Analysis of natural circulation behaviors and flow instabilities of passive containment cooling system design for advanced PWR using MARS-KS code

Several design basis accidents such as loss of coolant accident (LOCA) and main steam line break (MSLB) of pressurized light water reactor could threaten the integrity of containment building by increasing containment temperature and pressure. Although containment spray systems (CSS) have been installed for the depressurization of containment against the accidents, iPower nuclear power plant requires additional passive safety system design to provide for malfunction of CSS. Therefore, passive containment cooling system (PCCS), which consists of passive containment cooling tank (PCCT), heat exchanger modules, and connection pipelines, is under development. The natural circulation behavior of the PCCS is an important hydraulic phenomenon for the guarantee of decay heat removal capacity, required to satisfy the safety acceptance criteria. Especially, flow instabilities, which could threaten the structural integrity and enlarge the performance uncertainty of the system, have been reported for multi-channel heat exchangers operating with natural circulation like present PCCS design of iPower reactor. In this study, the natural circulation behaviors of iPower PCCS according to various operating conditions and design parameters (elevation of returning pipeline) were analyzed by MARS-KS code to observe the flow instability phenomenon inside the system. Two types of flow instabilities, flashing-induced instability and density wave oscillation were observed in the PCCS with different heights of returning pipeline. Through additional analysis on flow stability map of PCCS design, effects of mitigation strategies (system pressure and flow resistance) on natural circulation behavior of the system were quantified. The analysis results provide physical insight on possible flow instability and its mitigation strategies, that would be crucial information on designing natural circulation-driven multi-channel heat exchanging system consisting of more than two heat exchanger modules. (C) 2019 Elsevier Ltd. All rights reserved.