During the design and construction phases of nuclear power plants (NPPs), fire protection design principle requires that the safety classified equipment within the nuclear island, the service water pump building and galleries should meet the safety goal whenever a design basis fire occurs. The design of passive facilities, such as structure of buildings and physical separation of equipment, should prevent one single fire from failing all redundant equipment of the same safety function. When one single fire fails all redundant equipment of the same function, it is called a fire common mode. By performing fire vulnerability analysis of fire areas or fire zones, analysts can identify the possible fire common modes, and confirm if they are acceptable or not through further analysis. For NPPs in operation, the vulnerability analysis can be performed by examining the details of plant specific cables and equipment. The confirmed unacceptable common modes should be modified or protected through changing cable routing or adding fire coating, etc. Moreover, by employing internal fire probabilistic risk analysis (PRA), analysts can categorize the identified fire common modes according to severity, assess the changes in the risk of internal fire after elimination, and evaluate the benefit of performing elimination, which provides reference for engineering modification by showing the order of priority of each modification. This article introduces the process of identifying fire common modes, performing internal fire PRA and providing suggestions of modification, and illustrates the application of internal fire PRA in eliminating fire common modes and enhancing the safety level of NPPs.

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