ML20151G288
| ML20151G288 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 07/05/1988 |
| From: | Kuo A, Riccardella P, Tang S STRUCTURAL INTEGRITY ASSOCIATES, INC. |
| To: | |
| Shared Package | |
| ML20151G274 | List: |
| References | |
| SIR-88-020, SIR-88-020-R00, SIR-88-20, SIR-88-20-R, NUDOCS 8807280265 | |
| Download: ML20151G288 (50) | |
Text
{{#Wiki_filter:. 0 i Report No. SIR-88-020 Revision 0 Project No. CECO-09Q-7 July, 1988 Evaluation of End Cap eld 02A-S10 Weld Overlay Repair Considering Ultrasonic Inspection Findings During 1988 Outage Quad Cities Nuclear Power Station Unit 2 Prepared by: Structral Integrity Associates Prepared for: Commonwealth Edison Company Prepared by:
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c- , 9 TABLE OF CONTENTS Section Page
1.0 INTRODUCTION
. . . . . . . . . . . . . . . . . . . . 1-1 2.0 PROBLEM DESCRIPTION . . . . . . . . . . . . . . . . . 2-1 2.1 Geometry . . . . . . . . . . . . . . . . . . . 2-1 2.2 Ultrasonic Examination Evaluation . . . . . . 2-1 2.3 Evaluation Criteria . . . . . . . . . . . . 2-6 2.4 Applied Loading . . . . . . . . . . . . . . . 2-7 3.0 ANALYSIS . . . . . . . . . . . . . . . . . . . . . 3-1 3.1 Three-Dimensional Finite Element Analyais . . 3-1 3.1.1 Finite Element Model . . . . . . . . . 3-1 3.1.2 Loading Conditions . . . . . . . . . . 3-2 3.1.3 Modeling of UT Indications . . . . . . 3-2 3.1.4 Finite Element Results for Uncracked Structure . . . . . . . . . . , . . . 3-3 3.1.5 Finite Element Results for Cracked Structure . . . . . . . . . . . . . . 3-3 3.2 Net Section Collapse Analysis . . . . . . . . 3-4 3.2.1 Formulation of Multiple-Crack Net Section Collapse Equations . . . . . . 3-4 3.2.2 Verification of Multiple Crack Net Section Collapse . . . . . . . . . . . 3-6 3.2.3 Evaluation of Weld 02A-S10 . . . . . . 3-7 3.2.4 Analysis Results . . . . . . . . . . . 3-8 4.0 DISCUSSION OF RESULTS . . . . . . . . . . . . . . . . 4-1
5.0 CONCLUSION
S . . . . . . . . . . . . . . . . . . . . . 5-1
6.0 REFERENCES
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. LIST OF TABLES TABLE PAGE 2-1 Detailed Listing of IGSCC Indications in Repaired Weld overlay 02A-S10 . . . . . . . . . . . . . . . 2-3 2-2 Detailed Listing of "Contamination Crack" Indications in Repaired Weld Overlay 02A-S10 . . . . . . . . . . 2-4 3-1 Multiple vs. Single Flaw Net Section Collapse Comparlsvn . . . . . . . . . . . . . . . . . . . . 2-5 l
111 mmsm : 1 1
LIST OF FIGURES FIGURE PAGE 1-1 Sketch of Repair to End Cap Weld Overlay (02A-S10). . 1-3 2-1 Detailed Geometry of As-Repaired End Cap Weld Overlay 02A-S10 . . . . . . . . . . . . . . . . . . . 2-8 2-2 Approximate Locations of Ultrasonic Indications in Repaired Weld Overlay 02A-S10 (I = IGSCC Indication; C= "Contamination Crack") . . . . . . . . . . . . . 2-9 3-1 Finite Element Model of End Cap-to-Header Weld , Overlay 02A-S10 . . . . . . . . . . . . . . . . . . . 3-9 3-2 Sketch of Innermost Layer (No. 1) of Finite Element Model Showing Element Numbers . . . . . . . . . . . 3-10 3-3 Sketch of Layer No. 2 of Finite E1.ement Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-11 3-4 Sketch of Layer No. 3 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-12 3-5 Sketch of Layer No. 4 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-13 3-6 Sketch of Layer No. 5 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-14 3-7 Sketch of Layer No. 6 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-15 3-8 Sketch of Layer No. 7 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-16 3-9 Sketch of Layer No. 8 of Finite Element Model Showing Element Numbers . . . . . . . . . . . . . . . . . . . 3-17 3-10 Stress Intensity Contours in an Uncracked End Cap . . 3-18 3-11 Stress Intensity Contours in Layer No. 1 for Cracked End Cap . . . . . . . . . . . . . . . . . . . . . . . 3-19 3-12 Stress Intensity Contours in Layer No. 2 for Cracked End Cap . . . . . . . . . . . . . . . . . . . . . . . 3-20 3-13 Stress Intensity Contours in Layer No. 3 for Cracked End Cap . . . . . . . . . . . . . . . . . . . . . 3-21 iv DfTEGUTY ASSOCIATESINC
m . LIST OF FIGURES (continued) FIGURE PAGE 3-14 Stress Intensity Contours in Layer No. 4 for Cracked End Cap .
