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Investigating chemical and molecular changes in uranium oxyfluoride particles using NanoSIMS and Micro-Raman Spectroscopy

Abstract: 
Processes involving the treatment of nuclear material inherently release small amounts of this material to the environment. Environmental sampling is therefore an important tool in the detection of undeclared nuclear activities. Environmental samples taken at enrichment facilities typically contain particles of uranium oxyfluoride (UO2F2) formed from the hydrolysis of UF6. Somewhat surprisingly, these samples were also found to contain uranium-bearing particles without a measurable amount of fluorine, suggesting UO2F2 is unstable with respect to the loss of fluorine. As environmental sampling depends upon laboratory analysis of nuclear material that has often been exposed to the environment after it was produced, it is important to understand how those environmental conditions might have changed the material over time. Previous studies have shown that exposure to high temperature, high relative humidity and ultraviolet light accelerates the loss of fluorine in UO2F2 particles, yet the conditions under which this occurs and the chemical and molecular changes that result from this decomposition are not well understood. This work aims to provide a better understanding of the chemical and molecular changes in UO2F2 particles from the exposure to specific environmental conditions, and in particular from the exposure to air of different relative humidity. To this end, UO2F2 particulate material was prepared from the controlled hydrolysis of UF6 expressly for the purpose of these experiments. Particles were measured by ultrahigh spatial resolution secondary ion mass spectrometry (NanoSIMS) and micro-Raman spectroscopy before and after exposure to air at different relative humidity. These measurements demonstrated that even though the decomposition of UO2F2 is very slow, NanoSIMS and Raman spectroscopy can be applied to distinguish subtle differences depending on the environment to which the samples were exposed. The experiments described in this paper also demonstrated that a combination of analytical techniques is the best approach to characterize UO2F2 particles produced from UF6 hydrolysis.