Seismic Risk Quantification (Task 13)

Task Overview

Objective

The objective of this task is to quantify CDF and LERF frequency from the SPRA model.

Purpose

This section provides guidance for assembling the results of the seismic hazard analysis, fragility analysis, and seismic sequence models, after they are completed, to estimate the CDF and LERF.

Guidance

One approach used in SPRA is to develop a discrete hazard interval model using a PGA-based hazard curve as the base SPRA model quantification. The modeling approach is illustrated in the following figure.

Figure 13-1 Graphical depiction of a typical seismic probabilistic risk assessment modeling approach (from EPRI 3002000709)

The elements of the analysis are summarized below:

• Seismic initiators derived from site-specific probabilistic seismic hazard analysis (Seismic Hazard Analysis (Task 6))
• Seismic accident sequences developed by using a seismic initiating event tree (SIET) and a set of accident sequence event trees from the FPIE PRA (Fault Trees and Accident Sequences (Task 11))
• Seismic System fault trees (Relay Chatter Evaluation (Task 9), Seismic Fragility Calculations (Task 10), Fault Trees and Accident Sequences (Task 11))
• Fragility analysis (Seismic Fragility Calculations (Task 10)) is used to model seismically induced system failure modes in the event tree and fault tree models (Fault Trees and Accident Sequences (Task 11))
• Crew interface with accident mitigation systems is incorporated into the seismic system fault trees as modified by the fragility analysis (Fault Trees and Accident Sequences (Task 11), Seismic Human Reliability Analysis (Task 12))
• Software is used to process this information into a cohesive framework and quantify the models

Software

The analytic tools for the development of a quantified model may use the EPRI CAFTA code, augmented by the ACUBE Binary Decision Diagram (BDD) software (or an alternative BDD software). The CAFTA code is well tested and widely used in various industries and numerous countries. CAFTA is available together with ACUBE, PRAQuant, FRANX and other tools through the EPRI .

The SPRA model is typically developed in a modular approach. Event trees (converted to fault trees), event tree top logic (nodal fault trees), and system-level fault trees are all developed as distinct files. In addition, the FRANX tool may be used to develop a relational database for linking individual fragility events to existing, modeled basic events. This modular structure allows individual files to remain manageable and reviewable. A single-top model used for quantification may be developed by merging the previously described files. Merging the files into a single-top model is a standard CAFTA modeling technique that is performed by the analyst.

The model may be quantified using PRAQuant, from the Phoenix Architect software suite. Also, due to the special circumstances within seismic modeling (for example, overcounting caused by numerous high failure probability events), the ACUBE code, which uses the BDD algorithm, or an alternative software is used in model quantification to obtain a realistic assessment of the total CDF risk metric (or individual accident sequence frequencies as desired).

Supplemental Guidance

Related Element of ASME/PRA Standard

Part 5, Seismic Plant-Response (SPR)

EPRI Guidance

Seismic Probabilistic Risk Assessment Implementation Guide (3002000709), Appendix C Section C.2 describes software and quantification algorithms.

Phoenix Architect v2.0 Suite (3002023512)

Other Guidance

(none noted)