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Nuclear Energy

The JET nuclear fusion project makes a game-changing breakthrough

Print edition : Mar 11, 2022 T+T-
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Fig. 1: A photo grab of the video shot of the record fusion energy output pulse

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Fig. 6: The interior of JET with the new beryllium-tungsten wall and superimposed D-T plasma.

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Fig. 2: In D-T fusion, two isotopes of hydrogen, deuterium ( 2 H) and tritium ( 3 H), are brought together. The fusion reaction generates helium ( 4 He), a neutron and a large amount of energy, with the chargeless neutron carrying 80 per cent of it.

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Fig. 3: The new world fusion energy record of 59 megajoules (DTE2, in red) is over two-and-a-half times the previous record reached during JET’s earlier deuterium-tritium experiments in 1997 (DTE1, in grey).

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Fig. 4: The basic components of a tokamak include the toroidal field coils (in blue), the central solenoid (in green) and the poloidal field coils (in grey). The total magnetic field (in black) around the torus confines the path of travel of the charged plasma particles.

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Fig. 5: The fusion reaction rate increases rapidly with temperature until it maximises and then gradually drops off. The deuterium-tritium fusion rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

In December, the JET fusion energy project broke the 24-year-old record of energy output achieved by nuclear fusion. This has laid the foundation for the eventual large-scale production of fusion energy and possibly a path towards a sustainable and clean energy future.

AT any point of time since around the 1950s, we have heard scientists telling us that our ability to tap nuclear fusion, the process that powers the burning and shining of the sun and other stars, as a source of limitless, clean and safe energy is at least another 50 years away. Maybe, not any more. With the latest developments in fusion research at the experimental nuclear fusion reactor called the Joint European Torus (JET) in the United Kingdom, the feasibility of realising that dream is now perhaps closer than ever before.

On December 21, 2021, JET, which is operating at the Culham Centre for Fusion Energy (CCFE) of the U.K. Atomic Energy Authority (UKAEA) in Oxfordshire, broke the 24-year-old record of energy output achieved by the fusion of nuclei of deuterium (D) and tritium (T), the two heavier isotopes of hydrogen (H), the most abundant element in the universe (Fig. 1). On February 9, 2022, the UKAEA and EUROfusion—the European Consortium for the Development of Fusion Energy, which runs this facility and comprises 30 research organisations from the 27 European Union member states and 3 from the U.K., Switzerland and Ukraine—made public announcements of this landmark achievement.

The production of nuclear electricity that is delivered to homes at present is based on the process of nuclear fission where heavy nuclei break up into lighter nuclei, yielding energy. Nuclear fusion is its opposite, that is, lighter elements (lighter than iron), such as isotopes of hydrogen and helium, fuse together at very high temperatures—at about 100-150 million degrees Celsius—to form heavier elements with the release of energy in the form of heat. Fusion reactions involving the lightest of nuclei, namely hydrogen and its isotopes, yield the highest amount of energy (Fig. 2).

The problem in harnessing energy from nuclear fusion has been in making fusion reactions happen in a controlled and sustained fashion so that the energy released can be continuously tapped. If this can be achieved in suitably designed future fusion power plants, the world will have in its hands a near-limitless source of energy without the attendant huge problem of nuclear waste that one is faced with in operating the fission-based nuclear reactors of today. More importantly, fusion reactors will not undergo meltdowns as happened at the Chernobyl or Fukushima plants.

In a tweet on February 9, the UKAEA said: “Record-breaking 59 megajoules [MJ] of sustained fusion energy at world-leading UKAEA’s JET facility.” It posted a video of the record-breaking five-second pulse inside JET’s doughnut-shaped internal vessel, called a “tokamak”, at https://twitter.com/i/status/1491381459181248515. This fusion energy output of 59 MJ is nearly 2.7 times the output of 21.7 MJ that JET achieved in 1997 in a pulse that lasted about four seconds (Fig. 3).

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 $22-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.”

