First-principles study of the tritium species diffusion across Ni-Zircaloy-4 getter interface

Tritium (T) occurs only in trace amounts in the Earth’s environment. To make T in abundance, nuclear reactions are needed. In tritium-producing burnable absorber rods (TPBARs), γ-LiAlO₂ pellets enriched with ⁶Li isotope are used to produce T. Then, the produced T species (T, OT, T₂, T₂O, CT_x, etc.) react with the Ni-coated Zrcaloy-4 getter where it is captured and leads to formation of a metal hydride. However, accurate analysis of these T species transport through the ceramic pellets to getter is hampered by the lack of fundamental data for its solubility and diffusivity. Using first-principles density functional theory, we demonstrate how these T species diffuse from surface of γ-LiAlO₂ through the Ni layer into the Zircaloy-4 getter to form hydrides under different conditions (e.g. vacancy, impurities (Sn, Fe, C, etc.)). Based on the available experimental evidence, we built a Ni/Ni₁Zr₃/Zr interface model to simulate T diffusion from Ni coating layer across two interfaces into getter. The Sn impurity in the interface region causes an increase of T diffusion barrier while Zr vacancy could bring the barrier down. Our results provide a better understanding of T transport properties within TPBARs to improve performance and increase T production with high confidence.