title: Average structure and local configuration of excess oxygen in UO(2+x).
abstract: Determination of the local configuration of interacting defects in a crystalline, periodic solid is problematic because defects typically do not have a long-range periodicity. Uranium dioxide, the primary fuel for fission reactors, exists in hyperstoichiometric form, UO(2+x). Those excess oxygen atoms occur as interstitial defects, and these defects are not random but rather partially ordered. The widely-accepted model to date, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO(2+x) involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO(2+x).
authors.affiliation: 1] Department of Geology and Geophysics, Center for Computation and Technology, Louisiana State University, Baton Rouge, Louisiana 70803-0001, USA [2] Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA.
authors.forename: Jianwei
authors.initials: J
authors.lastname: Wang
authors.affiliation: 1] Department of Geological and Environmental Sciences, Stanford University Stanford, California 94305-2115, USA [2] Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA.
authors.forename: Rodney C
authors.initials: RC
authors.lastname: Ewing
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA.
authors.forename: Udo
authors.initials: U
authors.lastname: Becker