ABSTRACT
Tornadoes, tropical cyclones, and floods are considered among the most severe external events which may affect the safety of a plant. The extent of protection required against these events depends on their probability of occurrence and on the hazard the plant may present under their effects. Regarding nuclear power plants, it is required that “safety-related” structures, systems, and components be protected in order to carry out their safety function under the most severe natural events and in absence of all the external electrical supplies. In the deterministic approach the reference maximum event is derived from a comprehensive application of the most severe, reasonably possible meteorological and hydrologic factors. Design basis floods are commonly evaluated by extrapolating existing records and data up to probabilities regarded of interest for plant protection; all relevant events should be considered and an analysis should be performed to evaluate which events may occur simultaneously.
