  Uranium dioxide (UO$_2$), one of the most important nuclear fuels, can
accumulate excess oxygen atoms as interstitial defects, which significantly
impacts thermal properties. In this study, thermal conductivities and inelastic
neutron scattering measurements on UO$_2$ and UO$_{2+x}$ (x=0.3, 0.4, 0.8,
0.11) were performed at low temperatures (2-300 K). The thermal conductivity of
UO$_{2+x}$ is significantly suppressed compared to UO$_2$ except near the
N\'eel temperature TN= 30.8 K, where it is independent of x. Phonon
measurements demonstrate that the heat capacities and phonon group velocities
of UO$_2$ and UO$_{2+x}$ are similar and that the suppressed thermal
conductivity in UO$_{2+x}$ results from high phonon scattering rates. These new
insights advance our fundamental understanding of thermal transport properties
in advanced nuclear fuels.
