L-13-103, Rev 0 to Design Rept for Recirculation Sys Weld Overlay Repairs at Brunswick Steam Electric Plant Unit 1
| ML20087A031 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 11/04/1983 |
| From: | Charnley J, Sund H, Wenner T NUTECH ENGINEERS, INC. |
| To: | |
| Shared Package | |
| ML20087A025 | List: |
| References | |
| CPL-13-103, CPL-13-103-R, CPL-13-103-R00, NUDOCS 8403070318 | |
| Download: ML20087A031 (50) | |
Text
Controlled Copy No. CPL-13-103 Revision 0 November 1983 CPL 013.0103 4
DESIGN REPORT FOR RECIRCULATION SYSTEM WELD OVERLAY REPAIRS AT BRUNSWICK STEAM ELECTRIC PLANT UNIT 1 I
Prepared fo r Carolina Power and Light Company Prepared by
- NUTECH Engineers, Inc.
San Jose, California Prepared by: Issued by:
g- fN A t-b.
J E. Charnley, P.E.
- H.J. Sund Project Manager Project Engineer Approved by:
y . M Date: NOVEWh4R 9 s 19 N T. Y. Wenner, P.E.
Engineering Director 8403070318 840227 PDR ADOCK 05000325 G PDR nutggb
REVISION CONTROL SHEsT I
TITLE: Design Report for Recircu- DOCUMENT FILE NUMBER: CPL 013.0103 lation System Weld Overlay Repairs at Brunswick Steam Electric Plant Unit 1 J. E. Charnley / Staff Encineer NAME / TITLE INITIALS J. R. Taylor / Consultant II kg N AME / TITLE INITIALS T. Lem/ Consultant I 74 NAME/ TITLE INITIALS
.H. L. Gustin / Senior Engineer f INITIALS NAME / TITLE D. C. Talbott/ Consultant I ((
INITI A'LS NAME / TITLE AFFECTED DOC PREPARED ACCtJRACY CRITERIA REMARKS PAGE(S) REV SY/OATE CHECK SY / OATE CHECK BY / OATE ii NA NA 111 O
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TITLE: Design Report for Re- DOCUMENT FILE NUMBER: CPL 013.0103 circulation System Weld Overlay Repairs at Brunswick Steam Electric Plant Unit 1 AFFECTED DOC PREPARED ACCURACY CRITERIA REMARKS PAGE(S) REV SY / DATE CHECK SY / DATE CHECK BY / DATE
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l CERTIFICATION BY REGISTERED PROFESSIONAL ENGINEER 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.
Certified by:
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' DOC ## Registration No. 16340 Date 3 CPL-13-103 iv Revision 0 nutggh
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TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF FIGURES Vii
1.0 INTRODUCTION
1 2.0 REPAIR DESCRIPTION 4 3.0 EVALUATION CRITERIA 7 3.1 Strength Evaluation 7 3.2 Fatigue Evaluation 8 3.3 Fracture Mechanics Evaluation 9 4.0 LOADS 10 4.1 Mechanical and Internal Pressure Loads 10 4.2 Thermal Loads 10 5.0 EVALUATION METHODS AND RESULTS 12 5.1 28" Elbow Evaluation 12 5.1.1 Code Stress Analysis 13 5.1.2 Fracture Mechanics Evaluation 15 4
5.2 Effect on Recirculatioh and RHR Systems 18 6.0 LEAK-BEFORE-BREAK 26 6.1 Net Section Collapse 26 6.2 Tearing Modulus Analysis 27 6.3 Leak Versus Break Flaw Configuration 28 6.4 Axial Cracks 29 6.5 Multiple Cracks 30 6.6 Crack Detection Capability 30 6.7 Non-Destructive Examination 31 6.8 Leakage Detection 32 6.9 Historical Experience 33 7.0
