Status of U.S. NRC Research Activities for Primary Water Stress Corrosion Crack Growth Rate Testing
Greg Oberson, Jay Collins, Eric Reichelt, Eric Focht
U.S. Nuclear Regulatory Commission
2016 International LWR Materials Reliability Conference and Exhibition
Chicago, IL – August 1 – 4, 2016
The views expressed in this presentation are those of the authors, not necessarily those of the U.S. NRC.
Motivation for NRC Testing Program
• Establish independent and confirmatory technical basis for structural integrity analyses of nickel-based alloy components and welds
• Support an efficient and effective process for making regulatory decisions on issues such as in-service inspection relief requests and ASME Code actions
• Leverage resources to benefit complementary research programs, such as initiation, peening and subsequent license renewal
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Recent Testing Areas
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• Alloy 690– Cold work– Microstructure/thermomechanical
processing– Heat affected zone
• Alloy 52/152 weld metals– Chromium dilution– Repairs– Mitigations– Weld flaws– Welding processes
• Alloy 182 weld metals– Heat-to-heat susceptibility– Low K testing
CC N729 – upper heads CC N770 – piping welds
Half-nozzle repairsCC N847 – excavation and weld repair
Testing of Alloy 690• Current status:
– CGR tests on 40+ specimens from 10+ heats– Substantial microstructural analysis and characterization– No additional tests planned at this time
• Key findings: – Very low crack growth rates (~10-9 mm/s) in as-received condition– Crack growth rate increases with increasing cold work– HAZ does not appear to have notably higher susceptibility
• Important considerations for data analysis:– Cold work levels or equivalent strain in actual plant components– Test specimen orientation relation to crack growth directions in plant
components– Temperature and K dependencies
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PNNL Alloy 690 Data
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6
10-13
10-12
10-11
10-10
10-9
10 15 20 25 30 35 40 45 50
C690-LR-2, ValinoxC690-CR-1, ValinoxS690-CR-1. SumitomoHAZ, CF690-CR-1, ValinoxHAZ, CF690-CR-3, Valinox
Expe
rimen
tal C
GR
(m/s
)
Stress Intensity K (MPa·m1/2)
Alloy 690 and 690 HAZ (CRDM)Simulated PWR environment320°C
MRP-55Alloy 600
75 Percentile
Solid symbols: CL
All as-received
ANL Alloy 690 Data
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Alloy 690 Characterization
Grain misorientationmapping and hardness testing indicate strain in weld HAZ equivalent to 10-15% CW
Hot working imparts significantly less grain misorientation and hardening than cold work
Characterization across HAZ
Cold forged ~30% Hot forged ~30%
CGR > 100x lower
Crack Growth Rate Testing of Alloys 52/152• Current status:
– Completed tests on 20+ specimens from 10+ welds– Substantial microstructural analysis and characterization– About 10 weld specimens currently in test
• Key findings:– Most “typical” weld metal shows low crack growth rate (<10-8 mm/s), even in some cases with
high intergranular engagement – Limited data near Alloy 152-LAS and Alloy 52M-Alloy 182 interfaces give notably higher crack
growth rates, above 10-7 mm/s
• Important considerations for data analysis: – Effects of welding parameters– Presence of dilution zones and overlay/inlay/onlay configurations– Effects of embedded flaws and/or weld repairs– Temperature and K dependencies
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Alloy 52/152 Crack Growth Rate Data
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PNNL ANL
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Alloys 52/152 Characterization
Elemental mapping near Alloy 152M-LAS interface shows Cr dilution to ~20% in localized regions. Specimens are oriented to align cracks in this zone.
EBSD identifies features such as dendrite packet boundaries aligned parallel to the interface which may be a preferential crack path.
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Alloys 52/152 Characterization
Characterization identifies localized regions of high grain misorientation and hardness in welds.
Discontinuous nature of these regions mitigates against sustained, long crack growth in most cases.
Alloy 690
Alloy 52
Fusion Line
Recent Weld Dilution Testing
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LAS152
152
LAS
PNNL Test
ANL Test
• Multiple tests recently completed on Alloy 152-LAS weld dilution specimen that showed very high CGR in prior ANL testing
– PNNL tested two specimens in first weld bead layer
– ANL tested one specimen in second weld bead layer
• Much lower CGRs were measured, on the order of 10-9 mm/s
• Specimen characterizations still ongoing to help understand observations
Ongoing Weld Repair Testing
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LAS
152
PNNL Test
ANL Test
• ANL and PNNL are testing specimens from the EPRI 20% inner diameter Alloy 52M repair weld
– Across repair-to-original weld interface– In Alloy 52M/LAS dilution zone
• Small LOF cracks are present at Alloy 52M/LAS interface
• Data acquired to date show low CGR, consistent with typical Alloy 52M weld metal, but additional testing and post-test analyses will be needed.
