ML24172A306
| ML24172A306 | |
| Person / Time | |
|---|---|
| Issue date: | 06/27/2024 |
| From: | David Rudland, Dan Widrevitz Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML24172A304 | List: |
| References | |
| Download: ML24172A306 (1) | |
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Risk-Informing Materials Issues Risk Insights for Materials Issues NRC Perspective Dave Rudland Dan Widrevitz Technical Exchange Meeting June 27, 2024
Topics
- Introduction
- Motivation for Risk-Informed Materials Guidance
- PRA, PM, and SM for passive components
- Leveraging PFM in risk-informed issues
- NRC success stories
- Recent NRC Guidance Efforts
- LIC-206
- RG 1.245 2
Introduction Commission Policy Statement SRM-SECY-97-287 Final Regulatory Guidance on Risk-Informed Regulation: Policy Issues The Commission has approved the staff's recommendation to allow very small increases in core damage frequency (CDF) and large early release frequency (LERF) The staff should recognize that risk insights continue to be one input to the integrated decision-making process and the proposed values should be considered a guideline and not a hard cut-off.
3
Introduction
[NRC is] a risk-informed regulator that embraces a philosophy whereby risk insights are considered together with other factors.
The NRC can further enhance its effectiveness and efficiency by integrating risk insights into all programs and at all levels of the agency.
-Be riskSMART NUREG/KM-0016 4
Introduction Risk-Informed Decision Making (RIDM) and Materials Engineering RIDM balances Five Principles to reach risk-informed decisions Five Principles are mutually reinforcing and not separable RIDM does not draw conclusions solely on risk; that is risk-based Risk Analysis Current Regulations Met Performance Monitoring Safety Margins Defense-In-Depth 5
RIDM Guidance The relationship between RIDM, PRA, and materials engineering topics has been managed on a case-by-case basis The increased use of RIDM by applicants, licensees, and internal NRC processes has led to increasing requests for more materials engineering specific guidance 6
Materials Engineering - Continuous Change 7
The Good News The combination of experience, adaptations, and improved technology have resulted in significant safety improvements in passive systems, structures, and components (SSCs) over the decades Regulations have been continuously risk-informed in many areas to reduce conservatisms while maintaining safety If nothing changes, we could expect this high level of safety to continue indefinitely 8
The Bad News Change is inevitable The Wear Out phase of the bathtub curve is coming, but we do not know when Significant risk-informed regulations have already been implemented Burn In Maturity Wear out?
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PRA/PM/SM for Passive Components NRC Materials Engineer review typically focuses on the quality of component design, manufacture, and installation followed by periodic inspection. These efforts support maintenance of adequate safety margin.
NRC conducts development of probabilistic fracture mechanics tools to generate failure probabilities for passive reactor components for use in regulatory decision making.
Paraphrasing NUREG CR 6936 10
Using PFM to Risk-Inform Materials Topics Probabilistic Fracture Mechanics (PFM) is an analytical method for modeling degradation likelihood.
What can go wrong is an assumption in the analysis.
PFM results can be used as proxies for initiating event frequencies in providing risk-insights.
(The how often of the Risk Triplet) 11
Motivation for Probabilistic Fracture Mechanics
- Life Extension
- More data & knowledge support improved models - less uncertainty
- Original margin overly burdensome?
Do we change the margin with time?
- Issues
- Deterministic margins make all inputs conservative
- Deterministic approaches
- Not well suited to quantifying actual risk
- Solution: Probabilistic analyses -
- Properly account for true uncertainty 12
PFM and RIDM Update LOCA initiating event frequency Relaxations to design, inspection, maintenance requirements PFM analyses Very small change in failure frequency?
