Thermal Conductivity Of ThO2 Research Articles

Monte-Carlo modeling of phonon thermal transport using DFT-based anisotropic dispersion relations over the full Brillouin zone

We present a Monte Carlo (MC) approach to solve the phonon Boltzmann transport equation (BTE) in which the anisotropic phonon dispersion relations over the full Brillouin zone (BZ) are used. In this approach, the discretization of the BZ used to compute the phonon relaxation time places constraints on the direction of scattered phonons in the real-space simulation domain. The phonon dispersion and phonon relaxation times are calculated using the density functional theory (DFT) approach. The modified MC approach is validated by a close examination of its ability to simulate phonon transport in both the ballistic and diffusive regimes for multiple materials including GaAs, InAs, ThO2, and α-U. In doing so, the phonon thermal conductivities from 100 K to 1000 K are calculated and compared with traditional non-transport solution of the phonon BTE. It is found that the phonon thermal conductivities of α-U and ThO2 obtained from MC simulations using isotropic dispersion are larger than the values obtained using anisotropic phonon dispersion relations over the full BZ. The effect of phonon-defect scattering on the thermal conductivity of ThO2 is also studied as an application of the current MC approach and found to agree with previously computed values in the literature. The MC solver developed here has been parallelized as a step to demonstrate its potential to solving computationally intensive phonon thermal transport problems at the mesoscale.

Thermal conductivity of ThO2: Effect of point defect disorder

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.

The effect of SPS processing parameters on the microstructure and thermal conductivity of ThO2

Thorium dioxide (ThO2), is a nuclear fuel that is expected to play a vital role in the Generation IV nuclear reactors. One of the challenges to implementation of thoria for the fuel cycle has been the difficulty in fabricating dense pellets via conventional sintering techniques. In this study, the non-conventional sintering of thoria using spark plasma sintering (SPS) was explored. A systematic investigation into the influence of processing parameters on the densification, microstructure, grain size and thermal conductivity of ThO2 is presented. The range of sintering temperature, pressure, and hold time has been systematically varied between 1500 and 1800 °C, 50–70 MPa and 5–15 m, respectively. The results revealed that without the help of any sintering aid, pellets with a relative density of 95% theoretical density (TD) were fabricated at a sintering temperature of 1600 °C, sintering pressure of 50 MPa and a hold time of 10 min. Furthermore, the characterization of these specimens clearly indicates that by carefully selecting the processing parameters, the density, microstructure, grain size and the thermal conductivity of ThO2 can be suitably controlled. This study shows that the use of the SPS technique can potentially solve one of the primary concerns in the front end of the thoria fuel cycle.

MOLECULAR DYNAMICS STUDY OF THE THERMAL CONDUCTIVITY OF THORIUM-BASED MIXED OXIDE FUEL

A molecular dynamics (MD) simulation method of accurately calculating the thermal conductivity of thorium based mixed oxide fuel is introduced and two cases of ThO2 fuel doped with different Ce fraction and U fraction were considered, respectively. A random atomic distribution doping method was adopted, together with Coulomb-Buckingham potential adopted to describe the interaction of atoms. The calculated results indicate the Coulomb-Buckingham potential could accurately calculate the lattice parameter and thermal conductivity of ThO2, UO2 and CeO2 fuel，which is found to be consistent with experimental measurement. The thermal conductivity of thorium based mixed oxide fuel was further calculated based on this method. It was found that with the Ce doping fraction increased, the lattice parameter of (Th,Ce)O2 mixed oxide fuel was found to be decreased and the thermal conductivity was found to be increased gradually. While with the U doping fraction increased, the thermal conductivity of (Th,U)O2 mixed oxide fuel was found to have very tiny change and lattice parameter was found to be decreased gradually too. In addition, the calculated thermal conductivity of (Th0.8,Ce0.2)O2 fuel was found to be consistent with experimental measured data. Lastly, the effects of fission products on the thermal conductivity of ThO2 were also analyzed and discussed.

