L-13-104, Rev 1 to Design Rept for Weld Overlay Repairs at Brunswick Steam Electric Plant Unit 2

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Rev 1 to Design Rept for Weld Overlay Repairs at Brunswick Steam Electric Plant Unit 2
ML20080P426
Person / Time
Site: Brunswick Duke Energy icon.png
Issue date: 12/31/1983
From: Charnley J, Sund H, Wenner T
NUTECH ENGINEERS, INC.
To:
Shared Package
ML20080P415 List:
References
CPL-13-104, CPL-13-104-R01, CPL-13-104-R1, NUDOCS 8402220541
Download: ML20080P426 (84)
v • d • e


Text

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I I Controlled Copy No. CPL-13-104 I Revision 1 December 1983 CPL 013.0104 DESIGN REPORT FOR WELD OVERLAY REPAIRS I AT BRUNSWICK STEAM ELECTRIC PLANT UNIT 2 Prepared for Carolina Power and Light Company Prepared by NUTECH Engineers, Inc.

San Jose, California Prepared by: Issued by:

I Q9 hJ.

h E. Charnley, P.E.

warso H. J. Sund I

Project Engineer Project Manager Approved by:

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h. Wenner, P.E.

Date: bECt4BtCL 81 y 1963 I

Engineering Director 8402220541 340210 DRADOCK05000g g I .. _- .

I REVISION CONTROL SHEET TITLE: Design Report for Wold DOCUMENT FILE NUMBER: CPL-013.0104 I Overlay Repairs at Bruncwick Steam Electric Plant Unit 2 J. E. Charnley / Staff Engineer b NAME / TITLE k INITIALS T. LemAssociate Engineer -74.

NAME / TITLE INITIALS H. L. Gustin / Senior Encineer NAME/ TITLE INITIALS D. C. Talbott/ Consultant I [( 7 I B. Natarajan/ Consultant II NAME/ TITLE NAME/ TITLE INITIALS 41 O INITIALS M. Kleinsmith/ Consultant I hek NAME/ TITLE INITIALS AFFECTED DOC PREPARED ACCURACY CRITERIA REMARKS PAGE($) REV BY/OATE CHECK 8Y / OATE CHECK BY / DATE 11 0 hCU!YP3 N/g N/4 I v lii iv 0

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I REVISION CONTROL SHEET I TITLE: Design Report for Weld (CONTINUATION)

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I I TITLE:

REVISION CONTROL SHEET Design Report for Weld (CONTINUATION)

DOCUMENT FILE NUMBER: CPL 013.0104 I AFFECTED DOC Overlay Repairs at Brunswick Steam Electric Plant Unit 2 PREPARED ACCURACY CRITERIA REMARKS PAGE(S) REV BY/DATE CHECK BY / DATE CHECK BY / DATE iv 1 4 n/h/83 N/A N/A v 1 vi 1 vii 1 viii 1 ix U H 1

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I CPL-13-104 CEP 3-3.1.2 Revision 1 iv REV 1

I I CERTIFICATION BY REGISTERED PROFESSIONAL ENGINEER I I hereby certify that this document and the calculations contained herein were prepared under my direct supervision, reviewed by me, and to the best of my knowledge are correct and complete. I am a duly Registered Professional Engineer under the laws of the State of California and am competent to review this document.

I Certified by:

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Registration No. 16340 l

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,I ces-o-104 v Revision 1

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I I TABLE OF CONTENTS Page LIST OF TABLES viii I LIST OF FIGURES ix 4

1.0 INTRODUCTION

1 2.0 REPAIR DESCRIPTION 5 3.0 EVALUATION CRITERIA 10 3.1 Weld Overlay Repair Criteria 10 3.1.1 Strength Evaluation 11 3.1.2 Fatigue Evaluation 11 3.1.3 Fracture Mechanics Evaluation 12 3.2 Flawed Pipe Analysis Criteria 12 4.0 LOADS 13 4.1 Mechanical and Internal Pressure Loads 13 4.2 Thermal Loads 14 4.3 Weld Overlay Shrinkage Induced Loads 15 5.0 EVALUATION METHODS AND RESULTS 16 5.1 12" Elbow Evaluation 16 l

i 5.1.1 Code Stress Analysis 17 l 5.1.2 Fracture Mechanics Evaluation 19 l 5.2 Recirculation Inlet Safe End Evaluation 22 l

l 5.2.1 Code Stress Analysis 23 l

5.2.2 Fracture Mechanics Evaluation 24 ,

5.3 Reactor Water Clean Up System Evaluation 25 5.3.1 Code Stress Analysis 25 1 5.3.2 Fracture Mechanics Evaluation 27 l Effect on Recirculation, RWCU, and 5.4 28 RHR Systems CPL-13-104 vi Revision 1 nutggh

I TABLE OF CONTENTS (Continued)

I Page 5.5 Unrepaired Flaw Evaluation in Large 29 Diameter Pipe welds I 5.5.1 5.5.2 5.5.3 28" Recirculation Piping 22" Recirculation Piping 20" RHR Piping 30 34 35 I 5.6 Predicted Growth of Flaws in Unrepaired 12" and 6" Diameter Pipe 36 6.0 LEAK-BEFORE-BREAK 57 6.1 Net Section Collapse 57 6.2 Tearing Modulus Analysis 58 6.3 Leak Versus Break Flaw Configuration 59 6.4 Axial Cracks 60 6.5 Multiple Cracks 61 6.6 Crack Detection Capability 61 6.7 Non-Destructive Examination 62 6.8 Leakage Detection 63 6.9 Historical Experience 64 l 7.0

SUMMARY

AND CONCLUSIONS 70

8.0 REFERENCES

72 I

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I I LIST OF TABLES Number Title Page 1.1 List of UT Flaw Indications 3 5.1 Thermal Stress Results 38 5.2 12" Elbow Code Stress Results 39 5.3 Safe End Code Stress Results 40 5.4 Reactor Water Clean Up Code Stress 41 Results 5.5 Weld Overlay Residual Stress Effects on 42 Flawed Welds Within the Brunswick Unit 2 Recirculation Piping System 6.1 Effect of Pipe Size on the Ratio 65 of the Crack Length for 5 GPM Leak I Rate and the Critical Crack Length (Assumed Stress a = Sm/2 )

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I I LIST OF FIGURES Number Title Page 1.1 Conceptual Drawing of Recirculation System 4 2.1 Configuration of 12" Elbow Weld Overlay 7 I 2.2 Configuration of Safe End to Pipe Weld Overlay 8

2.3 Configuration of Reactor Water Clean Up 9 Weld Overlay 5.1 12" Elbow Finite Element Model 43 5.2 Weld Overlay Thermal Model 44 5.3 Thermal Transients 45 5.4 Recirculation Inlet Safe End Pinite 46 Element Model 5.5 Reactor Water Clean Up Finite Element 47 Model 5.6 Piping Model 48 5.7 Axial Weld Residual Stress in Pipe 49 I a Diameter of 20" to 28" l

5.8 Crack Growth Prediction for Cracks in 50 28" Diameter Pipe 5.9 Crack Growt1 Prediction for Cracks in 51 22" Diameter Pipe 5.10 Crack Growth Prediction for Cracks in 52 20" Diameter Pipe I 5.11 Axial Weld Residual Stress in 12" Diameter Pipe Welds 53 5.12 Growth of Circumferential Cracks in 54 Unrepaired 12' Diameter Pipe 5.13 Axial Weld Residual Stress in 6" Diameter 55 Pipe Welds CPL-13-104 ix Revision 1 g nute_9h

I I LIST OF FIGURES (Continued)

I Number Title Pace I 5.14 Growth of Circumferential Cracks in Unrepaired 6" Diameter Pipe Welds 56 I 6.1 Typical Result of Net Section Collapse Analysis of Cracked Stainless Steel Pipe 66 6.2 Stability Analysis for BWR Reoirculation 67 System (Stainless Steel) 6.3 Summary of Leak-Before-Break Assessment 68 of BWR Recirculation System 6.4 Typical Pipe Crack Failure Locus for Combined 69 Through-Wall Plus 360* Part-Through Crack I

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1.0 INTRODUCTION

This report summarizes analyses performed by NUTECH to assess weld overlay repairs and to evaluate unrepaired flaw indications in the Recirculation System, Residual Heat Removal (RHR) System, and Reactor Water Clean Up (RWCU) System at Carolina Power and Light Company's Brunswick Steam Electric Plant Unit 2 (Brunswick 2).