SUMMARY
AND CONCLUSIONS 39
8.0 REFERENCES
41 CPL-13-103 v Revision 0
1 LIST OF TABLES Number .
Title Page 5.1 Thermal Stress Results 20 5.2 28" Elbow Code Stress Results 21 6.1 Effect of Pipe Size on the Ratio 34 of the Crack Length for 5 GPM Leak Rate and the Critical Crack Length (Assumed Stress a = (Sm)/2 )
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LIST OF FIGURES Number Title Page 1.1 Conceptual Drawing of Recirculation System 3 2.1 Configuration of 28" Elbow-to-Valve Weld Overlay 6 5.1 28" Elbow-to-Valve Finite Element Model 22 l 5.2 Weld Overlay Thermal Model 23 5.3 Thermal Transients 24 5.4 Piping Model 25 6.1 Typical Result of Net Section Collapse 35 Analysis of Cracked Stainless Steel Pipe 6.2 Stability Analysis for BWR Recirculation 36 System (Stainless Steel) 6.3 Summary of Leak-Before-Break Assessment 37 of BWR Recirculation System i
6.4 Typical Pipe Crack Failure Locus for Combined 38
, Through-Wall Plus 360' Part-Through Crack s'
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1.0 INTRODUCTION
This report summarizes evaluations performed by NUTECH to assess weld overlay repairs in the Recirculation System at Carolina Power and Light Company's Brunswick Steam Electric Plant Unit 1 (Brunswick 1). Weld overlay repairs have been applied to address ultrasonic (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 4 to arrest any further propagation of the cracking, and to restore original design safety margins to the weld.
The flaws addressed in this report were detected during the October 1983 inspection.
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The required design life of each weld overlay repair is at least five years. The amount that the actual life i exceeds five years will be established by a combination of future analysis and testing.
Crack indications have been detected adjacent to two 28" elbow-to-valve welds. Both of these welds were
. repaired with weld overlay designs evaluated in this report.
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Ficure 1.1 shows the two elbow-to-valve welds and the three welds which were overlay weld repaired earlier (Reference 1) in relation to the Reactor Pressure Vessel and other portions of the Recirculation and Residual Heat Removal (RHR) Systems. All of the existing l
Recirculation System material is Type 304 stainless steel.
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INDICATIONS IN SAFE END 3O TD PIPL WELD
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INDICATIONS IN 28" %
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, , ., O INDICATIONS IN 28" ELB0W-TO-VALVE WELD 0g (28-A-14) gh ( INDICATIONS IN 28" ELB0W-TO-VALVE fI WELD 28-B-8 LOOP A INDICATIONS WillCil ARE IBOXEDI WERE FOUND IN Tile OCTOBER, 1983 INSPECTION.
O Figure 1.1 FCP'83 0684 CONCEPTUAL DRAWING OF RECIRCULATION SYSTEM R
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l 2.0 REPAIR DESCRIPTION l
The UT indications in the elbow weld heat-affected zones have been repaired by establishing additional " cast-in-place" pipe wall thickness from weld metal deposited 360 degrees around and to either side of the existing weld, as shown in Figure 2.1. The weld deposited band over the cracks will provide wall thickness equal to that enquired to restore the original design safety margins. In addition, the weld metal deposition will produce a favorable compressive residual stress pattern. The deposited weld metal will be Type 308L, which is resistant to propagation of IGSCC cracks.
. 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.
- 2) Volumetric examination of the completed weld overlay by the ultrasonic examination technique in accordance with ASME Section XI.
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- 3) Volumetric preservice examination of the weld overlay and existing circumferential pipe weld by 1
the ultrasonic examination technique in accordance
- with ASME Section XI.
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go NO MtN R OVERLAY TAPER A TRANSITIONS LONG RAOluS I E L80W Figure 2.1 CONFIGURATION OF 28" ELBOW-TO-VALVE WELD OVERLAY FcPL83.0645 CPL-13-103 6 Revision 0 nutp_qh
3.0 EVALUATION CRITERIA 9
This section describes the criteria that are applied in this report to evaluate the acceptability of the weld overlay repairs described in Section 2.0. 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 requirements 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. ?ss a further means of ensuring structural adequacy, criteria are also provided below for fracture mechanics evaluation of the repairs.
- 5. 3.1 Strength Evaluation Adequacy of the strength of the weld overlay repairs with respect to applied mechanical loads is demonstrated CPL-13-103 7 Revision 0
l with an ASME Boiler and Pressure Vessel Code Section III, Class 1 (Reference 2) analysis of the weld overlay repairs.
'l 3.2 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 2.
