Speciation mapping of the oxidation layer on aged uranium dioxide using scanning transmission x-ray spectromicroscopy

• A focused ion beam section was extracted from a UO 2 crystal following accelerated aging and was structurally and chemically characterized. • The UO 2 to U 4 O 9 oxidation reaction was successfully spatially characterized via oxygen K-edge x-ray absorption spectroscopy at the nanometer scale and correlated with TEM results. • The UO 2 – U 4 O 9 interface is sharp (∼200 nm) under the conditions employed. • The formation kinetics of U 4 O 9 were determined by the depth profile of the oxygen speciation. • Interfacial effects need to be accounted for in nuclear forensic characterization of particles. In this study, UO 2 was aged in humid air and prepared as a thin section using a focused ion beam (FIB) instrument. The specimen was measured using synchrotron radiation spectromicroscopy techniques at the Beamline 11.0.2 STXM end station of the Advanced Light Source (ALS). Non-negative matrix factorization (NMF) methods were used to identify and map three component x-ray absorption near-edge structure (XANES) spectra in the oxygen K-edge data, revealing a surface layer of U 4 O 9 with a thickness of 206 ± 21 nm, and the bulk of the sample remaining as UO 2 . Uranium N 4,5 -edge XANES spectromicroscopy supports these results. The diffusion-controlled parabolic rate constant for UO 2 oxidation to U 4 O 9 was calculated from the observed layer thickness and compared to literature values. Complementary transmission electron microscopy (TEM) was used to image the sample and identify the phases present in various regions, confirming the STXM results.