The study of the UO(2+x) crystalline structure with high-resolution neutron diffraction experiment

Abstract : The basic properties of UO2 are essential to understanding and predicting the in-reactor behavior of fuels. In particular, UO2 oxidation has been studied for many years in relation to both fuel fabrication and spent fuel storage issues. Phase transition properties of oxide fuels are fundamentally controlled by the nature and behavior of the atomic-scale defects on the anion sublattice. In this study we aim to characterize the local crystalline structure incorporated with oxygen rearrangement has occurred in the UO(2+x) (0< x <0.25) in order to better describe the formation of defects and atomic thermal motionas a function of temperature. Neutron diffraction is one of the rare analytical techniques which enables the bulk properties of the anion sublattice to be studied. We present here a neutron diffraction experiment in ILL Grenoble on several samples with different deviations-from-stoichiometry in a temperature range covering the phase transition between the two-phase (UO(2+x) and U4 O9) and single-phase (UO(2+x)) zones. The PDF (Partial Distribution Function) analysis and the Rietvield refinement on both UO(2+x) and UO2 fresh fuel specimens will also be performed to understand how and to what extent the temperatures distort the long-ranging ordering of the dispersed oxygen defects in the hyper-stoichiometric uranium dioxide. What is more, we focus particularly on the dependence upon temperature and deviation-from-stoichiometry of lattice parameter and the Debye-Waller factors of both uranium and oxygen sublattice between 296K and 1273K. A classic Debye behavior has been observed in UO2 which compares favorably to the previous research.