A new Method Of Characteristics (MOC) based computer code, USLAM, has been used to analyze the Fire Protection System (FPS) of a Nuclear Power Plant. Some unique features of this system are that it has a complex geometry, has many operating modes, is connected to an adjacent nuclear power plant FPS, and has a significant number of high elevation standpipes or risers. In a typical FPS, firefighting water is pumped from a low elevation reservoir at atmospheric pressure to the whole system including higher elevations regions where water hammer due to water column separation & rejoining may occur. A low capacity keep-full system is employed to keep the system pressurized during standby conditions. A loss of system pressure due to the opening of the pre-action or deluge valves can lead to void formation at high elevations whose collapse can result in severe water hammer. A catastrophic valve failure due to a water hammer event at a nuclear power plant (Arastu, et al, 1999) disabled the entire FPS.
The analysis presented in this paper is to evaluate the possibility of such a water hammer event and mitigate it in an effective manner. The mitigation method employed is the use of multiple vacuum breakers. As many as 11 vacuum breakers are used for this purpose. The paper discusses the analysis results without and with vacuum breakers for all operating modes. The basic methodology of the USLAM code is also discussed.