Weld overlay repairs hav2 been applied to address itltrasonic (UT) examination results believed to be indicative of intergranular stress corrosion cracking (IGSCC) in the vicinity of the welds. The purpose of each overlay is to arrest any further propagation of the cracking, and to restore original design safety margins to the weld. Certain flaw indications were determined by fracture mechanics evaluations to have acceptable design i safety margins without repair for at least the next six months. The flaw indications addressed in this report were detected during the October-November 1983 inspection.

Flaw indications have been detected adjacent to 15 welds in the Recirculation System, I weld in the RHR System, and 3 welds in the RWCU System. Figure 1.1 shows the location of these flaw indications relative to the Reactor Pressure Vessel and other portions of the CPL-13-104 1 I Revision 0 nutgd)

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I Recirculation, RWCU, and RHR Systems. All of the flaws are in Type 304 stainless steel material. Table 1.1 contains a description of each flaw indication as well ,

as the disposition of each.

The required design life of each weld overlay repair is at least five years. The amount that the actual life exceeds five years will be established by a combination I of future analyses and testing.

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1 M M M M M M M M M M M M C e M oo O *O

< t4 Ttati 1.1 P- I lA H oY

3 P LIST OF t!T FLM INDICATI0!ts O

A ytlpJ0. OnIENIATlON Lt NGill IlmguG8LRAI L DEPf 88 toCAl1QN Dl1PpS Ll_Ilhi ma b+

Z-QJ2-28"-A-8 45* skewed 1.0" Di _61 E h9v side Flaved Plce_ Analysis 2-DJ2-26"-A-14 Circinaferential 0.5" 2Q1 et [I bey __ilde F Igyed Pl og Aria _tyst,_____

Z-M)2-12"-C-4 n Circumfergntial 2.0" 141 ff/A P ipedigte Repaliyr Figure 2.2 C{rriefeigntial 1.25" 11 N/A Pipq_115fe Repair per Piqure 2.2 2:QJ2-J2"-st-3 C11gunforgntial 1.25" 51 N/A Elboy_gide Rgpajr per Figugg_2,1_,

2-Lll-29"-A-2 Itap3yerle 0,2}" 121 J61 Pipf_3Jde Flowed Pipe Analy11s'..

2;G]I 6"-10 Citglem f gten tia l I.25" 1]1 lifA [Ibow 31de Repa_f r per Figure 2.3 2:QJ2-ZQ"-A-4 Cligimfgygn Mal 1.0" 31 _91 Pj pq_3[de Floyed Pipe Analyds_

Citsumftf90Llal 1.5" 121 131 Elbgv side f l awed P ipe___Ana_ty1'is hM]2-12"-J-3 Circumfgrgn(lat 1.75" 111 N/A Elbow 1[de Repair per Figure 2.1 2-U]h28"-D-4 Cligumigigntlas I.0" 81 111 LibgwJ {de Flayed Pipe Analytt s _

Z-D}2:28"-D-5 Q.itgimf grentia l 2JIS" 101 221 Upitrgam_gide flawed Pipo Anatylls Ci rcumfgigatia l 3.0" 101 131 (Jp111 gam side F lawed Pipe Analy1[s_

Clfgumfe_tenglat 1.5" 101 121 Upstige_m side figwed Pigg Analyiis 2-Gil:20":D.-} Cittumferenting 0.F" 161 1Q1 E!bgw_gide F laygd_._f_lpe_ Ana_ty11s Circumferential enq_iraniyer_te 0.8" 51 121 Ejbgw_11de FIsyed Pipe Analysis Z-032-12"-1(-2 CIIt!=f tf e!!Lla t 1.25" 111 N/A ((bgy_113e f Regalper Figure 2.1 2-!!J 2:12"-2:2_ Ciggumferretlal 24 2 121 ff/A [1 hey _11sfe Recair per Piqure 2.!

Circimferential 11.5)"

2:E32:22"-AH-5 a!y1_$beyed I r} t tgel t ke.Qt 201 291 Pipe _1]de Flaved Pipe Analysis Z-81}]-6"-15 Clrgi!alefgntial 1.2)" 161 ff/A [1 bow 11de Repair Per Figure 2.3 Downstream 4 a

Z-G]l 6"-16 CirgunfgregLjal 0.75" 81 N/A side Repair per Figure 2.3 Z-012-22"-on-1 Circumf e_tentia l 1.2" 71 161 fnd_gao F l aygd_r_teg_Ana.1stis a

Z:032-20" .A!} Circumfgtrntial 0.5" 191 321 Pipe side Flayed Pipe Anajysis

-03 Whgytd l' 17 61 17 6% Pigg_1jde Flaygd__fleg Anajytis 2

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I 2.0 REPAIR DESCRIPTION The UT flaw indications in the weld heat-affected zones that required repair have been remedied by establishing additional " cast-in-place" pipe wall thickness from weld metal deposited 360 degraec around and to either side of the existing weld, as shown in Figures 2.1, 2.2 and 2.3. The weld-deposited band over the cracks provides wall thickness equal to that required to restore the original design safety margins. In addition, the weld metal deposition produces a fa mrable compressive residual stress pattern. The deposited weld metal is Type 308L, which is resistant to propagation of IGSCC cracks.

I The non-destructive examination of the weld overlays consisted of:

1) Surface examination of the completed weld overlay by the liquid penetrant examination technique in accordance with ASME Section XI.

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2) Volumetric examination of the completed weld overlay by the ultrasonic examination technique in I ^""'"""""""^"*"**""""-

CPL-13-104 5 Revision 0 nutp_cb

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3) Volumetric preservice examination of the weld overlay and existing circumferential pipe weld by the ultrasonic examination technique in accordance with ASME Section XI.

The UT flaw indications in the weld heat-affected zones that were not repaired have been determined by flawed pipe analysis to have acceptable design safety margins for at least the next 6 months.

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Figure 2.1 CONFIGURATION OF 12" ELBOW TO PIPE WELD OVERLAY CPL-13-104 7 Revision 0 nutp_qh I .. ._

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CONFIGURATION OF REACTOR WATER CLEAN UP WELD OVERLAY ir

3.0 EVALUATION CRITERIA This section describes the criteria that are applied in this report to evaluate the acceptability of the weld overlay repairs and flawed pipe analyses.

I Weld Overlay Repair Criteria 3.1 Because of the nature of these repairs, the geometric configuration is not directly covered by Section III of the ASME Boiler and Pressure Vessel Code, which is intended for new construction. However, materials, fabrication procedures, and Quality Assurance require-ments are in accordance with applicable sections of this Construction Code, and the intent of the design criteria described below is to demonstrate equivalent margins of safety for strength and fatigue considerations as provided in the ASME Section III Design Rules. In addition, because of the IGSCC conditions that led to the need for repairs, IGSCC resistant materials have been selected for the weld overlay repairs. As a further means of ensuring structural adequacy, criteria are also provided below for fracture mechanics evaluation of the repairs.

CPL-13-104 10 Revision 0 nutp_qh

I I 3.1.1 Strength Evaluation I Adequacy of the strength of the weld overlay repairs with respect to applied mechanical loads is demonstrated with an ASME Boiler and Pressure Vessel Code Section III, Class 1 (Reference 1) analysis of the weld overlay repairs.

I 3.1.2 Fatigue Evaluation The stress values obtained from the above strength evaluation were combined with thermal and other secondary stress conditions to demonstrate adequate fatigue resistance for the design life of each repair.

