© 2016 Electric Power Research Institute, Inc. All rights reserved.
International Light Water Reactor Materials
Reliability Conference and Exhibition 2016
McCormick Place
Chicago, IL, USA
Steve McCracken & Jon Tatman, EPRI
Excavate and Weld Repair (EWR)
for SCC Repair or Mitigation
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Background – Stress Corrosion Cracking (SCC)
SCC is the result of three drivers
‒ Susceptible Material
‒ Tensile Stress
‒ Corrosive Environment
SCC mechanisms in the nuclear power
generation industry are
– Primary water SCC (PWSCC) in a PWR
– Intergranular SCC (IGSCC) in a BWR
Mitigation typically results in a stress
improvement and / or applies a SCC
resistant material
Tensile Stress
σresidual ≈σyield
Corrosive
Environment
Susceptible
Material
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Background – SCC Repair & Mitigation Methods
Mechanical Stress Improvement Process (MSIP) Weld Overlay
(N-504-4, N-740-2, N-754, App. Q)
Inside Diameter Onlay (N-766)Inside Diameter Inlay (N-766)
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ASME Code Case N-847 provides rules and
requirements for the new EWR mitigation method
SCC susceptible material is excavated from OD
Excavation is filled with SCC resistant weld metal
EWR options:
– May be designed to promote stress improvement
– May be applied to reduce a flaw (stress corrosion
crack – SCC) to an acceptable size for continued
service
– May be applied as mitigation (no SCC detected)
– May be a full 360º excavation or a partial
circumferential excavation (partial arc EWR)
Excavate and Weld Repair (EWR) Method
EWR Weld Metal
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PWR Case
– Dissimilar metal weld (DMW) joining
low alloy steel to austenitic safe-end or
piping
– PWSCC susceptible Alloy 82/182 weld
metal
BWR case
– Similar metal weld (SMW) joining
stainless-to-stainless piping
– IGSCC susceptible sensitized stainless
steel base material heat-affected-zone
(HAZ)
Code Case N-847 Applies to Class 1 Butt Welds
EWR for Dissimilar Metal Weld
EWR for Similar Metal Weld
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EWR Condition / Description 1A 2A 1B 2B
Uncracked Butt Weld: Pre-EWR examination (1) performed and
no inside surface connected flaw or subsurface defect detected.X X
Cracked Butt Weld: Inside surface connected flaw or subsurface
defect detected by pre-EWR examination or pre-EWR examination
is not performed.
X X
EWR with Stress Improvement: Tensile residual stress ≤ 10 ksi
(69 MPa) on inside wetted surface as determined by stress
analysis.
X X
EWR with No Stress Improvement: Tensile residual stress >10
ksi (69 MPa) on inside wetted surface or residual stress analysis is
not performed.
X X
(1) Pre-EWR examination per ASME Section XI, Appendix VIII supplement.
Types of Full 360º EWR
EWR Condition / Description 1A 2A 1B 2B
Uncracked Butt Weld: Pre-EWR examination (1) performed and
no inside surface connected flaw or subsurface defect detected.X X
Cracked Butt Weld: Inside surface connected flaw or subsurface
defect detected by pre-EWR examination or pre-EWR examination
is not performed.
X X
EWR with Stress Improvement: Tensile residual stress ≤ 10 ksi
(69 MPa) on inside wetted surface as determined by stress
analysis.
X X
EWR with No Stress Improvement: Tensile residual stress >10
ksi (69 MPa) on inside wetted surface or residual stress analysis is
not performed.
