First-principles calculations of uranium diffusion in uranium dioxide

The present work reports first-principles DFT+U calculations of uranium self-diffusion in uranium dioxide (UO2), with a focus on comparing calculated activation energies to those determined from experiments. To calculate activation energies, we initially formulate a point defect model for UO2 +/- x that is valid for small deviations from stoichiometry. We investigate five migration mechanisms and calculate the corresponding migration barriers using both the LDA+U and GGA+U approximations. These energy barriers are calculated using the occupation matrix control scheme that allows one to avoid the metastable states that exist in the DFT+U approximation. The lowest migration barrier is obtained for a vacancy mechanism along the < 110 > direction. This mechanism involves significant contribution from the oxygen sublattice, with several oxygen atoms being displaced from their original position. The < 110 > vacancy diffusion mechanism is predicted to have lower activation energy than any of the interstitial mechanisms and comparison to experimental data for stoichiometric UO2 also confirms this mechanism.