Résumé
The oxidation of UO2 in Ar-O2 atmospheres was studied at temperatures between 350 °C and 600 °C and for oxygen partial pressures (pO2) ranging from 20 kPa to 71 kPa. The experiments were carried out on powder samples and discs cut from sintered pellets. Oxidation kinetics were monitored by TGA and the oxides formed were characterised by XRD, SEM and SSA measurements. Whatever the temperature or pO2 tested, total oxidation of UO2 to U3O8 is systematically observed. At all temperatures studied, the conversion of UO2 to U3O8 seems to involve the formation of an intermediate oxide, which could be U3O7. Experiments carried out on powders with different SSA appear 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 leads to an increase in the UO2 oxidation rate. On the other hand, an increase in the oxidation temperature seems 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.