An XPS study on the impact of relative humidity on the aging of UO₂ powders

High resolution x-ray photoemission spectroscopy (XPS) was used to characterize the chemical speciation of high purity uranium dioxide (UO₂) powder samples following aging for periods of up to one year under controlled conditions with relative humidity ranging from 34% to 98%. A systematic shift to higher uranium oxidation states, and thus an increase in the mean uranium valence, was found to directly correlate with the dose of water received (i.e. the product of exposure time and relative humidity). Exposure duration was found to have a greater impact on sample aging than relative humidity. Lastly, the sample aged at 98% relative humidity was found to have unique structural differences for exposure time beyond 180 days when observed by scanning electron microscopy (SEM).