an accident. Graphite found favor as material for short-lived experiments, but appears unlikely to become the primary plasma-facing material (PFM) in a commercial reactor.

Tungsten’s sputtering rate is orders of magnitude smaller than carbon’s, and tritium is much less incorporated into redeposited tungsten. However, tungsten plasma impurities are much more damaging than carbon impurities, and self-sputtering can be high, requiring the plasma in contact with the tungsten not be too hot (a few tens of eV rather than hundreds of eV). Tungsten also has issues around eddy currents and melting in off-normal events, as well as some radiological issues.

Reduced-activation ferritic/martensitic (RAFM) steel is the benchmark structural material for in-vessel components of fusion reactor. First wall (FW) concept is a thin tungsten (W) layer coated on a RAFM steel structural material. The purpose of the W layer on the FW is to (1) protect the blanket components from temperature excursions during any transient (edge localized modes, ELMs) and off-normal events (disruptions) in the device and (2) protect the blanket from erosion from ions impacting the wall. A thin W layer is advantageous to maximize the number of neutrons reaching the breeding zone, whereas a thicker W layer has the advantage of further reducing the temperature in the steel components of the FW. Additionally, the thickness of the W layer has to be sufficient to not be eroded during the lifetime of the component. The temperature gradient through the FW layer depends on the thickness of the layer, and thus different thicknesses would have a different stress state. In turn, designs with different temperature profiles could lead to different neutron irradiated property changes because irradiation defects have temperature dependent formation. The optimal FW layer thickness must be a balance of all these factors.

Bainitic steels containing 2.25 to 3 wt % chromium are attractive structural materials for lifetime component applications in a fusion power system because they have good high temperature structural properties, can be constructed into complicated components with welding, and potentially have a low capital cost.

Testing needs for FW and divertor materials:

-1. Mechanical properties, including fatigue, creep, tensile, and compression behavior, need to be recorded for any candidate W material both in the unirradiated and neutron irradiated conditions. Many advanced manufacturing and joining techniques create W microstructures that are dissimilar from standard powder metallurgical W, so each candidate W material needs to be tested to generate reliable data for use in designing the FNSF.

-2. Thermal properties, including thermal conductivity after neutron irradiation, response to thermal cycling, and effects of long-term use at elevated temperatures, need to be determined for any candidate W material.

-3. Response to neutron irradiation with a 14 MeV neutron source will be important for W because the transmutation cross sections are sensitive to energy, and the transmutation products are known to have a large negative effect on W thermomechanical properties.

-4. Response to plasma exposure, including erosion and retention measurements. Any advanced manufacturing technique, particle addition, or composite will change the microstructure of W and create additional internal interfaces that may result in additional trapping sites for tritium. Having data on the candidate W material’s interaction with deuterium and tritium is critical so that the tritium inventory can be accurately estimated.

-5. Research needs 1–4 could be advanced significantly with single-effects studies. With that starting database, it may be possible to design the first-phase fusion reactor components with enough confidence to build them. Since it may not be possible to test the full combined effects of neutron flux, particle flux, stress field, and temperature gradient until the operation of the fusion reactor, the data from the first phase of fusion reactor will be critical to inform and improve the design of the FW and divertor for successive phases.

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Section-10

Nuclear fusion projects:

IAEA’s role in fusion activities:

The IAEA has supported fusion energy research since its inception and helps Member States exchange and build knowledge on fusion science and technology. Worldwide research has made impressive progress in fusion and plasma physics. Many scientific questions have been solved in the last years. Controlled nuclear fusion and plasma physics research is currently carried out in more than 50 IAEA Member States. The challenge is to prove that fusion as an energy source is scientifically feasible. Since this will require large, complex and expensive devices to address reactor-relevant physics and technology challenges, international collaboration on fusion research and development is needed. The IAEA fosters international collaboration and coordination to help close the existing gaps in physics, technology and regulation and move forward in developing the peaceful use of fusion energy. The Agency’s activities in this field cover, among others, plasma physics and fusion power, technologies and material, both for magnetic and inertial fusion. The IAEA and the ITER Organization have had a close relationship from the very beginning, particularly in the areas of nuclear fusion research, knowledge management, human resources development, and educational activities and outreach. The IAEA hosts a variety of fusion-related forums, including the biennial Fusion Energy Conference, a series of workshops on the DEMO project, and many technical meetings. It also produces publications on fusion, such as the Fusion Journal and the Fusion Physics Book; creates networks of institutions and scientists to address key issues of common interest; maintains databases for the fusion community; and supports education and training activities on fusion.

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EUROfusion:

The EUROfusion roadmap forms the basis for the programme of EUROfusion and provides a clear and structured way forward to commercial energy from fusion.

Since 2012, when the first version of the Roadmap was published, the European fusion community has made significant advances in many areas. These include employing successfully tokamak walls made of metal, gaining a greatly advanced understanding of the material properties that will be required by ITER and DEMO, and completing the construction and operation of the Wendelstein 7-X stellarator. Recent highlights are the world record fusion energy of 59 MJoule achieved in the JET tokamak by burning only 170 micrograms of the deuterium-tritium fuel, and the completion of the pre-conceptual design phase of DEMO, the first device aimed to produce fusion electricity.

Roadmap Phases of EUROfusion are depicted in figure below:

Short Term:

Construction of ITER
Research & Development in support of ITER
Deuterium-tritium operation of JET
Concept Design phase of DEMO
Research & Development for DEMO
Construction of a fusion materials testing facility, IFMIF-DONES
Scientific and technological exploitation of the stellarator concept

Medium term:

First scientific and technological exploitation of ITER
First exploitation of IFMIF-DONES
Engineering Design phase of DEMO with industrial involvement
Development of power plant materials and technologies
Possible further development of the stellarator concept

Long term:

In the course of the roadmap implementation, the fusion programme will move from being laboratory-based and science-driven towards an industry- and technology-driven venture. The design, construction and operation of DEMO require full involvement of industry. This will ensure that after a successful DEMO operation industry can take on the responsibility for commercial fusion power. It not only emphasises the need to intensify collaboration with industry but also to seek all opportunities for collaboration outside Europe (Broader Approach).

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Fusion in the UK:

The United Kingdom’s fusion research programme is based at Culham Centre for Fusion Energy (CCFE) in Oxfordshire, the fusion research arm of the UK Atomic Energy Authority. The research is funded by the Engineering and Physical Sciences Research Council and by the European Union under the Euratom treaty.

The UK contributes to fusion research in two main ways:

-1. Its own fusion programme, centred on the MAST (Mega Amp Spherical Tokamak) Upgrade device. MAST Upgrade builds on the success of the original MAST tokamak (2000-2013) with major new capabilities in areas such as plasma stability and exhaust. The UK programme is also contributing to preparations for the international ITER project, and research on plasma physics and fusion materials and technology.

-2. Operating JET – the Joint European Torus, the world’s largest tokamak and Europe’s flagship experiment. JET is situated at Culham, where CCFE operates it on behalf of fusion researchers around Europe via a contract between the European Commission and the United Kingdom Atomic Energy Authority.

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China’s fusion energy programs:

China is growing as a hub for active fusion research, as its scientists and entrepreneurs are making significant investments in fusion energy. China has two main fusion enterprises driving scientific advances and investment: the government-funded research based at the Institute of Plasma Physics at the Hefei Institute of Physical Science and the privately-funded fusion research of ENN Group.

In Hefei, government-funded scientists operate the recently-upgraded Experimental Advanced Superconducting Tokamak (EAST). The machine is government-funded, through the National Nuclear Corporation, a large state-owned corporation. It cost nearly USD$900 million to build and operate through 2019. In the past year, the EAST device has achieved world records, maintaining a plasma temperature of 120 million degrees Celsius for 101 seconds and 160 million Celsius for 20 seconds. These achievements should be seen as a huge success for a fusion device and show the scientific prowess of China’s research teams.

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Until recently, progress toward fusion energy in the U.S. has been principally focused on pure fusion science. Funding toward engineering challenges has been periodically complicated by political issues, over both the cost of ITER and the direction of the country’s energy policies. Still, Congress has increased appropriations for the Department of Energy’s (DOE’s) Office of Fusion Energy Science (FES) 44% since 2015, to $671 million in fiscal year 2020. Funding for the U.S. commitment to ITER has likewise been increased.

A report from the National Academies of Science in 2019 strongly recommended that the U.S. remain a member of ITER but also pursue the goal of a compact pilot fusion plant that would have higher power density and lower capital cost than larger DEMO designs. This plant would likely have net generation of about 200 MW to 300 MW. The preference for a smaller design reflects the economic realities of electricity generation in the U.S. and the practical need to advance the technologies first at the least cost.

The DOE’s Fusion Energy Sciences Advisory Committee (FESAC) is currently working on a report that would consider the feasibility of such a pilot plant. The study is expected to be completed later this year.

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Fusion research projects:

A long-standing quip about fusion points out that, since the 1970s, commercial deployment of fusion power has always been about 40 years away. While there is some truth in this, many breakthroughs have been made, particularly in recent years, and there are a number of major projects under development that may bring research to the point where fusion power can be commercialised.

Several tokamaks have been built, including the Joint European Torus (JET) and the Mega Amp Spherical Tokamak (MAST) in the UK and the tokamak fusion test reactor (TFTR) at Princeton in the USA. The ITER (International Thermonuclear Experimental Reactor) project currently under construction in Cadarache, France will be the largest tokamak when it operates in the 2020s. The Chinese Fusion Engineering Test Reactor (CFETR) is a tokamak which is reported to be larger than ITER, and due for completion in 2030. Meanwhile it is running its Experimental Advanced Superconducting Tokamak (EAST). In the UK, Tokamak Energy has commissioned and is further developing its ST40 tokamak. In March 2022 the US Department of Energy announced up to $50 million of federal funding to support US scientists conducting experimental research in fusion energy science; $20 million of this would support tokamak facilities, while the other $30 million would support research to improve fusion performance and increase the duration of burning plasma scenarios.

Much research has also been carried out on stellarators. A large one of these, the Large Helical Device at Japan’s National Institute of Fusion Research, began operating in 1998. It is being used to study the best magnetic configuration for plasma confinement. At the Garching site of the Max Planck Institute for Plasma Physics in Germany, research carried out at the Wendelstein 7-AS between 1988 and 2002 is being progressed at the Wendelstein 7-X, which was built over 19 years at Max Planck Institute’s Greifswald site and started up at the end of 2015. Another stellarator, TJII, is in operation in Madrid, Spain. In the USA, at Princeton Plasma Physics Laboratory, where the first stellarators were built in 1951, construction on the NCSX stellerator was abandoned in 2008 due to cost overruns and lack of funding.

There have also been significant developments in research into inertial fusion energy (IFE). Construction of the $7 billion National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), funded by the National Nuclear Security Administration, was completed in March 2009. The Laser Mégajoule (LMJ) in France’s Bordeaux region started operation in October 2014. Both are designed to deliver, in a few billionths of a second, nearly two million joules of light energy to targets measuring a few millimeters in size. The main purpose of both NIF and LMJ is for research to support both countries’ respective nuclear weapons programs. In September 2021 the United Kingdom Atomic Energy Authority (UKAEA) opened the Fusion Technology Facility at the Advanced Manufacturing Park in Rotherham. It will house test rigs including the Combined Heating and Magnetic Research Apparatus (CHIMERA), which would be the only device capable of simulating conditions of a fusion power plant to test prototype components.

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ITER

Fusion scientists realized some time ago that existing tokamaks are simply not large or powerful enough to reach burning plasma conditions. In order to resolve the design of a power plant, research at power-plant scale is necessary. Thus, a long-term goal has been building a facility that would have the necessary capabilities.

The ITER project was born in November 1985, when Soviet General Secretary Mikhail Gorbachev proposed an international collaboration on fusion energy to President Ronald Reagan. The name was originally an acronym for International Thermonuclear Energy Reactor, though that has since been dropped in favor of another representation, that is, that iter is Latin for “the way,” namely, the way to fusion energy.

The ITER agreement was signed in 1987 by the U.S., the European Union (EU), Japan, and the Soviet Union (Russia has assumed the USSR’s membership role). Under the agreement, all members have equal access to the technology developed, though each member funds only a portion of the cost. The U.S. is responsible for about 9% of ITER funding, in a mix of cash and in-kind contributions.

Although initial work began in 1988, it took until 2001 before an engineering design was agreed upon. The U.S. withdrew from ITER in the late 1990s, though it would rejoin in 2003. China and the Republic of Korea also joined ITER in 2003, followed by India in 2005. This brought the coalition to seven groups comprising 35 nations, making ITER the largest multinational science project in history.

The current ITER agreement was signed in 2007, and a location near Aix-en-Provence in southern France was selected as the site of the facility. However, there were considerable challenges in getting such a large project with so many members off the ground. Construction proceeded somewhat fitfully for several years and fell badly behind schedule.

The platform measures 42 hectares and is one of the largest man-made levelled surfaces in the world. There are 39 buildings, facilities and power supplies which will be needed to operate the biggest fusion machine. More than 3000 people are contributing to ITER’s civil engineering works. The main worksite is a markedly sterile environment, where tremendous components are being put into place with the help of 750-ton cranes. Workers have already put together the shell of the tokamak, but they are still awaiting some parts, including a giant magnet from Russia that will sit at the top of the machine.

The dimensions are mind-blowing. The tokamak will ultimately weigh 23,000 tons. That’s the combined weight of three Eiffel towers. It will comprise a million components, further differing into no fewer than 10 million smaller parts.

This powerful behemoth will be surrounded by some of the largest magnets ever created. Their staggering size — some of them have diameters of up to 24 meters — means they are too large to transport and must be assembled on site in a giant hall.

Given the huge number of parts involved, there’s simply no room for error.

Even the digital design of this enormous machine sits across 3D computer files that take up more than two terabytes of drive space. That’s the same amount of space you could save more than 160 million one-page Word documents on.

Behind hundreds of workers putting the ITER project together are around 4,500 companies with 15,000 employees from all over the globe.

Thirty-five countries are collaborating on ITER, which is run by seven main members — China, the United States, the European Union, Russia, India, Japan and South Korea. It looks a little like the UN Security Council, though many have tried hard to keep geopolitics out of ITER entirely.

But as Russia seeks to redraw Europe’s map with its war in Ukraine, and even challenge the post-war world order, there are concerns over the country’s continued role in ITER, and just as many over its potential exclusion. Russia has been cut out of a number of other international scientific projects in the fallout of its war, but the European Commission has explicitly made an exception for ITER in its sanctions. Part of this is because Russia is inextricably linked with not only the project but fusion energy historically.

Some key figures about ITER:

24 m high

30 m wide

23000 tonnes weight

1000000 Number of components

830 m³ plasma volume

Figure above is February 2020 fish-eye view above the ITER tokamak chamber under construction and gives an idea of the impressive scale of the device. Initial ITER operations are scheduled for 2025, with DT operation set for 2035.

ITER will have many capabilities that go well beyond current tokamaks. It will be the first device that can generate a burning plasma and explore the fundamentals of how a tokamak contains the fusion reaction and the process of self-heating. Using DT fusion, ITER will produce 500 MW of fusion power at a Q value of 10—smashing the current world record of 16 MW at a Q value of 0.67, which was achieved on JET in 1997 for MCF. ITER’s thermonuclear fusion reactor will use over 300 MW of electrical power to cause the plasma to absorb 50 MW of thermal power, creating 500 MW of heat from fusion for periods of 400 to 600 seconds. ITER is designed to yield in its plasma a ten-fold return on power (Q=10), or 500 MW of fusion power from 50 MW of input heating power. As of 2022, the record for energy production using nuclear fusion is held by the National Ignition Facility reactor, which achieved a Q of 1.5 in December 2022 for ICF. Beyond just heating the plasma, the total electricity consumed by the reactor and facilities will range from 110 MW up to 620 MW peak for 30-second periods during plasma operation. As a research reactor, the heat energy generated will not be converted to electricity, but simply vented.

Figure above is cutaway schematic of the ITER facility that shows the tokamak in the center with a simulation of the fusion plasma inside the tokamak. The entire device is about five stories tall.

ITER will be by far the largest and highest magnetic field tokamak in the world, and it will be powered by a central solenoid that will be the most powerful pulsed superconducting magnet ever constructed. Fabricated from 36 km of superconducting cable, this 1,000-ton magnet will drive 15 million amperes of current through the plasma, far more than anything that has been possible before. In addition, ITER will serve as a test bed for a number of critical fusion technologies, including tritium breeding, plasma control, advanced diagnostics, and disruption mitigation. Though it will not operate as a power plant, ITER will test safety features that future fusion power plants will require.

What questions will be answered by ITER that have not already been answered by research to date?

ITER is the experimental step between today’s fusion machines, focused on plasma physics studies, and tomorrow’s fusion power plants. The plasma physics community will have access for the first time, in ITER, to a burning plasma. In a burning plasma, the energy of the helium nuclei produced by the fusion reactions is enough to maintain the temperature of the plasma, thereby reducing or eliminating the need for external heating. Self-heating plasmas will be the key in the future to producing electricity from fusion energy, allowing for sustained, ongoing fusion reactions.

To be able to create plasmas with dominant self-heating, ITER will be twice as large as the largest tokamak fusion experiment currently operating (JET in the UK), with ten times the plasma volume. This unique experimental machine has been designed to:

confine a deuterium-tritium plasma in which alpha-particle heating (self-heating) dominates
generate 500 MW of fusion power in its plasma from 50 MW of heating input power (Q≥10) for long durations (400 to 600 seconds)
contribute to the demonstration of the integrated operation of technologies for a fusion power plant
test concepts for a tritium breeding module
demonstrate the safety characteristics of a fusion device

Fusion is a promising option long-term for sustainable, global energy supply if the remaining technical challenges can be overcome. ITER’s stated mission is to demonstrate the feasibility of fusion power as a large-scale, carbon-free source of energy. The objectives of the ITER project are not limited to creating the nuclear fusion device but are much broader, including building necessary technical, organizational, and logistical capabilities, skills, tools, supply chains, and culture enabling management of such megaprojects among participating countries, bootstrapping their local nuclear fusion industries.

As a first-of-a-kind research and demonstration project, ITER is naturally quite expensive. Because most of the contributions are in-kind components produced in the member nations under different public financing approaches, an exact total of ITER’s costs is impossible to produce. However, a rough estimate generated by the ITER organization in 2016 was around $20 billion through start of DT operations in 2035. As large as that figure may seem, it is spread across a coalition of 35 nations, all of which will share in the technology ITER develops.

What will come after ITER?

The details are still to be determined, but a number of targets are in sight. If all goes well, the technology from ITER should enable electricity generation from fusion, and member nations are not waiting until the late 2030s to begin planning. Several follow-on devices that will be even higher performance than ITER are in development. The ITER coalition has referred to this next step as the DEMO phase, and several conceptual designs for DEMO devices are in development in the EU, U.S., Korea, and China. DEMO-stage devices are expected to be simpler and less expensive than ITER, because they will be designed for power generation rather than research, as well as being “always on” devices that operate in steady state rather than exploring different fusion regimes. Of these, China’s may be the closest to operation.

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JET

In 1978, the European Community (Euratom, along with Sweden and Switzerland) launched the Joint European Torus (JET) project in the UK. JET is the largest tokamak operating in the world today. A similar tokamak, JT-60, operates at the Naka Fusion Institute of Japan Atomic Energy Agency in Japan, but only JET has the facilities to use D-T fuel. JET produced its first plasma in 1983, and became the first experiment to produce controlled fusion power in November 1991, albeit with high input of electricity. Up to 16 MW of fusion power for one second and 5 MW sustained has been achieved in D-T plasmas using the device, from 24 MW of power injected into its heating system, and many experiments are conducted to study different heating schemes and other techniques. JET has been very successful in operating remote handling techniques in a radioactive environment to modify the interior of the device and has shown that the remote handling maintenance of fusion devices is realistic.

JET is a key device in preparations for ITER. It has been significantly upgraded in recent years to test ITER plasma physics and engineering systems. Further enhancements are planned at JET with a view to exceeding its fusion power record in future D-T experiments. A compact device – Mega Amp Spherical Tokamak (MAST) – was developed alongside JET at Culham, partly to serve the ITER project, and the substantial MAST Upgrade project is now being implemented in stages to increase neutral beam power from 5 to 12.5 MW and the energy deposited in plasma from 2.5 to 30 MJ. MAST Upgrade is focused on designing a plasma exhaust system or divertor that would be able withstand the intense power loads created in commercial-sized fusion reactors. It achieved first plasma in October 2020.

In 2019 the UK government committed £22 million over four years for the conceptual design of the Spherical Tokamak for Energy Production (STEP) at Culham. The technical objectives of STEP are: to deliver predictable net electricity greater than 100 MW; to exploit fusion energy beyond electricity production; to ensure tritium self-sufficiency; to qualify materials and components under appropriate fusion conditions of neutron flux; and to develop a viable path to affordable life-cycle costs. STEP is scheduled for completion in 2040.

On 21 December 2021, using deuterium-tritium fuel, JET produced 59 megajoules during a five second pulse, beating its previous 1997 record of 21.7 megajoules, with Q = 0.33.

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KSTAR

The KSTAR (Korea Superconducting Tokamak Advanced Research) at the National Fusion Research Institute (NFRI) in Daejeon produced its first plasma in mid-2008. It is a pilot device for ITER, and involves much international collaboration. It will be a satellite of ITER during ITER’s operational phase. The tokamak with 1.8 metre major radius is the first to use Nb3Sn superconducting magnets, the same material to be used in the ITER project. Its first stage of development to 2012 was to prove baseline operation technologies and achieved plasma pulses of up to 20 seconds. For the second phase of development (2013-2017), KSTAR was upgraded to study long pulses of up to 300 seconds in H mode (high-confinement mode) – the 100s target was in 2015 – and embark upon high-performance AT mode. It achieved 70 seconds in high-performance plasma operation in late 2016, a world record. In addition, KSTAR researchers also succeeded in achieving an alternative advanced plasma operation mode with the internal transport barrier (ITB). The device does not have tritium handling capabilities, so will not use D-T fuel. The KSTAR — a nuclear fusion reactor developed by researchers at the Seoul National University (SNU) in South Korea — has attained temperatures over 100 million degrees Celsius in a reaction that lasted for about 30 seconds.

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ALCATOR

At the Massachusetts Institute of Technology (MIT) since the 1970s a succession of small ALCATOR (Alto Campus Torus) high magnetic field torus reactors have operated on the principle of achieving high plasma pressure as the route to long plasma confinement. Alcator C-Mod is claimed to have the highest magnetic field and highest plasma pressure of any fusion reactor, and is the largest university-based fusion reactor in the world. It operated 1993-2016. In September 2016 it achieved a plasma pressure of 2.05 atmospheres at a temperature of 35 million degrees Celsius. The plasma produced 300 trillion fusion reactions per second and had a central magnetic field strength of 5.7 tesla. It carried 1.4 million amps of electrical current and was heated with over 4 MW of power. The reaction occurred in a volume of approximately 1 cubic metre and the plasma lasted for two seconds. Having achieved this record performance for a fusion reactor, government funding ceased.

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Large Helical Device – stellarator:

The Large Helical Device (LHD) at Japan’s National Institute for Fusion Science in Toki, in the Gifu Prefecture, was the world’s largest stellarator. LHD produced its first plasma in 1998 and has demonstrated plasma confinement properties comparable to other large fusion devices. It has achieved an ion temperature of 13.5 keV (about 160 million degrees) and plasma stored energy of 1.44 million joules (MJ).

Wendelstein 7-X stellarator:

Following a year of tests, this started up at the end of 2015, and helium plasma briefly reached about one million degrees centigrade. In 2016 it progressed to using hydrogen, and using 2 MW it achieved plasma of 80 million degrees centigrade for a quarter of a second. W7-X is the world’s largest stellarator and it is planned to operate continuously for up to 30 minutes. It cost €1 billion ($1.1 billion).

Heliac-1 stellarator:

At the Australian Plasma Fusion Research Facility at the Australian National University the H-1 stellarator has run for some years and in 2014 was upgraded significantly. H-1 is capable of accessing a wide range of plasma configurations and allows exploration of ideas for improved magnetic design of the fusion power stations that will follow ITER.

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Fusion research outputs have boomed over the last fifteen years since ITER was established, by looking the number of first authorship papers presented at the IAEA Fusion Energy Conference (FEC) between 2006 and 2021. The United States of America (USA) retains the top position with 152 first authorship papers for 2021. Japan and China are in second and third place, respectively. The Princeton Plasma Physics Laboratory (USA) and the Max Planck Institute for Plasma Physics (Germany) are the leading organizations in this index, with a total of 42 first authorship papers in 2021. The Institute for Plasma Research (India) and Southwestern Institute of Physics (China) are not far behind, with 37 and 35 first authorship papers in 2021, respectively.

A similar increasing pattern emerges when looking at private sector investments over the past two years. Over 30 private fusion companies can be found in Australia, Canada, China, France, Germany, Israel, Italy, Japan, United Kingdom, and USA.

The public and private investment in the fusion energy sector is rapidly growing, with private sector investment surpassing public funding for the first time in 2022. According to the Global Fusion Industry survey of 2022, public funding for the year stood at $4.7 billion, while private investments increased by 139% from 2021, reaching $4.8 billion.

Short summaries of some of the private fusion companies are given below.

TAE Technologies.

TAE has been working for about 20 years on an approach known as field-reversed configuration (FRC). TAE’s technology, rather than relying on deuterium-tritium (DT) fusion, instead seeks to fuse hydrogen and boron. Though this is a more difficult reaction to achieve—requiring temperatures at least an order of magnitude higher—it has the advantage of not producing the highly energetic neutrons that complicate DT fusion. FRC is a magnetic confinement method forming a toroidal plasma, but without a toroidal magnetic field. In order to achieve temperatures 10 times higher than anything the tokamaks of the world are even aiming for, TAE had to design a very different reactor. So it did, inspired by particle accelerators at CERN. CERN can accelerate soups of particles to astronomical conditions. Their marker, self-reported, is five trillion degrees, or the equivalent. TAE only need a billion by harvesting some of the building blocks of those principles and translate those into fusion. Rather than running its plasma around in a donut shape, TAE keeps it spinning in place, confined by powerful magnetic rings. And in terms of magnetic efficiency, which is a basic number you can get from looking at the geometry and design of the reactor, that’s about 10% in a tokamak, and about 90% in a design like TAE. TAE is based in Irvine, California. Its publicly announced funding totals $700 million, and known investors include Google.

Commonwealth Fusion Systems (CFS).

A spin-off from the Massachusetts Institute of Technology’s (MIT’s) Plasma Science and Fusion Center, CFS is pursuing a fairly conventional tokamak approach, but leveraging high-technology advances that came too recently to be incorporated into ITER. Foremost among these is the use of “high temperature” superconductors made with rare earth barium copper oxide (REBCO), ITER employs niobium-tin. It is hoped this will allow for smaller, more efficient, and less expensive magnets. CFS is continuing a collaboration with MIT to develop a design for a compact high-field tokamak, called SPARC, that would produce 50 MW to 100 MW of fusion power at a Q value of 3. Construction of SPARC is slated to begin soon. MIT is one of the investors in CFS; others include several venture capital funds.

General Fusion.

This Vancouver, British Columbia–based company is pursuing one of the more revolutionary approaches, which it calls magnetized target fusion (MTF). The MTF concept uses a sphere filled with molten lead-lithium, which is then pumped to form a vortex. A pulse of magnetically confined plasma fuel is injected into the vortex, and an array of pistons creates a shock wave in the liquid metal to compress the plasma to fusion conditions. Heat from the liquid metal will then be captured and used to generate electricity. The company is supported by Amazon CEO Jeff Bezos, Microsoft, and venture capital.

Tokamak Energy.

A UK company, Tokamak Energy is working on magnetic confinement fusion, but employing a tokamak with a more spherical shape, based on a concept developed in the U.S. and the UK. This device, called ST40, has been commissioned and research on it is currently ongoing. The advantage offered by spherical tokamaks compared to regular tokamaks is that they favor a very compact construction style which puts the magnets very close to the plasma, effectively making them more efficient in retaining the plasma, with less power required to maintain stable plasma. Tokamak Energy in 2022 announced plans to construct a high field spherical tokamak using high-temperature superconducting (HTS) magnets. Dubbed the ST80-HTS, the machine would demonstrate multiple technologies required to achieve commercial fusion energy, the company says. In July 2020 it was awarded £10 million from the UK Department for Business, Energy and Industrial Strategy (BEIS), as part of the government’s Advanced Modular Reactor project. The funds will contribute to core development work on high temperature superconducting (HTS) magnets and plasma exhaust system (divertor) technologies. The divertor must handle high levels of heat and particle bombardment while removing impurities and waste from the system. It aims to have a prototype delivering electricity to the grid by 2030.

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Several fusion researchers who don’t work for private firms said that, although prospects are undeniably exciting, commercial fusion in a decade is overly optimistic. “Private companies say they’ll have it working in ten years, but that’s just to attract funders,” says Tony Donné, programme manager of the Eurofusion consortium which conducts experiments at the state-run Joint European Torus, established at Culham in the late 1970s. “They all have stated constantly to be about ten years away from a working fusion reactor, and they still do.”

Timelines that companies project should be regarded not so much as promises but as motivational aspirations, says Melanie Windridge, a plasma physicist who is the FIA’s UK director of communications, and a communications consultant for the fusion firm Tokamak Energy, in Culham. “I think bold targets are necessary,” she says. State support is also likely to be needed to build a fusion power plant that actually feeds electricity into the grid, adds Ian Chapman, chief executive of the UK Atomic Energy Authority (UKAEA).

But whether it comes from small-scale private enterprise, huge national or international fusion projects, or a bit of both, practical nuclear fusion finally seems to be on the horizon. “I’m convinced that it’s going to happen”, says Chapman.

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Microsoft made a bet on nuclear fusion:

Microsoft signed a jaw-dropping agreement to purchase electricity from a nuclear fusion generator. A company called Helion Energy thinks it can deliver fusion power to Microsoft by 2028. It announced a power purchase agreement with Microsoft that would see it plug in the world’s first commercial fusion generator to a power grid in Washington. The goal is to generate at least 50 megawatts of power — a small but significant amount and more than the 42MW that the US’s first two offshore wind farms have the capacity to generate today. To say that’s a tall order would be the understatement of the year. “I would say it’s the most audacious thing I’ve ever heard,” says University of Chicago theoretical physicist Robert Rosner. “In these kinds of issues, I will never say never. But it would be astonishing if they succeed.”

Experts’ optimistic estimates for when the world might see its first nuclear fusion power plant have ranged from the end of the decade to several decades from now. Helion’s success depends on achieving remarkable breakthroughs in an incredibly short span of time and then commercializing its technology to make it cost-competitive with other energy sources.

The most advanced attempts at generating electricity through nuclear fusion involve shooting powerful laser beams at a tiny target or relying on magnetic fields to confine superheated matter called plasma with a machine called a tokamak.

Helion uses neither of those methods. The company is developing a 40-foot device called a plasma accelerator that heats fuel to 100 million degrees Celsius. It heats deuterium (an isotope of hydrogen) and helium-3 into a plasma and then uses pulsed magnetic fields to compress the plasma until fusion happens. Helion claims that the machine should eventually be able to recapture the electricity used to trigger the reaction, which can be used to recharge the device’s magnets. Getting enough helium-3 fuel could be another big challenge. It’s a very rare isotope that’s used in quantum computing and medical imaging. Helion, however, says that it has patented a process to make helium-3 itself by fusing deuterium atoms together in its plasma accelerator.

Assuming Helion can pull this all off, it still has to ensure that it can do so in an affordable way. The cost of the electricity it generates for consumers would need to be comparable to or cheaper than today’s power plants, solar, and wind farms. The company isn’t sharing what price it agreed to in its power purchase agreement with Microsoft, the company’s goal is to one day get costs down to a cent a kilowatt hour.

Helion’s funders include OpenAI CEO Sam Altman. Microsoft has made a multibillion dollar investment in OpenAI to boost its development of popular tools like ChatGPT. Altman is Helion’s board chair and largest investor, and he may have been involved in brokering Helion’s power purchase agreement with Microsoft.

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Section-11

Challenges to nuclear fusion:

The benefits of fusion reaction are immense apart from generating much more energy, fusion produces no carbon emissions, the raw materials are in abundant supply, it produces negligible radioactive waste compared to fission, and is considered much safer. The challenges to fusion on earth proved much greater than expected. We’re basically making stars on Earth. The fusion of two hydrogen atoms to make helium is the main process that powers the sun and other stars. When such light atomic nuclei combine, they release an immense amount of energy. But because these nuclei have positive electrical charges, they repel one another, and it takes tremendous pressures and temperatures to overcome that electrostatic barrier and get them to merge. If scientists can contain the fuel for fusion—a plasma mixture of deuterium and tritium, two heavy isotopes of hydrogen—the energy released in the reaction can make it self-sustaining. But how do you bottle a plasma at a temperature of around 100 million kelvins, several times hotter than the center of the sun?

No known material can withstand such extreme conditions; they would melt even extremely heat-resistant metals such as tungsten in an instant. The answer long favored for reactor design is magnetic confinement: holding the electrically charged plasma in a “magnetic bottle” formed by strong magnetic fields so it never touches the walls of the fusion chamber. The most popular design, called a tokamak and proposed in the 1950s by Soviet scientists, uses a toroidal (or doughnut-shaped) container.

The process requires exquisite control. The furiously hot plasma won’t stay still: it tends to develop large temperature gradients, which generate strong convection currents that make the plasma turbulent and hard to manage. Such instabilities, akin to miniature solar flares, can bring the plasma into contact with the walls, damaging them. Other plasma instabilities can produce beams of high-energy electrons that bore holes in the reaction-chamber cladding. Suppressing or managing these fluctuations has been one of the key challenges for tokamak designers. “The big success of the past 10 years has been in understanding this turbulence in quantitative detail,” says Steven Cowley, who directs the Princeton Plasma Physics Laboratory.

For fusion to be widely adopted as a clean, reliable and safe source of electricity, we need to start thinking of fusion not just in terms of awesome science, but also in terms of power plants. To support this, ITER and other government programs have focused more attention and resources on the engineering challenges associated with fusion. These include fuel production and handling, and materials for the “first wall” between the plasma and the energy conversion equipment.

One of the biggest obstacles to magnetic-confinement fusion is the need for materials that can withstand the tough treatment they’ll receive from the fusing plasma. In particular, deuterium-tritium fusion makes an intense flux of high-energy neutrons, which collide with the nuclei of atoms in the metal walls and cladding, causing tiny spots of melting. The metal then recrystallizes but is weakened, with atoms shifted from their initial positions. In the cladding of a typical fusion reactor, each atom might be displaced about 100 times over the reactor’s lifetime.

The consequences of such intense neutron bombardment aren’t well understood, because fusion has never been sustained for the long periods that would be required in a working reactor. “We don’t know and won’t know about materials degradation and lifetime until we’ve operated a power plant,” says Ian Chapman, CEO of the U.K. Atomic Energy Authority (UKAEA), the British government’s nuclear energy organization. Nevertheless, important insights into these degradation problems might be gleaned from a simple experiment that generates intense neutron beams that can be used to test materials. Such a facility—a particle-accelerator-based project called the International Fusion Materials Irradiation Facility–Demo Oriented Neutron Source—should begin operating in Granada, Spain, in the early 2030s. A similar U.S. facility called the Fusion Prototypic Neutron Source has been proposed but doesn’t yet have approval. There is still no guarantee that these material issues can be solved. If they prove insurmountable, one alternative is to make the reactor walls from liquid metal, which can’t be damaged by melting and recrystallization. But that, Cowley says, brings in a whole suite of other technical concerns.

Another major challenge is making the fusion fuel. The world has abundant deuterium: this isotope constitutes 0.015 percent of natural hydrogen, so the seas are literally awash in it. But tritium forms only in small quantities naturally, and it decays radioactively with a half-life of just 12 years, so it’s constantly disappearing and must be produced afresh. In principle, it can be “bred” from fusion reactions because the fusion neutrons will react with lithium to make it. Most reactor designs incorporate this breeding process by surrounding the reactor chamber with a blanket of lithium. All the same, the technology is unproven at large scales, and no one really knows whether or how well tritium production and extraction will work.

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Challenges to fusion are depicted in figure below:

Most important challenges are achieving good confinement (maintain plasma in the magnetic bottle long enough), ignition/Q factor and controlling capex and levelized costs.

Several challenges must be overcome to achieve commercial fusion, and stakeholders’ projections of this timeline range from 10 years to several decades.

One key scientific challenge is in the physics of plasmas, the state of matter needed for fusion. Researchers have made advancements in understanding the behaviour of burning plasmas but lack sufficient experimental data to validate their simulations.

One key engineering challenge is the development of materials that can withstand fusion conditions for decades, such as extreme heat and neutron damage, and no facility exists where materials can be fully tested.

More generally, the task of extracting energy from fusion to provide an economical source of electric power presents several complex systems engineering problems that have yet to be solved.

ITER problems have long been known:

In 2018, the estimated cost of the project was $22 billion. So far, it has also been said that the reactor should go into operation for the first time – with deuterium plasma – in 2025 and run with deuterium and tritium from 2035. Whether and how much this will change is still unclear. In fact, the ITER designers have various problems. On the one hand, the dimensions of two segments of the vacuum vessel deviate too much. On the other hand, there are signs of corrosion on the heat shield – helium could escape there. Last but not least, the French nuclear supervisory authority requires ITER designers to provide evidence of radiation protection measures.

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Challenges to fusion can be categorised in 4 groups:

-1. Physics challenges

-2. Material challenges

-3. Fuel challenges

-4. Engineering challenges

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Physics challenges:

Common responses to fusion range from a motivating “a cutting-edge energy source that isn’t quite ready yet, but will be soon” to the more cynical “the energy source of the future, and it always will be.” From outside of this research community, fusion appears to be an energy source that has not lived up to its potential; a commercial, net-electric fusion reactor does not presently exist. Why has this technology remained elusive? What makes fusion energy so difficult to achieve?

The complexities of plasma physics and fusion energy science are inexhaustibly fascinating in their own right, and there are still unknowns yet to be uncovered and understood. But, when considering the prospect of building a fusion reactor, the fundamental conditions that must be achieved are relatively easy to understand. In a sense, the complexities of fusion energy science regard how to achieve reactor conditions, not what those conditions are. Regardless of how you go about building your working fusion reactor, the fundamental fusion conditions that must be achieved underpin the challenge of fusion energy overall.

One of the central challenges of fusion energy is the temperature required to produce meaningful amounts of fusion power from an ionized gas that is commonly referred to as a plasma. The necessary temperatures for fusion energy production vary depending on the type of fusion being pursued. One of the most popular fusion reactions considered for first generation fusion systems is called deuterium-tritium (DT) fusion. One of the primary reasons for pursuing DT fusion first is that it requires the lowest plasma temperatures to make significant amounts of fusion power when compared to more exotic types of fusion. Even so, the plasma temperatures in the core of an eventual DT fusion reactor will be approximately 150 to 200 million degrees, which is amazingly hot compared to temperatures we encounter in everyday life. These high-temperatures are largely a non-negotiable requirement for fusion in the sense that at substantially lower temperatures, no matter how much fuel you put into the reactor, the amount of fusion power produced will be vanishingly small simply because fusion reactions are not likely to occur at those temperatures.

