Thermal conductivity of ThO2: Effect of point defect disorder

Abstract

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

Thermal Energy Transport in Oxide Nuclear Fuel

“…Broadly speaking, impurity defects have a lesser impact on thermal conductivity than interstitials and vacancies, and of the fission products, Xe has the most significant impact on the thermal conductivity of UO2 [25,26,555]. An additional complexity for ionic materials is the fact that scattering due to charge compensating defects must also be considered.…”

“…27,602 While dispersion curves can be validated by neutron scattering experiments, the data on three-phonon scattering are limited. This approach has been applied recently to ThO 2 555 to predict bulk single-crystal thermal conductivity in comparison to measured values. 179 Figure 28 compares some of the notable results from previous studies that considered the thermal conductivity of ThO 2 .…”

“…The expansion of phonon scattering rates in Table 2 to SMRT requires recasting them into a form that accounts for differences between v p and v g . Using dispersion curves fit to inelastic neutron scattering data, 50 Deskins et al 555 implemented the SMRT approximation in the case of uranium substitutional defects in ThO 2 and compared the resulting temperature dependence of the thermal conductivity to measured values 604 and those from MD simulations 603 (Figure 29). This comparison shows that in these fluorite oxide systems, making the SMRT approximation still produces a reasonable description of the thermal conductivity in defect-bearing systems across a wide temperature range through the BTE.…”

“…However, it has been recognized for some time that smaller radiation-induced defects such as dislocation loops and sub-TEM-resolution defect clusters contribute significantly to a reduction in thermal conductivity. Until recently, the impact of smaller defects has been neglected, as they are more difficult to characterize and require a more detailed accounting of phonon structure. ,,,− …”

Structural dynamics of Schottky and Frenkel defects in ThO2: a density-functional theory study

“…al. 14 predicts a decrease in the thermal conductivity of ThO 2 from point defects including oxygen and thorium vacancies. This has also been observed in other work on ThO 2 (ref.…”