Differences in oxidation behavior of used CANDU fuel during prolonged storage in moisture-saturated air and dry air at 150 °C

In the Canadian research and development program on fuel storage, used CANDU (Canada deuterium uranium) UO[sub 2] fuel bundles are being exposed in experimental vessels to both dry and moisture-saturated air environments at 150 C. At intervals of several years, individual fuel elements, which were deliberately defected before storage, are recovered for destructive examination to determine the extent of UO[sub 2] oxidation that has occurred. The most recent examinations took place after 99.5 and 69 months of storage under dry and moist conditions, respectively. The progress of oxidation in the two different storage environments is compared, and the results of fuel examination by optical microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) are described. Interpretation of the results is complicated by the different O[sub 2]/UO[sub 2] ratios in the two types of storage vessel and the fact that oxygen was completely consumed during at least some of the storage intervals. Nonetheless, it is clear that the presence of moisture promotes a more generally distributed oxidation of UO[sub 2] grain boundaries. The probably involvement of radiolytic processes in the moist oxidation reaction and possible reasons for the sensitization of certain regions of the fuel to moistmore » oxidation are discussed. In addition to oxidation of UO[sub 2], the XPS spectra provide evidence for the radiation-induced incorporation of oxygen and nitrogen into adventitious carbon (adsorbed hydrocarbons) on the UO[sub 2] surfaces.« less

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