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

In order to elucidate fuel wash-out behavior after the degraded second ary failure of LWR fuel rods, corrosion experiments in high temperatur e and high pressure water of 340 degrees C and 15 MPa were conducted f or unirradiated oxide fuels, two each of undoped UO2 with two differen t grain sizes, 5 wt% Gd2O3- and 10 wt% Gd2O3-doped UO2. In degassed wa ter of dissolved oxygen concentration (DO) equal to 10 ppb, no corrosi on or oxidation was found, but the corrosion became noticeable above D O = 3 ppm. Pellet oxidation proceeded with U4O9 formation along the gr ain boundaries and subsequent U4O9 growth into enveloped UO2 grains. C onsequently, the grain boundary penetration rate in the oxygenated wat er followed a diffusion-controlled parabolic law. Mismatch in the O/U ratio between the grain boundaries and interiors led to weakened bound aries, which is the cause of fuel wash-out in heavily degraded fuel ro ds. Both increases of grain size and Gd2O3 content were found to provi de improved corrosion resistance.