ML20095L134
| ML20095L134 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/30/1992 |
| From: | Woottten M WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML19354F333 | List: |
| References | |
| SG-92-04-023, SG-92-4-23, WCAP-13327, NUDOCS 9205060240 | |
| Download: ML20095L134 (128) | |
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V/ESTINGHOUSE CLASS 3 WCAP 13327 SG-92-04-023 North Anna Unit 11992 Steam Generator Operating Cycle Evaluation April 1992 Approved by:
. J. Wootten, M ager S am Generator T chnology & Engineering WESTINGHOUSE ELECTRICCORPORATION Energy Systems Business Unit Nuclear Services Division P.O. Box 355 Pittsburgh, PA, 15230 0355 C 1992 Westinghouse Electric Corporation All Rights Reserved
- TABLE OF CONTENTS Sftc!!on .Tlite Eaaft
1.0 INTRODUCTION
11 2.0
SUMMARY
AND CCNCLUSIONS 2-1 2.1 Overall Conclusions 2-1 2.2 Summary of Principal Results 2-3 3.0 EDDYCURRENT DATA REVIEW 31 3.1 Inspection Summary 3-1 3.2 Tube Plugging Summary 3-1 3.3 WEXTEX Indications 3-2 3.4 TSP C:rcumferentialIndications 3-5 3.5 TSP Axialindications ' 35 3.6 Multiple Circumferential Indications (MCI's) 36 3.7 Potential Combined Axial and CircumferentialIndications 3-7 t
3.8 RPC Resolution Considerations 3-7 3.9 WEXTEX Circumferential Crack Growth Rates 39 3.10 Tube Support Plate Circumferential Crack Growth Rates 3 10 3.11 Inspection Transient Considerations 3 10 i
j 4.0 OPERATING CYCLECCNSIDERATIONS 41 l 4.1 Planned Operating Conditions for the Remainder of 41
( Current Cycle 4.2 Comparison of Current and Prior Operating Parameters 41 4.3 Relative Corrosion Rate Modeling 42 4.4 Relative Corrosion Rates Between First and Second Half 43 of Current Fuel Cycle 5.0
SUMMARY
OF CRACK SIZES SATISFYlNG REG. GUIDE 1.121 5-1 5.1 Burst Tests for Multiple Circumferential Cracks 5-1 5.2 Fatigue Crack Growth Rate Tests for Multiple S-2 Circumferential Cracks i
TABLEOFCONTENTS Section Ii!!ft E202 5.3 Acceptable Single Crack Sizes for Tube Burst 5-2 5.4 Acceptable MCI Crack Conditions for Tube Burst 5-3 5.5 Acceptable Combined Axial and Circumferential Crack 5-4 onditions 5.6 Acceptable Single Crack Sizes, for Tube Vibration 5-6 6.0 TUBEINTEGRITY ASSESSMENT 6-1 6.1 - Crack Distributions for Second Half of Current Fuel Cycle 6-1 6.2 WEXTEX Tube Burst Assessment 63 6.3 TSP Circumferentiallndication Tube Burst Assessment 6-4 6.4 TSP AxialIndication Tube Burst Assessment 65 6.5 Potential TSP Mixed Mode Assessment 6-6 6.6 WEXTEX Tube Vibration Assessment G6 6.7 TSP Tube Vibration Assessment 6-7 6.8 MCI and Mixed Mode Tube Vibration Assessment 68 7.0 SLB LEAK RATE ANALYSES 71 7.1 Crack Distributions for SLB Leak Rates 7-1 7.2 Leak Rate Analysis Methods 7-2 7.3 Leak Before Break (LBB) Capability 7-4 7.4 Limiting Crack SLB Leak Rate 78 7.5 SLB Leak Rates for Crack Distributions 79 7.6 Conclusions - 7 10 l
l l
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1.0 INTRODUCTION
This report provides a tube integrity assessment to Reg. Guide 1.121 criteria for end-of Cycle 9 conditions of the North Anna Unit 1 steam generators. The 1992 mid-cycle inspection results are adjusted to obtain expected end-of-Cycle 9 (EOC 9) 1 maximum crack sizes and crack distributions. Considerations of reduced Thot and the demonstrated effects of an initial RPC Inspection (called an " inspection transient') on subsequent cycle indications are used to adjust the mid cycle inspection results to EOC 9 conditions. Assessments are provided to demonstrate EOC-9 margins against tube burst and tube vibration induced crack propagation. Potentialleak rates under SLB conditions are also evaluated for the EOC-9 crack distributions, The 1992 mid-cycle inspection program included full length testing with bobbin probes of 100% of the available tubes in each steam generator, RPC examination of all hot leg WEXTEX transitions and all hot leg TSP intersections, and RPC testing of all Row -
2 U-bends. The results of the inspection program are evaluated along with planned changes in operating conditions for the remainder of the current fuel cycle to determine the expected end of-cycle (EOC) crack distributions, including the number and sizes of circumferential cracks. The expected circumferential crack distribution is based on operating cycle length considerations, planned changes in operating conditions for the second half of the fuel cycle, and inspection transient considerations. Although the first and second halves of the current fuel cycle have approximately the same number of effective full power days of operation, a reduction in Th ot of 3'F for the second half of the cycle and a coastdown to lower power levels during the last four months of the cycle are expected to result in a reduction in the number and size of circumferential indications at the end of the current cycle. Furthermore, based on industry experience with inspection transients, the number and sizes of T .iP circumferentialindications at the end of the current cycle are expected to be smaller than the indications observed during the first 100% RPC inspection of the hot leg TSPs performed during the1992 mid cycle inspection.
The WEXTEX transition and TSP circumferential cracks can be considered ' strong" for tube burst considerations in that throughwall crack angles of [ )b,c meet three times normal operating pressure differentials and throughwall angles greater than
[ ]b,c meet SLB burst pressure conditions. North Anna inspection practices and 11
plugging of all circumferential cracks reduce the likelihood of tube burst at SLB conditions to extremely low levels. During the 1992 inspection program, two cracks which exceeded the 226' crack angle for 3AP burst capability were found. Thus, one of the principal objectives of this report is to demonstrate that the EOC circumferential crack distributions will satisfy 3aP and AP SLBburst pressure requirements. The EOC 9 crack distributions are developed from the 1992 mid-cycle 100% RPC inspections of the hot leg WEXTEX and TSP regions. Evaluations of multiple circumferential cracks and mixed mode (axial and circumferential) cracking are performed to demonstrate tube integrity for the EOC-9 conditions, The eva.uation of circumferential cracks also includes an assessment of the potential for crack propagation by tube vibration under normal operating and SLB flow conditions.
Tube vibration assessments are particularly sensitive to throughwall crack angles, as small ligaments such as 10% remaining wall or ligaments between microcracks add substantially to the tube stiffness and can effectively eliminate tube vibration concerns.
The RPC data provide very limited data on crack depth and generally overestin. ate the length of the throughwall portien of the indicated circumferential crack length. The tube vibration analyses to assess potential propagation of corrosion cracks are most confidently performed for assumed throughwall cracks. Thus it is necessary to bridge the information gap between RPC crack angles and potential throughwall crack angles which, if present, would be considerably smaller than the RPC crack angles. Based on the extensive North Anna inspections for circumferential cracks, it is reasonable to expect little throughwall crack penetratic , .he remainder of Cycle 9, particularly with operation limits on leakage of 50 g' ., However, it is the goal of this evaluation to provide conservative but realistic asses.. ients of potential crack propagation due to tube veration. To this end, assessments of the potential for vibration induced crack propagation in the WEXTEX and TSP regions are made using the tube vibration analysis methodology of WCAP 13034.
In the very unlikely event that throughwall or near throughwall cracks may be present that do not leak during normal operation but may leak during a postulated steam line break, SLB leak rates are calculated based on end-of cycle crack distributions. The SLB leak rates for axial and circumferential cracks are calculated for the expected EOC crack distributions, which are based on reductions in the number and sizes o,'
indications expected at the EOC-9 when compared to those found in the February 1992 1-2
mid-cycle insnoctions. In addition, bounding SLB leak rates are calculated based on the crack distributions found in the 1992 inspections.
Section 2 of this report intogratos the detailed results of this report into a summary tube integrity assessment and provides the overall conclusions of this evaluation. The 1992 inspection results for North Anna Unit 1 are described in Ser. tion 3. Changes in oporating conditions are avaluated in Section 4 to dotormino relative corrosion rates between the first and second ham of tho fuel cycle. A summary of crack sizes satisfying Reg. Guide 1.121 is presented in Section 5. The tubo int 90rity assessments for tube burst, LBB, and vibration induced crack propagation are developed in Section 6. -
Section 7 sunrnarlzes the steam line break leak rate analyses.
c 13 l
2.0 SUWAARY ANDCONCLUSIONS During January hbruary 1992, mld. cycle inspections of the steam generator tubes were performed for Cycle 9 of North Anna Unit 1 The eddy current inspe::tions included l RPC testing of 100% of the hot log TSP intersoetions and WEXTEX expansion transitions.
Tne number of tubes piugged for indications at the TSPs was slanificantly higher than the number plugged in previous inspections. The increase in the number of TSP 'ndications i and the large crack angles observed have been attributed 13 the initial application of RPC probes to all of the HL TSP Intersections;ilis judged that a significant number of the indications dotected in 1992 existed during previous inspections, but were not detected due to denting at the TSPs until the uso of the more sensitive RPC probes. This report evaluates the acceptability of continued operation for the romainder of Cyclo 9, and updates the Cyclo 9 operating cycle ovaluation performed in WCAP 13034. Cycle 9 prior to the mid cycle inspection included 254 effectivo full power days (EFPD) at i essennally 100% power. The romalnder of Cycle 9 includes 252 EFPD with about 2/3 of the cycle at 95% power and the remaining time in a coastdown mode at lower power levels.
2.1 Overall Conclusions s
From the results of this evaluation. it is conclw e " .
Operation of the North Anna Unit 1 SGs to the end of the second half of Cycle 9 is acceptable and Reg. Guide 1.121 criteria will be met at the end of the cycle. This conclusion is based on the expected fewer and smallt,r indications at EOC.9 compared to the m'*. . inspection results, which satisfy Reg.s Guido 1.121.
a Based on the reduced power level and four rnonth coastdown period planned for the second half of Cycle 9,it is expected that there will be at least a 7% roouction e in both the the number of newly initiatad cracks and 'ho growth rates of existing cracks which were undetected during the 1992 mid cycle inspections.
