Evidence of the negative thermal expansion of the UO2.00 fluorite local structure


Evidence of the negative thermal expansion of the UO2.00 fluorite local structure

Martin, P. M.; Prieur, D.; Epiphano, E.; Dardenne, K.; Rothe, J.; Hennig, C.; Scheinost, A. C.; Neuville, D.

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 as a fluorite structure (Fm-3m) from room temperature to the melting point (3147 ± 20 K), in which both interatomic distances and lattice parameters expands with the temperature. This view was challenged by more recent in situ synchrotron X-ray diffraction measurements, showing an unusual thermal shrinking of the U-O distances up to the melting point.
It was later confirmed later 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.
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 K to 1265 K. In the whole range, the U sublattice remains locally of the fluorite type. Whereas, results collected in Ar-4% H2 atmosphere at 298, 805, 1090 and 1265 K using a dedicated furnace, shown a decrease of the first U-O bond lengths with increasing temperature. The direct determinations of U oxidation state during the measurements clearly show that neither reduction nor reduction of the UO2.00 sample occurred ensuring that the work was really done on a stoichiometric compound. Furthermore, an increase of the disorder is observed with increasing temperature which we modelled using the Einstein model. These findings are of significant importance in order to understand and predict the thermal behaviour of nuclear fuel.

Keywords: thermal contraction; UO2; XAFS

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Publ.-Id: 27712