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Fuel for world’s largest fusion reactor ITER is set for test run

A section of pink plasma superimposed on JET vessel equipped with the ITER-Like wall

The Joint European Torus has started conducting experiments with tritium fuel.Credit: EUROfusion (CC BY 4.0)

A pioneering reactor in Britain is gearing up to start pivotal tests of a fuel mix that will eventually power ITER — the world’s biggest nuclear-fusion experiment. Nuclear fusion is the phenomenon that powers the Sun and, if physicists can harness it on Earth, it would be a source of almost limitless energy.

In December, researchers at the Joint European Torus (JET) started conducting fusion experiments with tritium — a rare and radioactive isotope of hydrogen. The facility is a one-tenth-volume mock-up of the US$22-billion ITER project and has the same doughnut-shaped ‘tokamak’ design — the world’s most developed approach to fusion energy. It is the first time since 1997 that researchers have done experiments in a tokamak with any significant amount of tritium.

In June, JET will begin fusing even quantities of tritium and deuterium, another isotope of hydrogen. It is this fuel mix that ITER will use in its attempt to create more power from a fusion reaction than is put in — something that has never before been demonstrated. The reactor should heat and confine a plasma of deuterium and tritium such that the fusion of the isotopes into helium produces enough heat to sustain further fusion reactions.

“It’s very exciting now to, at last, get to the point where we can put into practice what we’ve been preparing all these years,” says Joelle Mailloux, who co-leads the scientific programme at JET. “We’re ready for it.”

Trial run

JET’s experiments will help scientists to predict how the plasma in the ITER tokamak will behave and to craft the mega-experiment’s operating settings. “It’s the closest we can get to achieving ITER conditions in present-day machines,” says Tim Luce, chief scientist at ITER, near Cadarache in France. The experiments are the culmination of around two decade’s work, says Luce. ITER will begin operations with low-power hydrogen reactions in 2025. But from 2035, it will run on a 50:50 mix of deuterium and tritium.

Both ITER and JET, which is based at the Culham Centre for Fusion Energy (CCFE) near Oxford, use extreme magnetic fields to confine plasma into a ring and heat it until fusion occurs. The temperatures in JET can reach 100 million degrees, many times hotter than the Sun’s core.

The world’s last tokamak fusion experiments with tritium also took place at JET. The goal then was to hit peak power, and the facility succeeded in achieving a record ratio of power out to power in (known as a Q value) of 0.67. That record still stands today; 1 would be break-even. But this year, the aim is to sustain a similar level of fusion power for 5 seconds or more, to eke out as much data from the experiments as possible and to understand the behaviour of longer-lasting plasmas.

Working with tritium poses unique challenges — JET researchers have spent more than two years refitting elements of their machine and preparing to handle the radioactive material. The isotope decays quickly, so it occurs only in trace amounts in nature and is usually made as a by-product in nuclear-fission reactors; the world’s supply is just 20 kilograms.

Part of the challenge of handling tritium is that its reactions with deuterium produce neutrons at a much higher rate than deuterium reactions alone. Commercial reactors will capture the energy of these neutrons to generate electricity, but in JET, the high-energy particles will pepper the machine’s interior and damage diagnostic systems. That means that the JET team has had to move cameras and other instruments behind concrete shielding, says Ian Chapman, who leads the CCFE.

“We’ve had to refresh and renew all of our processes”, from storage to handling, Chapman says. Once tritium experiments start, neutron bombardment will make the inner facility radioactive, so it will become a no-go zone for humans for 18 months. Staff have therefore had to get used to a mindset similar to that of the engineers who send craft into space: “You can’t just go in and fix things, it has to work first time,” Chapman says.

Tritium pulses

JET’s campaign will use less than 60 grams of tritium, which it will recycle. Fuel containing a fraction of a gram of tritium will be pulsed into the tokamak 3–14 times a day. Each of these discharges will be an individual experiment with slightly different parameters, and will generate between 3 and 10 seconds of useful data, says Mailloux. “What we are after is physics information that we can use to validate our understanding, and then we can apply that to preparing the future machine,” she says.

Some experiments will use just tritium; others will combine deuterium and tritium in equal proportions. Both types of experiment are important, because a key goal is to understand the effect of tritium’s larger mass on plasma behaviour (tritium has two neutrons in its nucleus, whereas deuterium has one and hydrogen has none). That will help in predicting the impact of using different isotopes in ITER. The mass of the isotopes influences the conditions — such as magnetic field, current and external heating — needed for the plasma to reach a crucial state known as confinement. (In this state, the highest-energy particles remain within the ionized gas, and that is important for sustaining the plasma’s temperature.) “We want to explore this and understand why,” says Anne White, a plasma physicist at the Massachusetts Institute of Technology in Cambridge.

Another major difference from the 1997 experiments is that JET has been refitted so that the inner materials that protect the machine against the effects of heat and neutron bombardment, and remove impurities from the plasma, match those in ITER’s design. Because these materials could radiate back into the plasma and cool it down, understanding how they interact with the fusion process is crucial.

The latest generation of fusion scientists has never worked with tritium, which makes it all the more important to do the experiments, says Chapman. “It’s a big deal. People are watching,” adds Luce.

Nature 591, 15-16 (2021)

doi: https://doi.org/10.1038/d41586-021-00408-1

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