- The use of RPC probes to 'nspect 100% of the hot leg TSP intersections in all three SGs during February 1992 produced a significant increase in the number of tubes plugged for indications at the TSP elevations. Industry experience has shown that 21
l during the next inspection following the inillat application of more restrictive eddy current analysis guidelines or the use of the mere sensitive (at tube diametral changes such as donted intersoctions) RPC probo, thoro is a significant decrease in both the number and size of indications observed. Based on such inspecGon transients for the WEXTEX regions of the North Anna steam generators and for tubosheet and TSP regions of other oporhting plants, the number of axial and circumferentialindications at th9 iSPs at the end of the current cycle are expected to be reduced to about 2/3 of the mid-cycle inspection resu'ts. The inspection transient is also expected to result in a reduction of about 8% in the circumferential ;
crack angles at the TSPs. The 8% reduction in crack angles is based on North Anna Unit 1 experience for WEXTEX indicatior.s.
The 7% reduction in crack growth from the reduced power conditions would apply to both growth in depth and in crack angle. However, the RPC probes can only measure crack angle with confidence. Thus, the 7% reduction is only 9pplied to the crack '
angle growth rate. The largest crack growth rates found at the TSPs woro 39* and 45' for the first half of tne current fuel cycle. A 7% reduction in the largest crack growth rates would reduce growth by about 3*. Honce, the reduced power operating conditions are expected to result in a reduction in crack angles of about 3'in comparison to those measured in the 1992 mid cyclo inspection. This reduction is applied for circumferentialindications at the WEXTEX transition and at the TSPs.
The largest WEXTEX circumferentialindication observed in the 1992 mid cycle -
inspect!on was 199', which is less than the 226' and 297* required to moet 3aP l
bu;st capability Sr throughwall and segmented cracks, respectively. Two circumferentialindications at the TSPs had overall crack lengths (240* and 239')
which exceeded the 226* simiting crack angle for 36P for a uniform, throughwall crack. However, the deepest parts of these cracks metsured 134' and 96', much loss than the 226* limiting throughwall crack angle. Furthermore, applying the 8% >
- and 3* reductions in TS9 crack ang!es expected for the end of the current cycle results in maximum expected TSP circumferential crack angies of 218' and 217'.
Thorefore, all EOC circumferential indications are expected to meet the 3aP burst capability guideline of Reg. Guide 1.121.
22
, y The TSP intersections of the North Anna Unit 1 SGs have experienced denting to the extent that axlal cracks within the TSP thickness aro judged not to open, even during a postulated SLB event. The denting would prb Jnt the TSPs from moving relatNo to tha tubes during a SLB. Therefore, only axlat cracks extending above or below the TSP edges are accounted for in the tube integrity and SLB evaluations. The
- 'rinx! mum axial crack longth measured outsido the TSPs during the 1992 mid cycle inspection was 0.49 inch; this is less then the crack length of 0.72* which meets 3AP burst critoria for the segmented crack morphology. The axial crac k leagths from the 1992 mid-cycle inspection are conservatkely applied for the expected EOC crack distribu~. ion. Therefore, all EOC axialIndications are expected to meet 3AP
~
burst capability for Reg. Guido 1.121.
- Potentialloakage at SLB conditions can bo bounded by leakage from the estimated crack distributions at EOC conditions. The SLB leak rate for the expected EOC crack distribution is [ ]b.c. The bounding SLB leak rate, assuming the crack distributions from the 1992 inspection apply for the EOC,is calculated to bo l jb.c.
The administrative leak rate limit of 50 gpd provides large leak before break margins against tubo burst t t 3AP N .O. for assumed throughwall cracks and against burst at SLB conditiont,. even for leak limiting segmented crack morphologies.
Corrosion crack propagation duo to tube vibration is not expected for the range of circumferential crack sizes expected at the end of the current cycle. Even in the very unlikely event that crack propagation should occur, the North Anna leakage monitoring system and administrative procedures would provide detection and plant shutdown prior to a large leakage event approaching tube rupture. The North Anna leakage monitoring proceduros would initiato plant shutdown following leakage exceeding 50 gpd in a single SG; plant shutdown would be (cmpleted within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
2.2 Summary of Prinopal Results The January February 1992 mid-cycle inspection at North Anna Unit 1 encompassed fulllongth testing with bobbin probes of 100% of the available tubos in each steam generato.. The bobbin data was used to evaluate the condition of all free longth tubing.
23
. _ _ _ _ _ _ _ _ . _ . _ _ . _ . , __ _ ,_ _ _-. - _ . _ , . ~ .
including all U bends with the exception of Row 2. The Row 2 U bends were inspected with RPC probes.100% of the available hot leg WEXTEX regions were also inspected with RPC probes; this marked the second time 100% RPC inspection of the hot leg tubosheet region was performed at North Anna Unit 1, the first being in Spring 1991 prior to Cycle 9 oporation. During January 1992, comparison testing of the 8x1 arrayed pancake probo and the RPC probe on indications at the TSPs showed that in most cases,it was necessary to make conservative and probably false 8x1 calls to achlove more than 90% detect!on of the signals identified by RPC testing. This was attributed to the inability to discriminate effectively between liftoff signals at the dented TSP Intersections and crack indications at the TSP edges. Therefore, an inspection program encompassing RPC testing of 100% of the TSP intersections was undortakon. This represented the firt,t application of 100% RPC Inspection to the TSPs at North Anna Unit 1.
Table 21 summarizes the tube plugging summary for the January February 1992 inspection program. Thrt e tubos were plugged for free span bobbin indications. 36 tubes woro plugged for axial or circumferential RPC indications in the WEXTEX transition region. 469 tubes were plugged for RPC indications at the hot log TSP intersections.19 tubes wore plugged proventively.
Of the 36 tubes plugged for WEXTEX indications,31 were classified as circumferential cracks and 5 were classified as axial cracks. These indications were similar in appearance and EC characteristics to it.dications observed in prior inspections. The crack morphology is predominantly PWSCC. Tube pulls in prior outages have shown the cracks to be comprised of numerous microcracks with aspect ratios (length / depth) of 4/1 to 6/1, separated by nearly full depth ligaments. The segmentation of the WEXTEX cracks enhancos the strength and stiffness of the tube with recpoet to an entirely throughwall crack; this segmentation is accounted for in the assessment for vibratica induced crack propagation (Section 6.6) and the SLB leak rate analyses (Set, tion 7.0).
A total of 212 tubes were identified with circumferential crack indications at the TSP elevations. The phase angles observed in the 1992 TSP circumferentlal crack population are consistent with the behavior observed in prior inspections, and are indicativo of ODSCC. Pulled tube examinations during the 1991 inspection showed that '
60% of the total RPC are length was throughwall. A similar crack morphology is 2-4
i expected for the 1992 TSP eircumferentiat indications. Since the detectable threshold i depth for RPC probes is about 50% depth for OD circumferential cracks, the model for '
TSP circumferential cracks has 60% of the total RPC measured crack angle as though ;
wall and the portion of the tube circumference apparently free from degradation har.
50% deep undetected cracking. While this modelis applied for SLB leakage analysis, '
tube buts' capab!!ity for EOC 9 is conservatively demonstrated assuming the total RPC angle is throughwall. !
A total of 286 tubes were identified with axial cracks at the TSP elevations. Almost half of these indications were at the first TSP, and 77% were at the first two TSPsl this !
distribution was similar to those found in earlier inspections when dilection was accomplis 6 d by bobbin probes. The dominant crack morphology of the TSP axial indications is PWSCC, based on tube pulls performed it' "195 and 1987. The TSP PWSCC axlal cracks exhibit multiple initiation sites with - . erous microcracks comprising the macrocrack lengths identified in the GPC inspection. As observed in the WEXTEX PWSCC circumferential cracks, the microcracks comprising the TSP axial cracks exhibit aspect ratios of 4/1 to 6/1 and are separated by nearly full depth ligaments. Due to the presence of denting at nearly all of the TSP intersections, the axlal cracks witnin the dented TSP crevicos are not expected to opon up, ovon during '
postulated SLB conditions; the denting would keep the TSPs from moving relative to the :
tubes during a postulated SLB event. For the tubo burst and steam lino break leak rate analysos, only the portions of the ar.lal cracks ortond;ng above or below the edges of the TSP are considered. All of the axialindications detected at the TSP elevations have been plugged.
The largest WEXTEX circumferential indication, measuring 199', was observed in R25C36 of SG B. Throughwall crack angles of up to 226' and segmented crack angles of up to 297* are shown to meet the 3aP burst capability requirement of Reg. Guide 1.121.
Therefore, all of the WEXTEr' indications observed satisfied the 3AP burst requiroment.
Two circumforentiat indications at the TSP elevations had RPC crack angles which exceeded 226' for a uniform, throughwall crack; these tubes were R34042 2H (240')
and R18C39-1H (239'), both in SG A. The deepest. parts of these cracks measured
-134' and -96', respectively. Hence, the potential throughwall port'ons of theso
- cracks were less than the 226' throughwall angle required for 3AP burst capability.
l 2-5
__- __~____ _ -_- _ .- - _ _ ..-. - - _- _. - , -, . _ . .
Based on operating cycle considerations and inspection trans'ent considerations, the largost crack angles for the crack distribution at the end of the current operating cycle i would be smaller than found at the last inspection and are expected to meet 3aP burst capability even if the RPC crack angles are very conservatively assumed to be Uniformly throughwall cracks. These consdstations are discussed below.
During the second half of the current fuel cycle, the North Anna Unit 1 steam generators '
will be operated at reduced lead (95% power) with a lower primary coolant temperature than during the first half of the cycle. During the final four months of the fuel cycle, the plant will be in a coastdown modo at progressively lower power levels to -
conserve fuel. The durations of the first half of the current cycle (254 EFPD) and the second half of the cycle (252 planned EFPD) are approximately the same. Based on this data, the tolative corrosion rates during each half of the current fuel cycle have been evaluated, Assurning that the operating chemistries for the two periods are temparable.
the variation in relative corrosion for normalized corrosion activation energies ranging frem 160 kcat/ mole to 50 kcaPmole has boon determinod. For the highest activation onbrgy evaluated, the tots! corrosion during the second half of the fuel cycle is expected to be 63% of that observed during the first half of the cycle. Using the lowest, most conservative activation energy evaluated, the total corrosion during the second half of the fuel cycle is expected to be approximately 93% of that observed following the first half of the cycle, The variation in relative corrosion based on the most conservative activanon energy is applied for the EOC crack distribution, even though the reduction in the number of indications with TSP elevation would imply a larger activatior' energy.
