Effect of pores and He bubbles on the thermal transport properties of UO₂ by molecular dynamics simulation

The thermal conductivities of UO₂ single crystals containing nanoscale size pores and He gas bubbles are calculated using non-equilibrium molecular dynamics as a function of pore size and gas density in the bubble. As expected, the thermal conductivity decreases as pore size increases, while the decrease in thermal conductivity is determined to be more substantial than the predictions of traditional analytical models by Loeb and Maxwell-Eucken. However, the recent model of Alvarez, which is applicable when the phonon mean-free path is comparable to the pore size, is able to quantitatively reproduce the simulation results. The thermal conductivity of UO₂ of the small pores considered here is reduced further when the pore is filled with He gas. This surprising result is due to the penetration of the helium atoms into the lattice where they act as phonon scattering centers.