(ITER, designed to demonstrate the scientific and technological feasibility of fusion power, will be the world’s largest experimental fusion facility (“A nuclear leap”, Frontline , January 27, 2006). The participating entities in this international joint venture are Europe (which is contributing almost half the cost of its construction), China, India, Japan, the Republic of Korea, the Russian Federation and the United States (contributing equally to the rest). The facility is under construction in Saint-Paul-lez-Durance, southern France, where the French Cadarache nuclear research facility is located, and is fast nearing completion.)

Safe & sustainable

Ian Chapman, Chief Executive of the UKAEA, said in a statement: “These landmark results, have taken us a huge step closer to conquering one of the biggest scientific and engineering challenges of them all.” The results are being seen as the clearest demonstration in 25 years of the potential fusion energy has to deliver safe and sustainable low-carbon energy that will combat the effects of climate change by decarbonising energy production. In fact, given the spectacular advances in fusion research in recent years, particularly at JET, the UKAEA hosted a series of events in Glasgow in November 2021 at the United Nations Framework Convention on Climate Change’s Conference of the Parties (COP26) to convey the emerging importance of nuclear fusion in meeting future global energy demand. In a message to COP26, Chapman said: “Fusion energy is low carbon, safe, efficient and the fuels exist in abundance. It’s a game changer for our global energy future. I’m in no doubt that fusion will be a complementary part of the energy mix for generations to come. It’s one of the biggest scientific and engineering quests in history … and the rewards for success will be huge for our planet.”

Reproducing on the earth the conditions that one obtains in a star like the sun is not easy. Fusion reactions occur when two or more atomic nuclei come close enough so that the strong nuclear attractive force between them exceeds the repulsive electrostatic force pushing them apart and results in their fusion to form heavier nuclei. For this to happen, the fuel atoms need to be given sufficient energy to overcome the electrostatic barrier, and this is achieved by heating them to very high temperatures. In fact, the temperatures required on the earth are 5-10 times the temperature at the centre of the sun. This is because of the much higher density of particles in stellar bodies more massive than the earth, such as the sun, because of their stronger gravitational pull, and this aids in bringing the fusing atoms together.

At the high temperatures of 100-150 million °C required for fusion to occur, the atoms are stripped of their electrons, resulting in a hot gas of floating positively charged nuclei and negatively charged free electrons in a state known as plasma. Unlike a normal gas, plasma, being charged, is electrically conductive, and its motion can be controlled by externally applying very strong magnetic fields so that the hot bubbles of charged gas are confined within a designated volume.

One of the common ways of containing and confining plasma is by using a doughnut-shaped, or torus, device called a tokamak where magnetic field coils confine plasma particles to attain the conditions necessary for fusion (Fig. 4). The plasma particles float around mostly in the centre of the torus where fusion reactions between the nuclei occur in large numbers. Tokamaks have, in fact, emerged as the leading plasma confinement concept for future fusion power plants. The reactors of both JET and ITER are basically doughnut-shaped tokamaks.

At the core of JET is the toroidal vacuum vessel where the fusion plasma is confined by means of strong magnetic fields (of about four tesla) and plasma currents (of about five million amperes). In JET’s present configuration, the major and minor radii of the plasma torus are 3 metres and 0.9 m respectively, and the total plasma volume is 90 cubic metres. The plasma is heated using 4 MW power radio waves and by injecting a fast beam of hydrogen atoms. A divertor at the bottom of the vacuum vessel allows escaping heat and gas to be exhausted in a controlled way.

In operation since 1983, JET was originally designed to study plasma behaviour in conditions and dimensions similar to a typical large-scale fusion reactor. However, with the emergence of the idea of ITER as an international facility, JET’s chief aim today, as mentioned before, is to prepare for the construction and operation of ITER by serving as a test bed for ITER technologies and plasma-operating scenarios. It is the only fusion reactor in the world that currently operates with a D-T fuel mix, the fuel that will be used in ITER as well. While at its start in 2025, ITER will operate with low-power H reactions alone, from 2035 onwards it is expected to run on an even mix of D and T. ITER’s ultimate goal is to be able to achieve a Q-value of 10, producing 500 MW from 50 MW of injected thermal power to ignite the D-T plasma.

