Irradiation damage effects on helium migration in sintered uranium dioxide

In this study, the effects of radiation on helium migration are investigated through the analysis of polycrystalline uranium dioxide samples irradiated at fluences up to 5 × 1015 at. cm−2 with 8 MeV iodine ions. Following irradiation, samples are implanted with 500 keV 3He+ ions at fluences in the range of 1016 at. cm−2. Three nuclear reaction analysis (NRA) techniques are subsequently implemented using the 3He(2H,1H)4He reaction. The influence of temperature using NRA was first studied based upon 3He depth profile changes and the on-line monitoring of helium release. The effect of the sample microstructure was also investigated at the grain scale by performing analyses of the helium spatial distribution with a nuclear microprobe. Neither substantial helium release nor depth profile changes are observed at temperatures below 900 °C in irradiated samples. Following annealing at temperatures above 1000 °C, a substantial proportion of the implanted helium is released from the samples. From this temperature upwards, the two dimensional He cartographies reveal that the gas has been preferentially released in the vicinity of grain boundaries. These results can be interpreted in the light of previous studies in terms of gas precipitation and re-solution. Helium precipitation is enhanced in irradiated samples up to 900 °C because of the presence of irradiation induced defects. At temperatures in excess of 1000 °C, the precipitated helium is partly returned to the matrix hence it is preferentially released in regions adjacent to grain boundaries, which appear to act as defect sinks.