Assuming a particular plasma temperature in a fusion reactor must be maintained, how do we go about doing so? To begin answering this question, assume that you have a cup of coffee on your desk you would like to maintain at 150 F. If left on its own, the coffee would begin to cool, and continue doing so until it reaches room temperature. The amount of time required for it to cool will depend on how quickly the thermal energy leaks out of the cup into the ambient environment. We know there are ways to extend the time it takes for the coffee to cool, such as putting it in a heat retaining container (e.g. a Styrofoam cup, or a Thermos). But, even when using these thermal insulators, the coffee will eventually cool to room temperature unless we supply energy to the system to keep its temperature at 150 F. A solution for doing so, which also satisfies the everlasting need for coffee during rainy days, is to drink some of your coffee that has cooled slightly below 150 F, and then fill your cup back up with coffee that is warmer than 150 F. With a balance between energy input and output, the coffee temperature can be held nearly constant indefinitely; this thought experiment exemplifies the first law of thermodynamics, otherwise known as conservation of energy.

As simple as this example seems, it captures the salient features of an operating fusion reactor. In this analogy, the temperature of the coffee is to the temperature of the fusion plasma. The rate of leakage of thermal energy out of the coffee cup is called the energy confinement time in a fusion reactor. The warmer coffee that is poured into cup is the energy input into the fusion plasma, which is mostly by charged fusion products (e.g. helium nuclei in DT fusion), and auxiliary heating sources such as microwaves, fast neutral particle beams, or resistive heating from electrical currents flowing in the fusion plasma. All of these energy sources must balance the energy losses from the system to maintain the desired temperatures for fusion to occur. Lastly, the amount of fusion power produced is directly related to plasma density to the second power while operating at optimal plasma temperature of 150 to 200 million degrees.

Thus, in the previous paragraph we identified three quantities of interest for designing a fusion reactor, the plasma density, the plasma temperature, and the energy confinement time. Using the law of conservation of energy, just like in our coffee focused example, the result can be expressed as the product of being greater than or equal to a particular number for fusion “ignition” to occur. Fusion ignition is the point when the system becomes self-sustaining from an energy standpoint. This expression is called the “triple product,” which is also colloquially referred to as the “Lawson criterion” by some in the fusion research community today. This is the most important equation in fusion energy science since it provides the absolute plasma parameters that must be pursued in any fusion reactor system. A commercial fusion system will likely operate below ignition conditions, but it must be close to said parameters to allow for high power “gain,” meaning that significantly more power is produced than is required to keep the fusion reactor operating. The excess fusion power that is produced is ultimately converted into electricity for sale to consumers.

Figure above shows fusion triple-product performance in various devices over time.

Why is fusion energy so challenging to achieve?

The simple answer is that it has been particularly difficult to obtain high enough plasma densities, temperatures, and energy confinement times simultaneously for a reactor to approach ignition conditions. In particular, a sufficiently long energy confinement time (e.g. how quickly our fusion plasma (or coffee) cools) has been especially difficult to achieve while simultaneously reaching the temperatures and densities required for meaningful amounts of fusion power to be produced. However, we have made great strides in our understanding of plasma physics over the past decades and have dramatically improved “triple-product” plasma performance, as can be seen in the figure above. Plotted are the maximum values of triple-products obtained in various fusion experiments, and it is clear remarkable progress has been made. The ITER tokamak experiment, presently under construction in Cadarache, France, is slated to be the first fusion reactor to produce more energy than is put in, and by a factor of ten. ITER will mark another meaningful step towards an eventual commercial fusion power plant, and will be the best performing fusion experiment to date on the “triple-product” basis, as shown in above.

Confinement quality:

Confining a fusion plasma inside a magnetic field is a bit like squeezing water inside a balloon. Differences in pressure, temperature, and density can cause the fields to balloon outward or spring a leak.

Researchers have been able to confine fusion plasmas long enough to generate fusion reactions for many years. However, the quality of plasma confinement—defined as the time required to lose energy to the vessel walls—is a key element in the cost-effectiveness of a hypothetical fusion power plant. This confinement time needs to be long enough to allow sufficient plasma energy to circulate in the confined region so that confined ions are kept hot enough to maintain an appropriate level of fusion. Current devices have managed confinement times of about 0.3 seconds; fusion power plants will likely need times of a few seconds, levels they theoretically should achieve with their larger size and stronger magnetic fields.

Recent studies have identified confinement quality as the most important factor for reducing capital costs, because it has a direct impact on the necessary size of the tokamak as well as other critical elements of the plant, such as the handling of heat and particle loads. Further research is necessary to develop higher-quality confinement solutions that would reduce these costs.

Though high-temperature superconducting materials, which can generate much stronger magnetic fields, have created some excitement in the fusion community, it is not yet known how well these will perform in operation, and studies have suggested that the choice of magnet technology may have relatively little impact on cost-effectiveness.

Unpredictability of plasma:

The difficulty is learning how to hold and confine the super-hot plasma within an electromagnetic field. The plasma behaves like a weather system in terms of being incredibly hard to predict using conventional techniques. Scientists have not been able to control the turbulent plasma as it is heated to hundreds of millions of degrees, and the reaction simply stops. This unpredictability is attributed to the science of Magneto-Hydro Dynamics (MHD) and Gyrokinetics – the state of the plasma is changing all the time. However very recent advances in machine learning techniques may change playing field in favour of scientists.

High magnetic field tokamaks have its own challenges:

David Kramer wrote an interesting report on high-magnetic-field fusion devices for the August 2018 issue of Physics Today. The high magnetic field certainly does shrink the device’s plasma volume, but high magnetic field is a double-edged sword. It has significant disadvantages. The story points out only one: the increased pressure on the field coils.

Another disadvantage is that as one shrinks the device, the neutron wall loading increases. Take SPARC, the tokamak being developed by Commonwealth Fusion Systems (CFS). It has 1/70 the volume of ITER, the international prototype fusion energy reactor, but 10 times the power density. Whereas ITER hopes to achieve 500 MW of neutron power, SPARC hopes to achieve about 70 MW. If one assumes surface area scales as the 2/3 power of volume, SPARC’s surface area is about 1/17 of ITER’s. Hence SPARC, a small experimental device, will have about 2.5 times ITER’s wall loading of about 1 MW/m2! The problem will only get worse as CFS moves to devices like the ARC (affordable, robust, compact) reactor, which will produce commercially interesting amounts of power. Wall loading is a big issue, not a minor detail, in fusion physics.

In addition, whereas the plasma scales to smaller size with increasing magnetic field, the fusion blanket does not. No matter what the magnetic field, the blanket has to prevent leakage of uncharged neutrons out the other end. The minimum blanket thickness we have seen is about one and a half meters thick. The blanket alone dictates some minimum size for a power-producing fusion device. It is difficult to see how shrinking the minor radius to below a meter buys you very much if the blanket thickness is one and a half meters. That could be a problem especially for the Tokamak Energy device, a spherical tokamak, which relies on a thin center post that must remain superconducting in the presence of an intense neutron flux.

In fact, the most important advantage of using high-temperature superconductors (HTS), whether at 5 T or 10 T, is that the magnets could be disassembled and reassembled rather easily.

Energy loss making fusion facility due to physics challenges:

The recent “breakthrough” that NIF announced pertains to the term “physics challenges”. One can identify three stages of physics challenges.

The first challenge is to have enough fusion reactions in the pellet that is blasted by lasers to produce more energy than is put into the target. That was what seems to have been seen at NIF: the reports say that the lasers pumped in 2.05 megajoules of energy and about 3.15 megajoules came out. All of this over a time period of a few nanoseconds (a nanosecond is one billionth of a second). The figure of 3.15 megajoules might seem like a lot but it is only 0.875 kilowatt-hours, that too of heat, which would produce perhaps 0.3 kilowatt-hours of electricity if it was used to boil water and drive a turbine.

The second physics challenge is to produce more energy than is used by the facility as a whole. NIF is far from meeting this challenge. It admitted that just the 192 lasers consumed around 400 megajoules in the process of blasting the pellet. To this, we have to add all the energy that goes into running the other equipment and the facility as a whole.

The final physics challenge is to produce more energy than what is required to construct the facility and all the equipment. In the case of the ITER experiment, for example, it has been estimated that “the tokamak itself will weigh as much as three Eiffel towers i.e., 23,000 tons”. As Daniel Jassby, a retired physicist from the Princeton Plasma Physics Lab, put it, all this “must appear on the negative side of the energy accounting ledger”.

If these physics challenges are not met, of course, then one has a permanent loss-making facility in energy terms. NIF is far from meeting the latter challenges.

Huge parasitic power consumption:

In addition to the problems of fueling, fusion reactors face another problem: they consume a good chunk of the very power that they produce, or what those in the electrical generating industry call “parasitic power drain,” on a scale unknown to any other source of electrical power. An intractable operating expense is the 75-to-100 megawatts-e of parasitic electric power consumed continuously by on-site supporting facilities that must be purchased from the regional grid when the fusion source is not operating.

Fusion reactors must accommodate two classes of parasitic power drain:

First, a host of essential auxiliary systems external to the reactor must be maintained continuously even when the fusion plasma is dormant (that is, during planned or unplanned outages). Some 75-to-100 MWe (megawatts electric) are consumed continuously by liquid-helium refrigerators; water pumping; vacuum pumping; heating, ventilating and air conditioning for numerous buildings; tritium processing; and so forth, as exemplified by the facilities for the ITER fusion project in France. When the fusion output is interrupted for any reason, this power must be purchased from the regional grid at retail prices.

The second category of parasitic drain is the power needed to control the fusion plasma in magnetic confinement fusion systems (and to ignite fuel capsules in pulsed inertial confinement fusion systems). Magnetic confinement fusion plasmas require injection of significant power in atomic beams or electromagnetic energy to stabilize the fusion burn, while additional power is consumed by magnetic coils helping to control location and stability of the reacting plasma. The total electric power drain for this purpose amounts to at least six percent of the fusion power generated, and the electric power required to pump the blanket coolant is typically two percent of fusion power.

In inertial confinement fusion and hybrid inertial/magnetic confinement fusion reactors, after each fusion pulse, electric current must charge energy storage systems such as capacitor banks that power the laser or ion beams or imploding liners. The demands on circulating power are at least comparable with those for magnetic confinement fusion.

The power drains described above are derived from the reactor’s electrical power output, and determine lower bounds to reactor size. If the fusion power is 300 megawatts, the entire electric output of 120 MWe barely supplies on-site needs. As the fusion power is raised, the on-site consumption becomes an increasingly smaller proportion of the electric output, dropping to one-half when the fusion power is 830 megawatts. To have any chance of economic operation that must repay capital and operational costs, the fusion power must be raised to thousands of megawatts so that the total parasitic power drain is relatively small.

In a nutshell, below a certain size (about 1,000 MWe) parasitic power drain makes it uneconomic to run a fusion power plant.

The problems of parasitic power drain and fuel replenishment by themselves are significant. But fusion reactors have other serious problems that also afflict today’s fission reactors, including neutron radiation damage and radioactive waste, potential tritium release, the burden on coolant resources, outsize operating costs, and increased risks of nuclear weapons proliferation.

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Plasma disruptions:

Physicists have been exploring the properties of plasmas within tokamak devices since the 1960s. In tokamaks, disruptions are sudden losses of the thermal and magnetic energy stored within the plasma, which occur when operating near plasma stability limits or when systems malfunction and plasma control is lost. It is well known that beyond certain operational boundary conditions—for example, when plasma current, pressure or density rises too high for a given magnetic field—the plasma can become unstable. A disruption is an instability that may develop within the tokamak plasma. A Plasma instability is a region where turbulence occurs due to changes in the characteristics of a plasma (e.g. temperature, density, electric fields, magnetic fields). Disruptions lead to the degradation or loss of the magnetic confinement of the plasma, and because of the high amount of energy contained within the plasma, the loss of confinement during a disruption can cause a significant thermal loading of in-vessel components together with high mechanical strains on the in-vessel components, the vacuum vessel and the coils in the tokamak. In some cases, because of the large electric fields created during the disruptions, a relativistic electron beam (containing “runaway electrons”) forms that can penetrate several millimetres into the in-vessel components when it is eventually lost from the plasma.

Disruptions are catastrophic events due to the loss of the plasma confinement in a very short time window of the order of milliseconds. Such events stress the conductive walls, including the PFCs, with forces which magnitude grows non-linearly with the main plasma current. At JET, considering a plasma current on axis of 3 MA, forces of about 3 MN, that are comparable to the weight of a F15, have been experienced. ITER is expected to operate at 15 MA, therefore, considering the non-linear dependence on the plasma current, unintentional disruptions could cause serious damages to the device. It is straightforward then to require that such occurrences have to be avoided, at least during high power discharges.

A disruption is an abrupt termination of a tokamak discharge due to the loss of magnetic configuration stability. Usually they occur upon reaching tokamak operational limits: Greenwald density limit, current limit, beta limit, or because of the development of large magnetohydrodynamic (MHD) instabilities. The disruptive event is accompanied by the release of all plasma thermal and electromagnetic energy on a very short time scale compared to the duration of the discharge. In addition to this, eddy currents are induced in the vessel during a disruption and a large fraction of plasma current might be converted to runaway electrons. All these factors result in extreme thermal and structural loadings on the vacuum vessel components. While for small and medium size machines the loadings during disruptions are relatively minor, for large devices such as JET and ITER they are destructive and prevent sustainable operation of the machine. Disruptions might bring severe damage to the vessel, therefore the number of disruptions in future devices has to be limited. At the present time disruptions are common and unavoidable events. However, there is a possibility to ameliorate their consequences or to make a disruption less likely to occur. Unless mitigating action is taken, plasma-facing components can suffer local damage due to the thermal loads and to the deposition of runaway electrons during disruptions. In addition, in extreme cases, the mechanical strains on the components during disruptions may cause some deformation.

Disruptions are probably the most dangerous events preventing a safe and the steady state operation of a tokamak. However other designs of magnetically controlled fusion experiments exist, which might represent alternatives to the tokamak path. The most advanced competitor is the stellarator one, characterized by having field coils properly bended toroidally, shaping directly the magnetic field which confines the plasma. Such design allows avoiding the need of a toroidal current to induce a poloidal component as for tokamaks. Stellarators have a series of advantages, among which the most impressive one is indeed the almost absence of disruptions, due to the combination of the lack the toroidal current and the stabilizing effect of the magnetic shear on MHD instabilities. However, stellarators have both a challenging engineering design, as well as physics issues. Considering the latter, the most important one is due to the fact that at high electron temperature, i.e., low collisionality, the neoclassical transport losses are much higher than the tokamak ones. This is due to the fact that particles can be trapped in banana orbits in tokamaks, because of the axisymmetric magnetic field, while in stellarators particles are trapped in the so-called magnetic ripples, formed by the shape of the coils. Since the poloidal variation of their trajectories is small, such particles can be lost quite easily. However, W7-X has recently shown that it is possible to reduce such losses by working in a optimized neoclassical regime, reaching high temperature plasma conditions in the experimental campaigns conducted in 2017–2018.

Both tokamaks and stellarators are also affected by the so called Edge Localized Modes (ELMs). Edge Localised Mode (ELM) is an instability which occurs in short periodic bursts during the H-mode in divertor tokamaks. It causes sudden outbursts of the plasma thus expelling particles and depositing large heat flux onto the vessel wall. The plasma loses severe amounts of energy. In high-power fusion devices such as ITER or DEMO, ELMs are so powerful that they will cause erosion at the vessel wall. An ELM can expel up to 20 percent of the reactor’s energy. ELMs cause large transient heats and particle loads on the PFCs. Extrapolating the energies associated to each ELMs to ITER, values up to twenty times higher than the benchmarked limits of considered materials are expected. Energy densities up to 11 MJ m^−2 are indeed supposed to impact on the first wall. Therefore, methodologies to either suppress, such as Resonant Magnetic Perturbations, or mitigate, such as ELMs pacing have been tested by scientists in the last decades. Among the latter, ELMs pacing techniques have been extensively tested also on AUG, JET, DIII-D and EAST is thought to be one of the main tools for ELMs mitigation on ITER.

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Material challenges:

The design for the demonstration fusion power plant (DEMO) utilizes the tokamak concept, in which a burning plasma is contained in a torus-shaped vacuum vessel. The fuel – a mixture of Deuterium and Tritium – is heated to temperatures in excess of 150 million °C, forming hot plasma. Strong magnetic fields keep the plasma away from the walls. Fusion reactors experience many commonalities with advanced fission reactors and high power accelerator spallation targets. The operational requirements of the structural materials in fusion power plants are beyond today’s experience. This includes elevated operating temperature, cyclic operation with long hold time, prolonged periods of operation, steep temperature and stress gradients, high neutron irradiation damage and a very high production rates of helium and hydrogen as well as corrosion. Databases supporting mathematical models and designs of future fusion power plant are mainly derived from relatively few tests facilities. Individual countries are exploring ways to facilitate a faster research and technology development and share information in large collaborative programs. For example studies of H isotopes retention (also referred to as “fuel retention”) in Plasma Facing Components (PFC) are carried at the currently world’s largest nuclear fusion experimental reactor JET (Joint European Torus) as a part of the international effort under EUROFusion consortium agreement. There are numerous materials involved in a fusion reactor, ranging from electrical insulators to superconducting magnets. For DT fuels, the tritium is produced from lithium in what is called a blanket region. The blanket region has to be cooled, with the extracted heat going to the power-conversion system (e.g., a steam cycle). And, of course, there needs to be some structure surrounding all of this.

Nuclear fusion reactors are extremely hostile environments for plasma facing materials (PFM) and plasma-facing components (PFC). ITER or DEMO will withstand severe steady-state thermal loads up to about 20 MW m^−2 combined with transient ones up to GW m^−2 due to edge-localized modes (ELMs). In addition, off-normal events such as disruptions or vertical displacement events (VDEs) could take place, compromising the mechanical integrity of the reactor. Thermally induced erosion of plasma-facing material (PFM) and damage of the joints between the PFM and the heat sink are to be considered; material irradiation with hydrogen isotope ions (D+ and T+) and impurities’ particles will create hydrogen-induced and neutron-induced degradation of the wall, transmutation, and activation. The appropriate choice of PFM, the design and the joining technique of PFC are a challenging issue for fusion reactors’ successful operation.

Challenges for plasma-facing materials and components:

The interaction processes between the burning plasma and the first wall in a fusion reactor are diverse: the first wall will be exposed to extreme thermal loads of up to several tens of megawatts per square meter during quasistationary operation, combined with repeated intense thermal shocks (with energy densities of up to several megajoules per square meter and pulse durations on a millisecond time scale). In addition to these thermal loads, the wall will be subjected to bombardment by plasma ions and neutral particles (D, T, and He) and by energetic neutrons with energies up to 14 MeV. Hopefully, ITER will not only demonstrate that thermonuclear fusion of deuterium and tritium is feasible in magnetic confinement regimes; it will also act as a first test device for plasma-facing materials (PFMs) and plasma-facing components (PFCs) under realistic synergistic loading scenarios that cover all the above-mentioned load types.

The plasma-facing wall of future thermonuclear fusion reactors with magnetic confinement such as ITER or DEMO must withstand harsh loading scenarios. The so-called plasma–wall interaction (PWI) processes that are crucial at the interface between the hot plasma and the wall are associated with quasistationary thermal loads up to about 20 MW m^−2 combined with short, extremely strong thermal transients up to the gigawatts per square meter range during edge-localized modes (ELMs). In addition, irradiation effects resulting from the plasma species and the 14 MeV neutrons have a strong impact on the integrity of the wall armor materials. Therefore, synergistic effects resulting from simultaneous thermal, plasma, and neutron wall loads must also be evaluated in complex experiments. Under reactor-relevant conditions, the following are the most serious damaging mechanisms: thermally induced defects such as cracking and melting of the plasma-facing material (PFM); thermal fatigue damage of the joints between the PFM and the heat sink; hydrogen-induced blistering; helium-generated formation of nanosized clusters; and neutron-induced degradation of the wall armor via reduction of the thermal conductivity, embrittlement, transmutation, and activation.

Synergistic loading scenarios with a strong impact on performance and lifetime of PFCs:

Up to now, materials research in the field of thermonuclear fusion has been done primarily in laboratories and in test facilities that have focused primarily on individual effects only, such as thermal fatigue, thermal shocks during transient events, plasma exposure, and neutron irradiation tests. Today, emphasis is also laid on synergistic effects such as high thermal loads under plasma exposure or simultaneous thermal and neutron wall loads as seen in the figure below, as described in a study ‘challenge for plasma-facing components in nuclear fusion’, 2019.

Synergies between thermal loads plus plasma exposure: Here, plasma-induced processes such as blister or bubble formation, hydrogen embrittlement, or the growth of He nanobubble layers or of fuzz on the plasma-exposed surface have a negative impact on resistance to intense thermal loads.

Synergies between thermal loads plus neutrons: Neutron-induced material degradation such as reduced thermal conductivity, transmutation effects, embrittlement, and increased ductile–brittle transition temperature have a strong impact on the high-heat-flux performance of wall components, both under steady-state conditions and under intense transients.

Synergies between plasma exposure plus neutrons: In this area, future research should be directed toward the trapping of H and He in neutron-induced defects, the formation and influence of transmutation products, etc.

In a nutshell, material degradation is accelerated when synergistic effects are taken into consideration.

The neutron radiation produced by DT fusion is an order of magnitude more energetic than that produced by nuclear fission. In addition, the helium generated by the reaction, as well as excess heat and other impurities in the plasma, must be removed on an ongoing basis during operation. This exhaust path will be subject to extremely high temperatures and particle bombardment. No materials currently exist that can be confidently relied upon to survive these conditions over the life of a commercial power plant. Developing them is an active area of research, with work exploring new alloys, better materials, and even liquid surfaces and candidate solutions. Better understanding of how these materials behave in the reactor environment and their interaction with fusion performance is necessary.

The environment for fusion is a rather unique performance arena and poses a number of challenges. A DT reactor, for example, will experience unprecedented neutron radiation damage, on the order of 2 to 10 times greater than what core internal structures of existing fission reactors must cope with. In order for fusion to be economically competitive and acceptable to the public, the reactor structure must be made of low-activation materials (i.e., materials that will not become highly radioactive). There will also be quite high particle- and heat-flux conditions, and high operating temperatures will cause thermal creep. Unfortunately, in a lot of cases, materials developed for other technologies can’t be straightforwardly applied to fusion energy.

There are three specific areas where materials impact fusion reactor design: the plasma-facing region, where there is high heat flux and particles are impacting the metal structure; the plasma-diagnostic, heating, and magnet systems; and the structure of the blanket and first-wall region surrounding the plasma, which is the heart of the heat-extraction system.

Heat fluxes for fusion energy cover a wide spectrum. At the extreme end, when there is a disruption condition, the fluxes can be very high (about 103-104 MW/ m2). Fortunately, these occur for only a millisecond or so. Under steady-state conditions, the heat fluxes (about 1-10 MW/m2) are comparable to what you might find in a rocket nozzle application, or a bit higher than what occurs in a fission reactor.

Because the cost for fusion energy is going to be largely driven by the size of the plant, the reactor should be made as compact as possible. However, as you start decreasing the size, the heat flux on the first wall increases. Straightforward analyses reveal the maximum steady-state heat flux to be about 1-10 MW/m2 for a reasonable wall thickness (about 5 mm) in a fusion reactor, depending on the material.

Today, tungsten is considered to be the most reliable material for high-heat-flux components in future fusion reactors owing to its high melting point (Tmelt = 3422 °C) and a thermal conductivity of approximately 160 W m^−1 K^−1. For ITER and other large-scale confinement experiments, alternative candidate materials based on beryllium and carbon-fiber composites (CFCs) show promise. A major drawback of the application of beryllium in PFM is its relatively low melting point (Tmelt = 1287 °C).

Beside beryllium, carbon (in particular, fiber-reinforced graphite) is the most frequently used PFM in today’s magnetic confinement experiments. Depending on the selected fiber type and architecture, carbon-fiber reinforced graphite can be manufactured with thermal conductivities equal to or even better than that of copper (up to about 400 W m^−1 K^−1). However, such an excellent thermal conductivity will degrade rapidly under the influence of energetic neutrons. In D-T-burning fusion reactors with carbon walls, tritium-containing hydrocarbon deposits are formed on all in-vessel components. This will finally result in an inacceptable tritium inventory in the fusion reactor under current licensing laws and limits. For these two reasons, carbon has been discarded as a PFM for ITER.

Another technique that is being looked at is to put a fast-flowing liquid coolant on the first surface. This would be a very efficient way to take away the heat, but there are a number of technological challenges. For example, it’s hard to see how this could be applied to a toroidal geometry. This approach is being seriously considered in inertial confinement schemes, where there is a more suitable reactor shape.

Another of the key factors for plasma-facing materials is that neutral particles coming out of the plasma can sputter the surface atoms. In addition to erosion concerns, these sputtered atoms contaminate the plasma, since they absorb heat from the plasma fuel but do not fuse. The plasma power loss is proportional to NiZi, where Ni is the number of sputtered impurity atoms and Zi is the corresponding atomic number.

If you look at the sputtering behavior of various materials at fusion-relevant conditions (10-1,000 eV hydrogen ion energies), stainless steel is one of the worst possible plasma-facing materials. It has a high sputtering yield (probability of a surface atom being sputtered into the plasma by an incident hydrogen ion) and a high atomic number, making it impossible to use in the first wall of a fusion reactor due to unacceptable plasma power losses. The most attractive plasma-facing candidates at the present time are either beryllium or carbon, because of their low atomic number, or something like tungsten, because of its low sputtering yield.

On the one hand, materials research for plasma-facing components (PFCs) has become of great importance owing to the construction of ITER in Cadarache, France. On the other hand, materials research activities in the inertial confinement fusion (ICF) field are still rather scarce, although urgent material solutions will be needed the closer the performance of laser- or ion-driven fusion experiments approaches break-even. There is no doubt that the development of actively cooled wall targets will benefit significantly from progress in both the magnetic and inertial confinement fusion fields. The development of new plasma scenarios, ELM-suppression techniques, etc., will expose targets to less severe conditions, and better target design (improved geometries, new materials, etc.) will benefit the field of fusion in general. The loading conditions in both scenarios are similar (especially with regard to hydrogen, helium, and neutron loads); nevertheless, transient thermal loads differ greatly depending on the selected ICF concept and the operational situation.

Reactor Ports material:

One example of a success story is in the materials used to make the ports through which the energy beams are sent to heat up the plasma. The windows are needed in order to avoid atmospheric contamination of the plasma. One heating technique, electron cyclotron heating, uses high-frequency (about 140 GHz) radio waves. Up until about 5 years ago, engineers were limited in terms of the power density they could get through this window, since there was no available material that could withstand a high-power radio beam.

Recently, there have been some dramatic advances in chemical vapor deposition (CVD) diamond. The cost has dropped by more than an order of magnitude in the last 5 years and is expected to drop by another order of magnitude in the next 2 years with the construction of large-scale production plants. At the same time, the quality of the CVD diamond films has dramatically improved (in part due to the application of plasma technologies in the CVD process). Now, it is possible to purchase 46-inch-diameter free-standing diamond wafers that can be put in these beam ports.

Insulators:

Radiation can also have a negative effect on insulators, which are essential for monitoring and heating the plasma in a fusion reactor. If you expose any insulating material to an ionizing field, its electrical conductivity increases in a linear fashion. Conductivity increases of over 10 orders of magnitude have been observed at high dose rates. Fortunately, insulators such as aluminum oxide retain enough of their insulating properties to be useful for fusion applications. A few years ago, there was concern that nobody had ever taken one of these insulators and exposed it for a long time to an electric field during irradiation to simulate in-service conditions. Some electron-beam studies suggested insulators would suffer rapid electrical breakdown. If this were to happen, the fusion reactor would not operate.

Structural materials:

The structural materials used in a fusion reactor must address a number of concerns. They need to have a high thermal-stress capability. They have to be compatible with the coolant, which may be a liquid metal such as lithium or a gaseous coolant (He). They should be passively safe under accident conditions, environmentally friendly, and able to resist radiation damage. There are only a few structural materials that can adequately address all of these concerns. The three main candidates are ferritic martensitic steel, vanadium-based alloys, and SiC-fiber-reinforced SiC composites.

Many high performance structural materials used in other industries are not suitable for fusion applications. For example, titanium alloys used for aerospace applications have a very high solubility for hydrogen. This results in a lot of tritium leaving the plasma and entering the structure, creating an unsafe condition. (If there were an accident, the tritium could be released into the atmosphere.) Nickel-based superalloys are great materials in many respects, but when subjected to radiation, they have significant grain-boundary embrittlement problems.

Radiation can produce large changes in structural materials. At low temperatures (less than 0.3 Tm, where Tm is the melting temperature), the main concern is radiation hardening and embrittlement. As you go up in temperature, there is a phenomenon called radiation creep, which acts on top of thermal creep and can limit the amount of stress that can be put on the structure. Volumetric swelling is a significant concern for certain materials at intermediate temperatures (0.3-0.6 Tm). And, at very high temperatures (>0.45 Tm), there can be pronounced helium embrittlement at grain boundaries. So, the radiation environment in a fusion reactor is quite a bit more severe than it is for structural materials in existing fission reactors, and the challenges for materials scientists are also greater.

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Materials challenges for successful roll-out of commercial fusion reactors, a 2022 study:

As members of the UK fusion community (covering national laboratories, academia and industry), authors approached various colleagues to put together articles for this special issue of the Journal of Physics: Energy covering the materials challenges through to successful roll-out of fusion reactors. This paper serves to introduce the special issue and gives our opinion on the key challenges, many of which are covered in more detail in the submitted papers. Others may have differing opinions about what the key challenges are, but what authors will all agree on is that they are substantial and will require sizeable resources to be addressed. Further, while authors are all UK-based, all humankind will benefit from successful commercial roll-out of fusion for energy production, and the effort has been and will continue to be global. Fusion has entered the engineering era. Moving from plasma science to experiments demonstrating the benefits of modified torus shapes and advanced divertor geometries, the ‘field’ has become an ‘industry’. Investors now focus on whether superconducting magnet joints are feasible in large tokamak designs and how to deliver net energy to the grid. As with all technology trajectories, materials (both structural and functional) are the key enablers. For fusion materials, the three major challenges remain resilience to the combined damaging effects of tritium, transmutation and neutron bombardment (a veritable ‘triple whammy’), achieving suitable irradiation strategies for adequate damage studies (with optimal use of modelling as complementary science) and defining material safety and waste guidance in an era of evolving regulation. Authors have highlighted issues around ‘the triple whammy’, the resulting need for testing facilities and modelling proxies, and aspects of regulating materials in, and waste generated from, operating fusion reactors.

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Fuel challenges:

The deuterium-tritium reaction is favored by fusion developers because its reactivity is 20 times higher than a deuterium-deuterium fueled reaction, and the former reaction is strongest at one-third the temperature required for deuterium-only fusion. In fact, an approximately equal mixture of deuterium and tritium may be the only feasible fusion fuel for the foreseeable future. While deuterium is readily available in ordinary water, tritium scarcely exists in nature, because this isotope is radioactive with a half-life of only 12.3 years. The main source of tritium is fission nuclear reactors. Current methods rely on extraction from the coolant in heavy-water reactors or bombardment of lithium targets in light-water reactors.

A single 500-MW fusion power plant is expected to require about 35 kilograms (kg) of tritium fuel per year. This amount is not only far in excess of current global production capacity (which is roughly 2–3 kg/year from aging facilities at CANDU reactors in Canada and South Korea), it also represents a cost factor that would reach into billions of dollars. Thus, fusion power plants will need a method to breed tritium in situ. Fortunately, the fusion reaction itself offers a potential means to do so. Placing a lithium blanket around the tokamak would generate tritium (and further heat) as the fusion neutrons are captured by the lithium nuclei and spontaneously transition to tritium. Technology solutions to capture this tritium during operation are in development.

Tritium fuel cannot be fully replenished.

The tritium consumed in fusion can theoretically be fully regenerated in order to sustain the nuclear reactions. To accomplish this goal, a lithium-containing “blanket” must be placed around the reacting medium—an extremely hot, fully ionized gas called a plasma. The neutrons produced by the fusion reaction will irradiate the lithium, “breeding” tritium.

But there is a major difficulty: The lithium blanket can only partly surround the reactor, because of the gaps required for vacuum pumping, beam and fuel injection in magnetic confinement fusion reactors, and for driver beams and removal of target debris in inertial confinement reactors. Nevertheless, the most comprehensive analyses indicate that there can be up to a 15 percent surplus in regenerating tritium. But in practice, any surplus will be needed to accommodate the incomplete extraction and processing of the tritium bred in the blanket.

Replacing the burned-up tritium in a fusion reactor, however, addresses only a minor part of the all-important issue of replenishing the tritium fuel supply. Less than 10 percent of the injected fuel will actually be burned in a magnetic confinement fusion device before it escapes the reacting region. The vast majority of injected tritium must therefore be scavenged from the surfaces and interiors of the reactor’s myriad sub-systems and re-injected 10-to-20 times before it is completely burned. If only one percent of the unburned tritium is not recovered and re-injected, even the largest surplus in the lithium-blanket regeneration process cannot make up for the lost tritium. By way of comparison, in the two magnetic confinement fusion facilities where tritium has been used (Princeton’s Tokamak Fusion Test Reactor, and the Joint European Torus), approximately 10 percent of the injected tritium was never recovered.

To make up for the inevitable shortfalls in recovering unburned tritium for use as fuel in a fusion reactor, fission reactors must continue to be used to produce sufficient supplies of tritium—a situation which implies a perpetual dependence on fission reactors, with all their safety and nuclear proliferation problems. Because external tritium production is enormously expensive, it is likely instead that only fusion reactors fueled solely with deuterium can ever be practical from the viewpoint of fuel supply. This circumstance aggravates the problem of nuclear proliferation.

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Two private fusion efforts have decided to simply forgo tritium fuel. TAE Technologies, a California startup, plans to use plain hydrogen and boron, whereas Washington state startup Helion will fuse deuterium and helium-3, a rare helium isotope. These reactions require higher temperatures than D-T, but the companies think that’s a price worth paying to avoid tritium hassles. “Our company’s existence owes itself to the fact that tritium is scarce and a nuisance,” says TAE CEO Michl Binderbauer.

The alternative fusion reactions have the added appeal of producing fewer or even no neutrons, which avoids the material damage and radioactivity that the D-T approach threatens. Binderbauer says the absence of neutrons should allow TAE’s reactors—which stabilize spinning rings of plasma with particle beams—to last 40 years. The challenge is temperature: Whereas D-T will fuse at 150 million degrees Celsius, hydrogen and boron require 1.5 billion degrees.

Helion’s fuel of deuterium and helium-3 burns at just 200 million degrees, achieved using plasma rings similar to TAE’s but compressed with magnetic fields. But helium-3, although stable, is nearly as rare and hard to acquire as tritium. Most commercial sources of it depend on the decay of tritium, typically from military stockpiles. Helion CEO David Kirtley says, however, that by putting extra deuterium in the fuel mix, his team can generate D-D fusion reactions that breed helium-3. “It’s a much lower cost system, easier to fuel, easier to operate,” he says.

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Engineering challenges:

There are some really hard problems that block practical fusion, with no immediate solutions in sight. The only place we know of working, large-scale, energy-generating fusion reactions is at the center of a star, where the gravity of the massive star creates the necessary density, temperature, and time being held in that state at the core for fusion reactions to run continuously. This is called the Lawson criteria, and is hard to achieve. All of the layers of the star above that act as an energy exchanger, converting all the gamma rays and fast neutrons at the core into progressively less energetic wavelengths of light and motion of nuclei (heat) so by the time they reach the surface of the star, that energy is emitted as IR and visible light that heats and powers our planet’s biosphere, and can provide photovoltaic energy to us as well. All of our efforts in fusion have been to try and reproduce this process on earth, in a small reactor that can provide practical energy.

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-1. Magnetic Confinement Fusion:

Tokamak configuration has a donut-shaped fusion chamber surrounded by magnetic coils that generate a very strong magnetic field that compresses and contains a hot, burning plasma, and is there to keep it at the right density and temperature for long enough for fusion reactions to occur.

The first problem with magnetic confinement is fundamental – no matter how strong or well formed the magnetic field containing the plasma is, it will always leak, as positive nuclei or ions spiralling around the magnetic field lines collide and scatter, eventually drifting out of the containment field. The only known solution is to make the reactor larger so the scattering ions take longer to travel to the plasma boundary, and thus more fusion can happen during that time. The world’s most advanced magnetic confinement fusion power plant is ITER, which, to reduce the ion scattering problem mentioned above, is 6-stories tall and about the same dimension in diameter, containing the mass of 3 Eiffel towers, and it is still not expected to be large enough to contain a plasma long enough to sustain burning and produce continuous energy generation. Experience building other fusion reactors suggests that when machine size is doubled one achieves 8 times improvement in heat confinement.

The problem: loss of D+T:

Fusion uses hydrogen isotopes in a plasma, a kind of flowing gas that responds to electric fields, like the gas in fluorescent lights. A fusion reactor heats that gas to extremely high temperatures and compresses it with magnets.

But some of the material leaks out and slams into the tungsten armor tiles that line the walls inside of the donut-shaped reactor. A fusion reactor produces neutrons, which can penetrate deep into the walls and create a pathway for the deuterium and tritium to follow. That’s an inefficiency in the process and a possible safety issue. We are interested in looking at how much of that deuterium and tritium, that should be used as fuel is instead getting stuck in the tungsten tiles, and how deeply it is trapped in the tile.

From a regulatory point of view, operators are limited to a certain amount of deuterium and tritium in a facility. But those isotopes are intended to be used as the energy source. If some of it is stuck in the walls of the reactor vessel, the deuterium and tritium aren’t available to create heat and eventually electricity. Yet it still counts against the operator’s regulated limit. That’s an efficiency issue that must be overcome.

Permeation or penetration will occur. Knowing to what extent and how deep it is can allow scientists to engineer the systems to account for the loss of isotopes.

If the deuterium or tritium is going to permeate through the first layer, then we need to control that at the second layer behind that.

A more practical problem is how do we extract the energy?

Most of the energy in deuterium-tritium fusion reactions is released in fast, high-energy neutrons, which (because they are neutral, and have no electric charge) are not confined by the magnetic field, don’t heat up the plasma, and have to be stopped by a thick shield, which then heats up, and can vaporize water to steam in order to power turbines and electrical generators. The problem is the constant bombardment by neutrons makes the shield material degrade with time, and become very radioactive, posing a problem for removal and disposal.

Also, deuterium-tritium fusion reactions are currently used because they take place at the lowest energies and at the lowest plasma temperatures of all the possible fusion fuels, and thus are easier to ignite and keep lit. However, tritium is not found naturally in any significant quantities and needs to be manufactured in nuclear reactors. With today’s production capacity, we just cannot make enough tritium to power even ITER continuously. We could use better fuels, like Helium-3 or Boron-11, which do not emit neutrons and thus ‘burn clean’, but the plasma temperature must be much higher and the confinement much better to ignite fusion and sustain it with these fuels. And while Boron-11 is plentiful, Helium-3 is so rare that scientists think that the best source of it is lunar regolith that has been irradiated for billions of years on the moon’s surface. Any energy generation infrastructure that requires a regular supply of a moon-based fuel, even as energy dense as He3, will probably never work out.

A proposed solution to the Tritium deficit is to construct the shield surrounding the reactor with lithium, which is abundant, and fissions into tritium when struck by neutrons, thereby ‘breeding’ tritium within the reactor itself. Of course, this comes with its own set of problems of how to capture, purify, and contain the tritium after it is released into the tokamak reaction chamber from the walls.

