ML23010A250
| ML23010A250 | |
| Person / Time | |
|---|---|
| Issue date: | 01/11/2023 |
| From: | Division of Operating Reactor Licensing |
| To: | |
| References | |
| Download: ML23010A250 (1) | |
Text
NRC Public Meeting Discuss Risk-Informed High Energy Line Break Methodology Nuclear Regulatory Commission January 11, 2023 NRC Headquarters Rockville, MD (& Virtual)
Agenda
Purpose:
The purpose of the meeting is to discuss risk-informed high energy line break (HELB) methodology covering, but not limited to, overview of the supporting topical report, review of relevant regulations, and intended licensing applications, and related NRC staff activities Time Topic Speaker 1:00pm Introductions/Opening Remarks All 1:10pm Risk-informed HELB/BER Presentation Industry/EPRI 2:00pm NRC Research Activities related to HELB/Cumulative Usage Factors NRC 2:45pm Discussion All 3:00pm Break All 3:15pm Brainstorming Session - Approaches/Applications for Risk-Informed HELB/Break Exclusion All 4:00pm Opportunity for Public Comment Public 4:15pm Wrap Up / Action Items / Next Steps All 4:30pm Adjourn All
NRC Activities Related to Revising High Energy Line Break Methodology Rob Tregoning Bruce Lin Stewart Bailey Nuclear Regulatory Commission Public Meeting to Discuss Risk-Informed High Energy Line Break Methodology January 11, 2023 NRC Headquarters Rockville, MD (& Virtual)
Background:
Prior Public Meetings June 11, 2019: Category 2 Public Meeting o
Meeting summary - ML19214A095 o
NRC presented report on background and history of BTP 3-4 o
EPRI presented contents of EPRI Technical Report No. 1022873, Improved Basis and Requirements for Break Location Postulation o
Westinghouse presented impact of BTP 3-4 CUF criterion on design process for AP1000 o
Key outcome: continue the effort to re-evaluate the BTP 3-4 criteria March 1, 2021: Category 2 Public Meeting o
Meeting summary - ML21089A005 o
NRC discussed current HELB guidelines, proposed strategies for alternative criteria, and potential alternative approaches/technical considerations for operating and new LWRs o
EPRI summarized their HELB activities since the 2019 meeting o
Actions NRC: Evaluate possibility of phased approach, provide recent HELB submittals, consider other piping configurations (i.e., bolted and flanged connections)
Industry: Evaluate possibility of conducting a pilot-plant submittal All: Inform advanced non-LWR vendors and designers and consider their feedback
- Important considerations (discussed in 2019 public meeting) o No strong technical basis for existing BTP 3-4 criteria o Both CUF and stress criteria can be challenging to meet o Significant design challenges may arise in meeting existing criteria o EPRIs 2011 proposed risk-informed approach contains relevant, applicable concepts (Report No. 1022873) o Leverage both operating experience and insights from related risk-informed initiatives
- Formed working group to evaluate changes to BTP 3-4 and develop alternative framework and associated acceptance criteria for postulating pipe break and leak locations Framework for Recent NRC Efforts
Revising BTP 3-4: Underlying Philosophy Significantly revamp existing BTP and provide as a voluntary alternative for existing or new reactors o
Eliminate stress criteria, relax CUF criterion o
Consider other degradation mechanisms o
Consider operating experience o
Tailor CUF evaluations, as needed, to applicable systems/components Support operating reactor, new reactor, and advanced reactor applications o
Provide identical framework and approach o
Require unique evaluations or additional measures depending on reactor type Approach should be risk-informed o
Consider both failure consequences and susceptibility to degradation o
Utilize a graded approach consistent with the risk Broaden scope beyond classical, LWR welded-piping systems o
Include other types of connections (e.g., bolted) o Include other passive components/systems which primarily transport fluid Adopt a similar approach for both high-energy and moderate-energy breaks
Approach: Philosophy Consistent with both Deterministic and Risk-Informed Evaluations BreakPreclusion Demonstration Addressconsequencesof failureand,ifneeded, susceptibilitytoapplicable degradationmechanisms Screening Analyses Screening Asneeded,perform analyses,designchanges,or mitigationtodemonstrate thattheprobabilityofpiping failureisextremelylow Additional Measures SRP3.6.3-LBBDetermination Increasing Risk Increasing Rigor Yes No Yes
Alternative Framework: Scope and Objectives
- Scope o Provide alternative method to determine rupture locations in accordance with SRP 3.6.2 to demonstrate that GDC-4 requirements are met o Not intended as a substitute to 10 CFR 50.46 requirements
- Specific Objectives o Change CUF < 0.1 criterion to CUFen < 1.0 o In lieu of current stress criteria in BTP 3-4, require compliance with ASME Section III and predecessors (e.g., B31.1) o Leverage to the extent possible existing design (e.g., ASME Section III Appendix W) and operational (e.g., GALL, ASME Section XI/O&M) aging management practices/programs o Eliminate mandatory postulation of breaks at terminal end locations, at high-stress intermediate locations, and intermediate locations with CUF > 0.1 o Provide alternative, risk-informed guidance for postulating break locations for use in plant design and safety analyses
Alternative Framework: Applicability
- Light water reactors operating below 360 ºC o
While the initial applicability is limited to LWRs, this framework is intended to ultimately support establishing break locations for other reactor types (e.g., ANLWRs) as well.