Hence, the reducca power and EOC coastdown for the second half of the current fuel cycle are expected to lead to at least a 7% reduction in the number of newly intitiated cracks and in the growth rates of existing cracks which were undetected during the mid Cycle inspection. For the circumferential cracks at the WEXTEX region and TSPs, the 7%
reduction in growth rates would result in a reductinn of the largest crack angles by
- about 3',
in addition to a reduction in the sizes and number of indications due to op < iting cycle considerations, the TSP crack distribution for the end of the current cycle is also expecied to show a reduction in the number and size of TSP indications based on the first time application of RPC testing at 100% of the hot leg TSPs in 1992. Industry experience has shown that during the first use of revised EC analyst guidelines or the 26
-- . _. - . . - . - . ., . - ~ - - - _ . - ... -- - - -- - -.-
initial application of the more sensitive RPC probe to 100% of the tubesheet region, the number of indications increases subriantia0y over the indications observed during previous inspections. During the first inspection after the initial application of the revised analyst guidelines or more senshive probe, both the number and site of indications decrease substantially when compared to the previous outage. This phenomenon is called an ' inspection transiert'. Section 3.11 presents inspection transient data for the tubesheet regions of five plants, including North Anna Units 1 and 2, and for the "o regions of three plants, including North Anna Unit 1. Based on the data presented, it is judged that many of the tubes plugged for indications during the inspectiore translents had cracks which existed during prior inspections, but were not deteCled until the use of the more sensitive probe of revised analysis guidelines.
Furthermore,it is judged that the significant level of denting in the North Anna Unit 1 SGs has contributed to the inability to detect the cracks at the TLPs until the use of the more sensitive RPC probe at all TSP intersections. The number of TSP circumferential indications at the end of the current cycle is axpected to be less than 2/3 of the indications observed during the mid cycle inspections.
The first RPC inspection of 100% of the WEXTEX region at North /.nna Unit 1 omurred in the Spring 1991 inspections prior to Cycle 9 operation. Hence, tee second 100%
RPC Inspection of the WEXTEX transitions during January February It'92 resulted in
, fewer tubes plugged for WEXTEX circumferentiat indications; therefore, no change in tbc number of WEXTEX indications is expected for the end of the current cycle, other than that associated with the lower Thot as described above.
i For the TSP axialindications, the crack lengths observed during the 1992 inspections are conservatively applied for the endof cycle crack distribution. However, based on the significant increase in the number of axialindications at the TSPs as the result of the 100% RPC inspection transient, the number of axialIrdcations at the end of the current cycle ls expected to be ?/3 of that observed in the 1992 mid cycle inspection.
The crack distributions expected at the end of the current fuel cycle, based on operating cycle considerations and inspection transient considerations, are based or, modifying the 1992 mid cycle distr;butions as follows:
i 27 l
l L
\
i WEXTEX Circumferentia! indications
- reduction in crack angles by 3'
- no change in the number of indications TSP Circumferential Indications reduction in crack angles by both 3' and 8% of the last inspection results number of indications reduced to 2/3 of last inspection TSP AxlalIndications no change in crack length qumber of indications reduced to 2/3 of last inspection.
The reduction in WEXTEX Indications for EOC 9 is negligible and acceptable burst capability has been demonstrated for the indications found at the mid-cycle inspection.
For the TSP circumferential indications, the reduced Thot and the RPC inspection transient lead to expected EOC crack angles less than 218', Since the expected cracks are less than the 226' for 3AP buist capability of throughwall cracks, the EOC 9 conditions satisfy Reg. Guide 1.121 oven if the total RPC angle is assumed to be throughwall.
Steam line break leak rate analyses have been perfor .ed for the expected end of cycle crack distributions. The leak rate contributions from the expected WEXTEX circumferential, TSP circumferential and TSP axial crack distributions are calculated to be 0.6 gpm,12.8 ppm, and 3.4 gpm, respectively. Axtal cracks in or above the WEXTEX region are not considered to contribute to the SLB leak rate, since all axial cracks exceeding 40% throu{,h wall depth are plugged before retuming to power and throughwall cracks are not expected during the remaining one half cycle of operation.
The to*-' SLB leak rate for the expected EOC crack distribution is estimated to be 16.8 gpm. In addition, a bounding SLB 'eak rate is calculated by conservatively applying the1992 crack distributions for the WEXTEX and TSP regions for the end-of cycle. The bounding SLB leak rate would be approximately 30.5 gpm.
A tube vibration assessment has been performed to determine the potential for crack propagation driven by turbulent and fluidelastic tube vibration. The minimum crack angles for crack propagation are calculated for circumferential cracks at the WEXTEX 28
t transition and at the TSPs. None of the WEXTEX circumferential cracks observed after the first half of the current cycle had crack angles which were subject to crack propagation due to tube vibration. For the TSP regions, the potentla! for tube vibration induced crack propagation is significant only for indications at the bottom edge of the first TSP None of the circumferentiallndications observed at tbc bottem of the first TSP during the 1992 mid-cycle inspection program had crack angles v.hich exceeded those required for crack proptigation. As previously described, the creo.\ engles for the WEXTEX and TSP regions at the end of the current cycle are expected to 50 smaller than those observed during the 1992 mid cycle inspection. Therefore, none of the rJrcumferentialindications at the TSPs or WEXTEX transitions are expected to r experience tube vibration induced crack cropagation.
h 1
29
Table 21 North Anna Unit 1 Tube Plugging Summary February 1992 Number of Tubes Plugged indication EmhgIyne E SSB SIG WEXTEX (Hot Leg) RPC 15 10 11 TSP (Het Leg) RPC 143 148 178 Free Span Bobbin 1 0 2 Preventive 7 2 10 Total 1992 Plugging 166 160 201 Cumulative SG Plugging 629 585 851 (18f%) (17.3%) (25.1%)
2 10
3.0 EDDYCURRENT DATA REVIEW 3.1 Inspection Summary The 1992 North Anna Unit 1 steam generatcr inspection program encompassed full length testing with bobbin probes of 100% of the available tubes in each steam generator The data from this program was used to evaluate the condition of all free length tubing, including the U bend portions, except for Row 2 U bends. Comparison testing of the 8x1 arrayed pancake probe and the RPC probe was conducted on indications at the TSPs. The comparison tests showed that in most cases,it was necessary to make conservative and probably f alse 8x1 calls to achieve more than 90% detection of the signals identified by RPC testing. This was attributed to the inability to discriminate effectively between liftoff signals at the dented TSP intersections and crad indications at the TSP edges. Utilization of the 8x1 probes with a h(;h overcall rate and RPC verification was judged to be less efficit. :than RPC test.ng alone. Therefore, an
- lospection program encompassing RPC testirig of 100% of the TSP Intersections was und : ' ken. All hot leg (HL) WEXTEX transitions and HL tube support plate intersections, whether dented or not, were inspected with RPC probes. Additionally, all Row 2 tubes were tested through the U bends with RPC probes.
The eddy current (EC) analysts were required to pass site specific examinations approved by the Virginia Power NDE Level 111. All data was anah'xed independently by two analysts, Resolutions of conflicts arising between primary 6: J secondary analysts were performed by Westinghouse lead analysts, in concert with procodures jointly established with the Virginia Power NDE Level 111 and overseen independently by Virginia Power Quality Accurance.
3.2 Tube Plugging Summary Through the end of 1991, cumulative tube plugging of 15.1% (1538 tubes) had accrued; this was distributed among the steam generators as follows: SG A,13.67%
(463); SG B,12.54% (425); SG C,19.18% (650). The incremental plugging resulting from the January Feorvary 1992 inspection is summarized in Table 31. Of the 527 tubes plugged during this outage, most were identified by RPC inspection at the tube support plates (469) and the WEXTEX transition zones (36P 3 were identificd on 31
free length tubing by bobbin probes and 19 were preventively plugged. After completion of the 1992 plugging,2065 tubes (20.31%) nave been plugged cumulatively; thess are distributed among the three steam generators as follows: S3 A,18.6% (629); SG 0, 17.3% (585); SG C,25.1% (351).
3.3 WEXTEX Indications All flaw indications identified by RPC testing in the WEXTEX region were regarded as sufficient basis for plugging the tubes on which they were found. Of the 36 such tubes, 31 were classified as circumferential cracks and 5 as axial cracks. On fNe of the tubes, more than one circumferential crack was reported; these were designated MCI (multiple circumferentialindication) as distinguished from SCI for single circumferential indications; there were no multiple axial indications (MAI) reported. Table 3-2 summarizes the distribution of the WEXTEX RPC indications by type for each steam generator. Figures 31,3 2 and 3 3 present the plan views for steam generators A, B, and C respectis ely; these show the distribution of the WEXTEX indications.
3.3.1 1.ocation of WEXTEX Indications By f ar, the predominant location of th9 circumferential indications observed in the North Anna Unit 1 WEXTEX region is within the first 0.2" below the top of the tubesheet (TTS); only 2 of the 31 circumferentialindications reported were above this, and those were within 0.1* of the TTS. One circumferent!alindication was more than 0.2" below (0.28') the TTS. By contast,3 of the 5 axialindications were more thar. 0.1* above the TTS, while 2 were within the 0.1* just below the TTS.
3.3.2 Crack Morphology The WEXTEX crack indications recorded in the February 1992 inspection are similar in appearance and EC characteristics to the prior indications. The predominant aspect of these signals from EC data is the ID origin, indicated by the phase angt data associated with the RPC signals which, were classified mainly as circumferential, with a small number appearing axialin orientation. Although the phase angles are typically offset in combination with the transition effects, tne WEXTEX phase angle observations as a group lie in the ID range of flaw behavior, distinctly different from the phasa angles associated 32 1
with the TSP / dent circumferential signals which are grouped in tha OD range of flaw behador. The ID-originated cracks in the WEXTEX zone are characterized as cracks with multiple initiation sites, with resulting numerous microcracks rnmprising the macrocrack detected by the RPC probe. Tuoe pulls in 1985 and 1987 examined ID-originated cracks of this typo and established that the individual microcrads detected have aspect ratios of 4/1 to 6/1 separated by nearly full depth Ibaments. The deepest cracks are typically within a macrocrack or ac acent microcracks for which the separating ligament may have been lost by corrosion. No significant cracking is four,d other than tho essentially continuous macrocrack. As for the 1991 analysis, the PWSCC cracks are represented by a ligament model with 0.2 to 0.3 inch long, deep (assumed throughwall) segments separated by ligaments (assumed wall thickness). An elastic ligament modelis used to size the width of the ligaments such that the ligaments remain elastic under normal operating pressure differentials, which results in ligaments with about 0.050 inch width for 0.2 inch throughwall segments.
~3.3.3 WEXTEX Circumferential Arc Lengths Individual SCI's exhibit lengths measurud in degrees arc length, ranging from less than 60'(very few) to 199*. By contrast, the 1991 inspection results included WEXTEX crack are lengths up to 247' and the 1989 inspections included are lengths up to 300'.
