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.