The criteria for fatigue evaluation include:

1. The maximum range of primary plus secondary stress was compared to the secondary stress limits of Reference 1.

I

2. The peak alternating stress intensity, including l all primary and secondary stress terms and a l

l fatigue strength reduction factor of 5.0 to account for the existing crack, was evaluated using conventional fatigue analysis techniques. The l total fatigue usage factor, defined as the sum of I

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the ratios of applied number of cycles to allowable number of cycles at each stress level, must be less than 1.0 for the design life of each repair.

Allowable number of cycles was determined from the stainless steel fatigue curve of Reference 1.

I 3.1.3 Fracture Mechanics Evaluation A highly conservative method was used to demonstrate the adequacy of the weld overlay repair. All relevant UT flaw indications were assumed to be through-wall cracks. The maximum crack depth was actually measured i to be approximately 22% of pipe wall, but all cracks were assumed to be 100% of pipe wall. The weld overlay I

was then designed such that the net section limit load l

requirements of Reference 2 were satisfied.

l 3.2 Flawed Pipe Analysis Criteria The unrepaired flaws were shown to be adequate without repair for the next six months based on the criteria given in References 2 and 3. Reference 3 requir's that I the allowable flaw depth be equal to 2/3 of the Reference 2 allowable depth.

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I 4.0 LOADS I The loads considered in the evaluation of the UT flaw indications consist of mechanical loads, internal pressure, differential thermal expansion loads, and wsld overlay shrinkage induced loads. The mechanical and internal pressure loads used in the analyses are described in Section 4.1, an explanation of the thermal transient conditions which cause differential thermal expansion loads is presented in Section 4.2, and the loads induced by weld overlay shrinkage are explained in Section 4.3.

4.1 Mechanical and Internal Pressure Loads The design pressures of 1325 psi (pump discharge) and 1150 psi (pump suction) for the Recirculation, RHR, and RWCU Systems were obtained from Reference 4. The dead weight and seismic loads applied to each weld were

( obtained from Reference 5 for the Recirculation and RHR l

Systems. For the RWCU System, code allowables were assumed to obtain conservative loads.

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I I 4.2 Thermal Loads I The thermal expansion loads for each weld were obtained from a computer model of the Recirculation and RHR Systems. The NUTECH computer program PISTAR (Refer-ence 6) was used. Reference 4 defines several types of transients for which the Recirculation and RHR Systems are designed. These transients were conservatively grouped into three composite transients. The first composite transient is a startup/ shutdown transient with a heatup or cool down rate of 100*F per hour. The second composite transient consists of a 50*F step temperature with no change in pressure. The third composite transient is an emergency event with a 416*F step temperature change and a pressure change of 1325 psi. In the five year overlay design life, there are 38 startup/ shutdown cycles, 25 small temperature change cycles, and one emergency cycle.

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For the RWCU System, code allowables were assumed to obtain the thermal expansion loads.

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I I 4.3 Weld overlay Shrinkage Induced Loads I Each weld overlay causes a small amount of axial shrink-age underneath the overlay. This shrinkage induces bending stresses in the remainder of the piping system.

These shrinkage induced stresses are calculated with a PISTAR (Reference 6) piping model.

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I I 5.0 EVALUATION METHODS AND RESULTS I The evaluation of each of the weld overlay repairs consists of a code stress analysis per Section III (Reference 1) and a fracture mechanics evaluation per Section XI (References 2, 3, and 7). The flawed pipe analysis was performed per References 2 and 3.

5.1 I 12" Elbow Evaluation Circumferential UT flaw indications were found in the heat-af fected zone on the elbow side of elbow--to-pipe welds 12-K-2, 12-K-3, 12-J-2, and 12-J-3. The maximum length of any indication is 2.25 inches. The deepest indication was sized by UT to be 12% (0.07 inch) of the pipe thickness which is 0.568 inches. However, for analytical purposes, the crack was assumed to be through-wall for a length of 2.75 inches.

i During the overlay welding, one through-wall axial crack

, was observed in each of welds 12-K-2 and 12-K-3. The l

crack was repaired by grinding approximately 1/16 inch

! deep into base metal and filling the cavity via manual welding. After the first layer of manual welding, a PT 4

examination was performed to demonstrate that the crack had been sealed.

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I 5.1.1 Code Stress Analysis A finite element model of the cracked and weld overlaid region was developed using the ANSYS (Reference 8) computer program. The model was based on the design minimum overlay thickness of 0.20 inch, which is 9 percent smaller than the actual minimum average thickness of 0.22 inch. The finite element model was based on weld 12-J-2 as it has the largest crack. The small added weight (less than 25 pounds) of the weld overlay will not significantly change the existing seismic analysis. Figure 5.1 shows the model. This figure also shows the material that was removed to represent the crack.

I The stress in the overlaid elbow due to design pressure j and applied moments as described in Sections 4.1 and 4.2 was calculated with the finite element model.

I The weld overlay thermal model was taken to be axisymmetrical (Figure 5.2). The exterior boundary was assumed to be insulated. The temperature distribution 1

in the wold overlay, subject to the thermal transients defined in Section 4.2, can be readily calculated using Charts 16 and 23 of Reference 9. The maximum through-I CPL-13-104 17 Revision 0 l

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I I wall temperature difference was determined to be less than 4'F for the normal startup cycle, 40'F for the small temperature cycle, and 330'F for the emergency transient.

The maximum thermal stress for use in the fatigue analysis was calculated as follows:

EaAT EaAT 2

"

  • 2(1 v) +

lv (Reference 1)

Where:

6 E = 28.3 x 10 psi (Young's Modulus)

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a = 9.11 x 10 'F-1 (Coefficient of Thermal Expansion)

AT y = Equivalent Linear Temperature Difference AT 2 = Peak Temperature Difference l

I The values of AT1, AT2, and o are given in Table 5.2 for all three thermal transients.

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5 The results of a code stress analysis per Reference 1 are given in Table 5.2. The allowable stress values for Reference 1 are also given. The weld overlay repair satisfles the Reference 1 requirements.

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I I A conservative fatigue analysis per Reference 1 was performed. A fatigue strength reduction factor of 5.0 was applied due to the crack. The fatigue usage factor was then c?lculated assuming 38 startups, 25 small temperature change cycles and one emergency cycle every five years. These results are also summarized in Table 5.2.

I 5.1.2 Fracture Mechanics Evaluation The allowable crack depth was calculated based on References 2 and 3. Crack growth due to fatigue was determined based on Reference 17. Crack growth due to IGSCC was evaluated based on References 10 and 17.

5.1.2.1 Allcwable Crack Depth From Reference 5, the highest applied primary (pressure

+ dead weight + OBE) stresses in any 12 inch elbow in the recirculation system is 9860 psi. The allowable crack depth was calculated based on Reforcnces 2 and

3. A crack length of 2.75 inches, which i a 0 <. 3 inch longer than the measured UT length, was assumed to calculate the allowable depth.

I Stress Ratio =

Pm + Pb g,3 I

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Pm + Pb = Primary Stress Sm = Code allowable stress value

= 16,800 psi (at design temperature of 562*F)

Stress Ratio = 1 80g = 0.59 I Fractional Crack Length =

L = Crack Length = 2.75 irsches ,

D = Weld Diameter = 12 inches I Fractional Crack Length = = 0.07 I

Thus, per Reference 2, the allowable crack depth is 75%

of the pipe thickness. Similarly, for short (- 0.5 inch) through-wall axial cracks, the allowable depth is 75% of the pipe thickness. Therefore, the minimum weld overlay thickness was chosen to be equal to one third of the pipe thickness.

I PIPE 0.568 .

TOVERLAY"

" < 0.20 inch I

3 3 The minimum average as-built overlay thickness is 0.22 g

inch.

I I

CPL-13-104 20 Revision 0 nut.9_q.h_

g

I I 5.1.2.2 Crack Growth The existing cracks could grow due to both fatigue and stress corrosion. Fatigue growth due to the three types of thermal transients defined in Section 4.2 was calculated in Reference 17 The fatigue crack growth for five years of the cycles shown in Figure 5.3 was calculated to be less than 0.01 inch.

I IGSCC growth requires a susceptible material. By assumption, the cracks are through the original weld heat-affected zone. Any additional IGSCC growth would have to occur in the weld overlay which is not suscept-ible due to i*s high ferrite content and its duplex (austenite and delta ferrite) structure. Therefore, the cracks will not grow into the weld overlay.

