Elucidating the Roles of Redox and Structure in the Grain Growth of Mn-doped UO2

Abstract Mn-doped UO₂ is considered a potential advanced nuclear fuel due to ameliorated microstructural grain growth compared to non-doped variants. However, recent experimental investigations have highlighted limitations in growth mechanisms, apparently arising from misunderstandings of its redox-structural chemistry. To resolve this disparity, this investigation has used a combination of synchrotron X-ray diffraction and spectroscopy measurements supported by ab initio calculations to cross-examine the redox and structure chemistry of Mn-doped UO₂ single crystal grains and ceramic specimens. Mn was found to enter the UO₂ matrix divalently with additional formation of fluorite Mn⁺ ²O in the bulk material. Extended X-ray absorption near edge structure measurements indicated that due to the isostructural fluorite relationship between UO₂ and Mn⁺ ²O, grain enhancing diffusing U species, that should migrate to neighbour UO₂ grains during sintering, are inadvertently incorporating within MnO, inhibiting grain growth. The investigation consequently highlights the significance of considering total redox and structural chemistry of main and minor phases in the design of advanced materials.