ML24071A046
| ML24071A046 | |
| Person / Time | |
|---|---|
| Issue date: | 03/11/2024 |
| From: | Michelle Kichline NRC/NRR/DRA/APOB |
| To: | |
| Kichline M | |
| Shared Package | |
| ML24071A045 | List: |
| References | |
| Download: ML24071A046 (17) | |
Text
Risk-Informed Programs Used at the U.S. Nuclear Regulatory Commission Michelle Kichline Senior Reliability and Risk Analyst Office of Nuclear Reactor Regulation 1
2 Agenda
- Background on Risk-Informed Regulation
- Established Risk-Informed Programs
- Risk-Informed Categorization
- Passive Categorization Programs
- Aging Management Programs
- Key Messages
3 Risk-Informed Regulation
- The U.S. NRC licenses and regulates the civilian use of radioactive materials to provide reasonable assurance of adequate protection of public health and safety, to promote the common defense and security, and to protect the environment.
- In 1995, the Commission issued the Probabilistic Risk Assessment (PRA)
Policy Statement (60FR42622), which states, in part:
1)
The use of PRA technology should be increased in all regulatory matters to the extent supported by the state-of-the art in PRA methods and data and in a manner that complements the NRCs deterministic approach and supports the NRCs traditional defense-in-depth (DID) philosophy.
2)
PRA should be used in regulatory matters, where practical, to reduce unnecessary conservatism associated with current regulatory requirements, regulatory guides, license commitments, and staff practices.
4 Principles of Risk-Informed Decision Making Regulatory Guide 1.174 (ML17317A256) Figure 2
5 Established Risk-Informed Programs Over the last 20 years, the NRC has approved industrys requests for alternative inspection programs for passive components that leverage risk insights as part of their technical basis to ensure adequate protection is maintained. Examples include:
- Elimination of circumferential weld inspections in BWR pressure vessels (BWRVIP-05, EPRl TR-105697, ML032200246, and GL 98-01),
- Extension of the inspection interval for PWR pressure vessel welds from 10 years to 20 years (WCAP-16168-A, ML11306A084)
- Revised risk-informed inservice inspection (ISI) evaluation procedure (EPRI TR-112657 Rev. B-A, ML013470102)
- Extension of integrated leak rate testing (ILRT) from 3 inspections in 10 years to 1 in 10 years and then 1 in 15 years (NEI 94-01, ML12221A1202)
6 Basis for Risk-Informed Programs
- A bounding risk assessment showed the risk associated with failure of each component would be within acceptance guidelines
- the risk assessment evaluated the consequence (conditional core damage probability (CCDP)) associated with failure of the component
- Operating experience from completed inspections confirmed acceptable past performance.
- Risk assessment and operating experience were used to justify extension of inspections.
- Performance monitoring ensures the extensions remain appropriate.
7 Risk-Informed Categorization
- Title 10 of the Code of Federal Regulations (10 CFR) Section 50.69, Risk-Informed Categorization and Treatment of Structures, Systems, and Components for Nuclear Power Plants, (69FR68047) established a process for characterizing the safety significance of both active and passive components.
- The 10 CFR 50.69 categorization process relies on both risk assessment results and deterministic safety considerations to classify components as high or low safety significant (HSS or LSS).
- Risk alone is not sufficient to identify whether components are HSS or LSS, especially for passive pressure boundary components.
Robust Categorization Process Integrated Decision-making Panel (IDP)
- Final decisions using PRA insights and non-PRA aspects
- Highly experienced plant personnel with combined expertise in PRA, Safety Analysis, Operations, Design and System Engineering
- Guidance describes a well-defined, highly structured process
- Documentation requirements PRA / Risk Analyses Internal Events Other External Events Seismic Pressure Boundary Failures Sensitivity Studies Fire Qualitative Questions Non-PRA Defense-in-Depth Periodic Review and Update 8
9 RISC-1 Safety-Related, Safety Significant RISC-2 Non-Safety Related, Safety Significant RISC-3 Safety-Related, Low Safety Significant RISC-4 Non-Safety Related, Low Safety Significant 10 CFR 50.69 Categorization
10 Risk-Informed Alternative Treatments
- 10 CFR 50.69 allows the use of alternative treatments to meet certain requirements for components that are classified as LSS.
- Alternative treatments are allowed for items listed in 10 CFR 50.69(b)(1),
which includes the following requirements:
- Quality assurance (10 CFR 50 Appendix B)
- Environmental qualification (10 CFR 50.49)
- Containment leakage testing for certain penetrations and valves (10 CFR 50 Appendix J)
- Inservice testing, inservice inspection, and repair and replacement activities (RRA) (10 CFR 50.55a)
- Seismic qualification (10 CFR 100 Appendix A)
11 Risk-Informed Alternative Treatments
- Alternative treatments must ensure that safety related LSS components (RISC-3) remain capable of performing their safety functions under design basis conditions.
- Industry has found alternative treatments especially beneficial for quality assurance and RRA.
- The passive categorization process approved for use in 10 CFR 50.69 has been proposed for use in other areas, including aging management.
12 Passive Categorization Processes The NRC has accepted the following methods for categorization of passive ASME Code Class 2 and 3 components as part of 10 CFR 50.69:
- ASME Code Case N-660, Risk-Informed Safety Classification for Use in Risk-Informed Repair/Replacement Activities.
- ANO2-R&R-004, Revision 1, Draft ASME Code Case N-752, Risk-Informed Safety Classification and Treatment for Repair/
Replacement Activities in Class 2 and 3 Moderate Energy Systems, at Arkansas Nuclear One, Unit 2.
13 Categorization Process Differences
- 10 CFR 50.69(c)(v) states the categorization process must be performed for entire systems and structures, not for selected components within a system or structure.
- ASME Code Case N-752 differs from 10 CFR 50.69 in that it allows categorization of individual components.
- N-752 characterizes risk using CCDP and incorporates a multidisciplinary review to evaluate defense-in-depth.
14 Aging Management Programs
- License renewal aging management programs (AMPs) are used to manage aging of passive long-lived components within the scope of license renewal.
- Several AMPs are condition monitoring programs that use a sampling approach to ensure an aging affect is not being experienced.
- The sampling approach was often initially loosely based on providing a 90%
confidence that 90% of the population would not experience degradation.
- In some cases, the sampling philosophy has been relaxed in subsequent revisions to the AMPs, with the addition of compensatory measures for extent of condition.
15 Risk Informing Aging Management
- The passive categorization processes used in risk-informed RRA are a good basis for determining high and low safety significance, but special attention may be needed for passive components important for mitigating external events.
- Component risk categorization alone is not sufficient to determine appropriate sample sizes for identifying aging effects.
- Some aging effects may not become apparent until later in life.
- The sampling approach used in aging management could be more rigorously determined using statistical analysis of performance monitoring results, similar to the process used in probabilistic fracture mechanics.
16 Key Messages
- Probabilistic risk assessment results constitute one input to determining the overall safety significance of systems, structures, and components.
- Performance monitoring and maintaining adequate defense-in-depth and safety margins are key principles that must be considered when discussing risk significance, inspection frequency changes and aging management issues.
- Staff positions established in reviewing risk-informed inservice inspection relaxations are relevant but may require additional considerations to support assessing aging management related changes.
17 Questions?