ML26006A148
| ML26006A148 | |
| Person / Time | |
|---|---|
| Issue date: | 01/06/2026 |
| From: | Michelle Gonzalez NRC/RES/DRA, NRC/RES/DRA/PRAB |
| To: | |
| References | |
| Download: ML26006A148 (17) | |
Text
Accident Sequence Precursor (ASP) Program PRA Realism Public Workshop January 6, 2026 Christopher Hunter Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch
Topics
- Background
- Revised ASP Analysis Review Process
- Inclusion of Hazards Beyond Internal Events
- SPAR Model Feedback
- Common-Cause Failure (CCF) Modeling and Treatment
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Background===
- ASP Program analyses provide a holistic view of the risk significance of events and degraded conditions.
- SDP results are leverage whenever possible.
- Independent analyses are performed for:
- Degraded conditions with no licensee performance deficiency.
- Concurrent degraded conditions/unavailabilities, including equipment undergoing testing or maintenance.
- ASP Program results are used to evaluate industry-wide trends and insights of risk/safety significant events.
Revised ASP Analysis Review Process
- In 2024, all precursor analyses started being sent to licensees for a 30-day review period.
- Prior to 2024, only precursors with a CCDP/CDP 10-4 were sent for licensee review.
- This change was largely made due to ASP analyses now evaluating hazards other internal events.
- This evaluation includes applicable hazards that may not be included the SPAR model.
- Licensee response is optional.
- Provides an opportunity to ensure that analysis assumptions are appropriate for the event/degraded condition and the results are reflective of the as-built, as-operated plant.
Inclusion of Additional Hazards
- Historically, ASP analyses of degraded conditions only considered hazards that were included in all SPAR models.
- Not including applicable hazards in the ASP evaluations, especially internal fires, could have significant impacts on the results and insights.
- Seismic hazards started being considered in 2018.
- High winds (including hurricanes and tornadoes) started being considered in 2020.
- However, internal fires and floods are not included in all SPAR models.
- In addition, some SPAR models include these hazards that are based on older information.
Inclusion of Additional Hazards (cont.)
- If the SPAR model does not include a specific hazard, analysts leverage all available risk information.
- License amendment requests.
- Discussion with SRAs.
- Importance measures from licensee PRA.
- IPE/IPEEE submittals.
- Risk from these hazards can be treated quantitatively or qualitatively.
- Given the high uncertainties associated with this approach, the focus is to determine if unmodeled hazards would result in the risk of degraded conditions to exceed precursor thresholds.
SPAR Model Feedback
- Issues identified during analyses are considered for base SPAR model changes.
- Technical issues are identified and forwarded to the GREATR Group for consideration.
- Examples
- FLEX equipment mission time
- Safe/stable end state requirements
- AC power recovery requirements
CCF Modeling and Treatment
- Focus on reducing uncertainties within the current data collection and modeling approaches.
- Developed CCF parameters using component-specific priors.
- Use of the generic prior results in CCF impacts on components with dissimilar components that may have a stronger/weaker coupling.
- Developed revised guidance on common cause component group modeling.
- New guidance will reduce overcounting of CCF impacts.
- Developed interim approach to treatment of cross-unit CCF.
- Use of existing parameters will overestimate CCF across units in most cases.
- Initiated research to calculate cross-unit CCF parameters for key components.
Questions
RASP Handbook Update PRA Realism Public Workshop September 30, 2025 Christopher Hunter Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch
Overview
- Revision 3 of Volume 1 of the RASP Handbook for Internal Events was completed on December 19, 2025 (ML25344A036).
- Many relatively small revisions, including editorial changes, were made to add clarity and to improve the readability of the document.
- However, some sections needed more substantial revisions, including new guidance.
- Revised the section numbers to increase ease of reference and reduce the excessive use of bullets.
- Appendix A, Road Map for Risk Analysis of Operational Events, was eliminated.
- The material contained within was never considered formal guidance and was rarely (if ever) used by the SRAs.
- Updated the guidance for the various cases on revising mission times.
- Clarified that the revision of mission times should be rare and limited to components that are not required to operate for the complete PRA mission time.
- Provided updated guidance on revising mission times in the SPAR models.
- Multiple, redundant components for all accident sequences.
- Single component for all accident sequences.
- Mission time changes for limited set of accident sequences.
- Mission time changes for applicable cut sets.
- Section was completely rewritten to update guidance and increase readability.
- Added background material on the key CCF models (Alpha, MGL, Beta).
- Added background material on how CCCGs are modeled in the SPAR models and new guidance on CCCG modeling to reduce overcounting/overestimation of potential CCF.
- Added new guidance on treatment of intersystem CCFs including cross-unit CCF along with an interim approach for adjusting cross-unit CCF parameters.
CCF (cont.)
- Provided information on how different CCF parameters are calculated.
- Added a section on causal alpha factors, including their limitations, and the recommendation that they should not be used in most cases.
- Replaced the CCF ground rules with updated key principles from NUREG-2225, Basis for the Treatment of the Potential CCF in the SDP, (ML18274A198).
- Updated the CCF examples and included a detailed example of how SAPHIRE calculates the revised CCF probability.
LOOP Events
- Section was completely rewritten to expand guidance for all LOOP events (i.e., not just LOOP initiating events) and increase readability.
- Added key LOOP definitions.
- Added a section on modeling of FLEX mitigation strategies in the base SPAR models, along with several key areas that analysts may need to revise on a plant-specific basis or at least understand the uncertainties associated with the existing approach.
- Added a section that provides guidance for analyzing the various LOOP events.
Initiating Events, HRA, and SSIEs
- Reordered the three cases and eliminated Case 4, which was associated with SSIE component failures.
- Revised language to be inclusive of the three programs (SDP, ASP, and MD 8.3) and to match what is currently done in the SDP.
- Updated this section to include information on IDHEAS-ECA and the IDHEAS dependency method.
- SSIEs
- Revised and expanded the examples for the various SSIE ECAs.
Questions