excavate and weld repair (ewr) for scc repair or mitigation · –provides following welding...

© 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

2© 2016 Electric Power Research Institute, Inc. All rights reserved.

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


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)


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


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


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


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


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



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



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


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


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


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


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)


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)


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


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)


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


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


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


Questions or Comments?

Welding and Repair Technology Center


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