First-principles calculations of uranium diffusion in uranium dioxide

Abstract

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.

Effect of additives on self-diffusion and creep of UO 2

“…The most likely mechanism was found to be a vacancy mechanism along the 111 direction and comprises an important influence of the oxygen sublattice. First-principles electronic computations yielded migration barriers of 3.6 eV (GGA + U model) or 4.8 eV (LDA + U model) [66]. These values can be compared with the experimental-based values listed in Table 3, discussed above, or yet with another experimental work by Reimann and Lundy [67], which gave Q D = 4.3 eV.…”

“…The mechanisms for uranium self-diffusion in UO 2 are detailed in a recent study by Dorodo et al [66]. The most likely mechanism was found to be a vacancy mechanism along the 111 direction and comprises an important influence of the oxygen sublattice.…”

Mesoscale Benchmark Demonstration Problem 1: Mesoscale Simulations of Intra-granular Fission Gas Bubbles in UO2 under Post-irradiation Thermal Annealing

“…From both simulations [4,37,38] and experiments [39][40][41] it is clear that oxygen (O) vacancies and interstitials move several orders of magnitude faster through the UO 2 lattice than cations or fission gases. The migration barriers for anion vacancies and interstitials are 0.5 [39,42] and 0.9-1.3 eV [39,42,43], respectively, while the lowest barrier for cations (a cluster of two U vacancies that can form under irradiation) is predicted to be 2.6 eV [4] and the barrier for migration of single U vacancies is 4.5-4.8 eV [4,38].…”

“…Migration of U interstitials was recently investigated using DFT calculations [38] and the barrier was calculated to be 3.7 eV for the indirect interstitial mechanism, which is lower than the barrier for single U vacancies but about 1 eV higher than for clusters of U vacancies (see below). Under thermal equilibrium conditions the contribution from interstitials to cation diffusion is very small due to the negligible concentration of such defects compared to vacancies [38].…”

“…The migration barriers for anion vacancies and interstitials are 0.5 [39,42] and 0.9-1.3 eV [39,42,43], respectively, while the lowest barrier for cations (a cluster of two U vacancies that can form under irradiation) is predicted to be 2.6 eV [4] and the barrier for migration of single U vacancies is 4.5-4.8 eV [4,38].…”