High Phonon Scattering Rates Suppress Thermal Conductivity in Hyperstoichiometric Uranium Dioxide

TL;DR

This study investigates how excess oxygen in uranium dioxide increases phonon scattering, leading to reduced thermal conductivity, with implications for nuclear fuel performance.

Contribution

It provides new experimental evidence linking hyperstoichiometry to high phonon scattering rates and suppressed thermal conductivity in UO$_2$.

Findings

Thermal conductivity is significantly suppressed in UO$_{2+x}$ compared to UO$_2$.

High phonon scattering rates are responsible for reduced thermal transport.

Thermal properties are largely unaffected by excess oxygen near the Néel temperature.

Abstract

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.