A detailed study of UO2 to U3O8 oxidation phases and the associated rate-limiting steps

The kinetic and crystalline evolutions of UO2 during its oxidation into U3O8 at 250 °C in air were studied by isothermal thermogravimetry and calorimetry, coupled with an in situ synchrotron X-ray diffraction on the D2AM-CRG beamline at ESRF. This study was aimed at determining experimentally the validity of the kinetic assumptions made in existing literature to account for the oxidation of UO2 into U3O8 and also to determine precisely the structural evolution, in relation to the kinetic behaviour. Our results provide evidence of four distinct kinetic time domains, and the assumption of a single rate-limiting step is verified only for two of them. The crystalline phases associated with these domains are also identified. In fact, the first kinetic domain corresponds to the reaction of UO2 into U4O9; the second one is linked to the two simultaneous reactions, UO2 into U4O9 and U4O9 into U3O7. Finally, the transition from U3O7 into U3O8 corresponds to the third and fourth kinetic domains. These results show that the oxidation of UO2 into U3O8 cannot satisfactorily be described with modelling approaches used in the literature. A new general outline is proposed to study the oxidation of uranium oxides. This outline will improve both the understanding and predictions of oxidation processes at the relatively low temperatures that are expected during interim storage of spent nuclear fuel.