ML22097A107
| ML22097A107 | |
| Person / Time | |
|---|---|
| Issue date: | 04/13/2022 |
| From: | David Dijamco NRC/NRR/DNRL/NVIB |
| To: | |
| Dijamco D, (301) 415-1502 | |
| References | |
| Download: ML22097A107 (14) | |
Text
Use of Probabilistic Fracture Mechanics for ISI Extension A Regulatory Perspective David Dijamco 2022 International Inservice Inspection Workshop
- April 13, 2022 The views in the presentation herein reflect those of the presenter and not of the U.S. NRC.
U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Vessels and Internals Branch
What is the risk of extending ISI for a component?
Probabilistic fracture mechanics (PFM) has been used to answer this, at least partially. (Part 1)
To what extent can PFM address the question of risk? (Part 2) 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 2
PFM in a nutshell Fracture mechanics, crack driving force < material resistance In deterministic fracture mechanics, you apply safety factors on either side (or both) and perform one calculation.
PFM is performing many fracture mechanics calculations in which the crack driving force is compared to the material resistance, then counting how many failures you get.
4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 3
Many calculations are performed because the input parameters are not a single value, but a distribution of values that are sampled.
PFM illustration 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 4
Sample (trial or realization)
Crack driving force Material resistance Fail?
(crack driving force >
material resistance?)
1 8
75 No 2
15 50 No 3
75 25 Yes 10 million 65 100 No Probability of failure = number of failures ÷ number of samples
PFM is complex Performed with computer codes such as FAVOR and xLPR PFM input parameters, each with distributed values:
Flaw size/density Fracture toughness (or associated parameters such as RTNDT)
Probability of detection Weld residual stress Fluence Crack growth rate PFM guidance: Regulatory Guide 1.245 and NUREG/CR-7278 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 5
Two points in using PFM for ISI extension Effect of ISI and examination coverage need to be included If generic PFM results are used, the ISI and examination coverage in the generic analysis would need to closely align with the ISI and examination coverage that have been performed for the specific component.
Sensitivity analyses (SA) and sensitivity studies (SS) performed to address and understand uncertainties SA helps flush out the inputs that are driving the desired analysis output (e.g.,
probability of failure); therefore, this is a global look at the analysis.
SS helps in determining the effect of a particular input in the analysis (e.g.,
stress); this is a local look at the analysis.
4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 6
In a PFM analysis, probability of failure (PoF) is often queried as the output.
PoF is counting the number of failures out of all possible scenarios that arise from the uncertainties in the input parameters.
4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 7
Provided that the resulting PoF is less than the specified criteria, is going by PoF alone sufficient for extending ISI? (moving into Part 2)
Underlying concept behind ISI Monitor the condition of a component throughout its operating life. But why?
Components are designed with safety margins, and so at the start of operation (baseline condition) the component is deemed acceptable.
Monitor condition of the component to compare with the baseline condition to determine if there has been a change (which could mean presence of service-induced degradation that must be addressed). This ensures the safety margins in the component are maintained, so that it will continue to perform its intended function, and thereby provide reasonable assurance of overall plant safety.
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Extending ISI period reduces ISI examinations, and this impacts the intent of ISI Other aspects that must be addressed based on ISI concept:
- Monitoring condition
- Degradation
Risk of this reduction must be addressed.
This makes extending ISI a risk-informed decision.
Risk-Informed Decision Making (RIDM) 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 10 RIDM is integrated decision making.
Maintenance of Safety Margins Increase in risk is small Consistency with Defense-in-Depth Change meets current regulations Condition or Performance Monitoring PFM PFM addresses the risk principle and can inform the safety margin principle.
Risk-Informed Decision Making (RIDM) 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 11 RIDM is integrated decision making.
Maintenance of Safety Margins Increase in risk is small Consistency with Defense-in-Depth Change meets current regulations Condition or Performance Monitoring PFM PFM allows weight to be given to the risk principle when combined with performance monitoring.
Aspects of performance monitoring Starting with keywords from ISI concept
- Monitor condition for change in degradation
- Maintenance of safety margin Three key aspects of performance monitoring can be derived.
- Direct evidence of presence and/or extent of degradation
- Timely method to detect novel and/or unexpected degradation
- Validation/confirmation of continued adequacy of analyses Performance monitoring is important for ISI extension, especially extensions during long-term operation because of increased uncertainty.
4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 12
Examples of PFM + performance monitoring as basis for ISI extension 4/13/2022 Use of PFM for ISI Extension l April 2022 International ISI Workshop 13 Elimination ISI of BWR vessel circumferential welds (covered in presentation tomorrow) 20-year ISI extension of PWR vessel welds
- PFM: FAVOR analyses
- Performance monitoring: coordinated ISI of the same-fleet units to ensure regular fleet ISI data; one-time ISI for subsequent extensions to validate that generic flaw-distribution used in the PFM analysis is bounding for the unit.
Note: both are implemented through the alternative request process in NRC regulations.
Summary PFM alone is not an adequate basis for ISI extension.
For ISI extension, PFM allows weight to be given to the risk principle of RIDM when combined with performance monitoring.
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