I The longest measured UT flaw indication is 2.25 inches. Without the installation of a weld overlay, the ucack would be expected to lengthen. However, the weld overlay will significantly change the residual stress pattern near the inside of the pipe as shown in Refer-ence 17 In Reference 17, a 12 inch pipe-to-elbow weld l

which was overlaid with a 0.20 inch thick overlay was evaluated fr.r residual stress. The results show that the overlay changed the residual stress pattern I

CPL-13-104 21 I Revision 0 nutp_ql)

I I sufficiently to prevent crack growth. Therefore, the weld overlay will prevent further lengthening of the cracks. After the required five year design life, the crack depth would be equal to the original pipe thick-ness plus 0.01 inch, or 0.578 inches. The allowable crack depth is 0.591 inches (0.591 = (.75) (0.788)], as the as-built thickness of the overlay of weld 12-J-3 is 0.22 inch.

I 5.2 Recirculation Inlet Safe End Evaluation I The heat-affected zone on the piping side of weld I?-G-4 between the recirculation inlet safe end extension (pup piece) and the recirculation piping had a 14% through-wall 2 inch long crack and a 1.25 inch 7% through-wall crack. This weld is between the Type 304 stainless steel piping and an assumed low carbon content (0.03 weight percent) stainless steel pup piece. It is close to the NiCrFe safe end (Figure 2.2). Due to the location of the NiCrFe safe end, a nonstandard weld overlay was designed. The weld overlay is shorter than j a standard weld overlay on the pup piece side of the 12-G-4 weld. The short weld overlay was judged to be acceptable based on the low carbon content, and thus high resistance to IGSCC cracking, of the pup piece.

I CPL-13-104 22 Revision 0 nutE!!

I

5.2.1 Code Stress Analysis I The ASME Code stress analysis of the cracked and repaired 12-G-4 weld was performed with an ANSYS (Reference 8) finite element model. The model was based on an overlay thickness of 0.20 inch, which is 50 percent smaller than the actual minimum average thick-ness of 0.41 inch. The amall added weight (less than 25 pounds) of the weld overlay will not s igni f ica nt.ly change the existing seismic analysis. Figure 5.4 shows the model. The stress in the overlaid safe end due to design pressure and applied moments as described in Sections 4.1 and 4.2 was calculated with the finite element model.

The results of a code stress analysis per Reference 1 are given in Table 5.3. The allowable stress values for Reference 1 are also given. The weld overlay repair satisfies the Reference 1 requirements.

I A conservative fatigue analysis per Reference 1 was performed. In addition to the stress intensification factors requtred per Reference 1, and additional fatigue l

l I strength reduction factor of 5.0 was applied due to the crack. The fatigue usage factor was than calculated l assuming 38 startups, 25 small temperature change cycles CPL-13-104 23 Revi.sion 0 nutp_qb l

and one emergency cycle every five years. These results are also summarized in Table 5.3.

5.2.2 Fracture Mechanics Evaluation The allowable crack depth was calculated based on References 2 and 3. Crack grcwth due to fatigue was determined based on Reference 17. Crack growth due to

+ IGSCC was based on References 10 and 17 5.2.2.1 Crack Growth The existing cracks could grow due to both fatigue and IGSCC. From Reference 17, the calculated fatigue crack growth is less than 0.01 inch within the next five years. Since the overlay material is resistant to IGSCC growth, the crack will not propagate from IGSCC. The wall thickness for weld number 12-G-4 is 0.68 inch.

Therfore, the maximum crack depth after five years is predicted to be equal to the original pipe thickness plus 0.01 inch, or 0.578 inches. The allowable crack depth is 0.733 inches [0.733 = (.75)(0.978)], as the as-built thickness of the overlay is 0.41 inch.

I I

CPL-13-104 24 Revision 0 nutp_ch

I I 5.3 Reactor Water Clean Up System Evaluation I The largest circumferential UT flaw indication found in the 6" Reactor Water Clean Up System was 16% through-wall and 1.25 inches long. For analytical purposes, the crack was assumed to be through-wall for a length of 1.75 inches, which is 0.50 inch longer than the UT measured length.

I 5.3.1 Code Stress Analysis I A finite element model of the cracked and weld overlaid region was developed using the ANSYS (Reference 8) computer program. The model was based on the design minimum overlay thickness of 0.20 inch, which is 23 percent smaller than the actual minimum average thickness of 0.26 inch. The finite element model was based on weld 6-15 as it has the largest crack. The small added weight (less than 10 pounds) of the weld overlay will not significantly change the existing seismic analysis. Figure 5.5 shows the model. This figure also shows the material that was removed to represent the crack.

I CPL-13-104 25 I Revision 0 nutub I

I I Due to insufficient load information for the RWCU system, the ASME allowable stress for the pipe material was assumed and corresponding loads calculated. This assumption is highly conservative since it implies that the material is stressed to the allowable limit.

The results of a code stress analysis per Reference 1 are given in Table 5.4. The allowable stress values for Reference 1 are also given. Table 5.4 shows that the allowable of NB-3650 (Reference 1) is exceeded.

Reference 1 allows a simplified elastic-plastic analysis to be performed in this situation. Therefore, paragraph NB-3653.6 (Reference 1) which allows this alternate analysis technique, was used to calculate the primary and secondary stresses and the results are presented in Table 5.4. The weld overlay repair satisfies the Reference 1 requirements.

I A conservative fatigue analysis per Reference 1 was l

performed. In addition to the stress intensification f factors required per Reference 1, and additional fatigue 1

( strength reduction factor of 5.0 was applied due to the crack. The fatigue usage factor was then calculated assuming 38 startups, 25 cmall temperature change cycles and one emergency cycle every five years. These results are also summarized in Table 5.4.

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I I

Fracture Machanics Evaluation I 5.3.2 The allowable crack depth was calculated based on Re ferences 2 and 3. Crack growth due to fatigue was determined based on Reference 17. Crack growth due to ,

IGSCC was based on Re ferences 10 and 17.

Crack Growth I

5.3.2.1 The existing cracks could grow due to both fatigue and IGSCC. From Reference 17, the calculated fatigue crack growth is less than 0.01 inch within the next five years. Since the weld overlay material is resistant to IGSCC growth, the crack will not propagate due to IGSCC. The wall thickness for weld number 6-15 (16%

through-wall crack) is 0.432 inch. Therefore, the maximum crack depth after five years is predicted to be equal to the original pipe thickness plus 0.01 inch, or 0.442 inches. The allowable crack depth is 0.519 inches (0.519 = ( . 75) (0. 692) ] , as the minimum as-built thickness of the overlay is 0.26 inch.

I lI I 27 CP L- 13-104 Revision 0 g nut.es.h_

I I 5.4 Effect en Recirculation, RWCU, and RHR Systems I The offects of the radial shrinkage are limited to the region adjacent to and underneath the overlay. Based on Reference 11, the stresses due to the radial shrinkage are less than yield stress at distances greater than 4 inches from the ends of the overlay. Weld residual stresses are steady state secondary stresses and thus I are not limited by the ASME Code (Reference 1).

The effect of the axial weld shrinkage on the Recircula-tion, RWCU, and RHR Systems was evaluated with the NUTECH computer program PISTAR (Reference 6) and the piping model shown in Figure 5.6.

The measured shrinkages of all weld overlay repairs were imposed as boundary conditions on this model. Since the ASME Code does not limit weld residual stress, all stress indices were set equal to 1.0.

I The maximum calculated stress was less than 13.2 ksi.

The location of this stress is shown on Figure 5.6.

Steady state secondary stresses of 13.2 ksi are judged to have no deleterious effect on the Recirculation, I

I CPL-13-104 28 Revision 0 I "" U

I I RWCU, or RHR Systems. The value of the shrinkage induced stress at each location with an unrepaired flaw is tabulated in Table 5.5.