. . . . . . . . . . . . . . . . . . . . . 3-22 3-15 End Stress Intensity Contours in Layer No. 5 for Cracked Cap .
. . . . . . . . . . . . . . . . . . . . . . 3-23 3-16 Stress Intensity Contours in Layer No. 6 for Cracked End Cap . . . . . . . . . . . . . . . . . . . . . . 3-24 3-17 Stress Intensity Contours in Layer No. 7 for Cracked End Cap . . . . . . .. . . . . . . . . . . . . . . 3-25 3-18 Stress Intensity Contours in Layer No. 8 End Cap . . . . . . . . . . . . . . . . . . . . . . 3-26 3-19 Illustration of Circumferentially-Cracked Cross-Section Used in Net Section Collapse Analysis
. . . . . . . . . . . . . . . . . . . . . . 3-27 v
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' INTEGRFIT ASSOCWEIIC
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1.0 INTRODUCTION
A weld overlay repair was performed on a 22-inch recirculation header-to-end cap weld (02A-S10) during the 1986 refueling outage at Quad Cities Unit 2. Post repair ultrasonic (UT) examinations showed a single circumferential and several axial indications, which appeared to extend into the weld overlay material such that the minimum remaining ligament of these flaws was less than the full structural overlay thickness of the repair. These indications were evaluated and found to be acceptable for continued operation in Reference 1, using a variation of the ASME Section XI net section collapse approach. As a result of these flaws, however, the NRC required Commonwealth Edison to provide some action to address this flawed weld overlay during the 1988 refueling outage. The 1988 UT examination of this weld showed no significant changes in the flaws observed during the 1986 outage. Nonetheless, Commonwealth Edison decided to repair the overlay to correct these flaw indications. The repair consisted of machining off the entire flawed region of the overlay, including a portion of the underlying base metal wall thicknese (Figure 1-1). The base metal wall thickness was then reestablished by deposition of weld metal, and the overlay reapplied. The overlay was then reinspected, using UT procedures and techniques similar to those used before the repair. The original base metal IGSCC flaws were observe 6 in this examination, along with a significant number (approximately 50) of UT reflectors which were evaluated as "contamination cracks" in the weld overlay material. The minimum remaining ligaments reported for several of these indications were less than the standard design basis full structural weld overlay thickness for this repair. As a result of theso 2nspection findings, Commonwealth Edison embarked on a multi-faceted program to evaluate the repaired SIR-88-020 1-1
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overlay, which included metallurgical samples to confirm the ultrasonic findings, and an analysis to evaluate the acceptability of the weldment for continued operation,' assuming that the metallurgical samples would confirm the presence of actual cracks as reported by UT. This report presents the results of the analytical evaluation. Because of the magnitude and. extent of the UT indications reported, it was decided to perform a three-dimensional finite element model of the weld overlay repaired joint, with the portions of the weld containing UT indications "zerced-out" in the model (i.e. their elastic modulus, and thus their load carrying capacity set to zero). The remaining elements in the model were then assumed to carry the full structural load on the overlay, and a comparison of the stresses in these elements to ASME Section III Code allowables was performed. A second analysis was also performed, using an ASME Section XI net section collapse approach, with specially developed squations to consider the non-uniform distribution of the observed indications around the circumference of the overlay. Descriptions of both of these analyses are presented in the subsequent sections of this report, along with a summary of the significant results and comparison to ASME Code allowables. Conclusions are then presented as to the acceptability of the weld for continued operation in the event that the UT indications do turn out to be cracks or crack-like defects. SIR-88-020 1-2 DITEGRITY ASSOCIATESINC
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2.0 PROBLEM DESCRIPTION 2.1 Geometry The geometric configuration of the end cap-to-header weldment and as-repaired weld overlay is shown in Figure 2-1. Pertinent dimensions used in the analysis are given in this sketch. 2.2 Ultrasonic Examination Evaluation Ultrasonic indications in the repaired weld overlay were taken from Reference 2. These are summarized in Table 2-1 and 2-2, and are depicted graphically in Figure 2-2., Two types of indications were reported. Twenty-three were called "IGSCC Indications" by t.he examiners, and were believed to be associated with the original IGSCC that the overlay was intended to repair. These indications are in the underlying base material, and are thus assumed to communicate with the inside surface of the pipe. Review of the remaining ligament data in Table 2-1 for these "IGSCC Indications" reveals that none of them had measured depths below the surface of the overlay (i.e., remaining ligaments) which would violate the required thickness for a full structural overlay (reported in Reference 3 as 0.33 inches for this weld). Thus, no further evaluation would be required if these were the only indications present. The remaining fifty-three indications were identified as "Contamination Cracks" by the examiners, because they were restricted to the weld overlay material itself, and did not penetrate into the underlying base material of the pipe, as was the case for the "IGSCC Indications". (The designation "Contamination Cracks" stems from the method used to produce UT test samples at the J. A. Jones NDE Center, in which contaminants were introduced intentionally during the welding process so as to produce crack-like defects.) These latter indications do require SIR-88-020 2-1 M INTEGRITY ASSOCMTESINC
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further evaluation, since it can be seen from Table 2-2 that many of them do have remaining ligaments which violate the minimum required overlay thickness. An observation fror Figure 2-2 is that, in many regions of the overlay, the two types of indications co-exist close to one another, such that to be analytically conservative for structural purposes, one would have to combine them. Thus, the worst case condition of a flaw which progresses all the way from the inside surface of the pipe to a depth which violates the required minimum wall thickness of the overlay must be assumed in the analysis at some locations. As will be seen later in this report, this worst case combination was assumed where appropriate in the analyses. SIR-88-020 2-2 NM INTEGRITY ASSOCIATESIE 2
t i l i Table 2-1 !
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Detailed Listing of IGSCC Indicationc in Repaired Weld Overlay 02A-S10 Circum. IGSCC Location Indication Remaining Indication (inches from Length Ligament Side of Type of No. O Reference) (inches) (inches) Weld Reflector 1 32.4 11.6 1.10 UPST CIRC 2 45.8 4.2 1.30 UPST CIRC 3 0 1.1 1.O DNST CIRC 4 6 5.2 1.0 DNST CIRC 5 58.5 1.5 1.3 DNST CIRC 6 68 3.3 1.1 DNST CIRC 7 2.7 .90 8 3.7
.44 DNST AXIAL
.90 .45 DNST AXIAL 9 4.5 .75 .50 DNST AXIAL 10 6.6 .70 .50 11 DNST AXIAL 11.6 .80 .79 12 21.9 DNST AXIAL 1.05 .76 DNST AXIAL 13 22.3 1.0 .60 14 DNST AXIAL 24.1 .54 .62 DNST AXIAL 15 25.5 .25 .48 UPST AXIAL 16 27.6 .95 .62 DNST AXIAL 17 27.8 .85 .70 DNST AXIAL 18 34 .40 1.2 DNST AXIAL 19 36 .45 20
.82 DNST AXIAL-36.7 1.0 1.3 DNST 21 64.3 AXIAL 1.35 .59 DNST AXIAL 22 68.1 1.25 23 0
.52 DNST AXIAL
.90 .84 DNST AXIAL SIR-88-020 2-3 INTEGRITY ASSOCIATEINC