Figure 3 4 compares the size distributions for the 1991 and 1992 inspections. The smaller number of circumferential cracks reported in 1992 (31) compared to 1991 (216) reflects the shorter operating period prior to shutdown but more importantly, the effect of the 1991 inspection transient;i.e., the 100% RPC inspection performed in l-the WEXTEX region in 1991 was the first such inspectbn applied in North Anna Unit 1
! (see Section 3.11),
3.4 TSP CircumferentialIndications RPC examination of the assumed dented TSP's in the North Anna 1 steam generators l- resulted in identification of 212 tubes with circumferential crack indications (SCI &
MCl) Table 3 3 provides a breakdown of the distribution of these cracks for each of the steam generators with respect to TSP elevation. Figures 3 5,3-6 and 3 7 display the planar composite distribution of these indications for steam generators A, B and C respectively.
33 r
_ - - . - - - -- . _ _ _ . - _ - _ - - . - = . - . - - . - - .
The are length distribution of the observed TSP circumferential crack population is presented in Figure 3 8, together with the comparable 1991 distribution. It is seen that the 1992 population is much larger than that observed in 1991; this is a manifestation of the inspection transient resulting from the first application of 100%
RPC testing to the support plate intersections. The 1991 population (93 tubes) represents only those cracks confirmed by RPC examination after detection by bobbin or 8x1 probe testing. Beesuse prior detection by bobbin or 8x1 probes was required before RPC testing, only -540 Ir.tersections were tested with RPC. This excludes those 7th support plate locations RPC tested in conjunction with NRC Bulletin 88 02 concerns. The 1992 inspection on the affected support plates (15) encompassed more than 50,000 intersections, a nearly 10040ld increase in RPC scope.
The 1992 intpection transient resulted in increased numbers of indications as well as longer are lengths. Cracks which were previously undetected were repor1ed in the 1992 initial 100% RPC inspection, including some with long are lengths. The la: gest are lengths found were in SG A at R34C42 2H and R18f'391H, with RPC crack angles of 240' and 239', respectivety; these were the only two indications exceecing 226' for 3.iP burst capability. The characterization of 8x1 signals observed in 1991 was apparently unable to discriminate effectively between liftoff signals at the dented intersection and crack indications at the TSP edges. This effect was confirmed in 8x1 vs.
RPC comparison tests performed at North Anna 1 in January 1992, in these tests. it was necessary to make conservative and probably false 8x1 calls to achieve 290%
detection of the signals identified by RPC testing.
3.4.1 Crack Morphology for Tube integrity Assessments The TSP circumferential crack population observed in 1992 exhibits phase angles usually associated with OD origin degradation. This is consistent with the behavior of the 1991 populatiors as well as the findings of the tube examination (R110141H) for the circumferential cracks. In that examination, it was observed that 60% of the total arc length associated with the main crack at the upper edge was throughwalt. For the 1992 evaluation, it is assumed that the observed circumferential cracks are similar to those found in 1991, i.e., ODSCC consisting of macrocracks formed by linkup of individual microc'acks followed by corrosion of the separating ligaments. The portion of the tube -
34
circumference apparently free from degtadation based on RFC results is assumed to have 50% d9ep undetected cracking.
3.5 TSP AxialIndications The RPG oxamination of 100% of the hot leg support plate intersections in S/G's A, S and C resulted in identification of 286 tubes with axial cracks, with 98,87 and 101 indications in each SG, respectively. As indicated in Table 3 4, those indications were distributed with almost half at the 1H level and 77% at the first two plates. This distribution is similar to those found in earlier inspections (e.g. 1985,19t,',1989, i 1991) when detection was accomplished by bobbin probes. Figure? 19,310 ano 311 display the planar composite distributions for the TSP axlat indications. Again, the pattern is similar to those observed in prior inspections. Neither the circumferential ODSCC indications nor the axial (PWSCC) indications display any preference for localized clustering at the TSP elevations. Comparing 1992 inspection results with those from 1991 shows that the tubes plugged for axial cracks.In 1992 arise mainly (63%) from cracks not detected by bobbin at the edges of dented intersections while cracks in this category accounted for only 13% of 1991's tube plugging for axial cracks. Therefor- it is likely that the 286 tubes plugged in 1992 for axial cracking reflect the RPC inspection transient described above rather than accelerated PWSCC during the 9 month operating interval just prior to the inspection.
Cracks undetected by the bobbin probe are masked by denting at the TSP and the RPC probe is better suited to detect them because its surface riding feature reduces the influence from the dont interference. It is estimated that the 'new" axial cf ack population Hentifiable by bobbin in the 1992 inspection axounts for 120 of 287 tubes, approximately .ialf the number plugged on that basis in 1991, during an inspection that followed an uninterrupted oparating cycle.
3.L ' Crack Morphology Tube pulls performed in 1985 and 1987 have characterized the dominant morphology associated with axial cracks in the dented TSP intersections of North Anna 1 as PWSCC with multiple initiation sites and numerous microcracks adding up to the macrocrcck lengths typically identified in RPC inspections. Aspect ratios (length / depth) of $/1 to Gli are associated with the individual microcracks which are separated by nearly full 35
thickness ligaments. The deepest cracks are typically within a macrocrack or adjacent microcracks for which the separating ligament may have been lost by corrosion. No significant cracking is found other than the essentially continuous macrocrack. For analysis, tir "WSCC cracks can be represented by a ligament model with 0.2 to 0.3 inch long, deep (assumed throughwall) segments separated by ligaments (assumed full thickness). An elastic ligament modelis used to size the width of the ligaments such that the ligaments remain elastic under normal operating pressure differentials, which results in ligaments of about 0.050 inch width for 0.2 inch throughwall segmerna.
3.6 Multiple Circumforential indictions (MCI's) 3.6.1 WEXTEX MCI's Of the 31 tubes identified with circumferential cracks in the WEXTEX expansion zone, there wme 5 which exhibited MCI's. Table 3 5 provides a listing of the individual tubes, alon3J with the elevation of indications, the arc lengths and ligaments observed, ard the total crack are lengths. The smallest ligament observed was 12' on S/G A R24C33 which. adjusted for RPC resolution capability (see Section 3.8), represents
-42'separauen between cracks. The apparent total crack are length is 234' (149' +
85'); with resolution corrections for coil lead in and lead-out effec's assuming deep cracks, this is reduced to about 174', On this basis, all MCIligaments observed exceed the 30 35' ligament size required for MCI burst capability to exceed that of a single crack with a 70' ligament (see Section 5.4). All the observed WEXTEX MCfs meet the 3aP bufst capability (see Section 6.2).
3.6.2 TSP MCrs A total of 212 tubes were plugged for TSP circumferential cracks; of these 13 exhibited MCI signals on the same elevation (i.e. support plate edge). Table 3 6 provides a list of these tubes, the elevations, and the crack and ligament lengths observed. The minimum ligament observed in these MCis was -41', which amounts to -71' after RPC resolution adjustment for lead-in/ lead-out coil effects, Thus the observed ligaments exceed the 60 70* size required to establish MCI burst capability at 3AP conditions (Section 5.4);
hence, all of the TSP MCl's meet the 3AP burst capability requirements (Section S.3).
36
.,_.r r .. ,.-., -.- . , , . , _m.. _ , ,_ -, ,_, ,._..s_- , , _.m.,_ . ,.., _ _ , ,...,, _ _ __ _- - . - - , ., . _- _ , ,_
3.7 Potential Combined Axlal and CircumferentialIndications Of the 469 tubes plugged for axial and circumferentialindications at support plates, there were 13 tubes (14 TSPs) which displayed both axial and circumtstential indications at the sams TSP Seven (7) of these instancot involved axla! crads either entirely within the TSP dimensions or on the opposite edge from the circumferential indication. The other seven (7) occurrences involved situations in which the axial crack i intersected the same elevation ca which the circumferentialindication was observed; the locations of these tubes are shown in Table 3 7 and Figure 312. The data for all 14 TSP locations is summarked in Table 3-8; given are the tube, the elevation, the arc length of the circumf6tentiel indication, the distance between the indications' centers (expressed in degroes), the derbed ligament are length, and the sum of ligament angle measured plus 1/2 of the axialindication are length. The derived ligamer;t angle is obtained (rom the (, enter line to center line separation minus half the circumferential angle;ule provides a ligament angle comprised of the ligament plus half the RPC resoluticn capability for the single axial crack. The actual measured ligament plus half the RPC resolution capability for the axial crack provides a check against the derived arc length, it is seen that either the derived or the measured arc length provides sufhciently long ligaments even before adjustment for RPC lead IrVlead out effects, except for R30C67; after adjustment this tube also is shown to possess sufficient ,
ligament separating the two crack modes to meet the burst pressure requirements. All j- the ligaments in the potential mixed mode degradation cases observed exceed 30' and are j' expected to meet 3AP burst capability requirements (see Section 6.5).
3.8 RPC Resolution Considerstions Limitations on RPC resolution do not permit accurate sizing of ligaments between closely spaced eracks. RPC resolution tests performed for a ligame 1 between two throughwall
! circumferentialindications were described in WCAP 13034 and are summarized in this section for application to the structural assessment of MCis. RPC resolution tests were also performed for the 1:gament between a throughwall circumferential crack and a L throughwall axial crack to support the assessment of potential combined circumferential and axial indications at TSP intersections (Section 3.7).
i l
l 37 l
v-. . . . . ...m,.__ . . - - , _ , , - - , - c-y, _ -. - _, . _,. . ,, - , , . ,,,,.,,.,,..r,-.. m , , - . _ .-- , . . , , , -
p RPC resolution for a ligament between two circumferential indications was evaluated using EDM notch simulations for circumferential cracks. Ligament sizes of 0.125 and 0.250 inches between throughwall EDM notches were inspected with an RPC probe. The results are showr4 in Figure 313. It is caen that a ligament of 0.125 inch between the EDM circumferential notches cannot be resolved by the RPC probe. A ligament of 0.25 inch, corresponding to an angle of about 35',is required to clearly separate the circumferentialindications based en the RPC amplitude retuming to the ' null point
- for '
which the amplitude returns to the background level.
The North Anna RPC data analysis guidelines require the amplitude to return to the null point to call a circumferentialindication a MCl. The ligament size or angle is then defined as the width of the null point response between two circumferentialindications.
Because of the limited resolution, the RPC measured ligament at null point response significantly underestimates the actualligament between two deep circumferential cracks. The actualligament between two closely spaced, throughwall circumferential cracks can be expected to exceed the measured RPC ligament plus 30', This RPC resolution limitation results from the RPC coillead-in and lead out effects on measuring a crack size such that the crack angles for deep indications are overestimated.