I 5.5 Unrepaired Flaw Evaluation in Large Diameter Pipe Welds I

UT examination of the large diameter piping nf the Recirculation and RHR systems at Brunswick 2 has revealed eleven welds which have reportable flaw indications. Eight of the welds with flaw indica-tions are in the 28" Recirculation piping, two are in the 22" Recirculation piping, and one is in the 20" RHR piping. The location of each of these indications is shown in Figure 1.1.

IGSCC crack growth in large diameter piping is different than IGSCC crack growth in small diameter piping due to a significant difference in the original butt weld residual axial stress. In large diameter piping, there is a significant portion of the pipe thickness near the inside surface which is in compression as shown in Figure 5.7. Typically in small diameter piping, the inside one-half of the pipe thickness experiences axial residual tensile stress. Thus, shallow circumferential flaws in large diameter piping will generally grow significantly slower than similar flaws in small I

CPL-13-104 29 Revision 0 I ""' W

diameter piping. The following Sections provide the results of crack growth analyses of the flaws in large diameter Recirculation and RHR piping.

I 5.5.1 28" Recirculation Piping i

Eight welds in the 28" Recirculation piping have reportable UT flaw indications. The welds with flaw indications are pipe-to-pipe, pipe-to-valve, and pipe-to-elbow welds and the flaws are either circumferential, axial or skewed. The flaws are tabulated in Table 1.1 and their location is shown in Figure 1.1.

I Weld number 28-B-5 has the highest value of primary stress and contains the longest and deepest UT flaw indication. Thus, the allowable crack depth for weld 28-B-5 will be used with the length and stress as bounding for any of the crack indication locations in the 28" Recirculation piping. A ccdck length of 7.4 inches (the sum of the UT measurements plus 0.5 inch) is t assumed in the allowable crack depth calculations. The allowable crack depth was determined based on the primary stress values from Reference 5, and on the method in Reference 2.

I CPL-13-104 30 Revision 1 nutp_qh I -

I I From Reference 2 for a 7.4 inch long circumferential crack, the allowable depth is 75% of the pipe thickness. In accordance with Reference 3, two-thirds of the allowable crack depth is used or 50% thcough-wall. Based on a three-dimensional finite element analysis of a 28" pipe-to-elbow joint, no stress intensification factor is included in the above calculation for P +P'b I The prediction of crack growth for each of the existing UT flaw indications requires several inputs:

1) Steady state applied stress
2) Weld residual stress
3) Flaw characterization
4) Crack growth model
5) Crack growth law I The approach was to use conservative input for applied stress, residual stress, crack growth model and crack growth law. Thus, the result of the analysis is a very conservative prediction of crack size versus time.

I The steady state moment at each crack location due to operating pressure, dead weight and thermal expansion were obtained from References 5 and 6. In addition, the I

CPL-13-104 31 I Revision 0 nutp_q_h.

g

I I moment due to the axial weld shrinkage of the overlays (Section 5.4) was added to the other steady state moments.

I The weld residual stress was obtained from a set of NUTECH standard residual stress curves (Reference 18).

The residual axial stress curve for large bore piping from Reference 18 is shown in Figure 5.7. This residual stress curve was used for each crack growth analysis.

The flaw sizes are tabulated in Table 1.1. It was conseratively assumed that each crack was full depth for the entire crack length.

I A conservative crack growth law for weld sensitized material was used. It is given below:

~

0.llk

= 1.2 x 10 e I where:

I da = Differential crack size dT = Differential time k = Applied stress intensity factor l

I I

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I I The crack growth model is a linear interpolation between an inside diameter (I.D.) cracked cylinder and an edge-cracked plate. The crack growth model assumes a 360' crack. The magnification factors for both an I.D.

cracked cylinder with T/R = 0.1 and an edge-cracked plate (T/R = 0.0) were obtained from Reference 15.

I The predicted crack growth of each of the eight cracked welds was calculated with the NUTECH computer program NUTCRACK (Reference 15). Figure 5.8 is a plot of the predicted crack depth as a function of time for weld 28-B-5. Examination of Figure 5.8 will show that the UT flaw indication will not grow to the allowable size for at least 50 months.

Another way of expressing the same margin is to determine the crack size that would grow to the allowable crack size in the next 6-month fuel cycle.

From Figure 5.8 for weld 28-B-5, a crack size of 46%

would grow to the allowable of 50% in 6 months. Thus, the currently allowable crack size is 46%, which is 2.2 times the largest measured crack size.

I I

I CPL-13-104 33 Revision 0 nutp_qh I -

I I 5.5.2 22" Recirculati.on Piping i Two welds (22-AM-5 and 22-BM-1) in the 22" Recircula-tion piping have reportable UT flaw indications. Weld d\

22-BM-1 is c end cap-to-pipe weld and 22-AM-5 is a cross-to pipe weld. The majority of the UT flaw indications are circumferentially oriented. The flaws I are tabulated in Table 1.1 and their location is shown I in Figure 1.1.

Weld number 22-AM-5 has the highest value of primary stress and contains the largest UT indication. Thus, the allowable crack depth for weld 22-AM-5 will be bounding for both of the crack indication locations in the 22" Recirculation piping.

I The allowable crack depth for weld 22-AM-5 was determined based on the method in Section 5.1.2.1 to be 75% of the pipe wall thickness. In accordance with Reference 3, two-thirds of the allowable crack depth is used or 50% through-wall.

The predicted crack growth of weld 22-AM-5 was calculated using the method described in Section 5.5.1. The results are shown in Figure 5.9. The indication in the 22" piping will not grow to the r

I CPL-13-104 34 Revision 1 nutp_qb

I I allowable size in less than 5 years. Examination of Figure 5.9 shows that a crack size of 46% would grow to the allowable in 6 years, or the currently allowable crack size is 46% which is 2.3 times the largest measured crack size.

I 5.5.3 20" RHR Piping one pipe-to-valve weld (20-A-2) in the 20" RHR piping has a reportable UT flaw indication which is transversely oriented. The flaw is tabulated in Table 1.1 and th6 location is shown in Figure 1.1.

I The allowable crack depth for weld 20-A-5 was determined based on the method in Section 5.1.2.1 to be 75% of the pipe wall thickness. Based on Reference 3, 50% through-wall is used as the allowable crack depth.

l I The predicted crack growth for each of the cracks was calculated using the method described in Section 5.5.1. The results are shown in Figure 5.10. The indication in the 20" piping will not grow to the l

allowable size in less than 4 years. From Figure 5.10, a crack size of 45% would grow to the allowable in 6 months, or the currently allowable crack size is 45% which is 2.8 times the largest measured crack size.

I CPL-13-104 35 I Revision 1 nutggh

I -

I 5.6 Predicted Growth of Flaws in Unrepaired 12" and 6" I Diameter Pipe Analyses were performed for the flawed 12" elbows and safe end welds and the 6" RWCU welds to predict the crack growth due to IGSCC if the flaws were left unrepaired. The crack growth was calculated using the computer code NUPLAW (Reference 16). Linear weld residual throuch-wall stress distributions (shown in Figures 5.11 and 5.13) due to the butt welds and finite length cracks were used in the analyses. IGSCC crack growth was calculated using the crack growth law given below:

~

6 bdT= 2.27 x 10 K I da = Differential crack size dT = Differential time K = Applied stress intensity factor I

Thits crack growth law is slightly more conservative than that used in Section 5.5.1 in the K range applicable for these analyses. The axial residual stress distribution used for the 12" elbow and safe end analyses is shown in Figure 5.11. From Reference 5, the highest stress in the recirculation system occurs at the safe ends. Also, I

CPL-13-104 36 Revision 0 nutp_cb

I I the maximum crack depth (14% through-wall) and the maximum length (3.25 inches) occur at the 12-G-4 safe end weld. Therefore, the crack growth for weld 12-G-4 was used to bound all flaws in the 12" recirculation cystem. A crack length of 3.75 inch (0.5 inch greater than the measured UT indication length) was assumed.

The results are shown in Figure 5.12. Examination of Figure 5.12 shows that the crack will propagate to the allowable crack depth of 50% through-wall (per References 2 and 3) in less than two months.

