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

Monoclinic zirconia (m-ZrO$_2$) 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-ZrO$_2$ 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-ZrO$_2$ tetravalently, where its solubility is approximately 1.0 mol%, m-(Am$^{4+}$$_{0.011(7)}$Zr$^{4+}$$_{0.989(7)}$)O$_2$, attributed to the large Am$^{4+}$ cation, where excess Am, that is predominantly trivalent, adopts a C-type (Am$^{4+/3+}$$_{1-x}$Zr$^{4+}$x)$_2$O$_{3+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$_2$.