The RPC lead in or lead out effects are each typically found to be about a RPC coil diameter of about 0.1 inch diameter or about 15' each. Therefore,if it is assumed that the measured RPC crack angles are deep or throughwall cracks, the measured angles should bo decreased by 30* and the measured lignments betw9en cracks should be increased by 30'.
RPC resolution tests were also performed for a ligament between a circumferential and an axial crack. Again the cracks were simulated by throughwa!! EDM notches. Ugament slies of 0.125,0.250 and 0.400 inch were evaluated. The RPC inspection results for these tests are shown in Figure 314. For a 0.125 inch ligammt, the ligament between the axial and circumferential indications cannot be resoNed ';y RPC. For the 0.250 inch
(-37') ligament, the RPC amplitude retums to the null point with a 0.040 inch (-7*)
ligament detectable by RPC Thus the ligament angle is underestimated by about 30'.
For the 0,400 inch ligament, the RPC measured li, ament is between 0.110 (16') and 0.150 inch (22*) compared to the actualligament angle of 59' Thus the ligament size is underestimated by >35' and at least 30' must be added to the RPC measured ligament to obtain the minimum actual ligament size.
38
The ligament stze between axial and circumferential cracks can also be estimated by alternate methods which ignore the RPC crack angle obtained for axialindications, as given in Tal>le 3 8. In Table 3 8, two methods are applied to estimate the ligament size.
The first method derives the ligament angle as the conterline to centertino distance i between the axial and circumferent;al cracks minus half the circumferential angle. The second method utilizes the RPC measured ligament size plus half the RPC measured angle !
for the axial crack. Both of these methods tend to underestimate the ligament size by the overestimate of the circumferential angle resulting from the RPC lead in or lead-out effect. Thus the ligament sizes of Table 3 8 are general conservative underestimates of the actualligament compared to adding 30' to the RPC measured ligament size, 3.9 WEXTEX Circumferential Crack Growth Rates Tubes reported with WEXTEX circumferential cracks in 1992 were re analyzed from the 1991 EC data, in order to establish an estimate of azimuthal crack propagation; the ,
1992 inspection represented the second consecutive in service inspection of the WEXTEX transitions with RPC probes. Of the 31 tubes found with WEXTEX circumferentialindications (see Table 3 2),24 were evaluated on the basis of the 1992 data to exhibit visible circumferential features which were attributed to the precursor cracks. For these cases, the apparent azimuthal arc was calculated and compared to the arc lengths reported at the corresponding locations in 1992; these differences yleided an average change for WEXTEX crack growth of 12.8'i 12.2* for the operating interval (see Table 3 9). To compare with the prior cycles growth rate determined from 8 x 1/RPC correlations, the observed value is doubled to account for i the shortened operating interval to yloid a cycle average growth rate of approximately 26' for 1991 1992 compared to the 19891991 growth rate of 30'. Figure 315 compares the cumulative and interval growth rates for 1991 and 1992. The 95%
cumulative growth rate value for twice the 1992 result (66') compares favorably with the 1991 value (73') which was based on the 8 x 1/RPC correlation. The operating interval completed prior to the 1992 inspection was 254 EFPD versus SOS EFPD preceding the 1991 inspection.
- 39 y r .m-.--, ,,,,....c -
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3.10 Tube Support Plate Circumferential Crack Growth Rates There were 23 TSP circumferential indications reported in 1992 which had been RPC tested in 1991. Comparing the are lengths from the 1992 calls with the conosponding l
features in the 1991 data (Table 310) yloided comparisons which amounted to l 10.5'112.2' growth for the short cycle: doubling this estimate to enable comparison with the previous, full cycle average growth rate again
- esults in a favorable comparison . 21% for the 1991 1992 period vs. 30% for the 1989 1991 period.
Similarly the 95% cumulative probabilities for these periods were 63' and 67' respectively. Figure 316 presents the cumulative and interval growths compared between the 1992 RPC and 19918x1 data..
3.11 Inspection Transient Considerations During the December 1990 and February 1992 inspection programs at North Anna Unit 1, RPC probes were used more extensively than in previous outages; this resulted in one time occurrences of a large number of tubes plugged due to indications in the WEXTEX transition region (1990) and at the TSP intersections (1992), This behavior is typical of plants which experience inspection transier:ts. Specifically, the number of indications found during the first inspectbn using new NDE guidelines or a more sensitive probe type are typicany much larger than in the preceding inspections.
During the next inspection following the application of the revised guidelines or new probe, the number and size of indications have been observed to decrease substantially.
Experience has shown that many cracks detected during the first application of more stringent guidelines or with a more sensitive probe would also have been found in the preceding inspection, had the guidelines or probe been applied at that time, inspection transients for the tubesheet and TSP regions have been observed in several plants in recent years, following the implementation of 100% RPC inspection in the hot leg tubesheet or the use of revised EC analysis guidelines for the TSP regions. The levels of tube plugging at North Anna Unit 1 have been consistent with the inspection transient behavior observed at other plants. industry experience with inspection transients at several plants are compared to that of North Anna Unit 1 in the fallowing sections.
3 10
.. _ . . _ . _ _ _ . . . , ~ . . _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ . _ _ _ _ _
3.11.1 WEXTEX Region Inspection Transient at Plant L The first application of the RPC probe to 100% of the hot leg tubesheet regions in all four SGS at Plant L occurred during the 1989 inservice inspections.100% RPC inspections of the hot leg tubesheet regions were also periormed during subsequent inspections in 1990 and 1991. Table 311 Summarizes the tube plugging results for j tubesheet region cracking from 1987 through 1991. Prior to 1989, nine tubes were {
plugged for ODSCC in the tubesheet region. Upon implementation of 100% RPC l
Inspection of the hot leg tubesheet in 1989, a total of 90 tubes were plugged for axial and circumferentialindications in the WEXTEX transition region. Of this total,41 tubes were plugged for axial cracks on the OD of the tubes, and 56 tubes were plugged for ID circumferentialcracking. During the second 100% RPC inspection of the hot leg tubesheet region in 1990, a total of 29 tubes were plugged for cracking in the WEXTEX transition region; all but two of those tubes were plugged for axlat indications. 28 tubes were plugged for tubesheet indications in 1991.
Similar to the trend in numbers of indications for inspection transients, crack sizes tend to be the largest in the first RPC Inspection and smaller in subsequent inspections. For Plant L, the targest circumferential crack angles in the three sequentialinspections were 263',20' and 158'.
The above results for Plant L clearty demonstrate ine inspection transient experienced during the initial application of 100% RPC to the WEXTEX expansion region, when a significant increase in the number of both axial and circumferential mdications occurred. Subsecuent inspections using RPC to inc>pect 100% of the hot leg tubesheet have resulted in significantly lower numbers of tubes and smaller crack sizes for axial and circumferential cracking in the WEXTEX expansion region, compared to the first RPC inspection.
3.11.2 Inspection Transle.its at Plant A 1 Table 3-12 shows the tube plugging history for Plant A 1 from 1985 through 1991. A significant increase in the number of tubes plugged for tubesheet and TSP indications occurred in 1991. For the tubesheet region, this increase is the result of tha first
. application of RPC to 100% of the tubesheet hot leg. The significant increase in the 3 11
number of tubes plugged for indications at the TSPs is the result of a change in EC analyst guidelines. These inspection transients are described in detail below.
Prior to 1991, a modest number of tubes were plugged for OD indications above the tubesheet; sixteen of the 20 tubes plugged from 1985 to 1988 for ODSCC in the
, tubesheet region were for indications on the hot lag side in the sludge deposition region. l During the October 1989 inspection program,100% of the available tubes were -
inspected fulllength using a standard bobbin probe. Supplemental MRPC inspections were performed for distorted indications (Dl's) at the TSPS tsnd above the tubesheet. In l addition, a random sampling of 9% of the tubes in one SG were inspected with an 8x1 probe for possible circumferential cracking in the WEXTEX transition region. A total of 22 tubes were plugged for ODSCC in the sludge deposition region.
In order to increase the detectability of circumferential indications in the WEXTEX region,100% of the available tubes in the hot leg tubesheet region were inspected by RPC probes in April 1991;in addition, the bobbin inspection guidelines for indications above the tubesheet were altered (i.e., the voltage threshold for RPC inspection was elintinated), The resulting number of tubes plugged for ODSCC at the top of the tubetheet more than doubled, totalling 46 tubes plugged, Although no tubes had been plugged for PWSCC in the tubesheet in prior outages 72 tubes were plugged for PWSCC in the WEXTEX expansion zone (37 axialindications and 35 circumferentialindications). This behavior is typical of the inspection transients observed during first time application of 100% RPC or aher chsnges in bobbir' interpretation guidelines.
The number of tubes plugged for indications at the TSPs during April 1991 aiso increased as a result of the elimination of the bobbin voltage cutoff for requiring RPC inspection. The more stringent analyst guidelines and increased use of the RPC probe resulted in the plugging of 102 tubes for indications of ODSCC at the TSP elevations; only four tubes had been plugged for such indications during all prior inspections. Again, this clearly demonstrates the inspection transient effect of changes in analyst guidelines and more extensive use of the RPC probe.
l The next inspection program for the Plant A-1 steam generators will occur during the September 1992 refueling outage; hence, there is no data to demonstrate a decrease in
- the number of indications observed during the first outage following the inspection l
t 3 12
transient. However, the steem generators at Plant A 2 recently completed the first inspection after a $1milar inspection trantient; the results for Plant A 2 are described below.
3.11.3 Inspection Transients at Plant A 2 Table 313 shows the tube plugging history for Plant A 2 from 1985 through 1992. A significant increase in the number of tubes plugged for tubesheet and TSP indications ocx:urred in 1990. For the tubesheet region, this increase was the result of the first application of RPC to 100% of the tubesheet hot leg. The significant increase in the number of tubes plugged for indications at the TSPs was the result of a chang 3 in bobbin coil analyst guidelines. These inspection transients are described in detail below.
