Implementing first principles calculations of defect migration in a fuel performance code for UN simulations

Results are reported of first principles VASP supercell calculations of basic defect migration in UN nuclear fuels. The collinear interstitialcy mechanism of N migration is predicted to be energetically more favourable than direct [0 0 1] hops. It is also found that U and N vacancies have close migration energies, and O impurities accelerate migration of N vacancies nearby. These values are both in qualitative agreement with the effect of oxygen on the reduction of the activation energy for thermal creep reported in the literature, as well as in quantitative agreement with the experimental data when taking into account the uncertainties. The migration energies have been implemented in the thermal creep model of the TRANSURANUS fuel performance code. Therefore a concrete example is provided of how first principles computations can contribute directly to improve the design tools of advanced nuclear fuels, e.g. the predictions reveal a limited effect of oxygen on the thermo-mechanical performance of nitride fuels under fast breeder reactor (FBR) normal operating conditions.