PRS-08-003/1

ATTRIBUTES OF WELD OVERLAYS FOR LOWERING

PIPE RUPTURE PROBABILITY

Pete Riccardella, Bud Auvil, Nat CofieInternational Workshop on LBB in RCS

Components with Active Degradation ModesJanuary 9-11, 2008

PRS-08-003/2

Outline of Presentation

• Weld Overlays (the basics)• Verification of Weld Overlay Effectiveness

♦ Experimental♦ Field Experience♦ Analysis/Inspection Requirements

• Alternatives for Demonstrating LBB or LPR (Low Probability of Rupture)

PRS-08-003/3

Weld Overlays – The Basics• For IGSCC (in BWRs) and PWSCC (in PWRs), crack

growth rates are often too fast to permit successful flaw evaluation by ASME Section XI rules

• Weld overlays were invented in 1981, as a means of assisting flawed welds to pass Section XI evaluation♦ Additional structural reinforcement (with SCC resistant

material)♦ Favorable residual stress reversal to stop the growth of

existing cracks• Over 800 overlays applied during 25 year period in

BWRs, many still operating• Since 2003, ~180 overlays applied to PWR pressurizer

nozzles

PRS-08-003/4

ASME Section XI Flaw EvaluationBasic Concept

00.10.20.30.40.50.60.70.80.9

0 2 4 6 8 10 12

Time (yrs)

Cra

ck D

epth

(a/t)

End-of PeriodAllowableCrack Growth

Flaw Size from Inspection

Allow able Flaw Size

PRS-08-003/5

Weld Overlay Attributes

• Weld overlays (both repair and preemptive) possess attributes that effect an extremely low probability of rupture in underlying weld:♦ Structural reinforcement to ASME XI margins

with very large cracks assumed to be present ♦ Crack growth barrier of SCC resistant material♦ Favorable residual stress reversal (from tensile

to compressive) ♦ Enhanced inspectability

PRS-08-003/6

Piping Weld Overlay Schematic

PRS-08-003/7

Cross Section –DMW Nozzle Weld Overlay

PRS-08-003/8

WOL Design Concepts(as Defined in MRP-169)

Full Structural Overlay(FSWOL)

Optimized Overlay(OWOL)

PRS-08-003/9

Summary of WOL Terminology

Optimized

Full StructuralRepair(WOL)

Optimized

Full StructuralMitigation(PWOL)

WOL TypeWOL Function

PRS-08-003/10

Verification of Weld OverlayEffectiveness - Experimental

• 28-Inch Notched Pipe Test • Battelle/NRC Degraded Pipe Tests• EPRI/MRP Demonstration Program for

PWR PWOL s

PRS-08-003/11

28-Inch Notched Pipe Test

PRS-08-003/12

28-Inch Notched Pipe Test (Results)

Notch Tested pre-WOL Notch Tested post-WOL

PRS-08-003/13

Battelle/NRC Degraded Pipe Tests

PRS-08-003/14

Battelle/NRC Degraded Pipe Tests (Results)

PRS-08-003/15

EPRI PWOL Mockup (Showing Residual StressMeasurement Locations)

0°

45°

90°

135°

ID Weld Repair

PRS-08-003/16

EPRI PWOL Mockup Residual Stress Results

ID Surface Axial StressPre-Overlay Analysis vs. Measurements

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 0.5 1 1.5 2

Dist. from DMW Centerline (in)(towards nozzle)

Stre

sses

(ksi

)

Analysis

XRD 45

XRD 0

XRD 90

A-182 Thru-wall Butter Region on ID

SS Clad on ID

A-182 Clad on ID

0° 45° 90° 135°ID Weld Repair

ID Surface Axial StressPost-Overlay Analysis vs. Measurements

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 0.5 1 1.5 2

Dist. from DMW Centerline (in)(towards nozzle)

Stre

sses

(ksi

)

Analysis

XRD 45

XRD 135

Hole Drill

A-182 Thru-wall Butter Region on ID

SS Clad on ID

A-182 Clad on ID

Pre-Overlay Post-Overlay

PRS-08-003/17

Verification of Weld OverlayEffectiveness – Field Experience• BWRVIP-75

♦ Published in 1999♦ Evaluated WOL performance in 33 BWRs (262

overlays still in service)♦ 15 years of service history w/no evidence of

crack growth or new crack initiation♦ Included several A-82/52 overlays on DMWs (1st

performed at VY in 1986)♦ Accepted as basis to extend WOL inspection

schedules relative to NRC Generic Letter 88-01

PRS-08-003/18

Verification of Weld OverlayEffectiveness – Field Experience (cont’d)

