Local structure and charge distribution in mixed uranium-americium oxides: effects of oxygen potential and Am content

Partitioning and Transmutation (P&T) of minor actinides (MA) is currently studied to reduce the nuclear waste inventory. In this context, the fabrication of MA bearing materials is of great interest to achieve an effective recycling of these highly radioactive elements. In order to ensure the inpile behavior, these fuels have to respect several criteria including preservation of the fluorite structure and defined oxygen to metal ratio (O/M). In the case of Am bearing materials, such as U1-yAmyO2±x (y=0.10, 0.15, 0.20), the O/M determination is quite challenging using conventional methods (TGA, XRD) due to the particular Am thermodynamic properties. Despite the lack of experimental data in the U-Am-O system, thermodynamical modeling are currently developed to effectively assess the O/M ratio. In this work, the O/M ratios were calculated for various oxygen potentials using the cation molar fraction determined by XAS measurements. These results are an important addition to the experimental data available for the U-Am-O system. Moreover, XRD and XAS indicated that the fabrication of fluorite U1-yAmyO2±x solid solution can be achieved for all Am content and oxygen potentials investigated. Based on the molar fraction, a description of the solid solution was proposed depending on the considered sintering conditions. Finally, the occurrence of an expected charge compensation mechanism between Am(+III) and U(+V) was pointed out.