Atomic scale Monte-Carlo simulations of neutron diffraction experiments on stoichiometric uranium dioxide up to 1664 K

Abstract

The neutron transport in nuclear fuels depends on the crystalline structure of materials when neutron energies lie below a few eV. For that purpose, the theoretical formalism that describes the neutron elastic and inelastic scatterings by crystals has been implemented in the CINEL processing tool in order to provide temperature-dependent neutron cross sections usable by the Monte-Carlo code TRIPOLI4®. The performances of the Monte-Carlo calculations are illustrated with the analysis of neutron powder diffraction data on UO2 measured up to 1664 K with the D4 and D20 diffractometers of the Institute Laue–Langevin (Grenoble, France). The comparison of the experimental and simulated pair distribution functions confirms the unusual decrease of the U–O atomic distances with increasing temperature when an ideal fluorite structure (Fm3̄m space group) with harmonic atomic vibrations is assumed over the full temperature range. The flexibility of the CINEL code allowed to explore disorder or anharmonic oxygen vibrations in the Fm3̄m space group by using either a four-site model with a relaxation term or a structure factor equation with a non-zero anharmonic third-cumulant coefficient. As none of these models succeeded to improve the agreement with the experiments, recent works that propose other local crystalline symmetries for UO2 at elevated temperatures were investigated with the CINEL code. The case of the Pa3̄ symmetry is briefly discussed in this paper.