Evidence of the negative thermal expansion of the UO2.00 fluorite local structure
Résumé
The thermal properties of uranium dioxide UO2 are of significant importance in view of a safe use of the nuclear energy. Up to now, UO2 is considered to crystallize in a fluorite structure (space group Fm-3m) from room temperature to the melting temperature (3147 ± 20 K), in which both interatomic distances and lattice parameters expand with temperature. Recent in situ synchrotron X-ray diffraction measurements have tackled these literature results as an unusual thermal shrinkage of the U-O distances up to the melting point was recently observed [1]. It was confirmed by neutron pair distribution function results and interpreted as a consequence of the splitting of the U-O distances due to a change in the U local symmetry from Fm-3m to Pa-3 above 300 K [2]. In contrast to these previous investigations, we used an element-specific synchrotron-based spectroscopic method (X-Ray Absorption Spectroscopy) to probe in situ the uranium local environment in UO2.00 sintered pellet sample from 50 to 1265 K under controlled atmosphere. In this whole temperature range, the U sublattice remains locally in the fluorite. Whereas, results collected in Ar-4% H2 atmosphere at 298, 805, 1090 and 1265 K using a dedicated furnace, shown a decrease in the first U-O bond lengths with increasing temperature. The direct determinations of U oxidation state during the measurements show that neither reduction nor oxidation of the UO2.00 sample occurred ensuring that the acquired data really refer to a stoichiometric compound. Furthermore, an increase in the local disorder is observed with increasing temperature what phenomenon was modelled using the Einstein model. These findings are of significant importance in order to understand and predict the thermal behaviour of UO2 nuclear fuel.