Focused ion beam-scanning electron microscope examination of high burn-up UO2 in the center of a pellet

Focused ion beam-scanning electron microscope and electron backscattered diffraction examinations were conducted in the center of a 73 GWd/tU UO2 fuel. They showed the formation of subdomains within the initial grains. The local crystal orientations in these domains were close to that of the original grain. Most of the fission gas bubbles were located on the boundaries. Their shapes were far from spherical and far from lenticular. No interlinked bubble network was found. These observations shed light on previous unexplained observations. They plead for a revision of the classical description of fission gas release mechanisms for the center of high burn-up UO2. Yet, complementary detailed observations are needed to better understand the mechanisms involved.