Seismic PRA

This wiki is intended to serve the needs of a seismic risk analysis team by describing a structured framework for conduct of the overall analysis, as well as providing references to specific recommended practices to address each key aspect of the analysis.

This initial issue of the seismic probabilistic risk assessment (SPRA) Wiki provides overall guidance, with an emphasis on systems analysis, equipment list development, human reliability analysis, and model development and quantification. Subsequent releases will add additional guidance for seismic hazard and fragility guidance.

The figure below illustrates the major elements of an SPRA, all the way through to a complete consequence analysis. This wiki addresses the left most parts of this diagram up to the release frequency, focusing on the steps for performing an SPRA to determine core damage frequency (CDF), with consideration of large early release frequency (LERF). This is consistent with the criteria in the ASME/ANS Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment of Nuclear Power Plant Applications. After the plant damage state frequencies are known, at the utility’s option, the remaining consequence analysis and resulting risk curves from the SPRA release frequencies could be determined using similar processes as from other internal events or external events PRAs.

Seismic risk assessment methodology (from EPRI 3002000709 and multiple other sources)

The three key elements of an SPRA are shown in the figure as:

• Seismic hazard analysis (highlighted in yellow)
• Seismic fragility evaluation (highlighted in blue)
• Systems analysis and consequence analysis (highlighted in green)

The seismic hazard analysis evaluates the frequencies of occurrence of different levels of earthquake ground motions at the site. This includes a probabilistic evaluation of significant ground motions that could occur at the site due to the entire range of possible earthquake magnitudes. Structural frequencies between 0.5 Hz and 100 Hz are typically considered, although the hazard is typically characterized by a single parameter such as the peak ground acceleration (PGA), which the other structural frequencies carried along as a scale factor from the single parameter.

The seismic fragility evaluation estimates the probabilities of failure of important structures, systems, and components (SSCs) that contribute to mitigating a seismic event at the site. The fragility of an SSC is calculated using the ratio of the SSC seismic capacity to the SSC seismic demand at its mounting point. The SSC realistic seismic capacity is determined using a variety of information sources from seismic analyses, to shake table data, to earthquake experience data and is typically well above the demonstrated design basis capacity. The mounting point seismic demand is derived from the site seismic hazard and realistic seismic response analysis. The fragility is typically expressed as the median capacity (the input motion where there is a 50% chance of failure) and a characterization of the uncertainties.

The systems analysis models the combinations of structural and equipment failures that could initiate and propagate a seismic core damage sequence. The systems analysis includes the development of the event trees for accident sequence modeling, and of the logic models for each of the individual event tree top events. Both seismically induced failures and random failures must be considered in the logic. Some knowledge of the failure probabilities for seismically induced failures is required so that these failure modes can be considered in the logic. This element also includes the assembly of results from the other key elements to estimate the frequencies of core damage and plant damage states, as well as the uncertainties in those estimates.

The information in this wiki is laid out as a series of tasks to be performed in developing the SPRA. An overview of the tasks is presented in the Seismic PRA Development Tasks page, followed by a series of pages for each task.