Natural Circulation Performance of Supercritical CO ₂ Dual Loop System – A Numerical Study

In modern nuclear reactors, coupled natural circulation loops (NCLs) play a crucial role in core heat removal and passive safety, while supercritical CO2 (sCO2)-based natural circulation system supports sustainability development goals. This numerical study investigates the steady state thermohydraulic performance of a three-dimensional dual loop natural circulation system operating with sCO2. To evaluate the performance, two loop configurations are examined: Case I, featuring horizontal coolers, and Case II, featuring vertical coolers. Simulations are conducted at two heat inputs (1,000 W and 2,000 W) and four operating pressures (8 MPa–11 MPa). The analysis includes both local and bulk properties – temperature, density, velocity, mass flow rate, heat transfer coefficient (HTC), and Nusselt number. Streamline patterns and radial velocity profiles at the loop junctions are examined to understand flow dynamics. Results indicate that Case I exhibits superior thermal performance and circulation strength across all pressures, with lower bulk temperatures, higher mass flow rate, and more stable density gradient. Case II shows greater thermal sensitivity, especially under increased power, with noticeable performance degradation due to stronger thermal expansion. Overall, Case I demonstrates greater robustness and thermal efficiency under varying operational loads, making it the preferred design for high-power sCO2-based dual loop applications.