PWROG Flaw Tolerance Evaluationof CE and B&W Pump Suction andDischarge Nozzle DM Welds

Warren Bamford, Westinghouse

with input from Ashok Nana, Areva

August 2009

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Why We are Here

* To discuss large diameter RC Pump cold leg nozzles with

dissimilar metal (DM) welds

* To explain the results of these programs, to address the

challenges associated with the examination of these welds

* To describe plans to revise N-770 to address this

* To gain NRC staff feedback on the results of the program,

and the proposed changes to Code Case N-770

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Presentation Outline

* Inspection Challenges for CE Large DiameterNozzles

* Probability of Cracking in this Region

* The Safety Case: Defense in Depth

" CE Design RCP Flaw Tolerance Study" B&W Design RCP Flaw Tolerance Study" Proposed Revision to Code Case N-770

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CE Reactor Coolant Pump Nozzle DM WeldInspection Issues: an Example

RCP OutletNozzle DM

Weld

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CE Reactor Coolant Pump Nozzle DM WeldInspection Issues: another Example

RCP OutletNozzle DM

WeldInstrument

Nozzle

Permanent Interferences-01nre A

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Reactor Coolant Pump Nozzle DM Weld Inspectionand Mitigation Issues

- Cast Stainless Steel Safe End (One-side Inspection for CE Nozzles)

- Weld Contour/Nozzle Configuration Limit Inspection Coverage

- Lack of coverage cases considered from 11 % to 23% circumference,

covering the range of plant limitations

- Industry is working to reduce these limitations, and the maximum may

be14% at this time

- No Practical Mechanical Mitigation Solution exists for these Large

Diameter Low Susceptibility Nozzle DM Welds

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Mitigation Challenges

" Access to the ID of these nozzles is not available,so ID mitigations are not practical

" The diameter is very large, over 30 inches, or 76cm, and there are a number of penetrations in theimmediate vicinity of the DM Weld

" This makes OD solutions very complicated:- MSIP must be applied immediately adjacent to the DM weld, but on

the one side is Cast Stainless Steel, and the other has these

obstructions

- Weld overlay requires massive amounts of welding, complicated by

zsa1e same issuesPWROG

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The Big Picture

* Low Cold Leg Temperature Results in:- Lower probability of crack initiation

- Slower PWSCC growth rate

" These RC Pump Nozzle Welds are highly Flaw Tolerant,

due to the large diameter and wall thickness- Evaluations considered both FCG and PWSCC

- Range of flaw shapes

* Continued operation without repair can be justified, per

Section XI, for substantial flaw sizes

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Large DM Weld - Inspection Data

Reactor Coolant Pump Suction & Discharge DM Weld Inspections4 Plants with 26 total DM Welds inspected, all PDI qualified

0 DM Weld indications

Reactor Vessel DM Weld Inspections31 Plants with 183 total DM Welds inspected, all PDI qualified

6 DM Weld indications, in Outlet NozzlesSummer (x2), Salem 1, Ringhals 3 & 4, OHI 3

Steam Generator Inlet & Outlet DM Weld Inspections14 Plants with 116 total DM Welds inspected, ECT and UT

16 inlet DM Weld indications (at 7 Japanese Plants)

QNo PWSCC indications in Cold Leg Locations

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Best Estimate Weibull Parameters for theProbability of a 7% Flaw: DM Weld Inspections

.0L.

CL

E0

10%

9%

8%

7%

6%

5%

4%

3%

2%

1%

0%

0 10 20 30 40 50 @048F 60Effective Full Power Years (EFPY)

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Conclusions: Probabilistic Evaluation

* For best estimate Weibull parameter estimates:

There is less than a 1 % probability that the ith DM Weldwill develop a flaw on or before the 60th year (EFPY) oflife.

20 EFPY 0.04%

40 EFPY 0.24%

60 EFPY 0.66%

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Leak Detection Capability and Action Levels

* All plants have a leak detection capability of less than or

equal to 0.1 GPM, implemented under NEI-03-08 (needed)

* All plants also maintain 7 day moving averages of leak rates

" Action levels have been standardized for all PWRs, and are

based on deviations from:- Seven day rolling average

- Specific values

- Baseline Mean

" Action times vary, but are always within 24 hours

* A PWR recently shut down in July with 0.2 GPM unidentified leakage

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Leak Rate Action Levels (per WCAP 16465)

0 Action Levels on the absolute value of Unidentified RCS Inventory Balance (from

surveillance data):

- One seven (7) day rolling average of Unidentified RCS Inventory Balance values > 0.1

gpm.

