Experiment and Numerical Simulation Study on CCFL Characteristic of Hot Leg Section under Loss of Coolant Accident

This study aims to investigate the characteristics of counter current flow limitation (CCFL) in the hot leg section of a nuclear reactor under a small break loss of coolant accident (SBLOCA) scenario, thereby enhancing the safety of thermal equipment in nuclear power plants. CCFL, a phenomenon that occurs frequently during loss of coolant accident, significantly impacts the operational safety of nuclear reactors. To achieve this objective, a novel simulation approach based on the volume of fluid (VOF) method was developed and implemented using the FLUENT simulation software. This approach was employed to conduct computational fluid dynamics (CFD) modeling of the ACPR50s reactor’s hot leg section, where CCFL was prone to occur during SBLOCA. The simulation focused on understanding the flow characteristics within the hot leg, particularly under varying gas and liquid flow conditions. During the methodology phase, a detailed three-dimensional model of the hot leg section was created, and simplifications were made to the lower and side tanks to reduce computational complexity without compromising accuracy of the simulation results. The VOF method, known for its precise interface tracking capabilities, was chosen over the two-fluid model due to its ability to clearly capture and maintain the interface between different fluids. The simulation was set up as a three-dimensional transient simulation, with appropriate boundary conditions and a grid independence verification process ensuring the reliability of the results. The results indicate that the proposed simulation approach accurately captures the CCFL phenomenon and its impact on the flow characteristics within the hot leg section. When the gas flow rate is low, the liquid surface remains relatively undisturbed. As the gas flow rate increases, liquid droplets are entrained and deposited within the hot leg, leading to the formation of interface waves at the liquid outlet. Further increases in the gas flow rate result in the propagation of these interface waves to the bend of the hot leg, causing water surges and an elevation of liquid levels within the bend and horizontal sections. The comparison between simulation results and experimental data reveals a good agreement in the trend of the flow rate at the outlet of the left chamber of the hot leg. However, under low liquid flow rate conditions, the coupling between simulation and experiment is less satisfactory, with the simulation predicting the onset of CCFL earlier than observed in experiment. It is found that the simulation approach performs more accurately under large liquid flow rate conditions, suggesting that further improvements are needed for small liquid flow rate scenarios. In conclusion, this study demonstrates the effectiveness of the VOF-based simulation approach in investigating CCFL characteristics in the hot leg section of a nuclear reactor under SBLOCA conditions. While the approach shows good applicability in large liquid flow rate conditions, it requires refinement for more accurate predictions in small liquid flow rate regimes.