fib/sem examination of a high burn-up uo2 in the center of the pellet

FIB/SEM examinations were conducted in the central part of a 73 GWd/tU UO2 fuel. They showed the formation of sub-domains within the initial grains. Most of the fission gas bubbles were found to be situated on the boundaries. Their shapes were far from spherical and far from lenticular. No interlinked bubble lattice was found. These observations enlighten previous unexplained observations. They plead for a revision of the classical description of the fission gas release mechanisms for the central part of the high burn-up UO2. Yet, complementary detailed observations are needed to better understand the mechanisms involved.