This documentation summarized initial events assessment of auxiliary building as a result of internal flooding based on Chinese 1000MWe PWR. Detailed screening procedure of plant areas and quantitative frequency of each identified initial event are presented. In order to implement the analysis, the whole auxiliary building is divided into several areas, depending on elevation, isolation and function. Areas that are essential to plant operation and that house safe shutdown equipment are screened through a two-step process to determine the susceptibility to an internal flood. After this process, areas that are not eliminated are where flooding scenarios may occur. All possible flooding scenarios are reviewed to determine if they could result in any of the initiating events identified in the internal initiating events analysis which belongs to Level 1 PSA. For those flooding scenarios that do not result in an existing initiating event, a new initiating event is developed. Eventually, the frequency of each initiating event is achieved by identifying each flooding source and its failure mode, and correlating this to failure data. It must be paid attention to that the identification of internal flood hazards is plant specific since the routing of piping systems varies substantially from plant to plant. Therefore, the conclusion obtained from this issue may be not applicable for other nuclear power plant.

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