At Plant A 2, PWSCC has been observed in the hot tog mechanical expansion region since January 1985. Prior to 1990,86 tubes were plugged for PWSCC indications in the expanded region. During the inspection programs prior to 1990,100% of the available tubes were inspected fulllength using a standard bobbin probe. Supplemental MRPC inspections were performed for distorted indcations (Dl's) at the TSPs and above the tubesheet. During the December 1990 inspection program,100% of the available tubes in the hot leg tubesheet region were inspected using RPC probes; 328 tubes wer e niucced for indications of PWSCC at the roll transitions.100% RPC inspection of the roll transition regions in March 1992 resulted in only 60 pluggable tubes, all with axial indications in the roll transitions.
l l From 1985 to 1989, a total of 112 tubes were plugged for ODSCC at the TSP elevations of Plant A-2. In December 1990, revised inspection guidelines for the TSP regions eliminated bobbin coil thresholds and 243 tubes were plugged for ODSCC at the TSP elevations. These revised guidelines were also responsible for the April 1991 inspection transient described above _for Plant A 1. Again,inis clearly demonstrates the inspection _ transient effect of changes in analyst guidelines and more extensive use of the RPC probe, in March 1992, only 18 tubos were plugged for ODSCC at the TSPs.
However, this inspection implemented an a!!ernate tube plugging criteria; thus, the reduction in plugging from 243 to 18 is a combination of the inspection transient and plugging criteria.
3 13 i
t m
3.11.4 Inspection Transients and Conclusions for Nonh Anna Units 1 and 2 Table 314 summarizes the tube plugging due to circumferentialindications in the tubesheet and TSP regions of North Anna Units 1 and 2 through 1992. The table demonstrates that the Neith Anna steam generators have experienced similar inspection transient behavior to that seen in the other plants described above.
The first application of RPC to inspect 100% of the WEXTEX transitions in North Anna Unit 1 occurred in December 1991. The resulting inspection transient produced 216 circumferentiallndications. For the inspectiori following the first 100% RPC inspections, only 31 circumferential indications were found at Unit i t hence, the number of indications were reduced by a factor of 7. The first application of RPC to inspect 100% of the WEXTEX transitions in North Anna Unit 2 occurred in September 1990. The resulting inspection transient produced 26 circumferentialindications, up from 4 Indications in the preceding outage . For the inspection following the first 100%
RPC inspections,13 indications were found at Unit 2. For Unit 1, the maximum circumferential crack angle found in the f;rst RPC inspection was -247*, compared to 199* In the subsequent inspection. Except for a large circumferentialindication identified in the first Unit 2 RPC inspection but not called for plugging based on lack of phase angle response, the largest Unit 2 circumferential crack was reduced from 238' to 128' in the outage following the RPC inspection transient. >
The first application of the RPC probe to 100% of the hot log TSP intersections of North Anna Unit 1 occurred in February 1909 A totalof 212 tubes were plugged for ,
circumferentialindications at the TSP elevations. This represented a significant increase over the number of tubes plugged during any of the prior outages. This behavior is consistent with the inspection transient behavior observed for the TSP axial 4 indications of Plants A 1 and A 2.
Based on the data presented above for inspections following an outage in which revised inspection guidelines or more sensitive probes are used, it is judged that many of the tubes plugged for indications during the inspection translents had cracks which existed during prior inspections, but were not detected until the use of the more sensitive probe or revised analysis guidelines. The significant level of denting in the North Anna 1 steam generators has contributed to the limited detectability of the cracks at the TSP 3 14
i intersections until the use of the more sensitive RPC probe at all TSP intersections.
Therefore, it is judged that the number and size of indications which will exist at the end of the current operating cycle will be smaller than the cracks found during the 2/92 inspection program.
1 I
t l
l l 3 15
Table 31 i
NORTH ANNA #1 FEBRUARY 1992 TUBE PLUGGING
SUMMARY
A B C THROUGH 1991 463 425 650 WEXTEX (HL) (RPC) 15 10 11 TSP (HL) (RPC) 143 148 178 OTHER (BOBBIN) 1 0 2 PREVENTIVE 7 2 10 TOTAL 1992 166 160 201 CUMULATIVE 629 585 851
% PLUGGED 18.6% 17.3% 25.1%
3 16
- - - . ~ . - - = - . _ . . . - . - - _ . . - . . - . . - _ . . - ~ - - .
I l
l Table 3 2 i i
I i
NoRrw ANNA #1 FEBRUARY 1992
{
WEXTEX INSPECTION RESULT 5 i
- TusES A B C SINGLE AXIAL 3 0 2 .
-HULTIPLE AXIAL 0 0 0 :
SINGLE CIRCUMFERENTIAL 10 9 7 MULTIPLE CIRCUMFERENTIAL 2 1 2
~
o THERE WERE Q TURES WITH MIXED MODE DEGRADATION IDENTIFIED IN_THE WEXTEX TRANSITIONS.
l.
l.
- 3 17
- l
, . . , - . . - _ _ . . . . ~ . _ _ . _ . _ _ . . _ . . , _ - _ _ _ _ . . . .
V Ta'6s 3 3 NORTH ANNA #1 FEBRUARY 1992 TSP CIRcuMFERENTIAL CRACKS
-ELEVATION STEAM GENERATOR A B C SCI / MCI SCI / MCI SCI / MCI 1H 25/3 22/0 27/0 2H 15/0 33/3 42/3 3H 2/0 7/2 19/2-4H 3/0 0/0 3/0 5H 1/0 0/0 1/0 6H 0/0 0/0 0/0 7H 0/0 0/0 0/0 l
SCI - SINGLE CIRCUMFERENTIAL INDICATION MCI - MULTIPLE CIRCUMFERENTIAL INDICATIONS 3 -18
Table 3-4 NoRTu ANNA #1 FEBRUARY 1992 TSP AXIAL k VK5 ELEVATION STEAM GENERATOR A B C SAI/MAI SAI/MAI SAI/MAI 1H 36/0 53/4 37/5 2H 24/2 14/3 41/1 3H 7/0 6/0 5/1 ,
4H 5/0 5/0 8/0 5H .. 21/0 1/0 1/0 6H 3/0 1/0 2/0
--7H 0/0 0/0- 0/0 SAI - SINGLE AXIALINDICATION MAI . MULTIPLE AXtALINDICATIONS 3 -19
Table 3 5 North Anna 1 .
WEXTEX MC!s 10L11 luba location. Cire.1 Lig.d Circ.2 Litl h 5,,t Littnarator A R19C32 TS .12 1020 240 (540 )(I) 820 1520 1840 R24C33 T5 .08 1490 1140 85 0 0 0 0 12 (42 )(3) 2340 (174 )(1)
Steam Generator B f
R9C36 T5+.00 0 0 0 0 0 81 17 (47 )(I) 82 1800 163 Steam Generator _q R10C72 T5 .02 760 380 760 0
- 170 1510 R25C50 TS .04- 670 460 590 1780 '1260 Note 1. RPC measured ligament increased by 30' for resolution limitations (coil lead-in and lead-out effects) and each circumferential indication decreased by 30*.
3 -20
)
i Table 3 6 North Anna 1 ISP Multiple Circumferential Indicattons (NCis)
IM .g location Cire. 1 Llo, 1 Cire. 2 Lim 2
} team Ger3rator A R6C6 lH+.39 810 910 780 1100 R17C10 IH+.50 90 0 860 950 R29C30 lH+.26 - 890 1310 750 870 - 670 Steam Generator 4 RllC8 2H .21 1140 680 0 1170 61 2H+.23 1080 650 810 R12C8 0 1060 3H+.23 128 750 980 590 R22C24 2H .30 1320 730 1010 0 540 R2C41 3H+.29 3H .25 1380 66 (960 )(I) 1150 0 41 (710 )(')
910 880 1010 800 R36C62 2H ?3 830 690 1070 0 101 Steam Generator C R20C9 3H .27 1200 640 790 870 RlG,10 3H+.28 1050 8 92 880 - 750 R10C20 2H+.35 800 0 83 1070 - 900 R18C44 2H+.35 990 900 1090 - 62 0 R27C71 2H .30 .790 960 820 1030 Note 1. RPC measured ligament angles based on return of $19nal amplitude to background or null point. For deep circumferential incications, the actual ligament exceeds the measured ligament by t. bout 30' or more (see Section 3.11 and Figure 3-15) due to RPC coil lead-in and lead-out resolution limits on crack angles.
. 3 21
Table 3 7 NORTH ANNA #1 FEBRUARY 1992 TSPS WITH BOTH CIRCUMFERENTIAL AND AXIAL CRACKS NO CASES OF MIXED MODE DEGRADATION WERE DETECTED IN S/G A AND S/G B.
SEVEN (7) LOCATIONS WITH MIXED MODE DEGRADATION AT THE SAME TSP EDGE WERE OBSERVED IN S/G C.
MINIMUM ELEVATION TUBE EDGE
- CIRtuM1 SEPARATING LIGAMENT ** ,
TSP #1: R29C16 + 750 > 250 R3C43 + 730 > 390 R18C46 + 690 > 690 TSP #2: R10C29 - 1190 >1130 R8033 + 760 > 370 R29064 - 910 > 270 R30C67 + 1330 > 200 DATA WAS ACQUIRED WITH 2X RPC PROBE; FOR CLOSEST AXIAL-CIRCUMFERENTIAL PROXIMITY 3 COIL ZETEC PROBE WAS USED; NO APPARENT IMPROVEMENT IN RESOLUTION.
ALL THESE INTERSECTIONS WERE JUDGED TO BE ACCEPTABLE AGAINST TUBE BURST CRITERIA.
+ UPPER EDGE
- LOWER EDGr NOT ADJUSTED FOR RPC RESOLUTION CONSIDERATIONS 3 22
Table 3-8 North Anna-1 Mixed Mode Evaluation DerivedIIS Measured (2)
. Luke tocation circ. Anale C.t. to C.L. Lia.Anale Lio.+ 0.5 Axial 0 Resolution Steam Generator C 0 0 R20C9 3H .27 130 /19 Opposite Edges,No Concern 0
R14C10 2H+.20 101 Opposite Edges,No Concern 0
R5til IH .27 96 Axial inside ISP.No Concern R29C16 1H+.26 75 0 62 25 6 +28 0 Acceptable,large ligament P10C79 2H .29 119 0 90*+230 Acceptable,large ligament k6033 IH .30 85 0 Opposite Edges.No Concern 76 0 0 0 Y RBC33 2H+.18 82 440 10 +27 Acceptable large ligament N 0 R6C40 3H .26 104 Opposite Edges,No 'oncera R3C43 IH+.31 73 0 75 0
39 0 0 17 +20* Acceptable.large ligament IH+.33 69 0 0 0 R18C46 36 +33 Acceptable,large ligament 0 0 100+17 0 2H .33 0 R29064 91 75 30 Acceptable,largc ligament ,
2Ht.47 63 Opposite Edges,No Concern R30C67 2H+.26 1330 (103 )I3) 86 0 20*(35 )3 50+19 0 Acceptable,large ligament R30C70 2H+.31 95 0 Opposite Edges,No Concern Hotes:
- 1) Ligament angle obtained as circumferential crack centerline to axial crack centerline distance w us one-half the circumferential angle.