PNNL
ANL
52MLAS
LOF LOF
Ongoing Cold Worked Weld Testing
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LAS
152
PNNL Test
ANL Test
• Prior testing by GE on 20% CW Alloy 152 at constant K showed intermittent periods of very rapid crack growth >10-6 mm/s
• ANL and PNNL are testing specimens from various Alloy 52/152 welds with additional 15 to 20% CW.
• Welds with this extent of CW are not expected in plant service, but data could be used to develop important correlations between weld hardness and crack growth rate.
ANLIHI 152M 15%CF ENSA 52M butter 15% CF MHI 52 20% CF
Recently Completed Alloy 182 Testing
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• PNNL tested various heats of Alloy 182 to assess their susceptibility for use in initiation test program.
• In general, CGRs as a function of K are consistent with those reported in MRP-115.• NRC is aware of EPRI plans for updating MRP-55 and -115 models but are not
involved other than to share data from NRC-sponsored tests.
Testing Path Forward• Alloy 52M and 52i overlays on Alloy 182
• Alloy 52M narrow groove welds with ductility dip cracks
• Alloy 52/152 – stainless steel – low alloy steel triple interface
• CIEMAT/ENSA Alloy 690/52/152 CRDM mockup
• High heat input Alloy 52/152 weld(s)
• Envision CGR test program winding down in 2018
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Crack Growth Rate Expert Panel
• NRC supports the PWSCC Expert Panel via a Memorandum of Understanding with EPRI
• Primary participation is through NRC contractors at ANL and PNNL who are members of Data Evaluation Group
• NRC may observe the Data Applications Group
• NRC participation does not imply endorsement of Expert Panel conclusions, which may be subject to further review as part of a formal safety evaluation.
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Expert Panel Desired Outcomes
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• Clearly articulated technical basis to support model development
– Reasoning for selection/exclusion of data– Justifying functional dependencies– Accounting for test-to-test, heat-to-heat, and lab-to-lab data
variability– Addressing data uncertainties, particularly at very low CGR
• Acknowledgement of differing viewpoints on technical issues– Open, respectful discussion– Consensus agreement when unanimity is not reached– Documentation of differing viewpoints
Expert Panel Path Forward
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• NRC staff and contractor engagement with the Expert Panel will likely decrease towards the end of 2016.
– Data Evaluation group scoring for Alloys 690/52/152 should be completed by the Fall– Modeling approach for Data Applications group is becoming clearer and NRC will provide limited
input.
• It has not been requested by EPRI nor does NRC have the expectation to review the Expert Panel report prior to its publication.
• EPRI has not yet communicated to NRC whether the Expert Panel report will be submitted as a Topical Report nor how the findings will be engaged with licensing or ASME Code actions.
• EPRI plans that could affect the allocation of NRC staff time and budget should be communicated with reasonable notice.
Summary• PWSCC testing program supports technical bases for safety evaluations including for
in-service inspection requirements of upper head nozzles and piping welds.
• Testing on Alloy 690 confirms very low CGR on material with low to moderate levels of CW. No additional testing is planned at this time.
• Most Alloy 52/152 tests to date show very low CGR except for a few tests near weld interfaces. Ongoing and planned tests will address effects of dilution, repairs, and weld defects.
• NRC staff and contractors expect to continue participating in the PWSCC Expert Panel through the rest of 2016.
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Acronyms• ANL – Argonne National Laboratory• CC – ASME Code Case• CGR – Crack growth rate• CIEMAT – Centro de Investigaciones Energéticas, Medioambientales y Technológicas• CRDM – Control rod drive mechanism• CW – Cold work• EBSD – Electron backscatter diffraction• ENSA – Equipos Nucleares S.A. • EPRI – Electric Power Research Institute• GE – General Electric• HAZ – Heat affected zone• LAS – Low alloy steel• LOF – Lack of fusion• PNNL – Pacific Northwest National Laboratory• PWSCC – Primary water stress corrosion cracking
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