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article imageLoops of liquid lithium used to clean tritium in fusion reactions

By Karen Graham     Oct 17, 2017 in Science
Researchers have proposed a unique design that could improve the ability of future fusion power plants to generate safe, clean and abundant energy in a steady state or constant manner.
Steady-state fusion power plant designs present some major divertor technology challenges, including high divertor heat flux both in steady-state and during transients, as well as the technical issues associated with long-term dust accumulation, and the recycling of tritium and its safety issues.
Researchers led by the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) in collaboration with Oak Ridge National Laboratory, the University of Illinois at Urbana-Champaign and the National Institute for Fusion Science in Japan have proposed an innovative design that would allow for a steady state of operation in nuclear fusion power plants while addressing these major challenges.
Physicist Masa Ono with images reported in Nuclear Fusion paper.
Photo by Elle Starkman/Office of Co...
Physicist Masa Ono with images reported in Nuclear Fusion paper. Photo by Elle Starkman/Office of Communications
Princeton Plasma Physics Laboratory
Putting a star in a jar
The unusual design uses loops of liquid lithium to clean and recycle the tritium, a radioactive hydrogen isotope that fuels fusion reactions. The loops of liquid lithium also protect the divertor plates from the 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. “We wanted to see how we can protect the divertor plates and keep the fusion chamber clean.”
As we have learned, the sun and stars are powered by fusion, the merger of light elements to release energy. However, here on Earth, fusion power plants are used to combine tritium with its sister isotope deuterium to create the energy for generating electricity. Producing this kind of power is sometimes considered to be like “putting a star in a jar.”
JET  the Joint European Torus  is the world s largest operational magnetic confinement plasma physic...
JET, the Joint European Torus, is the world's largest operational magnetic confinement plasma physics experiment, located at Culham Centre for Fusion Energy in Oxfordshire, UK.
PPPL scientists' design calls for pumping liquid lithium, a silvery metal, into and out of the tokamak, a device that uses a powerful magnetic field to contain the hot plasma needed for producing controlled thermonuclear fusion power. The process cleans out the dust and other impurities from the plasma, thereby safeguarding the divertor.
Lithium has a number of functions
The lithium covers the divertor plates: Injection of the lithium into the tokamak divertor chamber plates will cover the plates with a film of lithium, protecting the plates from the heat and particles rising up from the core of the plasma. The lithium would also act like a sponge, capturing particles before they have a chance to bounce back into the plasma, cooling it down and reducing fusion performance.
“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.”
Outside view of the NSTX reactor at PPPL.
Outside view of the NSTX reactor at PPPL.
Princeton Plasma Physics Laboratory
Recycling tritium: While tritium is a key fuel that will fuse with deuterium to produce the fusion reaction, only about one percent of the tritium is expected to be consumed, meaning the unconsumed tritium must be removed and recycled to maintain the fusion reaction.
"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 crystallize out," say the researchers.
The system would then drain the tritium out reheat it and regenerate the tritium where it would them be moved to a separator that would get rid of the impurities, pumping the tritium back into the tokamak. The researchers also say that as an alternative, the loop could feed into a centrifuge that separated the tritium from the lithium and returned the isotope to the tokamak.
Removing dust: If left unchecked, tons of dust could build up inside the reaction chamber from interactions between the plasma and the fusion chamber walls. Using the same loop that recycles tritium, the loop would also deliver dust particles to a dust filter. “After the dust filter is filled, it must be replaced,” Ono said. “Since the filter would be relatively close to the fusion chamber, it must be replaced remotely.”
View of plasma after injection of a frozen deuterium pellet inside the tokamak fusion test reactor a...
View of plasma after injection of a frozen deuterium pellet inside the tokamak fusion test reactor at PPPL.
Princeton Plasma Physics Laboratory
Eliminating unwanted elements: When the plasma makes contact with the walls of the tokamak, impurities, like nitrogen and oxygen are formed that could cool the plasma. Again, the loop carrying liquid lithium would carry these impurities to the tritium separator.
“Since these impurities are expected to be relatively low level,” Ono said, “they could be handled after separation through specialized smaller cleaning loops attached to the main one.”
Laboratories around the globe are addressing the challenges presented by fusion power, and testing 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.”
This very interesting research was published in the journal Nuclear Fusion.
More about fusion reactions, liquid lithium, Tritium, tokamak, dust accumulation
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