No Yes CDF/LERF Determination CDF/LERF very small Integrated Decision Making Defense in depth Increase in risk is small Performance Monitoring Change meets current regulations Safety Margins PFM is only one Part of Risk-informed Decision Making 13
Licensing Reviews and Emergent Issues Risk/PRA Acceptability Risk information submitted outside of the formal risk-informed licensing basis change process Reliance on Licensee Risk Information Qualitative Quantitative Deterministic Reviews Relief Requests Requests with PFM LIC-206, RG 1.245, RIMA Process still being defined Process defined Emergent issues Process defined LIC-504 Risk-Informed License Basis Changes (RD 1.174/RG 1.200 TSTF-505 Risk-informed Completion times NFPA-805 Risk-informed Fire Protection 10CFR 50.69 SSC Categorization 5b Risk-informed Surveillance Frequencies Risk-informed Inservice Inspection Need adequate, consistent information and confidence in results 14
Timeline of PFM Applications Not exhaustive list 1990 2000 2010 2020 RPV Weld Inspect Relief RI-ISI 50.61a Transition Break Size PZR Inspect Relief SG Inspect Relief 50.61 50.46 Davis Besse Upper Head RG1.245 CRDM Thermal Sleeve French SCC Doel/Tihange CMAC Wolf Creek Cracking Future Applications Other Inspection Relief RG1.230 RG1.178 RIMA OE Application Guidance Regulation Peening Upper head inspect Davis Besse Upper Head 15
PFM Informs Risk Proxies The output of PFM is typically an expected time until impact on integrity (through wall cracking frequency, TWCF, LOCA, rupture, etc.)
or other measure concerning integrity of a component.
In risk-space, this informs potential initiating events such as LOCAs or loss of availability of trains of equipment.
Typically, PFM results are reported as best-estimates with sensitivity analysis/study results to demonstrate margin and/or management of uncertainties.
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Performance Monitoring Performance monitoring has always been a foundational aspect of safety bases Where structural integrity is important, performance monitoring generally takes the form of inspections or testing:
- RPV surveillance programs
- Etc.
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Performance Monitoring Performance monitoring supports materials-related risk-informed decision making in three primary ways:
- Direct evidence of presence and/or extent of degradation
- Validation/confirmation of continued adequacy of analyses
- Timely method to detect novel/unexpected degradation 18
Performance Monitoring Performance monitoring is not:
- Independent of the significance of a component (both quantity and quality of monitoring)
- Only one kind of monitoring (e.g. ultrasonic, visual, walkdown, etc.) there are many potential approaches
- Optional 19
RIDM - Mat. Eng/RA Comparison Topic How a materials engineer addresses problem using RIDM How a risk analyst addresses problem using RIDM What can go wrong?
Degradation can impact component structural integrity Component/system failure How likely is it?
Degradation initiation frequency estimated from data or models Frequency of component/system failure -
data or analysis What is the consequence?
Component failure proxy (leakage, rupture, LOCA etc.)
Core damage, etc.
Defining risk/risk metric Increase in risk of component failure due to alternative Increase in risk of core damage due to alternative Calculating Risk PFM PRA Risk acceptance criteria Does the alternative significantly increase the risk of component failure? No prescriptive guidance.
Per RG 1.174 20 20
RIDM - Mat. Eng/RA Comparison 21 Topic How a materials engineer addresses problem using RIDM How a risk analyst addresses problem using RIDM Safety margin Change in ASME code requirements impacts safety margins of component - less inspections yields less margin assurance Similar Performance monitoring Need monitoring to ensure PFM remains valid over time period alternative is approved and protection continues against unexpected degradation Same, but also required for PRA Regulations If staff finds the alternative provides acceptable level of quality and safety, the regulations are met Same Defense in Depth Failure of Class 1 component may impact Defense in Depth, but other barriers to release are unaffected.
Same 21
Materials RIDM - History of Success Risk-informed approaches are foundational to NRC Materials Engineering approach Stretching back to the beginning 10 CFR 50.61 was risk-informed in 1982 (SECY-82-465)
Risk driven probabilistic solutions are key to managing continuously changing degradation landscape 22
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Materials RIDM - History of Success Exemplary uses of risk in Materials Engineering history:
Right-sizing emergent degradation response Pressurized Thermal Shock Rules (10 CFR 50.61 and 50.61a)
International Op. Ex. (carbon macrosegregation, injection line cracking, etc.)
Etc.
Optimizing performance monitoring based on increased operating experience and risk modeling Reactor vessel inspections Risk Informed ISI Steam generator tube inspections Pressurizer, steam generator, and heat exchanger shell and nozzle inspections Etc.
