A computer simulation study of the accommodation and diffusion of He in uranium- and plutonium-doped zircon (ZrSiO 4)

Abstract

We report the results of an atomistic computational study of He accommodation and diffusion in the Pu 4+- and U 4+-doped zircon (ZrSiO 4). The He–cation potentials derived for this work give results of comparable accuracy to DFT calculations. We have calculated the structural features of doped lattices as well as He solution energies in interstitial sites in the perfect and doped zircon and its diffusion in these lattices. The mode of He accommodation in the perfect zircon is influenced mainly by the topological features of the lattice, promoting site preference of He towards accommodation in the interstitial sites present in the middle of c cylinder channels, whereas the presence of Pu 4+ and U 4+ dopants in the zircon lattice significantly affects the energetics of He accommodation and diffusion in the lattice. Doping causes strong local structural distortions, extending to next nearest-neighbour atoms of the dopants to a radius of up to ∼4 Å, in agreement with experimental results. The presence of dopants in the vicinity of He enhances the solubility of He in the lattice compared to the perfect lattice. The mechanism of diffusion is also affected, where the dopants can create a He trap along the most energetically favourable pathway in the (0 0 1) direction, which may slow down the movement of He along the c direction. The dopants also lower the energy barriers by ∼50% in the octahedral sites.