- 2. The peak alternating stress intensity, including all primary and secondary stress terms and a fatigue strength reduction factor of 5.0 to account for the existing crack, was evaluated using conventional fatigue analysis techniques. The total fatigue usage factor, defined as the sum of 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.
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l Allowable number of cycles was determined from the stainless steel fatigue curve of Reference 2.
3.3 Fracture Mechanics Evaluation A highly conservative method was used to demonstrate the adequacy of the seld overlay repair. All relevant UT indications were assumed to be through-wall cracks. The maximum crack depths were actually measured to be approximately 20%-30% of pipe wall, but they were assumed to be 100% of pipe wall. The weld overlay was then designed such that tue net section limit load requirements"of Reference 3 were satisfied.
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l 4.0 LOADS I The loads considered in the evaluation of the elbow-tc- ]
valve welds consist of mechanical loads, internal pressure and differential thermal expansion loads. The mechanical loads and internal pressures used in the analysis are described in Section 4.1, and an explanation of the thermal transient conditions which cause differential thermal expansion loads is presented in Section 4.2.
4.1 Mechanical and Internal Pressure Loads The design pressure of 1325 psi for the Recirculation System was obtained from Reference 4. The dead weight and seismic loads applied to each weld were obtained from Reference 5.
4.2 Thermal Loads The thermal expansion loads for each weld were obtained from a computer model of the Recirculation and RHR Systems. The NUTECH computer program PISTAR (Reference 6) was used. Reference 4 defines several types of transients for which the Recirculation System I
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is 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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! 5.0 EVALUATION METHODS AND RESULTS i
The evaluation of the weld overlay repcirs consists of a code stress analysis per Section III (Reference 2) and a fracture mechanics evaluation per Section XI (References 3 and 7).
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- 5.1 28" Elbow Evaluation T
The heat-affected zone on the elbow side of weld 28-A-14 l
(elbow-to-valve weld) has one circumferential UT indica-
- t. '.o n . The length of the indication is 23 inches. It was depth sized by UT to be 20% (0.25 inch) of the pipe thickness which is 1.26 inches. However, for all analytical purposes, the crack was assumed to be through-wall for its entire 23 inch length.
., The heat-affected zone on the elbow side of weld 28-B-8 (elbow-to-valve weld) has one circumferential UT indication. The length of the indication is 8 inches.
It was depth sized by UT to be 30% (0.33 inch) of the i
pipe thickness which is 1.088 inches. However, for all
- analytical purposes, the crack was assumed to be through I
4 wall for its entice 8" length.
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1 5.1.1 Code Stress Analysis A finite element model of the cracked and weld overlaid region was developed using the ANSYS (Reference 8) l computer program. The model was based on the design minimum overlay thickness of 0.42 inch, which is 9 percent smaller than the actual minimum average thickness of 0.46 inch. The finite element model was based on weld 28-A-14 as it has the largest crack and the highest pressure induced stress after installation of the weld overlay. The small added weight (less than 100 pounds) of the sold overlay will not significantly change the existing seismic analysis. Figure 5.1 shows the model. This figure also shows the material that was remo' sed to represent the crack.
The stress in the overlaid elbow due to design pressure and applied moments as described in Sections 4.1 and 4.2 was calculated with the finite element model.
1 The weld overlay thermal model was taken to be axisymmetrical (Figure 5.2). The exterior boundary was assumed to be insulated. The temperature distribution in the weld overlay, subject to the thermal transients defined in Section 4.2, can be readily calculated using CPL-13-103 13 Revision 0 i,
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Charts 16 and 23 of Reference 9. The maximum through wall temperature difference was determined to be less than 6*F for the normal startup cycle, 45'F for the small temperature cycle, and 377'F for the emergency trancient.
The maximum thermal stress for use in the fatigue 1
analysis was calculated as follows: (Reference 2)
EaaT 1 EaaT 2
- " +
2(1 v) lv Where:
E = 28.3 x 10 6 psi (Young's Modulus) a = 9.11 x 10-6.p-1 (Coefficient of Thermal Expansion)
AT 1 = Equivalent Linear Temperature Difference AT 2 = Peak Temperature Difference The values of ATg , AT 2 , and e are given in Table 5.1 for all three thermal transients.
