  This work presents the first principles calculations of the lattice thermal
conductivity degradation due to point defects in thorium dioxide using an
alternative solution of the Pierels-Boltzmann transport equation. We have used
the non-perturbative Green's function methodology to compute the phonon point
defect scattering rates that consider the local distortion around the point
defect, including the mass difference changes, interatomic force constants and
structural relaxation near the point defects. The point defects considered in
the work include the vacancy of thorium and oxygen, substitution of helium,
krypton, zirconium, iodine, xenon, in the thorium site, and the three different
configuration of the Schottky defects. The results of the phonon-defect
scattering rate reveals that among the considered intrinsic defects, the
thorium vacancy and helium substitution in the thorium site scatter the phonon
most due to substantial changes in the force constant and structural
distortions. The scattering of phonons due to the substitutional defects
unveils that the zirconium atom scatters phonons the least, followed by xenon,
iodine, krypton, and helium. This is contrary to the intuition that the
scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass
difference. This striking difference in the zirconium phonon scattering is due
to the local chemical environment changes. Zirconium is an electropositive
element with valency similar to thorium and, therefore, can bond with the
oxygen atoms, thus creating less force constant variance compared to iodine, an
electronegative element, noble gas helium, xenon, and krypton. These results
can serve as the benchmark for the analytical models and help the
engineering-scale modeling effort for nuclear design.

Materials Science