Corrosion behavior of unirradiated oxide fuel pellets in high temperature water

In order to elucidate fuel wash-out behavior after the degraded secondary failure of LWR fuel rods, corrosion experiments in high temperature and high pressure water of 340°C and 15 MPa were conducted for unirradiated oxide fuels, two each of undoped UO₂ with two different grain sizes, 5 wt% Gd₂O₃- and 10 wt% Gd₂O₃-doped UO₂. In degassed water of dissolved oxygen concentration (DO) equal to 10 ppb, no corrosion or oxidation was found, but the corrosion became noticeable above DO = 3 ppm. Pellet oxidation proceeded with U₄O₉ formation along the grain boundaries and subsequent U₄O₉ growth into enveloped UO₂ grains. Consequently, the grain boundary penetration rate in the oxygenated water followed a diffusion-controlled parabolic law. Mismatch in the O/U ratio between the grain boundaries and interiors led to weakened boundaries, which is the cause of fuel wash-out in heavily degraded fuel rods. Both increases of grain size and Gd₂O₃ content were found to provide improved corrosion resistance.