Simulating investigations on the start-up of the open natural circulation system

• A numerical code is developed to simulate the start-up process of the open NCLs.
• The resistance distributions of single-phase and two-phase flow are different.
• Effective method is found to improve the flow and heat transfer of the system.
• Flashing and flashing-induced instability are simulated.
• The effects of the structural parameters on the instability boundaries are studied.

Simulating investigations are carried out to study the resistance characteristics and flow instability during the start-up of the open natural circulation system, which is designed for the passive containment cooling system. Four different flow modes can be found during the start-up process: stable single-phase flow, flashing-induced intermittent flow, flashing-induced sinusoidal flow and stable two-phase flow. Flashing occurring in the riser section is the only phase-change phenomenon in this study. The flow resistance distributions of the loops under different flow modes have been studied to provide the foundation for choosing the structural parameters reasonably to improve the heat transfer capacity. The results show that the system has nearly the same resistance distribution in the steady single-phase flow under different containment conditions. The acceleration pressure resistance in the riser section is the main resistance in two-phase flow. The two-phase heat removal capability can be improved by increasing the diameter of the riser partially, of which the expansion section only needs to satisfy the condition that the length of expansion section is longer than that of flashing. The effects of the tube diameters and the inlet and outlet resistances of the heat exchanger on the flashing-induced instability region also have been studied. The results indicated that increasing the diameter of downcomer and decreasing the riser diameter can reduce the flow instability region. However, both increasing the inlet and outlet resistance coefficient of the heat exchanger will enlarge the flashing-induced instability region.