Helium release in uranium dioxide in relation to grain boundaries and free surfaces

Nuclear reaction analyses (NRA) based on the 3He(2H,4He)1H reaction were previously performed to follow the evolution of implanted 3He in polycrystalline UO2 samples. Experimental results pointed to an enhancement above 800 °C of the diffusion coefficient of helium over several microns in the vicinity of the grain boundaries, with respect to the diffusion coefficient within the grain. This was ascribed to the fact that grain boundaries are probably defect sinks which locally modify the defect concentrations.This study aims at demonstrating the particular effect of grain boundaries on helium migration. To this end, 3He implanted polycrystalline UO2 samples were cracked then annealed at 900 °C. Helium migration in the vicinity of the grain boundaries and near the crack was investigated by means of NRA microanalyses. Helium depletion extends over far larger distances in the vicinity of the grain boundaries than near the crack. Experimental evidence has been collected of the particular effect of grain boundaries on helium migration, which do not act as free surfaces at which helium atoms are simply released.