The exact nature of uranium oxides in spent nuclear fuel is a primary concern in the storage of nuclear fuels resulting from both the volumetric expansion of uranium oxides and change in solubility with an increase in oxygen content. In particular, the growth of hyper–stoichiometric uranium dioxide has been linked to fracturing of spent fuel, resulting in further oxidation and the release of short-lived radionuclides trapped within the UO2 matrix. Oxygen K-edge x-ray absorption spectroscopy (XAS), utilizing a scanning transmission x-ray microscope (STXM) at the Advanced Light Source, is an excellent method to distinguish between different uranium oxide stoichiometries, but specimens must be extremely thin as a result of the short penetration length of soft x-rays. Low burnup pressurized water reactor spent nuclear fuel pins were processed at Idaho National Laboratory utilizing focused ion beam (FIB) sectioning, resulting in specimens ideal for soft x-ray spectroscopy. Measurements of oxygen K-edge near-edge XAS, using the Beamline 11.0.2 STXM have been conducted on several FIB sections of spent nuclear fuel. The FIB sections present challenges in normalization and data analysis. The sensitivity of the oxygen K-edge XAS spectrum between the different uranium oxides has allowed for species-specific mapping of the spent nuclear fuel at the sub-micron scale, showing UO2 as the bulk of the fuel, with U3O7 localized near defects. Additionally, spectral results at the uranium N4,5 edges and the cerium M4,5 edges are analyzed and will be discussed.

Oxygen K-edge spectromicroscopy of uranium oxides in spent nuclear fuel

ACS Fall 2020 Virtual Meeting

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