A study on hydrogen absorption and dissolution in liquid lithium

Methods that plan to recover tritium from liquid lithium require intimate knowledge of the surface, sub-surface, and bulk chemistry associated with the interactions between hydrogen isotopes and lithium particles. Focusing on the lithium–lithium hydride system, previous studies have been able to determine concentrations associated with the liquidus curve, which separates the hydrogen dissolved in solution (known as the phase) from the hydrogen which precipitates out as lithium hydride (known as the phase). Knowledge of how these phases coexist in bulk melts is particularly important when the lithium is exposed to a hydrogen, deuterium, or tritium plasma, because they govern how quickly one can recover these isotopes in back-end processes for future lithium-walled fusion reactors. To this end, lithium samples were exposed to hydrogen plasmas in the Tungsten Fuzz Characterization of Nanofeatures (TUFCON) chamber at the University of Illinois. Each lithium sample was varied with respect to sample temperature, applied electrical bias, and length of sample exposure, and in each there coexisted a combination of the and phases. In all cases, two distinct absorption periods were observed during exposure. Similarly, two distinct desorption periods were observed during temperature-programmed desorption (TPD) scans. While similar desorption periods have been observed in the literature, changes in sample resistivity measured in the current study help to validate this behavior from a novel, condensed-phase perspective. The results of lithium exposures in TUFCON will be presented, along with a discussion on how the exposure conditions and phases affect recovery. Observations of superficial surface layers, and how they affect absorption and desorption, will be included in these discussions. How these results, along with the resultant marginally-enhanced dissolution behavior, can extend to tritium recycling efforts will also be explored. (paper)