Effects of carbon impurity on tritium diffusion and helium formation in γ-LiAlO2 pellets: A first-principles study

Abstract

Gamma-phase lithium aluminate (γ-LiAlO2) in the form of annular pellets is used as a breeding material in tritium-producing burnable absorber rods (TPBARs) for the production of tritium by thermal neutron irradiation of 6Li. Significant radiation damage occurs in γ-LiAlO2 due to neutrons, and tritium and helium (He) nuclei, which recoil through the crystal. The radiation-damaged crystal is rich with vacancies and impurities such as carbon (C) that can act as tritium trapping sites. The presence of C impurities can hinder 3H diffusivities significantly. Here, we studied the effect of C impurities on 3H diffusivity and energy barriers in γ-LiAlO2 using first-principles density functional theory. In addition, we studied the He diffusion in an undefective γ-LiAlO2. We calculated the energy barrier for diffusion in γ-LiAlO2 to be 0.98 eV for substitutional 3H in the presence of a C impurity. At 600 K, the corresponding diffusion coefficient was calculated to be 7.98×10-14m2/s. Similarly, we calculated the energy barrier for He diffusion to be 0.12 eV, which was along the crystallographic c-direction. The corresponding diffusion coefficient for He at 600 K was calculated to be 6.8×10-7m2/s. This work may open further theoretical and experimental avenues on impurity effects and release of He from γ-LiAlO2.