(287c) Separating Helium and Hydrogen Using Palladium-Based Metal Foil Pumps

Separating helium from hydrogen is a challenging separation that typically requires expensive cryogenics. It is also a critical enabling technology for future nuclear fusion power plants. The fusion plasma exhaust is expected to contain about 1% He in a mixture of hydrogen isotopes (D, T). The low pressure of the exhaust (~10 Pa) precludes the use of conventional pressure driven membrane processes. Metal foil pumps (MFPs) employ superpermeation, which is based on the direct absorption of superthermal hydrogen created by a plasma that increases solubility orders of magnitude beyond Sieverts' law expectations. Specifically, we focus on Pd and its alloys (PdAg, PdCu). With appropriate interface engineering all foils displayed similar temperature dependence with flux declining with temperature, suggesting that the primary rate-limiting step is absorption of superthermal hydrogen. Among these foils the hydrogen flux through PdCu was 3 - 5X greater than that of Pd or PdAg, which were similar. The superiority of PdCu is attributed to its superior hydrogen desorption kinetics. The fluxes achieved are the highest reported to date, and the results highlight the important roles of both surface and bulk properties. In addition, we demonstrate the ability to extract nearly all the H₂ from a feed stream of simulated fusion plasma exhaust. We discuss the potential impact of these technologies on the size and cost of the tritium plant, as well as additional benefits including reduced requirements for both the tritium startup inventory and tritium breeding ratio.