Insights into the Structural and Redox Chemistry of Cr-doped (Ln,U)O₂ Materials

Cr-doped UO₂ nuclear fuels have endured sustained interest from industry and researchers due to the superior in-reactor performance they possess over traditional non-doped variants (UO₂). The ubiquitous generation of spent nuclear fuel (SNF) is a challenge that impacts many nations, regardless of fuel type. Direct disposal of SNF into deep geological repositories is the preferred method for nations including German and Finland among many others but requires long-term understanding of the SNF stability that is often ensured for 1 000 000 years according to StandAG. Consequently, researching the structural and chemical stability of SNF is crucial aspect of nuclear waste management. Despite high, increased in-reactor performance and reduced in bulk waste generation of Cr-doped UO₂ fuels, their chemistry and structural properties within SNF are still poorly studied. Specifically potential interaction of Cr with trivalent cations of lanthanides (Ln) and minor actinides (MA) (e.g., Nd³⁺, Gd³⁺, Am³⁺, Cm³⁺) and Pu⁺³ may lead to the formation of alteration phases such as perovskite ((Ln³⁺/MA³⁺)Cr³⁺O₃) which may impact stability both in-reactor and as SNF. Accordingly, we have been exploring the formation of such phases via the preparation of model system materials, under conditions that are relevant to Cr-doped UO₂. This involves the systematic synthesis of Cr-doped ((U_1-xLn_x)O₂) for Ln = La, Ce, Pr, Gd, Ho, Lu. These materials have been subject to high resolution structural and redox analysis using synchrotron X-ray powder diffraction (S-PXRD) and high-energy resolution fluorescence diffraction-X-ray absorption near-edge structure spectroscopy measurements (HERFD-XANES) performed at BM20 beamline of the European Synchrotron Radiation Facility (ESRF). These measurements, supported by Raman spectroscopy mapping and scanning electron microscopy, allow the occurrence of specific alteration phases to be observed in Cr-doped UO₂ when encountering lanthanides. These results will be discussed in detail with respect to the current knowledge of Cr-doped UO₂ and classical UO₂ chemistry in the context of nuclear waste management. In summary the findings might potentially have important implications for the long-term performance of nuclear waste repositories and the design of new materials for nuclear applications