Nonstoichiometry in Uranium Dioxide

Abstract

An extension of Anderson's formulation of the defect theory of nonstoichiometry is used with an oxygen—interstitial oxygen-vacancy model to calculate the composition of the uranium dioxide phase at the lower phase boundary and the relative partial molar thermodynamic functions for oxygen at compositions ranging from the lower phase boundary to UO2.08. The model is used as a basis on which to argue that differing experimental results near UO2.00 are more likely due to differing, nonequilibrium, concentrations of uranium vacancies than to failure to reach oxygen equilibrium. The present form of the model, with an interstitial site density of one per uranium site fails sharply at about UO2.08 rather than gradually as the oxygen content is increased from UO2.00. Discussion of the effects of alternate site densities is used to explain the observed values of the partial molar entropy of the oxygen in the range from UO2.02 to UO2.24.