I For the 6" RWCU system, the bounding crack depth (16%

through-wall) and length (1.25 inch) was found at weld number 6-15. An assumed length of 1.75 inches (0.5 inch longer than measured UT indication) was used in the analysis. The ASME Code allowable (Reference 1) of 1.5 Sm (25.2 ksi) was assumed as the applied stress. The axial residual stress distribution shown in Figure 5.13 was used in the analysis. The results of the analysis is shown in Figure 5.14. Examination of Figure 5.14 shows that the crack will propagate to the allowable crack depth of 50% through-wall (per References 2 and 3)

I '" '""" '""" " "'"-

I I

CPL-13-104 37 I Revision 0 nut.qq_h g

I I

I I Table 5.1 THERMAL STRESS RCSULTS I

I SMALL NORMAL TEMPERATURE EMERGENCY STARTUP PARAMETER CHANGE CYCLE CYCLE CYCLE (CYCLE 1) (CYCLE 2) (CYCLE 3)

EQUIVALENT 3.25 F I

LINEAR 31 F 258*F TEMPERATURE AT y PEAK TEMPERATURE 0 9 71 F aT 2

,I l THROUGH

, WALL THERMAL 598 PSI 9,024 PSI 73,661 PSI l STRESS c F CP L83.08-13 i

I I

lI I CPL-13-104 38 Revision 0 nutggh ll

I I

Table 5.2 12" ELBOW CODE STRESS RESULTS I

I ACTUAL STRESS SECTION III EQUATION CATEGORY OR I NUMBER USAGE NB ALLOWABLE FACTOR I S N/A N/A S , = 14,300 PSI PRIMARY (9) 18,7000 PSI 21,450 PSI (10) 34,200 PSI 42,900 PSI S ARY PEAX CYCLE 1 (29,500)S*

l CYCLE 2 CYCLE 3 (11) (13,000)S*

'135,500)s*

N/A I USAGE FACTOR (5YR)

N/A 0.03 1.0

  • THE FACTOR OF 5 IS THE CONSERVATIVELY ASSUMED FATIGUE STRENGTH REDUCTION FACTOR.

I ** ALLOWABLE GASED ON TYPE 308L WELD OVERLAY FMTERI AL.

FCPla3.0814 I

I CPL-13-104 39 Revision 0 nutp_qh

I I

I Table 5.3 SAFE END CODE STRESS RESULTS I

ACTUAL STRESS SECTION III EQUATION I

CATEGORY NUMBER OR NB ALLOWABLE USAGE FACTOR S N/A N/A S, =14,300 PSI PRIMARY (9) 11,400 PSI 21,450 PSI I 1 PRIMARY +

(10) 26,300 PSI 42,900 PSI SECONDARY PEAK CYCLE 1 (21,600)5* N/A CYCLE 2 (11) (13,000)5*

I CYCLE 3 USAGE-(127,500)S*

I FACTOR (5 YR)

N/A .02 1.0 I

  • THE FACTOR OF 5 IS THE CONSERVATIVELY ASSUMED FATIGUE STRENGTH REDUCTION FACTOR.

FCPL83.08-15

I I

I CPL-13-104 40 Revision 0 nutp_qh I _ _ --

l l

Table 5.4 REACTOR WATER CLEAN UP CODE STRESS RESULTS ACTUAL STRESS EQUATION OR SECTION III CATEGORY NB ALLOWABLE NUMBER I USAGE FACTOR S N/A N/A S = 14,300 PSI P RIMARY (9) 17,700 PSI 21,450 PSI PRIMARY +

(10) 44,000 PSI 42,900 PSI SECONDARY PEAK (39,300)5*

CYCLE 1 (13,000)S*

CYCLE 2 (11) N/A (145,200)5*

I CYCLE 3 (12) 42,900 PSI PRIMARY + 18,200 PSI NA 38,300 PSI (13) 42,900 PSI USAGE I FACTOR

($ YR)

N/A .08 1.0 I *THE FACTOR OF 5 IS THE CONSERVATIVELY ASSUMED FATIGUE STRENGTH REDUCTION FACTOR.

      • ELASTIC-PLASTIC ANALYSIS WAS PERFORMED FCPL83.0816 SEE EQUATIONS 12 AND 13 BELOW I

I CPL-13-104 41 Revision 0 I ""'*ch

Table 5.5 WELD OVERLAY RESIDUAL STRESS EFFECTS ON FLAWED WELDS WITHIN THE BRUNSWICK UNIT 2 RECIRCULATION PIPING SYSTEM l

l s

WELD NUMBER INCREASED STRESS LEVEL 2 - 832 - 28"- A -14 27 psi 2 - B32 - 28"- A - 8 40 psi 2 - E11 - 20"- A - 2 412 psi 2 - B32 - 28"- A - 4 116 psi 2 - 832 - 28" - B - 4 559 psi 2 - B32 -28 "- B - 5 389 psi 2 - 832 -22"- B - 3 430 psi 2 - B32 -22"- AM - 5 168 psi 2 - B32 - 22"- BM - 1 0 psi 2 - B32 - 28"- A13 41 psi 2 - B32 - 28"- B9 386 psi l

FCPL83.0841 I C-- u --

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I Figure 5.1 12" ELBOW FINITE ELEMENT YODEL CPL-13-104 43 Revision 0 I

nut.e_qh

I INSULATION

+A WELD

, , , , , , , , , , , , , , , , , , , , , , , , , f,

,, , , , , , , . bin . .

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SECTION A-A FCPl.B3.08-22 Figure 5.2 WELD OVERL AY THERMAL MODEL I CPL-13-104 44 Revision 0 nutggh g

I l I  ;

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EMERGENCY -

I #

I I SMALL TEMPERATURE -

$ CHANGE b

'A STARTUP SHUTDOWN -

NORMAL -

OPERATION RESIDUAL - l 38 25 5

_ CYCLES CYCLES YEARS

( _

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'1' SE0VENCE CYCLE REPEATS lI l

TIME FCPLB3.08 23 lI I

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$ ID OD THICXNESS (T)

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T% STRESS (ksi) 0 +30.0 19 0.0 25 - 9.9 39 -14.2 l 50 -12.0 l

81 0.0 100 + 8.1 i

FCPL83.08-27 I

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Figure 5.7 AXlAL WELD RE0lDUAL STRESS PIPE DIAMETER OF 20" TO 28" CPL-13-104 49 l Revision 0 nute_qh lI

' ~

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I so _ _ ___ _ _ _ _ S'Ed8'Lof _ _ _ _ _ _ __

46

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I FCPL33.0845 Figure 5.8 I CRACK GROWTH PREDICTION FOR CRACKS IN 28" DIAMETER PIPE CPL-13-104 50 Revision 0 I

nutgg_h g

t I

I e I ALLOWABLE C F"TH 50----------------------------

48 l I

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FCPL83.0847 I

I Figure 5.9 I CRACK GROWTH PREDICTION FOR CRACKS IN 22" DIAMETER PIPE CPL-13-104 51 Revision 0

. nutg,gh 1

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I ALLOWABLE DEPTH 50 -- - - - - - - - - - - - - - - - - -

45 - _ _ l l

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Figure 5.10 CRACK GROWTH PRED!CT!ON FOR CRACKS IN 20" DlAMETER PIPE lI CPL-13-104 Revision 0 52 nutp_9b 1

I

I

, +35 -

l E

!E I a:

O io OD AXIAL RESIDUAL STRESS i

FCPLS3.0842 l

l l

l l

l l

l Figure 5.11 I AXIAL WELD RESIDUAL STRESS IN 12" DIAMETER PIPE WELDS CPL-13-104 53 Revision 0 nut,eSh

w e I

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g .0 I ------------------

ALLOWABLE CEPTH --

50 I

I 40 -

b a

g g 30 -

e l

! 20 -

!I g iO -

I

g e 2 TIME (MONTHS)

I FCPL83.0844 I

,1,_ s.,2 g

GROWTH OF CIRCUMFERENTIAL CRACKS IN UNREPAIRED 12" DIAMETER PIPE CPL-13-104 54 Revision 0 I

nutp_qb

I I

I I

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A I $

E ts 0

I I ID 00 AXIAL RESIDUAL STRESS I FCPL83.0643 I

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I Figure 5.13 AXIAL WELD RESIDUAL STRESS IN 6" DIAMETER PIPE WELDS CPL-13-104 55 Revision 0 I

nutgqh I -

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I I -

I ALLOWABLE DEPTH 50 - - - - - - - - - - - - - - - - - - - - - - - - - - - ----

I 4 40 -

d I !g 30 -

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0 , ,

0.0 0.5 1.0 TIME (MONTHS)

FOPL83.08 08 I

I Figure 5.14 GROWTH OF CIRCUMFERENTIAL CRACKS IN UNREPAIRED 6" DIAMETER PIPE WELDS I CPL-13-104 Revision 0 56 I

nutggh j

I I 6.0 LEAK-BEFORE-BREAK I 6.1 Net Section Collapse I The simplest way to determine the cffect of IGSCC on the structural integrity of piping is through the use of a simple " strength of materials" approach to assess the load carrying capacity of a piping section after the cracked portion has been removed. Studies have shown (References 12 and 13) that this approach gives a conservative, lower-bound estimate of the loads which would cause unstable fracture of the cracked section.

