Study of thermal stratification in the upper plenum of the liquid metal fast reactor under mixed convection – An LES of the flow in the E-SCAPE facility

Abstract

A numerical analysis of thermal stratification in the upper plenum of the European SCAled Pool Experiment (E-SCAPE) facility was conducted using Large Eddy Simulation (LES) to enhance the understanding of thermal hydraulics phenomena in liquid metal fast reactors (LMFR) under mixed convection conditions. The geometric complexity in the above-core structure region was represented using a porous medium model. The results provide insights into the overall flow phenomena in the upper plenum region, the thermal instability in the above-core structure region, as well as the characteristics of rounded jets that emerge from the barrel walls. Under mixed convection conditions (low flow rate conditions), the strong buoyancy causes hot lead–bismuth eutectic (LBE) to accumulate at the top first, then flowing downwards, and then exiting the region through the upper set of barrel holes. Conversely, unmixed cold LBE spreads through the lower set of barrel holes. This results in a stratified temperature distribution, with lower temperatures at the bottom, slightly higher temperatures in the middle, and the highest temperatures at the top, demonstrating thermal stratification in the upper plenum region. This stratification occurs because the jets are weak in strength, resulting in poor mixing in the upper plenum region. Flow movements are confined to regions close to the jets, while areas away from the jets experience almost no movements or negligible movements, referred to as dead zones. It is useful to note that strong circulations are observed in one of our previous studies in the same facility under forced condition, which results in good mixing and no thermal stratification. In the above-core structure region, large-scale temperature fluctuations in the form of Kelvin-Helmholtz (KH) instabilities and mixing layers have been observed when the hot fluid backflows and interacts with the cold fluid.

The behaviour of the flows in the upper plenum region is dominated by the influence of different types of jets, including horizontally issued jets, jets angled upwards, and jets impinging on the components of the upper plenum. Under mixed convection conditions, the jets behave similarly to negatively inclined positively buoyant jet with minimal interactions between the top and middle jets and between the middle and bottom jets, occurring only in the vicinity of the jets. The latter stages of the jets indicate very little or almost negligible background flow movement. Additionally, high turbulence is observed in the shear layer of the jet orifice, which transitions into mixing layers after six jet diameters along the trajectory for the upper set of jets.