i Table 2-2 Detailed Listing of "Contamination Crack" Indications in Repaired Weld Overlay 02A-S10 Circum. Contamination Location Indication Remaining Crack (inches from Length Ligament Side of Type of No. O Reference) (inches) (inches) Weld Reflector 1 1.5 .80 .35 UPST CIRC 2 3.0 .90 .24 UPST CIRC 3 7.1 .70 .26 DNST CIRC 4 10.2 .80 .23 UPST CIRC 5 24.1 1.7 .21 UPST CIRC 6 26.3 1.4 .18 DNST CIRC 7 28.1 .60 .25 DNST CIRC 8 29.1 1.1 .31 DNST CIRC 9 71.3 .70 .28 UPST CIRC 10 71.0 .90 .19 UPST CIRC 11 70.1 .80 .20 DNST CIRC 12 26.1 1.1 .25 DNST CIRC 13 27.3 .80 .21 DNST CIRC 14 28.5 1.3 .20 DNST CIRC 15 32 1.3 .16 DNST CIRC 16 32.5 1.3 .33 DNST CIRC 17 33.1 .90 .22 UPST CIRC 18 34.1 .90 .24 UPST CIRC 19 37.2 .90 .28 UPST CIRC 20 38 .70 .21 UPST CIRC 21 38.8 .80 .21 DNST CIRC 22 39.1 .80 .25 DNST CIRC 23 43 .70 .28 DNST CIRC 24 43.7 1.2 .29 UPST CIRC 25 44.7 1.1 .28 DNST CIRC SIR-83-020 2-4 - gg INTEGRITY ASSOCIATESINC
Table 2-2
, (continued)
Circum. Contamination Location Indication Remaining Crack (inches from Length Ligament Side of Type of ((o . O Reference) (inches) (inches) Weld Reflector 26 51 .90 .22 UPST CIRC 27 56.1 .90 .24 UPST CIRC 28 70.1 1.20 .25 UPST CIRC 29 11.0 .80 .28 UPST CIRC 30 11.2 .70 .21 UPST CIRC 31 13.3 1.1 .22 UPST CIRC 32 14.5 .50 .24 UPST CIRC 33 16 .90 .25 DNST CIRC 34 4.0 .90 .18 UPST CIRC 35 40 1.0 .16 UPST CIRC 36 51.5 .80 .12 UPST CIRC 37 52.7 1.0 .22 UPST CIRC 38 1.0 .95 .18 DNST AXIAL 39 2.1 .60 .58 UPST AXIAL 40 3.1 1.0 .56 DNST AX1AL 41 7.3 .80 .52 DNST AXIAL 42 11.3 .90 .28 DNST AXIAL 43 22.6 .90 .52 DNST AXIAL 44 24.8 .90 .51 DNST AXIAL 45 26.5 .90 .58 DNST AXIAL 46 27.9 .95 .58 DNST AXIAL 47 32 .90 .29 CNST AXIAL 48 43.2 .65 .20 DNST AXIAL 49 64.8 .90 .58 DNST AXIAL 50 66.8 .80 .57 DNST AXIAL 51 10.2 .90 .56 DNST AXIAL 52 34 1.0 .21 DNST AXIAL 53 68.1 .90 .55 DNST AXIAL SIR-88-020 2-5 - {,g INTEGRITY AfLMINC
2.3 Evaluation Criteria The basic criterion for this evaluation is to satisfy the 1 appropriate stress limits of the ASME Boiler and Pressure Vessel Code. This twquirement has been satisfied in two different ways for the two analyses performed. For the three dimensional finite element analysis, the applicable criteria are the primary stress limits for design loadings of ASME Section III (Reference 4, Paragraph 11B-3 221) . Application of other ASME Section III stress limits for secondary and peak stresses and for cyclic operation are not de ;raed necessary because of the Obsence of any loading conditions other than internal pressure in thic end cap weldment, ana due to the fact that there are no significant thermal tran icnts in this system. Thus, satisfying the primary stress limitations for the highest stressed element in the model is sufficient to show that all other Section III stress limits are satisfied. i For the not section collapse analysis, the alternative flaw evaluation criteria for stainless steel piping of ASME Sec*. ion XI (Reference 5, Paragraph IWB-3642) were applied. Ti. 'se require that a net section collapse analysis of the piping cross vection, containing the observed flaw indication, show a safety factor equal to greater than that inherent in the original design Code for the component (a factor of 3 for nornal operating and upset conditions). In keeping with standard overlay design practice, as documented in liUREG-0313, Revision 2 (Refercnce 6), the subject weld overlay was installed with carefully controlled weld wire and proced' ires, so as to provide a ductile repair which is highly resistant to further IGSCC. Thus, it is not necessary to consider non-ductile SIR-88-020 2-6 g INTEGRTFY ASSOCIATESINC
i i failure of the overlay, or further propagation of the flaw indications into the overlay as additional criteria for evaluation of the observed indications. 2.4 Applied Loading The only significant loading acting on weld 02A-S10, because of its pipe to end cap configuration, is internal pressure. The system design pressure of 1250 psig was used in the analyses, other loadings such as system operating thermal / pressure transients are not significant for this location. e SIR-88-020 2-7 6 INTEGRITY ATOC3ATESINC
