Containment Structures of U.S. Nuclear Power Plants: Background, Regulations, Codes and Standards, and Other Considerations
8 Design Basis and Beyond Design Basis Considerations of Natural Phenomena
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Containment structures are designed to remain functional when subjected to the loading from the natural events, such as earthquakes, flood water heights and associated effects, and high winds. Traditionally, the design bases for these natural hazards, particularly for seismic and flooding, have been evaluated using deterministic methods considering maximum historical events and accounting for uncertainties in characterizing such events. Our understandings of the natural hazard phenomena and our experiences from actual events have shown that the design bases are not bounding events and could be exceeded, albeit with low likelihoods. The evolution of probabilistic hazard methods has given us an ability to characterize hazard in terms of frequency of exceedance. These methods, in conjunction with the risk assessment methods, allow us to evaluate performance of a plant when subjected to design bases or beyond design bases natural events.