One of design features of AP1000 is its passive safety system. As the newly designed system, the performance of AP1000 under severe accidents causes a lot of concerns. To prevent or minimize being uncovered of the core fuel during LOCAs, many researchers have devoted themselves to simulate LOCAs. In this paper, based on RELAP5, the model of AP1000 was established and validated. LOCA simulations of AP1000 with different breaks were conducted, including LOCAs both on cold leg and hot leg with the break size of 0.1 ft2 and LOCAs with different break size as 0.01%, 0.1%, 1%, 10%, 20%, 40%, 50% of the cold leg cross section area on cold leg. The result indicates that, for AP1000, LOCA on cold leg is more critical, and the system shows great safety performance under LOCAs with different break sizes on cold leg, especially for LOCA with middle break size because the core decay heat is fully removed by coolant and the cladding temperature decreases soon once the accident happens during the process. The result also reveals that, during the process, the core decay heat is mainly removed by the discharge coolant for large break LOCA, but mainly removed by PRHR HX for small break LOCA.

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