Figure below shows various fusion fuel cycles:

Impurities in plasma:

The laws of physics dictate the maximum plasma density that can be achieved for a given current in a tokamak, which means that in ITER—as in other tokamak devices—there will be an upper limit to the number of atoms that can be confined.

Within this limit, it is important that the plasma contain as many atoms as possible that are capable of reacting to produce fusion—in ITER’s case, atoms of deuterium and tritium.

Even in trace amounts, other atoms (“impurities”) dilute the core of the plasma by taking the space that could be occupied by the fusion fuels, resulting in fewer reactions and a reduction in energy production. And because fusion reactions occur in a roughly proportional manner to the square of fuel density, the “multiplier” effect sets in quickly—fewer fuel atoms result in a dramatic drop-off in fusion reactions, while more fuel results in a rapid increase.

Impurities originate from vacuum vessel and the in-vessel component materials … iron from the steel components, beryllium from the top layers of the first-wall panels protecting the vacuum vessel, and tungsten from the divertor targets.

Impurities not only dilute the plasma but—depending on the physical properties of the atoms involved (the number of electrons)—they can also cool it to differing degrees. The process is similar to that in a fluorescent lamp. The electrons of the impurity atoms run into the electrons in the plasma and drain their energy, re-emitting it as electromagnetic radiation—including visible light.

The heavier elements, in particular, drain a lot of energy from the plasma through radiation because of a high number of electrons (tungsten has 74). The energy lost through impurity radiation cools the plasma down and the fusion reactions stop.

In ITER, to keep these radiative losses to a minimum, the divertor will be working from its position at the bottom of the machine to continually exhaust impurities from the plasma and limit contamination.

The very properties that make impurities unwelcome in the core of the plasma, however, can be applied to beneficial effect in the plasma edge region.

Because the energy confinement provided by the machine’s magnetic fields is not perfect, large power fluxes can find their way to the edge of the plasma and onto the divertor targets. To avoid localized depositions that would be too high for the material components to withstand, scientists will inject impurity gases at the plasma edge. The radiative properties of the impurities will act to reduce the power fluxes to the material elements by dissipating their energy over a larger zone.

As the plasma in this edge/divertor region is already at temperatures much lower than those required to produce fusion power, this plasma cooling will not affect fusion power production in ITER.

Dust:

Small particles (dust) exist in magnetic confinement fusion devices. Their origin is due to plasma–surface interactions. Dust particles may contain significant amounts of hydrogen isotopes, 50% of which will be tritium in future devices. It is important to assess and understand the processes by which dust is formed and by which it interacts with the fusion device and its plasma. Dust may be a safety hazard due to its high chemical reactivity and due to the mobile tritium inventory. Dust may influence the plasma performance and the operation of fusion devices.

Power Generation:

To be useful as a power plant, a fusion reactor obviously must generate electricity. Fusion researchers generally envision that heat from the tokamak will be used to drive turbine generators, but exactly how the heat off-take will function is still a matter requiring considerable engineering. While in a sense this is the most conventional part of the power plant design, the technological challenges remain significant, as for high efficiency, the device must operate at high temperatures. Most current designs envision a helium loop that would extract heat from the lithium blanket, and either drive a turbine directly or generate steam in a secondary loop.

Another issue is that when all of the superconducting magnets in ITER are running at full current (45 kA central solenoid, 68 kA toroidal field coil) to create the 13 Tesla toroidal magnetic field (13x that in an MRI machine) and to create the other plasma-shaping and heating fields, they are storing 60 GigaJoules, or around 12 Tons of TNT worth of energy. This is because the 180 kilometers of superconducting Niobium-Tin wires in all these massive magnet coils can carry enormous electrical current when supercooled with liquid helium.

But, if that cooling fails, the superconductor heats up, quenches, and becomes a normal conductor, and can no longer carry that enormous current. With 68,000 amps suddenly meeting resistance, the coil rapidly vaporizes, and causes a meltdown of the other coils, with a total energy release of 12 tons of TNT. This is very undesirable, especially when those coils are wrapped around a highly radioactive shield that also vaporizes and is expelled into the atmosphere. There are fault-detection systems, but it takes a while to dissipate that amount of energy safely, and there may not be enough time to react quickly enough in the event of a catastrophic magnet coil failure.

So, some disconcerting challenges are still there to be solved for practical fusion energy generation with magnetic confinement, with some perhaps not so easily solved.

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-2. Inertial confinement fusion:

The physics of laser-driven fusion is so complex and multifaceted that computer simulations of it often take more time than actual experiments. Early on, modellers were more often learning from the experiments rather than telling the experimenters what to do next. An increasing closeness between model prediction and experimental outcome has underpinned the recent success at NIF and bodes well for future improvements in target design.

Here powerful lasers are focused in on a tiny pellet of deuterium + tritium fuel to compress it very quickly and cause it to reach the required temperature and density (Lawson criteria) needed to undergo fusion, in the same manner as a thermonuclear weapon (or H-Bomb), but on a much smaller scale as seen in the figure below. However, there are fundamental problems with imploding the DT fuel pellet evenly, as the plasma becomes very unstable once compression starts, and unless the laser beams used are perfectly aligned and perfectly even (or flat), it is kind of like squeezing Jello with your fingers, and the plasma kind of bulges out where the lasers are slightly less intense, and it does not reach the ignition criteria before it is squeezed out of the gaps.

The “engineering challenge” that revolves around: how do you convert this experimental set up that produces energy for a microscopic fraction of a second into a continuous source of electricity that operates 24 hours a day and 365 days per year. To do that, these fusion reactions should occur several times each second, each second of the day, each day of the year. As of now, the lasers can fire only once a day, at a single target. To move from that state to what is required will need an improvement by a factor of over 500,000 (assuming around six shots per second).

But it is not just firing the laser. Each of these explosions produces a large amount of debris, which would have to be cleared. And then a new pellet has to be placed with utmost precision at the very spot where the lasers can focus their beams.

If all of this is not trouble enough, there is fuel procurement. NIF uses a “gold cylinder with a frozen pellet of the hydrogen isotopes deuterium and tritium”. Deuterium and tritium are isotopes of hydrogen. Deuterium is quite common but tritium is very scarce, because it decays radioactively with a half-life of only around 12 years. Fusion proponents often talk about generating tritium in situ, but this is an exceedingly difficult task. ICF also has many problems with energy conversion to electricity, and much worse wear and tear on the shielding from these small nuclear fusion explosions being set off, which also trashes all the precision equipment needed to hold the fuel pellet in the focus of the converging laser beams with extreme accuracy.

Another major limitation is that the NIF laser requires 300 million Joules of electrical input to provide two million Joules of laser light output—less than 1% efficiency. So the target would have to produce an unfeasibly large gain in order to produce more energy than went into powering the laser used in this instance.

However, the NIF laser is based on technologies that hark back to the 1980s. It uses flash lamps and amplifiers made from slabs of glass doped with the rare-earth element neodymium.

Modern high-power lasers using semiconductor technology can do far better, reaching around 20% efficiency. Given that laser-driven fusion targets are expected to be able to produce gains in excess of 100 when working optimally, using modern lasers would produce significant net energy output.

Another challenge for laser-driven fusion is bringing down the cost of the targets. The manpower involved in making the NIF targets means that each one costs as much as a brand new car.

A new target is required every time the laser fires. For actual power production, this would mean a new one several times a second. The targets used on NIF also rely on a technique known as “indirect drive” in which the target first converts the laser energy into X-rays that then implode the fusion fuel capsule inside the target. This adds both complexity and cost.

Many scientists consider that the way forward for laser-driven fusion energy would involve “direct drive” ignition. Here, the laser directly illuminates a simple, spherical fuel capsule. This approach to ignition has, however, yet to be demonstrated.

NIF’s fuel (deuterium and tritium) gives out much of its energy in the form of high-energy neutrons (particles which make up the atomic nucleus along with protons). The neutrons interact with the materials in the reactor vessel, changing their composition and microscopic structure.

This could pose serious challenges for optical components that must transmit or reflect laser light efficiently. Some scientists consider driving similar physics by alternative means, perhaps using pulsed electrical power directly, or focused beams of ions (charged atoms).

Even if one were to adopt the approach of watching superhero movies and willingly suspend disbelief to assume that all these engineering challenges are solved, then there is an even more difficult challenge: to make this incredibly complicated process into an economically competitive way of generating electricity. If one goes by history, the last could be a killer as has been the case with nuclear fission power, which is a far easier process in comparison to fusion.

Thus, recent advances can better be described as “micron-stones”, to coin a term, rather than milestones, and that too on a path that might never lead to economical electricity generation.

In the meanwhile, this recent NIF experiment is far more likely to be useful to nuclear weapons designers. Currently the main application of inertial confinement fusion is as an experimental testbed for calibrating computer codes used to simulate and design thermonuclear fusion weapons. As horrific as these weapons may be, the simulation codes developed for designing them, are the one of the greatest tools for innovation in fusion energy! There is a hydrodynamics code which simulates the behavior of the fissile (and fusion) materials under extreme conditions of pressure and temperature, as well as transport codes that model neutron transport and scattering in the materials under these conditions, and many other bits and bobs. They are all standard numerical techniques, and the really classified part is the integration and calibration of all these codes with previous nuclear weapon tests from the past, and with these small scale simulations in the present.

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So, in summary, we should not shut down or stop building today’s fission nuclear reactors, as commercial fusion reactors are not only far in the future, but also produce radioactive waste (their shielding, which needs to be periodically replaced), and Tokamaks can melt down catastrophically and explode if there is a cooling failure in the magnet coils during full power operation. They also do not run on seawater, like some say. We need expensive and difficult to produce tritium (which is also radioactive) to ignite fusion at the temperatures and densities that today’s tokamak magnetic fields can produce.

To advance fusion energy in your career, the best fields to study would be physics (classical, electrodynamics, plasma, quantum, nuclear…) and computer science, with a focus on numerical simulation. The actual physical fusion reactors are so expensive and time-consuming to build, test, and operate, that you may only get to iterate designs once or twice in a career, but computer simulations can take us in so many different directions more cheaply and easily by comparison.

Perhaps there is some exotic stellarator magnetic confinement design, or advanced Farnsworth electrostatic confinement design that has not been explored in depth, that will turn out to hold the key to fusion energy. Simulation is probably the best first step to find it, unless you have a few billion dollars and several hundred kilometers of Niobium-Tin wire and helium cooling tubing to play with.

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Section-12

Cost of nuclear fusion:

Economics of Nuclear Power:

Nuclear power plants are expensive to build but relatively cheap to run. In many places, nuclear energy is competitive with fossil fuels as a means of electricity generation. Waste disposal and decommissioning costs are usually fully included in the operating costs. If the social, health and environmental costs of fossil fuels are also taken into account, the competitiveness of nuclear power is improved.

The basic economics metric for any generating plant is the levelized cost of electricity (LCOE). It is the total cost to build and operate a power plant over its lifetime divided by the total electricity output dispatched from the plant over that period, hence typically cost per megawatt hour. It takes into account the financing costs of the capital component (not just the ‘overnight’ cost).

On a levelized (i.e. lifetime) basis, nuclear power is an economic source of electricity generation, combining the advantages of security, reliability and very low greenhouse gas emissions. Existing plants function well with a high degree of predictability. The operating cost of these plants is lower than almost all fossil fuel competitors, with a very low risk of operating cost inflation. Plants are now expected to operate for 60 years and even longer in the future. The main economic risks to existing plants lie in the impacts of subsidized intermittent renewable and low-cost gas-fired generation. The political risk of higher, specifically-nuclear, taxation adds to these risks.

Assessing the costs of nuclear power:

The economics of nuclear power involves consideration of several aspects:

Capital costs, which include the cost of site preparation, construction, manufacture, commissioning and financing a nuclear power plant. Building a large-scale nuclear reactor takes thousands of workers, huge amounts of steel and concrete, thousands of components, and several systems to provide electricity, cooling, ventilation, information, control and communication. To compare different power generation technologies the capital costs must be expressed in terms of the generating capacity of the plant (for example as dollars per kilowatt).
Plant operating costs, which include the costs of fuel, operation and maintenance (O&M), and a provision for funding the costs of decommissioning the plant and treating and disposing of used fuel and wastes. Operating costs may be divided into ‘fixed costs’ that are incurred whether or not the plant is generating electricity and ‘variable costs’, which vary in relation to the output. Normally these costs are expressed relative to a unit of electricity (for example, cents per kilowatt hour) to allow a consistent comparison with other energy technologies.
External costs to society from the operation, which in the case of nuclear power is usually assumed to be zero, but could include the costs of dealing with a serious accident that are beyond the insurance limit and in practice need to be picked up by the government. The regulations that control nuclear power typically require the plant operator to make a provision for disposing of any waste, thus these costs are ‘internalized’ as part of operating costs (and are not external).
Other costs such as system costs and nuclear-specific taxes.

In a nutshell

Nuclear power is cost competitive with other forms of electricity generation, except where there is direct access to low-cost fossil fuels & subsidized renewables.
Fuel costs for nuclear plants are a minor proportion of total generating costs, though capital costs are greater than those for coal-fired plants and much greater than those for gas-fired plants.
System costs for nuclear power (as well as coal and gas-fired generation) are very much lower than for intermittent renewables.

An analysis by the Brattle Group in 2016 showed that zero-emission credits for nuclear power could secure the economic viability of nuclear plants in competition with subsidized renewables and low-cost gas-fired plants. The actual near-term shortfall for a distressed nuclear plant tends to be relatively modest – typically around $10/MWh, which translates to $12 to $20 per ton of avoided CO2, depending on the size of the shortfall and the carbon-intensity of the affected region. This cost compares favorably with other carbon abatement options, the estimated social cost of carbon, and the cost of state policies designed to reduce CO2 emissions from the power sector. These findings demonstrate that the retention of existing nuclear generating plants, even at a modest premium, represents a cost-effective method to avoid CO2 emissions and enable compliance with any future climate policy … at reasonable cost. Sustaining nuclear viability in the interim will reduce near-term emissions, and is a reasonable and cost-effective insurance policy in the longer term.

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Comparing the economics of different forms of electricity generation:

In 2017 the US EIA published figures for the average levelized costs per unit of output (LCOE) for generating technologies to be brought online in 2022, as modelled for its Annual Energy Outlook. These show: advanced nuclear, 9.9 ¢/kWh; natural gas, 5.7-10.9 ¢/kWh (depending on technology); and coal with 90% carbon sequestration, 12.3 ¢/kWh (rising to 14 ¢/kWh at 30%). Among the non-dispatchable technologies, LCOE estimates vary widely: wind onshore, 5.2 ¢/kWh; solar PV, 6.7 ¢/kWh; offshore wind, 14.6 ¢/kWh; and solar thermal, 18.4 ¢/kWh. Lazard’s report on the estimated levelized cost of energy by source (10th edition) estimated unsubsidized prices of $97–$136/MWh for nuclear, $50–$60/MWh for solar PV, $32–$62/MWh for onshore wind, and $82–$155/MWh for offshore wind.

An October 2020 report from Lazard compared the LCOE for various generation technologies on the basis of its estimates, related to input from “a wide variety of industry participants”. For nuclear power (2200 MWe plant), capital cost including financing (at a high discount rate) ranged from $7675 to $12,500 per kilowatt, and the LCOE accordingly varied from $129 to $198/MWh. For a 600 MWe coal plant the capital cost ranged from $3000 to $8400/kW, giving an LCOE of $60 to $143/MWh. Gas combined cycle (550 MWe) capital cost was $700 to $1300/kW and LCOE $65 to $159/MWh. The purpose of the study was to compare these figures with “alternative energy technologies”, particularly wind and solar PV, but without taking account of system costs. The nuclear costs estimated by Lazard were well above those in the IEA-NEA study based on existing projects, with well-referenced data.

So far cost of nuclear means cost of nuclear fission electricity as discussed in above paragraphs. Now I discuss cost of fusion.

Since no fusion power plants have been built yet, we have two options for estimating what fusion power plants might cost in the future. First, we can review the few public engineering cost estimates that are available for fusion concepts. Where information is not available, we can look at the cost of projects with a similar scale and level of complexity (e.g. fission power plants), and extrapolate.

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Estimates of the cost of fusion electricity:

The cost of fusion electricity is driven principally by its capital cost and by how many hours the plant can run each year. Quantitative discussion focuses on “the levelized cost of electricity.” In essence, the levelized cost is the total cost of building a plant and running it over its lifetime, divided by the kilowatt hours of energy that the plant produces over its lifetime. Like its fission counterpart, the total cost of a fusion power plant is dominated by its initial capital costs. The kilowatt hours produced over its lifetime are affected primarily by the size of the plant, the number of hours that it is able to run each year, and the efficiency with which the thermal energy produced through fusion is converted into electricity.

There are many estimates of the capital cost of a fusion plant. The estimates range from $2,700 to $9,700 per kilowatt of capacity. The plants have a capacity between 1,000 and 1,500 megawatts. Assuming that the capital cost per kilowatt is roughly independent of the size across this small range of sizes, the estimated capital cost of a fusion power plant with 1,000 megawatts of capacity would range from 2.7 to 9.7 billion dollars.

The wide range of capital costs is partially explained by varying assumptions about how many plants of the same kind have been built prior to the plant whose cost is being estimated. Fusion plants are expected to become less expensive as more plants of a specific design are built. “Technological learning” captures this issue: cost models often assume that costs will fall at some well-defined rate as additional units of the same kind are installed.

The wide range of costs is also due to differences in the assumed technological maturity of the plant.

Fusion plants are likely to become less expensive as they incorporate successive advances in technology. For example, with maturity may come greater efficiency in converting the thermal energy of fusion into electricity. Nearly a factor of two is at stake, with conversion efficiency ranging between 30 and 60 percent. This efficiency depends especially on the temperature of the blanket; the blanket absorbs the thermal energy released in the fusion reactions and delivers most of that thermal energy to the turbine that produces electricity. The larger the difference between the temperature of the blanket and the temperature of the environment (ocean or river water, for example), the higher the efficiency of electricity generation. Efficiencies of 30 percent are representative when the blanket is water-cooled and maintained at a temperature of 300 degrees Celsius, so that steam enters the steam turbine at nearly 300 degrees Celsius. The higher 60 percent efficiency might be realized if a blanket could be maintained at much higher temperatures as a result of being cooled by a gas or a liquid metal.

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Nuclear Fusion Reactors are Expensive (Australian study):

The Lawrence Livermore Labs used 192 laser beams to blast a peppercorn-sized pellet stuffed full of hydrogen isotopes with x-rays. This heated it to 100 million degrees K and — briefly — initiated fusion. This was not easy to do. It was so tricky that laser blasting is unlikely to be used in future reactors as magnetic confinement inside a giant techno-donut is considered more practical. The technology was not cheap. Creating sustained fusion will be even more expensive. Because of the high-precision engineering, it will be a high-capital-cost form of generation.

The three major reasons why Earth-based nuclear fusion will never be an economical source of energy are:

-1. High Capital Costs: Reactors will be expensive to build.

-2. Fuel Costs: Despite many claims to the contrary, fuel for current fusion reactor designs is not cheap.

-3. A Poor Fit for Modern Grids: Grids with high — or moderate — amounts of solar and wind generation have extended periods of low or zero wholesale electricity prices. This is disastrous for the economics of an expensive-to-build energy source. Also, as the interior of nuclear fusion reactors will become radioactive over time, they’re also likely to have high decommissioning costs.

High Capital Costs:

Nuclear fission power generation has been around for 70 years and still isn’t cheap. This is because it costs a huge amount of money to build a fission reactor with a high level of safety. While the cost of building nuclear fission power stations could be reduced, it’s not possible for them to be as cheap as coal power stations because they are more complex and can go wrong in ways that coal power can’t.

Nuclear fusion reactors will cost more than fission because they are far more complicated. To create heat with fission, all you need to do is put the fuel rods close to each other, and it happens automatically. To create heat in a fusion reactor, advanced technologies such as immensely powerful lasers or magnetic confinement techno-donuts are needed. The cost of a power plant based on ITER would be approximately ten times the cost of a nuclear fission power plant.

Nuclear fusion doesn’t have the same level of radiation hazard as fission, but reactors’ interiors will still become radioactive. This means the costs of precautions against releasing radioactive material may be much lower, but they can’t be eliminated.

But even if fusion power plants could be built for the same cost as coal power stations they still won’t be able to pay for themselves. This is because new coal power stations are becoming uneconomic worldwide. In fact, any thermal generation — including nuclear fission and fusion power stations — will find it difficult to compete with solar PV and wind turbines that don’t require expensive heat exchangers, condensers and turbines.

Nuclear Fusion Fuel is not Cheap:

A widely repeated claim is that fuel for fusion is dirt cheap. But this is a techno-myth. It’s uneconomically expensive now and unlikely to ever be very cheap in the future.

Current nuclear fusion reactor designs use hydrogen isotopes for fuel. By weight, the fuel is two-fifths deuterium and three-fifths tritium. Deuterium is relatively cheap, but tritium is currently around $40,000 Australian per gram. At this price, when used in a fusion reactor that is 33% efficient at turning heat into electricity, the cost of tritium will be around 58 cents per kWh generated.

A paper from 1991 says the cost of producing tritium in a fusion reactor could be as low as $215-$300 US per gram. If we take the lowest end of that range and convert the $215 into 2022 dollars it becomes $640 US. At the current exchange rate, that’s $930 Australian. Assuming 100% of the tritium undergoes fusion, the fuel cost will be around 1.3 Australian cents per kWh generated. This is roughly the same fuel cost as for nuclear fission and around 4 times the cost of coal for Victorian power stations.

Coal, Fission, & Fusion don’t play well with Renewables:

In South Australia, they have long periods of low, zero, or negative wholesale electricity market prices. This is due to wind and solar power accounting for 70% of the state’s generation. This is terrible for the economics of baseload generators such as coal, nuclear fission, and future fusion.

Because they have high capital and low fuel costs, they save little money by shutting down during periods of very low electricity prices. But as they’re competing against solar and wind with zero fuel cost, their only options are to shut down or operate at a loss during these periods.

Even if nuclear fusion energy research receives hundreds of billions in funding, a prototype fusion power station able to provide more energy than it consumes is still decades away. In 30 years, the world’s electricity generation will be mostly renewable, and most of the time, that electricity will be extremely cheap. Even if fusion reactors are technically feasible, they won’t be profitable to build. If coal and fission can’t pay for themselves, neither will nuclear fusion.

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Nuclear Fusion Capital Costs are High but can be reduced to Economically Competitive Level, 2021 report:

LCOE for the development of a new generation of nuclear fusion reactors are high at around £100/MWh, but a substantial programme of standard build could bring them down to a viable target of £60-£70/MWh, a report published by engineering group Assystem says. According to the report, which examines the potential for fusion in the UK, the government has estimated the 2040 levelised costs of electricity (LCOE) for the UK for standalone offshore wind, onshore wind and large-scale solar of £40/MWh, £44/MWh and £33/MWh respectively. The £60-£70/MWh cost for fusion “provides the first target for nuclear fusion to be economically competitive”, the report concludes. It says fusion is uncompetitive today with other low-carbon options available in the UK – including wind and light-water nuclear fission reactors. The reason for this is the combination of a relatively high construction cost (£5,887/kWe) and a low capacity factor (56%). With an improved, large fusion design the construction cost decreases to £4,135/kWe and the capacity factor to 75%. These two effects improve the fusion economics, decreasing the LCOE into the range £60 to £97/MWh.

Capital costs of fusion are high, with the core device costs – magnets, vessel and divertor and blanket – being more than 66% of direct costs and almost 50% of total costs. Reducing the cost of these key components by innovation in either design or manufacture, and by production learning will have the most effect in making fusion competitive. Though fuel costs are low, other operations and maintenance costs are significant – particularly the cost of replacing life-limited vessel and blanket components.

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The value of fusion energy to a decarbonized United States electric grid, a 2023 study:

Jacob A. Schwartz, Wilson Ricks, Egemen Kolemen, Jesse D. Jenkins.

Highlights:

-If fission is an option, fusion will likely need similar or lower costs

-Pulsed and steady-state tokamaks have similar value, assuming the same power output

-A high variable operational cost would significantly decrease a plant’s value

-Integrated thermal storage could increase the value of fusion by 5%–10%

Fusion could be a part of future decarbonized electricity systems, but it will need to compete with other technologies. In particular, pulsed tokamak plants have a unique operational mode, and evaluating which characteristics make them economically competitive can help select between design pathways. Using a capacity expansion and operations model, authors determined cost thresholds for pulsed tokamaks to reach a range of penetration levels in a future decarbonized US Eastern Interconnection. The required capital cost to reach a fusion capacity of 100 GW varied from $2,700 to $7,500 kW^−1, and the equilibrium penetration increases rapidly with decreasing cost. The value per unit power capacity depends on the variable operational cost and on the cost of its competition, particularly fission, much more than on the pulse cycle parameters. These findings can therefore provide initial cost targets for fusion more generally in the United States.

Fusion energy is often hailed as a limitless source of clean energy, but new research from Princeton University suggests that may only be true if the price is right. In a study led by fusion expert Egemen Kolemen, associate professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment, and energy systems expert Jesse Jenkins, assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment, Princeton researchers modeled the cost targets that a fusion reactor might have to meet to gain traction in a future U.S. energy grid. The findings, published in Joule in 2023, illustrated that the engineering challenges of fusion energy are only part of the problem — the other part lies in economics.

“People will not pay an unlimited amount of money for fusion energy if they could spend that money to generate clean energy more cost-effectively,” said Jacob Schwartz, a former postdoc with Kolemen and Jenkins who led the modeling for the study and currently works as a research physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory, a national laboratory working to advance the science of fusion energy. “Above a certain cost, even if we can engineer them, not many developers will want to build them.”

The model results demonstrated that the niche for fusion in the U.S. depends not only on the price of building a reactor but hinges greatly on the energy mix of the future grid and the cost of competing technologies like nuclear fission.

If the market for fusion is favorable, then even with capital costs as high as around $7000 per kilowatt, fusion could still reach 100 GW capacity — about the current capacity of U.S. nuclear power plants, which supply about a fifth of today’s electricity needs. But supposing alternative technologies like nuclear fission, hydrogen, carbon capture and storage, or long-duration battery storage successfully take root, capital costs might have to be less than half that price for fusion to reach the same 100 GW capacity. “Fusion developers need to keep an eye on the competition,” Jenkins explained. “If successfully commercialized, fusion power plants are likely to look a lot like classic nuclear fission plants from the perspective of electricity markets and grids. Both resources are complex technologies with tight engineering margins for safety reasons, which translates to high upfront investment costs. If the variable costs of fusion power plants end up low, fusion plants will likely compete head-to-head with new fission power plants.”

Fusion’s place in a renewables-heavy grid:

Kolemen, who also holds a joint appointment with the Princeton Plasma Physics Laboratory, explained that the researchers’ approach helped establish concrete goals for prospective fusion developers to use as benchmarks when considering plans for reactors. “Many fusion startups are popping up, all with different reactor designs, saying they can build a reactor with this or that amount of money. But it’s almost impossible to assign dollar amounts to individual reactor components,” Kolemen said. “So instead, we considered a future grid model with more renewables like wind and solar and worked backward to understand the cost targets that fusion reactors might have to meet to have a place in that future grid mix.” By determining the cost targets for fusion reactors in a grid likely to be dominated by renewable electricity sources like solar and wind, the researchers also gained insight that could inform the design of future fusion reactors.

For example, Schwartz said fusion energy has traditionally been seen as a ‘baseload’ energy source that would operate almost continuously. With widespread solar adoption and subsequently cheap midday electricity prices, however, the research suggests it would be valuable to add functionality to a fusion reactor to enable it to toggle on and off more frequently or to consider ways to store the energy from the reactor and sell it when it is most needed. The researchers said integrating storage into the design of a fusion reactor could increase its value by up to $1,000 per kilowatt. “Capturing more economic value by coupling thermal storage and operating flexibly means the first fusion power plants can compete with higher initial costs,” said Jenkins. “That higher ‘go-to-market’ price point could be key to kickstart the fusion industry and help lower costs over time through experience and incremental innovation.”

The model also suggested that the most promising locations for fusion energy in the U.S. would be in the northeastern region, where local solar and wind opportunities are more limited than in other areas. “The northeast doesn’t have the same potential for geothermal or hydropower energy as the western U.S. does, so it’s the place where something besides solar and wind will be most needed,” said Wilson Ricks, a study co-author and graduate student in mechanical and aerospace engineering.

Exploring the pulsed vs. steady-state debate:

In addition to considering fusion energy in the context of a future energy grid, the model also returned initial results that the researchers said could help settle an important question in the fusion research community between the viability of steady-state versus pulsed fusion reactors.

Steady-state and pulsed represent two different design approaches to fusion reactors. A steady-state fusion reactor would operate like a traditional power plant, running almost full-time. A pulsed fusion reactor, on the other hand, would be required to shut down for a short time and then restart every hour or every few hours.

While some scientists have questioned whether the operating requirements for pulsed reactors would make them difficult to integrate with the rest of the energy grid, the model results suggested no significant difference between the two designs regarding their effects on the large-scale grid.

“There’s been somewhat of a design battle between the two technologies, so it was interesting that our initial findings suggest there wasn’t much of a difference in how they would operate with the rest of the grid,” Kolemen said. “From the viewpoint of the model, the best design is simply the cheapest one.”

“This research doesn’t claim to know when fusion will come online. Fusion reactors may still be decades away from having a large impact on the energy grid. What this research gives us is a clear target for fusion researchers and startups to aim for when they do come online,” Kolemen said. “The end goal of fusion research is to provide some value to the U.S. and the grid. Now, we actually have some numbers to guide us.”

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Can fusion energy be cost-competitive and commercially viable? An analysis of magnetically confined reactors, a 2023 study:

Driven from investments toward net zero transition global interest in fusion energy is growing. This policy perspective addresses challenges for its commercialization, given the potentially long timeframe to deployment and competing/complementary technologies. Authors focus on magnetically confined fusion power, specifically tokamaks, as the route to commercialization is clearer and there is some cost data available. For fusion to be competitive beyond 2040, costs will likely need to be at or below ∼$80–100/MWh at 2020 price. This will be hard to achieve for early fusion designs both small or large, for which modelling shows energy costs will be greater than $150/MWh even accounting for production learning. This is due to the low power availability from pulsed operation; frequent replacement of vessel components; and low efficiency power cycles. Technology improvements to improve both cycle efficiency and power availability, along with production standardization and long-life components, have the potential to reduce generation costs and enable magnetically confined fusion to be commercially viable. Authors therefore recommend focusing commercialization efforts on plants with higher thermal efficiency and potential for higher availability as these maximize the probability of fusion energy proving commercially viable. Authors also recommend that fusion energy be deployed within a proportionate regulatory regime that recognizes its relatively low radiological hazard. Finally, construction of fusion reactors should be planned in fleet/program terms, as commitment to constructing many units will be necessary for it to become commercially viable.

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Renewable cheaper than fusion:

The widespread adoption of non-nuclear renewable energy has transformed the energy landscape. Such renewables are projected to supply 74% of global energy by 2050. The steady fall of renewable energy prices challenges the economic competitiveness of fusion power.

Figure below shows levelized cost of energy (LCOE) for various sources of energy including wind, solar and nuclear energy

Some economists suggest fusion power is unlikely to match other renewable energy costs. Fusion plants are expected to face large start up and capital costs. Moreover, operation and maintenance are likely to be costly. While the costs of the CFETR are not well known, an EU DEMO fusion concept was projected to feature a levelized cost of energy (LCOE) of $121/MWh. Furthermore, economists suggest that fusion energy cost increases by $16.5/ MWh for every $1 billion increase in the price of fusion technology. This high Levelized cost of energy is largely a result of construction costs.

In contrast, renewable levelized cost of energy estimates are substantially lower. For instance, the 2019 levelized cost of energy of solar energy was estimated to be $40-$46/MWh, on shore wind was estimated at $29-$56/MWh, and offshore wind was approximately $92/MWh.

2050 predictions:

Based on the predictions from the US Energy Information Agency (EIA), the cheapest form of energy in 2050 will be solar with an LCOE of $0.026/kWh – it will be dispatched whenever possible. Onshore wind energy will cost on average $0.035/kWh and electricity from natural gas will cost about $0.045/kWh. It’s important to note that the LCOE is NOT the retail price on your electricity bill. The LCOE does not include transmission and distribution costs, taxes, and marketing — it’s just the cost of making the electricity. The LCOE for fusion plant is estimated to be $0.11/kWh. Considering EIA’s predictions of about $0.03–0.045/kWh for solar, wind and natural gas in 2050, it is unlikely that fusion will be the absolute cheapest source of electricity. Still, fusion may be economically attractive for small power plants in remote areas with low sunlight or access to other fuels.

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Section-13

Safety and environmental concerns of nuclear fusion:

The safety requirements for a fusion facility are:

To protect the public and the environment against radiological hazards
To protect the site workers against radiation exposure that must be maintained As Low As Reasonably Achievable (ALARA)
To take measures to prevent accidents and to mitigate their consequences should they occur
To avoid the need for public evacuation in any accident
To minimize the amount of fusion facility radioactive waste

From the ITER website:

It is absolutely impossible for a Fukushima-type accident to happen at ITER. The fundamental differences in the physics and technology used in fusion reactors make a fission-type nuclear meltdown or a runaway reaction impossible. The fusion process is inherently safe.
Even in the event of a cataclysmic breach in the tokamak, the levels of radioactivity outside the ITER enclosure would remain very low. The ITER Preliminary Safety Report presents an analysis of risks that demonstrates that during normal operation, ITER’s radiological impact on the most exposed populations will be one thousand times less than natural background radiation. For postulated “worst-case scenarios,” such as fire in the Tritium Plant, the evacuation of neighboring populations would not be necessary.
Fusion reactors, unlike fission reactors, produce no high activity/long life radioactive waste. The “burnt” fuel in a fusion reactor is helium, an inert gas. Because the half-life of most radioisotopes contained in waste is lower than ten years, within 100 years the radioactivity of the materials will have diminished in such a significant way that the materials can be recycled for use in other fusion plants.

Nuclear fusion will of course require regulation as a nuclear technology. The main risk to the public arises from the potential for leaks of tritium, and this drives the safety design for conceptual plants. Tritium decays to 3He through beta emission and as a hydrogenic species burns to form water, which is easily taken up by biological organisms. In addition, it easily permeates through many materials and systems exposed to it and they must be detritiated during decommissioning before they can be recycled. A fusion power plant may have > 10 kg of tritium on-site in various systems—and much effort is devoted to the engineering of systems, which should minimize this.

The design goal for a fusion power plant is zero evacuation—that is, even under the worst conceivable series of system failures, the amount of released radioactive material should not be enough to require evacuation of any surrounding area. This, of course, requires the engineering of multiple levels of safety systems. For example, it is possible that a plasma disruption could melt part of the interior of the reactor, rupturing coolant channels in the blanket and causing the release of tritiated steam at high pressure into the reactor. Safety engineers model such events and the resulting pressure rises in the components to calculate where to install rupture disks, expansion volumes, and condensation tanks to ensure the release would be safely contained with no risk of tritium leakage beyond a primary containment level—and in case this also failed, there is also secondary containment in line with conventional nuclear defense-in-depth principles.

Fusion plasmas, while containing a lot of energy, are intrinsically unstable and require active control. In any case of loss of control, the plasma may disrupt, possibly causing damage to the reactor, but it cannot sustain a runaway nuclear reaction. There is some residual nuclear decay heat in the materials following the collapse of the plasma (which also affects the maintenance of the plant), but this is dispersed across a large volume of material and even a failure of plasma control followed by a complete loss of coolant does not lead to melting of plant materials.

The neutrons produced by the D-T reaction cause activation of the materials surrounding the plasma to form low- and intermediate-level waste. It is possible to design materials (for example, so-called low-activation steels), which limit the quantity of long-lived radionuclides formed and are intended to permit recycling of most structural elements of a fusion reactor within hundreds of years, a far more tractable problem than the tens of thousands of years required for the storage of fissile waste. However, this does require adequate detritiation and separation of materials extracted from the reactor and the full feasibility of these processes is not yet clear. The greatest production of long-lived nuclear material in common concepts is carbon-14, produced by neutron radiation of nitrogen, found in water (used as a coolant) and as a common alloying element in steel. Although 14C occurs naturally in low levels (produced in the atmosphere by high-energy cosmic rays), it is readily taken up by biological organisms and can be incorporated into DNA, where its decay can be damaging (the half-life of 14C is 5730 years). Therefore, the lifetime production of 14C by a fusion power plant must be limited, leading to stringent manufacturing limits on structural alloys.

There are also other concerns, principally regarding the possible release of tritium into the environment. It is radioactive and very difficult to contain since it can penetrate concrete, rubber and some grades of steel. As an isotope of hydrogen, it is easily incorporated into water, making the water itself weakly radioactive. With a half-life of about 12.3 years, the presence of tritium remains a threat to health for about 125 years after it is created, as a gas or in water, if at high levels. It can be inhaled, absorbed through the skin or ingested. Inhaled tritium spreads throughout the soft tissues and tritiated water mixes quickly with all the water in the body. Although there is only a small inventory of tritium in a fusion reactor – a few grams – each could conceivably release significant quantities of tritium during operation through routine leaks, assuming the best containment systems. An accident could release even more. This is one reason why long-term hopes are for the deuterium-deuterium fusion process, dispensing with tritium.

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Safety implications:

Due to the presence of similar systems and components, the safety considerations for tokamak and stellarator are expected to be very similar. However, the different magnetic confinement concept makes event sequences in tokamaks more complex and serious than in stellarators. Actually, accidents which finally entail the ingress into the plasma of materials from plasma-facing components (PFCs) or from cooling systems will cause disruptions in tokamaks aggravating the event sequences, or at least requiring more demanding design solutions. This is because disruptions, accompanied by vertical displacement events, lead to enhanced energy fluxes to PFCs and large magnetic forces that may cause damages to PFCs such as coolant leaks. Further consequences could be increased dust production/ mobilization along with increased risk of combined hydrogen (D, T) and dust explosions. For the following it is assumed that at this stage of conceptual design the safety assessments for stellarators can be reasonably encompassed by tokamak related assessments.

Fusion Safety Characteristics:

The energies stored inside the plant can hardly compromise the overall reactor assembly when released on their physics-based time scales. To maintain the burning plasma of a commercial FPP, the plasma chamber contains the fuel tritium which is produced in the power plant so that major shipments from/to off-site facilities are avoided. The nuclear power densities in the plasma are low compared to fission power reactors. The spent fuel consists of stable helium. The isotopic mix of all radioactive materials results in a radiotoxicity, lower by several orders of magnitude than in a NPP.

Continuous operation of the plant is maintained by refuelling with the D-T mixture. The fuel inventory in the plasma chamber at any time is sufficient for only about 1 to 2 minutes. So, the plasma burn can be stopped on this timescale by terminating the fuel supply. In case of an event which would impair the integrity of plasma-facing components or cause their overheating, impurities would enter the plasma causing an immediate thermal quench (on the time-scales of energy and particle confinement which are a few seconds).

Since tritium and deuterium can accumulate inside the plasma vessel one may assume, to be conservative, that tritium and deuterium outgassing from the surfaces of PFCs could sustain plasma burn. This would require the tritium and deuterium to arrange itself in a suitable way inside the magnet field configuration so that it would effectively fuel the plasma. However, such an event is considered to be implausible.

Power excursions, due to over-fuelling have in fact been considered for ITER. This scenario – whichever way it is caused – is studied. The conclusion is that the hazard of prolonging plasma burn does not exist, rather there exists a certain possibility of a fusion power overshoot.

