Oxidation and Hydration of U₃O₈ Materials Following Controlled Exposure to Temperature and Humidity

Abstract

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.

Morphology of U3O8 materials following storage under controlled conditions of temperature and relative humidity

“…The preparation of a-U 3 O 8 (S1) is described elsewhere [2,16]. The phase purity was confirmed by p-XRD analysis.…”

“…Temporal changes in the morphology of uranium oxide materials could provide insight into process and/or storage history [1][2][3][4][5]. Process variables inherent in the synthesis of nuclear materials are known to influence the morphology of uranium oxide products [4,[6][7][8][9][10][11][12][13][14][15].…”

“…In particular, comparison of the morphology of precipitates with those of subsequent calcination products suggests that microstructures of the precursor precipitates are retained through heat treatment [6][7][8][13][14][15][16]. A number of studies have shown that U 3 O 8 , a common intermediate in the production of nuclear material, generates schoepite phases (UO 3 ÁxH 2 O) during storage under higher relative humidities presumably following oxidation and/or hydration [2,[17][18][19]. It would be valuable to understand the variables contributing to temporally-induced morphology changes, and if any changes occur, the prospect for using the temporal changes in morphology that arise during storage under different conditions for forensic analyses.…”

“…Sub-samples of the materials were stored under controlled temperature and relative humidity conditions for 2 years. The chemical speciation of these materials and any chemical changes in the materials following storage were detailed previously [2]. The morphology of the materials are characterized using a lexicon of descriptors for assessing any temporal changes [20].…”

“…The morphology of the materials are characterized using a lexicon of descriptors for assessing any temporal changes [20]. These results are compared with powder X-ray diffraction (p-XRD) patterns and Extended X-ray Absorption Fine Structure (EXAFS) spectra [2]. …”