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 information follows the general layout of SPRA development described in EPRI 3002000709. The methodology described here addresses the processes for the performance of a seismic probabilistic risk assessment (SPRA) that focuses on a Level 1 PRA (core damage frequency (CDF)) with consideration of large early release frequency (LERF). The wiki pages for each SPRA task describe the task overview (including objective, purpose, and scope), related element of the ASME/ANS PRA Standard, related EPRI guidance, and any supplemental guidance. The two most recent versions of the PRA Standard include ASME/ANS RA-Sa-2009 and ASME/ANS RA-Sb-2013. ASME/ANS RA-Sa-2009 was endorsed by the NRC in RG 1.200 Revision 2.

This initial issue of the 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.

SPRA Process Overview

The figure below is derived from EPRI 3002000709 and provides a flowchart of the seismic PRA process discussed in this SPRA wiki.

The following considerations are important in the use of this figure.

• A Seismic PRA is iterative. Certain tasks may need refinement after conducting one or more of the subsequent tasks. It may also be appropriate to incorporate only limited detail in the first pass through an analysis task, deferring the pursuit of additional detail pending the results of a later task. For example, the number of components in Task 2 (Seismic Equipment List) is likely to be revised after attempts at screening in Task 7 (Seismic Equipment List Screening). The flow chart does not attempt to incorporate potential feedback loops. Analyst judgment is needed to ensure that an appropriate overall analysis process is followed consistent with study objectives.
• Even though the process flow illustrated should work for predominant cases, users may find other analysis task sequences to be more appropriate for their objectives. Task sequence choices may, for example, be influenced by plant-specific seismic features as well as the availability and depth of plant information supporting the Seismic PRA. Each analysis task incorporates added detail into a given aspect of the Seismic PRA. Task ordering is subject to practitioner judgment.

The following is a short description of each technical task for the overall Seismic PRA methodology.

Technical Tasks

Safety Systems and Internal Events PRA (Task 1)

The first step in a Seismic PRA is to define the systems analysis or backbone of the model. A seismic PRA is usually constructed by modifying an existing Internal Events PRA model.

Develop Seismic Equipment List (Task 2)

The selection of components credited for plant shutdown following a seismic event is a critical step in any Seismic PRA. The Seismic Equipment List (SEL) includes the equipment and systems required to provide protection for all seismically induced initiating events and the structures that house them. The list should include all components needed to mitigate seismically induced fires and floods and to prevent early containment failure in an earthquake. Equipment in non-safety systems are also typically placed on the list, if credit for these systems is to be included to achieve a safe shutdown. Components on a previously developed Unresolved Safety Issue (USI) A-46 list or an IPEEE SMA list, if available, should also be considered for inclusion.

Seismic Induced Fire and Flood (Task 3)

Seismic activity may result in a pipe rupture which causes an internal flood or failure of a fuel tank resulting in a fire. This task is to identify potential plant vulnerabilities given the combined effects of a seismic event and consequential internal fire or internal flood hazards.

Seismic Walkdown (Task 4)

The plant walkdown is typically conducted by a team of systems engineers and seismic fragility analysts. All items on the initial Seismic Equipment List (Task 2) are usually physically examined for seismic vulnerabilities, if possible, and their locations should be recorded. During the walkdown, the access paths for human actions following plant trip should also be inspected to verify that the control stations can still be accessed following an earthquake.

Seismic Response Analysis and In-Structure Floor Spectra (Task 5)

This task involves the derivation of the best-estimate (or median-centered) seismic responses and their variability in the form of structural loads or floor response spectra. The loads and floor response spectra define the demand for which structures, systems and components (SSCs) are evaluated. These best-estimate loads and floor response spectra and their variabilities are typically obtained through simulation probabilistic response analysis, by new deterministic analysis with estimated variability, or by scaling of the safe-shutdown earthquake (SSE) responses and assigning variability. A common input for this analysis is the ground response spectrum median spectral shape for a 10,000-year return period along with variability estimates. Other input motions such as the ground motion response spectrum (GMRS) or a different return period could be justified. If available in time, results from the soil failures evaluation should also be considered.

