A Study on Initiating Event Models of Non-Mitigating System Induced by Fire in a Fire PSA

In this paper, if the initiating events (IEs) are modeled by only rooms ignition frequencies, this approach is called as 'lumped room IE model', and if IEs occurred by a fire are modeled by the fault trees (FTs) in which components are damaged by the fire, the approach is called as 'FT IE model' in the fire PRA. The compared results for non-mitigating systems are exactly same ones for the mitigating systems. For example, the lumped room IE model derives a conservative CDF, and the FT based IE model derives a more realistic CDF value for the nonmitigating systems. Since the lumped room IE model is relatively easy to build up, it is suitable in the automatic fire PRA modeling from internal events and screening analysis. In a new nuclear power plant, it may be difficult to develop a FT based IE model. In this case, the lumped room model for the initiating events is good conservative model. The disadvantage of the FT based IE model is that we should carefully review the cutsets since it is easy to make a wrong fault tree model for the IEs, and furthermore, if incomplete FTs are used for IEs, then CDF could be underestimated. For a complicated room where FT based IE model is not easily prepared, a lumped room IE model is good since the lumped room IE model is conservative.