ML18200A116
| ML18200A116 | |
| Person / Time | |
|---|---|
| Issue date: | 07/19/2018 |
| From: | David Rudland Division of Materials and License Renewal |
| To: | |
| Rudland D, 415-1896 | |
| References | |
| Download: ML18200A116 (15) | |
Text
U.S. Regulatory Perspective David L. Rudland Senior Technical Advisor for Materials Division of Materials and License Renewal Office of Nuclear Reactor Regulations United States Nuclear Regulatory Commission 26th International Conference on Nuclear Engineering July 22-26, 2018 London, United Kingdom
LBB in the U.S. Nuclear Regulations
- 10CFR50 App A, General Design Criteria 4 requires, in part,
- that structures, systems, and components important to safety be designed to accommodate the effects of postulated accidents, including appropriate protection against the dynamic effects of postulated pipe ruptures.
- Certain postulated accidents, such as large break LOCA, are part of the design basis.
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LBB in the U.S. Nuclear Regulations
- GDC-4 also allows the use of analyses, approved by the NRC, to demonstrate extremely low probability of pipe rupture for removal of protective hardware (dynamic effects)
- In 1984, LBB was accepted as an analytical procedure for demonstrating extremely low probability of rupture events
- Leak-before-break is not a design condition or a requirement. The analysis is an option to justify the removal of protective hardware.
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- In 1984, a five volume report was published on a review of the NRC requirements in the area of nuclear piping
- Volume 3 of this document reviews the evaluation of potential for pipe breaks
- Gives recommendation for application of LBB in the NRC licensing process
- The conclusions and recommendations from this volume were implemented into Standard Review Plan on LBB (3.6.3) in 1987 (updated 2007) 4 NUREG-1061 - Report of the US NRC Piping Review Committee, 1984
- The SRP is applicable to Class 1 piping with the following caveats:
- Must be applied to entire system
- Cannot be used for piping susceptible to SCC, erosion-corrosion, creep, etc. (i.e., no degradation mechanisms that can cause long surface cracks)
- Systems with a history of fatigue cracking cannot be considered
- Pipes with likely water hammer are not considered
- Piping systems with possible brittle fracture are not considered
- Indirect failure must be shown not to cause rupture
- Flaw tolerance analysis Screening Criteria SRP 3.6.3 5
Flaw Tolerance in SRP 3.6.3 SRP 3.6.3 LBB Analysis Choose and demonstrate leak rate and fracture analyses Identify material properties Specify type and magnitude of loads Postulate a leaking crack Leakage detection limit Margin of 10 Calculate critical flaw size Margin 2?
Acceptable Not Acceptable yes no Margin 1.4?
Determine applied load margin yes no Normal operating +
faulted/seismic Leak rate analyses Fracture analyses Flaw tolerance approach 6
Extremely Low Probability of Rupture
- ECCS is designed to handle break in the largest piping system in reactor coolant system
- ASME Section III and screening criteria in SRP 3.6.3 provide assurance of extremely low probability events
- Flaw tolerance (LBB) analyses add confidence that the probability of pipe rupture is extremely low 7
U.S. Accepted LBB Accepted LBB applications in U.S. (All PWRs)
SYSTEM NUMBER OF APPROVALS Primary Coolant Loop (Hot & Cold Legs) 76 Pressurizer Surge Lines 14 Safety Injection Accumulator Lines 11 Residual Heat Removal Lines 9
Safety Injection Charging Lines 4
Reactor Coolant Loop Bypass Lines 5
BWR Approvals - NONE Susceptibility to IGSCC Not Adequately Addressed Few requests for LBB 8
PWSCC and LBB PWSCC occurred in LBB approved piping systems at Alloy 82/182 welds SRP 3.6.3 stipulates that no active degradation is allowed LBB analyses assumes idealized through-wall crack for leakage and stability calculations - Flaws could grow non-idealized Current LBB analyses needs to be enhanced to address this type of behavior 9
Traditional LBB Issues/Gaps 10 Leakage - Crack morphology SCC leaking crack can be 89% longer than an air fatigue crack for the same leak rate Weld stress impacts on COD Piping restraint Through thickness welding residual stresses can affect the crack opening area Effect will be more pronounced for smaller diameter pipes where postulated leakage crack size is larger percentage of pipe circumference Model development and validation needed!
LBBs Future
- Flaw tolerance evaluation in SRP3.6.3 does a great job at demonstrating the toughness capacity of the material
- The true process is so much more than toughness
- How do we deal with limitations in analyses, or justify safety factors for emergent degradation
- Modelling the process with best-estimate models and realistic uncertainty representation is key 11
xLPR Code
- xLPR is a modular, probabilistic fracture mechanics computer code for evaluating the risk of pressure boundary integrity failure
- The code was developed in a team environment by NRC/RES and EPRI through a memorandum of understanding following a detailed QA program
- The current application (Version 2.0) is for users to directly assess compliance with 10CFR50 App-A GDC-4
- May be applicable to other needs - High Energy Line Break, Risk-informed In-service Inspection, Large-break-LOCA redefinition, etc.
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0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Crack Mechanism xLPR Technical Flow Loads Material Properties 13 GoldSim Environment
xLPR Status and Plans 14 Status:
- Code development, Version 2 fully complete
- Documentation - 17,000 total pages - final NUREG under development -
2019
- Maintenance and public distribution o
NRC and EPRI finalizing legal framework to enable joint maintenance and U.S. domestic distribution activities Plans:
U.S. domestic release of xLPR V2 - 2018 Develop framework for international release of xLPR V2 - 2019 Development of xLPR Users Group - 2019-2020 Continued benchmarking - 2018-2019 LBB sensitivity analyses, piping system analyses, generalization studies, and development of regulatory guidance - 2019-2021
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