Defects in uranium dioxide significantly suppress thermal conductivity

In nuclear reactors, controlling the operating temperature is crucial for safe and efficient power generation. Part of this involves characterizing the thermal transport properties, measures of heat transfer across nuclear fuels.

Uranium dioxide is the most widely used nuclear fuel. Depending on the temperature and oxygen pressure, it can accommodate a variable stoichiometry, meaning that the ratio of oxygen to uranium can increase slightly. This incorporates oxygen defects beyond the ideal crystal structure.

Ma et al. examined how the oxygen to uranium ratio affects the thermal properties of uranium dioxide at low temperatures (2-300 K). While studies have explored this behavior at high temperatures, the low temperature results are of critical importance for benchmarking theoretical models.

The team used inelastic neutron scattering experiments to provide insight on phonon properties that govern thermal conductivity behaviors.

“The steepness of the phonon dispersion tells us how fast the heat carrying phonons travel, and the linewidths tell us how long they travel before they scatter,” said author Hao Ma. “These factors are directly related to the contribution of phonons to the thermal conductivity.”

Thermal conductivity was significantly suppressed with the addition of oxygen, except near the Neel temperature, where the ratio became unimportant. The inelastic neutron measurements demonstrated that the heat capacity and phonon group velocity did not change much with the oxygen to uranium ratio. Instead, the suppression likely stems from shorter phonon lifetimes.

“We plan to continue to measure the impact of various types of environment induced disorder on thermal transport in nuclear fuel materials,” said Ma. “This includes both chemical and irradiation induced defects.”

Source: “Suppressed thermal conductivity in hyperstoichiometric uranium dioxide controlled by phonon lifetimes,” by Hao Ma, Matthew S. Bryan, Judy W. L. Pang, Douglas L. Abernathy, Daniel J. Antonio, Krzysztof Gofryk, and Michael E. Manley, Applied Physics Letters (2022). The article can be accessed at https://doi.org/10.1063/5.0096655 .