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
'towepA
AREVA * Westinghouse1
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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•O"ne, AREVA i Westinghouse8 ~nr
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
PWRnC.
-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
AARE VA (OWestinghouseOtvnerS, &
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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
"50'vners 4'
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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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