Weld Residual Stress Validation Update - NRC: Home Page06/06/2013 Background •Numerical Modeling...
Transcript of Weld Residual Stress Validation Update - NRC: Home Page06/06/2013 Background •Numerical Modeling...
06/06/2013 vg 1
Weld Residual Stress Validation Update
David Rudland Michael Benson U.S. NRC RES/DE/CIB
Paul Crooker EPRI
NRC/EPRI Materials Issue Program
Technical Information Exchange June 5-7, 2013
06/06/2013
Background • Numerical Modeling of Weld Residual Stress (WRS)
began in the 1970s
• Due to advancements in computers and technology, great strides in numerical modeling and direct measurement of WRS have been made over the last 10 years
• With the advent of SCC, researchers began to understand the importance of WRS on the SCC behavior
• Due to difficulty in predictions and measurement, ASME Section XI gives no direct guidance on WRS
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Wolf Creek and Advanced FEA • Ultrasonic
examinations at Wolf Creek in Fall 2006 found indications in pressurizer Alloy 600 dissimilar metal butt welds
• Staff and industry conducted analyses that demonstrated WRS has large impact on flaw evaluation
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NoRepair-NoSS (Left-Right) NoRepair-WithSS (Left-Right)
DEI NoRepair-NoSS DEI-NoRepair-WithSS
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Wolf Creek and Advanced FEA • NRC issued (2007) confirmatory action letters (CAL)
to 40 plants – Enhanced leak monitoring – Inspection/mitigation of the pressurizer welds for all the PWRs with
uninspected Alloy 82/182 pressurizer welds (including 9 plants scheduled for 2008 inspection/mitigation)
• Staff briefed ACRS on crack growth predictions for these indications and their implications to other plants on March 2007
• ACRS concluded: – Technical basis was sufficient – Additional work on residual stress including validation
is required. vg 4
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WRS Validation Program Joint NRC/EPRI WRS Validation Program
•Scientific Weld Specimens•Phase 1A: Restrained Plates (QTY 4)•Phase 1B: Small Cylinders (QTY 4)•Purpose: Develop FE models.
Phas
e 1 -
EPRI
•Fabricated Prototypic Nozzles•Type 8 Surge Nozzles (QTY 2)•Purpose: Prototypic scale under controlled conditions. Validate FE models.
Phas
e 2 -
NRC
•Plant Components•WNP-3 S&R PZR Nozzles (QTY 3)•Purpose: Validate FE models.
Phas
e 3 -
EPRI
•Plant Components•WNP-3 CL Nozzle (QTY 1)•RS Measurements funded by NRC•Purpose: Effect of overlay on ID.
Phas
e 4 -
EPRI
• Purpose: improve and independently validate WRS finite element analyses (FEAs) against physical WRS measurements
Work conducted through NRC/EPRI MOU Addendum
• First-of-a-kind double-blind finite element validation project on prototypic nuclear welds, leading to – Regulatory impact:
e.g., optimized weld overlay effectiveness
– Increased understanding of modeling uncertainty
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Sample Results
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B - KIN
C - ISO
C - KIN
D - KIN
E - ISO
E - MIXED
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F - ISO
G - ISO
H - ISO
I - ISO
I - KIN
iDHD #1
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Major Conclusions
• WRS program allowed the refinement of analysis and measurement for dissimilar metal welds
• Improved quality and confidence in – Modeling approaches – Constitutive models – Measurement methods
• Defined current state of uncertainty in WRS
predictions
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Major Conclusions
• Uncertainty in the WRS analyses can be large – ~200MPa – Lightly driven by weld process uncertainty – Highly driven by modeler uncertainty
• Uncertainty in WRS measurements between
techniques may also be large
• Despite large modeling uncertainty, measurements and models show reasonable trend agreement
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WRS Validation • In the four phases of the WRS Validation program,
NRC/EPRI generated experimental and numerical weld residual stress data for dissimilar metal weld locations
• How can we use this information??
• What are the gaps for use in regulatory space?
• Do we need guidance/guidelines?
• How do we account for or reduce uncertainty? vg 9
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MRP-287 • Recently, through the ongoing efforts in PWSCC flaw
evaluation, EPRI published MRP-287 which gives guidance on PWSCC flaw evaluation.
• Document incorporated NRC informal comments, but the report has not been formally reviewed by staff
• Document lists attributes of acceptable weld residual stress analyses – Geometry and materials – Weld configuration and fabrication sequence – Repairs – Safe-end to pipe weld
• Document recommends numerical procedure be benchmarked and validated against experiments vg 10
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Flaw Evaluation Relief Requests
• Typically when SCC is found and analyzed per ASME Section XI, the analysis is reviewed by NRR
• The licensee supplies data on WRS assumed – From literature on a weld with similar characteristics – Generic WRS analysis – Case specific WRS analysis
• No information is presented with respect to WRS uncertainty and only a single through-thickness representation is presented to NRR – contrary to MRP-287
• From a regulatory viewpoint, how can we be confident that the WRS provided by licensee is validated and conservative with respect to the uncertainties?
