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Fusion wall: The inside of the JET vessel after the installation of new tiles. The reactor will provide a crucial test for a full-scale fusion experiment.

A team of researchers has restarted the world’s largest fusion experiment—the Joint European Torus (JET) reactor, near Oxford, U.K. The move is a step forward in the quest for practical nuclear fusion.

The project was put on hold while a new lining was installed. This lining mimics the planned configuration of the International Thermonuclear Experimental Reactor (ITER), a full-scale experimental fusion reactor now under construction in southern France. As a consequence, the new undertaking at JET is being called the ITER-Like Wall Project.

The JET team says the lining, made of tiles of the light metal beryllium, should be better able to withstand the extreme conditions needed for a self-sustaining fusion reaction than the carbon-fiber composite tiles used before. The lining will also allow for laser-driven fusion experiments, similar to those underway at the National Ignition Facility in California.

JET is a tokamak—a device for carrying out magnetic confinement fusion. Its doughnut-shaped reactor contains plasma made from hydrogen that’s squeezed by powerful magnetic fields. Eventually, magnetic pressure and heat force the hydrogen nuclei to fuse into helium, releasing a burst of energy and freeing high-energy neutrons.

JET is the only tokamak in the world equipped to use tritium, the radioactive form of hydrogen containing two neutrons in its nucleus, as well as the single-neutron form, deuterium. Forcing these two forms of hydrogen to fuse produces large yields of energy. The ITER tokamak will also use this form of fusion once it’s complete.

Guy Matthews, director of the ITER-Like Wall Project, explains that ITER would not be able to operate with a carbon lining. “Electrons in the material tend to dilute the plasma—each one will displace a hydrogen nucleus,” he says. “This is why carbon was chosen; it has a low atomic number, and carbon fiber can withstand high temperatures.”

The problem comes with introducing tritium to the plasma. “If you have it in the wall, it will tend to form hydrocarbon compounds with the hydrogen in the reactor, and when the form of hydrogen is tritium, the hydrocarbons will be radioactive,” says Matthews. “That’s a radiological issue, a safety issue, and an economic issue, because there isn’t much tritium available, and you don’t want it trapped.”

For JET, this isn’t an overwhelming problem; the doughnut-shaped “torus” is relatively small—about two meters across—and only runs fusion pulses for a few tens of seconds. But ITER’s torus will be almost 10 times that size and run 10-minute pulses; and a commercial fusion reactor would have to run continuously.

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Credit: European Fusion Development Agreement

Tagged: Energy, energy, fusion reactor

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