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C4- CE 1 -- 120 h 11 2 l C5 li t l ils C41 l 52 H 11 7 118 l M -- Cl2 cg4 Cl3 38, , 115 l 2nd QU Adit ANT 6:00 3:00 42' 44" 46' 48" $0* 51' 54' 36* 38" 40" C48 12 11 C22 11 C2n C2s Ct2 C3S C25 --- Cl9 C21 C23 C36 C24 9 On 6 no 3rd QU Al)ll AN T fr' 42" 84* ti6" 64* 7na liefereirc 54" 54" 58' 6Q* 122 C49 C$n it) 16 _ 15 121 _- C27 3 mr C2P 12 00 9W 4th QU Al)ll AN T Figure 2-2. Approxirante I,ocations of Ultrasonic Indications in Repaired Weld Overlay 02 A-SIO (I = IGSCC indication; C = "Contamination Crack") M SIR-88-020 2-9 mg
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3.0 ANALYSES 3.1 Three-Dimensional Finite Element Analysis t 3.1.1 Finite Element Model The subject end cap plus veld overlay and a portion of the adjacent ring header pipe were modeled using the Algor Supersap general purpose finite element computer code (Reference 7). The model used for the analysis is illustrated in Figure 3-1. In order to enable modeling of the individual UT indications at discreet locations in the overlay and underlying weldment, a three dimensional modeling approach was used. However, in order to optimize computer memory size and execution time, only one quadrant of the end cap structure was included in the model. Symmetry is implicitly assumed for this quarter model of the end cap. As will be discussed in Section 3.1.3, UT indications from the worst quadrant of the actual overlay were used in this stress analysis model. Thus, it ic conservatively assumed in this analysis that the other three quadrants of the end cap bear the same cracking pattern as the worst quadrant. Inspection of Figure 3-1 indicates that four elements were used to model the original header and end cap material thickness and another four elements to model the weld overlay material thickness. Thus, there are a total of eight elements through the wall thickness in the overlay region. A total of 2170 nodal points and 1584 eight-node brick elaments were used in the finite element model. Sketches showing each of the eight element layers of the finite element model, including element numbers are illustrated in Figure 3-2 through 3-9, with the inner most element layer being layer No. 1. SIR-88-020 3-1 STRIJCFtJRAL INTEGRITY ASSOCIATESINC
1 3.1.2 Loading Conditions
. Since the end cap is at a free end of the recirculation piping system, the only applied load is an internal pressure of 1250 psig, which corresponds to the design pressure of the piping system.
This loading was input as an internal pressure on the inside surface of layer 1 in the finite element model. 3.1.3 Modeling of UT Indications By examination of the UT indication pattern shown in Figure 2-2, it was concluded that the 3 o' clock to 6 o' clock location (circumferential 1ccation 18 inch to 36 inch from o reference) is the quadrant with worst UT indications. This quadrant contains the most of both "IGSCC indications" (10) and "contamination cracks" (17), and two of the contamination crack indications in this quadrant were reported to have remaining ligaments of 0.16 inch and 0.18 inch. The symmetry assumption inherent in the model automatically implies that the same cracking pattern exists in the other three quadrants. Since only primary stress is of interest in this finite element analysis (see Section 2.3 of this report for details), elements which contains UT indications can be zerced-out by setting their Young's moduli to a very small number (i.e. E = 30 psi for elements cracked versus E = 30,000,000 psi for uncracked elements). Such a small Young's modulus, in ef fect, eliminates the load carrying capacity of the zerced-out elements. Other conservatisms inherent in this numerical approach are as follows: IGSCC indication No. 1, with remaining ligament of 1.1 inch was assumed to exist 360* around the circumference, e Some IGSCC and contamination crack indications were combined since they fell in the same element. This had the effect of having some regions in the model which SIR-88-020 3-2 M INTEGMTY ASSOCIATESINC
e-l Were zeroed-out all the way from the ID surface to
. within approximately 0.15 inch of the OD of the overlay.
