Simulation analysis of an open natural circulation for the passive residual heat removal in IPWR

In this paper, an innovative passive safety system (PSS) for the integrated pressurized water reactor (IPWR) IP200 was designed, which aims to compact the equipment layout of PSS and apply in floating nuclear power plants. In this design, the reactor vessel (RV) is connected directly to the containment to form an open loop, and a natural circulation can be established in the loop for the residual heat removal. In order to evaluate the mitigation of PSS conservatively, an extreme scenario of SBLOCA (small break LOCA) along with station blackout (SBO) is simulated by Relap5 code. The temperature distribution and flow characteristics during the transient and the long-term cooling are both calculated to explain the pivotal thermal-hydraulic phenomena in the PSS. Furthermore, characteristics under different thermal boundaries and structural parameters are compared to discuss the coupling effects between circulation capacity and heat exhaust. The results show that the accumulation of non-condensable gas in containment with a proper pressure is conducive to restrain the instability of two-phase natural circulation. Nevertheless, a single-phase circulation will be formed eventually to reduce the heat transfer efficiency if the initial pressure is too high. Besides, increasing the heat transfer area of ultimate heat sink helps to enhance the cooling performance of natural circulation during the start transient, but it will increase the size of heat exchanger and reduce the averaged heat transfer coefficient of convection. This study determines the key thermal parameters of PSS and can provide a reference for practical engineering design. (C) 2018 Elsevier Ltd. All rights reserved.