- 2) Ligament a..gie obtained as the measured ligament angle plus one-half the axial crack angle.
- 3) RPC measurements adjusted for resolution (coil lead-in and 0lead-out) in measuring deep crack angles. Resolution typically ~1 coil diameter (0.105", 15 ) at each end of crack.
Table 3 9 WEXTEX Circumferential Crack Growth from RPC Data (1991 1992)
Growth S3 Iyhtt Location Der Cvele*
(degrees)
A R21C23 TSH 30 A R15C26 TSH 1 A R24C33 TSH 16 A R24C33 TSH 21 A R20C45 TSH 1 v A R25C55 TSH 21 A R15C56 TSH 29 l A R19C32 TSH 7 A R10C71 TSH 11 A R20C23 TSH 15 B R9C36 TSH 0 B R9C36 TSH 7 P R25C36 TSH 24 b R28041 TSH 2 h B R14C54 TSH 5 B R16C56 TSH 30 B R16C22 TSH 0 B R25C8 TSH 48 B R12C74 TSH 0 C R21C27 TSH 7 C R19C45 TSH 10 C R27C46 TSH 12 C R25C50 TSH 11 C R25C56 TSH 0 Aversr,. Change': 12.8* a 12.2
- All negative changes set to zero.
3 24
Table 310 TSP Circumferential Crack Growth from RPC Data (1991 1992)
Growin -
33 T.:Lo, Location oer Ovels' (degrees)
A R22C24 2H 6 A R30024 ~ 2H 10 A R7C27 2H 20 A R39041 1H 0 B R15C9 1H 13 C .R6C3 1H 1 C RSC11 3H 39 C RSC11 3H 29 C- R24C27 2H 14 C R24C27 2H 0 C R24C36 1H 13 C R41C42 3H 11 C R37C49 2H 4 C R37C49 2H 0 C. R36C52 3H 5 C R18C54 2H 12 C R29C64 2H 0 C R29C64 2H 12 C R9 CSS 3H 0 C R27071 2H 5 C R27C71 2H 0 C R17C62 2H 3 C R27C75 2H 45 Average Change *: 10.5* ! 12.2'
- All negative changes set to zero.
3 25
- Table 3-11 Insp9ction Transient for WEXTEX Expansion Region of Plant L Date of insoection Axial Ind cations Cire IndicPli2H1 .Inlal April 1986 1 0 1 May 1987 0 0 0 May 1988 8 0 8 May 1989(1) 41 56 97 May 1990(2', 27 2 29 May 1"d1(2) 17 11 28 (1) First ai.? !ication of RPC to 100% of active tubes in hot leg tubesheet region.
(2) 100% of active tubes in hot leg tubesheet region inspected with RPC, 3 -26
Table 3 Tube Plugging History for Plant A 1 (1985 1991)
Number of Tubes Plugged Tubesheet Tubesheet TSP Dala IMSCL CDSCO Q1DC D.1 hat S/85 0 0 0 1 10/86 0 3 2 1 4/88 0 17 0 0 10/89 0 22 2 0 4/91(1) 72 46 102 47 (1) Inspection transient resulting from &st time application of RPC to 100% of availab!e tubes in hot leg tubesheet region, as well as revised EC analyst guidelines and more extensive use of RPC at TSP elevations.
3 -27
Table 313 Tube Plugging History for Plant A-2 (1985 1992)
Number of Tubes Plugged Tubesheet Tubesheet TSP Late PWSCC COSCO QJECC Other 1/85 1 0 0 13 4/86 40 0 28 2 11/87 29 1 74 5 4/89 16 0 10 1 12/90 328(1) 0 243 0 3/92 51(2) o 3g(3) 3 (1) Inspet non transient resulting from first time application of RPC to 100% of available tubes in hot leg tubesheet region, as well as revised EC aralyst guidelines and more extensive use of RPC at TSP eievations.
(2) Second application of RPC to 100% of avhaabte tubes in hot leg tubesheet region [ nard roll expansion).
(3) Tube plugging reflected alternate tube plugging criteria and thus was not nly dependent on inspection transient, _
{'
3 28
Table 314 Tuos Plugging for Tubesheet and TSP Circumferential Indications at North Anna Units 1 and 2 North Anna Unit 1 North Anna Unit 2 WEXTEX WEXTEX D?a Transition ISPP. Da!2 Transition 4/89 31 -
2/89 4 12/90 2160) 93 9/90 200) _
2/92 31 212(2) 3/92 13 (1) Inspection transient resulting from first time application of RPC to 100% of available tubes in hot leg tubesheet region.
(2) Inspection transient resutung from first time application of RPC to hot leg TSP intersections in
- 00% of available tubes.
3 29
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Chart WEXTEX Growtn Rates 2/27/92 3 44
Figure 316 TSP Cumulative and Interval Crack Growth Distributions 100.00% -
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4.0 OPERATN3 CYCE CONSIDEPAT10NS The North Anna Unit 1 steam generators wn! be operated at reduced load and primary coolant temperature conditions for the rest of the operating cycle. The reduced load and l temperadras favor lower corrosion rates. The planr9d operatHg conditions, a
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comparison with priu values, and thalt impact on tbo degradation are o.scussed below. I 4.1 Planned Operating Conditinns for the Remainder of Current Cycle The remainbg half of the current fuel cycle of Unit 1 operation will extend 10 months from early March,1992 to early JanuLry,1993. Unit 1 operation will be limited to a -
maximum load. Per this limit, the max! mum load on the unit will not exceed 95% of the cirrent rating. In addition to the 95% operating limit, the ostput will be further restrained by unit coast down, which is projected to start in September,1992. The projected load prohie is shown in Figure 41. Thus, operation at/near 95% load is expected fer only a Six month period. !
The first half of the current Unit 1 fuel cycle was completed with 254 effectko full mwer days (EFPD) of operation at 100% load. The remaining phrt of the Unit 1 fuel cycle will have 252 EFPD of operation. Of these EFPD, about 2/3 will be albelow 95%
loud and the remaining willin the coast down modo at even lower loads.
At the steam generator inlet, the primary coolant umperature (Thot) corresponding to the 95% load operation will be 612.6'F. This is 3'F lower than the 615.6'F Th ot used during the recently completed, first half of the fuel cycle. The primary in!st and average temperatures are also displayed in Figure 41. Table 41 lists the projected steam generator primary and secondary side paramete's at 95% load.
4.2 Comparison of Current and Prior Operating Parameters Table 4 2 shows the fullload operating conditions during the first half of the current fuel cycle. Crimparing this with Table 41, it is observed that the projected primary inlet temperature has decreased by 3*F from the 1991 full load operating condition.
. Because of changes in the number of plugged tubes and adjustments to the turbine govemor valve, the steam pressure is about the same as in the first half of the cycle.
41
4.3 Relative Corrosion Rate Modeling Since the type of tubing corrosion found at North Anna Unit 1 has not been systematically studied in the laboratory, the most relevant means of determining the effect of operating conditions on the oorrosion rate is to use the variation in the number of eddy current indications with tube support plate elevation. This variation is a function of the tubing temperature, the avMlabl0 superheat (as it affects the maximum chemical concentration in the crevices), and the applied stress (resulting from denting corrosion). The number of oddy current Indications at a given tube support plate eleva,lon follows:
HL Circumferential Axial ISE Cracks QaWi IQM '
.: 1 74 126 200 2 90 79 169 3 28 18 46 4 6 18 24 5 2 23 25 6 0 6 6 7 0 0 0 Total 200 270 470 ,
j The estimate does not include multiple crack Indications. An equ} valent Arrhenius activation energy can be calculated by estimating the tubing temperature in the crevices. '
Using laboratory test data for packed tube support plates, tubing tempe,atures of 585, 579, and 574'F were calculated for the first three tuba support plate elevations, respectively. Applying these temperatures to the above data produces a very high activation energy, in the vicinity 160 kcal/ mole.
As mentioned previously, this activation energy reflects possible contrioutions from temperature, chemical concentration, and stress on the variation in the number of
- indications. Since indicat!ons are only identified after the corrosion has progressed some distance through the tubing, the value also includes both initiation and propagation ,
i
~ effects. Previous evaluations of the denting distribution indicate relatively little variation between the hot leg tube support plate elevations, with the average eddf current signal voltags at the bottom support plate being slightly lower than at the others; consequently, variations in stress are unlii ely to be important.
4-2
4.4 Relative Corrosion Rates Between First and Second Half of Current Fuel Cycle The operating characteristics of the previous and current operating periods are presented in Table 4 3. The current operating cycle reflects 6 months of operation at 95% power, followed by a 4 monta coastdown to 45% power. This operating history is shown in Figure 41. The effect of this operating history on corrosion rates can be determined by comparing the time at. temperature of the current and previous operating periods for various Arrhenius coefficients. This prediction assumes that the operating chemistries during the two periods are comparable, as is the extent of corrosion at the beginning of each period.
The predictions are presented in Table 4 3 for Arrhenius coefficients varying between 50 and 160 kcal/ mole The 160 kcal/ mole value was derived from the variation in corrosion with tube support plate elevation, and includes both temperature and superheat effects. The 50 kcaVmote value is representative of laboratory test data 1 collected under isothermal conditions. As should be expected, the relative corrosion during the second half of the operating cycle increases as the Arrhenius coefficient decreases.
The lower bound coefficient of 50 kcal/ mole has been selected for conservatively estimating reductions in crack initiation and growth Since the superheat for the first six months of the current operating period is only slightly below that in the first half of the cycle, the calculatee coefficient of 160 kcal/ mole may be too high. During the coastdown, the superheat decreases as the power decrtsases, so that a high coefficient is likely more appropriate. Consequentry, the calculated normalized corrosion rate of 0.933 should 9xceed the actual rate.
i 43
. _ _ ._. .._ m.,___.._._.___ . . _ . _ _ , . _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ . . _ _ _ _ _ _
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Table 41 j Projected Operating Conditions at 95% Load !
I Thermalload, Percent 95 Thermalload, MWt 920 Primary pressure, psia 2250 Primary inlet temperaturo Rhot),'F C12.6 Primary outlet temperatare (Toold), 'F 546 Steam pressure, psia 777 Steam tempera:ure,'F 515 Circulation ratio 4.84 4
I r
4-4
Tabte 4 2 i
Full Load Operating Conditlons During 1991 !
l Thermalload, Percent 100 l Thormal load por steam generator, MWt 968 Primary pressure, psia 2250 Primary inlet temperature (Thot), 'F 616 Primary outlet temperaturo (Tcold), 'F 546 Steam pressure, psia 783 Steam temperature, 'F 516 Circulation ratio 4.58 4-5
, . _ _ . _.. _ . . - . . _ _ _ _ .. - ~
Table 4 3 Variation in Relative Corrosion with Activati.cn Enercy r
l Normalized Corrosion Operating Power Hot Log Tube Activatio1 Energy (kcal' mole) .
f ycle .. L%1 Duration IgmD ICED d2 .120 AQ EQ (days) ('F)- ('F) !
