Study of the correlation between fuel engineering properties and point defects
Résumé
Uranium dioxide is widely used as the fuel material in current civil nuclear reactors. The basic engineering properties of uranium oxide are essential to understand and predict the behavior of fuels under all conditions. The Uranium-Oxygen binary phase diagram shows the broad range of oxygen concentrations over which the UO2±x solid solution exists. In the hyperstoichiometric region, oxygen defects form clusters the behavior of which determines to a large extent a number of very important engineering properties such as ion diffusivity, electrical conductivity, creep, etc.
Previous electrical conductivity experiments and point defect modelling of UO2+x material indicate that oxygen interstitial defects may start to form clusters at a low deviation-from-stoichiometry. In this study we aim to characterize the local crystalline structure that develops and the oxygen rearrangement that occurs in UO2+x (0 < x< 0.25) in order to improve our description of the nature of oxygen defects, and eventually, correlate atomic rearrangements to fuel properties. Neutron diffraction is one of the rare analytical techniques which enables the bulk properties of the anion sub-lattice to be studied.
We present here an analysis of a neutron diffraction experiment carried out at ILL Grenoble on UO2 and UO2+x (x≈0.03) samples in a temperature range covering the phase transition between the two-phase (UO2+x and U4O9) and single-phase (UO2+x) domains. The Pair Distribution Function (PDF) analysis and the Rietveld refinement on both UO2+x and stoichiometric UO2 fuel specimens were applied to try to identify the local arrangement of atoms, i.e. the predominant defects and to understand how the temperature distorts the original fluorite structure. Data refinement for a UO2+x sample shows that di-interstitial defect clusters are consistent with the average defect structure of UO2.03 at high temperature in the homogenous single-phased domain. This conclusion is also consistent with the only other available neutron diffraction experiment data reported at higher deviations-from-stoichiometry..