• Experience since BWRVIP-75♦ Flawless performance of BWR overlays

has continued (some in place ~25 yrs)♦ Numerous DMW overlays applied to

PWRs (beginning w/TMI in 2003)♦ PWR overlay OE still young, but to date

has paralleled BWR performance

PRS-08-003/19

Verification of Weld OverlayEffectiveness – Analysis & Inspection

• Design/Analysis Requirements • Typical Design/Analysis Results• Inspection Requirements

PRS-08-003/20

MRP-169 Design/Analysis Requirements• Structural Sizing• Residual Stress Improvement• Inspectability Considerations• Fatigue and PWSCC Crack Growth

Analysis• Section III Stress & Fatigue Evaluation• As-built Reconciliation and Evaluation of

Potential Effects on Other Components

PRS-08-003/21

Typical WOL Design Analysis - FEM

Finite Element Model

Stress Paths for Fracture Mechanics

Evaluation

PRS-08-003/22

Typical Post-WOL Residual Stresses(at Normal Operating Pressure & Temp.)

Hoop Stresses Axial Stresses

PRS-08-003/23

Post-WOL Stress Intensity Factors (Including Pressure + Operating Loads)

Axial Flaw

-40

-30

-20

-10

0

10

20

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Crack Depth (in.)

Stre

ss In

tens

ity F

acto

r (ks

i-in^

.5)

Path 1 Path 2 Path 3

Orig. DMW thickness "t"0.75 t

Circumferential Flaw

-30

-25

-20

-15

-10

-5

0

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Crack Depth (in.)

Stre

ss In

tens

ity F

acto

r (ks

i-in^

.5)

Path 1 Path 2 Path 3

Orig. DMW thickness "t"

0.75 t

PRS-08-003/24

PWOL Inspection Requirements• Inspections immediately after application of PWOL

♦ Construction (acceptance exam) of WOL and underlying HAZ

♦ PSI exam - outer 25% (FSWOL) or 50% (OWOL) of original DMW thickness

♦ Establishes “buffer zone” (25% of original DMW thickness) between flaw size for which UT technique must be qualified for and overlay design basis flaw

• Subsequent ISI Requirements ♦ ISI Schedule dependent on WOL function (repair or

preemptive)♦ Exam Volume same as PSI

PRS-08-003/25

WOL Examination Volumes

PRS-08-003/26

WOL - Inspectability Considerations can Influence Design

Inconel Buttering

Weld Overlay Volume

Weld / Base Material Volume

Safe End

Ferritic Steel Nozzle

Solid Red Lines Indicate the 45°Lower Limit of PDI-UT-8 Examination Coverage

Cross-Hatched Green Lines Indicate the Examination Volume Where Coverage was

not Achieved Using PDI-UT-8 45° Lower Limit

PRS-08-003/27

MRP-139/169 Inservice Inspection Categories & Intervals

MRP-139 Inspection Category

Applies to: Examination Extent and Schedule

B Inspected, uncracked, Reinforced by FSWOL

(or OWOL)

Existing Code Examination Program or Approved

Alternative C Inspected, uncracked,

Mitigated by SI 50% within next 6 years; if clean, then Code program

or approved alternative F Inspected, cracked,

Reinforced by FSWOL (or OWOL)

Once in next 5 years; if no new indications/growth, then Code program or approved alternative

G Inspected, cracked, Mitigated by SI

100% at 2 RFO intervals. If no new indications/growth after 2 exams then Code

program or approved alternative

PRS-08-003/28

Alternatives for LBB or LPR Demonstration

• Update existing, conventional LBB analysis (SRP 3.6.3) to account for new material and geometry conditions produced by WOL♦ Addressed in Deardorff/Cofie Paper

• Document WOL effectiveness at eliminating PWSCC concern and thus producing very low probability of both leakage and rupture

• Perform generic or nozzle specific probabilistic fracture mechanics analyses♦ Compare probability of rupture of WOL to non-

susceptible component qualified by conventional LBB♦ Addressed in Riccardella/Dedhia/Harris Presentation