Multi-Unit Seismic Probabilistic Safety Assessment Initiating Event Models. Annex I

Annex I describes the treatment of seismically induced loss of coolant accident (LOCA) initiating event frequencies for multi-unit probabilistic safety assessments (MUPSAs) based on examples from the Seabrook MUPSA. Seismic events have the potential for wide ranging effects on nuclear power plants and the surrounding regions. They represent one of the most important types of common cause initiating event (CCIE) considered in a PSA. For an event to be classified as a CCIE, it has to be capable of causing an initiating event and failure of one or more systems or structures that are needed to prevent the initiating event from progressing into an accident and to mitigate the consequences. A seismic event certainly meets these criteria. Moreover, a seismic event at a multi-unit site can cause initiating events and accident sequences on each reactor unit as well as on multiple reactor units concurrently. This annex explores the modelling of seismically induced initiating events on a multi-unit site and considers the effects of seismic correlation between identical components that share the same fragility curves. Seismic correlation is often used to describe two or more identical components that share a common fragility curve which fail at the same time due to a seismic event because of common analysis elements considered in the fragility development. Such common elements include the frequency content of the seismic waves arriving at the site and the common responses of soil and structures to the ground motion. In this publication, this phenomenon is referred to as seismically induced common cause failure (CCF) because when considering the impact on the components in question, this is really no different than other types of CCF modelled in a PSA, except that in this case the cause of failure and the cause of the synchronization of the times of failure is a seismic event.