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Analysis of Events Related to Flooding at NPPs

Abstract: 

This summary report presents the selected results of a statistical and engineering analysis of flooding and flooding protection related events registered in the following four databases: IRSN (Institut de Radioprotection et de Sûreté Nucléaire), GRS (Gesellschaft für Anlagen- und Reaktorsicherheit mbH), U.S. NRC (Nuclear Regulatory Commission) Licensee Event Reports (LERs), and the IAEA/NEA International Reporting System (IRS) during the last 20 years. In total 263 events reports are analysed including potential flooding. The events were analysed and classified into thirteen categories. The frequencies of all categories and families were presented for each database separately and for all events combined. This allowed comparison of events characteristics among all databases and cumulative results. A trend analysis was also performed. Finally, selected events were used to derive findings (lessons learned). No trend was identified over the whole analysed period. However, it is noticeable that the number of events is decreasing for all databases in recent couple years. Most of the events occurred during power operation, for all except for the one database. In the majority of events a real flood occurred, except for one database where potential flood events are dominant. Valves and passive components are the dominant direct causes in all databases, while seals and drainage are also important for the two databases. Training, qualification and procedures, with design configuration and analysis, are the most important root causes and causal factors for all databases, while maintenance activity and the equipment performance are also important for one database. Systems most often affected by flood are (reactor) auxiliary systems for all databases and additionally the waste management systems for one database. The majority of events do not have safety relevance except for one database, where low safety relevant events are dominant. The main purpose of corrective actions is: prevention, early identification and reduction of flood; fulfilment of the probable maximum flood requirements; reducing flood risk from non-safety systems and improving maintenance, review and inspection. Flooding and flooding protection related selected events were analysed in detail and the main conclusions are presented as list of more than 90 specific and 16 generic lessons learned. Findings are grouped related to the systems that has main role during the flooding event (i.e., non-safety systems interaction, buildings infrastructure and major cooling systems) and related to the type of activities that are (root) cause for the flooding event (i.e., maintenance, inspection, operating experience, analysis and configuration management). Generic findings are also addressing lessons learned related to mitigation, communication, modifications, requirements and configuration management. The findings from this summary report could be used as recommendations and are expected to help the licensees and regulatory authorities to prevent flooding events from occurring, to protect safety systems from flood and to improve flooding protection and mitigation.