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The results of a code stress analysis per Reference 2 are given in Table 5.2. The allowable stress values for Reference 2 are also given. The weld overlay repair satisfies the Reference 2 requirements.
A conservative fatigue analysis per Reference 2 was performed. A fatigue strength reduction factor of 5.0 was applied due to the crack. The fatigue usage factor was then calculated assuming 38 startups, 25 small temperature change cycles and one emergency cycle every five years. The results are summarized in Table 5.2.
5.1.2 Fracture Mechanics Evaluation The allowable crack depth was calculated based on Reference 3. Crack growth due to fatigue was determined
- based on Reference 1. Crack growth due to IGSCC was evaluated based on References 1 a.td 10.
5.1.2.1 Allowable Crack Depth From Reference 5,'the highest applied primary (pressure
+ dead weight + OBE) stresses in any 28 inch elbow in the recirculation system is 9860 psi. The allowable crack depth was calculated based on Reference 3.
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Stress Ratio = Pm + Pb Sm Pm + Pb = Primary Stress Sm = Code allowable stress value
= 16,800 psi (at design temperature of 562*F)
Stress Ratio = 9,860 = 0.59 16,800 s
Fractional Crack Length = L xD L = Crack Length = 23 inches
- .- D = Weld Diameter = 28 inches i.
J' Fractional Crack Length = 23 = 0.26 I' x28 i
Thus, per Reference 3, the allowable crack depth is 75%
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of the pipe thickness. Therefore, the minimum weld '
overlay thickness was chosen to be equal to one third of the pipe thickness.
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TOVERLAY = TPIPE = 1.26 = 0.42 inch ,
3 3 The minimum average as-built overlay thickness is 0.46 inch.
5.1.2.2 Crack Growth P
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 1. The fatigue crack growth for five years of the cycles shown in Figure 5.3 was calculated to be less than 0.01 inch.
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 suscsptible due to its high ferrite content and its duplex (austenite and delta ferrite) structure.
Therefore, the cracks will not grow into the weld overlay.
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S The longest measured UT indication is 23 inches.
Without the installation of a weld overlay, the crack
- would be expected to lengthen. However, the weld overlay will significantly change the residual stress t
pattern near the inside of the pipe as shown in Reference 10. In Reference 10, a 28 inch pipe-to-valve weld which was overlaid with a 0.35 inch thick overlay was evaluated for residual stress. The results show that the overlay changed the residual stress pattern 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 woul'd be equal to the original pipe thickness plus 0.01 inch, or 1.27 inches. The allowable crack depth is 1.31 inches [1.31 = (.75) (1.75)], as the e as-built thickness of the overlay of weld 28-A-14 is O.49 inch.
5.2 Effect on Recirculation and RHR Systems Installation of the weld overlay repairs caused a small amount of radial and axial shrinkage underneath the overlay. Based on measurements of the weld overlays, the maximum axial shrinkage was 0.09 inch (weld 28-B-8).
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Ohe effects 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 are not limited by the ASME Code (Reference 2).
The effect of the axial weld shrinkage on the Recircula-tion and RHR Systems was evaluated with the NUTECH computer program PISTAR (Reference 6) and the piping model shown in Figure 5.4.
The largest measured axial shrinkage of a 28 inch elbow-to-valve weld overlay was 0.09 inch. The measured shrinkages of both weld overlay repairs as well as the shrinkages from the three previous overlay repairs (Reference 1) 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.
The maximum calculated stress was less than 4.0 ksi.
The location of this stress is shown on Figure 5.4 Steady state secondary stresses of 4.0 ksi are judged to have no deleterious ef fect on the Recirculation or- RHR i
systems.
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SMALL NORMAL TEMPERATURE EMERGENCY STARTUP PARAM E CYCL ~e CHANGE CYCLE CYCLE !
