Thermal conductivity of bulk and porous ThO₂: Atomistic and experimental study

Thorium dioxide (ThO₂)is proposed to play a vital role in the world's future energy needs and is considered a better and safer alternative to the currently used nuclear fuel, uranium dioxide (UO₂). Thermo-physical properties of ThO₂ are superior to UO₂, but the fundamental physics governing the heat transport in ThO₂ is still ambiguous, and the available data for the thermal conductivity (k)of ThO₂ was scattered. In this article, a systematic investigation regarding the lattice thermal conductivity (k_L)of the bulk and porous ThO₂ is carried out theoretically and validated with experiments. The phonon transport calculations were done using two different methods; ab-initio calculations combined with the Boltzmann transport equation (BTE)and the equilibrium molecular dynamics (EMD)simulations using Green Kubo (GK)approach. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space modes, Grüneisen parameter, and mean free path (MFP)distributions were analyzed to understand the underlying physics in the thermal transport of ThO₂. The effect of porosity on the k_L by measurements and molecular dynamics (MD)simulations was explored. The measurements were performed on specimens with different porosity, that were prepared by spark plasma sintering (SPS)using the laser flash (LFA)technique. The results obtained demonstrated that the k_L values predicted by both the BTE and the EMD simulations were in excellent agreement with our experimental measurements. Moreover, the model to simulate the 95% theoretical density (TD)using MD simulations also captured the decrease in thermal conductivity with porosity and agreed well with the measured results for 95% TD dense sintered pellets.