Deterministic and Probabilistic Evaluations of Structures and Components Credited for Seismic Design Extension Conditions

Abstract

There is “high confidence” in the ability of the structures, systems, and components (SSCs) of nuclear power plants (NPPs) to perform as designed for design basis accidents (DBAs). For design extension conditions (DECs), the SSCs are required to perform as designed with “reasonably high confidence.” DECs represent scenarios or accidents that are more severe than DBAs. Typically, a spectrum of initiating events including random failure of systems or components, internal and external hazards is used in defining scenarios leading to the DECs. Seismic events that exceed the design basis earthquake (DBE) of a station could be considered seismic DECs. A deterministic design method is proposed to address higher demands of seismic DECs in the new and existing Canada Deuterium Uranium (CANDU) NPPs. The deterministic method builds on the current requirements of applicable codes and standards and recommends more relaxed acceptance criteria. Nevertheless, a means to probabilistically evaluate built-in margin exceeding the demand induced by a seismic DEC would provide a measure of the confidence in a DEC-assigned structure or component performing its function. Therefore, a probabilistic method that estimates the probability of survivability for a structure or component when subjected to the demand induced by a seismic DEC is proposed. The probabilistic method could be used to indicate whether there is a need for applying design modification to existing design features to address demands of seismic DEC. The mean, 5th-percentile, and 95th-percentile fragility functions of these SSCs are used. These fragility functions are typically developed to determine the high-confidence-low-probability-of-failure (HCLPF) value associated with the contribution of a structure or component to the overall plant seismic risk. Sample cases for design features that were implemented in existing CANDU NPPs to address seismic DECs are presented. Both the deterministic and probabilistic methods are applied to cases of civil structures, passive mechanical and electrical components, as well as active control and instrumentation components.