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 UO with two different grain sizes, 5 wt% GdO- and 10 wt% GdO-doped UO. 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 UO formation along the grain boundaries and subsequent UO growth into enveloped UO 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 GdO content were found to provide improved corrosion resistance