Researchers led by the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) have proposed an innovative design to improve the ability of future fusion power plants to generate safe, clean and abundant energy in a steady state, or constant, manner. The design uses loops of liquid lithium to clean and recycle the tritium, the radioactive hydrogen isotope that fuels fusion reactions, and to protect the divertor plates from intense exhaust heat from the tokamak that contains the reactions.
"There are many challenges to developing fusion energy and the handling of heat on divertor plates is among them," said PPPL physicist Masa Ono, lead author of a paper about the design published in the journal Nuclear Fusion . "We wanted to see how we can protect the divertor plates and keep the fusion chamber clean."
Fusion, the merger of light elements to release energy, is the process that powers the sun and stars. Here on Earth, fusion power plants will combine tritium with its sister isotope deuterium to create the energy for generating electricity. Producing this power in a fusion device is sometimes called "putting a star in a jar."
The system that Ono and colleagues designed calls for pumping liquid lithium in and out of a tokamak, a type of magnetic fusion device, to maintain steady state operation while cleaning out dust and other impurities from the plasma and safeguarding the divertor. The lithium, a silvery metal that readily combines with other elements, would serve a number of functions:
"Even a thin layer of liquid lithium can protect the plates," said Ono. "It also has a promise of improving plasma performance as observed in the National Spherical Torus Experiment and Lithium Torus Experiment at PPPL and in other fusion experiments, and reduces the heat flux. And since liquid lithium evaporates, we must continually provide more to keep the plates moist."
To accomplish this task, the liquid lithium would combine with tritium in the tokamak and carry it with dust and other impurities to a filter outside the tokamak where the dust would be removed. The next stop would be a cold trap operating at 200 degrees Celsius that would allow the tritium to crystalize out. After draining lithium from the trap, the system would reheat and regenerate the tritium and bring it to a separator that would discard the impurities and pump the tritium back into the tokamak. Alternatively, the loop could feed into a centrifuge that separated the tritium from the lithium and returned the isotope to the tokamak.
Addressing such ideas are PPPL and groups around the world testing flowing liquid lithium concepts. "We are looking to the future to come up with solutions," said Ono. "These issues must be dealt with if we are to realize practical and attractive fusion power plants."
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Collaborating with Ono on this work were physicists at PPPL, Oak Ridge National Laboratory, the University of Illinois at Urbana-Champaign and the National Institute for Fusion Science in Japan. Support for the U.S. research comes from the DOE Office of Science.
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas -- ultra-hot, charged gases -- and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov .
Nuclear Fusion