First-principles calculations of uranium diffusion in uranium dioxide

Abstract

The present work reports first-principles DFT $+\phantom{\rule{0.16em}{0ex}}U$ calculations of uranium self-diffusion in uranium dioxide (UO${}_{2}$), 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 UO${}_{2\ifmmode\pm\else\textpm\fi{}x}$ that is valid for small deviations from stoichiometry. We investigate five migration mechanisms and calculate the corresponding migration barriers using both the LDA $+\phantom{\rule{0.16em}{0ex}}U$ and GGA $+\phantom{\rule{0.16em}{0ex}}U$ 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 $+\phantom{\rule{0.16em}{0ex}}U$ approximation. The lowest migration barrier is obtained for a vacancy mechanism along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction. This mechanism involves significant contribution from the oxygen sublattice, with several oxygen atoms being displaced from their original position. The $\ensuremath{\langle}110\ensuremath{\rangle}$ vacancy diffusion mechanism is predicted to have lower activation energy than any of the interstitial mechanisms and comparison to experimental data for stoichiometric UO${}_{2}$ also confirms this mechanism.