The zerced-out elements in the finite element model are shown as either shaded area (for IGSCC indications) or cross-hatched areas (for contamination crack indications) in Figures 3-2 to 3-9. 3.1.4 Finite Element Results for Uncracked Structure As a check of the finite element model, a finite element analysis for the uncracked end cap plus overlay was performed (i.e., no elements were zorced-out}. In this check run, an internal pressure of 1250 psig was ' applied and all the elements in the finite element model have a Young's modulus of 30,000,000 psi. Resulting contour pM.s of stress intensities (maximum-minimum principal stress) from this check run are illustrated in Figure 3-10. It is seen from this figure that the predicted stress in the straight pipe portion of the end cap structure is about 12,342 psi, which is very close to the thin shell solution of (pR/t) = 12,500 psi. As expected, the stresses are lower than these nominal stress levels under the overlay, and all stresses are essentially uniform around the circumference. Also, the average predicted stress in the sphere part of the end cap is very clcse to the thin shell solution of (pR/(2t)) = 6,250 psi. Therefore, the finite element model was deemed to be adequate and to yield sufficient accuracy for this problem based on these uncracked model results. 3.1.5 Finite Element Results for Cracked Structure Figures 3-11 to 3-18 present contour plots of resulting stress intensities from the analysis of the model with the cracked elements zerced out. Results are presented in these figures for each of the eight element layers, beginning from the innermost layer 1. The maximum resulting stress intensity seen in these SIR-88-020 3-3 DITEGRITY ASSOCIATESINC
4 figures is 22,584 psi, in layer 2 (Figure 3-12) and can be seen to occur near one of the regions with the deepest combination of IGSCC plus "contamination crack" indications. The ASME Code acceptability of these stress results is discussed later, in , Section 4 of this report. 3.2 Net Section Collapse Analysis A net section collapse evaluation was performed for the end cap weld overlay 02A-S10, using a variation of the approach used in ASME Section XI, Reference 5. The UT examination of the overlay repair of the end cap weld in the current outage presents a new challenge to the conventional approach of net section collapse evaluation as adopted in Reference 5. Due to the considerable number, 76, of observed UT indications, and the large differential among crack depths and lengths, it was desired to evaluate the indications as multiple, discreet cracks instead of conservatively combining them into a single crack of enveloping length and depth, as is done in the code net section collapse approach. 3.2.1 Formulation of Multiple-Crack Net Section Collapse Equations The approach used to establish the crack depth / length limits in Reference 5 for austenitic steel piping is derived for a single circumferential surface flaw, rectangular in shape and assuming plastic collapse stress distribution, Reference 8. Multiple flaws can be lumped to form a single flaw if they are close in proximity. Due to the large number of indications observed in the weld 02A-S10, it la judged to be too conservative to consider it as a single 360* flaw with the deepest observed depth as the crack depth for net section collapse evaluation. Therefore the formulation as presented in Reference 8 is extended and delineated in this section to the case of multiple cracks. SIR-88-020 3-4 S11RM,'ITTRAL INTEGRITY ASSOCIATESINC
Consider a degraded pipe section with multiple rectangular cracks as'shown in Figure 5.1. Let d i be the crack depth of crack i in the degraded section, (amin)1 and (amax)1 be the starting and ending angle of crack 1, measuring in the counterclockwise direction. Using thin shell theory, the total pipe wall area A t can be calculated as 2rRmt (1) where R ,= mean pipe radius t = pipe wall thickness The pipe area at the degraded cross section Ad is written as N A t" 5"max - om in)1 d i (2) i=1 where N = number of cracks Let F and M be the remote axial force and bending moment from the degraded section. At plastic collapse failure, the remaining pipe area in the degraded section is fully plastic and the degraded section forms a plastic hinge. Failure occurs where the stress in the degraded section reaches the material flow stress O g. The limiting moment M for the multiple cracks section at the incipient of failure can be determined by the following equation: 2r M= vf t ( r cos(a-6) +6)da 0 N
-(amax)1
-{i=1 '("min)i a f dy( r cos(a-8) +6)do (3)
SIR-88-020 3-5 DrrEGRITY ASSOCWESINC
r *: , where N = number of crack in the degraded section
. r = pipe mean radius 6 = distance of displaced neutral axis in the degraded section 8 = rotation of bending axis The eccentricity effect of the axial force (F6) is also included in the calculation of the limiting moment. The inclusion of angle 8 in the formulation allows the examination of the effect of the orientation of the limiting bending moment on the multiple cracks.
