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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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06/06/2013

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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Normalized Distance (along Defined Path from ID to OD)

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NoRepair-NoSS (Left-Right) NoRepair-WithSS (Left-Right)

DEI NoRepair-NoSS DEI-NoRepair-WithSS

06/06/2013

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

06/06/2013

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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06/06/2013

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Sample Results

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B - KIN

C - ISO

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iDHD #1

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06/06/2013

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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06/06/2013

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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06/06/2013

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

06/06/2013

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

06/06/2013

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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06/06/2013

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

06/06/2013

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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06/06/2013

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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06/06/2013

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

06/06/2013

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

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06/06/2013

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

06/06/2013

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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06/06/2013

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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