Enhancing Tritium Extraction from Molten Salts Using Metal Permeators

The breeding of tritium (T) in a molten salt breeding blanket, such as LiF-BeF2 (FLiBe), is expected to lead to the formation of tritium fluoride (TF), which must subsequently be reduced to T2 before it can be extracted from the salt. With the low solubilities of T2 and TF anticipated in the salt, vacuum extraction and inert gas sparging have long been viewed as viable tritium recovery options. The initial reduction of TF would ideally occur at designed locations with a sacrificial material such as Be metal (added for control of the salt reduction/oxidation state). Over time, it can occur at undesirable locations, such as pipe walls, blanket or chamber vessel walls, and heat exchanger tube walls.In this work, a vacuum permeator was galvanically coupled with a sacrificial anode to determine if such a strategy could increase the effectiveness of T extraction while simultaneously providing a preferential location for TF reduction. It is envisioned that TF would react with a sacrificial metal such as Be to form BeF2, while T2 would form at a nearby, galvanically coupled cathodic material (e.g. PdAg or α-Fe) that would also serve as a vacuum permeator.The sacrificial anode will preferably be a component of the salt and more reactive/anodic than any associated structural materials. Galvanically coupling the active metal to the permeator would lead to a mixed potential for the anode/cathode system while providing electrons for reduction and oxidation reactions at specific, intended locations.To evaluate the feasibility of the devised concept while mitigating chemical hazards, LiF-NaF-KF (FLiNaK) and Zr were used as substitutes for FLiBe and Be, respectively. In experiments conducted with FLiNaK at 550°C, a sharp increase in H2 permeation through PdAg was observed after galvanically contacting the PdAg with a sacrificial Zr anode. Sparging of the salt with HF gas further improved the extraction rate by up to seven times the initial rate. The results presented here demonstrate that vacuum permeators can be utilized as efficient extraction materials for hydrogen isotopes in the fusion fuel cycle.