Hydrogen comes in three forms or isotopes: the natural form (H, the simplest) has a single proton as its nucleus; the heavier D has one proton and one neutron; and T, the heaviest, has a proton and two neutrons. In earlier fusion experiments, including those at JET, only atoms of H and D were used to gain insight into practical ways of testing of plasma heating and its confinement and related control systems, but they hardly produced any fusion reactions. This is because, among the different compositions of plasma using hydrogen isotopes, the probability for a fusion reaction to occur (the cross section, as it is called) is the highest in plasma that uses a D-T combination as the fuel (Fig. 5). In fact, the cross section peaks at lower energies (equivalently, lower temperatures) compared with other combinations. This is why a D-T fuel mix is the fuel of choice for a fusion reactor.

Fortunately, deuterium is commonly available. About 1 in 5,000 hydrogen atoms in seawater is deuterium. When fusion power becomes a reality, just one litre of seawater could produce as much energy as about 80 L of petrol. But to have a D-T mix as the fuel poses a challenge because tritium is rare, radioactive and unstable, decaying with a half-life of 12.3 years. In nature, therefore, tritium occurs only in trace amounts. It is, however, artificially produced as a by-product in nuclear fission reactors where the more abundant element lithium is exposed to energetic neutrons. The current world’s supply, however, is just 20 kilograms. Thus, a D-T fusion reactor would also require an enriched lithium-based tritium production facility either as a part of the reactor itself (using a blanket of lithium on the inner wall of the torus) or as a separate one alongside it.

Fusion experiments at JET in the 1990s were already using D-T as the fuel. In fact, they were called the DTE1 experiments and they provided a great deal of experimental knowledge in handling tritium in tokamak reactors. The 1997 record was the culmination of a series of DTE1 experiments. The aim then was to maximise peak power. Although JET succeeded in achieving a record Q-value of 0.67, the peak lasted only for a fraction of a second.

The present JET results of the DTE2 campaign do not overturn that peak value. The Q-value attained this time around was only 0.33, but the average sustained power was more than double the 1997 level, and that too over a five-second pulse. Indeed, the aim of DTE2, which began in June 2021 with the injection of D-T fuel into the tokamak and ended with the December’s record-breaking result, was not to hit peak power but to achieve a sustained higher power over a longer period of about five seconds and to understand the behaviour of a long-lasting tritium plasma and its effect on the system hardware, which will be crucial in operating ITER.

In August last, the National Ignition Facility of the U.S. Department of Energy, which uses laser beams to ignite and control a D-T plasma, achieved its highest power output ever of 1.35 MJ, which was 70 per cent of the laser energy pumped in to ignite the fusion process; that is, a Q-value of 0.7, higher than JET’s 1997 record. But the output energy pulse event was extremely short-lived, lasting just less than four billionths of a second. This really puts JET’s achievement in perspective and is an indicator of progress in fusion research in the near future, especially at ITER.

A breakthrough

According to Fernanda Rimini, a plasma researcher at the CCFE, who was quoted in Nature, JET loses heat more easily than ITER, and achieving break-even was never the aim of the DTE2 run. “If engineers applied the same conditions and physics approach to ITER as to JET,” she told Nature, “it would probably reach its goal of a Q of 10, producing ten times the energy put in.” Thus, scientists regard a five-second pulse of a sustained D-T fusion with good power output as a breakthrough. Success was the result of 20 years of experimental optimisation and hardware upgrades, particularly a change in the materials used, for JET to become more ITER-like in order to conduct dedicated studies for ITER.

By far the most significant upgrade to JET was to make the reactor wall with the metals beryllium (Be) and tungsten (W) for it to be an ITER-like wall (ILW). During the DTE1 campaign in 1997, JET had an inner wall made of solid carbon. Although carbon is heat-resistant and does not melt, an H/D/T plasma in contact with a carbon surface will result in the formation of hydrocarbons and erode the wall. These hydrocarbons can get deposited as gunk in remote corners of the machine that will be hard to remove and the fuel too will end up being captured in hydrocarbons and deplete the total amount of fuel in the core, which will severely affect the performance of the reactor. Hence, the idea of replacing the carbon wall with a Be-W wall was to ensure much lower tritium retention in the walls. This also enables the wall to sustain larger temperatures and prevent erosion (Fig. 6). Together with upgraded heating power, this ILW enabled scientists to develop plasma scenarios that resembled as closely as possible those planned for ITER.

