The role of redox and structure on grain growth in Mn-doped UO2

Abstract Mn-doped UO 2 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 grain growth apparently arising from misunderstandings of its redox-structural chemistry. To resolve this, we use synchrotron X-ray diffraction and spectroscopy measurements supported by ab initio calculations to cross-examine the redox and structural chemistry of Mn-doped UO 2 single crystal grains and ceramic specimens. Measurements reveal Mn enters the UO 2 matrix divalently as $$({{{Mn}}}_{x}^{+2}{{U}}_{1-x}^{+4}){{O}_{2-x}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> <mml:msubsup> <mml:mrow> <mml:mi>U</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>O</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:math> with the additional formation of fluorite Mn +2 O in the bulk material. Extended X-ray absorption near edge structure measurements unveil that during sintering, the isostructural relationship between fluorite UO 2 and Mn +2 O results in inadvertent interaction and subsequent incorporation of diffusing U species within MnO, rather than neighbouring UO 2 grains, inhibiting grain growth. The investigation consequently highlights the significance of considering total redox-structural chemistry of main and minor phases in advanced ceramic material design.