Study of the microstructural mechanisms linked with the fission gases release of irradiated uranium dioxide

During the irradiation of uranium dioxide in PWR reactors, fission gases (krypton and xenon) are created. This study aims at finding a relation between their retention and release with the microstructural evolutions of the fuel. Coarse grain pellets, irradiated at low temperature up to a burnup level of 9 GWJ.T 1 U, have been annealed in vacuum at 1600 or 1700 deg. C. The growth of intergranular gas bubbles and their coalescence to form labyrinths have been observed by scanning electron microscopy. These phenomena depend on the crystallographic direction of grain joints, which is mainly due to the surface energy anisotropy. Precipitates of metallic fission products disturb the bubble coalescence, which precedes the gas release. Cesium, a fission product, is probably incorporated in the intergranular bubbles like the rare gases. The hypothesis, based on experimental results, that cesium forms a liquid phase (with molybdenum for instance) has been proposed. This phase dissolves the uranium dioxide at the periphery of bubbles and accelerates the porosity growth. Annealing in helium has been performed between 1400 deg. C and 1760 deg. C on coarse grain UO2 fuel pellets, irradiated like the previous ones, and on PWR pellets irradiated up to a 14 GWJ.T 1 U burnup. The release of 85Kr has been measured continuously during the annealings. An analytical model has been developed. It takes into consideration the trapping of the gas in the intergranular bubbles which depends on the crystallographic direction of the grain joints. This gas retention induces sometimes a considerable delay in its release. The intergranular bubbles at the center of the grains efficiently retain the gas. The gas release mechanisms are identical in the experimental fuel and in the PWR fuels. The over-stoichiometry principally influences the intergranular migration of gases. (J.S.)