ML23114A034
| ML23114A034 | |
| Person / Time | |
|---|---|
| Issue date: | 04/27/2023 |
| From: | Ali Rezai, David Rudland, Dan Widrevitz NRC/NRR/DNRL/NPHP |
| To: | |
| References | |
| Download: ML23114A034 (1) | |
Text
Probabilistic Fracture Mechanics and Performance Monitoring U.S. NRC Public Meeting April 27th, 2023
Overview The topics
- Probabilistic Fracture Mechanics
- Risk-Informed Decision Making
- Performance Monitoring
- Precedent Examples
- Example Sampling Approach 4/27/2023 2
Key Take-Aways Probabilistic fracture mechanics (PFM) is inherently risk-informed.
Precedent including PFM and similar approaches has consistently relied on performance monitoring to holistically provide assurance.
No amount of analysis can entirely substitute for performance monitoring OE has shown that assumptions about components can be incorrect
- Unexpected degradation mechanisms (French SCC)
- Unusually vulnerable heats of materials (Davis Besse Second RPV Head)
- Undocumented repairs (Numerous)
Statistical techniques can be used to justify performance monitoring proposals 4/27/2023 3
Probabilistic Fracture Mechanics PFM brings together information from the risk-triplet,
- What can happen
- How often
- What are the consequences For example, PFM can be used to estimate the probability of leakage or rupture of a pressure-boundary component.
The outcome of PFM is inherently a risk-insight.
Consequently, any submittal that has as its basis PFM technology, will be reviewed by the U.S. NRC as a risk-informed submittal.
4/27/2023 4
Probabilistic Fracture Mechanics U.S. NRC recognizes Probabilistic Fracture Mechanics (PFM) as a leading technique for managing risk-informed management of long-lived passive components.
The U.S. NRC has accepted and used PFM in many areas, such as:
- RPV ISI optimization (BWRVIP-05, WCAP-16168)
- Pressurizers and steam generators
- Peening The U.S. NRC has published RG 1.245, Preparing Probabilistic Fracture Mechanics Submittals, to provide guidance on preparing PFM submittals.
4/27/2023 5
PFM - RG 1.245 Regulatory Guide 1.245 provides guidance on preparing PFM submittals for the NRC.
NUREG/CR-7278 (the RG 1.245 basis) states, Analysis of a PFM framework allows for assessments of the structural integrity of components to enable risk-informed decisions in a regulatory application The NRC has progressed in its efforts to implement risk-informed approaches into its regulation This guidance is contained in regulatory guide, RG-1.245.
RG 1.245 states, PFM is likely to be used to risk-inform licensing applications.
4/27/2023 6
Risk-Informed Decision Making Risk-informed decision making (RIDM) is a holistic framework supporting decision making at the NRC. The RIDM framework enables holistic evaluation of risk-insights with other considerations.
Within this framework, PFM often supports Principle 4, Risk-Informed Analysis, in balance with the other Principles.
Improvements in analysis in Principle 4 are often paired with proposals to reduce actions taken supporting Principle 5, Performance Monitoring.
4/27/2023 7
Performance Monitoring (1/2)
Performance monitoring is focused on directly measuring real-world indicators.
This compliments the other Principles by monitoring and trending reality as compared to our expectations (from analysis, etc.).
Acceptable performance monitoring approach must provide:
- Direct evidence of presence and/or extent of degradation
- Validation/confirmation of continued adequacy of analyses
- Timely method to detect novel/unexpected degradation 4/27/2023 8
Performance Monitoring (2/2)
Performance monitoring goes by many names. Inservice Inspection, Condition Monitoring, Aging Management, etc.
Each of these are specific direct measurements linked to the necessary function of subject components.
Inservice Inspection - 10 CFR 50.55a Performance/Condition Monitoring - 10 CFR 50.65 Aging Management - 10 CFR Part 54 Performance monitoring is not one thing - it is matched to the function of the subject.
4/27/2023 9
Combining Analysis with Performance Monitoring (Example 1)
Extension of inservice inspection (ISI) interval for RPV welds
- Submittals use methodology in approved topical report WCAP-16168 as basis for extending ISI interval from 10 years to 20 years for RPV welds
- Analysis Probabilistic fracture mechanics analysis using FAVOR code that addresses risk delta of conditional RPV failure frequency due to different ISI scenarios
- Performance monitoring Extension of ISI interval to a maximum of 20 years. Fleet inspections are coordinated to ensure regular stream of monitoring data.
