Fins: Improving tritium extraction systems and permeation sensors with the adoption of extended surfaces

Tritium extraction from lithium-lead (PbLi, 15.7 at. % Li) and tritium concentration measurement in the eutectic alloy represent some of the most challenging aspects of the R&D activities aimed to the development of ITER and the European DEMO reactor. To efficiently design Permeator Against Vacuum (PAV) systems and Hydrogen isotopes Permeation Sensors (HPS), theoretical models for the evaluation of the permeation flux have been proposed in literature, but methodologies for the improvement of their performances are still lacking. In this paper a new concept of finned permeator is analysed, leveraging the analogy between mass transport and heat transfer. In PAV and HPS, the low-pressure side is usually kept under medium/high vacuum conditions and surface phenomena can play an important role, especially when the membrane presents oxidation. The fin approach is particularly effective in these cases, where transport kinetics is dominated by surface effects (diffusion in the bulk is relatively fast) and can be used as a method to increase the permeation of hydrogen isotopes with limited increase in system size. Within the paper, the mathematical model for the extended surface placed on the vacuum side is derived and simple relations for design parameters for the finned surface, like fin efficiency and effectiveness, are derived. The solution of this analytical model is compared with numerical results for a PAV system with niobium membrane under relevant conditions for DEMO reactor.