An improved model to evaluate the oxidation kinetics of uranium dioxide during dry storage

During dry air storage, the oxidation of the spent fuel in case of cladding and container failure (accidental scenario) could be detrimental for further handling of the spent fuel rod and for the safety of the facilities. Recently, the phase transition sequence during the first step of parabolic oxidation kinetic has been challenged again and two well-distinguished intermediate products, U4O9 and U3O7 have been identified. Moreover, these observations have shown that the three phases (UO2, U4O9 and U3O7) occur together. Starting from a previous model of grain oxidation based on finite difference approach, a new model, describing the parabolic oxidation kinetic, has been developed based on the oxygen atom diffusion. This model allows in one hand to take into account the occurrence of the three phases and in another hand to describe accurately the plateau behaviour. A comparison between the model and literature data obtained on non-irradiated powders has been carried out and shows that this model can describe the weight gain evolution as a function of time for different temperatures. The diffusion coefficients of oxygen in the two phases (U4O9 and U3O7) were obtained by fitting the model results to experimental data. The comparison with the values given in literature is quite good.