UK’s JET nuclear fusion reactor sets new world record for energy output

Inside the JET fusion reactor


The UK’s 40-year-old fusion reactor achieved a world record for 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 2021, 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 equates to a power output of 12.5 megawatts – enough to power 12,000 homes, said Mikhail Maslov of the UK Atomic Energy Authority at a press conference on 8 February.

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.

Tim Luce, deputy head of the ITER construction project, told the press conference that ITER will scale up the energy output to 500 megawatts, or possibly even 700.

“These are what I usually call power plant scale,” he said. “They’re at the lower end of what you would need for an electricity generating facility. In addition, we need to extend the timescale to at least 300 seconds for the high fusion power and gain but perhaps as long as an hour in terms of energy production. So what JET has done is exactly a scale model of what we have to do in the ITER project.”

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 the 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.



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