Safety assessment of external flooding protection for nuclear power plants

Methods to systematically analyse existing nuclear power plants (NPP) regarding the adequacy of their existing protection equipment against external hazards, e.g. flooding, can be of deterministic as well as probabilistic nature. In the past the adequacy of the protection measures has been assessed only on a deterministic basis. The German regulatory body has issued probabilistic safety assessment (PSA) guidelines, which had been elaborated for a comprehensive integrated safety review of all NPP in operation. Amongst others the guidelines imply, that probabilistic considerations regarding external flooding are required. This paper presents a newly developed graded approach for the probabilistic assessment of external flooding. Main aspects are explained such as the underlying probabilistic considerations and the mathematical procedures for the calculation of exceedance frequencies, which have recently been developed and issued as part of the German Nuclear Safety Standard. Exemplarily it has been investigated if extreme events such as tsunami waves could be a hazard for NPP at coastal sites in Germany. Here it could be shown that due to limited source mechanisms and the specific morphological conditions in the North Sea no dedicated measures for protection against tsunamis in the German Bight are necessary. (author)