Thermal expansion and thermal conductivity of (Th,Pu)O2 mixed oxides: A molecular dynamics and experimental study

Thermal expansion and thermal conductivity of (Th,Pu)O2 mixed oxides (MOX) have been investigated using molecular dynamics (MD) simulations and experimental study. MD simulations have been performed to determine lattice thermal expansion of Th1−xPuxO2 (x = 0, 0.03125, 0.0625 and 0.09375) solid-solutions using a potential model which combines Coulomb–Buckingham–Morse and many-body functional forms. Special quasi-random structures have been employed to establish solid-solution configurations. Experimental investigations have been conducted to study the effect of homogeneity (PuO2 distribution in ThO2 matrix) on the thermal properties of ThO2–1wt.%PuO2 fuel pellets fabricated by conventional powder and pelletization (POP) process as well as coated agglomerate pelletization (CAP) processes. Dilatometry and laser flash techniques, respectively, have been employed for measuring thermal expansion and thermal diffusivity of the ThO2, ThO2–1wt.%PuO2 (both POP and CAP fabricated) and ThO2–6wt.%PuO2 POP pellets. The study shows a difference of <4% in the thermal conductivity values of ThO2–1wt.%PuO2 fuel pellets fabricated through CAP and POP techniques at 873 K and the difference decreases with increase in temperature. The thermal conductivity of ThO2 and ThO2–6.25 at% PuO2 calculated under equilibrium conditions by Green–Kubo formalism shows a good agreement with our experimental measurements. Our MD calculated melting temperatures of pure ThO2, PuO2 and Th0.9375Pu0.0625O2 using two-phase simulation are between 3650-3675 K, 2800–2825 K and 3575–3600 K, respectively, in good agreement with reported experimental observations.

Experimental and molecular dynamics study of thermo-physical and transport properties of ThO2-5wt.%CeO2 mixed oxides

We have determined the thermo-physical (elastic modulus, specific heat, thermal expansion and thermal conductivity) and transport (ionic conductivity) properties of ThO2-5wt.%CeO2 mixed oxide (MOX) using a combined experimental and theoretical methodology. The specific heat, ionic conductivity and elastic properties of ThO2-5wt.%CeO2 pellets prepared by conventional powder metallurgy (POP) and coated agglomerate pelletization (CAP) routes (sintered in both air and Ar-8%H2 atmosphere) are compared with respect to homogeneity (CeO2 distribution in ThO2 matrix), microstructure, porosity and oxygen to metal ratio. The effects of inhomogeneity and pore distribution on thermal expansion and thermal conductivity of the mixed-oxide pellets are identified. Molecular dynamics (MD) simulations using the Coulomb-Buckingham-Morse-many-body model based interatomic potentials are used to predict elastic properties in the temperature range between 300 and 2000 K and thermodynamic properties, viz., enthalpy increment and specific heats of ThO2. Finally, the thermal expansion coefficient and thermal conductivity of ThO2 and (Th,Ce)O2 mixed-oxides obtained from MD are compared with available experimental results.

Thermal expansion and thermal conductivity of (Th,U)O2 mixed oxides: A molecular dynamics and experimental study

Thermal expansion and thermal conductivity of ThO2 and (Th,U)O2 mixed oxides (MOX) have been investigated by using molecular dynamics (MD) simulation and experimental study. MD simulation have been performed to determine lattice thermal expansion of Th1−xUxO2 (x = 0 to 0.3125) solid-solutions in the temperature range of 300–3000 K using a combination of Coulomb-Buckingham-Morse and many-body potential model with partial ionic charges. Special quasi-random structures were employed to establish solid-solution configurations. Experimental verification of the lattice thermal expansion behavior is carried out on polycrystalline sample of ThO2 -x wt.% UO2 (x = 0, 6, 13, 25 and 30) MOX from room temperature to 1273 K in vacuum using high temperature X-ray diffraction (HT-XRD). Our combined MD simulations and HT-XRD measurements indicate that incorporation of UO2 in ThO2 systematically increases coefficient of thermal expansion but the rate of increase is higher in low UO2 composition range (≤13 wt.%). A comparative evaluation of the thermal properties of ThO2-6wt.% UO2 pellets prepared by coated agglomerate pelletization (CAP) process and powder metallurgy routes (sintered in Ar-8%H2 atmosphere) have been performed with respect to micro-inhomogeneity, porosity, O/M and trace impurities. The thermal conductivity of ThO2 and ThO2-6 wt.% UO2 calculated under equilibrium condition by Green–Kubo formalism show good agreement with our experimental measurements.

