Intergranular fission gas bubbles and solid precipitates in UO2 irradiated at high burnup in various conditions

With a view to gaining a better understanding of fission gas release mechanisms at high burnups, the microstructures of three types of nuclear fuel of initial density close to 10.4 g.cm-3 were studied: spheres of 1 mm diameter irradiated up to 20 GWd/THM at 900, 1200 and 1500 deg. C, pellets of PWR type irradiated to 22 GWd/THM, with centre-line temperature of the order of 1500 deg. C, and PWR pellets irradiated to 35 GWd/THM in a power reactor and then subjected to a number of transients between 200 and 300 W.cm-1 in an experimental reactor. SEM examination of fracture surfaces of these various fuels shows that, below 900 deg. C, no bubbles are visible. At 1200 deg. C, the nucleation of intergranular bubbles is clearly apparent, accompanied by metallic precipitates. At 1400-1500 deg. C, apart from a tendency for grain growth, it was found that the bubbles coalesce to a considerable degree, with accompanying start of porosity interconnection and formation of tunnels at grain edges. In cases where power transients were applied, this interconnection phenomenon is particularly marked in areas having reached 1300 deg. C in the plateau phase. The metallic precipitates associated with the bubbles were analysed by the EDS SEM system. Three classes of precipitates were distinguished, with a typical composition being given for each of them. These results must be supplemented by ceramographs and TEM examination. (author). 11 refs, 4 figs, 3 tabs