Gas release mechanisms in UO2

The first and basic step in gas release is single gas atom diffusion. The lattice location of rare gas atoms in the UO2 lattice and the mechanism of this diffusion process are discussed. Trapping of single gas atoms is important for out-of-pile laboratory work and helps to explain much of the scatter in the reported data. It is less important in-pile due to irradiation induced resolution. The experimental techniques used to measure gas diffusion and release are critically discussed. The available data on fission gas bubble mobility are reviewed. The reliable knowledge is scarce. The effect of burn-up (change in O/M-ratio, in-growth of fission products) is treated. The relative contributions of single gas atom diffusion, fission gas bubble mobility, sweeping and inter-granular bubble growth are discussed for the cases of highly rated fast breeder fuel and moderately rated water reactor fuel. It is argued that single gas atom diffusion is contributing essentially to release in LMFBR fuel, whereas trapping is not typical for in-pile conditions. Finally, some implications of empirical gas release relations and computer codes are given. (author)