Effects of Temperature and O2 Partial Pressure on the Kinetics and Mechanisms of UO2 Oxidation to U3O8

The oxidation of UO₂ in Ar–O₂ atmospheres was studied at temperatures between 350 °C and 600 °C and for oxygen partial pressures (pO₂) between 0.20 atm and 0.70 atm. The experiments were carried out on both powder samples and discs cut from sintered pellets. Oxidation kinetics were monitored by TGA and the oxides formed were characterised by XRD, SEM and specific surface area (SSA) measurements. Whatever the temperature or pO₂ tested, total oxidation of UO₂ to U₃O₈ was systematically observed. At all temperatures studied, the conversion of UO₂ to U₃O₈ seemed to involve the formation of an intermediate oxide, which could be U₃O₇. Experiments carried out on powders with different SSA appeared to show that the effect of solid texture predominates over the effect of temperature in the formation (or non-formation) of intermediate oxides. An increase in pO₂ systematically led to an increase in the UO₂ oxidation rate. On the other hand, an increase in the oxidation temperature seemed to cause a decrease in the density of cracks propagating through the samples, probably due to an increase in the plasticity of U₃O₈. The inert marker experiment also showed an inward growth of U₃O₈ involving anionic sublattice point defects.