Evolution of Helium Bubbles Induced by Helium Irradiation in Polycrystalline Palladium: An Insight into Tritium Aging

Essential insight into evolution principles of helium (He) bubbles within palladium (Pd), a prominent hydrogen isotope storage and tritium handling material, is of great importance to understand the service performance of such transition metal in the field of nuclear industry. Particularly, the features of He bubbles and their evolution details have long been attracting broad interests. Herein, He bubbles with different structural features and corresponding distinct evolving processes within polycrystalline Pd were investigated by transmission electron microscopy (TEM), elastic recoil detection (ERD) and thermal desorption spectroscopy (TDS). He bubbles were induced and regulated by irradiation conditions. Comprehensive characterizing results indicate that bubbles with small sizes, spherical shape and uniform distribution, realized by low-fluence irradiation, are inclined to be stable in Pd matrix, while coarsened bubbles with faceted shape and regional distribution in the high-fluence condition are prone to release at relatively low temperatures. Meanwhile the evolving processes of He bubbles including migration, coarsening and releasing within polycrystalline Pd were analyzed. The impact of the structural characteristics such as surfaces or grain boundaries of Pd, acted as "fast path" for bubble diffusion, coarsening and release, was systematically discussed. Moreover, the correlation between tuned irradiation and actual tritium aging was discussed by analyzing He concentrations and bubble sizes in both processes. The study reveals important features of He bubble evolution in Pd, which will provide positive reference for understanding correlative behaviors in other irradiation conditions, especially tritium aging.