The remote axial stress Pm is expr.ssed as: P" Atension = 0.5((1 + ,f
) A T -AD) (4) where A T
= t tal Pipe area Atension " Pipe area taking tension A = pipe area taking compression D
3.2.2 Verification of Multiple Crack Net Section Collapse Since no other results and data are available for the multiple crack geometry, verification of the above, multiple crack series formulation has been performed by simulation of a single flaw ar a continuous multiple flaws with the same crack depth. The results were checked against the results from the conventional Section XI approach using pc-CRACK, Reference 9. Five verification cases were selected. They are as follow:
- 1. 360* circumferential crack, 0.5 inch depth
- 2. 2 circumferential cracks, 0-180* and 180*-360*
0.5 inch depth SIR-88-020 3-6 ,
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- 3. 36 cracks, each 10*, 0.5 inch depth
- 4. 2 cracks, 0* -4 5* and 315*-360*, 0.5 inch depth
- 5. 45* crack, 0.5 inch depth other inputs used in the verification problems are:
Radius = 22 inches Thickness = 1.0 inch tension stress = 8.5 ksi Flow stress = 50.7 ksi Th6 same problems were analyzed using pc-CRACK and the results are presented in Table 3-1. The results from the two approaches compare favorably, thus verifying the multiple crack formulation. Table 3-1 Multiole Cracks Dc-CRACK Bending Crack Bending Crack Stress Depth Stress Depth Case (ksi) (in) (ksi) f in) .. . 1 27.90 0.5 27.90 0 .4381 2 27.90 0.5 27.90 0.5381 3 27.90 0.5 27.90 0.5381 4 46.44 0.5 46.44 0.499 5 54.21 0.5 54.21 0.5454 3.2.3 Evaluation of Weld 02A-S10 The indications in the end cap weld have been bounded conservatively by 58 circumferential cracks. These 58 cracks take up the full 360* of the pipe weld. Axial offset among UT indications are neglected and assumed to be in one plane section. SIR-88-020 3-7 EC
Regions where no indications were ~ identified by UT were assumed to have a crack depth of the shallowest UT indication. The subsurface cracks (or the contamination cracks) in the weld overlay were assumed to be surface cracks extending to the pipe inside surface. The resulting composite crack is shown in Figure 3-19, in the form of crack depth versus circumferential distance l measured from the o' (top) of the pipe. The pipe wall thickness ! is 1.7 inches, the sum of the original pipe wall and the weld overlay thickness. The outside diameter is 23.2 inches. Flow stress of the pipe material was taken as 50.7 ksi. Assuming no bending loads acting on the end cap, there will be no bending stress in the end cap. The loading in the end cap is due to pressure only. Therefore, the formulation in Section 3.2.3 is iterated by increasing the nominal axial stress until the bending stress is zero or the limiting moment M is negligible. Therefore, a margin of safety can be estimated when compared the iterated axial stress to the axial stress due to the operating pressure. 3.2.4 Analysis Results Using the formulation in Section 3.2.1, the maximum nominal axial . stress that can be sustained is calculated to be approximately 19 ksi. Using the equation for thin wall cylinder, the nominal axial stress in the weld overlayed pipe due to a pressure of 1250 psig is calculate ( as: g . PR , 1250 (11.6) - = 4.27 ksi 2t 2 (1.7) Thus, the safety margin in the end cap due to the pressure is safety margin = " 4' 4 6 ' GIR-98-020 3-8 m. :ea H-e yr;<
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1 a- j 4.0 DISCUSSION OF RESULTS 4.1 Finite Element Analysis The results of the finite element analysis discussed in Section 3.1 are reviewed and compared to ASME Code allowables in this section of the report. Applicable ASME Section III allowable stress values for this stainless steel weldment at operating temperature (550* F) are as follows (Reference 4): Stress Condition Allowable Stress Primary Membrane S m (16.9 ksi) Primary Membrane plus Bending 1.5S, (25.4 ksi) Primary plus Secondary 3.0S, (50.7 ksi) Primary plus Secondary + Peak Usage Factor < 1.0 Review of the stress intensity contour plots of Figures 3-11 to 3-19 indicates that in all layers, the maximum resulting stress in the cracked weldment model is less than the above ASME Code limit for primary membrane plus bending stress of 25.4 ksi. This is true, even though the localized nature of these maximum stress levels would justify using one of the higher stress allowables such as primary plus secondary or peak. Owing to this favorable comparison to the Code primary stress limit, no further detailed analysis of the stress results is required. l It is noteworthy that even if the above primary stress limits were exceeded locally, they could be shown to be satisfied by averaging elemental stress around the circumference, and through the thickness, and then extracting membrane and bending stress components for comparison to Code allowables. In that case, the l secondary and peak stress limits would also have to be satisfied I 4-1 l SIR-88-020 6 DITEGRITY ASSOCIATESINC
U i l i on a local basis. Such additional detailed evaluation of the
. results was not deemed necessary owing to the low levels of even the localized stresses in the cracked structure model.
