The design and development of hydrogen isotope extraction technologies for a limit-style liquid lithium loop
As lithium has grown in popularity as a plasma-facing material, efforts have been placed on examining its viability as a first wall candidate. Lithium has proven over numerous studies to improve core confinement, while allowing access to operational regimes previously unattainable while using solid, high-Z divertor and limiter modules. These benefits are due to the fuel retention capabilities of lithium, which allow it to be an almost ideally absorbing boundary, which is both beneficial and problematic. While lithium exhibits a number of other advantages and disadvantages as a plasma-facing material, none is more important than the tritium retention problem. As such, extraction technologies must be constructed and verified within the scope of larger scale lithium loop systems that separate lithium impurities, recover deuterium and tritium, and recycle clean liquid lithium back to the plasma-material interface. Laboratory-scale and pilot-scale studies have been conducted at the Center for Plasma-Material Interactions at the University of Illinois to investigate a number of phenomena that influence the recovery of entrained tritium from lithium. While the ultimate goal is to develop a fully-functional liquid lithium loop for the Lithium Metal Infused Trenches plasma-facing component, complete with efficient hydrogen reclamation technologies, there exists a lack in understanding within the community of the thermochemical fundamentals that are envisioned to drive tritium reclamation. Of specific interest are the evolution fluxes of hydrogen isotopes from solutions of various concentrations of hydrogen in lithium, and the associated temperatures. The knowledge of how the isotopic fraction affects recovery is pivotal to determining the appropriate thermal treatment technique. The laboratory-scale experiments in this report aimed at filling in the knowledge gaps in the literature with regards to the thermochemistry of the hydrogen-lithium system. In all cases, hydrogen was used as an isotopic surrogate for deuterium and tritium. Success was based on an individual samples ability to evolve molecular hydrogen at rates that would match or exceed in-vessel wall losses, determined from a simulated Lithium-Walled International Thermonuclear Experimental Reactor scenario. The hydrogen degassing of pure lithium hydride was observed to exceed fuel loss by a factor of two or greater, at temperatures near the melting point for hydride. Samples of both solid and liquid lithium were subjected to different hydrogen environments under a variety of exposure conditions. During plasma exposures, evidence of saturation, where hydride layers are formed at or near the sample surface and inhibit hydrogen absorption, was witnessed for solid lithium samples. Liquid samples exhibited this behavior to a lesser degree; however, mass diffusion was able to transport the insulating species away from the surface and absorption was able to continue, albeit to a lesser extent than was initially detected. The sub-surface chemistry was found to still be limited by the thermodynamic solubility thresholds in a plasma environment, meaning enhanced hydrogen dissolution was not witnessed at ion energies relevant to these experiments. The presence of a plasma, however, did appear to enhance absorption rates above and beyond what was capable with hydrogen gas alone. During these tests, hydrogen evolution rates from the dissolved phase never approached the point of being able to balance losses at the plasma-material interface, being always less by a factor of two or more. It was therefore determined that supplementary methods were required to enhance thermal-based recovery in solutions with hydrogen molar ratios less than the solubility limits. This work culminated in the design, development, construction, and proof-of-concept testing of a distillation column. Envisioned to be an integrated treatment method in a fully functional lithium loop, the column was developed based on the need to recover tritium and recycle fresh lithium back into the reactor. The novelty in this design was in its use of induction heating drive and condensation stages. Proof-of-concept tests were performed in the fully constructed prototype with solutions of lithium and lithium hydride at various molar ratios. The system was observed to operate as intended during these initial runs, but requires further testing; however, the column marks the first system constructed for the sole purpose of recovering tritium from a lithium-walled reactor. Such a system will prove most effective if upstream separation and purification techniques are present to divert the lithium deuteride and lithium tritide-rich streams to the column for thermal decomposition and degassing. In the case where upstream purification modules are absent from the lithium loop, the column alone will be hard pressed to achieve recovery rates in far-from-saturated solutions that balance wall losses. A technique to supplement the induction heating drive was therefore proposed. Ultrasonic degassing of liquid metals is an industry-tested technique used to rid melts of dissolved gases by taking advantage of acoustically-induced cavitation. This process was theoretically applied to the hydrogen-lithium system, displaying evidence that degassing is most effective in the presence of heat, ultrasonic waves, and vacuum. This work laid the theoretical groundwork for future application. The results presented in this report show that using the appropriate combination of treatment methods, hydrogen, and by extension deuterium and tritum, can be recovered from lithium at rates that balance in-vessel wall loss. Future work will be needed to then integrate these methods into a fully functional liquid lithium loop.
Retention, Recovery, Recycling, Thermoelectic Magnetohydrodynamics, Dissolution, Precipitation, Plasma, Lithium, Hydrogen, Deuterium, Tritium, Flux, Exposure
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign