Ab initio study of hydrogen diffusion in Be and Be12Ti for fusion applications

Abstract

Beryllium and Be12Ti are considered as neutron multiplier materials in various tritium breeding blanket designs for future fusion reactors. Tritium and helium are formed due to neutron-induced nuclear transmutation and accumulated in beryllium-based materials. As a β-radioactive isotope, tritium represents additional safety concern with respect to sudden release during operation or hindering consequent recycling. Therefore, detailed study of its accumulation and release is indispensable. In particular, it is important to reveal which process controls tritium release: binding of tritium with traps or diffusion out of material bulk. Ab initio calculations of solution energies at all non-equivalent hydrogen interstitial sites confirm easier dissolution of tritium in Be12Ti in comparison with pure beryllium. The 17 considered diffusion barriers between adjacent hydrogen interstitial sites are calculated using a dimer method instead of a computationally more expensive nudged elastic band approach. Three possible diffusion paths in Be12Ti were revealed: two of them along the c-axis and another one along the a- and b-axes. Considering calculated hydrogen diffusion barriers in Be12Ti, one can conclude that at low hydrogen concentrations and low temperatures, diffusion occurs preferentially along the c-axis. The rate limiting diffusion barriers in Be12Ti are found to be in the range of 0.4–0.5 eV, which are comparable to 0.42 eV in pure beryllium suggesting that tritium diffusion in both materials proceeds at comparable rates. Therefore, binding of tritium atoms with vacancies and other lattice imperfections are expected to play a crucial role in tritium retention.