Evaluating the Effectiveness of the Warm Water Layer in the RSG-GAS Reactor: A CFD Thermal-Hydraulic Analysis

This study investigates the thermal–hydraulic behavior of the Warm Water Layer (WWL) system behavior of the Warm Water Layer (WWL) system in the RSG-GAS research reactor using CFD simulations based on actual operating data from five core configurations (Cores 101, 102, 103, 107, and 108). The focus is on how inlet temperature, flow rate, and outlet distribution affect thermal stratification and its interaction with the primary coolant. To better understand the observed flow behavior, a net flow parameter (NET) is introduced, defined as the difference between inlet and outlet flow rates. Rather than treating stratification only from temperature profiles, NET is used here to directly relate flow imbalance with mixing and penetration behavior. The results indicate that stable stratification is consistently obtained at an inlet condition of 60°C and 10 m3/h. Increasing the inlet flow rate generally strengthens the layer and improves stratification, but at the same time enhances turbulent mixing with the primary coolant. At lower flow rates, upward penetration becomes weaker, mainly due to stronger downward entrainment linked to higher NET values. Higher inlet temperatures further support vertical temperature separation, although local heat transfer also becomes more active. Overall, the results show that WWL performance is not controlled by a single parameter, but by the balance between stratification and mixing. In this context, NET provides a simple but practical way to interpret the behavior and can be used to evaluate WWL operation under realistic conditions.