Y. Duan, T. Jia, H. Paudel, Y.-L. Lee, D. Senor, A.M. Casella
National Energy Technology Laboratory,
Keywords: tritium-producing burnable absorber rods (TPBARs), tritium diffusion and formation, r-LiAlO2 pellets, density functional theory
Summary:Tritium (T) is an isotope of hydrogen and rarely occurs naturally in the Earth’s environment. To make T in abundance, nuclear reaction is needed. In tritium-producing burnable absorber rods (TPBARs), due to its high density, γ-LiAlO2 is used in the form of an annular ceramic pellet enriched with the 6Li isotope and is located between the zircaloy-4 liner and nickel-plated zircaloy-4 tritium getter with a gas gap in between. When irradiated in a pressurized water reactor (PWR), the 6Li pellets absorb neutrons, simulating the nuclear characteristics of a burnable absorber rod, and produce T through 6Li + n --> T + α. The T chemically reacts with the metal getter where it is captured and leads to formation of a metal hydride. However, 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. For TPBARs to enable effective tritium production in PWRs and to improve the performance of γ-LiAlO2 pellets, in this study, we employ first-principles density functional theory calculations to investigate the bulk properties, the defect chemistry, and the T 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 properties, thermal conductivity, T activation energy barriers, and the T diffusion coefficients in γ-LiAlO2 are in good agreement with the available experimental data. In the bulk, our results show that the smallest activation energy barrier is 0.63 eV for substitutional T diffusion with a diffusion coefficient of 3.25x10^-12 m^2/s. The smallest TO diffusion barrier is found to be 2.17 eV. After T diffused from bulk to the surface, it could form different species (such as T2, T2O, CT4) depending on the surface structure, vacancy types and the impurity carbon. The T atoms produced in the bulk are firstly diffused toward γ-LiAlO2 surface and then accumulated in the lithium vacancy (VLi) of surface, due to the high energy barrier of T desorption. With increasing the T accumulation on the surface, our calculations indicate that the T can be desorbed from the surface and mainly to form T2 molecule. When the number of VLi increases on the surface, the T can combine with O and desorb from surface to form T2O molecule. Obviously, with a better knowledge of T transport properties within the γ-LiAlO2 pellets through our modeling, its performances and higher T production rate can be further evaluated experimentally with a higher confidence.