Plasma burn has to be terminated reliably within about 3 seconds. This is the task of a “Fusion Power Shutdown System” (FPSS) which simply injects impurities. In addition, there exist several inherent feedback mechanisms (such as the release of material from the PFCs) which extinguish the fusion process in uncontrolled situations.

A criticality accident cannot develop. After burn termination, the residual power density due to the decay of activated structure materials is not high enough that – a proper design of the plant taken into account – melting of plant components is possible even in case of a total loss of cooling.

The plasma vessel also contains radioactive dust. These inventories are low by comparison with radioactive inventories of fission equivalents. The associated FPP radiotoxicity decays rapidly.

The properties outlined above support safety and are considered to be inherent (according to the definitions used in the German rules and standards of safety requirements for nuclear power plants). An inherently safe design is based on those principles of the laws of nature which by themselves have a safety-directed effect.

Even though the inherent properties may lead to the conclusion that active safety systems are not mandatory, they will be included in the design of commercial FPPs to help limiting damage and consequences at all levels of defence.

Active safety systems are also very important during normal operation to limit individual and collective doses to personnel. The active safety systems contribute to the implementation of the ALARA principle (to reach an exposure level “As Low as Reasonably Achievable”).

At present, a typical example is provided by ITER’s active detritiation systems (DS) that provide an essential element of the second confinement system. They will be highly reliable, with redundancy to cope with failures.

If the active DS are completely lost, ITER’s confinement is still maintained by the static leak-tightness of the building. Some leakage will occur if the internal pressure rises above its normal sub-atmospheric value, so the extent of the environmental release will depend on how long it takes to bring the DS back into service.

Energy Inventories:

Stored energies may have the potential to destroy the integrity of confinement systems and to mobilise hazardous materials, thus causing their release into the environment. Therefore, the most important energy inventories in a commercial FPP are compiled and discussed in the following.

The D-T fuel content in the plasma vessel can sustain the plasma burn for only 1 to 2 minutes. The energy stored by the fuel mixture in the vessel is 325 GJ for the reactor models used for the SEAFP / SEIF studies. The complete conversion of this energy in fusion heat would require active sustainment of the burn process. It should be noted that under normal conditions, only a burn fraction of 2 % can be reached resulting in an energy release of 6.5 GJ.

The hot plasma of a FPP would typically contain a thermal energy of 1 to 2 GJ. The associated volume averaged energy density is low corresponding to plasma pressure of 3 to 6 bar.

The large energies stored in the magnetic field of a fusion reactor amount to typically 200 GJ and are a potential hazard to the first confinement barrier. Therefore, the design of ITER’s superconducting magnet system includes multiple monitoring, fault detection and protection systems. In particular, there is a safety-grade quench detection and a fast discharge system for the toroidal field coils. Together with robust design, these measures aim at a minimization of the probability of magnet faults that could entail damage to the first confinement barrier.

Nonetheless, a hypothetical event sequence has been analysed to assess the bounding consequences of damage to confinement barriers. Together with other conservative assumptions, large holes (1 m^2) in the plasma and cryostat vessel walls were postulated, caused by a release magnet energy. The analysis showed that even in this case the radiological consequences would be below any evacuation limit. To put the case of magnet energy implications on an even firmer basis, additional detailed analyses are performed at present.

Typical values of energy inventories are shown by Table below, which is based on the SEAFP, SEIF and PPCS reports.

Main energy inventories in a FPP:

Energy Source	Energy	Reference
In-vessel fuel (DT)	~ 325 GJ	SEAFP, SEIF
Magnetic field	~ 200 GJ	SEAFP, SEIF
Plasma thermal energy	1 to 2 GJ	SEAFP, SEIF, PPCS
Primary coolant water enthalpy	~ 400 GJ	SEAFP, SEIF

Like in a NPP, the residual heat generation in the FPP is not stopped completely after shutdown, but will continue at a few percent level (e.g. less than 2 % level), and decrease exponentially for the time after. This heat generation is due to the radioactive decay of the materials that were exposed to the neutron irradiation. This heat source has to be considered in order to ensure the long term cooling of the reactor and can play an important role in aggravating the consequences of an accident if active cooling cannot be provided.

Additional energy can be released during accidents by chemical reactions; a typical example is the reaction of water (coolant) and Be (neutron multiplier) in solid breeder reactors in case of a loss of coolant accident in the Vacuum Vessel. Here, steam can have an exothermal reaction with Be at temperatures greater than 600°C.

Furthermore, chemical reaction of i.e. water at high temperature with metals (e.g. Be or W) can produce hydrogen. If this element is mixed with oxygen it can produce fire, detonation or deflagration, releasing mechanical and thermal energy.

Hence, depending on the materials used in the FPP [coolant (especially if water used), structural materials, breeder and neutron multiplier combination] potential release of chemical energy should be considered in the energy inventory.

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Accident potential:

Accident potential and effect on the environment are critical to social acceptance of nuclear fusion, also known as a social license. Fusion reactors are not subject to catastrophic meltdown. It requires precise and controlled temperature, pressure and magnetic field parameters to produce net energy, and any damage or loss of required control would rapidly quench the reaction. Fusion reactors operate with seconds or even microseconds worth of fuel at any moment. Without active refueling, the reactions immediately quench.

The same constraints prevent runaway reactions. Although the plasma is expected to have a volume of 1,000 m^3 (35,000 cu ft) or more, the plasma typically contains only a few grams of fuel. By comparison, a fission reactor is typically loaded with enough fuel for months or years, and no additional fuel is necessary to continue the reaction. This large fuel supply is what offers the possibility of a meltdown.

In magnetic containment, strong fields develop in coils that are mechanically held in place by the reactor structure. Failure of this structure could release this tension and allow the magnet to “explode” outward. The severity of this event would be similar to other industrial accidents or an MRI machine quench/explosion, and could be effectively contained within a containment building similar to those used in fission reactors.

In laser-driven inertial containment the larger size of the reaction chamber reduces the stress on materials. Although failure of the reaction chamber is possible, stopping fuel delivery prevents catastrophic failure.

Most reactor designs rely on liquid hydrogen as a coolant and to convert stray neutrons into tritium, which is fed back into the reactor as fuel. Hydrogen is flammable, and it is possible that hydrogen stored on-site could ignite. In this case, the tritium fraction of the hydrogen would enter the atmosphere, posing a radiation risk. Calculations suggest that about 1 kilogram (2.2 lb) of tritium and other radioactive gases in a typical power station would be present. The amount is small enough that it would dilute to legally acceptable limits by the time they reached the station’s perimeter fence.

The likelihood of small industrial accidents, including the local release of radioactivity and injury to staff, are estimated to be minor compared to fission. They would include accidental releases of lithium or tritium or mishandling of radioactive reactor components.

Can Chernobyl-type accidents occur?

First, the amount of fuel available at each instant is sufficient for only a few tens of seconds, in sharp contrast with a fission reactor where fuel for several years of operation is stored in the reactor core. In a fusion reactor, there will only be a limited amount of fuel (less than four grams) at any given moment. The reaction relies on a continuous input of fuel; if there is any perturbation in this process and the reaction ceases immediately.

Second, fusion reactions take place at extremely high temperatures and the fusion process is not based on a neutron multiplication reaction. With any malfunction or incorrect handling, the reactions will stop. An uncontrolled burn (nuclear runaway) of the fusion fuel is therefore excluded on physical grounds.

Even in case of a total loss of active cooling, the low residual heating excludes melting of the reactor structure. Even in the event of the total loss of the cooling function, ITER’s confinement barriers would not be affected. The temperatures of the vacuum vessel that provides the first confinement barrier would under no circumstances reach the melting temperatures of the materials.

What would be the danger of an earthquake occurring near ITER, or a double disaster like earthquake and flooding?

The ITER facility is designed to resist an earthquake of amplitude x40 and energy x250 higher than any earthquake for which we have historical or geological references in the area of Saint Paul-lez-Durance, France. The ITER Tokamak Building will be made of specially reinforced concrete, and will rest upon bearing pads, or pillars, that are designed to withstand earthquakes (this technology is used to protect other civil engineering structures such as electrical power plants from the risk of earthquake). The risk of flooding, too, has been taken into account in ITER’s design and Preliminary Safety Report. In the most extreme hypothetical situation—that of a cascade of dam failures north of the ITER site—more than 30 metres remains between the maximum height of the water and the first basement of the nuclear buildings.

Could ITER explode?

In a tokamak fusion device, the quantity of fuel present in the vessel at any one time is sufficient for a few-seconds burn only. It is difficult to reach and maintain the precise conditions necessary for fusion; any disruption in these conditions and the plasma cools within seconds and the reaction stops, much in the same way that a gas burner is extinguished when the fuel tap is turned off. The fusion process is inherently safe; there is no danger of run-away reaction or explosion.

What about the issue of nuclear decay heat that was so serious at Fukushima?

It’s true that continued cooling is required in a fission reactor because, even after shutdown, there is a substantial decay heat to be eliminated that is produced by the fission decay of the tons of nuclear fuel present in the vessel.

In ITER or in future fusion power plants, this kind of scenario is impossible. The thermal power induced in the ITER vacuum vessel will be low. Even if no active cooling of the vacuum vessel is provided, as in the case of total failure of the cooling system, the resulting temperature would not threaten the integrity of the vacuum vessel.

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Magnet quench:

A magnet quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil exits the superconducting state (becomes normal). This can occur because the field inside the magnet is too large, the rate of change of field is too large (causing eddy currents and resultant heating in the copper support matrix), or a combination of the two. More rarely a magnet defect or cooling failure can cause a quench. When this happens, that particular spot is subject to rapid Joule heating from the current, which raises the temperature of the surrounding regions. This pushes those regions into the normal state as well, which leads to more heating in a chain reaction. The entire magnet rapidly becomes normal over several seconds, depending on the size of the superconducting coil. This is accompanied by a loud bang as the energy in the magnetic field is converted to heat, and the cryogenic fluid boils away. The abrupt decrease of current can result in kilovolt inductive voltage spikes and arcing. Permanent damage to the magnet is rare, but components can be damaged by localized heating, high voltages, or large mechanical forces.

In practice, magnets usually have safety devices to stop or limit the current when a quench is detected. If a large magnet undergoes a quench, the inert vapor formed by the evaporating cryogenic fluid can present a significant asphyxiation hazard to operators by displacing breathable air.

A large section of the superconducting magnets in CERN’s Large Hadron Collider unexpectedly quenched during start-up operations in 2008, destroying multiple magnets. In order to prevent a recurrence, the LHC’s superconducting magnets are equipped with fast-ramping heaters that are activated when a quench event is detected. The dipole bending magnets are connected in series. Each power circuit includes 154 individual magnets, and should a quench event occur, the entire combined stored energy of these magnets must be dumped at once. This energy is transferred into massive blocks of metal that heat up to several hundred degrees Celsius—because of resistive heating—in seconds. A magnet quench is a “fairly routine event” during the operation of a particle accelerator.

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Radiation hazards and Radioactive wastes from fusion energy:

The fusion reaction releases neutrons, the energy of which will be used in future power stations to heat water to heat drive the power plant. The neutrons would be quite dangerous to humans, but when the plant is turned off the production of neutrons ceases within milliseconds.

The neutron bombardment also affects the vessel itself, and so once the plant is decommissioned the site will be radioactive. However the radioactive products are short lived (50-100 years) compared to the waste from a fission powerplant (which lasts for thousands of years). Also, the radioactivity in a fusion powerplant will be confined to the powerplant itself.

The basic fuels (D and Li) as well as the direct end product (He) of the fusion reaction are not radioactive. However, a fusion reactor will require radiation shielding since it has a radioactive inventory consisting of (i) tritium and waste contaminated by tritium and (ii) reactor materials activated by the neutrons of the fusion reaction. Studies indicate, however, that an adequate choice of the latter can minimise the induced radioactivity such that recycling should become possible after some decades to a century. Thus, radioactivity does not have to be inherent to nuclear fusion, in contrast to nuclear fission where the fission reaction itself leads to dangerous long-lived radioactive products.

The tritium cycle is internally closed, and the total tritium inventory in the fusion power plant will be on the order of a few kg, of which only about 200 grams could be released in an accident. Special permeation barriers will have to be used to inhibit discharge into the environment of tritium diffusing through materials at high temperature. As tritium is chemically equivalent to hydrogen, it can replace normal hydrogen in water and all kinds of hydrocarbons. It could thus contaminate the food chain when released in the atmosphere. The absorption of tritium contaminated food and water by living organisms is a potential hazard. However, possible damage is reduced owing to the short biological half-life of tritium in the body of about 10 days.

Neutrons:

First, let’s take a foray into the world of neutrons. Lone—or “free”—neutrons are created naturally by cosmic rays interacting with the upper layers of the atmosphere. At their initial speed, it’s only a short trip to the Earth’s surface. But on the way they have every chance of encountering the nitrogen, oxygen or carbon particles present in the air, getting absorbed and forming an isotope … or bouncing off the surface of the particle nucleus and losing energy in the process.

As a result, only a few of these “space” neutrons ever reach the Earth’s surface—approximately 100-300 neutrons per second per square metre. If they have retained enough energy, they may be absorbed by elements present in the soil such as iron, silicon, potassium, etc. Or they’ll die a quick death, decaying into a proton, an electron and a neutrino.

Neutrons will also be generated by the fusion reaction inside of ITER. At full power, the ITER machine will generate some one hundred billion billions of highly energetic neutrons per second travelling at approximately 51,000 kilometres per second (17 percent of the speed of light!). Instead of the thin air of outer space, however, fusion neutrons will face a succession of daunting physical obstacles, some exceptionally dense.

Beryllium in the shielding blankets; high-strength copper and stainless steel in the first wall of the vacuum vessel; ultra-dense neutron-hungry borated concrete in the bioshield—these materials will contribute to absorbing the neutron flux from the fusion reaction and keep radiation from escaping to the environment.

But given the proportion of void in even the densest materials, won’t some neutrons pass all the obstacles unscathed? Yes, but not enough to worry about however—the survivors will be so few that they will be indistinguishable from the natural background “noise” of neutrons.

Fusion comes with negligible radiation:

JET, the Joint European Torus, one of the nuclear fusion devices that has already achieved fusion, producing 16MW in 1997, relies on the principle of bringing together (fusing) deuterium and tritium nuclei to produce helium and highly energetic neutrons (14.1 MeV). Magnetic confinement is used to overcome the mutual electric repulsion between tritium and deuterium, both of which are positive ions. The handling of massive amounts of tritium and the production of highly energetic neutrons are the main radiation protection concerns faced by fusion experiments using tritium. Although tritium has a very low radio-toxicity –it emits radiation which is not able to penetrate the skin – it easily forms organic compounds and diffuses through materials. So, due to the large quantity of tritium in fusion reactors (for example in ITER, about one kilogram per cycle plus a few kilograms stored on site), power plant staff members use protective clothing to avoid inhalation and direct contact with the material. Moreover, pressure systems have been designed to limit tritium spread outside the power plant. During normal operation, tritium is continually released. Although tritium is volatile and biologically active, the health risk posed by a release is much lower than that of most radioactive contaminants, because of tritium’s short half-life (12.32 years) and very low decay energy (~14.95 keV), and because it does not bioaccumulate (it cycles out of the body as water, with a biological half-life of 7 to 14 days). ITER incorporates total containment facilities for tritium. High energy neutrons with a high fluency rate induce radioactivity in reactor materials. The radioactivity remains even after the experiment has been closed down. Specifically shielding, optimisation programmes of tritium handling and storage, protective clothing for the staff and a remote maintenance plan will result in negligible radiation risks for both the public and the power plant staff.

Radioactive waste:

Fusion reactors create far less radioactive material than fission reactors. Further, the material it creates is less damaging biologically, and the radioactivity dissipates within a time period that is well within existing engineering capabilities for safe long-term waste storage. In specific terms, except in the case of aneutronic fusion, the neutron flux turns the structural materials radioactive. The amount of radioactive material at shut-down may be comparable to that of a fission reactor, with important differences. The half-lives of fusion and neutron activation radioisotopes tend to be less than those from fission, so that the hazard decreases more rapidly. Whereas fission reactors produce waste that remains radioactive for thousands of years, the radioactive material in a fusion reactor (other than tritium) would be the reactor core itself and most of this would be radioactive for about 50 years, with other low-level waste being radioactive for another 100 years or so thereafter. The fusion waste’s short half-life eliminates the challenge of long-term storage.

Radiation hazards of fission:

Nuclear fission, on the other hand, is the physical phenomenon employed in nuclear reactors and also, when intentionally “uncontrolled”, in nuclear weapons. A large nucleus, for example of uranium-235, when bombarded with neutrons will fission, or split, into two smaller nuclei, called fission products, also emitting a few neutrons and gamma photons. These fast neutrons can themselves induce fission in other uranium nuclei, creating a chain reaction process. Avoiding a runaway chain reaction and managing radioactive fission products are the main issues of fission power. Uncontrolled chain reactions in reactors may, in fact, cause meltdowns and damages (however not comparable with the effects of nuclear weapons), while radioactive fission products are high-level radioactive waste that contains 95 percent of the radioactivity arising from nuclear power (according to the World Nuclear Association) and requires safe storage for more than a thousand years.

Fusion reactors, unlike fission reactors, produce no high activity/long-lived radioactive waste. The “burnt” fuel in a fusion reactor is helium, an inert gas. Activation produced in the material surfaces by the fast neutrons will produce waste that is classified as very low, low, or medium activity waste. All waste materials will be treated, packaged, and stored on site. Because the half-life of most radioisotopes contained in this waste is lower than ten years, within 100 years the radioactivity of the materials will have diminished in such a significant way that the materials can be recycled for use (in other fusion plants, for example). This timetable of 100 years could possibly be reduced for future devices through the continued development of “low activation” materials, which is an important part of fusion research and development today. The activation or contamination of in-vessel components, the vacuum vessel, the fuel circuit, the cooling system, the maintenance equipment, or buildings will produce an estimated 30,000 tonnes of decommissioning waste that will be removed from the ITER scientific facility and processed.

To produce usable heat, the neutron streams carrying 80 percent of the energy from deuterium-tritium fusion must be decelerated and cooled by the reactor structure, its surrounding lithium-containing blanket, and the coolant. The neutron radiation damage in the solid vessel wall is expected to be worse than in fission reactors because of the higher neutron energies. Fusion neutrons knock atoms out of their usual lattice positions, causing swelling and fracturing of the structure. Also, neutron-induced reactions generate large amounts of interstitial helium and hydrogen, forming gas pockets that lead to additional swelling, embrittlement, and fatigue. These phenomena put the integrity of the reaction vessel in peril.

In reactors with deuterium-only fueling (which is much more difficult to ignite than a deuterium-tritium mix), the neutron reaction product has five times lower energy and the neutron streams are substantially less damaging to structures. But the deleterious effects will still be ruinous on a longer time scale.

The problem of neutron-degraded structures may be alleviated in fusion reactor concepts where the fusion fuel capsule is enclosed in a one-meter thick liquid lithium sphere or cylinder. But the fuel assemblies themselves will be transformed into tons of radioactive waste to be removed annually from each reactor. Molten lithium also presents a fire and explosion hazard, introducing a drawback common to liquid-metal cooled fission reactors.

Bombardment by fusion neutrons knocks atoms out of their structural positions while making them radioactive and weakening the structure, which must be replaced periodically. This results in huge masses of highly radioactive material that must eventually be transported offsite for burial. Many non-structural components inside the reaction vessel and in the blanket will also become highly radioactive by neutron activation. While the radioactivity level per kilogram of waste would be much smaller than for fission-reactor wastes, the volume and mass of wastes would be many times larger. What’s more, some of the radiation damage and production of radioactive waste is incurred to no end, because a proportion of the fusion power is generated solely to offset the irreducible on-site power drains.

Although nuclear fusion does not produce long lived fission products and actinides, neutron capture by the reactor structural materials and components forms short, moderate and some long lived activation products. Thus, in addition to tritium emissions and contaminated materials, there will be a need to manage radioactive materials and wastes produced by neutron activation, within regulatory controls, over the whole life cycle of a fusion reactor. The fusion community has adopted the position that radioactive waste produced from the first generation of commercial fusion reactors should be classified as low-level waste (LLW) within 100 years after the end of life (EOL), i.e. after cessation of commercial power generation. Radioactive wastes arising from operation and decommissioning of the JET experimental nuclear fusion reactor, located at Culham, are already factored into the UK radioactive waste inventory. Forecast LLW and ILW packaged volumes are 4,120 m3 and 480 m3, respectively; activated steels and alloy plant and equipment, including the JET vacuum vessel, are a major contributor to the ILW arising. Development of the future ITER and DEMO reactors is incorporating the experience of radioactive waste management developed in the context of JET, in particular detritiation technology.

The primary components of the fusion reactor system are likely to require disposal, including the activated front wall, blanket, divertor and vacuum vessel materials. From an activation perspective, the key nuclides of potential concern, as identified from recent activation modelling studies, are summarised in Table below.

Table below shows summary of key nuclides of concern for management of wastes from nuclear fusion.

Nuclide	Half life	Production route	Comments
³H	β^–, 12.3 y	²H(n,γ)³H ⁶Li(n,α)³H, ⁷Li(n,nα)³H	Unlimited solubility, no sorption. Could be managed by extended decay storage. Currently managed by nuclear industry through “dilute and disperse” approach.
⁶³Ni	β^–,100 y	⁶²Ni(n,γ)⁶³Ni	High solubility, strongly sorbing. Decays within engineered barriers of a GDF system.
14C	β^–, 5,700 y	¹⁴N(n,p)¹⁴C	High solubility and moderately sorbing as carbonate species, may be released as methane gas from corroding metal. In post closure DSSC for GDF, migration of ¹⁴C, and radiological risk, depends on speciation and geological environment.
⁹⁴Nb	β^–, 20,000 y	⁹³Nb(n,γ)⁹⁴Nb	Low solubility, strongly sorbing. In generic post-closure DSSC for GDF, ⁹⁴Nb migrates through lower strength sedimentary host rock, but calculated mean radiological risk is insignificant.

Management of radioactive waste arising from nuclear fusion reactors will depend on the availability of disposal routes, regulatory standards and practice in the country of origin. The development of nuclear fusion systems, however, is a uniquely collaborative international effort due to the technical challenge and cost. The natural misalignment of standards and practice between such international collaborators may result in different classification and management of otherwise identical conceptual wastes in the respective nations. This is no different from the management of radioactive wastes from current fission reactors, with the exception that such wastes are rarely directly comparable between nation states.

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Overview on the management of radioactive waste from fusion facilities: ITER, demonstration machines and power plants, a 2022 study:

In the absence of official standards and guidelines for nuclear fusion plants, fusion designers adopted, as far as possible, well-established standards for fission-based nuclear power plants (NPPs). This often implies interpretation and/or extrapolation, due to differences in structures, systems and components, materials, safety mitigation systems, risks, etc. This approach could result in the consideration of overconservative measures that might lead to an increase in cost and complexity with limited or negligible improvements. One important topic is the generation of radioactive waste in fusion power plants. Fusion waste is significantly different to fission NPP waste, i.e. the quantity of fusion waste is much larger. However, it mostly comprises low-level waste (LLW) and intermediate level waste (ILW). Notably, the waste does not contain many long-lived isotopes, mainly tritium and other activation isotopes but no-transuranic elements. An important benefit of fusion employing reduced-activation materials is the lower decay heat removal and rapid radioactivity decay overall. The dominant fusion wastes are primarily composed of structural materials, such as different types of steel, including reduced activation ferritic martensitic steels, such as EUROFER97 and F82H, AISI 316L, bainitic, and JK2LB. The relevant long-lived radioisotopes come from alloying elements, such as niobium, molybdenum, nickel, carbon, nitrogen, copper and aluminum and also from uncontrolled impurities (of the same elements, but also, e.g. of potassium and cobalt). After irradiation, these isotopes might preclude disposal in LLW repositories. Fusion power should be able to avoid creating high-level waste, while the volume of fusion ILW and LLW will be significant, both in terms of pure volume and volume per unit of electricity produced. Thus, efforts to recycle and clear are essential to support fusion deployment, reclaim resources (through less ore mining) and minimize the radwaste burden for future generations.

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Nuclear weapons proliferation:

Nuclear fusion is the name for a chemical process by which two atoms collide with sufficient energy to fuse together into one atom. The easiest fusion reaction to achieve is one between two different isotopes of Hydrogen – Deuterium (Hydrogen with one neutron) and Tritium (Hydrogen with two neutrons) – that comprise two protons and three neutrons in total. The products are a Helium atom (two protons and two neutrons) and a lone neutron that has an energy of 14.1 MeV.

This lone neutron is essential to understanding both the potential and the dangers of nuclear fusion. On one hand, it can create energy by heating up water to power a steam turbine; this is how normal nuclear power plants operate right now, except their neutrons are produced by nuclear fission rather than nuclear fusion. On the other hand, this neutron also has enough energy to make U-238 (harmless on its own) into Pu-239, which can be material for a nuclear bomb; in other words, this lone neutron can be used to make “fissile material.” The same process applies to Th-232, which can be made into U-233.

The state of world politics right now is such that the easiest and most effective way to stop nuclear bombs from being built is to restrict access to fissile material. The International Atomic Energy Agency has even set limits of what it considers to be “significant quantities” of fissile materials that form the lower limit of what could be used to make a nuclear bomb.

There are two main ways that nuclear fusion can be dangerous: firstly, the more we understand nuclear fusion, the easier it is to build a weapon that incorporates fusion. The second way that fusion can be dangerous is through its ability to produce fissile material which could then be used in a nuclear bomb.

Some researchers have considered “hybridizing” fusion and fission. In principle the neutrons from fusion can be used for three purposes related to fission power: 1) multiplying the ~20MeV total nuclear energy output associated with each fusion reaction by inducing fission reactions (~200 MeV each) in a sub-critical fission blanket surrounding the fusion system; 2) breeding fuel for fission systems by transmuting 238U or thorium to plutonium or 233U; and/or 3) using the energetic neutrons from fusion to “burn” plutonium and other transuranics or even long-lived fission products recovered from the reprocessed spent fuel of fission power plants. Combinations of these have also been examined. Relative to fission without reprocessing, some proposed approaches would reduce the need for uranium enrichment, and so would reduce the risk associated with clandestine centrifuge systems derived from national efforts. The risk of diversion of weapon-usable material does not appear to be qualitatively different from fission systems with reprocessing, since substantial processing of nuclear fuels would be required in all cases, although hybrids may allow different processing options. The risk of breakout would be similar to fission with reprocessing. Some forms of fission-fusion hybrid would reduce the long-term risk associated with Pu in stored waste. Overall, however, hybrid systems appear to inherit the main risks of fission with reprocessing, although more analysis should be done for specific proposals.

The easiest way to produce fissile material would be to use a fusion-fission hybrid system, which is an idea in which you combine fusion and fission reactions in order to produce both energy and fissile material. But, fusion-fission hybrids face an uphill battle engineering-wise and may never become a commercial product because the performance probably wouldn’t justify the extra cost over a normal fission system. The more likely scenario is that a fusion reactor is modified to become a breeder of fissile material, as is examined in Glaser and Goldston’s 2012 paper. The operation of pure (i.e. non-hybrid) fusion reactors is not accompanied by the production of fissile materials required for nuclear weapons. Only a significant modification of the fusion reactor – the introduction of a special breeding section containing fertile material – would make the production of weapons grade fissile materials possible.

Glaser and Goldston put a high estimate on hybrid production at 2.85 kg of fissile material per MW-year of fusion energy. Their estimate for a modified fusion reactor is much smaller; they ran a simulation on a 2 GWt fusion plant and found that it could produce 20 kg per week. That is enough for almost three nuclear bombs per week, which definitely qualifies as a proliferation risk. On the bright side, Glaser and Goldston highlight that it would take weeks to refit a fusion reactor for breeding, and that we can take precautions when building larger reactors to make sure that they are much more difficult to outfit for breeding. Also, according to the conclusion of experts, the presence of such a section (in an environment where none at all should be present) could be easily discovered by qualified inspectors. This is in sharp contrast to a fission reactor where production of these materials occurs in the reactor core itself and where in addition a delicate balance has to be made of large inventories of ingoing and outcoming nuclear material to discover any possible diversion of fissile material.

The open or clandestine production of plutonium 239 is possible in a fusion reactor simply by placing natural or depleted uranium oxide at any location where neutrons of any energy are flying about. The ocean of slowing-down neutrons that results from scattering of the streaming fusion neutrons on the reaction vessel permeates every nook and cranny of the reactor interior, including appendages to the reaction vessel. Slower neutrons will be readily soaked up by uranium 238, whose cross section for neutron absorption increases with decreasing neutron energy.

In view of the dubious prospects for tritium replenishment, fusion reactors may have to be powered by the two deuterium-deuterium reactions that have substantially the same probability, one of which produces neutrons and helium 3, while the other produces protons and tritium. Because tritium breeding is not required, all the fusion neutrons are available for any use—including the production of plutonium 239 from uranium 238.

It is extremely challenging to approach energy breakeven with deuterium-deuterium reactions because their total reactivity is 20 times smaller than that of deuterium-tritium, even at much higher temperatures. But a deuterium-fueled “test reactor” with 50 megawatts of heating power and producing only 5 megawatts of deuterium-deuterium fusion power could yield about 3 kilograms of plutonium 239 in one year by absorbing just 10 percent of the neutron output in uranium 238. Most of the tritium from the second deuterium-deuterium reaction could be recovered and burned and the deuterium-tritium neutrons will produce still more plutonium 239, for a total of perhaps 5 kilograms. In effect, the reactor transforms electrical input power into “free-agent” neutrons and tritium, so that a fusion reactor fueled with deuterium-only can be a singularly dangerous tool for nuclear proliferation.

A reactor fueled with deuterium-tritium or deuterium-only will have an inventory of many kilograms of tritium, providing opportunities for diversion for use in nuclear weapons. Just as for fission reactors, International Atomic Energy Agency safeguards would be needed to prevent plutonium production or tritium diversion.

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Other non-nuclear risks:

Reactor designers will have to minimise non-nuclear risks such as Li-fires, release of chemical toxins like Be, sudden loss of vacuum or cooling liquids, etc… But none of the possible issues currently appear to be sufficiently serious to weigh importantly in societal discussions about the attractiveness of fusion compared to other energy systems.

Environmental pollution?

The primary fuels (D and Li) and the direct end product (He) are not radioactive, do not pollute the atmosphere, and do not contribute to the greenhouse effect or the destruction of the ozone layer. Helium is in addition chemically inert and very useful in industry. There are no problems with mining (Li) and fuel transportation. There also exist no ecological, geophysical and land-use problems such as those associated with biomass energy, hydropower and solar energy.

Measures for tritium containment and detritiation of substances contaminated with tritium will have to be taken. During normal operation the dose for the public in the neighbourhood of the plant will only be a fraction of the dose due to natural radioactivity.

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Additional disadvantages shared with fission reactors:

Tritium will be dispersed on the surfaces of the reaction vessel, particle injectors, pumping ducts, and other appendages. Corrosion in the heat exchange system, or a breach in the reactor vacuum ducts could result in the release of radioactive tritium into the atmosphere or local water resources. Tritium exchanges with hydrogen to produce tritiated water, which is biologically hazardous. Most fission reactors contain trivial amounts of tritium (less than 1 gram) compared with the kilograms in putative fusion reactors. But the release of even tiny amounts of radioactive tritium from fission reactors into groundwater causes public consternation.

Thwarting tritium permeation through certain classes of solids remains an unsolved problem. For some years, the National Nuclear Security Administration—a branch of the US Energy Department—has been producing tritium in at least one Tennessee Valley Administration-owned fission power reactor by absorbing neutrons in lithium-containing substitute control rods. There has been significant and apparently irreducible leakage of tritium from the rods into the reactor cooling water that’s released to the environment, to the extent that annual tritium production has been drastically curtailed.

In addition, there are the problems of coolant demands and poor water efficiency. A fusion reactor is a thermal power plant that would place immense demands on water resources for the secondary cooling loop that generates steam, as well as for removing heat from other reactor subsystems such as cryogenic refrigerators and pumps. Worse, the several hundred megawatts or more of thermal power that must be generated solely to satisfy the two classes of parasitic electric power drain places additional demand on water resources for cooling that is not faced by any other type of thermoelectric power plant. In fact, a fusion reactor would have the lowest water efficiency of any type of thermal power plant, whether fossil or nuclear. With drought conditions intensifying in sundry regions of the world, many countries could not physically sustain large fusion reactors.

Fusion reactor operation will require personnel whose expertise has previously been required only for work in fission plants—such as security experts for monitoring safeguard issues and specialty workers to dispose of radioactive waste. Additional skilled personnel will be required to operate a fusion reactor’s more complex subsystems including cryogenics, tritium processing, plasma heating equipment, and elaborate diagnostics. Fission reactors in the United States typically require at least 500 permanent employees over four weekly shifts, and fusion reactors will require closer to 1,000. In contrast, only a handful of people are required to operate hydroelectric plants, natural-gas burning plants, wind turbines, solar power plants, and other power sources.

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In a nutshell:

Naturally occurring thermonuclear fusion reaction (of light atoms to form a heavier nucleus) in the sun and every star in the universe, releases incredible amounts of energy. Demonstrating the controlled and sustained reaction of deuterium-tritium plasma should enable the development of fusion as an energy source here on Earth. The promising fusion power reactors could be operated on the deuterium-tritium fuel cycle with fuel self-sufficiency. The potential impact of fusion power on the environment and the possible risks associated with operating large-scale fusion power plants is being studied by different countries. The results show that fusion can be a very safe and sustainable energy source. A fusion power plant possesses not only intrinsic advantages with respect to safety compared to other sources of energy, but also a negligible long term impact on the environment provided certain precautions are taken in its design. One of the important considerations is in the selection of low activation structural materials for reactor vessel. Selection of the materials for first wall and breeding blanket components is also important from safety issues. It is possible to fully benefit from the advantages of fusion energy if safety and environmental concerns are taken into account when considering the conceptual studies of a reactor design. The significant safety hazards are due to the tritium inventory and energetic neutron fluence induced activity in the reactor vessel, first wall components, blanket system etc. The potential of release of radioactivity under operational and accident conditions needs attention while designing the fusion reactor.

To sum up, fusion reactors face some unique problems: a lack of a natural fuel supply (tritium), and large and irreducible electrical energy drains to offset. Because 80 percent of the energy in any reactor fueled by deuterium and tritium appears in the form of neutron streams, it is inescapable that such reactors share many of the drawbacks of fission reactors—including the production of large masses of radioactive waste and serious radiation damage to reactor components. These problems are endemic to any type of fusion reactor fueled with deuterium-tritium, so abandoning tokamaks for some other confinement concept can provide no relief.

If reactors can be made to operate using only deuterium fuel, then the tritium replenishment issue vanishes and neutron radiation damage is alleviated. But the other drawbacks remain—and reactors requiring only deuterium fueling will have greatly enhanced nuclear weapons proliferation potential.

These impediments—together with the colossal capital outlay and several additional disadvantages shared with fission reactors—will make fusion reactors more demanding to construct and operate, or reach economic practicality, than any other type of electrical energy generator.

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Section-14

Advantages of fusion power:

With increasing concerns over climate change and finite supplies of fossil fuels, we need new, better ways to meet our growing demand for energy. As the world moves towards net-zero emissions, sustainable and affordable power sources are urgently needed by humanity. One of the most promising technologies, fusion, has attracted the attention of governments and private companies like Chevron and Google. In fact, Bloomberg Intelligence has estimated that the fusion market may eventually be valued at $40 trillion.

The benefits of fusion power make it an extremely attractive option:

No carbon emissions. The only by-products of fusion reactions are small amounts of helium, an inert gas which can be safely released without harming the environment. Fusion power barely releases any CO2, in fact, there is no direct release of carbon dioxide whatsoever (Whyte, 2016). The most promising avenue of fusion is the use of hydrogen isotopes called deuterium and tritium (D-T) to fuse into helium & neutron, a very simple reaction that involves no carbon dioxide. The plant would even supply its own power, leaving very few places where CO2 emissions could be of concern. Indeed, the only way carbon dioxide is involved are the transport vehicles bring the fuel to the plant!
Abundant fuels. Deuterium can be extracted from water and tritium will be produced inside the power station from lithium, an element abundant in the earth’s crust and seawater. Even with widespread adoption of fusion power stations, these fuel supplies would last for many thousands of years.
High energy density. One kilogram of fusion fuel could provide the same amount of energy as 10 million kilograms of fossil fuel. A one-Gigawatt fusion power station will need less than one tonne of fuel during a year’s operation. Consequently, the amount of fuel needed in a nuclear plant is much smaller compared with those of other types of power-generating facilities.
Less radioactive waste than fission. There is no radioactive waste by-product from the fusion reaction. Only reactor components become radioactive; the level of activity depends on the structural materials used. Research is being carried out on suitable materials to minimise decay times as much as possible.
Safety. A large-scale nuclear accident is not possible in a fusion reactor. The amounts of fuel used in fusion devices are very small (about the weight of a postage stamp at any one time). Furthermore, as the fusion process is difficult to start and keep going, there is no risk of a runaway reaction which could lead to a meltdown.
Reliable power. Fusion energy is on-demand and independent from the weather, making it an excellent option in a dependable portfolio for power generation. Fusion power plants will be designed to produce a continuous supply of large amounts of electricity. Once established in the market, costs are predicted to be broadly similar to other energy sources.
Scalable power. Fusion scales effectively with a small land footprint that can be located close to cities.
Limited risk of proliferation. Fusion doesn’t employ fissile materials like uranium and plutonium. (Radioactive tritium is neither a fissile nor a fissionable material.) There are no enriched materials in a fusion reactor like ITER that could be exploited to make nuclear weapons.
No risk of meltdown. A Fukushima-type nuclear accident is not possible in a tokamak fusion device. It is difficult enough to reach and maintain the precise conditions necessary for fusion—if any disturbance occurs, the plasma cools within seconds and the reaction stops. The quantity of fuel present in the vessel at any one time is enough for a few seconds only and there is no risk of a chain reaction.
Sustainable. All we know is that the energy from the nuclear fusion power plant is not renewable. Though, it can become possibly sustainable by using fusion and breeder reactors. By controlling atomic fusion similar to the sun, we can have an unlimited energy source.

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Drawbacks of Nuclear Fusion:

High Initial Cost.

Although the operational cost is relatively low, the equipment, infrastructure, and conditions required for the reaction to be carried out safely need billions of dollars to start with. Developing the technology to harness nuclear fusion is currently very expensive, and it is not yet clear if it will be economically viable in the long term.

It requires almost as much energy to create nuclear fusion as the energy it creates.

To fuse two atoms together, high levels of heat are required. In order to create this heat, a large energy investment must be made. This means the reaction from nuclear fusion produces just barely more than is required to make it, so with our current technology, it is not really a plausible energy creation method.

It is extremely difficult to achieve.

In stars, strong gravitational forces and high temperatures naturally create a fusion environment. But here on Earth, we are facing the challenge to make nuclear fuel hot and confined enough to start a self-sustaining ignition. Imagine trying to contain the plasma (a gaseous mixture of deuterium, tritium atoms and ions, and helium the fusion product) at 100 million degrees Celsius. No material can withstand that temperature. So, scientists attempt to keep the plasma (being electrically charged and having a magnetic field of its own) suspended in a magnetic field produced by superconducting magnets around the fusion chamber/vessel. This method is tough to achieve compared to nuclear fission.

It produces radioactive waste.

