An approach to perform internal flooding PSA for power plants

Abstract

Pertaining to nuclear power plant (NPP) safety, the flooding has been realized as one of the important phenomenon after Fukushima Daiichi accident, as inundation of Fukushima site caused an extended SBO up to 14 days for few units. It is important to analyze all possible internal and external flood hazards which may impact the safety of nuclear facility. This article covers Probabilistic Safety Assessment (PSA) of an internal flooding case only. There are numerous water storages in nuclear power plants, which may cause internal flooding. This paper gives a detailed approach to perform internal Flood PSA highlighting all tasks such as criteria for selection of flooding sources, type of floods according to break sizes, calculation of flooding frequency based on failure probabilities of large volume tanks & high pressure pipelines. The impact of flooding on plant safety has been assessed through flood mitigation by operator action or plant systems, which have been modeled in event tree of the Risk spectrum software. A case study of internal flooding due to component cooling water pipes and failure of high pressure piping of different systems has been performed to analyze impact of flooding on plant safety. The features to control and overcome this phenomenon in plant are also suggested in this paper.