- Two consecutive Unidentified RCS Inventory Balance values > 0.15 gpm.

- One Unidentified RCS Inventory Balance value > 0.3 gpm.

0 Action Levels on the Deviation from the Baseline Mean:

- Nine (9) consecutive Unidentified RCS Inventory Balance values > baseline mean [p]

value.

- Two (2) of three (3) consecutive Unidentified RCS Inventory Balance values > [p + 2a],

where a is the baseline standard deviation.

- One (1) Unidentified RCS Inventory Balance value > [p +3a].

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Leak Rate Calculations

" Leakage calculations used typical licensed methodology

" Critical through-wall lengths used ASME Code calculational

methods, eg. Code Case N-513

" Growth from Leakage to critical flaw length used both

fatigue and SCC

* Results show the time from leak to critical length, for a

range of Leakage rates

" FEA Crack verified these simplified calculations

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Time From Leakage to Critical Circ.Flaw Length(No Residual Stress case)

Time to Reach Critical Length vs. Initial Crack Leakage Rate

20

18

16

14

12

• 10.E

8

6

4

2

0

---- Minimum NoP Load

--- Maximum NoP Load

0 1 2 3 4

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Leak Rate (gpm)

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Conclusions: Defense in Depth

" Utility action levels ensure a very high likelihood of dealingwith leak rates in the range of 0.05 to 0.1 GPM

" Calculations show that a very long time is required to growa flaw leaking at 0.1 GPM to a size which could causefailure (over 15 years)

" A similar conclusion would be reached with the licensedleak rate capability of one GPM (over 10 years)

" Thus we have a very large margin between the action timefor a plant and the time to grow to flaw instability

" jT4U strongly supports defense in depth for this regionPWROG

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Section Xl Flaw Tolerance Calculations

* Allowable flaw size, per Appendix C

" Fatigue crack growth determined to be negligible

" PWSCC growth calculated with fixed flaw shape as well as

advanced finite element techniques (FEA Crack)

* Results presented in terms of the largest initial flaw which is

acceptable, for a range of time periods

" Axial and circumferential flaws considered

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Residual Stress Considerations

" Allowable flaw size, per Appendix C, is not affected by residual stress

* FCG and PWSCC are affected, PWSCC most strongly

" Residual stresses from fabrication for CE DM welds obtained from

Report MRP 113

" Residual stresses from postulated 15% ID weld repair obtained fromMRP-113 for CE design

" Residual stresses from postulated 50% ID weld repair obtained fromfinite element work for B&W design

" Stress distributions go strongly negative in mid wall, thus preventingcrack growth

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Residual Stresses from Fabrication: MRP 113.Stainless Steel Field Weld

no

a-in

Axto

80

60

40

20

0

-20

-40

-60

--- Hoop (ksi)

-U- Axial (ksi)

* Westinghouse

PWRn c.aft Ratio

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Residual Stresses from Postulated 15% Repair:MRP-1 13, No Stainless Steel Field Weld

80

60

40

20

.uU) 0

• -20

-40

-60

-4 Hoop (ksi))-- Axial (ksi)

*)Wesfinghouse

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-80

a/t Ratio

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Effect of Stainless Steel Weld on WRS50% ID Repair Case, B&W analysis

60000

50000

40000

0 300000.

