Nuclear fuel pins exhibit distortions in the UO2 lattice in response to temperature gradients, defects and the introduction of fission product (FP) gases. These distortions can have a significant influence on the activation barriers associated with fission product gas diffusion. A predictive understanding of this relationship is particularly relevant to anticipating the evolution of fission product gases during rapid temperature transients. Density Functional Theory (DFT) has the capacity to provide a relationship between lattice distortion and FP gas diffusivity by generating estimates of dilation dependent activation energies.

As a first step in this direction, the relation between lattice dilation and activation energy for isolated vacancies within an otherwise pristine block of alpha-quartz is quantified, where precise experimental data is readily available. The results lend confidence to the basic approach which is based on a one-shot transition state method, developed to lessen the computational resources required by the full transition state method. This technique is first applied to α-quartz for O vacancy hopping and diamond-Si for Si vacancy hopping. The method is subsequently extended to consider isolated uranium vacancies in UO2. This in turn is further generalized to estimate the activation volume for Kr atoms in UO2. Thus two different types of defects are considered; those of species native to the material and, in the case of UO2, FP gases introduced through the fission process.