• A smaller sibling of the ITER fusion reactor is testing heavy tritium fuel.
  • It’s been 24 years since scientists set the existing fusion record using tritium.
  • Tritium has two neutrons compared with deuterium’s single neutron.

    Nuclear scientists will soon test the special fuel mix that will eventually power the gigantic International Thermonuclear Experimental Reactor (ITER). The U.K.’s Joint European Torus (JET) reactor is holding a “dress rehearsal” for ITER by testing the fuel first. If the dry run goes well, that’s very good news for the future of fusion.

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    ITER is a huge, internationally enmeshed nuclear fusion project. After spending billions of dollars over decades, the reactor is scheduled to switch on for its first productive (“ignited”) fusion in 2035. Until then, teams will work around the clock on construction and testing. That means assembling the gigantic, donut-shaped tokamak reactor, where nuclear fuel will be turned into sun-hot plasma and circulated in a magnetic field.

    The magic fuel mix combines tritium and deuterium. There are just 20 kilograms of tritium, a fragile isotope of hydrogen with an extremely short half life, in the entire world. Deuterium, meanwhile, is stable and the most abundant natural isotope of hydrogen. ITER plans to use a 50/50 mix of both fuels beginning in 2035, which means the reactor must be able to withstand a special beating from swirling tritium neutrons that will bombard the exterior of the fusion chamber.

    Scientists at JET, then, are beginning to remotely run experiments with different tiny quantities of tritium and different numbers of plasma pulses, all with the goal to find the exact right mix and numbers for ITER to eventually use.

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    While deuterium is stable, it’s far lower-powered than tritium, which is much heavier and more massive. Imagine shaking a coffee can with 50 pennies inside versus 50 quarters: the shaking is the same, but how the can feels, how it sounds, and how much effort you must use to shake it are all different because of mass.

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    The goal is for the JET reactor to “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,” Nature’s Elizabeth Gibney explains:

    “[I]n JET, 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.”

    In a fully realized commercial fusion reactor, these particles are what will make the electrical energy, but JET is a test reactor that seeks scientific data, not electrical power. Tokamaks burn so fast and hot that it’s very easy for them to damage interior equipment and stop running. The magnetic field can easily destabilize and bring down the rest of the ongoing reaction. Tritium is powerful enough for the whirling neutrons to make JET’s entire facility radioactive and unsafe for scientists.

    JET is a scale replica of ITER in important ways that make it a great proving ground. In fact, the world’s existing fusion reaction record, a 0.67:1 ratio of fusion output to input, was set at JET using tritium in 1997. (For a fusion reactor to make power, it must reach a ratio of 1.01:1, for example, or higher.) Scientists say tritium in the mix makes a tokamak more likely to reach this goal, but it’s been 24 years since anyone lit up a tokamak using tritium.

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