Analysis On The Accident Of AP1000 Main Feedwater Line Break

The AP1000 is a nuclear power plant designed as the first Generation III reactor in support of the government. As the first AP1000 is still under building, the safety of the AP1000 needs to be simulated and verified by the mechanical transient program. The MFLB accident is a typical loss of heat sink accident and belongs to the fourth type DBA, and it should be noticed in the analysis of the API 000 accidents.In this paper, to investigate the design features of the AP1000, RELAP5/MOD3.4 was applied to model refering to the process of MFLB accident from the westinghouse, set the trigger signal of the security system and the auxiliary system and calculate the figures of the transient condition. The transient results of RELAP5 show that the established AP1000 model has the capacity to analysis MFLB accident. Those indicate that the heat can be removed successfully and the integrity can be maintained in the extreme accident. Furthermore,considering the asymmetric equipments of two loops in AP1000, the effect of the break in different loops was studied, and the results are changeless in RELAP5.Finally, the sensitivity was analyzed in several aspects. For one thing, the effect of main steam isolating valve's different response time was studied and its effect is not obvious. For another thing, the transient results indicate that the PRHR system and CMT play the leading roles in the later period. Considering the results, the research was done in two aspects. On one hand, CMT failing in the MFLB accident, the enhanced natural circulation of PRHR can enhance heat transfer, which matches with the decay heat in advance. On the other hand, with the resistance of PRHR increasing, the heat transfer reduces, so the PRHR and CMT need more time to remove the decay heat. The consequences indicate that PRHR's heat transfer ability plays a dominant role in the core's long-term natural circulation.In conclusion, the results of MFLB accident model applied by RELAP5 can be referred to the safety analysis of AP1000.