First-principles investigation of neutron-irradiation-induced point defects in B ₄ C, a neutron absorber for sodium-cooled fast nuclear reactors

Abstract

Abstract Boron carbide (B 4 C) is a leading candidate neutron absorber material for sodium-cooled fast nuclear reactors owing to its excellent neutron-capture capability. The formation and migration energies of the neutron-irradiation-induced defects, including vacancies, neutron-capture reaction products, and knocked-out atoms were studied by density functional theory calculations. The vacancy-type defects tend to migrate to the C–B–C chains of B 4 C, which indicates that the icosahedral cage structures of B 4 C have strong resistance to neutron irradiation. We found that lithium and helium atoms had significantly lower migration barriers along the rhombohedral (111) plane of B 4 C than perpendicular to this plane. This implies that the helium and lithium interstitials tended to follow a two-dimensional diffusion regime in B 4 C at low temperatures which explains the formation of flat disk like helium bubbles experimentally observed in B 4 C pellets after neutron irradiation. The knocked-out atoms are considered to be annihilated by the recombination of the close pairs of self-interstitials and vacancies.