Defect segregation facilitates oxygen transport at fluorite UO₂ grain boundaries

An important challenge for modelling transport in materials for energy applications is that in most applications they are polycrystalline, and hence it is critical to understand the properties in the presence of grain boundaries. Moreover, most grain boundaries are not pristine stoichiometric interfaces and hence dopants are likely to play a significant role. In this paper, we describe our recent work on using atomistic molecular dynamics simulations to model the effect of doped grain boundaries on oxygen transport of fluorite structured UO₂. UO₂, much like other fluorite grain boundaries, are found to be sinks for oxygen vacancy segregation relative to the grain interior, thus facilitating oxygen transport. Fission products further enhance diffusivity via strong interactions between the impurities and oxygen defects. Doping produces a striking structural alteration in the Σ5 class of grain boundaries that enhances oxygen diffusivity even further.