Background: The open natural circulation system has been used gradually in the passive safety system after severe accidents such as passive containment cooling system. Purpose: The aim is to investigate the start-up process of the open natural circulation system

and analyze the experimental phenomena and flow characteristics under different heating power and inlet subcooling conditions in detail. Finally

the map of the instability boundary during the startup of the natural circulation system is given. Methods: Experimental facility was set up and numbers of experiments under different conditions were conducted. The flow characteristics and mechanisms with different flow phenomena are analyzed in detail combining with the measurement results of the fluid temperature

pressure and mass flow rate. Finally

all the flow behaviors were classified under different conditions. Results: The results are as follows: the system which operates under low-power conditions will experience the stable single-phase

two-phase oscillation and stable two-phase flow sequentially with the inlet subcooling increasing. During the two-phase oscillation phase

two flow modes including the flashing-induced instability and the combination of the boiling eruption and flashing-induced instability occur. Conclusion: The instability region of the system enlarges with the increase of the heating power

which even makes it difficult to form the stable two-phase flow. During the two-phase oscillation phase

both the flashing-induced instability and the combination of the boiling eruption with flashing-induced instability belong to the density wave instability.Cited