Fission gas release

The fission event in oxide fuels gives rise to at least two fission products. The peaks of the fission product yield curves occur in the vicinity of the xenon and krypton isotopes and approximately thirty of these atoms - fission gases - are created per hundred fissions. This amounts to approximately 30 ml of fission gas at STP for every MWd of burn-up and poses a number of problems in the operation of reactors, particularly at higher burn-ups. The fission gases aggregate into two types of porosity. Within the grains, intra-granular pores are created and these can grow rapidly during transients to produce large swellings, which may cause strong pellet-clad interactions (PCI) and fuel failure. Inter-granular porosity occurs on the grain faces and the coalescence and subsequent venting to the grain edges leads to the development of continuous release pathways. This report describes the development of the different types of porosity and how this affects the subsequent release of fission gas. The fission gas release threshold is shown to be a natural consequence of the storage capacity of the grain boundaries and the effects of different models for irradiation induced re-solution are highlighted. The release of short-lived fission products, in particular, the iodine isotopes, is controlled by the microstructure created during the release of stable gases. It is shown that a model based on the fractal nature of the surface of UO2 can be used to reconcile the conflicting results obtained for the diffusion coefficients of short-lived and long-lived fission gas atoms. (author). 65 refs., 37 figs., 3 tabs