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X-ray spectroscopy unravels the chemical complexity of Cr-doped UO2 nuclear fuels

Cr-doped UO2 is an advanced nuclear fuel, yet questions remained concerning its complex chemistry. X-ray absorption spectroscopy measurements of Cr-doped UO2 single crystals resolved the Cr chemistry in UO2, creating key knowledge for the safe deployment of Cr-doped UO2 nuclear fuels.

To assist in removing the global reliance on fossil fuels, the nuclear power industry has turned to advanced nuclear fuel blends, which offer increased in-reactor efficiencies, improved safety performance and reduced waste accumulation. Cr-doped UO2, involving the addition of ~700 1000 ppm Cr, is one such popular variety due to its superior fuel performance over traditional fuel forms. It has garnered significant interest from industry and researchers alike since the 1980s, yet surprisingly, consensus is yet to be reached on the Cr chemistry in the fuel structure, including the oxidation state and lattice incorporation mechanism of Cr into UO2.

The challenge arises from the complexity of Cr states that are adopted within the bulk fuel structure during fuel preparation. This includes metallic (Cr0), oxide (Cr+32O3) and eutectic (Cr+2, Cr+32O3, Cr+2,+33O4) compositions that occur across grain boundary and precipitate regions of the fuel, in addition to lattice-incorporated Cr (Cr,UO2).

Figure 112 illustrates this chemical complexity. Measuring such a complex system and trying to identify the lattice- incorporated Cr state inevitably involves convoluted measurements. Reliably discerning one Cr chemical environment from another is a challenging task and one that has remained elusive for nuclear material scientists.

To overcome the challenge of convoluted measurement of bulk Cr-doped UO2, single-crystal grains were carefully extracted mechanically and measured, along with the bulk mother material, using high-energy-resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy at beamline BM20. The collected HERFD-XANES spectra displayed significant differences in the line shape between the single crystal and bulk material (Figure 113), reflecting the starkly different number and types of chemical environments possessed in the two related but different specimen types.

The HERFD-XANES and EXAFS analysis of the Cr- doped UO2 single crystals, which were supported by electron paramagnetic resonance (EPR) measurements, revealed that the Cr chemical state within the UO2 fuel lattice structure was singular and described by (Cr+3xU+41-x)O2-0.5x, contradictory to previous studies. In contrast to the Cr-doped UO2 single crystals, the bulk material was found to contain a plethora of Cr states, including metallic Cr (Cr0), oxide-related Cr+2 and Cr+32O3, attributed to grain boundary species and precipitates,

Fig. 112: a) Graphical representation of the Cr-U-O complex chemical environments in fresh Cr-doped UO2 nuclear fuel determined from this investigation. b) Scanning electron microscope backscattered electron (SEM-BSE) images of bulk Cr-doped UO2 and a single crystal grain, highlighting the morphology, and energy-dispersive X-ray spectroscopy (EDS) Cr K-edge maps illustrating the plethora

of secondary Cr phases present and absent in the bulk and single crystal grain respectively. b) is adapted from the original article (Murphy et al.) under the Creative Commons Attribution 4.0 International License.