X X
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Excavation extends less than 360º of
circumference
Weldment not fully mitigated
SCC reduced to acceptable size
Provides timely option when emergent ISI
examination reveals rejectable SCC
indication
Provides time for deployment of more
permanent repair
Circumferential overlap is critical design
parameter which likely will define partial arc
EWR design life
Consideration to deploy partial arc EWR with
SMAW temper bead (code case N-839)
Cross-Section of Partial Arc EWR
Partial Arc EWR DescriptionPartial
Arc EWR
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Welding Requirements
– Similar welding requirements as other SCC repair/mitigation cases such as N-740-2,
N-504-4 (Section XI App. Q), N-754, and N-766-1
– Permits using weld metal with 26% Cr (Filler Metal 52i)
– Provides following welding provisions
• Temper bead welding on low alloy steel
• Buffer layers to manage hot cracking
• Mechanical peening and leak sealing of flaws open at bottom of EWR excavation
Design Requirements
– Minimum excavation depth for a full 360º EWR is the minimum thickness of SCC-
resistant weld metal that can support the design loads including an allowance for SCC
and fatigue
– Design must consider all loads and load combinations (e.g., Service Levels A, B, C
and D as defined in ASME Section III)
N-847 General Requirements
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N-847 General Requirements
Residual Stress Analysis
– Must include as-welded condition prior to the EWR as well as any influence from
machining or prior weld repairs
– Must include as-welded stress distribution of a 50% through-wall 360º repair (must
include actual repair if construction records show a more severe repair condition)
– Effects of post weld heat treatment may be included
– A bounding analysis is permitted provided the analysis bounds the plant specific
conditions
– If a residual stress analysis is not performed yield-strength-level residual stresses are
assumed over the entire EWR and underlying material (accepted practice – MRP-113)
– 3-D residual stress analysis is required for a partial arc EWR
EWR Axial and Circumferential Overlay
– Axial overlap is required to ensure an existing or potential flaw in the existing
weldment is blunted and does not propagate
– Circumferential overlap is required on a partial arc EWR
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N-847 General Requirements
Flaw Growth and Acceptable Flaw Size
‒ SCC and fatigue crack-growth is evaluated in accordance with Section XI, Appendix C
(2010 Edition with 2011 Addenda)
‒ Flaw growth analysis defines life of the EWR and is based on minimum EWR
thickness established by design
Postulated Flaw Considerations
– Axial flaw length is 1.5” (38mm) or width of SCC susceptible material if greater
– Circumferential flaw length is assumed to extend 360º
– Flaw depth varies as follows:
• Actual flaw depth when pre-EWR examination detects cracking
• 10% of weldment thickness if no flaws detected
• Up to bottom of EWR excavation if no pre-EWR examination is performed
• Up to bottom of EWR excavation in most limiting direction for weldments with cast
austenitic stainless steel material
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Purpose of EWR Acceptance is Twofold
– Ensure new EWR deposit is free from weld
metal defects
– Identify EWR flaws that may adversely impact
the ISI examination
UT Examination
‒ Procedures must be demonstrated per Appendix
VIII, Supplement 11
‒ Five additional EWR type flaws required for
procedure demonstration
General Acceptance Standards
‒ Planar flaws are dispositioned per ASME Section
XI, IWB-3514
‒ Laminar flaws cannot exceed 10% of EWR
surface area and cannot reduce PSI / ISI
examination coverage by more than 10%
EWR Acceptance Examination
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EWR Preservice and Inservice Examination
PSI / ISI Examination Volume
– Inner 1/3 thickness (same as for all
Class 1 piping butt welds)
• Appropriate since any SCC would
initiate from ID wetted surface
• Same UT examination procedures for
ISI of un-mitigated Class 1 butt weld
can be used for the EWR weldment
– Any flaws that extend outside
examination volume must be fully
characterized
• This is consistent with current ISI
requirements for all butt welds
Procedures and Personnel Qualified per Section XI Appendix VIII
‒ Supplement 2 for similar metal austenitic but welds
‒ Supplement 11 for dissimilar metal high-chromium nickel alloy welds
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Extent & Frequency of Examination (PWR & BWR)
N-847 N-770-5 (PWR) BWRVIP-75-A (BWR)
EWR Type Item No. Table 1 Category Table 3-1 NWC (4) Table 3-1 HWC/HMCA (5)
1A (1,2,3) M-125%
every 10 yr
C
Uncracked MSIP
25%
every10 yr
10% every
10 yr
2A (1,2) M-2100%
every 10 yr
C
Uncracked MSIP
25%
every 6 yr
10% every
10 yr
1B (1,3) N-125%
every 10 yr
E
Cracked WOL
25% every 10yr & at least
12.5% in 1st 6 yr
10% every
10 yr
2B (1) N-2100%
every 10 yr
E
Cracked MSIP
100%
every 6yr
100% every
10 yr, at least 50% in
1st 6 yr
Partial Arc O
Same as
unmitigated butt
weld
F
Cracked no repair
Every refueling
outage
Every
refueling outage
(1) Full 360º EWR with SCC resistant weld metal
