Abstract
Soft X-ray spectromicroscopy at the O K-edge, U N4,5-edges and Ce M4,5-edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub-micrometer scale. To analyze these data, a modification to non-negative matrix factorization (NMF) was developed, in which the data are no longer required to be non-negative, but the non-negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K-edge to distinguish between two components, one present in the bulk of the sample similar to UO2 and one present at the interface of the sample which is a hyperstoichiometric UO2+x species. The species maps are consistent with a model of a thin layer of UO2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in thespent fuel.
Highlights
Spent nuclear fuel is a complex environment of uranium, plutonium, minor actinides and a host of fission products, combined with stresses and large thermal gradients (Kleykamp, 1985; Olander, 1976; Bruno & Ewing, 2006; Janeczek et al, 1996)
There is ongoing debate over whether uranium oxide can reduce Ce in a spent fuel environment, with several idealized UxCe1–xOy systems studied with X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and density functional theory (DFT) (Griffiths et al, 1994; Eloirdi et al, 2018; Antonio et al, 1996; Tracy et al, 2015; Suresh Kumar et al, 2004; Bera et al, 2009)
These studies show that reduction of Ce in a UO2 matrix is possible, but this depends on factors such as temperature, the exact redox potential in the sample’s environment, and the relative concentrations of Ce and U
Summary
Spent nuclear fuel is a complex environment of uranium, plutonium, minor actinides and a host of fission products, combined with stresses and large thermal gradients (Kleykamp, 1985; Olander, 1976; Bruno & Ewing, 2006; Janeczek et al, 1996). There is ongoing debate over whether uranium oxide can reduce Ce in a spent fuel environment, with several idealized UxCe1–xOy systems studied with X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and density functional theory (DFT) (Griffiths et al, 1994; Eloirdi et al, 2018; Antonio et al, 1996; Tracy et al, 2015; Suresh Kumar et al, 2004; Bera et al, 2009) These studies show that reduction of Ce in a UO2 matrix is possible, but this depends on factors such as temperature, the exact redox potential in the sample’s environment, and the relative concentrations of Ce and U. To the best of our knowledge, prior to this work there have been no direct measurements of the Ce oxidation state in spent fuel
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