Structural Dynamic Transient Analysis of Fire Protection System at a Nuclear Power Plant

NRC Information Notice 98-31[1] describes a water hammer event that occurred at Columbia Generating Station. In this event, actuation of a Fire Protection System (FPS) led to voiding in a tall dead-ended riser with subsequent water hammer in the riser when the main fire pumps were activated. Due to the severity of the event, NRC requires that FPS with similar configurations at other nuclear power plants be analyzed for similar events. This paper describes an evaluation of two selected riser piping systems performed to quantify the susceptibility of the potential water hammer events in the FPS at a nuclear power plant. The FPS was modeled in a proprietary computer program, USLAM (Hydraulic Analysis Code), and selected operational scenarios (analysis cases) were analyzed to quantify the potential waterhammer loads (fluid forces). A tall riser piping system in the Reactor Building was chosen as a sample system for structural dynamic analysis. Based on the results of the sample system, it was concluded that the fire protection piping systems could experience piping stress far exceed the faulted allowable loads as allowed by the ASME/ANSI piping code. A cost-effective mitigation scheme was then proposed plan in paper ICONE65-82622[2] as Part 1 of this study with consideration only hydraulic loading where 11 vacuum breakers are to be installed at various locations of the FPS. The structural analyses discussed only piping stress in this paper and demonstrate for the effectiveness of the proposed mitigation scheme from the revised hydrodynamic loads. As the continuation of Ref. 2, the results from this study validate the acceptance of these two piping systems.