Seismic Hazard Analysis (Task 6)

This task involves the development of a family of seismic hazard curves for the site in terms of the selected ground motion parameter (such as PGA). This task is usually carried out by specialist geotechnical engineers. Along with the family of hazard curves for horizontal ground motion, there should be guidance on how the fragility analysts are to account for vertical ground motion. The seismic hazard task also typically provides uniform hazard spectra (UHS) at a number of return periods.

Soil Failure Evaluation (Task 7)

The potential for soil liquefaction, slope failures, and damage to buried pipelines is assessed in this task. This task is usually carried out by specialist geotechnical engineers. To the extent that soil failures may impact plant structures housing PRA components, this task is usually performed early in the assessment. The structures and components of interest are provided by the Seismic Equipment List (Task 2).

Seismic Equipment List Screening (Task 8)

Certain high-capacity components may be screened out of the PRA components list based on a review of seismic qualification criteria and qualification documents and the walkdown screening. The screening level is typically chosen so that the contribution of screened components can be judged not significant to the final seismic CDF or LERF. The contribution of screened components can be estimated by bounding the frequency of seismic-caused component failure.

Relay Chatter Evaluation (Task 9)

This task involves identification and evaluation of relays whose chatter during an earthquake could result in adverse effects on plant safety. The identification of relays and the evaluations of the consequence of chatter on the electrical circuits are typically performed by the systems analysts with support from electrical engineers and plant operations staff. The seismic ruggedness of the relays, including the amplification of response through the cabinet into the relays, is typically evaluated by the seismic fragility analysts. In this Wiki, all contact chatter events are identified as relay chatter.

Seismic Fragility Calculations (Task 10)

This task is to calculate the conditional probabilities of structural or equipment failures for a given level of seismic ground motion for the screened-in components (those from Task 8).

Fault Trees and Accident Sequences (Task 11)

This task is to modify the internal events accident sequence models to address seismic initiating events. Assumptions about how to include each seismic failure mode into the seismic sequence models are typically made to account for the dependencies between trains in multi-train systems and for any other correlations identified by the fragility analysts. In addition to CDF, the seismic sequence model is usually capable of computing LERF. Therefore, this task includes an effort to adopt results from the internal events Level 2 analysis so that it can be used for seismic events.

Seismic Human Reliability Analysis (Task 12)

Operator actions in the Internal Events PRA are typically adjusted for seismic influences. In some cases, detailed seismic human reliability analysis (HRA) events or specific seismic HRA actions may be developed.

Seismic Risk Quantification (Task 13)

The task description provides recommendations for quantification and presentation of seismic risk results. Uncertainty and sensitivity analyses are also typically addressed in this task.

Seismic PRA Outputs (Task 14)

Given that the typical focus of the SPRA is not on bottom-line numbers (that is, the seismic CDF and LERF) but on the insights of the examination, a number of intermediate results are typically utilized. These results may include the contributions of each seismic failure mode to CDF and LERF and the relative contributions of each seismic interval to the total risks. Risk significant sequences contributing to CDF and LERF are also often identified.

Seismic PRA Report (Task 15)

This task involves documenting the SPRA methodology used and the results of the study. The form of the report is typically generated to be useful for peer review and suitable to update and apply to risk-informed applications.

Peer Review (Task 16)

After the draft report is prepared, a peer review of the procedures, numerical results, and insights obtained from the PRA is usually conducted. This is a culmination of the review process that has been implemented throughout the earlier tasks. The peer review is expected to produce a short report evaluating the SPRA study and identifying potential enhancements (e.g., Facts and Observations (F&Os)). The review is typically performed using the criteria of Section 5-3 of ASME/ANS Standard, except those that do not apply to seismic events.