But running JET with ILW led to surprising discoveries, according to Guy Matthews, the ILW project leader at JET. “Even though it was only the tokamak wall that [had] changed, the physics of the plasma itself changed quite a lot. Nobody had expected that. For instance, it became harder to achieve high plasma temperatures, and JET physicists had to develop new ways to achieve high fusion performance,” he said.

D-T fusion reactions produce neutrons at a much greater rate than with D (or H) atoms alone. While greater neutron generation does mean increased energy output, since neutrons have no charge, they cannot be contained by the magnetic field. This means they would hit against the walls of the reactor, causing damage. Renovation of the machine, particularly in terms of material used inside the reactor, was thus necessary to prepare it for the much heavier neutron bombardment and much greater heat. Although the superheated plasma is mostly contained within a magnetic field, the surrounding reactor material will still be subject to huge temperatures. At ITER, the hottest part of the machine would reach around 1,300 °C.

So the other major difference from the 1997 experiments was refitting the inside of JET’s reactor with materials that matched ITER’s design so that the effects of heat and neutron bombardment and removal of impurities from the plasma could be studied. But these new materials can affect the plasma itself, and understanding how they interacted with the fusion process was also an essential part of the run-up to the DTE2 campaign.

In the build-up to the DTE2 campaign, two other important issues were studied: one, the impact of the so-called “isotope effect” and, two, how to increase the fusion energy output by continued heating of the plasma with the product of the D-T fusion reactions, namely, the 3.5 million electronvolt helium nuclei, or alpha particles (Fig. 1). This is the ignition concept where the plasma burning is sustained without external heating depending on the ability of the reactor to confine the alpha particles as well in the torus.

The isotope effect was studied in 2020 to answer the following question: what would be the effect of changing the average atomic mass of the plasma? The average atomic mass of a full hydrogen plasma would be 1, the atomic weight of hydrogen. Similarly, a full D plasma will have an average atomic mass of 2, and a full T plasma will have an average atomic mass of 3. Since, for a plasma in transport, the isotope composition can have a strong effect on various processes in the plasma such as the heat, and particle and momentum confinement, it was necessary to understand or predict how they would vary for a 50:50 D-T plasma, which will have an average atomic mass of 2.5. After campaigns involving full H plasma and full D plasma, in December 2020, JET scientists began their experiments with full T plasma.

“We did this T-T campaign for two reasons,” said Joëlle Mailloux, a UKAEA nuclear physicist in a release issued on February 14, “one, to prepare for the D-T campaign; and two, to understand the physics of tritium in the reactor better.” One of the main issues in fusion is core confinement: how well can you make magnets hold a plasma with temperatures of 150 million °C. Once you achieve that with an H plasma, how do you translate elements of the technique evolved to T, which is three times heavier than H?

With D-T plasma, the fusion product helium nuclei also need to be kept track of. According to theory, these should affect the magnetic confinement owing to electromagnetic effects. According to Joëlle Mailloux, even while the whole effort is to fuse D and T, in experiments, it is difficult to disentangle which effect is which. In pure tritium plasma, there will be some T-T fusions, but since it does not produce any alpha particles, it lets you do just that, she said.

“We had some unexpected results and adapted procedures and experiments, but no big physics surprises, which is very encouraging,” Joëlle Mailloux said. “And although a full analysis of the results will take months to years to complete, the first lessons of the TT-campaign are already incorporated in the DTE2-campaign that just concluded.” And indeed, as its results show, the campaign was concluded with remarkable success.

The T-T studies and the record-smashing D-T fusion results may be the final experiments at JET. EUROfusion decided last year to end JET’s operations by 2023 end, and the UKAEA has stated that it plans to decommission the experiment. JET began in 1983 and has ended its four-decade-long experimental campaign in fusion research with remarkable results. These have laid the foundation for the large-scale production of fusion energy using the next-generation fusion reactors, and with that a path towards a sustainable and clean energy future may have emerged.

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