Impact of fuel heterogeneities on fission gas release for LWR U-Pu mixed oxide fuels

Different fabrication routes for mixed oxide fuels can give large differences in pellet microstructure and degree of heterogeneity. An heterogeneity in the plutonium distribution in the fuel matrix leads to non-uniform fission rates and fission product distribution across the pellet and perturbs the processes which determine the fission gas release (FGR). The relative effect of the heterogeneity increases at lower fuel temperatures, as is the trend in modern LWR fuels. In the first part of this paper, a model is presented which describes quantitatively the effects of an heterogeneous plutonium distribution on the fission gas release. In the second part, COMETHE III-L predictions based on this model are compared with experimental data for different manufacturing processes. The main conclusion that can be drawn from this study is that it is essential to take into account the heterogeneity of the plutonium distribution in the calculation of the fission gas release, as soon as the plutonium aggregate size is larger than about 10 μm. The dissolution of the plutonium aggregates in the UO2 matrix with the burnup, and the mean free path of the fission fragments from the site of the fission, must be considered in any consistent FGR model designed for mixed oxide fuels. (author)