Temperature-induced atomic-structure modifications in UO$_{2}$ and UO$_{2+x}$

Abstract

UO$_{2}$ and UO$_{2.07}$ were characterized from 25-1000$\,^{\circ}\mathrm{C}$ using neutron total scattering in order to evaluate effects of temperature and phase boundaries on local and average structures. Analyses of unit cell parameters showed that both materials exhibit very similar thermal expansion behavior and thermal expansion data lay along the upper bound of uncertainty for standard empirical models, indicating a slightly faster thermal expansion rate. Atomic displacement parameters of UO$_{2.07}$ showed evidence for U$_{4}$O$_{9}$ phase boundaries in accordance with the established phase diagram, despite the suppression of U$_{4}$O$_{9}$ superlattice peaks in the measured diffraction patterns. Pair distribution functions revealed that the differences in local structure between UO$_{2}$ and UO$_{2.07}$ were very small and PDF features are dominated by thermal effects. The rate of contraction of the first nearest-neighbor U-O distances of UO$_{2}$ and UO$_{2.07}$ between 25-1000$\,^{\circ}\mathrm{C}$ were shown to agree with molecular dynamics simulations and local structure analyses previously performed on UO$_{2}$ above 1000$\,^{\circ}\mathrm{C}$.