Europe's JET Set Ponders Future of World's Largest Fusion Reactor

The Joint European Torus (JET), the world's largest fusion reactor, celebrated its 30th anniversary last week. But the birthday party was marked by growing debate over the future of the landmark machine, which uses powerful magnetic fields to contain a plasma of hydrogen isotopes so that they will fuse, releasing energy—a similar reaction to the one which powers the sun.

Initially designed in the 1970s, the doughnut-shaped tokamak reactor in Culham, United Kingdom, created its first plasma in June 1983. And at an anniversary colloquium on 25 June, researchers recalled JET's many triumphs, in particular its 1997 world record of the greatest power output achieved by any fusion reactor: 16.1 megawatts. But another hot topic was how long JET's career could continue, and whether it should be closed down in 2018 to concentrate effort and funding on its successor, the more powerful ITER fusion reactor now under construction in France.

JET has had a long and productive working life because its designers built in plenty of margin. In the 1990s, the interior of the tokamak was refitted with a structure known as a divertor to remove exhaust plasma and excess energy and allow for longer fusion pulses. In 2004, researchers and funders decided to keep JET running so that it could carry out preparatory experiments for ITER. Among all the world's tokamaks, JET is the most similar to ITER in terms of size, shape, and plasma conditions. It is also the only tokamak currently equipped to operate with a full fusion fuel made of an equal mixture of the hydrogen isotopes deuterium and tritium. Tritium is radioactive, so requires specialist handling, and burning D-T fuel makes the reactor itself radioactive, so subsequent work inside the vessel is more complicated.

Between 2009 and 2011, JET was shut down to upgrade the heating systems and refit the interior with a new lining of the type that will be used in ITER—a mixture of beryllium and tungsten. Researchers have since learned important lessons about the possible performance of ITER using this "ITER-like wall." Over the next few years, the plan is to keep pushing JET's performance to see what it can do with the new wall, culminating in a series of D-T burning experiments in 2017.

After that, Euratom (European Atomic Energy Community), the European Union's nuclear directorate, is considering closing JET. In part, that's because future JET repairs and modifications will be slower and more expensive as a result of radioactivity in the vessel. Euratom is no doubt also thinking about the cost of keeping a large facility like JET running while the €16 billion ITER is at the peak of its construction expenditure. Once JET closes, fusion researchers should focus their attention on ITER, JET associate leader Francesco Romanelli said last week.

Many at the meeting expressed concern about that plan, however. If ITER's planned November 2020 startup should be delayed, they argued, there will be an awkward gap which may cause some researchers to find employment elsewhere. Some also thought it would be valuable for the two reactors to overlap, so that new approaches could be tried out on a smaller scale on JET without disrupting the ITER schedule.

Euratom is investigating the possibility of recasting JET as an international facility after 2018, asking the other six ITER partners—China, India, Japan, Russia, South Korea, and the United States—to contribute to the cost of keeping it running. But with ITER already expected to cost several times the original estimate, the partners may not be keen to shoulder the extra burden.

Daniel Clery

Daniel is Science’s senior correspondent in the United Kingdom, covering astronomy, physics, and energy stories as well as European policy.