4.2 Net Section Collapse Analysis The process of zeroing-out elements used in the finite element analysis of Section 3.1 does not fully account for the cracktip stress concentrating effect which could be prosant if the material were really cracked as reported by the UT inspections. The degree of refinement of the finite elemenu grid dictated by computer size and time limitations was insufficient to resolve the very steep stress gradients which would occur in the presence of the cracktips. However, for a highly ductilo material such as Type 304 stainless steel, and associated weld metals, the finite element modeling approach used gives an accurate representation of the potential impact of the observed UT indications on the load carrying capacity of the joint, and can be used to illustrate, conservatively, that ASME Code primary stress limits are satisfied. To further address the potential crack-like nature of the indications, an ASME Section XI net section collapse analysis was also performed, with specially developed equations which consider the non-uniform distribution of the observed crack indications around the circumference, as discussed in Section 3.2 of this report. The applicable ASME Code criteria for this evaluation are those of paragraph IWB-3642 of Reference 6, which state that the piping is acceptable for continued operation if safety factors of 3 can be shown for normal operating conditions, and 1.5 for emergency and faulted conditions. The analysis results presented in Section 3.2.4 of this report indicate that a safety factor of 4.46 is maintained for an internal design pressure loading of 1250 psig. This exceeds the pressure in the system for all normal operating conditions, and SIR-38-020 4-2 INTEGRITY ASSOCIATESINC
r , since there are no other applied loadings on the pipe, the emergency and faulted condition loadings will not exceed this stress level either. Thus the ASME Section XI requirements are satisfied by a comfortable margin by the net section collapse analysis, even if the UT indications are assumed to be cracks or cracklike defects. l l 1 l l l SIR-88-020 4-3 m INTEGRITY ASSOCIATESINC
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5.0 CONCLUSION
S Conservative analyses have been performed to evaluate the acceptability of 22-inch recirculation header-to-end cap weld overlay 02A-S10 at Quad Cities Unit 2. The analyses addressed a large number of UT indications observed in this overlay after repairs performed during the current outage . The analyses were performed in parallel with metallurgical examinations conducted to confirm the ultrasonic results. The analyses showed that all applicable ASME Code limits are satisfied, even if the UT indications are conservatively assumed to be cracks or cracklike defects. The analyses consisted of a three dimensional finite element model of the weld plus weld overlay, with the structural load carrying capacity of the elements containing UT indications Zerced-out. This portion of the evaluation demonstrated compliance with ASME Section III primary stress limits, aven with several conservative interpretations of the UT findings. The evaluation also included an ASME Section XI net section collapse analysis, using specially developed equations to address the non-uniform nature of the observed indications. This analysis also incorporated several conservative assumptions, and showed that the weldment satisfies ASME Section XI requirements for such an evaluation by a comfortable margin. Thus, it has been demonstrated that continued operation of the plant with this weld overlay, considering the the worst possible interpretation of the post-repair ultrasonic indications, does not represent a reduction in design basis safety margins of the recirculation system piping. SIR-88-020 5-1 6 INTEGRITY ASSOCIATESINC
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6.0 REFERENCES
- 1. Nutech Report CEC-73-205, Rev. O, "Fracture Mechanics Analysis of Weld 02A-S10, Quad Cities Nuclear Power Plant, Unit 2", June, 1987.
- 2. GE Nuclear Energy Indication Resolution Sheet No. R-236, ;
QCNPS Unit 2, Recirc. System Weld No. 02A-S10, May 31,1988. I
- 3. Nutech Report CEC-73-203, Rev. O, "Evaluation and Disposition of Flaws at Quad Cities Nuclear Power Plant, Unit 2 (1986 Outage)".
- 4. ASME Boiler and Pressure Vessel Code, Section III, "Rules I for Construction of Nuclear Power Plant Components", 1983 Edition with Addenda through Winter 1985. !
- 5. ASME Boiler and Pressure Vessel Code, Section XI, "Rules for Inservice Inspection of Nuclear Power Plants", 1983 Edition with Addenda through Winter 1985.
- 6. NURFG-0313, Rev. 2, "Technical Report on Material Selection and Processing Guidelines for BWR Coolant Pressure Boundary Piping", U.S. Nuclear Regulatory Commission, January, 1988.
- 7. ALGOR Supersap Reference Manual, ALGCR Interactive Systems, Inc., Pittsburgh, PA, November 24, 1986.
- 8. Section XI Task Group for Piping Flaw Evaluation, ASME Code, "Evaluation of Flaws in Austenitic Steel Piping", Journal of Pressure Vessel Technology, August 1986, Vol. 108.
- 9. pc-Crack User Manual, Version 1.2, March, 1987.
l l i SIR-88-020 6-1 INTEGIUTY ASSOCIATESINC
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