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 ‘tokomak’ 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.

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