Effect of Xe bubbles on the thermal conductivity of UO2: Mechanisms and model establishment

Abstract

The thermal conductivities of UO2 containing Xe bubbles were investigated using non-equilibrium molecular dynamics (MD) simulations as a function of porosity, bubble radius and the ratio of Xe atoms to U vacancies (Xe:VU). It was found that when the pores are filled with Xe atoms to form bubbles, the porosity and radius of the bubble both increase with the increase in Xe:VU. Based on the equilibrium porosity and bubble radius, a previous model for porous UO2 was used to predict the thermal conductivity. This model predicted only a slight change in the thermal conductivity with Xe:VU, which is induced by the changes in porosity and bubble radius. However, according to MD simulations, the thermal conductivity of UO2 containing Xe bubbles decreases obviously with Xe:VU. This indicates that in addition to porosity and bubble size effects, there exists another mechanism for the degeneration in UO2 thermal conductivity due to the presence of Xe bubbles. MD details revealed that many defects are produced on UO2/bubble interface, the concentration of which increases with Xe:VU. This leads to the increase in interfacial thermal resistance (ITR), thus resulting in the decrease in overall thermal conductivity. Based on the previous model for porous UO2, a model considering the above mechanism was established for UO2 containing Xe bubbles through phenomenological phonon hydrodynamics. In this new model, the increase in ITR is reflected by multiplying the thermal-drag force by a correction factor. The proposed model can be implemented in fuel performance codes to improve calculations of fuel thermal conductivity.