The nature of uranium oxidation is a critically important process across a variety of fields. It is a key component to understanding the nuclear fuel cycle, the fate and transport of uranium in the environment, and important signatures for nuclear forensics investigations. The oxidation of uranium dioxide is a complicated process, depending on factors including humidity, temperature and microstructure. The reactions of UO₂ with air and water vapor during storage and handling can provide useful insights into the recent history of UO₂materials; these reactions tend to form a layer of higher oxides at the sample surface, and the thickness, morphology, and composition of this layer are all promising potential signatures for recent storage conditions. At room temperature, the humidity of the environment is more important to oxidation than the oxygen content of the atmosphere, so understanding uranium oxidation can offer important insights into the provenance of a specimen.

The scanning transmission x-ray microscope (STXM) at Advanced Light Source (ALS) Beamline 11.0.2 is an instrument capable of chemical speciation mapping to the 25 nm scale, whereas the microX-ray fluorescence (XRF) Beamline 10.3.2 at the ALS can conduct chemical speciation and XRF with 2 um spatial resolution. UO₂ powder was allowed to age and agglomerate at 98% relative humidity for 378 days, then a focused ion beam (FIB) was used to section a thin slice of this agglomeration for study at the ALS STXM. Results at the oxygen K-edge and uranium N₅-edge identify a bulk, UO₂ phase and an interfacial, schoepite phase, located in distinct regions of the sample. Similarly, a UO₂ wafer was exposed to 20% oxygen for ~0.25 yr at 145 °C and a FIB section was extracted, yielding U₄O₉ and UO₂ phases via STXM. A modified non-negative matrix factorization (NMF) method was used to separate out the spectral components and identify the phases present. Use of the STXM to identify the uranium phases, particularly when coupled with FIB sectioning and NMF analysis is a promising method for the evaluation of spectromicroscopy result.

Scott Donald

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