Storage of Irradiated CANDU Fuel in Dry and Moist Air

Abstract

The long-term behaviour of irradiated CANDU fuel bundles stored in concrete canisters in dry air and in air saturated with moisture at 150°C is being assessed in a joint program between Atomic Energy of Canada Limited (AECL) and Ontario Hydro (OH). Undefected and intentionally defected irradiated fuel bundles from Ontario Hydro's Pickering and Bruce Nuclear Generating Stations are being used in the research program. Before the bundles were loaded into the canisters, they were extensively characterized to establish their pre-storage condition (1).

In 1984, the bundles were retrieved from the canisters for their first interim examination. No significant change was observed in the condition of the undefected fuel elements. Ceramographic evidence of U₃O₇ and microcracks and/or a higher oxide phase was observed in the UO₂ grains in the dry-air experiment. Grain boundary oxidation and microcracks and/or a higher oxide phase were also observed in the fuel stored in moist air. X-ray photoelectron spectroscopy (XPS) analyses of the fuel from the defect regions of both experiments indicated surface oxidation of the fuel to U₃O₈ or beyond. Although U₃O₇ was undetectable by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and by chemical analysis, indicating that its concentration in the fuel samples was extremely small, α-U₃O₇ was detected by XRD in the fuel from the defect regions in the dry-air experiment.