(2) Uncracked butt weld mitigated with EWR
(3) Residual stress ≤ 10ksi (69MPa) on ID of SCC
susceptible material
(4) BWR normal water chemistry (NWC)
(5) BWR hydrogen water chemistry (HWC) and noble
metal chemical application (NMCA) for IGSCC
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Comparison Between Mitigation Methods (N-770-5)
Mitigated Butt Weld with No Cracking
Mitigation
Method
1st
Inspection
2nd
Inspection
Resistant
Material
≤ 10ksi
on Inside
FSWOL 25% sample each interval No change Yes Yes
OWOL (2) 100% in 1st interval25%
Sample (1) Yes Yes
MSIP 100% 3rd RFO - 10 yr25%
Sample (1) No Yes
Inlay/Onlay 100% 3rd RFO - 10 yr25%
Sample (1) Yes No
EWR Type 1A (3) 100% 3rd RFO - 10 yr25%
Sample (1) Yes Yes
EWR Type 2A (3) 1st or 2nd RFO 100% Each Interval Yes No
(1) If no indication of new cracking
(2) OWOL extent and frequency of examination per proposed revision to N-770-4 (see ASME Rec. No. 16-57)
(3) EWR extent and frequency of examination per Case N-847 (see ASME Rec. No. 10-1845)
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Comparison Between Mitigation Methods (N-770-5)
Mitigated Butt Weld with Cracking
Mitigation
Method
1st
Inspection
2nd
Inspection
Resistant
Material
≤ 10ksi
on Inside
FSWOL 1st or 2nd RFO 25% sample (1) Yes Yes
OWOL (2) 1st or 2nd RFO 25% sample (1) Yes Yes
MSIP 1st or 2nd RFO 25% sample (1) No Yes
Inlay/Onlay 1st or 2nd RFO 25% sample (1) Yes No
EWR Type 1B (3) 1st or 2nd RFO 25% sample (1) Yes Yes
EWR Type 2B (3) 1st or 2nd RFO 100% each Interval Yes No
Partial Arc EWR (3) 1st RFO Same as unmitigated butt weld No No
(1) If no indication of new cracking
(2) OWOL extent and frequency of examination per proposed revision to N-770-4 (see ASME Rec. No. 16-57)
(3) EWR extent and frequency of examination per Case N-847 (see ASME Rec. No. 10-1845)
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Partial Arc EWR Mockups
Residual Stress Evaluation
– Main purpose of mockups was to compare and validate FEA
residual stress simulations with residual stress
measurements
• Modeling performed by Structural Integrity Associates
(see PVP2016-63815)
• Measurements performed by Hill Engineering
(see PVP2016-63197)
‒ Demonstrate temper bead capability in EWR cavity
UT Demonstration
‒ EWR partial arc mockup fabricated to demonstrate UT
examination capability
‒ Proof of concept UT demonstrations completed
‒ Full scale partial arc EWR mockup fabrication is in-process
• Mockup is intended for Appendix VIII qualification
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FE Model vs. Measured Residual Stress
General agreement in shape of stress field for FE model and measured residual stress– FE model and measurement details are provided in two companion presentations by Structural Integrity
Associates (SIA) and Hill Engineering, respectively
Measured
FE
Data shown is for Mockup EWR-1, Contour Plane 1 (transverse cross-section at end slope of partial arc EWR), longitudinal stress (z-direction)
Line plots of z-direction (hoop) stress
along excavation interface from 316L
SS side to SA-508 side (see dashed
horizontal line in contour plot)
Line plots of z-direction (hoop) stress
along center line of EWR from bottom
(ID) to top (OD) of the mockup weld (see solid vertical line in contour plot)
18© 2016 Electric Power Research Institute, Inc. All rights reserved.
N-847 Approved by SC-XI May 2016 on
recirculation ballot (Record # 10-1845)
– SC-XI Vote:
31 - approved
1 - disapproved
1 - not returning
– Only negative was NRC
N-770-5 Disapproved by SC-XI June 2016
on 1st consideration letter ballot (Record #
14-2233)
– SC-XI Vote:
29 - approved
1 - disapproved
6 - not returning
– Only negative was NRC
Status of N-847 & N-770-5 in ASME Section XI
19© 2016 Electric Power Research Institute, Inc. All rights reserved.
EPRI Reports (search keyword EWR on epri.com)– Report 3002007901, Jun 2016, WRTC: Technical Basis and Residual Stress Studies to
Support the Excavate and Weld Repair (EWR) Methodology for Mitigation of SCC in
ASME Class 1 Butt Welds
– Report 3002005518, Sept 2015, WRTC: Excavate and Weld Repair Demonstration
Mockup Results – Preliminary Report
– Report 1021012, Dec 2010, Topical Report: Application of the Excavate and Weld Repair
Process for Repair and Mitigation of Alloy 182 and 82 in PWRs
2016 PVP Conference– PVP2016-63769, Technical Basis for Code Case N-847 – Excavate and Weld Repair
(EWR)
– PVP2016-63815, 3D Residual Stress Simulation of an Excavate and Weld Repair Mockup
– PVP2016-63197, Residual Stress Mapping for an Excavate and Weld Repair Mockup
– PVP2016-64041, Technical Basis for Stress Levels Needed to Mitigate PWSCC in Alloy
82/182/600
EWR Reference Documents
20© 2016 Electric Power Research Institute, Inc. All rights reserved.
Steve McCracken, Jon Tatman, Carl Latiolais, Jack Spanner
EPRI
Pete Riccardella, Richard Smith, Francis Ku
Structural Integrity Associates
Michael Hill, Mitchel Olson, Adrian Dewald
Hill Engineering
Key Contributors to EWR Project & ASME Case N-847
21© 2016 Electric Power Research Institute, Inc. All rights reserved.
Questions or Comments?
Welding and Repair Technology Center
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Together…Shaping the Future of Electricity