After the Fukushima accident, there were two important feedbacks. One was the simultaneous occurrence of serious accidents of multiple units at the same plant site, and the other was the emergency diesel generator was unavailability under a serious accident. The events at Fukushima highlight the possibility that extreme natural phenomena could challenge the prevention, mitigation and emergency preparedness defense-in-depth layers. In the design of CPR1000 Nuclear power unit, the important contributor is the integrity of the reactor coolant system considering potential leaks from the reactor coolant pump seals following a station blackout (SBO) or loss of heat sink accident. After the SBO or loss of heat sink of whole plant occurs, normal main pump shaft seal injection will be lost. The hydrostatic test pump will start automatically to ensure emergency shaft seal injection. But the flow of hydrostatic test pump can only meet the requirements of one unit shaft seal injection, and the other unit shaft seal injection cannot be guaranteed, which may lead to loss of coolant accident (LOCA). If the LOCA is not controlled, it will lead to reactor core damage. At present, the improvement of shutdown seal used in the main pump of the new power plant can effectively solve this problem and reduce the frequency of reactor core damage. This paper mainly analyzes the technical characteristics of shutdown seal and the structure and function of main pump, so as to determine the feasibility and necessity of the improvement of shutdown seal of CPR1000 main pump.

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