Gamma radiolysis studies of uranyl fluoride

Summary

The safe handling and storage of radioactive materials require an understanding of the effects of radiolysis on those materials. Radiolysis may result in the production of gases (e.g., corrosives) or pressures that are deleterious to storage containers. A study has been performed to address these concerns as they relate to the radiolysis of residual fluoride compounds in uranium oxides. Samples of UO₂F₂·xH₂O and U₃O₈ (with ∼1.4 wt.% fluorine content) were irradiated in a ⁶⁰Co source and in spent nuclear fuel (SNF) elements from the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory. Container pressures were monitored throughout the irradiations, and gas and solid samples were analyzed after the irradiations. The irradiation of UO₂F₂·xH₂O produced O₂ – with G(O₂)-values ranging from 0.007 to 0.03 molecules of O₂ produced per 100 eV. Neither F₂ nor HF was produced by the irradiations. Chemical analyses of solid samples showed that some of the uranium was reduced from U(VI) to U(IV). A saturation damage limit for the UO₂F₂·xH₂O was demonstrated by using the HFIR SNF elements, and the limit was found to be 7–9% at ∼10⁸ rad/h). It is shown that the covalently bonded oxygen is more susceptible to radiation damage than is the ionically bonded fluorine. Irradiation of U₃O₈ (with ∼1.4 wt.% fluorine content) resulted in neither gas production nor a pressure increase. These experiments led to the conclusion that during long-term storage U₃O₈ is safe from overpressurization and the production of corrosives caused by gamma radiolysis of residual fluorides.