Lionel Desgranges1 Yue Ma1 Philippe Garcia1 Gianguido Baldinozzi2 David Simeone1 Henry Fischer3

1, CEA, Saint-Pal lez Durance, , France
2, Centre National de la Recherche Scientifique (CNRS), Saclay, , France
3, Institut Laue-Langevin, Grenoble, , France

In the last years a great effort was devoted to the modeling of uranium dioxide, starting from the atomic scale up to the fuel rod, with the aim of predicting the behavior of nuclear fuel and improving its safety [[i],[ii]]. All the modelling work has been performed with the hypothesis that UO2 has Fm-3m, fluorite type, crystalline structure, but the existence of a lower symmetry local order in UO2 was recently demonstrated [[iii]] and the consequences of this finding are still to be evaluated. Here we prove using neutron diffraction that the crystalline structure of UO2 at 1000°C can be described as nanodomains having Pa-3 symmetry separated by domain walls, having a specific symmetry. This hypothesis of a nanodomain structure dramatically changes the interpretation of UO2 structural changes as a function of temperature, which was previously based on local distortion due to polarons or point defects. These two interpretations are discussed.

[i] Linking atomic and mesoscopic scales for the modelling of the transport properties of uranium dioxide under irradiation
By: Bertolus, Marjorie; Freyss, Michel; Dorado, Boris; et al.
JOURNAL OF NUCLEAR MATERIALS Volume: 462 Pages: 475-495 Published: JUL 2015

[ii] Predicting material release during a nuclear reactor accident
By: Konings, Rudy J. M.; Wiss, Thierry; Benes, Ondrej
NATURE MATERIALS Volume: 14 Issue: 3 Pages: 247-252 Published: MAR 2015

[iii] What Is the Actual Local Crystalline Structure of Uranium Dioxide, UO2? A New Perspective for the Most Used Nuclear Fuel
By: Desgranges, L.; Ma, Y.; Garcia, Ph; et al.
INORGANIC CHEMISTRY Volume: 56 Issue: 1 Pages: 321-326 Published: JAN 2 2017