An open natural circulation system applied to the passive containment cooling system is proposed for the large dry double-wall concrete containment after severe accidents. In order to investigate the operating behaviors of the open loop, a numerical code is developed to simulate the transient flow characteristics from the startup to the long-term operation. The results show that the open system may experience such four stages as startup process, quasi-steady single-phase flow, transition stage and quasi-steady two-phase flow. Flashing is the dominated factor in the last two flow phases. Additionally, the effects of some systematic parameters on the mass flow rate are analyzed in detail. Both increasing the condensation heat transfer coefficient and the containment temperature can enhance the flow effectively whichever stage the system operates in. But the the initial fluid temperature can only affect the duration of the single-phase flow but has little impact on the mass flow rate. Moreover, greater riser height can lead to bigger single-phase flow rate, but nearly has no influence on the long-term two-phase flow and the flashing region.

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