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| − | 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 [http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=3002000709 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 [https://www.nrc.gov/docs/ML0904/ML090410014.pdf RG 1.200 Revision 2]. | + | 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. |
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| − | 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.
| + | 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. |
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| − | ==SPRA Process Overview==
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| − | The figure below is derived from EPRI [http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=3002000709 3002000709] and provides a flowchart of the seismic PRA process discussed in this SPRA wiki.
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| − | The following considerations are important in the use of this figure.
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| − | *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.
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| − | *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.
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| − | <imagemap>
| + | 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. |
| − | File:SPRA_Flowchart_for_Wiki.jpg|800px
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| − | rect 13 13 321 124 [[#Safety Systems and Internal Events PRA (Task 1)]]
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| − | rect 448 147 755 256 [[#Develop Seismic Equipment List (Task 2)]]
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| − | rect 12 225 319 338 [[#Seismic Induced Fire and Flood (Task 3)]]
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| − | rect 449 307 756 420 [[#Seismic Walkdown (Task 4)]]
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| − | rect 448 470 755 583 [[#Seismic Response Analysis and In-Structure Floor Spectra (Task 5)]]
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| − | rect 884 473 1191 586 [[#Seismic Hazard Analysis (Task 6)]]
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| − | rect 884 717 1191 830 [[#Soil Failure Evaluation (Task 7)]]
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| − | rect 449 635 756 748 [[#Seismic Equipment List Screening (Task 8)]]
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| − | rect 14 716 321 829 [[#Relay Chatter Evaluation (Task 9)]]
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| − | rect 449 797 756 910 [[#Seismic Fragility Calculations (Task 10)]]
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| − | rect 449 962 756 1075 [[#Fault Trees and Accident Sequences (Task 11)]]
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| − | rect 14 961 321 1074 [[#Seismic Human Reliability Analysis (Task 12)]]
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| − | rect 448 1124 755 1237 [[#Seismic Risk Quantification (Task 13)]]
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| − | rect 448 1287 755 1400 [[#Seismic PRA Outputs (Task 14)]]
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| − | rect 448 1449 755 1562 [[#Seismic PRA Report (Task 15)]]
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| − | rect 884 1450 1191 1563 [[#Peer Review (Task 16)]]
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| − | desc none
| + | [[File:00_SPRA_Overview.jpg|500px|thumb|right|Seismic risk assessment methodology (from EPRI 3002000709 and multiple other sources)]] |
| − | </imagemap>
| + | 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) |
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| | + | 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. |
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| | + | 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. |
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| − | The following is a short description of each technical task for the overall Seismic PRA methodology. | + | 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. |
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| − | ==Technical Tasks==
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| − | ===[[Safety Systems and Internal Events PRA (Task 1)]]===
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| − | 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.
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| − | ===[[Develop Seismic Equipment List (Task 2)]]===
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| − | 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.
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| − | ===[[Seismic Induced Fire and Flood (Task 3)]]===
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| − | 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.
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| − | ===[[Seismic Walkdown (Task 4)]]===
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| − | 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.
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| − | ===[[Seismic Response Analysis and In-Structure Floor Spectra (Task 5)]]===
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| − | 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.
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| − | ===[[Seismic Hazard Analysis (Task 6)]]===
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| − | 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.
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| − | ===[[Soil Failure Evaluation (Task 7)]]===
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| − | 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).
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| − | ===[[Seismic Equipment List Screening (Task 8)]]===
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| − | 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.
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| − | ===[[Relay Chatter Evaluation (Task 9)]]===
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| − | 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.
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| − | ===[[Seismic Fragility Calculations (Task 10)]]===
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| − | 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). | |
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| − | ===[[Fault Trees and Accident Sequences (Task 11)]]===
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| − | 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.
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| − | ===[[Seismic Human Reliability Analysis (Task 12)]]===
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| − | 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.
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| − | ===[[Seismic Risk Quantification (Task 13)]]===
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| − | The task description provides recommendations for quantification and presentation of seismic risk results. Uncertainty and sensitivity analyses are also typically addressed in this task.
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| − | ===[[Seismic PRA Outputs (Task 14)]]===
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| − | 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.
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| − | ===[[Seismic PRA Report (Task 15)]]===
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| − | 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.
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| − | ===[[Peer Review (Task 16)]]===
| + | 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. |
| − | 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.
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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.
