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)Zr⁴⁺_0.989(7))O₂, attributed to the large Am⁴⁺ cation, where excess Am, that is predominantly trivalent, adopts a C-type (Am^4+/3+_1-xZr⁴⁺_x)₂O_3+x phase in space group Ia-3. The known reversible high temperature phase transformation of m-ZrO₂ to tetragonal is further shown to be reduced from 1150 ^oC to 1050 ^oC via Am⁴⁺ incorporation. The investigation provides critical insight into the chemical reactivity and speciation of minor actinide elements with nuclear fuel cladding related m-ZrO₂.