Steam oxidation of fuel in defective LWR rods

Oxidation of UO2 by pure steam at pressures of 7 and 70 atm and 500 C and 600 C was measured in a thermo-gravimetric apparatus. The kinetics are linear, vary as the square root of the steam pressure, and are consistent with initial rates extrapolated from higher-temperature experiments in 1-atm steam. At temperatures characteristic of normal operation of defective fuel rods, the rate of hydrogen production by thermal oxidation of the fuel in steam is small compared with that due to cladding corrosion. The presence of H2 in the steam has a much greater retarding influence on fuel oxidation than on cladding oxidation. Other potential sources of fuel chemical reactivity in steam, including reaction in cracks in the hot pellet interior and radiolysis of steam by recoiling fission fragments, do not result in significant fuel oxidation. During the incubation stage of fuel-rod degradation, the bulk of the evidence indicates that fuel oxidation is not a major source of the hydrogen in the fuel-cladding gap that eventually may cause secondary-hydriding failure of the rod. (orig.)