Structural and chemical insights on the incorporation of americium into zircaloy-derived monoclinic zirconia

Monoclinic zirconia (m-ZrO₂) forms on the internal surface of nuclear fuel Zircaloy cladding, acting as a critical barrier against radionuclide release at the fuel-cladding interface. However, the incorporation of minor actinide elements like americium in m-ZrO₂ and resultant structural chemistry remains poorly understood. Using a combination of diffraction and high-resolution X-ray spectroscopic techniques, we have examined m-ZrO2 with 5 mol% Am doping. We show Am enters m-ZrO₂ tetravalently, where its solubility is approximately 1.0 mol%, m-(Am⁴⁺0.011(7)$Zr$⁴⁺0.989(7))O₂, attributed to the large Am⁴⁺ cation, where excess Am, that is predominantly trivalent, adopts a C-type (Am4+/3+₁₋ₓ$Zr$⁴⁺$x)$_2O₃₊ₓ$ phase in space group Ia-3. The known reversible high temperature phase transformation of m-ZrO$_2$ to tetragonal is further shown to be reduced from 1150 oC to 1050 oC via Am$⁴⁺$ incorporation. The investigation provides critical insight into the chemical reactivity and speciation of minor actinide elements with nuclear fuel cladding related m-ZrO$_2$.