Oxidation and hydration of U3O8 materials following controlled exposure to temperature and humidity.

Chemical signatures correlated with uranium oxide processing are of interest to forensic science for inferring sample provenance. Identification of temporal changes in chemical structures of process uranium materials as a function of controlled temperatures and relative humidities may provide additional information regarding sample history. In this study, a high-purity α-U3O8 sample and three other uranium oxide samples synthesized from reaction routes used in nuclear conversion processes were stored under controlled conditions over 2-3.5 years, and powder X-ray diffraction analysis and X-ray absorption spectroscopy were employed to characterize chemical speciation. Signatures measured from the α-U3O8 sample indicated that the material oxidized and hydrated after storage under high humidity conditions over time. Impurities, such as uranyl fluoride or schoepites, were initially detectable in the other uranium oxide samples. After storage under controlled conditions, the analyses of the samples revealed oxidation over time, although the signature of the uranyl fluoride impurity diminished. The presence of schoepite phases in older uranium oxide material is likely indicative of storage under high humidity and should be taken into account for assessing sample history. The absence of a signature from a chemical impurity, such as uranyl fluoride hydrate, in an older material may not preclude its presence at the initial time of production. LA-UR-15-21495.