Thermal expansion and thermal conductivity of (Th,Ce)O2 mixed oxides: A molecular dynamics and experimental study

Thermal expansion and thermal conductivity of ThO2 and (Th,Ce)O2 mixed oxides (MOX) have been investigated by using molecular dynamics (MD) simulation and experimental study. The MD simulations have been performed on ThO2, CeO2 and (Th,Ce)O2 using Coulomb–Buckingham type potential function with partial ionic charges. The interatomic potentials of ThO2 and CeO2 are derived in this study and special quasirandom structures are employed to establish solid solution configurations. In order to evaluate the effect of inhomogeneous distribution of CeO2 in ThO2 matrix and change in oxygen to metal ratio (O/M) on thermal properties of MOX, the samples of (Th,Ce)O2 were prepared by conventional powder metallurgy route as well as coated agglomerate pelletization (CAP) route and sintered in air as well as Ar–H2 atmospheres. The thermal expansions of ThO2 and (Th,Ce)O2 (3, 5 and 8wt% CeO2) are measured using dilatometry and thermal diffusivities of ThO2 and ThO2–5wt% CeO2 were measured using a laser flash technique. The experimental results of thermal properties of (Th,Ce)O2 showed good correlation with those of MD simulations. It was found that the effect of inhomogeneity on thermal expansion and conductivity was not very significant, while porosity as well as O/M (stoichiometry) of the pellet majorly affected thermal properties.

Modelling heat capacity, thermal expansion, and thermal conductivity of dioxide components of inert matrix fuel

Based on a simplified model of the phonon spectrum, on the statistical thermodynamics, and on the generalised Klemens model for thermal conductivity, some useful relationships bounding the specific heat capacity, the thermal expansion coefficient, the bulk modulus and the thermal conductivity of dioxides, often used as components in inert matrix fuel, were deduced in a quasi-harmonic approximation. The developed models were first verified with urania UO2, then applied for prediction of the isobaric specific heat, the isobaric thermal expansion coefficient, and the thermal conductivity of ThO2 and of one inert matrix material: ZrO2. The similarity principle was used in the cases where the input data were missing. The obtained results were compared with the available experimental data, and satisfactory agreement was demonstrated in the temperature range of 30–1600K. In most of the cases the predicted values were within the region of the dispersion of the experimental data. The largest difference was observed for the thermal conductivity of ZrO2 at high temperatures, which could be explained by unaccounted photon contribution.

Thermal Conductivity of ThO2-UO2 System

The thermal diffusivity of the ThO2-UO2 system in solid solution was measured by laser pulse method at temperatures ranging from 20° to 500°C. Reproducibility of the data was confirmed to be within 3%. The compositions of the samples were ThO2, ThO2-1%UO2, ThO2-5%UO2 and ThO2-10%UO2. The thermal conductivity was calculated from measured thermal diffusivity data and the specific heat data available in literature and corrected to zero porosity by using Loeb's equation, in which the shape factor is unity. The values on ThO2 thus obtained agreed very well with the data found in literature, throughout the range of temperature of the experiments. The thermal conductivity of ThO2, ThO2-1%UO2, ThO2-5%UO2 and ThO2-10%UO2, at 20°C, were 0.0312, 0.0288, 0.247 and 0.0184 cal·cm/sec· °C·cm2, respectively.

Effect of Porosity on the Thermal Conductivity of ThO2

The eflect of porosity on the thermal conductivity of sintered ThO2 of densities ranging from 90 to 95% of theoretical density was investigated at 20°C. The thermal conductivity was determined by means of laser pulse. Discrepancies were observed between the experimental results and the relation derived by Loeb. This discrepancy was explained by applying a modified Maxwell formula derived by Brailsford et al. The samples were also examined metallographically. It is concluded that the existence of fluid continuous regions contribute significantly to the thermal resistance of such materials as ThO2.