Typical results of such an analysis are indicated in Figure 6.1 (Reference 12). This figure defines the locus of limiting crack depths and lengths for circumferential cracks which are predicted to cause i failure by the net section collapse method. Curves are presented for both typical piping system stresses and stress levels equal to ASME Code limits. Note that a very large percentage of pipe wall can be crackec' before reaching these limits (40% to' 60% of circumference for through-wall cracks, and 65% to 85% of wall thickness l

for 360' part-through cracks).

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I Also shown in Figure 6.1 is a sampling of cracks which have been detected in service, either through UT examination or leakage. In each case there has been a comfortable margin between the size crack that was observed and that which would be predicted to cause failure under service loading conditions. Also, as discussed below, there is still considerable margin between these net section collapse limits and the actual cracks which would cause instability.

6.2 Tearing Modulus Analysis Elastic-plastic fracture mechanics analyses are presented in Reference 13 which give a more accurate representation of the crack tolerance capacity of stainless steel piping than the net section collapse approach described above. Figures 6.2 and 6.3 (Reference 13) graphically depict the results of such an analysis. Through-wall circumferential defects of arc length equal to 60' through 300* were assumed at various cross sections of a typical BWR Recirculation System.

Loads were applied to these sections of sufficient magnitude to produce net section limit load, and the resulting values of tearing modulus were compared to that required to cause unstable fracture (Figure 6.2).

Note that in all cases there is substantial margin, I

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I I indicating that the net section collapse limits of the previous section are not really failure limits. Figure 6.3 summarizes the results of all such analyses performed for 60' through-wall cracks in terms of margin on tearing modulus for stability. The margin in all cases is substantial.

I 6.3 Leak Versus Break Flaw Configuration 0

Of perhaps more significance to the leak-before-break argument is the flaw configuration depicted in Figure 6.4. This configuration addresses the concerns raised by the occurrence of part-through flaws growing, with respect to the pipe circumference, before breaking through the outside surface to cause leakage. Figure 6.4 presents typical size limitations on such flaws based on the conservative, net section collapse method of Section 6.1. Note that very large crack sizes are predicted. Also shown on this figure are typical detectability limits for short through-wall flaws (which are amenable to leak detection) and long part-through flaws (which are amenable to detection by UT). The margins between the detectability limits, and the conservative, net section collapse failure limits are substantial. It is noteworthy that the likelihood of flaws developing which are characterized by the vertical

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f axis shown in Figure 6.4 (constant depth 360* circumfer-ential cracks) is so remote as to be considered impossible. Material and stress asymmetries always tend to propagate one portion of the crack faster than the bulk of the crack front, which will eventually result in

" leak-before-break." This observation is borne out by extensive field experience with BWR IGSCC.

6.4 Axial Cracks The recent IGSCC occurrences at Monticello and Hatch 1 were predominately short, axial cracks which grew through the wall but remained very short in the axial direction. This behavior is consistent with expectations for axial IGSCC since the presence of a sensitized weld heat-affected zone is necessary, and this heat-affected zone is limited to approximately 0.25 inch on either side of the weld. Since the major loadings in the above net section collapse analysis are bending moments on the cross section due to seismic loadings, and since these loads do not exist in the circumferential direction, the above leak-before-break arguments are even more persuasive for axially oriented cracks. There is no known mechanism for axial cracks to lengthen before growing through-wall and leaking, and I

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I I the potential rupture loading on axial cracks is less I than that on circumferential cracks.

6.5 Multiple Cracks Recent analyses performed for EPRI (Reference 14) indicate that the occurrence of multiple cracks in a weld, or cracking in multiple welds in a single piping line do not invalidate the leak-before-break arguments discussed above.

I 6.6 Crack Detection Capability

,I IGSCC in BWR piping is detected through two means: non-destructive examination (NDE) and leakage detection.

! Although neither is perfect, the two means complement one another well. This detection capability, combined l

with the exceptional inherent toughness of stainless steel, results in essentially 100% probability that IGSCC would be detected before it significantly degraded the structural integrity of a BWR piping system.

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I I 6.7 Non-Destructive Examination I The primary means of non-destructive examination for IGSCC in BWR piping is ultrasonics. This method has been the subject of considerable research and develop-ment in recent years, and significant improvements in its ability to detect IGSCC have been achieved.

Nevertheless, recent UT experience at Brunswick 1, Monticello, Hatch 1, and elsewhere indicate that there is still considerable room for improvement, especially in the ability to size cracks and to distinguish cracks or crack-like indications from innocuous geometric conditions. Because of the difficulty in sizing, the weld overlays described herein do not depend on UT sizing as the cracks are assumed to be through-wall.

Figure 6.4, however, illustrates a significant aspect of UT detection capability with respect to leak-before-break. The types of cracking most likely to go undetected by UT are relatively short circumferential or axial cracks which are most amenable to detection by leakage. Conversely, as part-through cracks lengthen, and thus become more of a concern with respect to leak-a before-break, they become readily detectable by UT, and are less likely to be misinterpreted as geometric conditions.

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Leakage Detection I

6.8 Typical leakage detection capability for BWR reactor coolant system piping is through sump level and drywell activity monitoring. These systems have sensitivities on the order of 1.0 gallon per minute (GPM) of unidentified leakage (i.e., not from known sources such as valve packing or pump seals). Plant technical specification and administrative limits typically require investigation / corrective action at 5.0 GPM unidentified leakage, or when there is a 2.0 GPM increase in unidentified leakage in a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period.

Table 6.1 provides a tabulation of typical flaw sizes which cause 5.0 GPM leakage in various size piping assuming a membrane stress of Sm/2 (Reference 12).

I Also shown in this table are the critical crack lengths for through-wall cracks based on the net section collapse method of analysis discussed above. For conservatism, the leakage values are based on pressure stress only, while the critical crack lengths are based on the sum of all combined loads, including ceismic.

Considering other normal operating loads in the leakage i

analysis would result in higher rates of leakage for a I

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given crack size. Note that there is considerable margin between the crack length which produces 5.0 GPM leakage and the critical crack length, and that this margin increases with increasing pipe size.

6.9 Historical Experience The above theories regarding crack detectability have been borne out by experience. Indeed, of the approximately 400 IGSCC incidents to date in BWR piping, all have been detected by either UT or leakage, and none have even come close to violating the structural integrity of the piping (Reference 14).

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I Table 6.1 EFFECT OF PIPE SIZE ON THE RATIO OF THE CRACK LENGTH FOR 5 GPM LEAK RATE AND THE CRITICAL CRACK LENGTH I (ASSUMED STRESS a = Sm / 2)

I CRACK LENGTH FOR CRITICAL CRACK gjg NOMINAL c

PIPE SIZE 5 GPM LEAK (in.) LENGTH te (in.)

4" SCH 80 4.50 6.54 0.688 10" SCH 80 4.86 15.95 0.305 24" SCH 80 4.97 35.79 0.139 F CPL 83.08G I

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i I 6h i

'x N

d I ,

I i

1.0 6 4 gO

\

.\ .