The radioactive waste produced with fusion is not the same as with fission, and the two are often confused. With a nuclear fission reactor, the radiation is alpha particles, beta particles, and gamma rays (which can penetrate your skin and break apart the bonds in your DNA structure, giving you all kinds of cancer). In contrast, in a nuclear fusion reactor, the vessel wall is the only part that will be bombarded by the high energy neutrons, and if, in the worst case, all the protective layers surrounding the main fusion vessel fail, the neutron radiation will stop as soon as fusion reaction stops. In a fission reactor, the cancer-causing radiation still exists even in the waste materials, which means that extreme measures are needed to bury the waste to keep it as far away as possible from humans. In the case of nuclear fusion, the activated materials (i.e., the metal vessels which have been bombarded by neutrons) can be stored safely for about 100 years, after which the radiation level becomes so low that they can be reused in the fusion reactor again.

More research is needed to solve its issues.

To make fusion work, enough intelligent minds need to cooperate to address and solve its challenges.

Its practical energy returns are still quite unreachable.

Existing methods for artificially igniting nuclear fusion still require large amounts of energy input even on a small scale for a brief moment. “Igniting” it means achieving self-sustainable fusion reactions. In fact, the world’s biggest tokamak-type fusion reactor today is studying plasma on a large scale, but is still short of the dimensions needed for a productive fusion reactor.

The process is still in infancy.

The reactors able to contain nuclear fusion are in the development phase, and we may have to wait before generating profitable energy from this available resource.

There may be unanticipated consequences to using nuclear fusion.

The fact is that we don’t really know much about this form of energy creation. Are there dangers that we simply do not know yet and cannot predict?

Limited commercial deployment.

While nuclear fusion has been demonstrated in controlled laboratory experiments, it has not yet been deployed on a commercial scale. It will likely be many years before nuclear fusion becomes a practical energy source.

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Section-15

Will nuclear fusion be a sustainable energy source?

Why haven’t we attained fusion power yet?

Fusion research requires the use of expensive nuclear reactors (such as Tokamaks) which can contribute to construction costs upwards of hundreds of million pounds (even reaching the billion count for larger international projects). Requiring this massive upfront investment may result in fusion energy researchers and organisations becoming trapped in the ‘valley of death’ – the space between research and commercialisation. For firms which cannot prove a pathway to commercialisation, enticing investment to cross this valley is a challenge – for fusion energy (in which commercially viable fusion energy is not yet attained), this is especially difficult.

This high level of required investment is coupled with fusion reactors being large, complex projects with long development and construction times, making fusion energy a long-term high-risk investment. Not only are investors unlikely to receive a short-term return on their investments, but there is also the potential that these innovations may be entirely incapable of achieving fusion energy at a commercially viable level. Coupled with the perception that fusion energy is ‘always 30 years away’ (and potentially unattainable), investors with low appetites for risk may be dissuaded from the fusion research space.

Another problem preventing the attainment of fusion energy is its slow rate of innovation implementation. Fusion innovation occurs in cycles, with the construction of new generations of fusion reactors only commencing once the previous generation has started operation. No major design evolution can be implemented before the preceding generation has proven the efficacy (or lack thereof) of the current generation. This causes a technology lock-in, where new discoveries and innovations struggle to be implemented in a timely manner, slowing the rate of innovation. Compared to other energies (such as wind), the rate of growth, innovation, deployment, and adoption is significantly slower.

However, once major fusion reactors are established, they can be used to test new innovations with continuing upgrades and capital investment. First proposals for the JET were completed in 1975 with its first experiment commencing 8 years later in 1983. Regular experiments and upgrades continued to be ran at JET in the following years, including a 15-month shut down in 2009 to construct upgrades needed to test concepts from the upcoming ITER design. These fusion experiments and reactors can continue to be useful for many years – especially with further investment into upgrades and new innovations; however, this does not preclude fusion’s need for both lengthy initial construction times and continuing capital investment required to research new innovations.

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To decarbonize the power system, will fusion energy help?

Power generation currently accounts for approximately 30 percent of global CO2 emissions. To meet the Paris Agreement’s target of full decarbonization by 2050, many governments and utilities are shifting away from fossil fuels as a primary energy source and turning to renewable-energy technologies. The goal for many power sector players and their regulators is a zero-carbon energy grid. Volatility in the energy markets and geopolitical challenges may have complicated the transition to net zero in the short run, but in the longer run, the economics of renewable-power sources will drive likely investment into them.

In addition, as other industries transition away from fossil fuels, the demand for zero- or low-carbon electricity will increase. For example, as electric vehicles replace internal-combustion vehicles, more electricity generation will be required. McKinsey’s Global Energy Perspective 2022 projects that power consumption could triple by 2050. For countries to hit their decarbonization goals, it is thus essential that not just existing but also all added generation be zero carbon.

Renewable energy from wind and solar is currently the most cost-efficient form of new zero-carbon electrical generation, and by 2030 it is expected be the lowest-cost of any kind of generation in most markets. The continued development of wind and solar technologies and construction techniques is expected to continue, which means that the bulk of new near-term clean-electricity generation will probably come from these two sources. But wind and solar have their limitations: they are nondispatchable—that is, they generate electricity when the wind blows or the sun shines, not necessarily when the grid needs it. One example of such limitations: energy shortages in Europe that began in 2019—before Russia’s invasion of Ukraine—that were partly caused by historically low wind speeds lasting for months.

Other forms of dispatchable zero-carbon energy, such as geothermal or tidal power, are encouraging. But they are generally more expensive than wind and solar, can function only in a limited number of sites, and are less technologically mature. Grid-scale batteries and other forms of energy storage are increasingly promising, but they are still cost prohibitive at the required durations and have not yet reached the level of technological readiness for large-scale deployment.

Nuclear-fusion energy could help provide flexibility for zero-carbon electricity grids. Fusion—different from nuclear fission, which releases energy by splitting an atom in two—creates energy by combining two atoms, typically hydrogen isotopes. Fusion is dispatchable, which means that, unlike wind and solar, it does not rely on environmental or other external variables to generate power. The process of producing fusion energy creates no carbon emissions and no long-lived nuclear waste from spent fuel.

Historically, fusion machines have not been technically viable, because the energy input required to power the reaction has been larger than the energy produced by the machine. But in the last five years, fusion energy has reached a turning point in its development.

A zero-carbon energy grid requires flexibility:

A grid that relies on nondispatchable energy sources, like wind and solar, typically requires alternative sources of power generation. In other words, the minute-by-minute matching of the supply of wind and solar power to demand cannot occur in the way that energy from baseload-generating plants fueled by coal or natural gas can. Wind and solar are therefore sometimes referred to as variable renewable energy (VRE). A zero-carbon electrical grid cannot operate in a stable way with 100 percent VRE and will require sources of flexibility to back up, or firm, the VRE generation. In addition, land use for VRE and the required transmission infrastructure to connect it are increasing concerns. In Germany, for example, the construction of wind projects has not only slowed but also failed to achieve deployment goals for the past four years.

Flexibility—the ability to manage the intermittency of nondispatchable wind and solar—is thus crucial to achieving a zero-carbon energy grid. The real-time matching of supply and demand can be ensured in a number of ways. Gas and coal plants can adjust production up or down to smooth out fluctuations in the output of wind and solar power, for example, but relying on fossil fuels is not acceptable in a zero-carbon generation system unless carbon capture technology is used. Transmission lines can balance production across geographies. Well-designed incentives can encourage users to modify their consumption via demand-side management programs. Battery storage can serve the power system as both a generator (when discharging) and a consumption point, or load (when charging). This need for flexibility is creating a wave of innovation across different technologies, including dozens of types of batteries and other energy storage technologies, natural-gas combustion with carbon sequestration, and digitally enabled demand-side aggregators.

The most valuable resources of a decarbonized grid are dispatchable zero-carbon generation sources—technologies that can be turned on during periods of low wind and solar output. Unlike batteries or other electricity storage devices, which must be charged from the grid, these technologies generate net electrical energy and thus play a unique role in a decarbonized grid: they can be turned on during periods of low wind and solar output, without the duration constraints of batteries and with a high assurance of availability. If this kind of dispatchable zero-carbon generation were available at a sufficiently low cost, the energy system in Europe, for example, could consist almost entirely of some VRE plus dispatchable sources. Finding an affordable, scalable, and safe dispatchable zero-carbon generation technology would constitute a gigantic step toward a sustainable energy future for humanity. Fusion is potentially one of these technologies.

Fusion energy is potentially at a turning point:

Fusion energy, also known as controlled nuclear fusion, has been pursued since the 1950s, first as a classified program and then, since a landmark conference in 1958, as an open, collaborative international effort. Simply put, fusion works by combining light atoms, such as hydrogen, into heavier products, such as helium. The reaction releases enormous amounts of energy, which is then captured and converted into useful electricity by a fusion machine. There are many fusion machine designs, such as magnetic confinement (tokamaks and stellarators), inertial confinement, and magnetized target fusion.

Fusion energy has some critical advantages over fission as a zero-carbon power source. It is fully controllable and thus creates dispatchable power with a ramp rate fast enough to complement renewables in a VRE-heavy grid. The fuel is readily available to all nations—for example, the required hydrogen isotopes for one commonly proposed fusion reaction can be extracted from seawater; some companies, such as TAE Technologies, are proposing fuel cycles using ordinary materials like hydrogen and boron. The fuel is slowly metered into the machine in a way that makes meltdowns or runaway events essentially impossible. And fusion creates no carbon emissions and minimal nuclear waste—only the vessel itself. Unlike today’s fission plants, fusion produces no long-lived fuel waste.

These advantages are so substantial that fusion energy has long seemed the holy grail of energy technology. However, creating the temperature and pressure conditions required to initiate the reaction in the fusion machine is a great scientific and technological challenge—so much so that after more than 70 years of experimentation, a marketable solution remains elusive. Skeptics point out that practical fusion energy has been 20 years away for the past 50 years. The basic problem is the difficulty of preventing energy from leaking out, which means that fusion machines, so far, consume more energy than they create.

In addition, the machines are complex: they require the world’s most advanced magnets, hard-to-engineer materials that can withstand the intense temperatures on the machine’s inside wall, and submillimeter precision for machined parts several meters across. Conceptual designs, when costed out, have seemed too expensive to be competitive with other forms of generation. From the 1970s onward, governmental enthusiasm for funding fusion projects has therefore waned. Numerous privately funded start-ups now hope to operate commercial fusion machines before the end of the 2020s.

Now there are reasons to believe that fusion energy could be at a turning point. These include the following:

-1. The development of enabling technologies has allowed fusion to break new barriers. For example, additive manufacturing—often called 3-D printing—allows the complex geometrical shapes of parts required for the walls of fusion machines to be produced at low cost and designs to be iterated quickly. Rapidly increasing computer capacity has made it possible for simulation codes to represent fusion reactions in greater detail, so predictions about performance can be made without the expense of building large experiments. And rapid digital controls are improving the suppression of the vibrations that cause energy to leak out of the core fusion reaction. These and other technological advances have created the conditions in which fusion can develop more rapidly. In the same way, advancing lithium-ion battery technology has allowed electric vehicles to start proliferating after failing to achieve widespread adoption for 100 years, and manufacturing breakthroughs in low-cost polysilicon solar panels (by Chinese and other OEMs) from 2005 to 2008 largely enabled the massive scale-up of solar deployment in the past decade.

-2. There has been a sea change in the orientation of fusion research programs. Most fusion research used to occur in science-oriented, publicly funded labs. Now there is a new wave of privately backed programs, as well as programs working toward commercial viability in both the public and private sectors. This development has been driven partly by the fact that enabling technologies have progressed enough for private investors to step in with confidence. The result is that numerous privately funded start-ups now hope to operate commercial fusion machines before the end of the 2020s. A few major public labs are also launching programs to build commercially viable operational fusion machines. For example, the United Kingdom’s STEP program (out of the fusion research center at Culham) seeks to build a new fusion machine by 2040, and the Chinese fusion program recently accelerated plans for the China Fusion Engineering Testing Reactor, a prototype commercial fusion machine.

-3. Investment in private fusion funding has greatly accelerated. Private investment in fusion energy has surged over the past 20 years, with the value of investments nearly tripling in 2021. This increased funding is a combination of traditional technology venture funding; strategic investments by incumbent energy companies, such as Eni’s and Equinor’s investments in Commonwealth Fusion Systems; seed investments by ultra-high-net-worth individuals, such as Sam Altman’s 2021 investment in Helion Energy; and government investments, such as the participation of the UK Innovation & Science Seed Fund in Tokamak Energy’s series A venture round. Access to capital is allowing private companies to construct larger components for fusion machines and to design full-scale prototypes for construction later this decade.

The next five to ten years will be critical for the development of fusion energy. Some key benchmarks over the next decade will help indicate whether fusion is truly on a trajectory toward implementation. Over the next five years, we would expect to see these advances:

-1. Demonstration of the core temperature and pressure conditions required for the fusion energy produced to exceed the heating energy injected into the reaction. The temperatures required to produce energy from a fusion reaction are on the order of 150 million degrees Celsius. The hotter the core of a fusion machine can get, and the more pressure it can withstand without leaking energy, the more net energy it can produce. This level of confinement, long a challenge in achieving net energy gain, is a necessity.

-2. Confirmation of component-level performance of various fusion concepts. By 2025, leading private fusion players plan to demonstrate, or in some cases have already demonstrated, major subsystems. These include powerful high-temperature superconducting magnets (achieved by Commonwealth Fusion Systems in 2021), plasma injectors (such as the P13 injector demonstrated by General Fusion in 2017), radio frequency heating systems, and new wall materials that can survive the intense heat of a fusion machine’s interior. Successful tests of these major subsystems and components by 2025 would mean that operational prototype plants could be functioning by the decade’s end.

-3. Demonstration of fusion in conditions relevant for power plants and validation of the economics. By 2026, we would expect to see at least one player integrating all major subsystems into a functioning prototype that can validate system level performance. Such a prototype would also make it possible, for the first time, to conduct a Class 4 (feasibility-study level) estimate of the costs of a fusion machine’s parts manufacturing and assembly. This would be the first model of a fusion power plant’s economics that could truly inspire confidence.

The technology still needs to develop, and there is no guarantee that recent fusion concepts will ultimately produce net energy. Yet this time may be different for the longtime dream of fusion energy. Fusion’s characteristics as a zero-carbon, dispatchable source of electricity would allow it to play a key role in grids, along with wind, solar, carbon capture, and other technologies.

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Re-examining the role of nuclear fusion in a renewables-based energy mix, a 2021 study:

Fusion energy is often regarded as a long-term solution to the world’s energy needs. However, even after solving the critical research challenges, engineering and materials science will still impose significant constraints on the characteristics of a fusion power plant. Meanwhile, the global energy grid must transition to low-carbon sources by 2050 to prevent the worst effects of climate change. Authors review three factors affecting fusion’s future trajectory: (1) the significant drop in the price of renewable energy, (2) the intermittency of renewable sources and implications for future energy grids, and (3) the recent proposition of intermediate-level nuclear waste as a product of fusion. Within the scenario assumed by their premises, authors find that while there remains a clear motivation to develop fusion power plants, this motivation is likely weakened by the time they become available. Authors also conclude that most current fusion reactor designs do not take these factors into account and, to increase market penetration, fusion research should consider relaxed nuclear waste design criteria, raw material availability constraints and load-following designs with pulsed operation.

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Section-16

Nuclear fusion research:

History of fusion energy research:

The fusion process has been studied in order to understand nuclear matter and forces, to learn more about the nuclear physics of stellar objects, and to develop thermonuclear weapons. During the late 1940s and early ’50s, research programs in the United States, United Kingdom, and the Soviet Union began to yield a better understanding of nuclear fusion, and investigators embarked on ways of exploiting the process for practical energy production. Fusion reactor research focused primarily on using magnetic fields and electromagnetic forces to contain the extremely hot plasmas needed for thermonuclear fusion. Researchers soon found, however, that it is exceedingly difficult to contain plasmas at fusion reaction temperatures because the hot gases tend to expand and escape from the enclosing magnetic structure. Plasma physics theory in the 1950s was incapable of describing the behaviour of the plasmas in many of the early magnetic confinement systems.

The undeniable potential benefits of practical fusion energy led to an increasing call for international cooperation. American, British, and Soviet fusion programs were strictly classified until 1958, when most of their research programs were made public at the Second Geneva Conference on the Peaceful Uses of Atomic Energy, sponsored by the United Nations. Since that time, fusion research has been characterized by international collaboration. In addition, scientists have also continued to study and measure fusion reactions between the lighter elements so as to arrive at a more accurate determination of reaction rates. The formulas developed by nuclear physicists for predicting the rate of fusion energy generation have been adopted by astrophysicists to derive new information about the structure and evolution of stars.

Work on the other major approach to fusion energy, inertial confinement fusion (ICF), was begun in the early 1960s. The initial idea was proposed in 1961, only a year after the reported invention of the laser, in a then-classified proposal to employ large pulses of laser energy (which no one then quite knew how to achieve) to implode and shock-heat matter to temperatures at which nuclear fusion would proceed vigorously. Aspects of inertial confinement fusion were declassified in the 1970s and, especially, in the early 1990s to reveal important aspects of the design of the targets containing fusion fuels. Very painstaking and sophisticated work to design and develop short-pulse, high-power lasers and suitable millimetre-sized targets continues, and significant progress has been made.

Although practical fusion reactors have not been built yet, the necessary conditions of plasma temperature and heat insulation have been largely achieved, suggesting that fusion energy for electric-power production is now a serious possibility. Commercial fusion reactors promise an inexhaustible source of electricity for countries worldwide. From a practical viewpoint, however, the initiation of nuclear fusion in a hot plasma is but the first step in a whole sequence of steps required to convert fusion energy to electricity. In the end, successful fusion power systems must be capable of producing electricity safely and in a cost-effective manner, with a minimum of radioactive waste and environmental impact. The quest for practical fusion energy remains one of the great scientific and engineering challenges of humankind.

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Technology from Fusion Research:

Nuclear fusion is the power of the sun. When hydrogen is compressed and heated, its atoms will fuse together to form a heavier element, helium. This reaction releases the huge amount of energy stored in the nucleus.

If harnessed by humanity, fusion promises nearly unlimited energy, without pollution. One pound of hydrogen fusion fuel is capable of yielding as much energy as is contained in 10 million pounds of coal.

However, a fusion reaction will only take place at extremely high temperature and pressure: about 100 million degrees and 1000 times normal solid densities. In the sun, gravity is used to confine hydrogen at the density necessary for fusion. In a thermonuclear weapon, a nuclear fission explosion creates the temperatures and densities necessary for fusion. Unfortunately, neither the gravity of the sun nor the forces of a nuclear weapon are replicable on earth in the controlled manner necessary for energy generation. There are two practical ways being closely studied today to contain the pressure and temperature necessary to harvest energy from fusion; lasers can be used to ignite and inertia to confine the fuel or magnets can be used to confine plasma that is heated by an electric current. The issue is that these magnets, electric currents and lasers require more energy to generate and contain the extreme heat and pressure of a fusion reaction than gained from any experimental fusion reactions.

The first man-made nuclear fusion reactions happened on the Eniwetok Atoll in 1951 and 1952 during testing of the hydrogen bomb. Since this demonstration, the United States government has funded basic research into how to harness the power of fusion for peaceful energy production. Over these decades, the U.S. government has appropriated a total of approximately $36.4 billion in research funding (in inflation-adjusted 2010 dollars); an average of $639 million per year. The funding is split into two tracks: the Magnetic Fusion Energy (MFE) program – which focuses on using superconducting magnets to contain the plasma – and the Inertial Confinement Fusion (ICF) program – which uses lasers. The peak year for fusion funding was 1982, when the U.S. spent $1.3 billion. In 2010, the U.S. government appropriated $884 million for fusion funding (all numbers are 2010 dollars).

The benefits of decades of fusion research are not simply deferred to the future when fusion power will be commercially available. Other technologies have developed out of fusion research that have important applications across American society. Because of the very high requirements of confining fusion, many of the technological developments we enjoy because of the fusion program are in laser or magnet technology.

Sectors that have benefitted from technological spin-offs from fusion research include medicine, manufacturing, electricity transmission, environmental cleanup, and even national security. Some of the benefits we see every day, some are hidden, and still others have not yet achieved their promise. Even though fusion reactions have not yet powered so much as a light bulb, the impact of decades of fusion research is apparent across the American economy.

Impacts on Medicine:

Modern medicine has benefitted from the advances in magnets and lasers that fusion research has generated. Advances in magnet technology have made identifying and treating tumors easier, and advances in laser technology have created a better way to cut through skin and tissue.

One of the most revolutionary tools in modern medicine is the magnetic resonance imaging (MRI) machine. MRIs utilize powerful magnets and harmless (non-ionizing) radiation to create an image of a targeted area within the body. From their first use in 1973, MRIs have become a common tool for doctors to identify tumors or injuries that earlier could have required invasive surgery. Fusion research drove the advances in large bore superconducting magnets that made the invention of MRIs possible.

Proton radiotherapy is a type of radiation therapy for cancerous tumors that allows doctors to target only the tumor. Traditional radiation therapy uses X-ray radiation that can damage the surrounding, healthy tissue. Proton therapy can more specifically target the tumor. However, proton therapy requires a large cyclotron to isolate and accelerate protons into a beam. Due to their size – about 1000 tons – and capital cost – roughly $100 to $150 million – there are only 37 proton therapy centers in the world. However, advances in superconducting coils for magnetic fusion experiments at MIT’s Plasma Science and Fusion Center may allow for the creation of a smaller, less expensive “compact synchrocyclotron” that will allow many more hospitals to offer proton therapy.

Lasers are also useful in modern medicine. The ‘Ultra-Short Pulse Laser’ was developed in the 1990s as part of the Inertial Confinement Fusion program at Lawrence Livermore National Laboratory. It utilizes pulses of energy that last for just 50 to 1,000 femtoseconds (quadrillionths of a second). These pulses of energy are not long enough to do lasting damage to surrounding tissue. These lasers have been very effective in delicate surgeries, such as laser eye surgery or tonsillectomies.

Impacts on Manufacturing:

The high pressure and heat resulting from the plasma necessary for a fusion reaction have placed a demanding requirement for materials capable of withstanding such extremes. Some areas of a fusion reactor that face plasma will have withstand heat of more than 2000°C. Very few materials can withstand this heat and maintain their structural integrity.

However, the combination of two materials—carbon fiber composite coated with tungsten—has proved to be able to withstand the heat. The development of new materials like this has important spin-off effects on other manufacturing processes that need to withstand high heat.

Carbon fiber composite is increasingly being used as a material in airline engines. Carbon fiber technology originally developed for the plasma-facing tiles inside a reactor is now being used for high performance brakes in most airplanes.

In addition to alternative uses for the materials used to build a reactor, technology that was developed to heat plasma, called “ion source technology,” has been adapted to harden materials or to microscopically sculpt surfaces. Ion beams sculpt the surface of a material at the molecular level. This technology is now used to improve titanium hip implants, to improve the lenses of lasers, and even to manufacture consumer eyewear.

Impacts on Electricity Transmission:

The “smart grid” is a term for how to move and integrate fluctuating renewable energy generation into an existing electricity grid. A smart grid adapts to fluctuations in electricity generation by moving electricity over long distances and storing excess capacity. Many of the applications that will make a smart grid possible come from advances generated by fusion research.

High-temperature superconducting (HTS) wires are being developed jointly by Oak Ridge National Laboratory and private industry out of technology originally developed by the Applied Superconductivity Group, a part of the Department of Energy’s Fusion Energy division. HTS wires can conduct five times the electricity with half the transmission loss compared to traditional copper cable. These wires will allow power companies to create the “electricity superhighways” along which the smart grid will distribute electricity to where it is most needed.

Energy storage is a problem for fusion reactors. A reactor can require a pulse of energy upwards of 50 megawatts for less than one second of plasma discharge. This is often too much for a local power grid to handle. That means that the reactor has to develop a way to store energy in preparation for that pulse of electricity. Massive new flywheel machines are being developed to supply fusion reactors, some with a rotating mass exceeding 30 tons. They can store energy over time, to be released when needed.

Energy storage is a key part of making any electricity grid “smart” as well. If wind and solar power are going to be a large part of electricity generation, it will be important to build up reserves for when it is neither sunny nor windy. By addressing the problem of how to power fusion reactors, emerging flywheel technology will help with a smart grid.

Nuclear Security:

In the 1990s, scientists at the Princeton Plasma Physics Laboratory (PPPL) needed a way to determine the amount and type of residual radioactive elements that remained inside a decommissioned fusion reactor. They developed a device that could be lowered into the reactor and could identify each of the types of radiation on the inside of the reactor.

In late 2001, after the September 11 attacks, the United States government issued a call for proposals for a portable device that would be able to detect radioactive materials. It is important to be able to detect radiation because terrorists could try to detonate either a nuclear weapon or a ‘dirty bomb’ (a conventional bomb that spreads radioactive material) inside the United States. The scientists at PPPL have since adapted their device so that it can be used around the country. It is now called the “Miniature Integrated Nuclear Detection System” (MINDS). MINDS units have been deployed to ports and areas of critical infrastructure around the United States.

In a nutshell:

The American government, along with governments around the world, have been funding research to utilize the almost unlimited potential energy that fusion could provide for civilian energy generation. This is a vast technological and engineering challenge and it remains incomplete. However, history has shown that large scientific endeavours can lead to new and unexpected advances. The laser and superconductor industries have been the main beneficiaries of fusion research, but it is increasingly reaching across all sectors. Key sectors that have benefitted from technological spin-offs from fusion research include: medicine, manufacturing, electricity transmission, environmental cleanup, and national security. In addition to these contributions, fusion research funding has supported and developed the skills of generations of scientists and engineers. The “human capital” developed in their research on fusion energy is transferable across many other scientific fields.

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Demystifying vortex rings in nuclear fusion, supernovae, a 2023 study:

The vortex rings move outward from the collapsing star, populating the universe with the materials that will eventually become nebulae, planets and even new stars — and inward during fusion implosions, disrupting the stability of the burning fusion fuel and reducing the efficiency of the reaction.

This research, which elucidates how such vortex rings form, can help scientists understand some of the most extreme events in the universe and bring humanity one step closer to capturing the power of nuclear fusion as an energy source.

Nuclear fusion pushes atoms together until they merge. This process releases several times more energy than breaking atoms apart, or fission, which powers today’s nuclear plants. Researchers can create this reaction, merging forms of hydrogen into helium, but at present, much of the energy used in the process is wasted.

Part of the problem is that the fuel can’t be neatly compressed. Instabilities cause the formation of jets that penetrate into the hotspot, and the fuel spurts out between them – it can be compared to trying to squish an orange with your hands, how juice would leak out between your fingers.

Vortex rings that form at the leading edge of these jets, the researchers have shown, are mathematically similar to smoke rings, the eddies behind jellyfish and the plasma rings that fly off the surface of a supernova.

Better understanding the formation of swirling, ring-shaped disturbances — known as vortex rings — could help nuclear fusion researchers compress fuel more efficiently, bringing it closer to becoming a viable energy source. A mathematical model linking these vortices with more pedestrian types, like smoke rings, could help engineers control their behavior in power generation and more. Perhaps the most famous approach to fusion is a spherical array of lasers all pointing toward a spherical capsule of fuel. This is how experiments are set up at the National Ignition Facility, which has repeatedly broken records for energy output in recent years. The model developed by researchers at the University of Michigan could aid in the design of the fuel capsule, minimizing the energy lost while trying to ignite the reaction that makes stars shine. In addition, the model could help other engineers who must manage the mixing of fluids after a shock wave passes through, such as those designing supersonic jet engines, as well as physicists trying to understand supernovae. The model can also help researchers understand the limits of the energy that a vortex ring can carry, and how much fluid can be pushed before the flow becomes turbulent and harder to model as a result. In ongoing work, the team is validating the vortex ring model with experiments.

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Magnetic control of tokamak plasmas through deep reinforcement learning, a 2022 study:

The inside of a tokamak—the doughnut-shaped vessel designed to contain a nuclear fusion reaction—presents a special kind of chaos. Hydrogen atoms are smashed together at unfathomably high temperatures, creating a whirling, roiling plasma that’s hotter than the surface of the sun. Finding smart ways to control and confine that plasma will be key to unlocking the potential of nuclear fusion, which has been mooted as the clean energy source of the future for decades. At this point, the science underlying fusion seems sound, so what remains is an engineering challenge. We need to be able to heat this matter up and hold it together for long enough for us to take energy out of it. That’s where AI comes in.

In stars, which are also powered by fusion, the sheer gravitational mass is enough to pull hydrogen atoms together and overcome their opposing charges. On Earth, scientists instead use powerful magnetic coils to confine the nuclear fusion reaction, nudging it into the desired position and shaping it like a potter manipulating clay on a wheel. The coils have to be carefully controlled to prevent the plasma from touching the sides of the vessel: this can damage the walls and slow down the fusion reaction. (There’s little risk of an explosion as the fusion reaction cannot survive without magnetic confinement). But every time researchers want to change the configuration of the plasma and try out different shapes that may yield more power or a cleaner plasma, it necessitates a huge amount of engineering and design work. Conventional systems are computer-controlled and based on models and careful simulations, but they are complex and not always necessarily optimized.

Tokamaks are torus-shaped devices for nuclear fusion research and are a leading candidate for the generation of sustainable electric power. A main direction of research is to study the effects of shaping the distribution of the plasma into different configurations to optimize the stability, confinement and energy exhaust, and, in particular, to inform the first burning-plasma experiment, ITER. Confining each configuration within the tokamak requires designing a feedback controller that can manipulate the magnetic field through precise control of several coils that are magnetically coupled to the plasma to achieve the desired plasma current, position and shape, a problem known as the tokamak magnetic control problem.

The conventional approach to this time-varying, non-linear, multivariate control problem is to first solve an inverse problem to precompute a set of feedforward coil currents and voltages. Then, a set of independent, single-input single-output PID controllers is designed to stabilize the plasma vertical position and control the radial position and plasma current, all of which must be designed to not mutually interfere. Most control architectures are further augmented by an outer control loop for the plasma shape, which involves implementing a real-time estimate of the plasma equilibrium to modulate the feedforward coil currents. The controllers are designed on the basis of linearized model dynamics, and gain scheduling is required to track time-varying control targets. Although these controllers are usually effective, they require substantial engineering effort, design effort and expertise whenever the target plasma configuration is changed, together with complex, real-time calculations for equilibrium estimation.

A radically new approach to controller design is made possible by using reinforcement learning (RL) to generate non-linear feedback controllers. The RL approach, already used successfully in several challenging applications in other domains, enables intuitive setting of performance objectives, shifting the focus towards what should be achieved, rather than how. Furthermore, RL greatly simplifies the control system. A single computationally inexpensive controller replaces the nested control architecture, and an internalized state reconstruction removes the requirement for independent equilibrium reconstruction. These combined benefits reduce the controller development cycle and accelerate the study of alternative plasma configurations. Indeed, artificial intelligence has recently been identified as a ‘Priority Research Opportunity’ for fusion control, building on demonstrated successes in reconstructing plasma-shape parameters, accelerating simulations using surrogate models and detecting impending plasma disruptions. RL has not, however, been used for magnetic controller design, which is challenging due to high-dimensional measurements and actuation, long time horizons, rapid instability growth rates and the need to infer the plasma shape through indirect measurements.

In this work, authors introduce a previously undescribed architecture for tokamak magnetic controller design that autonomously learns to command the full set of control coils. This architecture meets control objectives specified at a high level, at the same time satisfying physical and operational constraints. This approach has unprecedented flexibility and generality in problem specification and yields a notable reduction in design effort to produce new plasma configurations. Authors successfully produce and control a diverse set of plasma configurations on the Tokamak à Configuration Variable, including elongated, conventional shapes, as well as advanced configurations, such as negative triangularity and ‘snowflake’ configurations. Their approach achieves accurate tracking of the location, current and shape for these configurations. Authors also demonstrate sustained ‘droplets’ on TCV, in which two separate plasmas are maintained simultaneously within the vessel. This represents a notable advance for tokamak feedback control, showing the potential of reinforcement learning to accelerate research in the fusion domain, and is one of the most challenging real-world systems to which reinforcement learning has been applied.

Here researchers have taught a deep reinforcement learning system to control the 19 magnetic coils inside TCV, the variable-configuration tokamak at the Swiss Plasma Center, which is used to carry out research that will inform the design of bigger fusion reactors in the future. AI, and specifically reinforcement learning, is particularly well suited to the complex problems presented by controlling plasma in a tokamak. Fusion offered a particular challenge to scientists because the process is both complex and continuous. The state of a plasma constantly changes and to make things even harder, it can’t be continuously measured. It is what AI researchers call an “under–observed system.” Researchers taught a reinforcement learning algorithm to control the fiery plasma inside a tokamak nuclear fusion reactor.

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Nuclear fusion and space exploration:

On Dec. 5, 2022, a team at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) achieved the milestone. The nuclear fusion feat has broad implications, fueling hopes of clean, limitless energy. As for space exploration, one upshot from the landmark research is attaining the long-held dream of future rockets that are driven by fusion propulsion. But is that prospect still a pipe dream or is it now deemed reachable?

Richard Dinan is the founder of Pulsar Fusion in the United Kingdom. He’s also the author of the book “The Fusion Age: Modern Nuclear Fusion Reactors.” “Fusion propulsion is a much simpler technology to apply than fusion for energy. If fusion is achievable, which at last the people are starting see it is, then both fusion energy and propulsion are inevitable,” Dinan said. “One gives us the ability to power our planet indefinitely, the other the ability to leave our solar system. It’s a big deal, really.” Exhaust speeds generated from a fusion plasma, Dinan said, are calculated to be roughly one-thousand times that of a Hall Effect Thruster (conventional ion thrusters), electric propulsion hardware that makes use of electric and magnetic fields to create and eject a plasma. Approaching the operation of the reactor, the machinery works creating plasma into electrically charged particles which then uses a rotating magnetic field to generate the thrust. “The financial implications that go with that make fusion propulsion, in our opinion, the single most important emerging technology in the space economy,” Dinan said. Pulsar Fusion has been busy working on a direct fusion drive initiative, a steady state fusion propulsion concept that’s based on a compact fusion reactor. According to the group’s website, Pulsar Fusion has proceeded to a Phase 3 task, manufacturing an initial test unit. Static tests are slated to occur next year, followed by an in-orbit demonstration of the technology in 2027. Pulsar Fusion is working to build a rocket powered by nuclear fusion that could reach speeds of 500,000 mph which could cut the time to fly to Mars by half. Astronauts could reach the red planet in weeks instead of months.

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Section-17

Hype and skepticism of fusion power:

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Fusion skepticism:

For most people, fusion remains a futuristic pipe dream, constantly lurking around the corner, never materializing. There are reasons for skepticism: Few scientific endeavours have been dogged by so many dead ends and false claims. But this has blinded us to the fact that, disappointments aside, scientists have been making slow but steady progress on fusion far longer than many people realize.

In the 1930s, chemist Ernest Rutherford and two collaborators began conducting experiments with a heavy isotope of hydrogen known as deuterium. In 1934, the team slammed deuterium atoms together, turning the isotope into helium while simultaneously producing what they described as “an enormous effect”—a blast of energy. This was fusion in miniature. Four years later, German physicist Hans Bethe figured out the precise subatomic sequence of events that undergird the process. That same year, two young scientists read Bethe’s article on the subject and resolved to put his ideas into practice.

The eccentric duo, Arthur Kantrowitz and Eastman Jacobs, worked at a government research facility focused on aircraft performance. Building a fusion reactor had nothing to do with their jobs, so they dubbed their creation a “Diffusion Inhibitor,” a vague but pretentious phrase that deterred superiors from asking too many questions. Their design, foreshadowing later developments, featured a metal donut, or “torus,” lined with magnets designed to contain and control the reaction. Lasers hadn’t been invented, so they opted for radio waves to superheat the hydrogen. This consumed so much power that they had to conduct experiments at night to avoid taking down the power grid. Ultimately, they flipped the switch and nothing happened. Not long afterward, their superiors caught on and shut down the project. No one realized it at the time, but the pair had come remarkably close to building the first fusion reactor, save for some flaws in the containment structure.

In the late 1940s, Argentina’s populist dictator, Juan Domingo Perón, funded the fusion research of an obscure Austrian scientist named Ronald Richter. In 1951, Peron proudly announced that Richter—who had close ties to former Nazis—had created the world’s first fusion reactor. Subsequent scrutiny unmasked Richter’s research as fundamentally flawed, if not fraudulent. The following year, however, two developments underscored why fusion could not be ignored. First came news that the United States had detonated the world’s first hydrogen bomb—effectively, an uncontrolled fusion reaction. No less consequential was the work of theoretical physicist Lyman Spitzer at Princeton University on how to control the superheated gas, or plasma, at the heart of the fusion reactor. This state of matter is like a subatomic orgy, where atomic nuclei and electrons, formerly monogamous, promiscuously mingle. In order to contain the chaos, Spitzer sketched out a figure-eight apparatus he called the stellarator. Through the 1950s, Spitzer and his collaborators built a series of prototypes that marked a giant leap forward. At the same time, a group of physicists in the Soviet Union led by Andrei Sakharov and Igor Tamm developed their own model, known as a Tokamak, a Russian acronym referring to a gigantic magnetic donut, or torus.

So began a new phase in fusion research as scientists built ever larger stellarators and tokamaks. From the late 1950s onward, fusion moved from a theoretical, fanciful concept to something concrete. Unfortunately, these advances also led flamboyant promoters to get ahead of themselves, imagining a future defined by inexpensive, limitless power. Typical of the genre was a breathless article in Popular Mechanics in 1959, “Fusion Power for the World of Tomorrow,” predicting, “It may come sooner than you think!” This hype proved damaging as well as unrealistic. Many commentators from the 1960s onward became increasingly disenchanted with fusion. Though the energy shortages of the 1970s led to more funding and renewed hopes, these inevitably fell short, bolstering the cynical view.

Lost in all the hoopla was the fact that scientific teams around the world continued to make slow but steady progress on turning fusion into a reality, gradually solving the technical challenges associated with containment while producing ever larger bursts of energy. These piecemeal advances, not especially eye-catching when viewed in isolation, were overshadowed by failures and frauds like the infamous “cold fusion” controversy of 1989, when two researchers erroneously claimed they had created a stable fusion reaction at room temperature. Fusion skeptics also delighted in pointing out that decades of research had never managed to achieve a so-called “net-energy gain.” Anytime researchers fired up hydrogen isotopes into a frenzy, they always ended up with less energy than when they started.

So not everyone is sold on the fusion dream. Critics argue that the massive amounts of money required to investigate this unproven technology would be better spent on proven renewable energy technology that can be deployed right away.

Matt Orsagh, a senior advisor with the consultancy Responsible Alpha, believes the success of the NIF experiment may be leading to overexcitement from investors. “The science behind this is amazing, but the practical applications of this experiment are still decades away,” he cautioned in January 2023. “Before December, fusion as a power source was all theoretical. Now, we know it can be done – but will it ever be practical for us as an energy source? That is still an open question.”

In the UK, the Scottish government opposed the national government’s attempt to open a demonstration fusion plant in Scotland. “We don’t have time to waste by pouring billions of pounds of public money into unproven technology,” Green member of the Scottish Parliament Mark Ruskell said in a press release. “Fusion may have a role in the future, but there is a long way to go before we will know if it is safe or viable. We cannot pin our hopes for decarbonising our economy on technology that is still years away… the UK government should instead focus on the major investment we need in renewables.”

