Transient analysis of AP1000 NPP under the similar Fukushima accident conditions

Highlights: • Simulation calculation to achieve stress tests on AP1000 NPP. • Simulation calculation to investigate the effectiveness of PXS in mitigating the Fukushima nuclear accident. • If the integrity of the primary system is not destroyed during the accidents similar to the Fukushima nuclear accident, the residual heat of AP1000 NPP can be removed by CMT and PRHR without operator interventions and the transient process can be developed to the hot shutdown condition successfully. • However, damages to the RCS pressure boundary (e.g. PRHR-HX tube), induced by the earthquake, and would result in core melt accident in a short time. • Measures to promote the safety of AP1000. - Abstract: Stress tests are conducted on AP1000 NPP to investigate the effectiveness of Passive Core Cooling System (PXS) in mitigating the similar Fukushima accident conditions, using thermal hydraulic analysis code RELAP. Key transient parameters, including the Reactor Coolant System (RCS) pressure, core coolant temperature and passive safety system flow, were obtained and the results were analyzed. The calculation results show that the AP1000 NPP can reach hot shutdown status when the Passive Residual Heat Removal System (PRHR) is available. However, the transient would develop to severe accident within five hours if double-ended rupture of one PRHR-HX tube occurred. Several suggestions are given based on the analysis results. Additional protections resulted from the earthquake are necessary for PRHR, especially for the tubes of the heat exchanger. Furthermore, reliability of the battery sets is of great importance, such special attentions should be paid to improve the availability of the battery sets under accident conditions.