Grain-boundary oxidation of used CANDU fuel exposed to dry air at 150 degrees C for a prolonged period

The grain-boundary chemistry of used CANDU fuel exposed to dry air at 150 degrees C for a prolonged period has been investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). High degrees of surface oxidation have been determined using the chemical-shift effects for the uranium photoelectron emission, but these must be largely restricted to thin films. The observed distribution of segregated fission products implies an absence of major fuel restructuring and SEM examinations revealed mainly subtle changes in the UO2 grain structure. These findings are consistent with metallographic evidence of pervasive grain-boundary attack, despite only slight bulk alteration of the fluorite-lattice structure. (author)