Extreme multi-valence states in mixed actinide oxides U1-yMyO2±x

In order to ensure the safety of nuclear reactors using oxide fuels, knowledge of the atomic-scale properties of U1−yMyO2±x (with M=Pu, Am) materials is essential. These compounds show complex chemical properties, as actinides may be present in different oxidation states. During irradiation in reactors, UO2±x and various ternary (or higher order) U1−yMyO2±x solid solutions can be encountered, such as fresh fuels (M = Pu), transmutation targets (M = Np or Am) or irradiated fuels (M = fission products, as d-transition metals and lanthanides). These materials are formed by substitution of uranium atoms (U4+) with other cations, which are not necessarily in a tetravalent state. In these U1−yMyO2±x compounds, presenting mostly an ionic character, a direct connection exists between the cationic oxidation state and the oxygen stoichiometry, generally referred to as Oxygen/Metal (O/M) ratio. This is a crucial point for the safety assessment of nuclear fuels as changes in the O/M ratio can dramatically affect their thermal properties, for instance the melting point and the thermal conductivity, which determine their behaviour in reactors and safety margins. ... mehrBy combining XRD, Raman microscopy, X-ray Absorption Spectroscopy (XANES, HERFD-XANES and EXAFS) in-depth study of atomic-scale properties of U1−yMyO2±x compounds can be performed. The first application of this methodology is dedicated to U1−yAmyO2±x on a wide compositional domain (y≤0.70) [1]. We provide evidences of the systematic co-existence of at least three different cations (U4+, U5+ and Am3+), even for stoichiometric compounds (O/M ratio = 2.00). For higher Am contents (y≥0.50), the presence of four cations (U4+, U5+, Am3+ and Am4+) is demonstrated. Nevertheless, the fluorite structure is maintained throughout the whole composition range. Indeed, both XRD and EXAFS results show that the cationic sublattice is unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice. The second application focuses on Pu-rich U1−yPuyO2-x fuels dedicated to fast neutron reactors. In such materials, the accepted convention is a U4+-Pu4+ configuration for stoichiometric (O/M=2.00) and the coexistence of Pu3+, Pu4+ and U4+ for hypo-stoichiometric face centred cubic materials. However, our recent results clearly show a situation more similar to U1−yAmyO2-x. Thanks to the gain in sensitivity provided by HERFD-XANES at actinide M4,5 edges, the systematic presence of U5+/Pu3+, even for stoichiometric samples (O/M ratio = 2.00), is evidenced for Pu content up to 40 mol%. We observe that the relative concentration of U5+ and Pu3+ increases with the Pu content, reaching 10% for Pu/(U+Pu) = 0.40. One should note that the O/M ratios calculated from O/Pu, O/U and O/Am determined by HERFD-XANES are equal to the Raman [5] an XRD derived [6] values. Furthermore, the analysis of the micro-structure (EPMA, µ-Raman) demonstrates that this multi-valence phenomenon cannot be attributed to the slight inhomogeneity of the samples, but to a real charge-compensation mechanism.
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