The Lead-based Fast Reactor (LFR) is a highly promising fourth-generation nuclear energy system with excellent passive safety features and economic viability. However, a significant thermal stratification phenomenon occurs in the upper plenum chamber after an emergency reactor shutdown. Thermal stratification can impede the removal of residual core heat, leading to temperature accumulation and even the risk of core meltdown. Additionally, the temperature gradients within the thermal stratification in the upper plenum chamber can induce thermal stress on the internal structural components, thereby increasing the overall thermal load and causing thermal deformation of these components.

This study aims to delve into the dynamic process of LBE (Lead-Bismuth Eutectic) thermal stratification and interface evolution characteristics in the upper plenum chamber after LFR shutdown.

Firstly, a simplified 1/6 scale model of the upper plenum chamber was established and computational fluid dynamics (CFD) software STAR-CCM+ was employed to conduct large eddy simulation studies on the thermal stratification process within the LFR's upper plenum chamber post-shutdown. Then, based on relevant experimental data, the accuracy of the model calculations was validated by simulating the reactor shutdown. Simulations were performed for both normal reactor operation and post-shutdown conditions, analyzing the temperature and velocity distributions within the plenum. Finally, the thermal stratification interface was defined, and its evolution was observed.

Simulation results indicate that under normal operating conditions, the small holes in the inner cylinder do not significantly affect the flow of LBE within the upper plenum chamber; around 120 s after shutdown, a thermal stratification interface forms, and around 400 s, this interface rises to the top of the inner cylinder. Moreover, the small holes in the inner cylinder can significantly slow down the rate at which the thermal stratification interface rises.

The findings of this study suggest that there are large temperature gradients and irregular vortices at the thermal stratification interface, laying the foundation for subsequent analyses of thermal stress and thermal fatigue on structural components within the upper plenum chamber.