Nuclear Technology / Volume 147 / Number 2 / August 2004 / Pages 258-268
Technical Paper / Nuclear Plant Operations and Control
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.