Abstract
To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1−yMyO2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U1−yAmyO2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the co-existence of four different cations (U4+, U5+, Am3+, Am4+) in U1−yMyO2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice.
Highlights
To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1−yMyO2±x materials is essential
The (U,Am)O2±x solid solution constitutes a special case, exhibiting unique features in comparison to the other actinide dioxides: XAS measurements reported on oxygenstoichiometric (O/M = 2.0) U1−yAmyO2 with y ≤ 0.2 have shown that americium is purely trivalent, while uranium is partially oxidised to the pentavalent state, with very close Am3+ and U5+ contents[4,5]
Good agreement between data and fits were obtained performing Rietveld refinements with a single fluoritephase. These results corroborate the existence of the (U,Am) O2±x solid solution for americium contents above 50 mol.%15,16
Summary
To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1−yMyO2±x materials is essential. In oxide fuel-based reactors, UO2±x as well as various ternary (or higher order) U1−yMyO2±x solid solutions can be encountered, as fresh fuel (M = Pu), transmutation targets (M = Np or Am) or irradiated fuel (M = fission products, as d-transition metals and lanthanides) These materials originate by substitution of uranium atoms (U4+) with other cations which are not necessarily in a tetravalent state. In these U1−yMyO2±x compounds, which present mostly ionic character, a direct connection exists between the cationic oxidation state and the oxygen stoichiometry, generally indicated with the Oxygen/Metal ratio (O/M ratio) This is a crucial point to understand for the safety assessment of nuclear fuels because their thermal properties, as for instance the melting point and the thermal conductivity, which determine their behaviour in reactors and safety margins, are dramatically affected by changes in the O/M ratio.
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