Water Sorption and Gamma Radiolysis Studies for Uranium Oxides

Experiments have been performed in our laboratory on water sorption and radiolysis for uranium oxides. For the water sorption experiments, uranium oxide samples were prepared and exposed to known levels of humidity to establish the water uptake rate. Subsequently, the amount of water removed was studied by heating samples in an oven at fixed temperatures and by differential thermal analysis/thermogravimetric analysis. It was demonstrated that heating at 650°C adequately removes all moisture from the samples. Uranium-238 oxides were irradiated in a ⁶⁰Co source and in the high-gamma-radiation fields provided by spent nuclear fuel elements of the High Flux Isotope Reactor. For hydrated samples of UO₃, the primary gas produced was H₂; however, the maximum pressure increase reached a steady-state value of ~500 torr (10 psi). This H₂ production appears to be a function of the dose and the amount of water present. Oxygen in the hydrated UO₃ sample atmosphere was typically depleted, and no significant pressure rise was observed. Heat treatment of the UO₃xH₂O at 650°C results in conversion to U₃O₈ and eliminates the H₂ production. For all of the U₃O₈ samples loaded in air and irradiated with gamma radiation, a pressure decrease was seen and little, if any, H₂ was produced - even for samples with up to 9 wt% moisture content. Hence, these results demonstrated that the efforts to remove trace moisture from U₃O₈ are not necessary to avoid pressurization of stored uranium oxides caused by gamma-induced radiolysis. In fact, this system can tolerate several percent of sorbed moisture.