Oxygen diffusion in hypostoichiometric uranium dioxide

The tracer oxygen diffusivity in UO/sub 2-x/ has been measured along the lower two phase boundary. The diffusion couple consisted of two matched hypostoichiometric UO2 wafers, one enriched with 18O and the other normal. These two were pressed together with a bond of liquid U in between. After a diffusion anneal the 18O concentration profile was determined by ion microprobe mass analysis, from which diffusion coefficients were obtained. The results showed much higher diffusion coefficients than those of stoichiometric UO2. This directly proved that the major defect species in UO/sub 2-x/ is the anion vacancy. Activation energy of anion vacancy migration was measured to be 11.7 +- 3.0 kcal/mole. A diffusion model established for UO2 and UO/sub 2+-x/ showed that in stoichiometric UO2 both interstitials and vacancies contribute significantly to oxygen diffusion and neither can be neglected; at 14000C their contributions are about equal. This model was extended to nearly stoichiometric UO/sub 2+-x/ to predict oxygen diffusion coefficients in these stoichiometry ranges. Also deduced from the model were the Frenkel defect energy and entropy of 85.6 +- 9.2 kcal/mole and 18.2 +- 7.3 eu, respectively. Using these values, the contribution of Frenkel disorder to the excess enthalpy of UO2 was evaluated. Calculation showed that Frenkel disorder accounts for 87% of the excess enthalpy at 30000K. A simple two band model for electronic excitation, with a band gap of 2.0 eV and effective electron mass of 7.6 m/sub e/, accounted for the remainder of the excess enthalpy