On the rim effect in high burnup UO2LWR fuels

Abstract Ion implantation experiments with energetic Xe-ions were performed in order to better understand the possible mechanisms leading to the formation of an outer shell of small grains and large porosity in high burnup UO 2 (the so-called “rim effect”). It is shown that grain subdivision related to the formation of highly pressurized (probably solid) Xe-precipitates occurs at implantation doses corresponding to fission product concentrations reached at 7 to 8 at% burnup. Fracture or cleavage of UO 2 grains lead to a structure of small grains similar in size to those observed in irradiated high burnup UO 2 . The limitations of such simulation experiments are discussed and the parameters are defined for conclusive reactor irradiation experiments to finally understand the underlying mechanism.