AP1000 Accident Analysis of Small and Medium LOCA Coupled with Failure of Half Depressurization

Automatic depressurization system (ADS) is a very important part of passive core cooling system (PXS) in passive safe nuclear plant. During accident, opening ADS valves can induce effective injection of makeup tank (CMT), accumulators and in-containment refueling water storage tank (IRWST). In order to evaluate the consequences of failure of partial ADS of AP1000, which is about to operating in China, the plant model is built by using SCDAP/RELAP5/MOD3.4 code to simulate small and medium LOCA which are selected as base accident followed by failure of half fourth stage ADS. The analytical results indicate that long time core cooling is postponed greatly as a result of ADS1, 2, 3 intentional depressurization. In addition, LOCAs with equivalent diameter 25.4cm and 34.1cm will not lead to core heating while 5.08cm break LOCA will. Meanwhile, high water level in the pressurizer will appear during all of three break LOCAs.