(CYCLE 1) (CYCLE 2) (CYCLE 3) j U
EQUIVALENT 6F 36 F 302 F LINEAR TEMPERATURE AT t i PEAX 0 9F 75 F TEMPERATURE AI 2
. . THROUGH 1,100 PSI 9,940 PSI 83,240 PSI I
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Table 5.1 THERMAL STRESS RESULTS CP L-13-10 3 20 Revision 0 l
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ACTUAL l SECTION III EQUATION STRESS CATEGORY NUMBER OR N8 ALLOWABLE THICXNESS S N/A N/A S, = 14,300** PSI PRIMARY (9) 18,600 PSI 21,450 PSI (10) 35,200 PSI 42,900 PSI RY PEAK CYCLE 1 (30,500)S* gfg (11) (14,600)5 CYCLE 2 CYCLE 3 (150,700)5 USAGE FACTOR N/A 0.04 1.0
- (5 YR) ;
- THE FACTOR OF 5 IS THE CONSERVATIVELY ASSUMED FATIGUE STRENGTH REDUCTION FACTOR.
- THE HIGHEST STRESS OCCURS IN THE 308L WELD METAL.
FCT1.83 M 1 Table 5.2 28" ELBOW CODE STRESS RESULTS m 12-1e>
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Figure 5.4 PIPING MODEL Fcrua.os.io CPL-13-103 25 Revision 0 nutggb
6.0 LEAK-BEFORE-BREAK 6.1 Net Section Collapse The simplest way to determine the effect 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 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 cracked before reaching these limits (40% to 60% of circumference for through-wall cracks, and 65% to 85% of wall thickness for 360* part-through cracks).
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Also shown in Figure 6.1 is a sampling of cracks chich 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 servico loading conditions. Also, as discussed below, there is still considerable margin between these net section collapse limits and the actual cracks which would cinuse 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 graphically depict the results of such an analysis (Reference 12). 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 CPL-13-103 27 Revision 0 nutagh
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k (Figure 6.2). Note that in all cases there is substantial margin, indicating that the net section !
l 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.
j The margin in all cases is substantial. l i
i 6.3 Leak Versus Break Flaw Configuration 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 i . 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 i 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 CPL-13-103 28 Revision 0
I substantial. It is noteworthy that the likelihood of flaws developing which are characterized by the vertical 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 CPL-13-103 29 Revision 0
cracks. There is no known mechanism for axial cracks to lengthen before growing through-wall and leaking, and j the potential rupture loading on axial cracks is less 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.
6.6 Crack Detection Capability 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 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.
CPL-13-103 30 Revision 0 1
r N. l i
6.7 Non-Destructive Examination 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 d1fficulty 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-CPL-13-103 31 Revision 0
l l
be f ore-b reak , they become readily detectable by UT, and l are less likely to be misinterpreted as geometric conditions.
, 6.8 Leakage Detection
\
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 j unidentified leakage (i.e., not from known sources such i as valve packing or pump seals). Plant technical specification limits typically require investigation /
corrective action at 5.0 GPM unidentified leakage.
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).
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 l stress only, while the critical crack lengths are based
' 1 on the sum of all combined loads, including seismic. )
CPL-13-103 32 )
Revision 0 YYl J l
l (Considering other normal operating loads in the leakage analysis would result in higher rates of leakage for a 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 theoria.s 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).
CPL-13-103 33 Revision 0
1 l
l CRITICAL CRACK NOMINAL CRACK LENGTH FOR gj;C PIPE SIZE 5 GPM LEAK (in.) LENGTH Ec (IU*)
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 FCPLA3.C643 Table 6.1 l
EFFECT OF PIPE SIZE ON THE RATIO OF THE CRACK LENGTH j l
FOR 5 GPM LEAK RATE AND THE CRITICAL CRACK LENGTH l (ASSUMED STRESS a = Sm/2) l l CP L-13-10 3 34 l Revision 0
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- o Field Data - Laaks 3
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@ ag = 48.0 ksi 0.2 -
Values at SECC F 0
O 0.2 0.4 0.6 0.8 1.0 Fraction of Circumference 6/r FCPL83.0611 i
Figure 6.1 TYPICAL RESULT OF NET SECTION COLLAPSE ANALYSIS OF
- CRACKED STAINLESS STEEL PlPE CPL-13-103 35 Revision 0 )
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50.0 12.5 25.0 82.5 100.0 137.5 175.0 212.5 250.0 T
FCPL83.0612 Figure 6.2 STABILITY ANALYSIS FOR BWR RECIRCULATION SYSTEM (STAINLESS STEEL)
CPL-13-103 36 Revision 0 nutech
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== =5 80 8m <a m5
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er := =
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CPL-13-103 37 Revision 0
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l 1 l l t- a PIPE CROSS SECTION
- 0. 7. -
0.6 1
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=/w FCPL33.0614 Figure 6.4 TYPICAL PIPE CRACK FAILURE LOCUS FOR COMBINED THROUGH-WALL PLUS 3600 PART-THROUGH CRACK CPL-13-103 38 Revision 0
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SUMMARY
AND CONCLUSIONS y' The evaluation of the repairs to the Recirculation System reported herein shows that the resulting stress
,u.