- High and moderate-energy Class 1 and 2 piping systems within the break exclusion region (BER) and high and moderate-energy Class 1, 2, and 3 piping systems outside of the BER.
- Nominal Pipe Size - 2 and greater o
Although it is recognized that demonstration of low-likelihood of rupture associated with degradation is more challenging for smaller piping and connections.
- Piping systems that are joined using welded or bolted flanged connections
- Piping systems designed using NRC-approved codes and standards (i.e., ASME Section III or B31.1)
Alternative Framework: Screening Evaluation (One Potential Risk-informed Approach)
Identify failure locations o
Higher design basis or operational stresses o
Higher likelihood of damage (during fabrication or serviced induced) o Less failure margin (i.e., ratio of design requirement to material properties)
Perform rupture consequence analysis o
Consider worse-case location and failure orientation o
Evaluate both rupture and leakage (including potential for long-term) effects o
Impacts of separation and shielding o
Address defense-in-depth: primary failure and consequential damage Conduct degradation susceptibility analysis (as needed) o Consider all possible failure mechanisms (e.g., SCC, fatigue, embrittlement, FAC, creep) o Evaluate susceptibility risk factors: Material, environment, stresses o
Evaluation could be qualitative or semi-quantitative o
Credit effects of operating experience and past inspection results (existing plants), and aging management programs or mitigative measures Increasing Susceptibility Increasing Consequence Decreasing Mitigation
Alternative Framework: Additional Measures Options available if it cannot be demonstrated that consequences are insignificant or significant degradation is extremely unlikely o
Enact design changes or other mitigative measures (e.g., pipe whip restraints) to decrease failure consequences o
Enact additional mitigative measures (e.g., increased inspection periodicity) to make it extremely unlikely that degradation will cause pipe rupture o
Perform a more rigorous risk analysis to demonstrate that risks are insignificant o
Conduct an advanced deterministic leak-before-break (LBB) analysis to demonstrate that the likelihood of a rupture is extremely low Other options, as proposed by licensee or applicant, may also be acceptable if they demonstrate that probability of piping/connection rupture is extremely low such that GDC 4 is met.
Consequences Rupture Likelihood Analysis Complexity
Alternative Framework: Requirements and Additional Considerations Requirements o
Approved system design (e.g., protection against overpressure transients), fabrication, (e.g., no partial penetration welds unless justified), and inspection (e.g., pre-service and in-service) practices o
System has no active degradation mechanisms unless appropriate aging management or other mitigative measures are in place o
Approved leak detection systems, as applicable (e.g., meeting RG 1.45 requirements)
Additional considerations o
Specific acceptance criteria and technical basis are coupled Criteria dependent on the degradation mechanism Criteria could be specific (e.g., CUFen < 1.0) or broad (e.g., demonstrate extremely low failure likelihood) o Technical basis development Complexity increases with the number of possible degradation mechanisms Complexity increases as the pipe size decreases Crediting time to initiate flaws will be more challenging, especially at welds o
Graded analysis approaches are appropriate Simpler deterministic bounding or binning analyses Probabilistic analyses
Advanced LBB Evaluation Conducted piping analyses using advanced leak-before-break (LBB) evaluation to demonstrate robustness of piping systems Selected challenging but representative systems/locations for analysis o
Smaller diameter systems o
Low toughness and high inertial loading Conducted analysis on actual piping systems o
PWR case - NPS 3 Schedule 160, TP304, CVCS - charging line o
BWR case - NPS 4 Schedule 160, A106B, RCIC system - Test to HPCI test line Report documenting the initial approach, methodology, and flaw stability assessment issued Oct 2022 (ML22277A504)
Advanced LBB Analysis Approach A robust LBB procedure developed from the IPIRG (International Piping Integrity Research Group) program o
Uses FE analysis of pipe system to get elastic uncracked pipe stresses o
Allows for nonlinear stress-strain curve of the material o
Inserts the circumferential TWC at the worst location in the FE analysis Analysis Steps o
Develop and verify finite element models o
Check ASME Code stress limits (Primary, Primary + Secondary, Service Level D) o Perform uncracked analysis using elastic and elastic-plastic materials properties o
Perform cracked analysis using elastic-plastic material properties at highest stress location
Initial Results As crack becomes larger, the applied moments drop from system flexibility changes Results show that even with the upper-bound allowable SL-D stresses, when accounting of non-linear material behavior and changes in system compliance, piping failure requires extremely long circumferential TWC lengths o
No crack instability even for TWC of 270-degrees (75%
of circumference) o At TWC=75%, the applied moment is about 1.5% of the uncracked elastic SL-D moment Currently conducting limited sensitivity studies to better understand the implications of the results Peak moment as a function of crack size for the BWR system Peak moment as a function of crack size for the PWR system at different applied frequencies
Next Steps
- Complete technical criteria o Evaluate/develop guidance on degradation mechanisms (ASME Section III Appendix W, ASME Section XI, ASME O&M, GALL) o Review relevant operating experience
- Develop Risk-Informed Approach o Maintains appropriate safety margins, defense in depth, performance monitoring
- Broaden scope beyond LWR welded piping systems
- Update regulatory guidance documents