With the increasing human activities and natural environment changes in the vicinity of nuclear power plants, intake structure blockage has become a major risk to challenge the nuclear power plant operation safety. This risk has been ignored for long time. There are a lot of external events that may cause intake structure blockage, such as biological events, floating objects, sand sedimentation, oil spilling, etc. The root causes to form these events are very complex and they are also very site-specific. As these external events occurrence frequencies are very difficult to be predicted due to very limited acknowledgement, current practice seldom uses detailed quantitative method to evaluate the intake structure blockage risk. This study proposed a probabilistic method in this risk analysis. The process includes external events identification and screening, plant response and quantification. A nuclear power plant was selected in this analysis as a case study. This method took a systematic process and identified some external events that were dominant contributors to the intake structure blockage. The result indicated that intake structure blockage caused by external events could be dominant risk contributors to the plant and their risk should not be ignored. Some recommendations were provided to reduce the risk. Engineering practice proves that this method is very useful to identify the plant weak points for the external events and can help to improve the plant safety.

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