Simulation on Thermal and Flow Characteristics Investigation of Natural Circulation Under Different Heating Powers using FASSIP-06 Ver.03 Loop

Abstract

Natural circulation systems are passive cooling mechanisms that operate based on density differences caused by temperature variations. These systems play a crucial role in ensuring the safety and efficiency of nuclear reactors, particularly during transient cooling phases following total power loss, where active systems fail, and passive cooling becomes essential. This study aims to analyze the thermal and flow characteristics of single-phase natural circulation in the FASSIP-06 Ver.03 loop. The investigation was conducted using CFD simulations in ANSYS Fluent, with heater power variations from 750 W to 1050 W. A three-dimensional model of the loop geometry was developed and the RNG k-ε turbulence model with scalable wall functions. Boundary conditions included a constant cooling temperature of 10°C and adiabatic walls, with water as the working fluid. The analysis focused on transient and steady-state temperature distribution, flow velocity patterns, and the relationship between heater power and Reynolds number. The results show that increasing heater power enhances the temperature difference between the hot and cold legs, intensifies the circulation flow, and increases Reynolds numbers in the cold leg from 3414 to 4759, indicating a transition toward turbulent flow due to its smaller diameter. In contrast, the hot leg flow remains laminar, with Reynolds numbers below 1739. These findings confirm that CFD simulation effectively captures the thermohydraulic behavior of natural circulation and validates the FASSIP-06 Ver.03 loop as a reliable model for passive cooling system analysis.