Theoretical study of tritium diffusion and formation from bulk and defective surface of γ-LiAlO₂ pellets

In tritium-producing burnable absorber rods (TPBAR), γ-LiAlO₂ is used in the form of an annular ceramic pellet enriched with the ⁶Li isotope. When irradiated in a pressurized water reactor (PWR), the ⁶Li pellets absorb neutrons and produce tritium (³H) through ⁶Li + n à ³H + α. The ³H chemically reacts with the metal getter where it is captured and leads to formation of a metal hydride. For TPBARs to enable effective tritium production in PWRs, we investigated the ³H diffusion pathways in the bulk and surface of γ-LiAlO₂ with different concentrations of lithium defects. The calculated results for bulk and low-index surfaces, thermal conductivity, ³H activation energy barriers, and the ³H diffusion coefficients in γ-LiAlO₂ are in good agreement with the available experimental values. In the bulk, our results show that the smallest activation energy barrier is 0.63 eV for substitutional ³H diffusion with a diffusion coefficient of 3.25x10^-12 m²/s. After ³H diffused from bulk to the surface, it could form different species (such as ³H₂, ³H₂O, C³H₄) depending on the surface structure, vacancy types and the impurity carbon. Our results indicate that the ³H₂ is the main product from γ-LiAlO₂ pellets. As the number of V_Li vacancies and ³H atoms increases under irradiation (³H-rich condition), ³H₂O release could increase from the surface of γ-LiAlO₂ pellets.