One-time inspection for subsequent extensions to validate that generic flaw-distribution used in report bounds plant-specific distribution per 10 CFR 50.61a(e).
4/27/2023 10
Combining Analysis with Performance Monitoring (Example 2)
Extension of inservice inspection (ISI) interval for pressurizer (PZR) and steam generator (SG) welds and nozzles
- Alternative requests submitted under 10 CFR 50.55a(z) to extend ISI interval from 10 years to 30 years for subject welds and nozzles Some submittals, instead of proposing to extend ISI interval,
- Analysis proposed to defer inspections to the end of licensed period.
Probabilistic fracture mechanics analysis with different ISI scenarios using PROMISE code
- Performance monitoring Plant, Component 10-year Approved Inspections within Percent of ASME ASME required inspections:
interval alternative approved alternative required duration of interval interval inspections see Pressurizer Shell Weld alternative Inspections - Example slides Two-unit PWR, SG 4th, 5th, 6th 30 years Two SGs 33%
One-unit PWR, SG 5th, 6th 30 years One SG (4th) 33%
Two-unit PWR, PZR 4th, 5th 30 years > Two PZRs > 33%
One-unit PWR, PZR and SG 4th, 5th 20 years One PZR, One SG 50%
4/27/2023 11
Combining Analysis with Performance Monitoring (Example 3)
ASME Code Cases N-770 and N-729 were developed to handle components with known active degradation mechanism.
These Code Cases base reinspection frequency on:
- Postulated flaw below qualified detection limit
- Material
- Temperature
- Conservative Crack Growth rates Repaired components still subject to inspections.
4/27/2023 12
SG Regulatory Framework (Example 4)
Until 2005, the SG surveillance requirements (i.e., performance monitoring) in the technical specifications (TS) were prescriptive.
- TS sample size was small and varied from 3 - 6% of tubes.
Process was not directly focused on ensuring tube integrity would be maintained until the next scheduled inspection.
- This shortcoming often necessitated actions beyond minimum TS requirements to ensure tube integrity.
With NRC approval in 2005, industry implemented TS that were performance-based, with some prescriptive requirements.
4/27/2023 13
SG Tube Integrity Performance Criteria Licensees use standard statistical methods to show that SG tubes maintained the TS-required Integrity Performance Criteria in the prior cycle of operation (Condition Monitoring Assessment) and will continue to maintain the TS-required Integrity Performance Criteria until the next scheduled inspection (Operational Assessment).
- Structural Integrity Performance Criterion: Provides a safety factor against burst under normal steady state full power operation and a safety factor against burst applied to design basis accidents.
- Accident Induced Leakage Performance Criterion: Limits calculated accident-induced leakage rate for any design basis accident, other than a SG tube rupture, to a rate less than that assumed in the accident analysis in terms of total leakage rate for all SGs and leakage rate for an individual SG.
- Operational Leakage Performance Criterion: Limits daily operational leakage.
4/27/2023 14
Aging Management (Example 5)
License Renewal Guidance (NUREG-1801, NUREG-2192)
- Condition monitoring samples based, in part, on EPRI TR-107514
- 90% confidence / 90% of population has no degradation
- Later simplified in NUREG-1801 (GALL, Rev 2) to 20%, maximum of 25 inspections One-Time Inspection Program - verifies absence of aging and the effectiveness of preventive programs (water chemistry, lubricating oil)
- Basis for other AMPs when aging may be expected 20%, maximum of 25, every 10 years 3%, maximum of 10, every 10 years e.g., Bolting Integrity, Closed Treated Water Selective Leaching Program (subsequent renewal)
Systems, Electrical Insulation for Cable and heavily informed by qualitative risk susceptibility Connections Programs and engineering judgement
- How should expanded use of risk insights factor into a technically sound sampling approach?
EPRI TR-107514: Age-Related Degradation Inspection Method and Demonstration:
In Behalf of Calvert Cliffs Nuclear Power Plant License Renewal Application (1998) 4/27/2023 15
Example Sampling Approach How much performance monitoring is enough?
There may be many ways to define a performance monitoring program - This example demonstrates one acceptable method.
