CORROSION BEHAVIOR OF IRRADIATED OXIDE FUEL PELLETS IN HIGH-TEMPERATURE WATER

In order to elucidate fuel wash-out behavior after the degraded second ary failure of BWR fuel rods, corrosion experiments in high temperatur e and high pressure water of 340 degrees C and 15 MPa were conducted f or UO2 and UO2-10 wt%Gd2O3 fuel pellets with different grain sizes, wh ich had been irradiated in a commercial BWR or a test reactor (burnup: 32-34 GWd/tU). The fuel corrosion was characterized by the occurrence of microcracks along the grain boundaries due to preferential U4O9 fo rmation, and became noticeable in the oxygenated water of more than se veral ppm dissolved oxygen concentration. The grain boundary penetrati on depths for the irradiated pellets were approximately equivalent to, or somewhat deeper than the values for previous unirradiated pellets reported by the present authors, Consequently, fuel irradiation, i.e., accumulation of fission products had no significant effect on the fue l corrosion Enlarging grain size and 10 wt% Gd2O3 doping distinctly im proved the corrosion resistance, The Kr-85 release into the hot water, which had been accumulated in the grain boundaries during base irradi ation, increased linearly with corrosion penetration depth.