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| Hauptseite > Publikationsdatenbank > New insights into the microstructure and redox chemistry of MIMAS MOX model systems supporting assessment of spent fuel behaviour |
| Hauptseite > Publikationsdatenbank > New insights into the microstructure and redox chemistry of MIMAS MOX model systems supporting assessment of spent fuel behaviour |
| Conference Presentation (Other) | FZJ-2026-00368 |
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2025
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Please use a persistent id in citations: doi:10.34734/FZJ-2026-00368
Abstract: As a recycling product of spent UO2 fuel, mixed oxide (MOX) nuclear fuel, which blends UO2 and PuO2 with varying Pu contents, has been often used in nuclear power plants worldwide. While several blending and mixing routes were developed, in Germany, mainly MIMAS MOX has been used for nuclear energy generation (Micronized Master Blend) [1]. Respectively, MIMAS MOX makes up roughly 7% of Germans SNF interim storage. Cross-sectioned MIMAS MOX fuel pellets show typically heterogeneous microstructure, with regions rich or depleted in Pu, separated by so-called interdiffusion zones with gradients in Pu content [2]. Understanding their chemical properties, in particular the local chemistry and redox states of these regions is critical to ensure safe management and eventual disposal of MOX when occurring as spent nuclear fuel (SNF). However, a paucity of information remains regarding these phenomena in MIMAS based fresh and spent MOX owing to challenges investigating the material, requiring strategic methods through accurate, bench marked model system approaches. In this presentation, the industrial MIMAS process has been downscaled to the laboratory level for a Ce surrogate method and bench marked against real Pu-bearing MIMAS MOX. The aim is to generate model systems that are reminiscent of the microstructure of MOX in a simplified way for single-effect study implementation and understanding their redox behaviour. Emphasis has been placed on understanding the influence of the feed UO2 powder origin on the microstructure and chemistry of the final MIMAS MOX ceramic, namely via the conversion of either ammonia diuranate (ADU) or ammonia uranyl carbonate (AUC). Despite using identical processing methods, the synthesized MIMAS MOX model systems were found to exhibit subtle but significantly different structural and redox behaviour as shown by high resolution powder synchrotron X-ray diffraction (S-PXRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD)and high-energy-resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) measurements. In particular, the results indicate variability in both U and Ce redox depending on processing route that should be reflected in genuine Pu-based MOX. These results are discussed and benchmarked against actual Pu based MIMAS MOX that is jointly examined via microstructural examinations and XRD analysis. This presentation will discuss both the techniques and challenge of producing representative MOX based ceramic materials supporting safety assessments for MOX SNF disposal.[1] D. Haas, A. Vandergheynst, J. van Vliet, R. Lorenzelli, J.-L. Nigon, Nuclear Technology 1994, 106, 60.[2] R. Delville, M. Verwerft, Microscopy and Microanalysis 2023, 29, 78.
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