Hypothetical example of the development of inspections needed to satisfy performance monitoring requirements
- Pressurizer shell weld inspections - ASME Section XI category B-B and B-D
- Volumetric exam required per ASME every 10 years
- Requested inspection every 30 years
- Three cases considered - submittal with
- Full US fleet
- 10 units
- 1 unit 4/27/2023 16
Example Sampling Approach, Contd
- Several initial assumptions
- Appropriate analyses have been conducted to demonstrate low probability event with known degradation
- Substantial multi-decade multi-design operational history verifies analyses assumptions (e.g. middle of the bathtub curve)
- Other measures such as walkdowns, system leak tests, etc. will continue SONGS SG Tube Failure FAC in steam drum
- Use statistical approach to estimate number of inspections needed Chance of failure
- Use currently accepted inspection techniques
- Provide on-going analyses validation
- Provide capacity to identify unexpected degradation should it occur Burn-in Maturity Wear-out 4/27/2023 17
Binomial Distribution
- The binomial distribution is frequently used to model , , = 1 the number of successes in a sample of size n drawn with replacement from a population of size N !
=
- Can be used to find # of inspections needed to find a k= number of successes (cracks found) crack n=number of trials (inspections) p= probability of success on an individual trial (% of population cracked)
- Only a function of the number of inspections and the If k=0 then this is the probability of no
% cracked successes is:
, = 1
- Does not demonstrate the impact of population size and therefore, the probability of at least one success is:
- Very easy to use 1 ,
4/27/2023 18
Monte Carlo Analysis Sample a weld Sample Inspection of population with x%
- Same idea can be developed through a cracked y% of welds MC analysis p=count/n
- Allows maximum flexibility in analysis MC Loop, n realizations yes
- Binomial response can be recreated no Done
?
Loop on weld population no Done yes
?
inspected?
no yes cracked? no yes For large populations For small populations count=count+1 4/27/2023 19
Pressurizer Shell Weld Inspections -
Example
- Analysis Inputs
- 61 PWRs in the US fleet - one pressurizer (PZR) per PWR
- All units have previously inspected PZRs
- All welds under consideration have similar materials, similar stresses, similar environments (in part handled by analyzing an inspection as full set per PZR)
- A 5% population incidence of novel degradation, with a 90% probability of detecting at least one occurrence in a PZR sampled
- Three 10-year inspection intervals under consideration
- Across US Fleet that is 183 required PZR inspections
- Using the binomial distribution, 45 inspections are required (41 inspections are needed when MC is used) - yielding about 25% of the ASME required inspections
- It would be assumed that all plant-specific submittals would implement this reduction 4/27/2023 20
Pressurizer Shell Weld Inspections -
Example
- If a licensee submitted an alternative for a small subset of the PWR PZR fleet
- Submittal with 10 units
- 30 required ASME PZR inspections
- Using the analyses, 8 inspections are required for PM
- Submittal with 1 unit Rounding up
- 3 required ASME PZR inspections
- Using the analyses, 1 inspection is required for PM
- Why are subsets different?
- Units must have acceptable performance monitoring programs in their own licensing space
- Approvals can only be granted on information contained within application 4/27/2023 21
Pressurizer Shell Weld Inspections -
Example
- Later inspections are more likely to detect novel degradation (if it is growing),
and consequently are more impactful
- Would conduct these inspections in the 1st Period of the last ASME Code Interval for the unit at which the inspection is to be performed (based on current 10-year intervals)
- Would not inspect a PZR more than once (e.g. purpose is to sample population not monitor individual PZRs)
- If novel degradation is found, substantial inspection scope expansion would occur
- This example presumes the natural cadence of applications and licensing periods would span out inspections to provide continuous monitoring and trending of PZR population 4/27/2023 22
Key Take-Aways Probabilistic fracture mechanics (PFM) is inherently risk-informed.
Precedent including PFM and similar approaches has consistently relied on performance monitoring to holistically provide assurance.
No amount of analysis can entirely substitute for performance monitoring OE has shown that assumptions about components can be incorrect
- Unexpected degradation mechanisms (French SCC)
- Unusually vulnerable heats of materials (Davis Besse Second RPV Head)
- Undocumented repairs (Numerous)
Statistical techniques can be used to justify performance monitoring proposals 4/27/2023 23