Utilization of dual-column focused ion beam and scanning electron microscope for three dimensional characterization of high burn-up mixed oxide fuel

Chemical and elemental mapping of spent nuclear fuel sections by soft X-ray spectromicroscopy

Soft X-ray spectromicroscopy at the O K-edge, U N_4,5-edges and Ce M_4,5-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 UO_2+x species. The species maps are consistent with a model of a thin layer of UO_2+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 the spent fuel.

Best practices for preparing radioactive specimens for EBSD analysis

A wide variety of specimen preparation techniques are available for ensuring that specimen surface finish has the acceptable quality for electron backscatter diffraction (EBSD) analysis. These techniques include but are not limited to vibratory polishing, broad, and focused ion beam milling. They have been widely implemented in the field of nuclear materials science with a varying degree of success. However, a systematic investigation of the effectiveness of each technique for preparation of highly radioactive specimens has not been conducted to date but would be beneficial during selection of the specimen preparation methodology. Multiple preparation techniques have been evaluated in this contribution with the final goal of determining the most effective technique for preparing radioactive specimens for EBSD analysis. This paper discusses the advantages and disadvantages of each technique and recommends best practices for preparing radioactive specimens for surface-based analysis techniques.