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White Paper for FESAC June 20, 2007 David N. Ruzic The Case for Liquid Lithium This white paper is in response to the question raised by FESAC, “What research is needed for DEMO that will not be learned from ITER?” DEMO is supposed to show the world that fusion power is not only possible, but could be practical. A power plant has to be economic, long-lived and safe. It is possible that flowing-molten- lithium plasma-facing components will allow all three of those requirements to be accomplished simultaneously. Economics: Costs for a given class of structures scales with volume. Only a small fraction of the plasma volume in ITER is planned to be hot and dense enough to achieve thermonuclear fusion. Most of the plasma volume is used to create the profiles which will ensure high confinement times. That space is also needed to ensure that the cold neutral atoms and molecules coming from the walls and divertor do not cool the core plasma. It takes space to maintain the temperature and density profiles which are cold at the wall and very hot in the center. If a wall and divertor could be found that had zero recycling, the temperature profiles could be flat. Cold particles (or any particles) would not return from the wall. Virtually the whole plasma volume would be a thermonuclear core. Hot temperatures would extend all the way to the walls. Therefore to produce the same power, the volume of the plasma, the volume of the magnetic field coils, the volume of a cryostat, the volume of the structural materials, etc. would all be greatly reduced – so would the cost. Molten lithium is such a zero-recycling surface for hydrogenic species. Small amounts of Li have improved energy confinement in TFTR, CDX-U[3,4] and NSTX due to recycling control. Lifetime: The planned first wall and divertor components have a limited lifetime in terms of number of shots, and particularly number of continuous hours of operation. This lifetime is severely reduced if there are disruptions, or significant ELMS. The baseline operating scenario for ITER even calls for a plasma with ELMS. Clearly ELMy plasmas can not be the baseline for DEMO. Even with ELM-free operation and robust disruption mitigation, sputtering, neutron damage, and thermal effects from normal operation will take their toll and limit DEMO’s lifetime. If an off-normal event does occur, the wall could be ruined. All solid surfaces suffer the same difficulty. A flowing molten system on the other hand is continuously replaced. Off-normal events may disrupt operation, but any material loss from the walls is repaired as a mater of course. The ability to handle high heat fluxes, especially in a compact design, is another great advantage of a flowing system. Surface temperatures are constant due to continuous replacement. Heat extraction can be done remotely in the liquid reservoir. While the temperature of liquid lithium will probably be constrained to 400C or so, thermal efficiency of the reactor needn’t suffer. Since 80% of the fusion energy is extracted from the blanket modules which could be run with gas cooling at very high temperatures, a Brayton cycle could be used. Then, the lower temperature lithium loop could be used as a pre-heat, or reheat source, increasing efficiency. Safety: Tritium inventory is a significant worry in a solid-wall device. Even if tungsten is used, displacements from neutron bombardment create traps, and those traps lock tritium away forever. Surface inventory could be cleaned, but periodic cleaning of the walls will cut into the duty cycle and utility of the power plant. Flowing lithium surfaces absorb even more tritium, but that tritium can be chemically removed as part of the tritium plant. It is even possible that the same extraction method and machinery used to remove tritium from the breeding blankets could be used to cleanse the plasma-facing components thereby reducing costs. Lithium is much less reactive with water or water vapor than sodium, and molten sodium has been used safely in nuclear facilities for years (Phenix and Super Phenix in France for instance). In terms of plasma performance, the low-Z nature of lithium allows a higher core contamination of it than with any other potential PFC material. Lithium is less-toxic than beryllium and widely available. There is a great deal of research which needs to be done on liquid lithium plasma- facing-components before the vision presented in this white paper become reality. However, that is exactly why it is being written! Can a flowing surface be designed to overcome J x B forces from eddy currents which may propel it off the wall? MHD modeling of free surface conducting liquids must be improved to find out. Can the surface be maintained below 400 C, or evaporation contained in some other way? Will temperature-enhanced sputtering limit its lifetime and utility?[7,8] Does it indeed retain sufficient helium to allow sufficient helium exhaust? Is the power handling capability high enough? Will temperature-gradient driven surface forces help or hinder such a system? What happens if ELMS still exist and strike the surface? Are there other plasma instabilities that will appear in flat temperature and density profile plasmas which may impact the walls? Will the flat profiles materialize in the first place? If ones wants DEMO to be a model of a low-cost, long-lived, safe fusion power plant, the option for it to use molten flowing lithium should not be ignored. However, for DEMO to be a model of anything other than ITER, much work needs to be done in the intervening years. For DEMO to contain molten flowing lithium, even more work is needed. Research into molten flowing systems is and has been done in the US program. However, it is being diminished since it is not considered “ITER relevant”. Lithium research is one area where the US is still considered a leader, but only continued support into lithium related research will keep us in such a position. I applaud FESAC for entertaining such notions. Nothing about flowing molten walls or the impact of lithium on plasma performance will be learned on ITER. References:  L. Zakharov et. al., “Ignited Spherical Tokamaks and Plasma Regimes with Li Walls”, Fusion Eng. and Design”, 72 (2004) 149.  D.N. Ruzic, M.C. Allain, and R.V. Budny, “The Effect of Lithium Wall Conditioning in TFTR on Plasma-Surface Interactions,” J. Nucl. Mater., 266-269 (1999) 1303.  R. Majeski, R. Doerner, T. Gray, R. Kaita,1 R. Maingi, D. Mansfield, J. Spaleta, V. Soukhanovskii,J. Timberlake,and L. Zakharov, “Enhanced Energy Confinement and Performance in a Low-Recycling Tokamak”, PRL 97 (2006) 075002.  R. Kaita, R. Majeski, T. Gray, H. Kugel, D. Mansfield, J. Spaleta, J. Timberlake, L. Zakharov, R. Doerner and T. Lynch, R. Maingi, V. Soukhanovskii, “Low recycling and high power density handling physics in the Current Drive Experiment-Upgrade with lithium plasma-facing components”, Phys. Plasmas 14 (2007) 056111.  H. Kugel and R. Majeski, Personal communications and ANL PFC workshop, June 2007.  M. Narula, M.A. Abdou, A. Ying, N.B. Morley, M. Ni, R. Miraghaie and J. Burris, “Exploring Liquid Metal PFC Concepts – Liquid Metal Film Flow Behavior under Fusion Relevant Magnetic Fields”, Fusion Eng. and Design, 81 (2006) 1543.  J.P. Allain, D.N. Ruzic, M.R. Hendricks, “Measurements and Modeling of D, H, and Li Sputtering of Liquid Lithium”, J. Nucl. Materials, 290-293 (2001) 1809.  J.P. Allain, M.D. Coventry, D.N. Ruzic, “Temperature-Dependence of Liquid Lithium Sputtering”, J. Nucl. Mater. 313-316 (2003) 641.  M. Nieto, D.N. Ruzic, W. Olczak, R. Stubbers, “Measurement of Implanted Helium Particle Transport by a Flowing Liquid Lithium Film”, J. Nucl. Mater., 350 (2006) 101.
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