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Materials RIDM History of Success Key uses of risk in materials engineering history (contd):
Actively develop improved probabilistic analyses capabilities:
- FAVOR/FAVPRO Reactor Vessel embrittlement and transient analysis code
- xLPR pipe degradation analysis code 25
NRC Guidance Efforts Guidance development for risk-informed applications continues forward Relevant recent developments:
- LIC-206, Rev. 1
- RG 1.245 Ongoing:
- RIMA (next presentation) 26
NRC Guidance Efforts - LIC-206 Improve early integration of review teams when risk-insights would lead to need for integration Improve use of risk-insights to scope and depth of review 27
NRC Guidance Efforts - LIC-206 Appendix C Sections 5.1 and 5.2 provide generic reviewer guidance to leveraging risk insights related to initiators, frequency, consequence, and the summation of these inputs Examples of scenarios provided in these sections 28
Regulatory Guide 1.245 - Graded Approach for PFM
- The depth and breadth of a PFM analysis might vary widely depending on several factors PFM is complex
- for PFM analyses themselves
- for the level of detail to be presented as part of an evidence package It makes sense to take a graded approach
- Higher safety significance
- Higher complexity
- Higher level of novelty General Principles Topics Covered
- Models
- Inputs
- Uncertainty Propagation
- Convergence
- Sensitivity Analyses
- QoI Uncertainty Characterization
- Sensitivity Studies Not Covered
- Application specific guidance More analyses, more documentation Higher burden to create defensible and rigorous evidence 29 29
NRC Guidance Efforts - RG 1.245 RG 1.245 provides substantial actionable guidance regarding the minimum necessary components of acceptable bases for use of PFM in a regulatory application RG 1.245 is not a holistic guide regarding all aspects of reviews containing PFM element, The NRC has an approved methodology for risk-informed decision making for design-basis changes [RG 1.174], and PFM may be used as a tool within that framework. (RG 1.245, Rev. 0) 30
Key Take-Aways RIDM requires integrated contributions from all Five Principles NRC Materials Engineering reviews tend to focus on safety margin and performance monitoring with inputs from risk analysis NRC Materials Engineering findings have particular impact on defense-in-depth, safety margin, risk analysis, and performance monitoring 31
BACKUP SLIDES NRC RG 1.174
Definitions Risk (Triplet)
What can go wrong?
How likely is it?
What are the consequences?
Risk Assessment Systematic method for addressing risk triplet as it relates to the performance of a particular system. To understand likely outcomes, sensitivities, areas of importance, system interactions and areas of uncertainty.
Definitions Deterministic Regulations/Guidance/License Conditions A method through which prescriptive criteria are established based on an evaluation of what can go wrong, including implied but unquantified examination of frequencies.
Probabilistic Risk Assessment A risk assessment generally conducted at the functional level of plant systems (e.g. not by modeling individual physical components such as pipes). Includes explicit examination of frequencies.
Definitions Risk Insights Results and findings that come from risk assessment. Results may relate directly to Commissions Safety Goals for Operations of Nuclear Power Plants or may take other forms.
Risk-Based Approach Sole reliance on numerical results of a risk-assessment.
Commission does not endorse this approach.
Definitions Risk-Informed Approach Approach whereby risk insights are considered together with other factors to establish requirements.
Defense-in-Depth Fundamental tenet of regulatory practice in the nuclear field.
Must be balanced with risk insights.
Definitions Performance-Based Approach An approach relying on measurable (or calculable) outcomes be met.
Definitions Five Principles of Risk-Informed Decision Making (RIDM)
An integrated risk-informed approach to decision making.
Principle 1: The proposed licensing basis change meets the current regulations unless it is explicitly related to a requested exemption.
Principle 2: The proposed licensing basis change is consistent with the defense-in-depth philosophy.
Principle 3: The proposed licensing basis change maintains sufficient safety margins.
Principle 4: When proposed licensing basis changes result in an increase in risk, the increases should be small and consistent with the intent of the Commissions policy statement on safety goals for the operations of nuclear power plants.
Principle 5: The impact of the proposed licensing basis change should be monitored using performance measurement strategies.
NRC RG 1.174