Accommodation of multivalent cations in fluorite-type solid solutions: Case of Am-bearing UO2

The radiotoxicity of spent nuclear fuel is mainly due to the content of minor actinides, which could be substantially reduced by Partitioning and Transmutation. A possible transmutation method would be to employ americium-bearing uranium oxide materials as blanket fuels in fast neutron reactors. In order to maintain fuel performance and reactor safety, it is mandatory to control the structural homogeneity and oxygen stoichiometry during the sintering process. In this work, U0.85Am0.15O2−x materials, fabricated by a solid state chemistry process, were sintered at 2023 K under three oxygen potentials, i.e. −375, −350 and −325 kJ mol−1, thereby significantly extending the range of a previous study. By coupling X-ray diffraction and X-ray absorption spectroscopy measurements, it was shown that fluorite solid solutions are obtained whatever the sintering conditions. The presence of U(+V), pointed out in a previous work for oxygen potentials equal to −520 and −450 kJ mol−1, was confirmed. This result constitutes an experimental proof of the existence of U(+V) in An-doped UO2 fluorite-type structure materials. These experimental data are major results in view of the developing thermodynamical model of the U–Am–O system. Considering the now available extended range, the effect of the oxygen potential is discussed in terms of charge distribution and local structure.

Graphical abstractThe presence of U(+V) in An-doped UO2 solid solution was demonstrated using XAS. This study was conducted in a wide range of oxygen potential, allowing to discuss the effect of this parameter and the accommodation in the fluorite structure. The presence of U(+V) in these materials is a major result which has to be consider for the development of thermodynamical modeling in the system U–Am–O, and ultimately for the fabrication of these materials in view of the spent fuel recycling.Figure optionsDownload full-size imageDownload as PowerPoint slide