The controversy over fusion investment is not just in national capitals. It has also become a heated topic at the UN’s annual climate conferences. Attempts to add it to the agenda have seen pushback from NGOs and Green politicians. German Green member of the European Parliament Rasmus Andresen has consistently opposed such efforts, calling fusion a “false climate solution” and saying “the money we are spending on projects like ITER could be used for other developments”. “Nuclear fusion is an idea that is as old as the nuclear industry, which somehow always seems to be 50 years away,” observes Mehdi Leman from Greenpeace. “The cost and uncertainty of fusion mean investing in thermonuclear reactors at the expense of other available clean energy options.” Garribba from the Commission disagrees. He says the EU may follow the US and UK in adopting a more conducive regulatory structure for fusion. “At the moment we are reflecting on whether the time has come to take a holistic look at the whole picture and how we see in Europe a further development.”

Although the science underpinning fusion research is solid, the amount of energy these reactions produce has yet to exceed the amount needed to instigate them. Breaking through this barrier and achieving a self-sustaining reaction (a process known as ignition) is the holy grail of fusion research. Some scientists say we’ll never reach this point, that fusion power is nothing but an expensive pipe dream. Their scepticism is partly due to the overly optimistic attitude of scientists in the 1950s who, having cracked open atoms, thought they’d also be able to fuse them together in a similar timeframe, with commercial fusion reactors on the grid by the 1970s. But the 70s came and went and today, by most estimates, we’re still another 40 or 50 years away from fusion power.

Misleading Media:

If you thought nuclear fusion had the potential to provide cheap energy on Earth, it could be the result of endless articles with headlines like these:

Breakthrough in nuclear fusion could mean ‘near-limitless energy’
Why fusion could be a clean-energy breakthrough?
Is nuclear fusion a clean-energy planet saviour?
Fusion: have we achieved the ‘holy grail’ of physics?

Many stories are just the product of talking to someone enthused about fusion or regurgitating PR announcements; adding a question mark to the headline as a get-out-of-jail-free card.

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Fusion ignition: a hyped breakthrough:

In December 2022, U.S. Department of Energy officials announced that the National Ignition Facility at the Lawrence Livermore National Laboratory in California finally attained “fusion ignition” – a long-awaited achievement for nuclear fusion researchers around the globe. The lab’s press release called it a “potentially world-changing breakthrough” in the quest for “limitless” energy that might “set the stage for fusion to someday become a viable clean-energy option.” Countless media outlets were happy to repeat the good news in big bold headlines and urgent push notifications. But what does this “breakthrough” actually mean? Does it really live up to the hype?

The facility has, at last, achieved slightly more fusion output than laser input: it should be called breakeven rather than ignition. On paper that is a major symbolic victory. In practice, it’s of little consequence. Here’s why.

The laser energy delivered to the target was 2.05 MJ, and the fusion output was likely about 3.15 MJ. According to multiple sources on NIF’s website, the input energy to the laser system is somewhere between 384 and 400 MJ. Consuming 400 MJ and producing 3.15 MJ is a net energy loss greater than 99%. For every single unit of fusion energy it produces, NIF burns at minimum 130 units of energy.

In terms of electrical power, 3.15 MJ would not quite power one 40-watt refrigerator light bulb for a day. Charging NIF steadily over the same day would draw 4,600 watts from the power grid. (NIF is actually charged much more quickly, but at the cost of a much higher draw in watts — more energy per unit time, over less time — but the total energy is the same.)

If you use a match to light another match—you’re not gaining much from that. But if you use a match to start a big fire, well then you’re gaining a lot because you have a fire. If gain meant producing more output energy than input electricity, then NIF fell far short. Its lasers are inefficient, requiring hundreds of megajoules of electricity to produce the 2 MJ of laser light and 3 MJ of fusion energy. The NIF scheme has another inefficiency. It relies on “indirect drive,” in which the laser blasts the gold can to generate the x-rays that actually spark fusion. Only about 1% of the laser energy gets into the fuel. There is “direct drive,” approach pursued by another lab, where laser beams fire directly onto a fuel capsule and deposit 5% of their energy.

Getting to viable fusion power:

Where such uncomfortable knowledge about feasibility is tackled in depth, it is only by critics. One physicist thus suggested commercial feasibility would demand an increase in fusion output of 100,000 per cent [other estimates suggest fusion would have to increase output by a factor of 317, or 31,700%], a mastery of exceedingly strict conditions vis a vis temperature, shape of target capsule and vacuum chamber, a solution to the problem that the machine breaks when it works and requires hours to recover, and an overcoming of the low supply of tritium fuel and its prohibitive cost. That’s an enormous scientific challenge to resolve before commercial operation can even be considered. The scientific challenge is equalled and possibly exceeded by others. A power plant needs to produce steady power. NIF currently executes, at best, one experimental blast per day. A commercial plant would need to blast fusion-producing capsules at a rate of tens of thousands per day.

Each blast requires strict conditions: temperatures a few degrees (Kelvin) above absolute zero; a spherical capsule, mechanically perfect in shape with an error of less than 1% the width of a hair; and a vacuum chamber environment. Most blasts suffer from slightly imperfect conditions and produce less fusion.

Either way, the machine takes hours to recover from each experiment. The fact that NIF is able to do this once per day is a technical achievement that took years to perfect. The problem with the laser method is that large equipment is unable to continuously fire beams, while smaller equipment lacks the output needed to ignite fusion. Making it happen 10,000 times faster is absurdly difficult. If it could be done, still more engineering then would be required to extract the energy in the form of heat for practical electricity generation.

Finally, there is a supply problem. The pellets contain deuterium and tritium. Deuterium is plentiful, but the world’s entire supply of tritium is something like 50 pounds. In 2020, the market cost of tritium was nearly $1 million per ounce. Livermore scientists estimate that a commercial operation modelled on NIF would require two pounds per day. Producing more tritium itself will be a challenge.

Nonetheless we should laud the scientific accomplishments of NIF. Many years (and careers) of hard work are producing progress on one of the most difficult applied science problems ever tackled. Scientifically, it’s symbolic progress. But it’s not a breakthrough, a game-changer, or the herald of imminent clean fusion power. NIF is still decades away from economically viable fusion.

In fact, NIF’s true value to the U.S. government is derived from its ability to test nuclear weapons without actually testing nuclear weapons. Currently, NIF is “the only facility that can perform controlled, experimental studies of thermonuclear burn, the phenomenon that gives rise to the immense energy of modern nuclear weapons.” The laser-mediated fusion reaction achieved at LLNL occurred at a lab called the National Ignition Facility, which touts its work on fusion for energy, but is primarily dedicated to nuclear weapons research. Prof. M. V. Ramana of the University of British Columbia, whose recent article was posted on the newly revived ZNetwork, explains, “NIF was set up as part of the Science Based Stockpile Stewardship Program, which was the ransom paid to the US nuclear weapons laboratories for forgoing the right to test after the United States signed the Comprehensive Test Ban Treaty” in 1996. It is “a way to continue investment into modernizing nuclear weapons, albeit without explosive tests, and dressing it up as a means to produce ‘clean’ energy.” Ramana cites a 1998 article that explained how one aim of laser fusion experiments is to try to develop a hydrogen bomb that doesn’t require a conventional fission bomb to ignite it, potentially eliminating the need for highly enriched uranium or plutonium in nuclear weapons.

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ITER, the epitome of fusion drawbacks:

When you look at the whole fusion enterprise objectively, you will feel that a working, everyday, commercial fusion reactor would cause more problems than it would solve. We can investigate a prototypical fusion reactor facility in the real world: the International Thermonuclear Experimental Reactor (ITER), under construction in Cadarache, France. Even if actual operation is still years away, the ITER project is sufficiently advanced that we can examine it as a test case for the doughnut-shaped design known as the tokamak—the most promising approach to achieving terrestrial fusion energy based on magnetic confinement.

Plasma physicists regard ITER as the first magnetic confinement device that can possibly demonstrate a “burning plasma,” where heating by alpha particles generated in fusion reactions is the dominant means of maintaining the plasma temperature. That condition requires that the fusion power be at least five times the external heating power applied to the plasma. Although none of this fusion power will actually be converted to electricity, the ITER project is mainly touted as a critical step along the road to a practical fusion power plant, and that claim is our concern here.

Let us see what can be deduced about some possibly irremediable drawbacks of fusion facilities by observing the ITER endeavour, concentrating on six areas: electricity consumption, tritium problems, neutron activation, cooling water demand, safety and breakdowns.

-1. Electricity consumption:

Adjacent to ITER is a 10-acre electrical switchyard with massive substations handling up to 600 megawatts of electricity, or MW(e), from the regional electric grid, which is enough to supply a medium-sized city. This power will be needed as input to supply ITER’s operating needs; no power will ever flow outward, because ITER’s internal construction makes it impossible to convert fusion heat to electricity. Remember that ITER is a test facility designed purely to show proof of concept as to how engineers can mimic the inner workings of the Sun to join atoms together in the real world in a controlled manner; ITER is not intended to generate electricity.

The electrical substation hints at the vast amount of energy that will be expended in operating the ITER project—and indeed every large fusion facility. Fusion reactors and experimental facilities must accommodate two classes of electric power drain:

First, a host of essential auxiliary systems such as cryostats, vacuum pumps, and building heating, ventilation and cooling must be maintained continuously, even when the fusion plasma is dormant. In the case of ITER, that non-interruptible power drain varies between 75 and 110 MW(e), wrote J.C. Gascon and his co-authors in their January 2012 article for Fusion Science & Technology, “Design and Key Features for the ITER Electrical Power Distribution.”

The second category of power drain revolves directly around the plasma itself, whose operation is in pulses. For ITER, at least 300 MW(e) will be required for tens of seconds to heat the reacting plasma and establish the requisite plasma currents. During the 400-second operating phase, about 200 MW(e) will be needed to maintain the fusion burn and control the plasma’s stability.

But much of the information about power drains—and the distinctions between ITER’s expected generation of heat instead of electricity—has gotten lost when the project was described to the public.

A typical widespread statement is that “ITER will produce 500 megawatts of output power with an input power of 50 megawatts,” implying that both numbers refer to electric power.

Actually the expected 500 megawatts of output refer to fusion power (embodied in neutrons and alphas)—which has nothing to do with electric power. In fact, electric power would be 1/3 of fusion power. The input of 50 MW referred to here is the heating power injected into the plasma to help sustain its temperature and current, and it’s only a small fraction of the overall electric input power to the reactor. The latter varies between 300 and 400 MW(e), as explained earlier.

We must recognize the colossal electrical power demanded by any fusion facility. In fact, it has always been recognized that a huge amount of energy is required to start up any fusion system. But tokamak fusion systems also require an unceasing hundreds of megawatts of electric power just to keep them going.

Yet there are far more serious issues with ITER’s advertised operation than the misleading labeling of projected input and output powers. While the input electric power of 300 MW(e) and more is indisputable, a fundamental question is whether ITER will produce 500 MW of anything, a query that revolves around the vital tritium fuel—its supply, the willingness to use it, and the campaign needed to optimize its performance.

ITER’s thermonuclear fusion reactor will use over 300 MW of electrical power to cause the plasma to absorb 50 MW of thermal power, creating 500 MW of heat from fusion for periods of 400 to 600 seconds. This would mean a ten-fold gain of plasma heating power (Q), as measured by heating input to thermal output, or Q ≥ 10. Beyond just heating the plasma, the total electricity consumed by the reactor and facilities will range from 110 MW up to 620 MW peak for 30-second periods during plasma operation.

In my view, input electric power of 300 MW (e) and output electric power of 166 MW (e) [i.e., 500 MW fusion thermal power] makes no sense at all.

At Q-physics of 10 as touted by ITER, the power output for the whole operation would ultimately be less than the total amount of power it took to run the reactor. That means overall power loss and commercial fusion is unfeasible at that Q.

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-2. Tritium troubles:

The most reactive fusion fuel is a 50/50 mixture of the hydrogen isotopes deuterium and tritium; this fuel (often written as “D-T”) has a fusion neutron output 100 times that of deuterium alone, and a spectacular increase in radiation consequences.

Deuterium is abundant in ordinary water, but there is no natural supply of tritium, a radioactive nuclide with a half-life of only 12.3 years. The ITER website states that the tritium fuel will be “taken from the global tritium inventory.” That inventory consists of tritium extracted from the heavy water of CANDU nuclear reactors, located mainly in Ontario, Canada, and secondarily in South Korea, with a potential future source from Romania. Today’s “global inventory” is approximately 25 kilograms, and increases by only about one-half kilogram per year, notes Muyi Ni and his co-authors in their 2013 journal article, “Tritium Supply Assessment for ITER,” in Fusion Engineering and Design. The inventory is expected to peak before 2030.

While fusioneers casually talk about fusing deuterium and tritium, they are in fact intensely afraid of using tritium for two reasons:

First, it is somewhat radioactive, so there are safety concerns connected with its potential release to the environment.

Second, there is unavoidable production of radioactive materials as D-T fusion neutrons bombard the reactor vessel, requiring enhanced shielding that greatly impedes access for maintenance and introducing radioactive waste disposal issues.

In 65 years of research involving hundreds of facilities, only two magnetic confinement systems have ever used tritium: the Tokamak Fusion Test Reactor at the Princeton Plasma Physics Lab, and the Joint European Tokamak (JET) at Culham, UK, way back in the 1990s.

ITER’s present plans call for the acquisition and consumption of at least 1 kilogram of tritium annually. Assuming that the ITER project is able to acquire an adequate supply of tritium and is brave enough to use it, will 500 MW of fusion power actually be achieved?

Analyses of D-T operation in ITER indicate that only 2 percent of the injected tritium will be burned, so 98 percent of the injected tritium will exit the reacting plasma unscathed. While a high proportion simply flows out with the plasma exhaust, much tritium must be continually scavenged from the surfaces of the reaction vessel, beam injectors, pumping ducts, and other appendages for processing and re-use. During their several dozen traverses of the Tritium around the plasma, vacuum, reprocessing and fueling systems, some tritium atoms will be permanently trapped in the vessel wall and in-vessel components, and in plasma diagnostic and heating systems.

The permeation of tritium at high temperature in many materials is not understood to this day, as R. A. Causey and his co-authors explained in “Tritium barriers and tritium diffusion in fusion reactors.” The deeper migration of some small fraction of the trapped tritium into the walls and then into liquid and gaseous coolant channels will be unpreventable. Most implanted tritium will eventually decay, but there will be inevitable releases into the environment via circulating cooling water.

Designers of future tokamak reactors commonly assume that all the burned tritium will be replaced by absorbing the fusion neutrons in lithium completely surrounding the reacting plasma. But even that fantasy totally ignores the tritium that’s permanently lost in its globetrotting through reactor subsystems. As ITER will demonstrate, the aggregate of unrecovered tritium may rival the amount burned and can be replaced only by the costly purchase of tritium produced in fission reactors.

A shortage of tritium fuel may leave fusion energy with an empty tank:

In 2020, Canadian Nuclear Laboratories delivered five steel drums, lined with cork to absorb shocks, to the Joint European Torus (JET), a large fusion reactor in the United Kingdom. Inside each drum was a steel cylinder the size of a Coke can, holding a wisp of hydrogen gas—just 10 grams of it, or the weight of a couple sheets of paper. This wasn’t ordinary hydrogen but its rare radioactive isotope tritium, in which two neutrons and a proton cling together in the nucleus. At $30,000 per gram, it’s almost as precious as a diamond, but for fusion researchers the price is worth paying. When tritium is combined at high temperatures with its sibling deuterium, the two gases can burn like the Sun. The reaction could provide abundant clean energy—just as soon as fusion scientists figure out how to efficiently spark it. Last year, the Canadian tritium fueled an experiment at JET showing fusion research is approaching an important threshold: producing more energy than goes into the reactions. By getting to one-third of this breakeven point, JET offered reassurance that ITER, a similar reactor twice the size of JET under construction in France, will bust past breakeven when it begins deuterium and tritium (D-T) burns sometime next decade. “What we found matches predictions,” says Fernanda Rimini, JET’s plasma operations expert.

But that achievement could be a Pyrrhic victory, fusion scientists are realizing. ITER is expected to consume most of the world’s tritium, leaving little for reactors that come after.

Fusion advocates often boast that the fuel for their reactors will be cheap and plentiful. That is certainly true for deuterium: Roughly one in every 6700 hydrogen atoms in the oceans is deuterium, and it sells for about $13 per gram. But tritium, with a half-life of 12.3 years, exists naturally only in trace amounts in the upper atmosphere, the product of cosmic ray bombardment. Nuclear reactors also produce tiny amounts, but few harvest it. Most fusion scientists shrug off the problem, arguing that future reactors can breed the tritium they need. The high-energy neutrons released in fusion reactions can split lithium into helium and tritium if the reactor wall is lined with the metal. Despite demand for it in electric car batteries, lithium is relatively plentiful.

But there’s a catch: In order to breed tritium you need a working fusion reactor, and there may not be enough tritium to jump-start the first generation of power plants. The world’s only commercial sources are the 19 Canada Deuterium Uranium (CANDU) nuclear reactors, which each produce about 0.5 kilograms a year as a waste product, and half are due to retire this decade. The available tritium stockpile—thought to be about 25 kilograms today—will peak before the end of the decade and begin a steady decline as it is sold off and decays, according to projections in ITER’s 2018 research plan.

The dwindling tritium supply is depicted in figure below:

The few kilograms of commercially available tritium come from CANDU plants, a type of nuclear reactor in Canada and South Korea. According to ITER projections, supplies will peak this decade, then begin a steady decline that will accelerate when ITER begins burning tritium.

ITER’s first experiments will use hydrogen and deuterium and produce no net energy. But once it begins energy-producing D-T shots, Alberto Loarte, head of ITER’s science division, expects the reactor to eat up to 1 kilogram of tritium annually.

To compound the problem, some believe tritium breeding—which has never been tested in a fusion reactor—may not be up to the task. In a recent simulation, nuclear engineer Mohamed Abdou of the University of California, Los Angeles, and his colleagues found that in a best-case scenario, a power-producing reactor could only produce slightly more tritium than it needs to fuel itself. Tritium leakages or prolonged maintenance shutdowns will eat away at that narrow margin.

If not for CANDU reactors, D-T fusion would be an unattainable dream. “The luckiest thing to happen for fusion in the world is that CANDU reactors produce tritium as a byproduct,” Abdou says. Many nuclear reactors use ordinary water to cool the core and “moderate” the chain reaction, slowing neutrons so they are more likely to trigger fission. CANDU reactors use heavy water, in which deuterium takes the place of hydrogen, because it absorbs fewer neutrons, leaving more for fission. But occasionally, a deuterium nucleus does capture a neutron and is transformed into tritium. If too much tritium builds up in the heavy water it can be a radiation hazard, so every so often operators send their heavy water to the utility company Ontario Power Generation (OPG) to be “detritiated.” OPG filters out the tritium and sells off about 100 grams of it a year, mostly as a medical radioisotope and for glow-in-the-dark watch dials and emergency signage. “It’s a really nice waste-to-product story,” says Ian Castillo of Canadian Nuclear Laboratories, which acts as OPG’s distributor. Fusion reactors will add significantly to the demand. OPG Vice President Jason Van Wart expects to be shipping up to 2 kilograms annually beginning in the 2030s, when ITER and other fusion startups plan to begin burning tritium. “Our position is to extract all we can,” he says.

But the supply will decline as the CANDUs, many of them 50 years old or more, are retired. Researchers realized more than 20 years ago that fusion’s “tritium window” would eventually slam shut, and things have only got worse since then. ITER was originally meant to fire up in the early 2010s and burn D-T that same decade. But ITER’s start has been pushed back to 2025 and could slip again because of the pandemic and safety checks demanded by French nuclear regulators. ITER won’t burn D-T until 2035 at the earliest, when the tritium supply will have shrivelled.

Once ITER finishes work in the 2050s, 5 kilograms or less of tritium will remain, according to the ITER projections. In a worst-case scenario, “it would appear that there is insufficient tritium to satisfy the fusion demand after ITER,” concedes Gianfranco Federici, head of fusion technology at the EuroFusion research agency.

Fusion reactors generally need a large startup tritium supply because the right conditions for fusion only occur in the hottest part of the plasma of ionized gases. That means very little of the tritium in the doughnut-shaped reactor vessel, or tokamak, gets burned. Researchers expect ITER to burn less than 1 to 2% of the injected tritium; the rest will diffuse out to the edge of the tokamak and be swept into a recycling system, which removes helium and other impurities from the exhaust gas, leaving a mix of D-T. The isotopes are then separated and fed back into the reactor. This can take anywhere from hours to days.

DEMO’s designers are working on ways to reduce its startup needs. “We need to have a low tritium [starting] inventory,” says Christian Day of the Karlsruhe Institute of Technology, project leader in the design of DEMO’s fuel cycle. “If you need 20 kilograms to fill it, that’s a problem.”

One way to tame the demand is to fire frozen fuel pellets deeper into the reactor’s burning zone, where they will burn more efficiently. Another is to cut recycling time to just 20 minutes, by using metal foils as filters to strip out impurities quickly, and also by feeding the hydrogen isotopes straight back into the machine without separating them. It may not be a perfect 50-50 D-T mix, but for a working reactor it will be close enough, Day says.

But Abdou says DEMO’s appetite is still likely to be large. He and his colleagues modeled the D-T fuel cycle for power-producing reactors, including DEMO and its successors. They estimated factors, including the efficiency of burning D-T fuel, the time it takes to recycle unburnt fuel, and the fraction of time the reactor will operate. In a paper published in 2021 in Nuclear Fusion, the team concludes that DEMO alone will require between 5 kilograms and 14 kilograms of tritium to begin—more than is likely to be available when the reactor is expected to fire up in the 2050s.

Even if the DEMO team and other post-ITER reactor designers can cut their tritium needs, fusion will have no future if tritium breeding doesn’t work. According to Abdou, a commercial fusion plant producing 3 gigawatts of electricity will burn 167 kilograms of tritium per year—the output of hundreds of CANDU reactors.

The challenge for breeding is that fusion doesn’t produce enough neutrons, unlike fission, where the chain reaction releases an exponentially growing number. With fusion, each D-T reaction only produces a single neutron, which can breed a single tritium nucleus. Because breeding systems can’t catch all these neutrons, they need help from a neutron multiplier, a material that, when struck by a neutron, gives out two in return. Engineers plan to mix lithium with multiplier materials such as beryllium or lead in blankets that line the walls of the reactors.

ITER will be the first fusion reactor to experiment with breeding blankets. Tests will include liquid blankets (molten mixtures of lithium and lead) as well as solid “pebble beds” (ceramic balls containing lithium mixed with balls of beryllium). Because of cost cuts, ITER’s breeder systems will line just 4 square meters of the 600-square-meter reactor interior. Fusion reactors after ITER will need to cover as much of the surface as they possibly can to have any chance of satisfying their tritium needs.

The tritium can be extracted continuously or during scheduled shutdowns, depending on whether the lithium is in liquid or solid form, but the breeding must be relentless. The breeding blankets also have a second job: absorbing gigawatts of power from the neutrons and turning it into heat. Pipes carrying water or pressurized helium through the hot blankets will pick up the heat and produce steam that drives electricity-producing turbines. “All of this inside the environment of a fusion reactor with its ultrahigh vacuum, neutron bombardment, and high magnetic field,” says Mario Merola, head of engineering design at ITER. “It’s an engineering challenge.”

For Abdou and his colleagues, it is more than a challenge—it may well be an impossibility. Their analysis found that with current technology, largely defined by ITER, breeding blankets could, at best, produce 15% more tritium than a reactor consumes. But the study concluded the figure is more likely to be 5%—a worrisomely small margin. One critical factor the authors identified is reactor downtime, when tritium breeding stops but the isotope continues to decay. Sustainability can only be guaranteed if the reactor runs more than 50% of the time, a virtual impossibility for an experimental reactor like ITER and difficult for prototypes such as DEMO that require downtime for tweaks to optimize performance. If existing tokamaks are any guide, Abdou says, time between failures is likely to be hours or days, and repairs will take months. He says future reactors could struggle to run more than 5% of the time.

To make breeding sustainable, operators will also need to control tritium leaks. For Jassby, this is the real killer. Tritium is notorious for permeating the metal walls of a reactor and escaping through tiny gaps. Abdou’s analysis assumed a loss rate of 0.1%. “I don’t think that’s realistic,” Jassby says. “Think of all the places tritium has to go” as it moves through the complex reactor and reprocessing system. “You can’t afford to lose any tritium.” Jassby says that the tritium supply issue essentially “makes deuterium-tritium fusion reactors impossible.” Alternative fuels for fusion reactors are also under development, based on radioactive helium or boron, but these require temperatures up to a billion degrees to trigger a fusion reaction.

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-3. Neutron Radiation and radioactive waste from fusion:

As noted earlier, ITER’s anticipated 500 MW of thermal fusion power is not electric power. But what fusion proponents are loathe to tell you is that this fusion power is not some benign solar-like radiation but consists primarily (80 percent) of streams of energetic neutrons whose only apparent function in ITER is to produce huge volumes of radioactive waste as they bombard the walls of the reactor vessel and its associated components.

Just 2 percent of the neutrons will be intercepted by test modules for investigating tritium production in lithium, but 98 percent of the neutron streams will simply smash into the reactor walls or into devices in port openings.

In fission reactors, at most 3 percent of the fission energy appears as neutrons. But ITER is akin to an electrical appliance that converts hundreds of megawatts of electric power into neutron streams. A peculiar feature of D-T fusion reactors is that the overwhelming preponderance of thermal energy is not produced in the reacting plasma, but rather inside the thick steel reactor vessel as the neutron streams smash into it and gradually dissipate their energy. In principle, this thermalized neutron energy could somehow be converted back to electricity at very low efficiency, but the ITER project has opted to avoid addressing this challenge. That is a task deferred to delusions called demonstration reactors that fusion proponents hope to deploy in the second half of the century.

A long-recognized drawback of fusion energy is neutron radiation damage to exposed materials, causing swelling, embrittlement and fatigue. As it happens, the total operating time at high neutron production rates in ITER will be too small to cause even minor damage to structural integrity, but neutron interactions will still create dangerous radioactivity in all exposed reactor components, eventually producing a staggering 30,000 tons of radioactive waste.

Surrounding the ITER tokamak, a monstrous concrete cylinder 3.5 meters thick, 30 meters in diameter and 30 meters tall called the bioshield will prevent X-rays, gamma rays and stray neutrons from reaching the outside world. The reactor vessel and non-structural components both inside the vessel and beyond up to the bioshield will become highly radioactive by activation from the neutron streams. Downtimes for maintenance and repair will be prolonged because all maintenance must be performed by remote handling equipment.

For the much smaller Joint European Torus experimental project in the United Kingdom, the radioactive waste volume is estimated at 3,000 cubic meters, and the decommissioning cost will exceed $300 million. Those numbers will be dwarfed by ITER’s 30,000 tons of radioactive wastes. Fortunately, most of this induced radioactivity will decay in decades, but after 100 years some 6,000 tons will still be dangerously radioactive and require disposal in a repository, says the “Waste and Decommissioning” section of ITER’s Final Design Report.

Periodic transport and off-site disposal of radioactive components as well as the eventual decommissioning of the entire reactor facility are energy-intensive tasks that further expand the negative side of the energy accounting ledger.

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-4. Water demand:

Torrential water flows will be needed to remove heat from ITER’s reactor vessel, plasma heating systems, tokamak electrical systems, cryogenic refrigerators, and magnet power supplies. Including fusion generation, the total heat load could be as high as 1,000 MW, but even with zero fusion power the reactor facility consumes up to 500 MW that eventually becomes heat to be removed. ITER will demonstrate that fusion reactors would be much greater consumers of water than any other type of power generator, because of the huge parasitic power drains that turn into additional heat that needs to be dissipated on site. (By “parasitic,” we mean consuming a chunk of the very power that the reactor produces.)

Cooling water will be taken from the Canal de Provence formed by channeling the Durance River, and most heat will be discharged into the atmosphere by cooling towers. During fusion operations, the combined flow rate of all the cooling water will be as large as 12 cubic meters per second (180,000 gallons per minute), or more than one-third the flow rate of the Canal. That level of water flow can sustain a city of 1 million residents. (But the actual demand on the Canal’s water will be only a very small faction of that value because ITER’s power pulse will be just 400 seconds long with at most 20 such pulses daily, and ITER’s cooling water is recirculated.)

Even while ITER is producing nothing but neutrons, its maximum coolant flow rate will still be nearly half that of a fully functioning coal-burning or nuclear plant that generates 1,000 MW(e) of electric power. In ITER as much as 56 MW(e) of electric power will be consumed by the pumps that circulate the water through some 36 kilometers of nuclear-grade piping.

Operation of any large fusion facility such as ITER is possible only in a location such as the Cadarache region of France, where there is access to many high-power electric grids as well as a high-throughput cool water system. In past decades, the great abundance of freshwater flows and unlimited cold ocean water made it possible to implement large numbers of gigawatt-level thermoelectric power plants. In view of the decreasing availability of freshwater and even cold ocean water worldwide, the difficulty of supplying coolant water would by itself make the future wide deployment of fusion reactors impractical.

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-5. ITER is not safe:

ITER creates two wholly new safety issues: plasma disruptions and quenching. If disruptions accidentally happen, it would be expensive and dangerous. The heat in a disrupted plasma can be ten times higher than the melting point of the first wall and the divertor. Imagine the problems that creates for regulators, managers and workers. The second problem is quenching. This is when a superconducting magnet suddenly becomes a normal electromagnet – and releases its energy. ITER’s coils contain the same energy as 12 tons of TNT. This has already happened 17 times in tokamaks. This causes overheating and melting of components; it may even start dangerous fires.

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-6. Breakdowns could plague fusion power plants:

Some of fusion’s fitfulness is innate to the design of doughnut-shaped tokamak reactors. The magnetic field that confines the ultrahot, energy-producing plasma is generated in part by the charged particles themselves, as they flow around the vessel. That plasma current in turn is induced by pulses of electrical current in a coil of wire in the doughnut’s hole, each lasting a few minutes at most. In between pulses the magnetic field ebbs, interrupting tokamak operations—and power delivery. The repetitive starts and stops of the reactor’s powerful magnetic fields also generate mechanical stresses that could eventually tear the machine apart.

In theory, the beams of particles and microwaves used to heat the plasma can also drive the plasma current. So can a quirk of plasma physics called the bootstrap effect. Near the edge of the plasma, a sharp pressure gradient causes the particles to spiral in such a way that they interfere with each other and push themselves—by their own bootstraps—around the ring. Using a combination of beams and bootstrap, researchers at ITER think they can get hourlong runs. But the bootstrap effect works best at high pressures and can push the plasma out of control, potentially damaging the reactor.

Such outbursts of turbulent plasma are another headache for reactor operators, because they can scour metal off the vessel’s inner wall, not only threatening its integrity, but also poisoning the plasma. At the Joint European Torus (JET), a U.K.-based tokamak with a reactor wall made of beryllium and tungsten, an automated protection system injects gas into the plasma to staunch the bursts—but not always successfully. “You get drops of beryllium everywhere,” says Fernanda Rimini, JET’s plasma operations expert.

ITER operators hope to quell the disruptions by firing frozen deuterium pellets into the plasma and applying an additional magnetic field. Both measures should make the edge of the plasma slightly leaky, so breakouts are small and manageable rather than big and damaging.

The flood of high-energy neutrons produced by fusion reactions pose another threat. The neutrons are a “double-edged sword,” says materials scientist Andy London of the UK Atomic Energy Authority. On the one hand, they dump heat in the reactor wall that ultimately generates electricity, and they can bombard lithium to breed tritium fuel. But they can also penetrate the reactor walls and lodge in surrounding steel structures, knocking atoms out of position and weakening the material. Nuclei in the structures sometimes absorb the neutrons, creating radioactive isotopes that do further damage. For example, neutron bombardment can turn the nickel in many steel alloys into a form that gives off helium, causing the steel to swell perceptibly. “The metal turns into a sponge,” London says. Finding tougher materials is a challenge, London says, because “we don’t have the luxury of a fusion reactor we can test materials in.” A planned European accelerator facility in Spain, dubbed IFMIF-DONES, is supposed to test fusion materials with the world’s most intense neutron beam. But construction has not started.

Fixing damaged or weakened reactor components will be slow. Because of the hostile radioactive environment, repairs will rely on robots or remote handling arms that can navigate the narrow access ports of a tokamak. Mohamed Abdou, a nuclear engineer at the University of California, Los Angeles, believes future reactors may operate less than 5% of the time. Compare this, he says, with today’s fission reactors. They can keep running even when individual fuel rods fail. Cranes can swap out fuel rods in just a couple of days. Availability can be as high as 90%. Achieving something similar for fusion will be “very challenging,” Abdou says.

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ITER’s influence:

The goal of ITER has never been to get fusion power. The goal is to solve all the problems fusion can create. Problems like: the material science, making high temperature magnets, controlling the plasma, building a trained workforce, finding modes of operation and making better models. Whether ITER performs poorly or well, its most favorable legacy is that, like the International Space Station, it will have set an impressive example of decades-long international cooperation among nations both friendly and semi-hostile. Critics charge that international collaboration has greatly amplified the cost and timescale but the $20-to-30 billion cost of ITER is not out of line with the costs of other large nuclear enterprises, such as the power plants that have been approved in recent years for construction in the United States (Summer and Vogtle) and Western Europe (Hinkley and Flamonville), and the US MOX nuclear fuel project in Savannah River. All these projects have experienced a tripling of costs and construction timescales that ballooned from years to decades. The underlying problem is that all nuclear energy facilities—whether fission or fusion—are extraordinarily complex and exorbitantly expensive.

A second invaluable role of ITER will be its definitive influence on energy-supply planning. If successful, ITER may allow physicists to study long-lived, high-temperature fusioning plasmas. But viewed as a prototypical energy producer, ITER will be, manifestly, a havoc-wreaking neutron source fueled by tritium produced in fission reactors, powered by hundreds of megawatts of electricity from the regional electric grid, and demanding unprecedented cooling water resources. Neutron damage will be intensified while the other characteristics will endure in any subsequent fusion reactor that attempts to generate enough electricity to exceed all the energy sinks identified herein.

ITER, the world’s biggest fusion experiment co-funded by 35 nations in the South of France, is already enormously over budget and behind schedule. Furthermore, the sector faces significant regulatory challenges, as current frameworks guiding nuclear fission will not be applicable to nuclear fusion. This means there is a lot of lengthy and pricey bureaucratic and policy work to be done before commercial fusion could become a possibility.

Over the last decade, solar costs have declined 85% while wind costs have declined 45%. Today, building new wind and solar infrastructure costs less than adding the equivalent capacity in coal or gas in two-thirds of the world. Most experts agree that we’re unlikely to be able to generate large-scale energy from nuclear fusion before around 2050 (the cautious might add on another decade). Given that the global temperature rise over the current century may be largely determined by what we do—or fail to do—about carbon emissions before then, fusion can be no saviour. Considering how far nuclear fusion still has to go, I don’t think it will be our solution to decarbonizing the economy in the timeframe needed to prevent the worst effects of climate change from occurring. Instead, I believe expanding cheap wind and solar is our best bet for achieving our climate goals. Fusion remains enormously expensive, and the achievement of net energy production remains elusive. In fact, the real fusion energy breakthrough is still decades away.

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Hype of nuclear fusion:

Nuclear fusion holds a promise of cheap, abundant, and safe energy for ages to come. That promise goes back to the 1950s and from the beginning, the time horizon for delivery was some 30 years ahead. It still is. Of course, progress has been made. But the program started in a post-war era that oozed a limitless optimism about a future in which people would live on Mars by now. In present times people are happy to be alive at all in large areas of the planet. And their outlook is grim. Promises for a faraway future have no meaning to them.

We have been at the doorstep of fusion hype repeatedly. In fact, ever since the 1950s fusion power has been just over the horizon. The fusion illusion has become its own cottage industry, with competing fusion research teams over-calling each other in a series of breakthroughs and decisive advances that generate hype, but no electricity.

For instance, on 9 February 2022 the Joint European Torus (JET) fusion reactor in the UK announced that it had produced 59 Megajoules of energy and that this indicated ‘powerplant potential’. Yet JET consumed significantly more power than it produced. So the claim of a net power gain was a form of hyped science communication in which future promise colonises present limitations.

Researchers at LLNL’s National Ignition Facility (NIF) are the most recent hype-mongers. In their 13 December announcement of NIF’s experimental result, the US Department of Energy (DOE) advertised the result as a ‘game changer’ and quoted a host of US politicians directly linking the result to commercial fusion power and the goal of a ‘net-zero carbon economy’. Media outlets which really should adopt stricter editorial standards gushed about the result implying ‘limitless, zero-carbon power’ or stating that it ‘changes everything’ and heralds a decisive step towards ‘carbon-free energy’ for ‘everyone’ for ‘millions of years’.

The only thing limitless and free about fusion power is the hype it generates.

We are told that a fusion reaction would have to occur “many times a second” to produce usable amounts of energy. But the blast of energy from the LLNL fusion reactor actually only lasted one tenth of a nanosecond – that’s a ten-billionth of a second. Apparently other fusion reactions (with a net energy loss) have operated for a few nanoseconds, but reproducing this reaction many times every second is far beyond what researchers are even contemplating.

We are told that the reactor produced about 1.5 times the amount of energy that was input, but this only counts the laser energy that actually struck the reactor vessel. That energy, which is necessary to generate temperatures over a hundred million degrees, was the product of an array of 192 high-powered lasers, which required well over 100 times as much energy to operate.

We are told that nuclear fusion will someday free up vast areas of land that are currently needed to operate solar and wind power installations. But the entire facility needed to house the 192 lasers and all the other necessary control equipment was large enough to contain three football fields, even though the actual fusion reaction takes place in a gold or diamond vessel smaller than a pea. All this just to generate the equivalent of about 10-20 minutes of energy that is used by a typical small home. Clearly, even the most inexpensive rooftop solar systems can already do far more. And Prof. Mark Jacobson’s group at Stanford University has calculated that a total conversion to wind, water and solar power might use about as much land as is currently occupied by the world’s fossil fuel infrastructure.

The fuss is all about a gain of a blip of energy barely enough to heat few kettles of water. Getting there has cost society a whopping $ 3.5 billion. That sum would buy some 2000 MW of wind power capacity that almost instantly delivers enough power to supply some 1.2 million households or a city the size of Chicago for at least 30 years. Invested in solar farms could produce even more. Or it would buy a factory to supply batteries for 1 million electric vehicles. While it seemed easy to find a dozen experts willing to gush on record about how remarkable it was to spend $3.5 billion to produce an energy output that might boil a few kettles, frank assessments of future prospects are confined to scattered observations by disconnected critics. You may consider a 3.5 billion investment to be a minor concern, but LLNL is not a major facility. The much-criticized nuclear fusion test facility ITER in France, – still to be completed – will run way over 50 billion according to some US estimates.

Daniel Jassby, a principal research physicist at the Princeton Plasma Physics Lab until 1999, worked for 25 years in plasma physics. He already performed a reality check for nuclear fusion.

He says: “… it is time to ask: Is fusion really a “perfect” energy source? After having worked on nuclear fusion experiments for 25 years at the Princeton Plasma Physics Lab, I began to look at the fusion enterprise more dispassionately in my retirement. I concluded that a fusion reactor would be far from perfect, and in some ways close to the opposite.”

His conclusions are based on obvious facts, that rarely make it to the media. Like this one: an energy source consisting of 80 percent energetic neutron streams is not an electrical energy source. Neutron streams lead to problems with nuclear energy, as we know from nuclear fission: radiation damage to structures; radioactive waste; the need for biological shielding; and the threat of nuclear weapons proliferation.

