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

Abstract 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- ZrO₂ and resultant structural chemistry remains poorly understood. Using a combination of diffraction and high-resolution X-ray spectroscopic techniques, we have examined m- ZrO 2 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 o C to 1050 o C 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- ZrO₂.