Modelling intra-granular bubble movement and fission gas release during post-irradiation annealing of UO2 using a meso-scale and spatialized approach

Abstract

The effective diffusion theory, which is generally used in the modelling of base-irradiation of nuclear fuel, cannot predict the intra-granular fission gas release during post-irradiation annealing tests. From this discrepancy between experiments and usual theory, several alternative scenarios emerged. The purpose of this work is to model these scenarios, as mechanistically as possible, and to distinguish those that could really explain the observations. The difficulty is that the fission gas bubbles in irradiated UO2 are extremely small and numerous (mean distance between nano-bubble centers is only of the order of 10–20 nm) while the grain radius is about 5μm. A new spatialized mesoscale model was developed where individual bubbles are described, along with the diffusion of vacancies from each bubble to the other, as well as from the free surface. Random movement and coalescence of the bubbles have also been included in the model. Based on this principal, two scenarios, and the combination of those, could be assessed: (a) the movement of bubbles in a vacancy gradient, and (b) the Brownian movement of bubbles. It was demonstrated that neither of these two scenarios, nor the combination of them, could explain the large fission gas release obtained during post-irradiation annealing in our reference experiment. This encourages us to consider additional mechanisms, involving dislocations for instance, that could explain the high fission gas release.