'm 20000

"R 10000

a 0

-10000

-20000

-30000

Normalized Distance Through the Thickness

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Conclusions: Residual Stresses

" Three cases were studied:

* Fabrication only

* 15% ID repair

* 50% ID Repair, with and without stainless steel field weld

* Each repair case induces compressive stress in mid wall, just beyondthe repair

" The stainless steel field weld is effectively a mitigation, causing largecompressive axial stress at the pipe ID, thus preventing crack initiation,and depressing the hoop stress as well

* This is consistent with the results on smaller welds in MRP-216

Sis is also consistent with operating experience

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Allowable ASME End-of-Evaluation Period FlawDepths (% Wall Thickness): CE Design Pumps

Flaw Orientation Pump Suction

and Discharge

Axial 75

Circumferential 73 to 75

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Allowable ASME End-of-Evaluation Period Flaw Depths(% Wall Thickness): B&W Design Pumps

Flaw Orientation Pump Suction

and Discharge

Axial 75

Circumferential 70 to 75(20 = 600)

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Maximum Acceptable Initial Axial Flaws:CE Designs, Accounting for SCC and FCG

(with Fabrication Residual Stress)

Time (months) to Reach

0.9 ASME Allowable Crack Depth

S0.8 -. 2• 24

- -360.7

-r- 48C'

," 0.6

- 0.5

CL

S0.4 -

CUU.3

- 0.2

0.1

00 0.1 0.2 0.3 0.4

Crack Depth / Length Ratio, a/fPWROG

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Maximum Acceptable Initial Circumferential Flaws:CE Designs, Accounting for SCC and FCG

(No Residual Stress)

1

0.9

- 0.86

o0.7U)

0)• 0.6

CL)-c

S0.4

o 0.3

0.2

0.1

0

Time (months) to ReachASME Allowable Crack Dept[

24

-- 36

---i- 48

0 0.1 0.2 0.3

Crack Depth / Length Ratio, a/l

0.4

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Maximum Acceptable Initial Circumferential Flaws:CE Designs, Accounting for SCC and FCG

(Fabrication Residual Stress + 15% ID Repair Residual stress)

1

0.9

.0.8

0.7

0 0.6

0.5

o0.4

00.3

0.2

0.1

Time (months) to ReachASME Allowable Crack Der

-- 24

-- 36- 48

0

0 0.1 0.2 0.3 0.4

Crack Depth / Length Ratio, a/f

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Maximum Acceptable Initial Axial Flaws:B&W Designs, Accounting for SCC

(with Fabrication Residual Stress, no stainless steel weld)

1.0

0.9 Time (months) to ReachEU ASME Allowable Crack Depth0-= 0.8 - 2412

w 0.7 - 36U)

0 -~-48

0.6 4

,•0.5

0.4

-6 0.3

0.2 -_ _-_-

" 0.1

0.0 .....0 0.1 0.2 0.3 0.4 0.5

Crack Depth/Length Ratio, ale

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Maximum Acceptable Initial Axial Flaws:B&W Designs, Accounting for SCC(Fabrication Residual Stress + ID Repair residual stress, no stainless steel weld)

1.0

0.9 lime (months) to ReachASME Allowable Crack Depth

00.8 g24

,0.7 36

S0.6 -a---480-

0.5 ft.4-

0.3 -

U 0.2 - ------ --a._

0E 0.1

0.0

00.1 0.2 03 0.4 0.5

Crack Depth/Length Ratio, ale

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Maximum Acceptable Initial CircumferentialFlaws: B&W Designs, Accounting for SCC

(with Fabrication Residual Stress, no stainless steel weld)

1.0

0.9

0.8

= 0.7

0.62l 0.5

S0.4z•(ots

All Time to ReachASME Crack Depth

0.3-.--o-2]4-& 4-E 0.2-3

0.1

0.0 ...

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Crack Depth/Length Ratio, a/l

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Maximum Acceptable Initial CircumferentialFlaws: B&W Designs, Accounting for SCC(Fabrication Residual + 50%1D Repair residual , no stainless steel weld)

1.0-

0.9,

0 0.8-6

0.7,

0.6.

0.5_

.ime (months)to Reach0ASME Allowable Crack Depth

1! 0.3-

S0.2-3

0.1 48

0.0 10.0 0.1 0.2 0.3 0.4 0.5 0.6

Crack Depth/Length Ratio, a/l

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Conventional SCC Growth Evaluation

2c, 2c 0

a,

a0

s distribution:

L

" Typical ASME Section XI SCC analyses

* Summary of SIF expressions in non-dimensional

form (SIF Database - discrete a/t, a/c)

* Solutions typically available for only 1 D stress

distribution

" SIF calculated at surface and deepest point of

crack

ilh SIF Databases limitations (curve-fitting required,

I component size, crack size)OW

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Natural SCC Growth Evaluation

