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This work presents the first principles calculations of the lattice thermalconductivity degradation due to point defects in thorium dioxide using analternative solution of the Pierels-Boltzmann transport equation. We have usedthe non-perturbative Green's function methodology to compute the phonon pointdefect scattering rates that consider the local distortion around the pointdefect, including the mass difference changes, interatomic force constants andstructural relaxation near the point defects. The point defects considered inthe work include the vacancy of thorium and oxygen, substitution of helium,krypton, zirconium, iodine, xenon, in the thorium site, and the three differentconfiguration of the Schottky defects. The results of the phonon-defectscattering rate reveals that among the considered intrinsic defects, thethorium vacancy and helium substitution in the thorium site scatter the phononmost due to substantial changes in the force constant and structuraldistortions. The scattering of phonons due to the substitutional defectsunveils that the zirconium atom scatters phonons the least, followed by xenon,iodine, krypton, and helium. This is contrary to the intuition that thescattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the massdifference. This striking difference in the zirconium phonon scattering is dueto the local chemical environment changes. Zirconium is an electropositiveelement with valency similar to thorium and, therefore, can bond with theoxygen atoms, thus creating less force constant variance compared to iodine, anelectronegative element, noble gas helium, xenon, and krypton. These resultscan serve as the benchmark for the analytical models and help theengineering-scale modeling effort for nuclear design.

First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2