Effects of Cr on H and He trapping and vacancy complexes in V in a fusion environment: a first-principles study

First-principles density functional theory was used to investigate the interaction between hydrogen (H) and helium (He) in V-Cr alloy, which is a potential structural material for use in fusion reactors. When vacancies are present in the V-Cr alloy, a single He atom prefers to occupy the octahedral site near the Cr atom rather than vacancy centre, which differs from the cases of iron and tungsten. Because of the decrease of the electron density around the He atom, there was a strong interaction between He and H. In the vicinity of He-vac complexes, H atoms tend to stay in the tetrahedral site rather than occupy the octahedral-interstitial site. A single He-vac complex can trap as many as six H atoms, which is more than can be trapped by an empty vacancy in the V-Cr alloy because of the electronic density redistribution of vacancy vicinity. This strong attraction explains the enhanced retention of H and He observed near the surface of V and V-based alloys under both sequential and simultaneous bombardments. The results provide useful insight into the application of the V-based alloys as candidate structural materials in fusion Tokamaks.