First principles density functional theory study of tritium species adsorption on Ni(111) surface and diffusion in nickel-sublayer for tritium storage

The nickel-plated zircaloy-4 is used as a tritium (³H) getter in the tritium-producing burnable absorber rods (TPBARs) to capture ³H produced in the ⁶Li-riched annular γ-LiAlO₂ pellet under neutron irradiation. The experimental data and our previous theoretical results showed that the ³H species produced from the γ-LiAlO₂ pellet were mainly ³H₂ and ³H₂O. These ³H species diffuse from the surface of the LiAlO₂ pellet across vacuum to the nickel-plated zircaloy-4 getter and then further diffuse into the getter to chemically form metal hydrides. While a number of studies show that oxygen binds strongly as compared to ³H on the nickel (Ni) layer, the detailed mechanism of ³H species absorption and diffusion across the Ni plate and Ni/Zr interface are still unclear. By employing density functional theory calculations, here we explored the ³H₂ and ³H₂O species adsorption and dissociation on the Ni(111) surface and diffusion into the Ni sublayer. Our results indicated that the ³H₂ and ³H₂O dissociate on the Ni(111) surface. The NiO_<i>x</i> and Ni(O³H)_<i>x</i> could be formed in the Ni layer due to the higher oxygen (O) diffusion energy barrier and formation of Ni vacancy defects. The oxygen was found to be retained in the Ni layer from diffusing across the Ni-Zr interface. This was revealed by comparing the diffusion barriers for ³H with O. ³H was found to have nearly three times smaller diffusion barrier than for O, making ³H comparatively easier to diffuse through the Ni layer. The obtained results provide guidelines for experimental measurements on ³H retention behavior in TPBARs and may open further avenues to explore the impurity effects on ³H diffusion and storage at the Ni/zircaloy interfaces.