Investigation of O/M ratio effect on thermal conductivity of oxide nuclear fuels by non-equilibrium molecular dynamics calculation

The thermal conductivities of UO_2+x, (U_0.8,Pu _0.2)O₂ × and UO₂ (induced Schottky defects) have been investigated by non-equilibrium molecular dynamics (NEMD) simulation from the viewpoints of the oxygen-to-metal ratio (O/M ratio), temperature, and point defect concentration. In the NEMD simulation, the thermal conductivity was determined by the temperature gradient and heat flux using Fourier's law. The calculated thermal conductivity decreased with increasing temperature and defect concentration i.e. excess oxygen and oxygen vacancy concentrations. The calculated thermal conductivity decreased with increase of the deviation x from stoichiometry and number of defects. By comparing the hypersto-ichiometric and hypostoichiometric oxides, it was found that the effect of Pu³⁺ and oxygen vacancy on the thermal conductivity was larger than that of U⁵⁺ and excess oxygen. Further, the cuboctahedral defect clusters were observed in the hyperstoichiometric UO_2+x supercell.