Contribution to the study of fission gas release and swelling of UC and (Usub(0.8)Pusub(0.2))C fuels

Irradiation swelling of carbide chiefly results of the precipitation in bubbles of the fission gas Xe and Kr, which are practically insoluble into the matrix. The growth of these bubbles according to various mechanisms, results in an instability of the fuel dimension. Swelling evolution has been studied by isochronal heatings of UC pellets irradiated at temperatures so that the gas atoms remain in supersaturation in the matrix. Two steps of swelling have been observed: a normal swelling 0<S<8%, a breakaway swelling 0<S<30% the last one corresponds to the interconnection of bubbles, and leads to an important release of fission gas through the created channels. The first pins of carbide fuel (UPuC) irradiated in Rapsodie reactor have been examined. They have shown that the swelling, in that conditions, is athermal and increases proportionally with the burn up. The swelling rate which has been measured (1.45%/fissioned at%) can be explained by a part of fission gas remaining in supersaturated solution. Measurements of the released gas volumes have shown the influence of irradiation activated diffusion. At irradiation temperatures below 1,850 deg F, the released gas fraction is independent of temperature and proportional to the square root of fission rate. Between 1850 deg F and 2200 deg F the gas release is controlled by thermal and activated diffusion. Above 2200 deg F only thermal diffusion remains