(305a) Density Functional Theory Study of the Tritium Formations on the Surfaces of ?-LiAlO2

One of the important components of tritium-producing burnable absorber rods (TPBAR) is the γ-LiAlO₂ layer used in the form of an annular ceramic pellet enriched with the ⁶Li isotope. Within TPBAR architecture, this oxide is located between the zircaloy-4 liner and nickel-plated zircaloy-4 tritium getter. When irradiated in a pressurized water reactor (PWR), the ⁶Li pellets absorb neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produce tritium (T). The T specie chemically reacts with the metal getter, leading to the formation of a metal hydride. Accurate analysis of the T transport through the ceramic pellets and the barrier/cladding system is hampered by the lack of fundamental data about the hydrogen isotope solubility and diffusivity. In TPBAR architecture, between the γ-LiAlO₂ pellet and the Ni-plated Zircaloy-4 getter (or liner), there is a gas gap. After T diffuses from bulk γ-LiAlO₂ to its surface, it is possible that different T species (such as T, T₂, T₂O, and OT, etc.) are formed on this surface which can then get into the gas gap region. Furthermore, upon recombination and desorption the molecular tritium can be formed on the γ-LiAlO₂ surface. Therefore, exploring the formation of T-related species on γ-LiAlO2 surface becomes very important as an integral part of the study of T solubility and diffusivity in the pellets. Based on the density functional theory, in this study, we investigate the stability and structural properties of γ-LiAlO₂ surface, the most possible adsorption sites, and the formation of T on surface. From the calculated surface free energies, the most stable surfaces were identified from all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces. By calculating the binding energies of T adsorbed on top, bridge, and hollow sites of LiAlO₂ (100) surface, we obtained four stable adsorption sites (three O-top sites and one Al-top site) and the most stable one is O-top site. From the Bader charge analysis, T adsorption on O-top site is a reduction process, while on Al-top site is an oxidation one. In addition, the zero-point energy correction (ΔE_Z) on the stability of adsorption is investigated. The obtained results showed that the ΔE_Z influence of T adsorption on O-top site is larger than that on Al-top site, but the trend of stability keeps unchanged.