Phonon Anharmonicity Effects at Elevated Temperatures in Actinide Oxides and Surrogates

Since thermophysical properties such as thermal conductivity and heat capacity are dictated by lattice vibrations in actinide oxide nuclear fuels, understanding anharmonic interactions of phonons at elevated temperature is crucial for developing accurate predictive tools for fuel behavior at operating temperatures. Raman spectroscopy is a facile means toward studying anharmonic phonon-phonon interactions via changes in the position and linewidth of zone-centered optical phonons with temperature. Here, we investigate the anharmonic behavior of the first-order Raman-active mode in two nuclear fuels (uranium dioxide and thorium dioxide) and a fuel surrogate (cerium dioxide) from room temperature to 1000°C. By examining the temperature-induced changes of the vibrational properties of these fuels/surrogates, the contribution of volume expansion and anharmonic inter-mode coupling to the Raman frequency shift and line width in these systems are reported. Experimental measurements are compared with first principles calculations of phonon dispersion linewidths that account for higher-order phonon scattering processes.