ML22089A037
| ML22089A037 | |
| Person / Time | |
|---|---|
| Issue date: | 04/14/2022 |
| From: | David Rudland NRC/NRR/DNRL |
| To: | |
| Rudland D | |
| References | |
| Download: ML22089A037 (11) | |
Text
Can ISI be Eliminated with Risk-Informed Decision Making David L. Rudland U.S. Nuclear Regulatory Commission International ISI Workshop April 11-14, 2022 Virtual Workshop The views expressed herein are those of the authors and do not represent an official position of the U.S. NRC
- First published in 1970
- A component, as designed and constructed, is acceptable but a preservice baseline examination is required. Subsequent inservice examinations are compared to this pre-service exam
- Similar components in similar conditions should act the same - Sampling programs were developed
- NDE methods were developed to look for service induced degradation April 11-14, 2022 2
ASME BPV Section XI - Sampling
- Originally all vessel welds were inspected at 5% for circ welds and 10% for longitudinal welds - NRC requested 100% of vessel welds - ASME agreed in hopes automation would allow this without burden - Didnt happen!
- Pipe welds - Class 1 - 25%, Class 2 - 7.5% - due to lesser safety significance
- 10-year interval was chosen based on historical failure rate from non-nuclear steam and petrochemical systems - Not a strong basis
- Degradation is not solely random - sampling programs may not always be effective at revealing degradation April 11-14, 2022 3
Risk-informed Modifications to Inspection
- Degradation occurs where the conditions necessary for a particular mechanism exist - typical ISI programs are not focused.
- Locations that have a higher failure potential with a significant consequence of that failure (i.e., risk) can be targeted for inspection April 11-14, 2022 4
- NRC uses a risk-informed decision-making methodology to determine impact
- But we only know what we learned from operational history Integrated Decision Making Defense-in-depth Safety Margins Increase in risk or CDF is small Monitoring Change meets current regulation
Using Risk Arguments for Inspections April 11-14, 2022 5
Operational History April 11-14, 2022 Data from Nuclear Energy Agency (NEA) has operated an event database project, Component Operational Experience, Degradation and Aging Program (CODAP)
Due to IGSCC 1.5 mechanisms/year The potential for new mechanisms must be considered 6
6 mechanisms/year
Risk Arguments for ISI April 11-14, 2022 7
Monitoring Analysis (PRA, PFM, etc)
Result Snapshot in time Real time Results
- Probabilistic fracture mechanics and other advanced analytical techniques model the state-of-the-art with the current-day knowledge
- Modelling the unknown is always a major question and limitation of these analyses
- Monitoring provides the necessary feedback
Examples
- Risk-informed ISI for Piping Uses evaluation of pipe failure consequence, and risk impacts of inspection changes to develop programs Any reduction in inspection is continually monitored through the performance monitoring program Updates to the RI-ISI program are performed at least on a 10-year interval and includes any industry OE Appropriate modifications of the ISI plan developed if new or unexpected degradation mechanisms occur Experienced-based and leverage the highly reliable performance and relatively low risk of nuclear power plant piping April 11-14, 2022 8
Examples
- Class 1 BWR Vessel welds BWRVIP-05 provided the basis for the elimination of circumferential weld inspections in BWR pressure vessels - used probabilistic fracture mechanics (PFM)
NRC staff did independent analyses and used principles of risk-informed decision making Demonstrated the failure frequency of the circumferential welds without ISI was well below the acceptance criteria defined in NUREG-1806 (pressurized thermal shock)
Axial welds and approximately 3% of circumferential welds are still inspected Allows performance monitoring of the circumferential welds to verify that the analyses used in the failure predictions remains accurate April 11-14, 2022 9
Examples
- 10 CFR 50.69, Risk-informed categorization and treatment of structures, systems and components for nuclear power reactors Reduces unnecessary regulatory burden for system, structures and components (SSCs) of low safety significance by removing these SSCs from the scope of special treatment requirements Blends risk insights, new technical information, and operational experience For ISI, safety-related SSCs that perform low safety significant functions (RISC-3), and nonsafety-related SSCs that perform low safety significant functions (RISC-4) can voluntarily comply with ISI rules RISC-3 category still need to undergo periodic inspections to verify that they can still perform as expected April 11-14, 2022 10
Summary
- ISI has been a safety tool for detecting changes in passive components due to material degradation since the 1970s
- The operational experience in the U.S. indicates that new degradation continues to present on a periodic basis, and the current analytical assumptions may not account for these future changes
- For safety related and/or safety significant passive components, performance monitoring is needed even if the change in risk is considered acceptable when ISI is eliminated
- Performance monitoring can verify risk analysis assumptions, identify possible unknown or unexpected emergent degradation, and verify continued safety of the plant April 11-14, 2022 11