Probing the Defect Structure in Single-Phase UO_2+x Systems

Oxidation of uranium dioxide (UO₂) nuclear fuel occurs under normal operation and is most evident at high burnup levels or during accident conditions. The excess oxygen is incorporated into the fluorite structure and the resulting atomic-scale defect configuration significantly influences important bulk properties such as thermal conductivity and fission gas release. Previous experimental and modelling efforts have proposed distinct oxygen defect cluster configurations; however, most characterization techniques lack sensitivity to the local atomic structure or the oxygen sublattice and the resulting data cannot be used to validate predicted defect clusters. Here, we present results on single-phase UO_2+<i>x</i> systems (<i>x</i> = 0.07 and 0.15) combining advanced experimental and modelling techniques to create high fidelity atomistic models of the oxygen defect clusters. <i>In situ</i> high-temperature neutron total scattering measurements with high sensitivity to the oxygen sublattice were performed at the Nanoscale-Ordered Materials Diffractometer (NOMAD) instrument at the Spallation Neutron Source (Oak Ridge National Laboratory). The data acquired at 600 °C and 1000 °C were analyzed <i>via</i> Reverse Monte Carlo modelling techniques which consider both the long- and short-range structures. The analysis reveals evolving behavior as a function of oxygen content with simple clusters in the low O:M regime (UO_2.07) and more complex, extended defects for higher oxygen concentrations (UO_2.15). Our findings have implications in improving and validating potentials for Molecular Dynamics simulations to advance larger fuel performance codes.