Gamma radiolysis studies of uranyl fluoride

Abstract

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(O 2)-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.