In addition, fusion reactors share some of the other serious problems that plague fission reactors, including tritium release, coolant demands, and high operating costs. There will also be additional drawbacks that are unique to fusion devices: the use of fuel (tritium) that is not found in nature and must be replenished by the reactor itself; and unavoidable on-site power drains that drastically reduce the electric power available for sale. To make up for the inevitable shortfalls in recovering unburned tritium for use as fuel in a fusion reactor, fission reactors must continue to be used to produce sufficient supplies of tritium. That leads to a perpetual dependence on fission reactors, with all their safety and nuclear proliferation problems.

As Jassby explains, inside the Sun, which is powered by fusion, normal hydrogen atoms, each consisting of nuclei containing one proton, are fused together and produce helium plus energy. Here on planet Earth, fusion reactors “burn neutron-rich isotopes [that] have byproducts that are anything but harmless: Energetic neutron streams comprise 80 percent of the fusion energy output of deuterium-tritium reactions and 35 percent of deuterium-deuterium reactions.” (Deuterium is a hydrogen atom consisting of one proton and one neutron in its nucleus. Tritium is a radioactive form of hydrogen having one proton and two neutrons.)

Jassby details the consequences:

These neutron streams lead directly to four regrettable problems with nuclear energy [both fission and fusion]: radiation damage to structures; radioactive waste; the need for biological shielding; and the potential for the production of weapons-grade plutonium 239—thus adding to the threat of nuclear weapons proliferation, not lessening it, as fusion proponents would have it.

So, you may be asking:

Why don’t scientists just use ordinary hydrogen instead of deuterium and tritium isotopes?

Our attempts to re-create the Sun’s power on Earth face “much lower particle densities and much more fleeting energy confinement,” Jassby explains. That’s why scientists use deuterium and tritium “which are 24 orders of magnitude more reactive than ordinary hydrogen.” That’s 10^24 times more reactive and therefore 10^24 times EASIER to fuse under the considerably less favorable conditions we can create here on Earth.

“This gargantuan advantage in fusion reactivity allows human-made fusion assemblies to be workable with a billion times lower particle density and a trillion times poorer energy confinement than the levels that the sun enjoys,” Jassby explains.

The neutrons which are liberated in this type of fusion have to go somewhere. Over time, just like in nuclear fission plants, these neutrons damage the reactor vessel wall. One design addresses this issue by encapsulating the fusion fuel in a “one-meter thick liquid lithium sphere or cylinder.” This will create tons of radioactive waste that has to be removed annually. Without this approach the vessel walls will have to be replaced periodically and then transported to waste disposals sites. Scientists are working on better reactor vessel materials.

This problem is less pronounced using just deuterium as fuel. But deuterium alone is 20 times LESS reactive than a deuterium-tritium mix making it harder to successfully create deuterium-only fusion. In addition, deuterium-only reactors make ideal breeding environments for plutonium-239, atomic bomb material that can be made by introducing uranium-238 into the reactor. Uranium-238 is much cheaper and far more plentiful than uranium-235—which makes up only 0.7 percent of mined uranium and which is the only naturally-occurring fissile material. Bombarding uranium-238 with neutrons is a good way to make plutonium-239, a fissionable product suitable for atomic bombs.

As Jassby sums up, fusion reactors face some unique problems: a lack of a natural fuel supply (tritium), and large and irreducible electrical energy drains to offset. Because 80 percent of the energy in any reactor fueled by deuterium and tritium appears in the form of neutron streams, it is inescapable that such reactors share many of the drawbacks of fission reactors—including the production of large masses of radioactive waste and serious radiation damage to reactor components. These problems are endemic to any type of fusion reactor fueled with deuterium-tritium, so abandoning tokamaks for some other confinement concept can provide no relief.

A final form of uncomfortable knowledge includes drawbacks, which are typically managed through practices that include denial (avoiding acknowledging information even if others bring it to collective attention), dismissal (manufacturing justifications for rejecting a counter-claim), diversion (distracting via a decoy issue) and displacement (swapping problems).

Two examples will suffice.

One is the deuterium-tritium fuel needed for any future fusion reactor. It scarcely exists in nature (a fact met with denial) and must be produced either in heavy water reactors or by breeding it from enriched lithium-6, which is in short supply (met with dismissal), and, no, it is not solved by speculations about extracting the fuel from sea water (a diversion).

A second drawback is that nuclear fusion may be not the perfect energy source for a climate crisis but, as a former fusion physicist put it, is ‘in some ways close to the opposite’. Put succinctly, the fact that neutron streams comprise 80 per cent of fusion energy output in deuterium-tritium reactions makes it an odd electrical energy source. The neutron streams damage the structure of the machine, produce relatively bulky radioactive waste, require biological shielding, and constitute a proliferation risk (Pu-239). The fusion reactor itself has a high parasitic power consumption, a scarce fuel supply, and likely high operating costs due to continual radiation damage.

Yet when managing such uncomfortable knowledge via the strategy of displacement, we substitute a more manageable surrogate. Ambiguity about net gain is that surrogate. Net gain in fusion research today exploits holes in our broader culture about what we do not know we know. It is unevenly known that more power is consumed than is produced by fusion experiments. The process of manufacturing ignorance about that unevenly known fact turns on excluding uncomfortable knowledge because of the way that knowledge might threaten fusion-related institutional goals and interests.

We are not ignorant of fusion gaslighting because of some natural but temporary state of maldistribution of knowledge, nor because we just happen to have not done the relevant work of knowing. Instead, fusion hype actively makes and sustains broader ignorance. Manufacturing ignorance is an achievement which in the case of fusion relies on fuzzy measures today being masked by heroic projections about tomorrow, aided by omitting the uncertainties attending fusion technology.

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Misleading people who don’t know difference between Q-physics and Q-engineering:

ITER’s made a bold promise: to produce 10 times the amount of energy it consumed. Even though ITER was only a test reactor that would never actually connect to the grid and produce electricity, such a result would be a record-smashing number for fusion reactors compared to its predecessor, a reactor called JET in the U.K. That one couldn’t even breakeven — meaning it produced less power than it consumed. ITER’s remarkable energy-production upgrade is thanks to the reactor’s scaled-up design. When it comes to doughnut-shaped fusion reactors called tokamaks, such as ITER and JET, size is a limiting factor. It is ITER’s enormous magnitude, nearly 240 feet tall and weighing 23,000 tons, that allows the organization to make such big claims. And the ITER organization has done so, frequently touting its 10-times power gain number, often called the Q ratio.

Steven B. Krivit is an author and publisher who has been writing about nuclear energy research for more than 20 years. When Krivit needed to double-check some figures for a book he was writing, a closer look at the Q ratio ITER promised revealed something concerning, he said. “I assumed that everybody knew the rate of power that went into these reactors. But the scientists that I spoke to said, ‘Well, actually, we don’t measure the rate of power that goes into the fusion reactors.’ And I’m going, `What are you talking about?’” Krivit said. “We all thought that the rate of power that you talked about from the JET reactor was a comparison of the power coming out versus the power coming in. And they said, ‘No.’ That power ratio doesn’t compare the rate of power coming out versus power coming in. It only compares the ratio of the power that’s used to heat the fuel versus the thermal power that’s produced by the fuel.”

That is Q-physics.

In reality, the Q ratio only speaks to what happens deep inside the reactor when fusion occurs, not the total amount of energy it takes to run the whole operation, or the actual usable electricity the fusion reaction could produce. That would be Q-engineering.

“And I’m thinking, wow, this is a pretty serious misunderstanding that I had, that everybody else that I know had,” Krivit said. So the first thing he did was get in touch with the scientists at JET, the U.K. reactor that held the previous record for Q. He wanted to find out the ratio for total electrical power input versus total electrical power output had the test reactor been connected to the grid. “The efficiency is 1%,” Krivit said. That was far off from the 67% Krivit had previously thought it was. That means if JET were hooked up to the grid, the reactor would lose almost all the power it used to operate. “My first reaction was ‘Oh, shit.’ There’s a real serious discrepancy here.” Krivit said.

What did all that mean for ITER, and its claim to be able to produce 10 times the power it used?

Everywhere the Q ratio was discussed, others seemed to understand it the same way Krivit had. He pointed to articles in the New York Times, Nature, the Wall Street Journal, and other outlets that conflated the amount of power injected directly into the fuel inside the reactor with the total rate of electrical power to operate ITER.

“All of these news outlets reported that the rate of power that ITER would require to operate was 50 megawatts,” Krivit said. Fifty megawatts in for 500 megawatts out — that’s how ITER said it would achieve a Q ratio of 10. It appears lawmakers might have believed the same. During a congressional hearing on ITER in 2014, California Rep. Eric Swallwell said, “ITER is designed to produce at least 10 times the energy it consumes,” and Texas Rep. Eddie Bernice Johnson added that ITER scientists are “confident it is now possible to actually build a full-scale test reactor that produces far more energy than it uses.” A member of ITER’s governing body who testified at the hearing did not correct these statements.

Will ITER actually produce 10 times more energy than it consumes?

Mark Henderson said the answer depends on what you mean by energy. “One of the confusions on this factor is that there’s really a Q-physics and, to some degree, there’s a Q-engineering.” he said. According to Henderson, physicists and non-physicists think of Q differently.

“For me to put in 50 megawatts of power, I need to pull from the grid about 150 megawatts. And so an engineer would say, ‘Well, Mark, wait a minute, you know, you’re pulling 150 megawatts from the grid, you convert it to 50 megawatts, it gives me 500 megawatts out, but then those 500 megawatts are going to heat water that turns a turbine that then generates electricity that would put back to the grid roughly 150 megawatts,’” he said. “So from an engineer’s perspective, if ITER was a [commercial] fusion reactor, it would be giving a Q-engineering roughly of about one.”

A Q-physics ratio of 10, but a Q-engineering ratio of 1.

Henderson doubted whether actual engineers who work on fusion sites correctly understood this difference, let alone the public. But it’s an important difference, nonetheless. [Note that nobody is talking about parasitic power drain.]

“That means the ITER reactor is effectively a zero-power reactor,” Krivit said. “If the design works as expected, we’re going to get the same rate of power coming out as the power going in. Zero extra power that would be available to use for any practical purposes.” Krivit noted that if ITER were hooked up to the grid, it would effectively be a massive, expensive, complicated, scientifically remarkable extension of electrical wire. But, of course, ITER was never going to be hooked up to the grid anyway. So that, in itself, wasn’t a huge issue.

To Krivit, the issue was one of misrepresentation. He argued that ITER should have known the Q-physics ratio was going to be misunderstood by the public as Q-engineering; that the organization’s language describing power output and Q was always way too squishy for non-experts to reach any other conclusion than that Q and “fusion power” referred to usable energy rather than heated plasma, especially when those terms were so often cited in the context of “virtually unlimited energy” as a feasible solution to the impending climate crisis that deserved more funding.

Henderson said that if there was confusion, it was all just a misinterpretation. There was no purposeful misrepresentation. “It is very easy in any field to be misinterpreted. And unfortunately, it’s easier for people to take potshots at fusion, just because [of] the amount of invested money to build a machine,” he said. “People will say, ‘Well, why are you building a fusion reactor compared to, for example, putting the money into a solar field or to a wind field?’ And my answer is to say that, you know, we should be looking at a short-term objective and a long-term objective.”

Still, during a TED Talk in 2019 entitled, “The Dawn of the Fusion Age,” Henderson told an audience, and the internet, that ITER is designed to “have 1 watt going in equals 10 watts out.”

Though that claim is technically true, did the audience of non-physicists know he was referring to directly injected energy and thermal power? Or did they assume he was talking about overall electrical power?

“I don’t like conveying information in a way that can be misinterpreted,” Henderson said. “And I think one of the key issues in communication is to be able to understand who your audience is, and to give that audience enough information to be able to understand what this thing called capital Q is. So obviously, fusion has hurt itself in how the capital Q has been interpreted in the community or in public.”

Henderson said a commercial fusion reactor would need a Q-physics of about 40, four times the Q of ITER, to generate sufficient power to be viable.

So why all the fuss about Q=10 from ITER?

Because from a selling point of view, it’s quite attractive. You will produce 500 megawatts from just 50 megawatts injected. So a gain factor of 10. This is very interesting, very promising but very misleading.

ITER recently withdrew its claim of net energy gain—of 500 MW of fusion power from 50 MW of input power (a Q value of 10)—and now says that ITER is ‘the investigation and demonstration of burning plasmas’, in which the energy of helium nuclei produced by fusion reactions is enough to maintain plasma temperature.

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Moral of the story:

-1. Plasma is one of the four common states of matter – solid, liquid, gas, and plasma. With increasing temperature, all materials are transformed successively from the solid, to liquid and then gaseous state. If the temperature is increased even more, a plasma is formed. Most plasmas are created when extra energy is added to a gas, which can occur when gases are heated to high temperatures. Almost all of the observable matter in the universe is in the plasma state. Formed at high temperatures, plasmas consist of freely moving ions and free electrons. They are often called the “fourth state of matter” because their unique physical properties distinguish them from solids, liquids and gases. Plasma is an electrically charged gas. Because plasma particles have an electrical charge, they are affected by electrical and magnetic fields. Plasma densities and temperatures vary widely, from the relatively cold gases of interstellar space to the extraordinarily hot, dense cores of stars and inside a detonating nuclear weapon.

-2. Nuclear fusion is the reaction in which two or more nuclei combine, forming a new element with a higher atomic number (more protons in the nucleus). Nuclear fusion is the process by which two or more light atomic nuclei join together to form a single heavier nucleus and release of large amounts of energy. The origin of the energy released in fusion of light elements is due to an interplay of two opposing forces: the nuclear force that draws together protons and neutrons, and the Coulomb force that causes protons to repel each other. A substantial energy barrier of electrostatic forces must be overcome before fusion can occur.

-3. Fusion reactions are of two basic types: (1) those that preserve the number of protons and neutrons and (2) those that involve a conversion between protons and neutrons. Reactions of the first type are most important for practical fusion energy production (DT reaction), whereas those of the second type are crucial to the initiation of star burning (PP reaction).

-4. Nuclei are made up of protons and neutrons, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. It is invariable for all the atoms. Nuclear binding energy is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. or, equivalently, the energy that would be liberated by combining individual protons and neutrons into a single nucleus. The nucleons are held together through forces which we refer to as the strong nuclear force. The greater the nucleus components are bound, the greater will be the binding energy which it requires in order to separate them. The nuclear binding energy for hydrogen nucleus, a proton, is zero because there is only one particle in the nucleus.

-5. The nuclear binding energies are on the order of a million times greater than the electron binding energies of atoms. The forces that bind nucleons together in an atomic nucleus are much greater than those that bind an electron to an atom through electrostatic attraction. Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17.6 MeV released in the deuterium–tritium (D–T) reaction.

-6. The binding energy per nucleon (BEN) is the average energy required to remove an individual nucleon from a nucleus—analogous to the ionization energy of an electron in an atom. If the BEN is relatively large, the nucleus is relatively stable.

A graph of binding energy per nucleon (BEN) versus atomic mass (A) is considered by many physicists to be one of the most important graphs in physics. The graph rises at low A, peaks very near iron (Fe, A=56), and then tapers off at high A. The peak value suggests that the iron nucleus is the most stable nucleus in nature (it is also why nuclear fusion in the cores of stars ends with Fe). The reason the graph rises and tapers off has to do with competing forces in the nucleus. At low values of A, attractive nuclear forces between nucleons dominate over repulsive electrostatic forces (coulomb forces) between protons. But at high values of A, repulsive electrostatic forces between forces begin to dominate, and these forces tend to break apart the nucleus rather than hold it together.

Since smaller nuclei have a larger surface-area-to-volume ratio, the binding energy per nucleon due to the nuclear force generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons. The electrostatic force, on the other hand, is an inverse-square force, so a proton added to a nucleus will feel an electrostatic repulsion from all the other protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei atomic number grows. The net result of the opposing electrostatic and strong nuclear forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei.

Fusion of light nuclei to form medium-mass nuclei converts mass to energy, because binding energy per nucleon is greater for the product nuclei. The larger nucleus has a greater binding energy and less mass per nucleon than the two that combined. Thus mass is destroyed in the fusion reaction, and energy is released. Once the size of the created nucleus exceeds that of iron, the short-ranging nuclear force does not have the ability to bind a nucleus more tightly, and the emission of energy ceases. In fact, for fusion to occur for elements of greater mass than iron, energy must be added to the system!

-7. When two nuclei are brought together for fusion, as they approach each other, all the protons in one nucleus repel all the protons in the other. Not until the two nuclei actually come close enough for long enough so the strong nuclear force can overcome (by way of quantum tunneling) the repulsive electrostatic force. Consequently, even when the final energy state is lower, there is a large energy barrier that must first be overcome. It is called the Coulomb barrier. The Coulomb barrier is smallest for isotopes of hydrogen, as their nuclei contain only a single positive charge. Using deuterium–tritium fuel, the resulting energy barrier is about 0.1 MeV. The (intermediate) result of the fusion is an unstable 5He nucleus, which immediately ejects a neutron with 14.1 MeV. The recoil energy of the remaining 4He nucleus is 3.5 MeV, so the total energy liberated is 17.6 MeV. This is many times more than what was needed to overcome the energy barrier. Other fusion reactions involving elements with an atomic number above 2 can be used, but only with much greater difficulty. This is because the Coulomb barrier increases with increasing charge of the nuclei, leading to the requirement that the plasma temperature exceed 1,000,000,000 K if a significant rate is to be achieved.

-8. The major obstruction to fusion is the Coulomb repulsion between nuclei. Since the attractive nuclear force that can fuse nuclei together is short ranged, the repulsion of like positive charges must be overcome to get nuclei close enough to induce fusion. The electrostatic force between the positively charged nuclei is repulsive, but when the separation is small enough, the quantum effect will tunnel through the wall. Therefore, the prerequisite for fusion is that the two nuclei be brought close enough together for a long enough time for quantum tunneling to act. At nucleus radii distances, the attractive nuclear force is stronger than the repulsive electrostatic force. Therefore, the main technical difficulty for fusion is getting the nuclei close enough to fuse. If the nuclei have enough kinetic energy to get over the Coulomb repulsion hump, they combine, release energy, and drop into a deep attractive well. Nuclear fusion involves the collision of two nuclei at high speeds. In order for them to fuse, they need to overcome their coulombic barrier. This would be energetically impossible – if not for a quantum tunneling effect which lowers the energy needed for fusion. The tunneling only happens when the distance between nuclei is on the order of 1 femtometer. Despite the tendency for strong repulsion of the positively charged protons, quantum mechanical tunneling (of the wave functions of the interacting protons) through the Coulomb barrier can occur. The closer reactants get to one another, the more likely they are to fuse. That closeness is directly related temperature i.e., kinetic energy of fusing nuclei. Thus most fusion in the Sun and other stars takes place at their centers, where temperatures are highest. Quantum mechanical tunneling is what makes fusion in the Sun possible, and tunneling is an important process in most other practical applications of fusion, too.

-9. In a deuterium–tritium fusion reaction, the energy necessary to overcome the Coulomb barrier is 0.1 MeV. Converting between energy and temperature shows that the 0.1 MeV barrier would be overcome at a temperature in excess of 1.2 billion kelvin. There are two effects that are needed to lower the actual temperature. One is the fact that temperature is the average kinetic energy, implying that some nuclei at this temperature would actually have much higher energy than 0.1 MeV, while others would be much lower. It is the nuclei in the high-energy tail of the velocity distribution that account for most of the fusion reactions. The other effect is quantum tunnelling. The nuclei do not actually have to have enough energy to overcome the Coulomb barrier completely. If they have nearly enough energy, they can tunnel through the remaining barrier. For these reasons fuel at somewhat lower temperatures will still undergo fusion events, at a lower rate. This is how we are doing D-T fusion in tokamak at 150 million K.

Note:

The formula to convert Kelvin into Celsius is C = K – 273.15. At millions of degrees, 273 addition or subtraction does not matter. So practically 150 million K is almost same as 150 million C.

-10. Nuclear fusion occurs in the core of the Sun. It is there where there is so much pressure and heat that atoms of hydrogen fuse and become helium. The sun weighs about 333,000 times more than Earth does. That mass creates powerful gravitational forces that produce extreme pressures. Overcoming that innate repulsion happens in the Sun’s core because it is under immense pressure of gravity as well as heat – around 265 billion bar of pressure and 15 million degrees Celsius temperature. Massive gravitational forces create these conditions for nuclear fusion. On Earth, it is impossible to achieve such conditions. Normally, fusion is not possible because the strongly repulsive electrostatic forces between the positively charged nuclei prevent them from getting close enough together to collide and allow fusion to occur. The mechanism to overcome the coulomb barrier is by the temperature and by the pressure. Temperature gives kinetic energy to particles and pressure confines particles. Temperature alone won’t do as particles will fly away, so pressure will confine them. In general, proton-proton fusion can occur only if the kinetic energy of the protons is high enough to overcome their mutual electrostatic repulsion. As a result they collide at high speeds overcoming the natural electrostatic repulsion that exists between the positive charges and subsequently fuse to form the heavier helium. At close distances, the attractive nuclear force allows the nuclei to fuse. During most of the Sun’s life, energy has been produced by nuclear fusion in the core region through the proton-proton chain. This process converts hydrogen into helium and produces about 3.6 × 10^11 kJ of energy per mole of helium produced on sun. Of all of the mass that undergoes this fusion process, only about 0.7% of it is turned into energy. By this process our Sun converts 600 million tons of hydrogen into 596 million tons of helium every second. Using the mass-energy equivalence, we find that this 4 million tonnes of matter is equal to about 3.6 x 10^26 joules of energy released per second! The release of fusion energy produces an outward radiation pressure and thermal gas pressure that prevents the Sun from gravitational collapse.

-11. The production of new elements via fusion nuclear reactions is called nucleosynthesis. Many heavy elements found on Earth and throughout the universe were originally synthesized by fusion within the hot cores of the stars by process known as nucleosynthesis. A star’s mass determines what types of nucleosynthesis occurs in its core (or during explosive changes in its life cycle). Different reaction chains are involved, depending on the mass of the star and therefore the pressure and temperature in its core. Hydrogen fusion is the fundamental nuclear reaction in stars. The smallest stars only convert hydrogen into helium. When hydrogen is depleted in medium sized star, helium is converted into oxygen and carbon. Massive stars can fuse carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Later reactions transform these elements into calcium, iron, nickel, chromium, copper and others. When these old, large stars with depleted cores supernova, they create heavy elements (all the natural elements heavier than iron) and spew them into space, forming the basis for life. Each of us is made from atoms that were produced in stars and went through a supernova. It would not be wrong if I say that we are children of the stars in the sky dissolving the manmade barriers of race, religion, language and culture.

-12. In the sun, gravity is used to confine hydrogen at the density necessary for fusion. In a thermonuclear weapon, a nuclear fission explosion creates the temperatures and densities necessary for fusion. Unfortunately, neither the gravity of the sun nor the forces of a nuclear weapon are replicable on earth in the controlled manner necessary for energy generation. There are two practical ways being closely studied today to contain the pressure and temperature necessary to harvest energy from fusion; lasers can be used to ignite and inertia to confine the fuel or magnets can be used to confine plasma that is heated by an electric current. The issue is that these magnets, electric currents and lasers require more energy to generate and contain the extreme heat and pressure of a fusion reaction than gained from any experimental fusion reactions.

-13. To achieve fusion on Earth, one has to create a plasma of the fusion reactants of sufficiently high temperature and density, and also hold it confined for a sufficiently long time away from any surrounding material walls. While the sun’s massive gravitational force naturally induces fusion, without that force a temperature even higher than in the sun is needed for the reaction to take place. So on Earth, we need temperatures of around 150 million degrees Celsius to make deuterium and tritium fuse, while regulating pressure and magnetic forces at the same time, for a stable confinement of the plasma and to maintain the fusion reaction long enough to produce more energy than what was required to start the reaction. In order for the fusion reaction to take place, the constituent nuclei of the fuel (isotopes of hydrogen) must be very hot, and they must collide with each other frequently enough. Fusion thus requires high temperature and density simultaneously, which when multiplied together is defined as pressure. Note that density is directly proportional to pressure and indirectly proportional to temperature. As pressure increases, with temperature constant, density increases. In fusion’s case the pressure is in a plasma unlike air where it is in a gas, but the principle holds the same way. Thus, high pressures (>2atm) and temperatures (>100 million degrees K, several times hotter than the center of the sun) are required to obtain fusion reactions in a magnetic confinement device. In addition to being required for obtaining fusion, the pressure determines the rate of fusion reactions taking place inside the fusion reactor once the desired temperature is reached. The reaction rate, and thus the fusion power of the device, goes approximately as the pressure squared, so a doubling of pressure leads to a quadrupling of the fusion power. Therefore, techniques that enable increased pressure or that obtain high pressure using cost-effective technologies, improve the overall economics of a fusion reactor. The economics require that magnetic fusion reactors achieve pressures of 3 to 10 atmospheres.

-14. Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. Such temperatures cause the matter to become a plasma and, if confined, fusion reactions may occur due to collisions with extreme thermal kinetic energies of the particles. There are two forms of thermonuclear fusion: uncontrolled, in which the resulting energy is released in an uncontrolled manner, as it is in thermonuclear weapons (“hydrogen bombs”) and in most stars; and controlled, where the fusion reactions take place in an environment allowing some or all of the energy released to be harnessed for constructive purposes.

-15. Fusion reaction requires immense temperatures of tens of million degrees Celsius to provide atomic nuclei with enough energy to overcome the natural repulsion that exists between them in order for them to fuse. The heating of fuel gases (D + T) can be achieved by coupling radio-frequency waves or microwaves to the fuel particles, by injecting energetic beams of neutral atoms that become ionized and heat the fuel gases, by magnetically compressing the gases, or by the ohmic heating (also known as Joule heating) that occurs when an electric current passes through the gases. At these temperatures, fuel gases become a plasma in which atomic components, such as nuclei and electrons are unbound. It is impossible for any known material to withstand direct contact with a substance as hot as these plasmas. No known material can withstand such extreme conditions; they would melt even extremely heat-resistant metals such as tungsten in an instant. The answer long favored for reactor design is magnetic confinement: holding the electrically charged plasma in a “magnetic bottle” formed by strong magnetic fields so it never touches the walls of the fusion chamber. The properties of plasma are very different to those of a normal gas. For example, a plasma is electrically conductive; its motion can be influenced by electric and magnetic fields. The fusion devices exploit this particular property of plasma: they confine the hot plasma in a “magnetic field cage”. For energy production the plasma has to be confined for a sufficiently long period for fusion to occur. At ITER, a device called the tokamak uses a strong magnetic field to confine the plasma used for fusion experiments. However, achieving the precise spatial configuration and strength of this magnetic field, as well as heating the plasma to the required temperature, poses significant challenges.

-16. The three keys to practical fusion energy generation are to achieve the temperatures necessary to make the reactions likely, to raise the number density (not mass density) of the fuel, and to confine it long enough to produce large amounts of energy. These three factors—temperature, number density, and time—complement one another, and so a deficiency in one can be compensated for by the others.

Lawson showed that magnetically confined fusion plasmas can indeed give net energy, if it meets the triple product, n_iT_iτ_E > 5×10²¹keV m⁻³s, (n_iis the ion density, T_iis the ion temperature, and τ_Eis the energy confinement time) for a Deuterium-Tritium based plasma. Decades of Tokamak research has confirmed that a D-T fusion product (density-confinement time-temperature n_iT_iτ_E) of ≥ 5×10²¹ m^-3 s KeV as a minimum requirement for ignition. JET has reached values of n_iT_iτ_E over 10²¹ m^-3 s KeV.

To realize a net power output from such a fusion power plant—allowing for plasma radiation and particle losses and for the somewhat inefficient conversion of heat to electricity— plasma temperatures of about 150,000,000 K (12.9 keV) and a product of particle density times containment time of about 3×10^20 particles per cubic metre times seconds are necessary. For example, at a density of 3×10^20 particles per cubic meter, the containment time must be one second. Such figures are yet to be reached, although there has been much progress.

-17. At such high temperatures, particles have a large kinetic energy, and hence velocity. If unconfined, the particles will rapidly escape, taking the energy with them, cooling the plasma to the point where net energy is no longer produced. A successful reactor would need to contain the particles in a small enough volume for a long enough time for much of the plasma to fuse. In ITER and many other magnetic confinement reactors, the plasma, a gas of charged particles, is confined using magnetic fields. The plasma must be confined so that the energy released by fusion reactions, when deposited in the plasma, maintains its temperature against loss of energy by such phenomena as conduction, convection, and radiation. The keys to generating usable amounts of fusion energy are to attain a sufficient plasma temperature and a sufficient confinement quality. The quality of plasma confinement—defined as the time required to lose energy to the vessel walls—needs to be long enough to allow sufficient plasma energy to circulate in the confined region so that confined ions are kept hot enough to maintain an appropriate level of fusion. The plasma needs to conserve the energy that is supplied to it by various methods. To understand how good the insulation is, in plasma physics, the energy confinement time parameter is used. If fusion reactor has a very long confinement time it will take little power to heat it, but if the confinement time is a low value it will take a lot of energy to maintain the temperature.

-18. Cross-section, in nuclear physics, is probability that a given atomic nucleus or subatomic particle will exhibit a specific reaction (for example, absorption, scattering, or fission) in relation to a particular species of incident particle. The probability that a reaction will occur is analogous to the probability of a random bullet hitting its target. Values of cross sections depend on the energy of the bombarding particle and the kind of reaction. The concept of a nuclear cross section can be quantified physically in terms of “characteristic area” where a larger area means a larger probability of interaction. The reaction cross section is usually not the same as the geometric cross-sectional area of the target nucleus or particle. A reaction’s cross section, denoted σ, measures the probability that a fusion reaction will happen. The reaction cross section (σ) is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution, then it is useful to perform an average over the distributions of the product of cross-section and velocity. This average is called the ‘reactivity’, denoted ⟨σv⟩. The fusion reactivity increases rapidly with temperature until it maximizes and then gradually drops off. The reaction rate (fusions per volume per time) is ⟨σv⟩ times the product of the reactant number densities. The D-T fusion is most conveniently achievable because the collision cross-section of the D-T fusion reactions is the highest and occurs at the lowest temperature.

-19. The main fuels used in nuclear fusion devises are deuterium and tritium, both heavy isotopes of hydrogen. The Deuterium (D) – Tritium (T) reaction has the largest cross section (in other words, the probability of a reaction to take place) and also the largest Q-value (the released energy of a reaction) of all varieties of fusion reactions. It produces an alpha particle (or Helium-4 nucleus) and a neutron, and releases 17.6 megaelectron volt (MeV) of energy in the form of kinetic energy of the products (3.5 MeV to alpha particle and 14.1 MeV to neutron). The development of fusion as a viable energy source is based on reactions between high temperature deuterium and tritium plasma ions resulting in the formation of 3.5 MeV α-particles and neutrons of 14.1 MeV kinetic energies, respectively. Each D-T fusion reaction releases 17.6 MeV compared with 3-4 MeV for D-D fusion and 200 MeV for U-235 fission. Based on the mass, the D-T fusion reaction releases about four times as much energy as the uranium fission reaction although individual fission reaction releases more energy per reaction than individual fusion reaction. D-T fusion reaction proceeds with a mass loss of 0.0188 amu, corresponding to the release of 1.69 × 10^9 kilojoules per mole of helium formed.

-20. Why fuse deuterium and tritium?

Deuterium is a hydrogen atom consisting of one proton and one neutron in its nucleus. Tritium is a radioactive form of hydrogen having one proton and two neutrons. The current best bet for fusion reactors is deuterium-tritium fuel. By weight, the fuel is two-fifths deuterium and three-fifths tritium. This fuel reaches fusion conditions at lower temperatures compared to other elements and releases more energy than other fusion reactions. Reactions between deuterium and tritium are the most important fusion reactions for controlled power generation because the cross sections for their occurrence are high, the practical plasma temperatures required for net energy release are moderate, and the energy yield of the reactions are high—17.6 MeV for the basic D-T fusion reaction.

The heavy isotopes of hydrogen contain only one proton in the nuclei. So the repelling force is relatively low. The deuterium-tritium reaction is the most useful for fusion energy because it most easily overcomes the Coulomb repulsion, and it has the highest energy release among laboratory-feasible reactions.

It should be noted that any plasma containing deuterium automatically produces some tritium and helium-3 from reactions of deuterium with other deuterium ions.

Other fusion reactions involving elements with an atomic number above 2 can be used, but only with much greater difficulty. This is because the Coulomb barrier increases with increasing charge of the nuclei, leading to the requirement that the plasma temperature exceed 1,000,000,000 K if a significant rate is to be achieved.

Why not fuse ordinary hydrogen?

Deuterium and tritium are 24 orders of magnitude more reactive than ordinary hydrogen. That’s 10^24 times more reactive and therefore 10^24 times easier to fuse under the considerably less favorable conditions. This huge advantage in fusion reactivity allows human-made fusion assemblies to be workable with a billion times lower particle density and a trillion times poorer energy confinement than the levels that the Sun enjoys for fusing ordinary hydrogen. The reaction ¹H + ¹H → ²H + e⁺+ v_e has a very low probability of occurring. It must be noted a deuteron-producing event is very rare due to it being initiated by a weak nuclear force. The proton-proton cycle is not a practical source of energy on Earth, in spite of the great abundance of hydrogen (¹H). The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton. It has not been possible to measure the cross-section of this reaction experimentally because it is so low but it can be calculated from theory.

Why not D-D reaction?

The deuterium-tritium reaction is favored by fusion developers because its reactivity is 20 times higher than a deuterium-deuterium fueled reaction, tritium-suppressed D-D fusion requires an energy confinement that is 10 times longer compared to D-T and double the plasma temperature. Each D-T fusion reaction releases 17.6 MeV compared with 3-4 MeV for D-D fusion. Energetic neutron streams comprise 80 percent of the fusion energy output of deuterium-tritium reactions and 35 percent of deuterium-deuterium reactions. If reactors can be made to operate using only deuterium fuel, then the tritium replenishment issue vanishes and neutron radiation damage is alleviated. But the other drawbacks remain—and reactors requiring only deuterium fueling will have greatly enhanced nuclear weapons proliferation potential.

JET, so far the only operational fusion experiment capable of producing fusion energy, is routinely operated with Deuterium only, for a number of reasons. This minimises activation (from D-T neutrons and from Tritium retention in walls etc.), enabling to upgrade JET easily and minimising decommissioning issues at the end of JETs operational life. The plasma temperatures of 150 – 200 million degrees C will enable lots of D-T fusion – but not very much D-D fusion as D-D needs much higher temperatures of 400 – 500 million degrees C.

Why not Proton Boron-11 reaction?

Their fusion releases no neutrons, but produces energetic charged alpha (helium) particles whose energy can directly be converted to electrical power but the optimum temperature for this reaction of 123 keV is nearly ten times higher than that for pure hydrogen reactions, and energy confinement must be 500 times better than that required for the D-T reaction. In addition, the power density (w/m3) is 2500 times lower than for D-T, although per unit mass of fuel, this is still considerably higher than for fission reactors. Whereas D-T will fuse at 150 million degrees Celsius, hydrogen and boron require 1.5 billion degrees. Advantages of this aneutronic fusion include greatly diminished material science problems (reduced radioactive contamination of reactor components) and reduced non-proliferation concerns.

-21. In a commercial fusion power station the fuel will consist of a 50-50 mixture of deuterium and tritium (D-T), since this mixture fuses at the lowest temperature and its energy yield is the largest compared with other fusion reactions. Deuterium can easily be extracted from seawater, where 1 in 6700 hydrogen atoms is deuterium. It is a widely available, harmless, and virtually inexhaustible resource. It sells for about $13 per gram. In every cubic metre of seawater, there are 33 grams of deuterium. Tritium can be produced from lithium, which is widely distributed in the Earth’s crust. Thus, the primary fuels for D-T fusion reactors are so abundant in nature that, practically speaking, D-T fusion is an inexhaustible source of energy for global energy requirements. For comparison, if the deuterium in 50 cups of seawater were used in a D-T fusion reactor, the energy produced would be equal to that gained from the burning of 2 tonnes of coal. In addition, the primary fuels (deuterium, lithium) and the direct end product (helium) of fusion are neither toxic nor radioactive, and they do not produce atmospheric pollution nor do they contribute to the greenhouse effect.

-22. A fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1, when the power being released by the fusion reactions is equal to the required heating power, is referred to as breakeven, or in some sources, scientific breakeven. But because of heat losses, burning plasmas are not reached until about Q = 5.

Ignition is defined to occur when the reactions produce enough energy to be self-sustaining after external energy input is cut off. This goal, which must be reached before commercial plants can be a reality, has not been achieved. Ignition corresponds to infinite Q, in which case no energy input is required to start self-sustaining fusion reactions in the plasma, with fresh fuel then being added to continue it. The thermonuclear fusion plasma can be confined in an ignited state when P aux (the auxiliary power supplied from outside to sustain the reaction) reaches 0 which happens for the D-T fusion reaction, for example, when the output alpha particles from the fusion reaction lose all of their energy in keeping the thermonuclear plasma hot and alpha power also accounts to sustain the plasma temperature against thermal conduction/convection losses plus the radiative power losses. In such a scenario, the fusion reaction is completely self-sustained by the alpha power and no external heating power is required.

When heating by the helium nuclei (“alpha heating”) is dominant (over 50 percent), the plasma is said to be a “burning plasma.” This is a state of matter that has never been produced in a controlled manner on Earth. At Q = 5, approximately 50 percent of the plasma heating is contributed by the alpha particles. At Q = 10 (ITER), this percentage rises to 66 percent. At Q=20 alpha heating represents 80 percent.

For over two decades since 1997, the record for Q was held by JET at Q = 0.67. The recent record for Q was held by JT-60 (Japan Torus-60) with Q = 1.25, slightly besting JET’s earlier Q = 1.14. In December 2022, the National Ignition Facility reached Q = 1.54 with a 3.15 MJ output from a 2.05 MJ laser heating, which remains the record as of 2023. It was overstatedly labelled as ignition achieved. The hydrogen bomb is the only device currently able to achieve fusion energy gain factor significantly larger than 1.

-23. How do both fission and fusion generate energy?

Both fission and fusion on earth entails strong nuclear force. Fusion and fission do not release energy for the same process. For instance, you can release energy by fusing hydrogen atoms, but you cannot release energy by helium fission. Similarly, you can release energy through the fission of uranium, but you cannot release energy by fusing constituent atoms together to make uranium. While it might seem confusing that energy can be generated by both fusion and fission, as they appear to be quite opposite processes, the explanation lies in the size of the nuclei. The general rule is that for elements lighter than iron, fusion is exothermic; for elements heavier than iron, it is fission that’s exothermic. This rule is approximate, and the details differ for certain isotopes.

Fission releases energy, because a heavy nucleus (like Uranium-235) took energy to squeeze all those protons and neutrons hard enough together to make them barely stick (by the nuclear force) against the natural tendency for all those protons to fly violently apart because of their electrostatic repulsion. When struck by an incoming neutron, a heavy unstable nucleus like uranium splits into two smaller nuclei releasing energy.

Both fission and fusion are nuclear reactions that produce energy, but the applications are not the same. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together forming a new element with a higher atomic number (more protons in the nucleus). Fission is used in nuclear power plant (NPP) since it can be controlled, while fusion is not utilized to produce power since the reaction is not easily controlled and is expensive to create the needed conditions for a fusion reaction.