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DM WRS Validation
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Surge Nozzle
Small symbols - analysis
• Trends very similar
• Scatter can be large
• Sensitivity analyses or probabilistic analyses may be needed
WRS1 WRS2
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Flaw Evaluation • Circumferential crack
in DM weld • 10% deep, c/a=2 • MRP-115 crack
growth
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MRP 287 suggests ±10ksi-in0.5 as validation criteria
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Flaw Evaluation • Axial crack in DM
weld • 10% deep, c/a=2 • MRP-115 crack
growth
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MRP 287 suggests ±10ksi-in0.5 as validation criteria
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Thoughts • To add confidence in WRS predictions
– Minimize model uncertainty – Develop reliable and consistent numerical procedures
– Robust WRS validation methods – Minimize measurement uncertainty – Develop appropriate criteria for validation
• For flaw evaluations – Use best estimate WRS from numerical
procedures that are reliable, consistent and validated
– If not possible, use conservative WRS • Yield level • Geometry specific and bounding WRS
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Ongoing ASME code
work
Completed Future Developed procedure/ methods
Reduce uncertainty
Define uncertainty and drivers
Develop criteria
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WRS Validation Needs • Short Term
– Reduce model uncertainties – Develop consensus approach to handle discrepancy between
model and measurement – Develop WRS validation acceptance criteria – 3-D moving arc analysis for partial arc repairs
• Long Term – Continue WRS guidance in ASME code – Continue monitoring international validation efforts – Need to understand WRS validation for other geometries
(CRDM, BMI, etc.)
• Very Long Term – Develop residual stress driven SCC data in DM welds
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1-2 years
2-3 years
When I win the lottery
Dep
ende
nt o
n fu
ndin
g!
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Status of the Program
• MOU addendum for WRS on hold due to impacts from sequestration
• NRC WRS NUREG – Initial draft complete – Provided to NRR, EPRI, and measurement experts for informal comments – Interactions between RES and NRR on NUREG are ongoing. – Tentative final publication: Fall 2013
• Phase 2b Round Robin
– Will be used to demonstrate reduction in modeling uncertainty – Draft modeling package to be complete in June
• Measurements
– Deep hole drilling measurements on Phase 2b nozzle complete – Contour and slitting measurements on Phase 2a and Phase 2b nozzles are
nearing completion
Latest Deliverables
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Select Publications • H. J. Rathbun, L. F. Fredette, P. M. Scott, A. A. Csontos, and D. L. Rudland, "NRC Welding Residual
Stress Validation Program International Round Robin Program and Findings," Proceedings of the ASME Pressure Vessels and Piping Conference, Baltimore, MD, PVP2011-57642, July 2011
• M. Kerr, D. L. Rudland, M. B. Prime, H. Swenson, M. A. Buechler, and B. Clausen, “Characterization of a Plate Specimen from Phase I of the NRC/EPRI, Weld Residual Stress Program,” Proceedings of the ASME Pressure Vessels and Piping Conference, Baltimore, MD, PVP2011-57687, July 2011
• Lee F. Fredette, Matthew Kerr, Howard J. Rathbun, and John E. Broussard, “NRC/EPRI Welding Residual Stress Validation Program – Phase III Details and Findings,” Proceedings of the ASME Pressure Vessels and Piping Conference, Baltimore, MD, PVP2011-57645, July 2011
• J. Broussard, ‘Materials Reliability Program: Welding Residual Stress Dissimilar Metal Butt-Weld
Finite Element Modeling Handbook (MRP-317),” Report 1022862, Electric Power Research Institute, Palo Alto, CA, December 2011
• J. Broussard, “Materials Reliability Program: Finite-Element Model Validation for Dissimilar Metal Butt-Welds (MRP-316),” Electric Power Research Institute, EPRI 1022861, December 2011
• M. Kerr and H. J. Rathbun, “Summary of Finite Element Sensitivity Studies Conducted in Support of the NRC/EPRI Welding Residual Stress Program,” Proceedings of the ASME Pressure Vessels and Piping Conference, Toronto, Canada, PVP2012-78883, July 2012
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Using WRS in Regulatory Space • Reduce uncertainty in industry submitted
deterministic flaw evaluation – Lessons learned from Phase 1-4 – Incorporate tiered WRS structure in ASME Section XI code
(ongoing) and 10CFR50.55a
• Incorporate WRS uncertainty in analyses – xLPR for leak-before-break
• Best Practices on new and repair fabrication – Learn from operating plant experiences – Don't repeat deleterious fabrication methods of the past – Learn from the lead in other industries.....
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