Gamma radiolysis studies of uranyl fluoride

Abstract

Gamma radiolysis / Uranyl fluoride / Uranium reduction / Uranium oxideSummary. 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 UO 2 F 2 ·xH 2 O and U 3 O 8 (with ∼ 1.4 wt. % fluorine content) were irradiated in a 60 Co 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 UO 2 F 2 ·xH 2 O produced O 2 -with G(O 2 )-values ranging from 0.007 to 0.03 molecules of O 2 produced per 100 eV. Neither F 2 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 UO 2 F 2 ·xH 2 O was demonstrated by using the HFIR SNF elements, and the limit was found to be 7%-9% (at ∼ 10 8 rad/h). It is shown that the covalently bonded oxygen is more susceptible to radiation damage than is the ionically bonded fluorine. Irradiation of U 3 O 8 (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 U 3 O 8 is safe from overpressurization and the production of corrosives caused by gamma radiolysis of residual fluorides.

Alpha Radiolysis of Sorbed Water on Uranium Oxides and Uranium Oxyfluorides

“…Neither fluorine nor HF was seen in the gas analyses. The results for the alpha irradiation of UO 2 F 2 are in sharp contrast to those seen for gamma irradiation, 3,5 where oxygen was found to be released from the UO 2 F 2 matrix. Therefore, the type of radiation (alpha vs gamma) appears to play a significant role in the net radiolytic effect.…”

“…Experimental studies have been conducted on the gamma radiolysis of sorbed water on uranium oxides and on fluoride impurities in these oxides (primarily on uranium oxyfluorides). [3][4][5] These experiments have demonstrated that, with respect to gamma radiation, these materials can be safely stored. To complete the understanding of radiolysis in a 233 U oxide system, a similar set of experiments have been pursued to explore the effects of alpha radiolysis.…”

“…The fact that neither HF nor fluorine was found is significant because these corrosive species were a concern for the long-term storage of uranium oxides that contain fluoride impurities. 3,5 Finally, the hydrogen production for each of the wetted samples is shown in Table 3.9 by estimating the hydrogen production as a mole percentage of the available water. This table shows that only a very small amount of the water was radiolyzed to form hydrogen.…”

“…Note that production of these corrosive species was a concern for the long-term storage of uranium oxides containing fluoride impurities. 3,5 The fact that none were found further demonstrates that these concerns were unwarranted.…”

“…3, the source of some of the hydrogen for moist UO 2 F 2 was corrosion of the sample container. In a similar fashion, corrosion may also play some role in the production of hydrogen in the alpha radiolysis experiments.…”