On the synergistic effects of helium and hydrogen in bubble formations and materials degradation

Cavities (voids and helium bubbles) can form in metals, which are subjected to neutron irradiation, by the agglomeration of vacancies produced by neutron bombardment and/or helium atoms generated by neutron-induced transmutation. The volumetric changes and degradation in mechanical properties because of swelling and cavities, respectively, have been of intense interest in nuclear applications [1]. One of the most studied materials is reduced activation ferritic martensitic (RAFM) steel, which is a candidate structural materials for use in the blankets of fusion reactors. RAFM steels will be exposed to high temperature and neutron irradiation. Helium and hydrogen atoms are generated through nuclear interactions of host atoms with the incident neutrons via (n, a) and (n, p) reactions [2]. In this study, we present the surface swelling induced by helium implantation for RAFM steels with and without pre-implanted hydrogen. Regular surface patterns with a step height of a few tens of nanometers and enhanced hardness are generated by masking the surface of RAFM steels with a square mesh transmission electron microscopy (TEM) grid during helium implantation. The cause of the difference in surface-step heights is evaluated using cross-sectional TEM observation and attributed to the change in microstructural features in the steels. Pre-implantation of hydrogen enhances the surface swelling with only one-twentieth dose of post-implanted helium ions. Possible mechanisms are postulated in this study to explain the synergistic effect of pre-implanted hydrogen.