levels are acceptable for all design conditions. The i stress levels have been assessed from the standpoint of load capacity of the components, fatigue, and the 3: resistance to crack growth.
Acceptance criteria for the analyses have been
~
established in Section 3.0 of this report which demonstrate that:
7
'd
- 1. There is no loss of design safety margin over that
..D 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.
D l
,w d
Analyses have been performed and results are presented
- i a which demonstrate that the repaired welds satisfy these !
l i
w
?i CPL-13-103 39
- Revision 0 l~
l
.., 1
= .. - - . - . - _ .. - .- . -
i 4
.J criteria by a large margin, and that the design life of
- each repair is at least five years.
]J Furthermore, it is concluded that the recent IGSCC experienced in the Reactor Recirculation System at
- ] Brunswick 1 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 J- been repaired at Brunswick 1, and the likeliood that g"
other similar cracking may have gone undetected. The conclusion is based primarily'on the extremely high innerent toughness and ductility of the stainless steel j piping material; the tendency of cracks in such piping to grow through-wall and leak before af fecting its
.. structural load carrying capacity (which indeed was the m case in the defects observed at Brunswick 1 earlier this year) andr the fact that as cracks lengthen and are less 7 likely to " leak-before-break", they become more amenable Iw, to detection by other NDE techniques such as UT and RT.
7 I i
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l j CPL-13-103 40 Revision 0 7
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8.0 REFERENCES
f 1 NUTECH Report CPL-09-102, Revision 0, " Design Report for Recirculation System Weld Overlay
.i Repairs at Brunswick Steam Electric Plant, Unit 1,"
May 1983.
- 2. ASME Boiler and Pressure Vessel Code Section III, Subsection NB, 1974 Edition with Addenda through
}
~~
Summer 1975.
l
- 3. ASME Boiler and Pressure Vessel Code Section XI, r,
b Paragraph IWB-3640 (Proposed), " Acceptance Criteria
, for Austenitic Steel Piping" (Approved by Main
- Committee for incorporation into Section XI in n- 1983),
u
]
i.,
- 4. General Electric Design Specification 22A1417, Revistor. 2.
Il O
. 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.
7 d
CPL-13-103 41 Revision 0 T .
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O O
. 6. NUTECH Computer Program PISTAR, Version 2.0, Users
- Manual, Volume 1, TR-76-002, Revision 4, File Number 08.003.0300.
- 7. ASME Boiler and Pressure Vessel Code Section XI, j 1980 Edition with Addenda through Winter 1981.
- 8. ANSYS Computer Program
- Swanson Analysis Systems, m Revision 4.
d
- 9. Schneider, P.J., " Temperature Response Charts,"
j John Hiley and Sons, 1963.
J
- 10. NUTBCH Letter, TVA-01-031, Overlay Design Methodology Licensing Support for the Browns Ferry n Unit 1 Weld Repair Program, July 25, 1983.
ll BJ
{ 11. NUTECH Report NSP-81-105, Revision 2, " Design Report for Recirculation Safe End and Elbow I
j Repairs, Monticello Nuclear Generating Plant,"
l December 1982, l
i c
, 12. EPRI-NP-2472, "The Growth and Stability of Stress Corrosion Cracks in Large-Diameter BWR Piping,"
July 1982.
b.1 l CPL-13-103 42 Revision 0 l
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184 l:
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t
'= 13. EPRI-NP-2261, " Application of Tearing Modulus Stability Concepts to Nuclear Piping," February
=
1982.
a .
Nuclear Regulatory Commission, " Status of BWR IGSCC
- Development Program,* October 15, 1982.
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