2cc

IN

O'ýý1

" SIF calculated by FEA

" FEA available for arbitrary 2D stress

distribution

" Evaluation at various crack front nodal

coordinates

* Growth Rate determined based on CGR

correlation for various crack front locations

'" Flaw shape/size updated and growth

calculation continues

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IHighss distribution: 5 I

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Natural SCC Growth Evaluation

* Growth evaluated at various crackfront locations, with arbitrary 2Dstress distributions

Growth Step Ida

dt=f(K)

CGRCorrelation withcrack front SIF

Growth Step 2 IResultantGrowth rates

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Circumferential Flaw Tolerance using FEACrack

* FEACrack was used to verify the conclusions of thedeterministic calculations, for the no residual stress case

" Only the 'No Residual Stress' Cases were run* For a flaw of depth 20% of the wall and length equal to 14%

of the circumference, more than 10 years are required toreach the Code allowable depth of 75%

" For a flaw of depth 20% of the wall and longer length equalto 23% of the circumference, 9.6 years are required toreach the Code allowable

" Only PWSCC was considered, since Fatigue Crack Growthis negligible for at least 10 years, the period of interest

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Circumferential Flaw Tolerance using FEACrack

Initial alt Initial Time to Time to

length/circumference a/t = .75 aft = 1.0

.20 .14 10.68 yrs. 12.52 yrs.

.20 .23 9.6 11.1

.30 .14 7.44 9.34

.30 .23 6.45 7.85

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Conclusions: Flaw Tolerance Evaluation

" Flaw Tolerance in these locations is very high, for both axial andcircumferential orientations, of realistic configurations

* A 50% ID repair was evaluated, but is not considered realistic

* The stainless steel weld significantly reduces the stresses, andproduces compression at the ID. These effects were not consideredin the analyses shown here

* Fatigue Crack Growth is negligible, so growth is dominated byPWSCC

• Residual stresses tend to arrest cracks in mid wall, and stainlesssteel field weld acts as a mitigation

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Key Objective of this Work

* Provide technical basis for changes in inspectionrequirements for large diameter low-susceptibilitynozzles

" Such changes are planned for Code Case N-770

" This would make it consistent with MRP Butt WeldInspection and Evaluation Guidelines (MRP-139, R 1)

" It will also provide a basis for continued operation for aperiod of time, to allow repair methods to bedeveloped if needed

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Flaw Tolerance Option For Code Case N-770

" Subpara. (d) is proposed for - 2500 of the case:

" For Cold Leg locations, with diameters >14 inches, achieve

maximum coverage possible, and perform a flaw tolerance

evaluation for circumferential flaw limitations

- Axial Flaws: Achieve maximum coverage possible, and document

the limitations, provided.90% Circ. coverage is achieved

- For Circumferential Flaws, If inspection coverage < 90%, and is a

result of permanent obstructions, the following requirements ensue,

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Option Proposed for Code Case N-770:Circumferential Flaw Tolerance

* Assume a through-thickness flaw for the area not inspected

" Calculate the critical flaw size for the location, using the

methodology of Code Case N-513

" Show that the time to reach critical flaw size is greater than

the time to the next inspection, or adjust the inspection time

accordingly

" Perform VT-2 Exams of the region every refueling outage

" Evaluate the risk of leakage occurring between inspections,

and document leakage monitoring action levels

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Option Proposed for Code Case N-770:Circumferential Flaw Tolerance (cont'd)

* Assume a part through flaw for the area not inspected

" Calculate the Section XI Allowable flaw size for the location

* Show that the time to reach the allowable flaw size is

greater than the time to the next inspection , or adjust the

inspection timing accordingly

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Summary

" Introduced the challenges of large diameter cold leg DM

Welds

" Explained the goals and results of the program, to address

those challenges

" Described our plans to revise Code Case N-770

" Comments on our approach to dealing with this issue in the

Code?

4:.rA

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Conclusions

" Flaw Tolerance Is A Viable Approach-Justify Reduced Examination Coverage and/or

larger Inspection Intervals- Justify Continued Operations/Delay Repairs

" Beneficial to Low Susceptibility Large DiameterNozzles With No Practical Mechanical MitigationOptions

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