Charge distribution and oxygen diffusion in hyperstoichiometric uranium dioxide UO2+x (x ≤ 0.25)

Quantum-mechanical techniques were used to determine the charge distribution of U atoms in UO2-x (x <= 0.25) and to calculate activation-energy barriers to oxygen diffusion. Upon optimization, the reduction in unit-cell volume relative to UO2, and the shortest < U-0 > and < 0-0 > bond-lengths (0.22 and 0.24 nm, respectively) are in good agreement with experimental data. The addition of interstitial oxygen to the unoccupied cubic sites in the UO2 structure deflects two nearest-neighbor oxygen atoms along the body diagonal of uranium-occupied cubic sites, creating lattice oxygen defects. In (1 x 1 x 2) supercells, the partial oxidation of two U4+ atoms is observed for every interstitial oxygen added to the structure, consistent with previous quantum-mechanical studies. Results favor the stabilization of two U5+ over one U6+ in UO2-x. Calculated activation energies (2.06-2.73 eV) and diffusion rates for oxygen in UO2. support the idea that defect clusters likely play an increasingly important role as oxidation proceeds. (C) 2011 Elsevier B.V. All rights reserved.