FUEL OXIDATION AND IRRADIATION BEHAVIORS OF DEFECTIVE BWR FUEL-RODS

The fuel oxidation of UO2 pellets in two types of defective Zircaloy-c lad BWR fuel rods with small leaks has been examined along both pellet diametral and fuel rod axial directions. The post-defect irradiation time was a few months for the base-irradiated full length rod and seve ral minutes for the power-ramped segment rod. No phase change to highe r order oxides of U4O9 or U3O8 was found, but hyperstoichiometric UO2x with fluorite structure was still present for both fuels. The fuel o xidation significantly depended on the defect size and distance from t he defect. The pellet volume-averaged O/M ratios at various axial loca tions were in the range of 2.02-2.06 for the base-irradiated fuel, and about 2.01 for the power-ramped fuel. The data revealed that pellet o xidation by steam proceeded notably even in a short period of several minutes and played a more important role for generating liberated hydr ogen, which could cause secondary hydriding of Zircaloy cladding, in c omparison with the inner wall oxidation of cladding.