-24. The greater the amount of binding energy per nucleon, the more stable the atom. Furthermore, atoms attempt to become more stable by increasing their binding energy per nucleon. So very large nuclides spontaneously split apart to form lighter, more stable, nuclides. Spontaneous fission is a relatively slow process except for the heaviest isotopes. Even when they do occur, these reactions are often very slow. The half-life for the spontaneous fission of 238U, for example, is 4468 billion years.

Fission occurs when a neutron slams into a larger atom, forcing it to excite and split into two smaller atoms—also known as fission products. Additional neutrons are also released that can initiate a chain reaction. When each atom splits, a tremendous amount of energy is released.

When 235U absorbs a thermal neutron, it splits into two particles of uneven mass and releases an average of 2.5 neutrons. The induced fission of this isotope releases an average of 200 MeV energy per atom, or 80 million kilojoules per gram of 235U. The attraction of nuclear fission as a source of power can be understood by comparing this value with the 50 kJ/g released when natural gas is burned. The energy released by fission is a million times greater than that released in chemical reactions; but lower than the energy released by nuclear fusion; for equal weights of fuel, fusion releases four times more energy than fission.

-25. Fusion reactions have an energy density per mass many times greater than nuclear fission; the reactions produce far greater energy per unit of mass even though individual fission reactions are generally much more energetic than individual fusion ones. However, fuels for fusion reaction are gases while fuels for fission reaction are solid, so energy density per volume is higher for fission than fusion. The energy concentration of fusion reactions in gas is less than for fission reactions in solid fuel, and the heat yield per each reaction is 11 times less. Therefore nuclear fission will always have a much larger power density (w/m3) than thermonuclear fusion, which means that we need a more extensive and thus more costly fusion reactor than a fission reactor with the same power output.

-26. The history of fusion research is the opposite of fission research. With fission, the reactor came first, and then the bomb was built. With fusion, the bomb was built long before any progress was made toward the construction of a controlled fusion reactor. More than 70 years after the first hydrogen bomb was exploded, the feasibility of controlled fusion reactions is still open to debate.

-27. Game of neutrons:

At full power, the ITER machine will generate some one hundred billion billions of highly energetic neutrons per second travelling at approximately 51,000 kilometres per second (17 percent of the speed of light!). The reaction of tritium and deuterium produces one neutron which carries most of the energy of the reaction. Neutrons are uncharged particles, and as such they are able to escape the fields confining the plasma. As a consequence, the neutrons are suitable to carry the energy out of the plasma to the wall of the reactor where the kinetic energy of the neutrons can be transformed to heat and used for energy production through a turbine. Furthermore, the neutrons can be indirectly used for the breeding of tritium from lithium. The energy which remains with the helium nucleus cannot escape the magnetic confinement and so it contributes to the necessary – as the plasma continuously loses energy through radiation – heating of the plasma.

Additionally, neutrons get absorbed in the surrounding fusion chamber (for example in tokamaks in the surrounding mostly metallic structures such as blankets and vessels). The ocean of slowing-down neutrons that results from scattering of the streaming fusion neutrons on the reaction vessel permeates every nook and cranny of the reactor interior, including appendages to the reaction vessel. The effect of neutron activation on metals is roughly that they become hardened, brittle, as well as radioactive. The fusion neutrons will produce nuclear transmutation reactions and atomic displacement cascades inside the various materials encountered, and therefore irradiated materials yielding a degradation of their physical and mechanical properties and enhancing eventually corrosion effects. Structural material stability is a critical issue. Materials that can survive the high temperatures and neutron bombardment experienced in a fusion reactor are considered key to success.

Also, neutrons generated in D-T fusion has enough energy to make U-238 (harmless on its own) into Pu-239, which can be material for a nuclear bomb; in other words, this lone neutron can be used to make “fissile material.” The clandestine production of plutonium 239 is possible in a fusion reactor simply by placing natural or depleted uranium oxide at any location where neutrons of any energy are flying about. But high energy fusion neutrons can also cause fission of U-238 although Uranium-238 cannot maintain a self-sustaining fission reaction. Note that fusion core of hydrogen bomb is surrounded by U-238, and the induced fission gives about half of the energy. In other words, if someone clandestinely try to make fissile material in fusion reactor, U-238 may undergo fast-fission creating explosion.

In fission reactors, at most 3 percent of the fission energy appears as neutrons. D-T fusion power consists primarily (80 percent) of streams of energetic neutrons. Neutron streams lead to problems with nuclear energy, as we know from nuclear fission: radiation damage to structures; radioactive waste; the need for biological shielding; remote handling and safety; and the threat of nuclear weapons proliferation. The neutron bombardment affects the fusion vessel itself, and so once the plant is decommissioned the site will be radioactive. However the radioactive products are short lived (50-100 years) compared to the waste from a fission powerplant (which lasts for thousands of years). Also, the radioactivity in a fusion powerplant will be confined to the powerplant itself.

A peculiar feature of D-T fusion reactors is that the overwhelming preponderance of thermal energy is not produced in the reacting plasma, but rather inside the thick steel reactor vessel as the neutron streams smash into it and gradually dissipate their energy. In principle, this thermalized neutron energy could somehow be converted back to electricity at very low efficiency. An energy source consisting of 80 percent energetic neutron streams is not an electrical energy source.

-28. A tokamak is a machine that confines a plasma using magnetic fields in a donut shape that scientists call a torus. In a tokamak, magnetic field coils confine plasma particles to allow the plasma to achieve the conditions necessary for fusion. ITER will use four types of magnets to contain the plasma: a central solenoid magnet, poloidal magnets around the edges of the tokamak, 18 D-shaped toroidal-field coils, and correction coils. One set of magnetic coils generates an intense “toroidal” field, directed the long way around the torus. A central solenoid (a magnet that carries electric current) creates a second magnetic field directed along the “poloidal” direction, the short way around the torus. The two field components result in a twisted (helical) magnetic field that confines the particles in the plasma. A third set of field coils generates an outer poloidal field that shapes and positions the plasma. Twisting the magnets can also produce the helical shape without the need for a transformer (central solenoid) – this kind of configuration is called a stellarator. In tokamaks the twisting is produced by a toroidal plasma current and in stellarators by external non-axisymmetric coils. In all types of stellarators, the poloidal field component is generated by external coils. Therefore, such systems can be operated without any externally driven plasma current. Hence, stellarators have an inherent potential for stationary operation. In addition, current driven instabilities, in particular disruptions, do not exist in stellarators.

-29. It is now known within a factor of two or so what it will take to achieve ignition or the burning plasma state quite close to it. That requirement, simplified drastically, is that the product of magnetic field strength, B, and the major radius of the plasma torus, R, must exceed roughly 20 tesla-meters. JET has a BR product of approximately 10. The Tokamak Fusion Test Reactor and JT-60 had similar values. None are sufficient to enter the burning plasma regime. The International Thermonuclear Experimental Reactor (ITER), a giant tokamak under construction in France by a collaboration of 35 nations, is designed to achieve and study burning plasmas. It will have approximately B = 5.7 tesla and R = 6 meters, for a BR of 34.2. The niobium-titanium superconductor envisaged for the ITER test reactor is to generate a magnetic field of 13 tesla on the coil, i.e. 5.7 tesla on the magnetic field axis. Commonwealth Fusion Systems (CFS) has demonstrated that, by using high-temperature superconductors, much higher magnetic field strength in the tokamak configuration can be generated without significant dissipation. This increase makes the proposed SPARC experiment feasible, which is a design with roughly B = 10 and R = 2.2, so BR = 22.4

-30. ITER’s thermonuclear fusion reactor will use over 300 MW of electrical power to cause the plasma to absorb 50 MW of thermal power, creating 500 MW of heat from fusion for periods of 400 to 600 seconds. ITER is designed to yield in its plasma a ten-fold return on power (Q=10), or 500 MW of fusion power from 50 MW of input heating power. As of 2022, the record for energy production using nuclear fusion is held by the National Ignition Facility reactor, which achieved a Q of 1.54 in December 2022 for ICF. ITER will be by far the largest and highest magnetic field tokamak in the world, and it will be powered by a central solenoid that will be the most powerful pulsed superconducting magnet ever constructed. Fabricated from 36 km of superconducting cable, this 1,000-ton magnet will drive 15 million amperes of current through the plasma, far more than anything that has been possible before. In addition, ITER will serve as a test bed for a number of critical fusion technologies, including tritium breeding, plasma control, advanced diagnostics, and disruption mitigation. Though it will not operate as a power plant, ITER will test safety features that future fusion power plants will require. The goal of ITER has never been to get fusion power. The goal is to solve all the problems fusion can create. Problems like: the material science, making high temperature magnets, controlling the plasma, building a trained workforce, finding modes of operation and making better models.

-31. Inertial confinement fusion (ICF) is how hydrogen bombs work. It relies on reactions at densities roughly a trillion times higher than magnetic fusion and taking place so quickly that most of the fuel reacts before the high-pressure assembly explodes apart. NIF focused 192 laser beams on a cavity, converting the laser energy into X-rays that heated a 2mm metal shell placed in the centre. As the outside of the shell rapidly expanded due to the heating, the inside was driven inwards – reaching density of one-hundred times that of lead, around 1000 g/cm³and temperatures around 100 million kelvins – creating a tiny piece of matter as found in dwarf stars for just a nanosecond. To produce useful energy, as opposed to devastation, the idea of inertial fusion energy (IFE) is that each explosion’s size should be scaled down to something that is manageable and repeated once per second or faster to provide a source of useful average power. That might sound a bit crazy, but currently almost all the power for ground transport works on the principle of repetitive explosions: the internal combustion engine.

-32. The amount of tritium consumed in producing one joule of fusion energy is exactly the same for MCF as for ICF. The modest amount of tritium in each small ICF capsule makes it practical to use tritium on every shot but NIF currently executes, at best, one experimental blast per day while JET can run 30 shots per day. Also, the pulse length of JET is approximately a trillion times longer than that of ICF. Huge inefficiencies in NIF’s lasers and in the conversion of fusion heat to electrical power mean its design is inherently impractical. A commercial laser-fusion power plant would need to generate 100 times more energy from each target than was input, and its lasers would need to fire around 10 times per second. This means designing a system that can accurately focus and fire the lasers on hundreds of thousands of targets each day. The problem with the laser method is that large equipment is unable to continuously fire beams, while smaller equipment lacks the output needed to ignite fusion. The big scaling advantage in favour of MCF is that the geometry remains substantially unchanged as you increase the size, where as in ICF it does not. As you scale up the size of the pellet, the size and complexity of the required laser system and optics would almost certainly increase disproportionately. The optics of ICF systems are necessarily very complex to produce the smoothest most even illumination, even slight changes in the geometry would require extensive redesign. So MCF is more likely to succeed than ICF for fusion power. Magnetic fusion not only produces scientific successes, but also seems very attractive, at least to private investors: a total of around 30 private companies are now working on small fusion reactors – most of them in the field of magnetic fusion – which have already raised almost five billion dollars in capital.

-33. The two varieties of fusion examined so far, magnetic and inertial confinement, exist at extreme ends of the parameters permitted by the triple product, extremes that are complex and expensive to achieve. Other approaches explore the middle ground between these extremes at more moderate levels of confinement and density. Many groups are investigating this middle ground in the hope of finding approaches that are faster, simpler, and cheaper than the multi-billion-dollar facilities like ITER and NIF.

Commonwealth Fusion Systems (CFS) has used high-temperature superconductor material to achieve a higher magnetic field in a smaller device, equalling the performance that would be achieved in an apparatus 40 times larger in volume using conventional low-temperature superconducting magnets. CFS is increasing the amount of fusion power per volume [from ITER’s] by more than a factor of 10. Fusion power per volume is the closest thing you can come up with to indicate the amount of economic output versus cost.

Tokamak Energy has developed a prototype for a compact and relatively simple fusion reactor called the ST40, which is much smaller and cheaper than other currently existing reactors.

TAE has been working for about 20 years on an approach known as field-reversed configuration (FRC). TAE’s technology seeks to fuse hydrogen and boron. Though this is a more difficult reaction to achieve—requiring temperatures at least an order of magnitude higher—it has the advantage of not producing the highly energetic neutrons that complicate DT fusion.

In Helion’s novel system, as deuterium and helium-3 fusion energy is created, the plasma expands. As the plasma expands, it pushes back on the magnetic field. By Faraday’s law, the change in field induces current, which is directly recaptured as electricity. Other fusion approaches aim to generate heat, in order to boil water and power steam turbines, which make electricity — like at traditional nuclear power plants but Helion can do it with no steam turbines or cooling towers.

Several fusion researchers who don’t work for private firms said that, although above mentioned projects are undeniably exciting, commercial fusion in a decade is overly optimistic and it would be astonishing if they succeed.

-34. In proposals of a roadmap towards the development of commercial magnetic fusion energy (MFE), a demonstration power plant (DEMO) is defined along the next step after the ITER tokamak project. DEMO refers to a proposed class of nuclear fusion experimental reactors that are intended to demonstrate the net production of electric power from nuclear fusion. EU DEMO should produce at least 2000 megawatts (2 gigawatts) of fusion power on a continuous basis. DEMO will be constructed once solution to many challenges of current fusion reactors are found.

-35. On earth, the potential advantages of energy by controlled nuclear fusion are manifold:

Limitless energy production, available all over the world, not subject to local or seasonal variations;
No greenhouse gas emissions;
No accidents such as melt-downs or explosions, due to an intrinsically safe physical process;
No production of radioactive waste with long half-lives; and
No or only a modest proliferation risk.

-36. Challenges to fusion can be categorised in 4 groups:

(1. Physics challenges

(2. Material challenges

(3. Fuel challenges

(4. Engineering challenges

Most important challenges are containing the plasma fuel at high temperatures, maintaining a great enough density of reacting ions, capturing high-energy neutrons from the reaction without melting the walls of the reactor, achieving good confinement (maintain plasma in the magnetic bottle long enough), ignition/Q factor, managing plasma instabilities and controlling capex and levelized costs.

-37. Plasma is difficult to hold: The plasma used in fusion-energy research is hundreds of millions of degrees K. You can’t hold it using a solid container, because the container would just melt. Instead, physicists have to corral it using electromagnetic fields (MCF) or work with it so quickly (in less than a billionth of a second) that holding it isn’t an issue (ICF).

Plasma is difficult to compress: If you don’t compress plasma from all sides perfectly evenly, it will squish out wherever it can. Imagine holding a large, squishy balloon. Now squeeze it down to as small as it will go. No matter how evenly you apply pressure, the balloon will always squirt out through a space between your fingers. The same problem applies to plasmas. Anytime scientists tried to clench them down into a tight enough ball to induce fusion, the plasma would find a way to squirt out the sides.

Creating the temperature and pressure conditions required to initiate the reaction in the fusion machine is a great scientific and technological challenge—so much so that after more than 70 years of experimentation, a marketable solution remains elusive. The basic problem is the difficulty of preventing energy from leaking out, which means that fusion machines, so far, consume more energy than they create. It is exceedingly difficult to contain plasmas at fusion reaction temperatures because the hot gases tend to expand and escape from the enclosing magnetic structure. The basic problem with magnetic confinement is fundamental – no matter how strong or well formed the magnetic field containing the plasma is, it will always leak, as positive nuclei or ions spiralling around the magnetic field lines collide and scatter, eventually drifting out of the containment field. Fusion plasma confinement must be good enough that plasma heat and particles escape more slowly than they react. Otherwise, like a pile of damp sticks to which the flame of a match is held, the reaction will fizzle out when the match—in this case, externally supplied plasma heating—is removed, because the reaction heat cannot overcome losses.

ITER is running over-budget and over schedule, with its projected launch date in 2040s. One of the reasons that ITER is late is that it is really, really hard; and what scientists & engineers are doing is fundamentally pushing the barriers of what’s known in the technology world.

-38. There are obstacles to obtaining high plasma pressures in fusion devices. As the plasma pressure is increased inside the magnetic bottle the plasma becomes more and more prone to slowly leaking heat, thus taking more input power to sustain the pressure and temperature. Eventually the pressure becomes large enough for the plasma to suddenly become unstable and the plasma pressure is lost, like popping a balloon. The worst form of this is termed a disruption and can do damage to the internal components of the machine, but does not affect the safety of the device. Additionally, as pressure is increased the plasma can develop turbulence that transports energy from the center of the plasma, cooling it.

-39. The fusion reactor lining that face plasma is known as the first wall. Exposure of first wall materials to fusion plasmas and their ensuing property degradation has long been recognised as one of the most important challenges facing fusion energy. Nuclear fusion reactors are extremely hostile environments for plasma facing materials (PFM) and plasma-facing components (PFC). The so-called plasma–wall interaction (PWI) processes that are crucial at the interface between the hot plasma and the wall are associated with quasistationary thermal loads up to about 20 MW m^−2 combined with short, extremely strong thermal transients up to the gigawatts per square meter range during edge-localized modes (ELMs). In addition to these thermal loads, the wall will be subjected to bombardment by plasma ions and neutral particles (D, T, and He) and by energetic neutrons with energies up to 14 MeV. Therefore, synergistic effects resulting from simultaneous thermal, plasma, and neutron wall loads must also be evaluated in complex experiments. Material degradation is accelerated when synergistic effects are taken into consideration. Under reactor-relevant conditions, the following are the most serious damaging mechanisms: thermally induced defects such as cracking and melting of the plasma-facing material (PFM); thermal fatigue damage of the joints between the PFM and the heat sink; hydrogen-induced blistering; helium-generated formation of nanosized clusters; and neutron-induced degradation of the wall armour via reduction of the thermal conductivity, embrittlement, transmutation, and activation. In addition, off-normal events such as disruptions or vertical displacement events (VDEs) could take place, compromising the mechanical integrity of the reactor. One of the biggest obstacles to magnetic-confinement fusion is the need for materials that can withstand the tough treatment they’ll receive from the fusing plasma. There is still no guarantee that these material issues can be solved.

-40. Plasma impurities:

To achieve maximum fusion efficiency in a tokamak device it is essential to limit the impurities in the plasma. But this can be a challenge, as interaction between the hot plasma and the material surfaces of the vacuum vessel causes material particles to detach and enter the swirling cloud of gas. Even in trace amounts, other atoms (“impurities”) dilute the core of the plasma by taking the space that could be occupied by the fusion fuels, resulting in fewer reactions and a reduction in energy production. And because fusion reactions occur in a roughly proportional manner to the square of fuel density, the “multiplier” effect sets in quickly—fewer fuel atoms result in a dramatic drop-off in fusion reactions. Impurities not only dilute the plasma but—depending on the physical properties of the atoms involved (the number of electrons)—they can also cool it to differing degrees. The electrons of the impurity atoms run into the electrons in the plasma and drain their energy, re-emitting it as electromagnetic radiation—including visible light. The heavier elements, in particular, drain a lot of energy from the plasma through radiation because of a high number of electrons (tungsten has 74). The energy lost through impurity radiation cools the plasma down and the fusion reactions stop.

-41. Plasma instabilities and Disruptions:

A Plasma instability is a region where turbulence occurs due to changes in the characteristics of a plasma (e.g. temperature, density, electric fields, magnetic fields). The furiously hot plasma won’t stay still: it tends to develop large temperature gradients, which generate strong convection currents that make the plasma turbulent and hard to manage. Such instabilities, akin to miniature solar flares, can bring the plasma into contact with the walls, damaging them. Other plasma instabilities can produce beams of high-energy electrons that bore holes in the reaction-chamber cladding. The thermal and magnetic energy in the tokamak can drive plasma instabilities that lead to disruptions, a central science and engineering challenge facing practical power production from nuclear fusion. Disruptions abruptly destroy the plasma’s magnetic confinement, thus terminating the fusion reaction and rapidly depositing the plasma energy into the confining vessel. At JET, considering a plasma current on axis of 3 MA, forces of about 3 MN (million newtons), that are comparable to the weight of a F15, have been experienced. ITER is expected to operate at 15 MA (15 mega amperes), therefore, considering the non-linear dependence on the plasma current, unintentional disruptions could cause serious damages to the device.

Suppressing or managing these fluctuations has been one of the key challenges for tokamak designers. The magnetic coils have to be carefully controlled to prevent the plasma from touching the sides of the vessel: this can damage the walls and slow down the fusion reaction. Finding smart ways to control and confine plasma will be key to unlocking the potential of nuclear fusion. Fusion offered a particular challenge to scientists because the process is both complex and continuous. The state of a plasma constantly changes and to make things even harder, it can’t be continuously measured. At the present time disruptions are common and unavoidable events.

-42. Parasitic power drain:

Fusion reactors face another problem: they consume a good chunk of the very power that they produce, or what those in the electrical generating industry call “parasitic power drain,” on a scale unknown to any other source of electrical power.

First, a host of essential auxiliary systems external to the reactor must be maintained continuously even when the fusion plasma is dormant (that is, during planned or unplanned outages). Some 75-to-100 MWe (megawatts electric) are consumed continuously by liquid-helium refrigerators; water pumping; vacuum pumping; heating, ventilating and air conditioning for numerous buildings; tritium processing; and so forth. When the fusion output is interrupted for any reason, this power must be purchased from the regional grid at retail prices.

Due to high parasitic power drain, it is uneconomic to run a fusion power plant below 1,000 MW size.

-43. Tritium supply:

Tritium is a radioactive isotope of hydrogen, but because it has a short half-life of 12.32 years, it is hard to find, store, produce, and is expensive. Tritium is an exceptionally pricey substance: a single gram is currently worth around $30,000. While deuterium is readily available in ordinary water, tritium scarcely exists in nature. The main source of tritium is fission nuclear reactors. A single 1000-MW fusion power plant is expected to require about 70 kilograms (kg) of tritium fuel per year. This amount is not only far in excess of current global production capacity (which is roughly 2–3 kg/year from aging facilities at CANDU reactors in Canada and South Korea), it also represents a cost factor that would reach into billions of dollars. Thus, fusion power plants will need a method to breed tritium in situ. Fortunately, the fusion reaction itself offers a potential means to do so. Placing a lithium blanket around the tokamak would generate tritium (and further heat) as the fusion neutrons are captured by the lithium nuclei and spontaneously transition to tritium. But there’s a catch: In order to breed tritium you need a working fusion reactor, and there may not be enough tritium to jump-start the first generation of power plants.

Replacing the burned-up tritium in a fusion reactor, however, addresses only a minor part of the all-important issue of replenishing the tritium fuel supply. Only 2 percent of the injected fuel will actually be burned in a magnetic confinement fusion device before it escapes the reacting region. The vast majority of injected tritium must therefore be scavenged from the surfaces and interiors of the reactor’s myriad sub-systems and re-injected 10-to-20 times before it is completely burned. If only one percent of the unburned tritium is not recovered and re-injected, even the largest surplus in the lithium-blanket regeneration process cannot make up for the lost tritium. By way of comparison, in the two magnetic confinement fusion facilities where tritium has been used (Princeton’s Tokamak Fusion Test Reactor, and the Joint European Torus), approximately 10 percent of the injected tritium was never recovered.

To make up for the inevitable shortfalls in recovering unburned tritium for use as fuel in a fusion reactor, fission reactors must continue to be used to produce sufficient supplies of tritium—a situation which implies a perpetual dependence on fission reactors, with all their safety and nuclear proliferation problems. The aggregate of unrecovered tritium may rival the amount burned and can be replaced only by the costly purchase of tritium produced in fission reactors.

-44. Magnetic quench:

Another issue is that when all of the superconducting magnets in ITER are running at full current (central solenoid 45 kA, toroidal field coil 68 kA) to create the 13 Tesla toroidal magnetic field (13x that in an MRI machine) and to create the other plasma-shaping and heating fields, they are storing 60 GigaJoules, or around 12 Tons of TNT worth of energy. This is because the 180 kilometers of superconducting Niobium-Tin wires in all these massive magnet coils can carry enormous electrical current when supercooled with liquid helium. A magnet quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil exits the superconducting state (becomes normal) due to various reasons including cooling failure. The superconductor becomes a normal conductor, and can no longer carry that enormous current. With 68,000 amps suddenly meeting resistance, the coil rapidly vaporizes, and causes a meltdown of the other coils, with a total energy release of 12 tons of TNT. This is very undesirable, especially when these coils are wrapped around a highly radioactive shield that also vaporizes and is expelled into the atmosphere.

-45. The engineering challenges of fusion energy are only part of the problem — the other part lies in economics. Fusion energy is often hailed as a limitless source of clean energy, but research suggests that may only be true if the price is right. People will not pay an unlimited amount of money for fusion energy if they could spend that money to generate clean energy more cost-effectively. Fusion power electricity in 2050 is predicted to be three times expensive than electricity from solar, wind and natural gas.

-46. The major reasons why Earth-based nuclear fusion will never be an economical source of energy are:

(1. High Capital Costs: Reactors will be expensive to build. Nuclear fusion reactors will cost more than fission because they are far more complicated. To create heat with fission, all you need to do is put the fuel rods close to each other, and it happens automatically. To create heat in a fusion reactor, advanced technologies such as immensely powerful lasers or magnetic confinement techno-donuts are needed. The cost of a power plant based on ITER would be approximately ten times the cost of a nuclear fission power plant.

(2. Fuel Costs: Despite many claims to the contrary, fuel for current fusion reactor designs is not cheap and tritium is currently around $30,000 per gram.

(3. As the interior of nuclear fusion reactors will become radioactive over time, they’re also likely to have high decommissioning costs.

(4. A Poor Fit for Modern Grids: Grids with high — or moderate — amounts of solar and wind generation have extended periods of low or zero wholesale electricity prices. This is disastrous for the economics of an expensive-to-build energy source. Because they have high capital and low fuel costs, they save little money by shutting down during periods of very low electricity prices. But as they’re competing against solar and wind with zero fuel cost, their only options are to shut down or operate at a loss during these periods.

-47. The fission waste consists largely of the fission products from the fuel, over which the plant designer has little control, whereas the fusion waste consists of structural components which have been activated by neutrons. Through the proper choice of fusion reactor materials, the amount of long-term waste can be greatly reduced relative to a fission reactor. With a nuclear fission reactor, the radiation is alpha particles, beta particles, and gamma rays (which can penetrate your skin and break apart the bonds in your DNA structure, giving you all kinds of cancer). In contrast, in a nuclear fusion reactor, the vessel wall is the only part that will be bombarded by the high energy neutrons, and if, in the worst case, all the protective layers surrounding the main fusion vessel fail, the neutron radiation will stop as soon as fusion reaction stops. In a fission reactor, the cancer-causing radiation still exists even in the waste materials, which means that extreme measures are needed to bury the waste to keep it as far away as possible from humans. In the case of nuclear fusion, the activated materials (i.e., the metal vessels which have been bombarded by neutrons) can be stored safely for about 100 years, after which the radiation level becomes so low that they can be reused in the fusion reactor again.

Tritium is a radioactive form (isotope) of hydrogen. However, the amount used is limited to a few grams of tritium for the reaction and a few kilograms on site. During operation, the radiological impact of the use of tritium on the most exposed population is much smaller than that due to natural background radiation. Tritium is a relatively weak source of beta radiation, which itself is too weak to penetrate the skin. Tritium can be inhaled, absorbed through the skin or ingested. Tritium damage is reduced owing to the short biological half-life of tritium in the body of about 10 days.

Specifically designed shielding, optimisation programmes of tritium handling and storage, protective clothing for the staff and a remote maintenance plan will result in negligible radiation risks for both the public and the staff of fusion power plant.

-48. A fusion reaction is about four million times more energetic than a chemical reaction such as the burning of coal, oil or gas. With fusion, one gram of combustible D-T fuel could generate 90,000 kilowatt hours of energy in a power plant, which is equivalent to the combustion heat of eleven tons of coal. While a 1000 MW coal-fired power plant requires 2.7 million tons of coal per year, a fusion plant of the kind envisioned for the second half of this century will need less than one ton of fuel during a year’s operation.

-49. Can Chernobyl or Fukushima type accidents occur in fusion power plant?

Very Unlikely.

The reasons that have made fusion so difficult to achieve to date are the same ones that make it safe: it is a finely balanced reaction which is very sensitive to the conditions – the reaction will die if the plasma is too cold or too hot, or if there is too much fuel or not enough, or too many contaminants, or if the magnetic fields are not set up just right to control the turbulence of the hot plasma.

The amount of fuel available at each instant is sufficient for only a few tens of seconds, in sharp contrast with a fission reactor where fuel for several years of operation is stored in the reactor core. In a fusion reactor, there will only be a limited amount of fuel (less than four grams) at any given moment. The reaction relies on a continuous input of fuel; if there is any perturbation in this process and the reaction ceases immediately. The fuel needs to be constantly fed into the reactor to keep fusion happening, making a runaway reaction impossible.

Fusion reactions take place at extremely high temperatures and the fusion process is not based on a neutron multiplication reaction. With any malfunction or incorrect handling, the reactions will stop. It is difficult to reach and maintain the precise conditions necessary for fusion; any disruption in these conditions and the plasma cools within seconds and the reaction stops, much in the same way that a gas burner is extinguished when the fuel tap is turned off. Unlike fission reactors, fusion reactors do not operate through a chain reaction, since the reaction products in fusion reactions do not themselves then initiate further fusion reactions. Thus, there is no danger of a runaway chain reaction causing a fusion reactor to melt down. Fusion reactions cannot be maintained spontaneously: any disturbance or failure stops the reaction. Moreover, since the energy confinement time in even a large fusion reactor would be short (a few seconds), the total energy stored in the reactor medium would be small, and the afterheat in the blanket would also be much smaller than in a fission reactor. Thus, the worst possible accidents in a fusion reactor should be of significance only to the reactor, not to the society. The fusion process is inherently safe; there is no danger of run-away reaction or explosion.

Measures can be taken to tackle magnetic quench. In practice, magnets usually have safety devices to stop or limit the current when a quench is detected. There is a safety-grade quench detection and a fast discharge system for the toroidal field coils. Should a quench event occur, the entire combined stored energy of these magnets must be dumped at once into massive blocks of metal that heat up to several hundred degrees Celsius—because of resistive heating—in seconds. However, if these measures fail, Tokamaks can melt down catastrophically and explode if there is a cooling failure in the magnet coils during full power operation.

-50. Fusion can be combined with fission in what is referred to as hybrid nuclear fusion where the blanket surrounding the core is a subcritical fission reactor. The fusion reaction acts as a source of neutrons for the surrounding blanket, where these neutrons are captured, resulting in fission reactions taking place. These fission reactions would also produce more neutrons, thereby assisting further fission reactions in the blanket. Hybrid fusion-fission fuel cycles, in which the neutrons emitted in D-T or D-D reactions are used to induce fissions or to create fissile isotopes (uranium-233 or plutonium-239) from fertile thorium-232 or uranium-238 in a blanket surrounding a fusion core, are possible. The fissile material produced can be fissioned in place or removed to fuel nonbreeder fission reactors elsewhere. The result, in effect, is to multiply the energy release per fusion reaction by about an order of magnitude. This makes the conditions that must be achieved in the fusion core less demanding than those for a pure fusion reactor, but the additional engineering complexity of combining fusion and fission technologies in a single device will at least partly offset this advantage and may overwhelm it.

-51. Fusion energy has some critical advantages over fission as a zero-carbon power source. It is fully controllable and thus creates dispatchable power with a ramp rate fast enough to complement renewables in a VRE-heavy grid. The fuel is readily available to all nations—for example, the required hydrogen isotopes for one commonly proposed fusion reaction can be extracted from seawater; some companies, such as TAE Technologies, are proposing fuel cycles using ordinary materials like hydrogen and boron. The fuel is slowly metered into the machine in a way that makes meltdowns or runaway events essentially impossible. And fusion creates no carbon emissions and minimal nuclear waste—only the vessel itself. Unlike today’s fission plants, fusion produces no long-lived fuel waste.

However, the necessary combination of temperature, pressure, and duration has proven to be difficult to produce in a practical and economical manner.

-52. History has shown that large scientific endeavours can lead to new and unexpected advances. The benefits of decades of fusion research are not simply deferred to the future when fusion power will be commercially available. Other technologies have developed out of fusion research that have important applications across human society. The laser and superconductor industries have been the main beneficiaries of fusion research, but it is increasingly reaching across all sectors. Sectors that have benefitted from technological spin-offs from fusion research include medicine, manufacturing, electricity transmission, environmental cleanup, and even national security. For example, one of the most revolutionary tools in modern medicine is the magnetic resonance imaging (MRI) machine. Fusion research drove the advances in large bore superconducting magnets that made the invention of MRIs possible.

-53. Fusion remains enormously expensive, and the achievement of net energy production remains elusive. Some scientists say that fusion power is nothing but an expensive pipe dream. Green members of Europe have consistently opposed fusion efforts, calling fusion a “false climate solution” and saying “the money we are spending on projects like ITER could be used for other developments.” The massive amounts of money required to investigate this unproven technology would be better spent on proven renewable energy technology that can be deployed right away. The cost and uncertainty of fusion mean investing in thermonuclear reactors at the expense of other available clean energy options. Over the last decade, solar costs have declined 85% while wind costs have declined 45%. Today, building new wind and solar infrastructure costs less than adding the equivalent capacity in coal or gas in two-thirds of the world. Considering how far nuclear fusion still has to go, fusion will not be our solution to decarbonizing the economy in the timeframe needed to prevent the worst effects of climate change from occurring. Instead expanding cheap wind and solar is our best bet for achieving our climate goals.

-54. In December 2022, U.S. Department of Energy officials announced that the National Ignition Facility at the Lawrence Livermore National Laboratory in California finally attained “fusion ignition” – a long-awaited achievement for nuclear fusion researchers around the globe. The laser energy delivered to the target was 2.05 MJ, and the fusion output was likely about 3.15 MJ. But the input energy to the laser system is around 400 MJ. Consuming 400 MJ and producing 3.15 MJ is a net energy loss greater than 99%. For every single unit of fusion energy it produces, NIF burns at minimum 130 units of energy. If gain meant producing more output energy than input electricity, then NIF fell far short. Its lasers are inefficient, requiring hundreds of megajoules of electricity to produce the 2 MJ of laser light and 3 MJ of fusion energy. NIF currently executes, at best, one experimental blast per day. A commercial plant would need to blast fusion-producing capsules at a rate of tens of thousands per day. If it could be done, still more engineering then would be required to extract the energy in the form of heat for practical electricity generation.

NIF breakthrough is all about a gain of a blip of energy barely enough to heat few kettles of water. Getting there has cost society a whopping $ 3.5 billion. That sum would buy some 2000 MW of wind power capacity that almost instantly delivers enough power to supply some 1.2 million households in a city for at least 30 years. Invested in solar farms could produce even more. Or it would buy a factory to supply batteries for 1 million electric vehicles.

We are told that nuclear fusion will someday free up vast areas of land that are currently needed to operate solar and wind power installations. But the entire NIF facility needed to house the 192 lasers and all the other necessary control equipment was large enough to contain three football fields, even though the actual fusion reaction takes place in a gold or diamond vessel smaller than a pea. All this just to generate the equivalent of about 10-20 minutes of energy that is used by a typical small home. Clearly, even the most inexpensive rooftop solar systems can already do far more.

In fact, NIF’s true value to the U.S. government is derived from its ability to test nuclear weapons without actually testing nuclear weapons. It is a way to continue investment into modernizing nuclear weapons, albeit without explosive tests, and dressing it up as a means to produce ‘clean’ energy. One aim of laser fusion experiments is to try to develop a hydrogen bomb that doesn’t require a conventional fission bomb to ignite it, potentially eliminating the need for highly enriched uranium or plutonium in nuclear weapons.

-55. It has always been recognized that a huge amount of energy is required to start up any fusion system. But tokamak fusion systems also require an unceasing hundreds of megawatts of electric power just to keep them going. A typical widespread statement is that “ITER will produce 500 megawatts of output power with an input power of 50 megawatts,” implying that both numbers refer to electric power but both are thermal power. ITER’s thermonuclear fusion reactor will use over 300 MW of electrical power to cause the plasma to absorb 50 MW of thermal power, creating 500 MW of heat from fusion for periods of 400 to 600 seconds.

In my view, input electric power of 300 MW (e) and output electric power of 166 MW (e) [i.e., 500 MW fusion thermal power] makes no sense at all. At Q-physics of 10 as touted by ITER, the power output for the whole operation would ultimately be less than the total amount of power it took to run the reactor. That means overall power loss and commercial fusion is unfeasible at that Q. In fact, commercial fusion reactor would need a Q-physics of about 40, four times the Q of ITER, to generate sufficient power to be viable. Then, 50 MW thermal input will give 2000 MW thermal output. And so total 300 MWe input electric power will give 666 MWe (i.e., 2000 MW fusion thermal power) output electric power with Q-engineering of 2. However, it is also possible that at Q-physics of 40, alpha heating will maintain plasma temperature without external 50 MW heating. Then, total 150 MWe input electric power will give 666 MWe output electric power with Q-engineering of 4. Experience building other fusion reactors suggests that when machine size is doubled one achieves 8 times improvement in heat confinement. Increasing Q would require an increase in the major radius or in the magnetic field strength. Either approach would increase the cost of the device. This is the best we can ever achieve in DEMO if all challenges are resolved. And it will be very expensive compared to electric power from fission, natural gas and renewables (wind & solar).

-56. Nuclear fusion may be not the perfect energy source for a climate crisis but, as a former fusion physicist put it, is ‘in some ways close to the opposite’. Put succinctly, the fact that neutron streams comprise 80 per cent of fusion energy output in deuterium-tritium reactions makes it an odd electrical energy source. The neutron streams damage the structure of the machine, produce relatively bulky radioactive waste, require biological shielding, and constitute a proliferation risk (Pu-239). The fusion reactor itself has a high parasitic power consumption, a scarce fuel supply, and likely high operating costs due to continual radiation damage.

ITER’s power pulse will be just 400 seconds long with at most 20 such pulses daily. Future reactors may operate less than 5% of the time. The cost of a fusion power plant based on ITER would be approximately ten times the cost of a fission power plant. On the other hand, today’s fission reactors can keep running even when individual fuel rods fail. Cranes can swap out fuel rods in just a couple of days. Availability can be as high as 90%. And only 1 ton or about 50 dm^3 of highly radioactive waste is produced per GWyr (the total volume after packaging for disposal becomes about 4m^3). Moreover, the danger of this waste is known and new methods are being developed to store it in a safe way, or even to eliminate it by transmutation thereby producing energy. This is in sharp contrast with the large amount of waste produced by burning fossil fuels: gigatons of CO2 spread around the world.

In a nutshell, energy efficiency, renewables, advanced nuclear fission, switch from coal to natural gas and direct electrification are the bulk solutions to climate change and not fusion.

_____

Dr. Rajiv Desai. MD.

July 30, 2023

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Postscript:

The harsh realities of fusion contradict the claims of its proponents of unlimited, clean, safe and cheap energy. Nonetheless, we must continue fusion research as we have tremendously benefitted from technological spin-offs from fusion research and developed the skills of generations of scientists and engineers; but getting a light bulb glowing in our homes economically from fusion reactor having burning plasma appears to be a pipe dream. Yes, governments and companies can try out different approaches and despite their powerful tools and creative approaches, many of these new ventures will fail. But if just one succeeds in building a reactor capable of producing electricity economically, it could fundamentally transform the course of human civilization.

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Footnote:

For a small doctor from small town to write article on ‘nuclear fusion’ is a bit too much. I regret any mistake. Scientists and engineers are requested to read this article and their comments/criticisms may be conveyed to me at my email [email protected]

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Mirage of Fusion Power

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