N 0.8 -9 9 N

g P

m = 6 ksi, Pb=0 i @@ *%

= 6 ksi, Pm+Pb = 1.5 S m I

P

$ 0.6 -

m

.a P=

~j @@@

  • 0,4 __ @ Field Data - Part-E

'8 Through Flaws

  • O Field Data - Leaks

] @

$ S m = 16.0 ksi ag = 48.0 ksi I 0.2 -

Values at 5500 F I

0 O 0.2 0.4 0.6 0.8 1.0 Fraction of Circumference, e/r FCPL83.08 28 Figure 6.1 TYPICAL RESULT OF NET SECTION COLLAPSE ANALYSIS OF CRACKED STAINLESS STEEL FiPE CPL-13-104 66 Revision 0 I

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3 I 30*\

L po 4" f*

- 21o*

I go "s

DlM p _

V"

~

3ao T=o I 25o -

Material N 2ao - (Unstable)

I N s

15o-2e - 12ao (StaNel 2s - soo

- . -2e - 240a 29 = 1800 I -- ...... s,.

. 29 = 3o00 sio .,is 2s.o ais ioo.o iss iso 2ss 2so.o g

T FCPLB3.0810 l Figuro e.2 STABILITY ANALYSIS FOR BWR RECIRCULATION SYSTEM (STAINLESS STEEL)

CPL-13-104 67 Revision 0 4

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N$z sa l m e =

G o

ss <M Em OE aze l x us as vz

=5 se m mg

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e gg y av

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.5 S e

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m oe se g

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' ~

~ s%

c sno uva asso ao aaawns g

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I 2=

I I t .

PIPE CROSS SECTION I 0.7 I 0.6

f I 0. 5 - 1 I

l c' 0.4 -

l I

g 0.3- 1 I

0.2 -

I tSt LEAK MONITOR i

1 I

0.1-0.0 , , , , ,

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

.i, FCPL83.08-12 Figure 6.4 TYPICAL PIPE CRACK FAILURE LOCUS FOR COMBINED THROUGH WALL PLUS 3600 PART THROUGH CRACK CPL-13-104 69 Revision 0 nutp_9h g

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I 7.0

SUMMARY

AND CONCLUSIONS The evaluation of the repairs to the Recirculation, RHR, and RWCU Systems reported herein shows that the result-ing stress levels are acceptable for all design conditions. The stress levels have been assessed from the standpoint of load capacity of the components, fatigue, and the resistance to crack growth.

I Acceptance criteria for the analyses have been established in Section 3.0 of this report which demonstrate that:

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1. There is no loss of design safety margin over that provided by the current Code of Construction for Class 1 piping and pressure vessels (ASME Section III).
2. During the design lifetime of each repair, the observed cracks will not grow to the point where the above safety margins would be exceeded.

Analyses have been performed and results are presented which demonstrate that the repaired welds satisfy these criteria by a large margin, and that the design life of each repair is at least five years. Analyses have also I

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I been performed which demonstrate that the unrepaired welds satisfy these criteria by a larger margin for at least the next six months.

I Furthermore, it is concluded that the recent IGSCC experienced in the Reactor Recirculation, RHR, and RWCU Systems at Brunswick 2 does not increase the probability of a design basis pipe rupture at the plant. This conclusion expressly considers the nature of the cracking which has been repaired at Brunswick 2, and the likelihood that other similar cracking may have gone undetected. The conclusion is based primarily on the extremely high inherent toughness and ductility of the stainless steel piping material; the tendency of cracks in such piping to grow through-wall and leak before affecting its structural load carrying capacity (which indeed was the case in the defects observed at Brunswick 1 earlier this year) and; the fact that as cracks lengthen and are less likely to " leak-before-break",

they become more amenable to detection by other NDE techniques such as UT and RT.

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8.0 REFERENCES

1. ASME Boiler and Pressure Vessel Code,Section III, Subsection NB, 1974 Edition with Addenda through Summer 1975.

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2. ASME Boiler and Pressure Vessel Code Section XI, Paragraph IWB-3640, " Acceptance Criteria for Austenitic Steel Piping" (Approved by Main Committee for Incorporation into Section XI in 1983).
3. NRC Policy Issue No. SECY-83-267C dated November 7, 1983 along with Attachments.
4. General Electric Design Specification 22A1417, Revision 2.
5. General Electric letter G-KB1-1-193, December 30, 1981, " Transmittal of GE Design Memo 170-17 Rev. 1, on Seismic Reevaluation of Recirculation Piping System for Brunswick Units 1 and 2."

I 6. NUTECH Computer Program PISTAR, Version 2.0, Users Manual, Volume 1, TR-76-002, Revision 4, File Number 08.003.0300.

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7. ASME Boiler and Pressure Vessel Code Section XI, 1980 Edition with Addenda through Winter 1981.
8. ANSYS Computer Program, Swanson Analysis Systems, Revision 4.

-9. Schneider, P.J., " Temperature Response Charts,"

John Wiley and Sons, 1963.

10. NUTECH Letter, TVA-01-031, Overlay Design Methodology Licensing Support for the Brcwns Ferry Unit 1 Weld Repair Program, July 25, 1983.
11. NUTECH Report NSP-81-105, Revicion 2, " Design Report for Racirculation Safe End and Elbow Repairs, Monticello Nuclear Generating Plant,"

December 1982.

12. EPRI-NP-2472, "The Growth and Stability of Stress Corrosion Cracks in Large-Diameter BWR Piping,"

July 1982.

I 13. EPRI-NP-2261, " Application of Tearing Modulus Stability Concepts to Nuclear Piping," February 1982.

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14. Presentation by EPRI and BWR Owners Group to U. S.

Nuclear Regulatory Commission, " Status of BWR IGSCC Development Program," October 15, 1982.

15. NUTECH Computer Program NUTCRAK, Revision 0, April 1978, File Number 08.039.0005.
16. NUTECH Computer Program NUFLAW, Revision 0, November 1983.

I 17. NUTECH Report CPL-09-102, Revision 0, " Design Report for Recirculation System Weld Overlay Repairs at Brunswick Steam Electric Plant, Unit 1,"

May 1983.

18. NUTECH Report GPC-07-102, Revision 0, " Design Report for Weld Overlay Repairs and Flaw Evaluations in Recirculation and RHR Systems at E. I. Hatch Nuclear Power Plant Unit 2", June 1983.

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Corporate Office I

NUTECH, Inc.

6835 Via del Orn San Jose, CA 95119 Phone: (408) 629-9800 Telex: 352062 Operaung Subsidiaries:

NUTECH Engineers,Inc. NUTECH Intemational,Inc.

6835 Via del Oro 6835 Via del Oro San Jose, CA 95119 San Jose, CA 95112 Phone: (408) 629-9800 Phone: (408) 629-9800 Telex: 352062 Telex: 352062 NUTECH Engineers Atlanta NUTECH International Madrid 1765 The Exchange, Suite 310 Orense,68-4' Atlanta, GA 30339 Madrid, Spain Phone: (404) 955-1275 Phone: 279-3508 Telex:46642 FATS NUTECH Engineers Bethesda 7910 Woodmont Avenue NUTECH Intermtional Seoul Bethesda, MD 20814 Korea Paint Building, Room 406 Phone: (301) 652-1610 204-6 Nonhyun-Dong Gangnam-Ku Seoul 134, Korea NUTECH Engineers Chicago Phone: 82-2-555-5887 225 N. Michigan Avenue,16th Floor Telex: K28143 Chicago,IL 60601 Phone: (312) 565-2900 NUTECH International Taipel 14th Floor,147 Chien-Kuo North Road l NUTECH Engineers Minneapolla Section 2 331 Second Avenue South Taipel. Taiwan ROC Minneapolis, MN $5401 Phone: 531-6226 Pnone: (612) 339-1100 Telex 28514 NUTECH Testin0 Corporation E 6830 Via del Oro San Jose, CA 95119 Phona: (408) 62'J-9800

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