UK’s JET nuclear fusion reactor smashes its own records in final tests

The UK’s 40-year-old fusion reactor smashed its own record for both reaction duration and energy output in its final runs before being shut down for good, scientists have announced.

The Joint European Torus (JET) in Oxfordshire began operating in 1983. When running, it was temporarily the hottest point in the solar system, reaching 150 million°C.

The reactor’s previous record was a reaction lasting for 5 seconds in 2o21, producing 59 megajoules of heat energy. But in its final tests in late 2023, it surpassed this by sustaining a reaction for 5.2 seconds while also reaching 69 megajoules of output, using just 0.2 milligrams of fuel. This amount of energy is roughly equivalent to the electricity consumed by an average person in the UK in a single day.

Today’s nuclear power plants rely on fission reactions, where atoms are smashed apart to release energy and smaller particles. Fusion works in reverse, squeezing smaller particles together into larger atoms.

Fusion can create more energy with none of the resulting radioactive waste created by fission, but we don’t yet have a practical way to harness this process in a power plant.

JET forged together atoms of deuterium and tritium – two stable isotopes of hydrogen – in plasma to create helium, while also releasing a vast amount of energy. This is the same reaction that powers our sun. It was a type of fusion reactor known as a tokamak, which contains plasma in a donut shape using rings of electromagnets.

Scientists ran the last experiments with deuterium-tritium fuel at JET in October last year and other experiments continued until December. But the machine has now been shut down for good and it is being decommissioned over the next 16 years.

Juan Matthews at the University of Manchester, UK, says JET will reveal many secrets as it is dismantled, such as how the lining of the reactor deteriorated through contact with plasma and where valuable tritium – worth around £30,000 a gram – has embedded in the machinery and can be recovered. This will be vital information for future research and commercial reactors.

“It’s great that it’s gone out with a little flourish,” says Matthews. “It’s got a noble history. It’s served its time and they’re going to squeeze a bit more information out of it during its decommissioning period as well. So it’s not something to be sad about; it’s something to be celebrated.”

A larger and more modern replacement for JET, the International Thermonuclear Experimental Reactor (ITER) in France, is nearing completion and its first experiments are due to start in 2025. Another reactor using the same design, the Korea Superconducting Tokamak Advanced Research (KSTAR) device, recently managed to sustain a reaction for 30 seconds at temperatures in excess of 100 million°C.

There are other approaches to creating a working fusion reactor being pursued around the world as well, such as the National Ignition Facility at Lawrence Livermore National Laboratory in California. This bombards capsules of fuel with immensely powerful lasers, a process called inertial confinement fusion, and has managed to unleash almost twice the energy that was put into it.