Streaming Liquid Metal Could Clean Ash in Future Fusion Reactors

Replacing a solid metal plate with a stream of liquid metal could help remove “ash” from future fusion reactors, according to recent research in Argonne’s Fusion Power Program. Fusion, the source of the sun’s energy, involves combining two light atoms into a single heavier atom. Argonne is part of a national team studying fusion as a potential source of commercial electricity for later this century. Fusion reactors offer a clean, potentially limitless source of electricity.

A magnetic fusion reactor would create a plasma—a hot gas of charged deuterium and tritium ions (particles), which are two forms of heavy hydrogen. Within the plasma, colliding deuterium and tritium nuclei would fuse into helium nuclei and release energy to be converted into electricity.

The plasma particles must be heated to very high temperatures, approximately 100 million degrees, to fuse. Since the plasma is too hot to be contained by any material, the charged plasma particles are confined on magnetic field lines produced by powerful magnets. The plasma in a 1,000 megawatt fusion plant would contain only about a gram of hydrogen fuel at any given time. Too many impurities, including the helium “ash” or waste product, can “quench” the plasma and shut down the reaction. Argonne leads a project to solve the ash-removal problem, which is one of the challenges in developing fusion reactors.

Magnetic field lines near the edge of the plasma guide a small fraction of the plasma particles, including the helium ash, to a divertor region where they are removed by vacuum pumps. The diverted plasma particles, including the helium, strike metal divertor plates as they are pumped out of the plasma chamber.

The plasma particles entering the divertor deposit high heat loads on the divertor plates and cause erosion, which can contaminate the plasma and limit the plates’ lifetime. The Argonne researchers propose to replace the solid divertor plates with a flowing liquid metal to eliminate the erosion problem and to accommodate the high heat loads.

“The idea is to use a kind of showerhead to produce liquid jets that form a liquid plate,” said engineer Rich Mattas, who heads Argonne’s effort. “The liquid would be continuously retrieved, reprocessed and recycled. Argonne computer models indicate that the plasma can tolerate the presence of certain liquid metal divertor materials.”

Leading candidates for the liquid metal divertor are lithium and a mixture of tin and lithium. Gallium is another candidate, Mattas says.

This work was done as part of the U.S. Department of Energy’s National Advanced Limiter/Divertor Plasma Facing Program.

For more information please contact Dave Baurac at 630-252-5584

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