Previous 100 254 615.6 584.6 1.00E+00 1.00E400 1.00E+00 1.00E+ 00 Current 95 182 612.6 582.2 5.20E 01 S.87E-01 6.11C 01 6.48E 01 .
1 Current 88 31 608.0 580.0 6.60E 02 8.31E 02 8.98E 02 1.01E 01 Current 70 30 595.0 572.7 2.38E 02 4.34E 02 S.31E 02 7.16E 02 ;
Current 58 31 586.0 567.5 1.21E 02 2.88E 02 3.84E-02 5.93E 02 Current - 45 30 580.0 565.7 9.13E-03 2.39E 02 3.28E 02 5.31E 02 Total for Curis.it Cycle 0.631 0.766 0.825 0.933 46
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SUMMARY
OF CRACK SIZES SATISFYlNG REG. GUIDE 1.121 The tube integrity limits for singic wal and circumferentla! cracks have been developed previously in Section 6.0 of WCAP 13034. The following sections summarize burst tests and crack propagation tests for multiple circumferential cracks. The acceptable crack conditions for multiple circumferential inoications (MCis), as well as for combined axial and circumferential(mixed mode) indications are presented. The acceptable sings crack sizes which satisfy the tube burst requirements of Reg. Guide 1.121 are also summarized, from WCAP 13034, along with the acx:eptable single crack sizes which are expected to preclude vibration induced crack propagation.
5.1 Burst Tests for Multiple Circumferential Cracks Room temperature tests of tubes with multiple coplanar circumferential EDM slits were conducttd to verify the calculational methodology used to compute the pressure cap?fD 9t af tubes with multiple circumferential stress corrosion cracks (M0ls). At very large crack sizes, the burst pressure previously (WCAP 13034) was found to be proportional to the not remaining areti, in this case a conservattve burst pressure is obtained by setting the not section stress equal to the flow strength of the material.
- Figure 51 shows sketches of the large coplanar EDM slits and smallligament geometries of the present test series. Note that some tigaments are only partialwall thickness ligaments simulating a 50% deep crack in the rcmaining ligaments. The burst pressure test results for two repeats of each configuration are also shown in Figure 51.
When results are plotted in terms of the sum of al.lrateumterentialcrack angles the burst pressures for the sum of the multiple slits exceed thor,e fonho single L circumferential clits. This is illustrated in Figure 5 2. Noto that at large total crack angles, the lower bound burst pressur6 s simply the not ares times the flow strc;,gth of 8
the material. Ugament location and bending tend +ncy (multiple ligaments reduce the l bending tendency osmpared to a single slot) havo en:y a second order effect on the burst -
L pressure. This is logical ghten that the presence of 09 lubs i.upport plate restricts l-L lateral motion and severely inhibits bending of the Nhc # ihn cracked section.
Application of these test results to acceptable MCI cv1%urathns is addressed in Section 5.4.
B-i -
5.2 Fatigue Crack Growth Rate Tests For Multiple Circumferential Cracks Just as the burst pressure calculational methodology was benchmarked with test data for rnultiple coplanar circumferential cracks, the fatigue crack growth analysis procedure was benchmarked also. Figure 5 3 shows sketches of the EDM starter notches used in the fatigue crack growth rate tests. Under fatigue loading due to turbulence or fluidelastic vibration, the centrallssue is whether or not growth will occur. If the applied ctress intensity range is above the threshold value, fatigue crack growth will f occur and high vibration frequencies can lead to extensive growth in a relatively short l per!od of time compared to the operational cyclei Rather than attempting time j
consuming fatigue crack growth threshold tests, the fatigue analysis was benchmarked
)
by demonstrating that the fatigue crack growth rates measured on severalligament geometries correlated with results on a conventional geometry using stress intensity range as the mrrelating parameter; that is, the agreement of fatigue crack growth results of ligament and conventional geometries demonstrates the acceptability of stress intensity factor calculations for ligament geometries. For smallligaments, there are severallimiting cases for stress intensity solutions for tension and bending. These were applied to the present case and worked well. For worst case service conditions, a minimum of three 35' ligaments are needed for fatigue stability and the largest crack ,
angle between ligaments must not exceed the single circumferential crack limit for initiating crack propagation. Experimental work shows that the presence of the third ligament converts the behavior of the tao to essentially the single crack case and the minimum size for the third ligament is very conservative.-
Figure F 4 illustrates a plot of measured fatigue crack growth rates versus the computed stress intensity range obtained by applying the emailligament solutions. The
_ good agreement of results for a variety of crack geometries supports the acceptability of the stress intensity range calculations and their comparison with fatigue crack growth threshold values.
5.3 Acceptable Single Crack Sizes for Tube Burst The burst strengtm of tubing with single circumferential cracks and single axial cracks have been evaluated previously in WCAP.13034. Sections 6.1 and 6.2 of WCAP 13034 summarize the burst tests performed for single circumferential and 52
l axial cracks. The burst test date are compared with analytical predictions, and the burst l capabilities of tubes with axial or circumferential cracks are estabilished in Section 6.4 1 of WCAP 13034.
1 Table 51 summarizes the acceptable crack sizes for single circumferential and axial cracks with either single throughwall crack or segmented crack morphologies. The limiting circumferential crack angle for a single crack is 226' for 3AP burst capability; a segmented crack satisfies 3AP for crack angles up to 297', The maximum acceptable crack angles for APSLB are substantially higher; thus,'.he 3AP burst capability requirement provides the limiting circumferential crack angles for tube burst considerations. Single throughwall axial cracks of up to 0.435 inch in length will satisfy 3AP; for segmented crack morphologies, single axial cracks of up to 0.72" long are expected to meet 3aP. The acceptable single axial crack lengths for AP SLBburst capability are substantially higher; hence,3AP is also the limiting condition for single axial cracks. !
5.4 Acceptable MCI Crack Conditions for Tube Burst As developed in Section 3.8, the RPC inspection definition of an MCI requires tho amplitude to return to the null point, and flPC resolution limitations imply a ligament size of the measured RPC ligament plus 30'. Thus, en MCI would have at least 35' ligaments, which were burst tested as described in Section 5.1. As shown in Figure 5 2, the burst pressure for two 35' full thickness ligaments exceeds that for a singie 290* crack. Thus, the burst pressere for an MCI exceeds SLB pressure differentla!s which correspond to a 321' throughwall single crack with full wall thickness ligaments.
Thus an MCI for WEXTEX indications, for which cracning in the remaining ligamnnts is not expected, would inherently exceed SLB burst capability. The burst test results of Figure 5-2 for 35' ligaments are equivalent to burst pressures for cingle throughwall cracks of less than the 226' crack angle for 3AP burst capability. However, not enough tests were performed to strongly support 3AP burst capability for 35' ligaments. For the present study,11is conservative and at, equate to evaluate WEXTEX MCl burst t
l ccpability as that for a single crack equal to the sum of the MCI crack ang!as after reduction of the RPC crack angles by 30' for coillead in and feed-out effects (see Section 3.8); that is, WEXTEX MCI RPC crack angles tota"t about 286*
(226'+30'+30') or ligaments tota!!ing 7t* would meet 3AP burst capability. This 53 1.
h analysis assumes that the cracks are very deep or throughwall, such that the RPC lead in and lead-out reductions of RPC angles are applicable, if crack depths at the edges of the crack are less than about 50% depth, the actual crack angles can be equal to or greater than the RPC angle. In this case, the burst capability would also exceed that associated with the RPC angle so that it would be appropriate to reduce the RPC angle for comparisons with throughwall burst pressures. Thus, total RPC crack angles of 286' for MCI or ? 6' for an SCI would be expected to meet 3aP burst capability.
For ODSCC circumferential cracks, for which a 50% deep crack is assumed in the remaining ligaments, the ligament sizes mus* be targer than the 35' for WEXTEX MCis.
The tests of Sec; ion 5.1 were performed for two 70' ligaments wh 50% deep slots in the ligaments. The burst pressures for this casc exceeded that of a 220' slag!e throughwall crack which meets 3AP burst capability. For the present report, it is conservative and adequate to require the RPC ligaments for TSP MCis to total 80' (resolution adjusted -140*) or the RPC crack angles totalling <280' for 36P burst capability. More simply,if the minimum measured RPC ligament for TSP MGs is 40' or larger, the indication would nieet W burs: capability.
5.5 Acceptable Combined Axial and Circumferential Crack Conditions As noted in Section 3.7, no intersecting circumferential and axialindications have been fcund in the North Anna Unit 1 SGs at either WEXTEX or TSP locations. An RPC-dc'9ctable ligament separating ar'al and circumferentialindications has been identified for a!! mixed mode ind; cations at the same edge of a TSP. No axial and circumfe.ential indications have been faund at the same e'avation in the WEXTEX expansions, so that mixed modo cracking is not a concern for the North Anna Unit 1 WEXTEX expansions.
Based on the RPC resolution tests of Section 3.8 for mixed mode indications, the actual ligament size corresponds to the RPC measured ligament plus 30' (-0.2"). Scoping analyser. indicate that a minimum liga nent of about three wall thicknesses (0.15' or
, ~22') is expected to be sufficient to provkie burst capability comparable to the axial crack aler.e. This is supporteU by tests for smallligaments (EPRI NP-63681, Rev.1)
- that show that li0 aments approaching the tube wall thickness between throughwalt
- circumferential and ax!al(L-shaped) cracks begin recovery of the mixed mode burst pressure loss compared to the axial crack alone. Thus, it is reasonable to expect that 1-S.4
.. - -.....-. a _ _ ._.-..- . . . - ~ - - . - - _ . = . - - . . - - - - _ -
f about a 22' ligament is adequate to obtain burst capability equivalent to the more limiting of the axial or circumferential crack.
Since the presence of an RPC measured ligament between deep axial and circumferential cracks implies a >30' ligament, the presence of an RPC ligament is sufficient to preclude significant reductions in burst pressures due to mixed mode cracking. !
Alterciately stated, if a RPC ligament is found, the burst capability can be evaluated separately for the axial or circumferential crack and no mixed mcde burst assessment is necest iry.
Even if intersecting mixed mode cracking should occur, the burst capability is not affected unless the :hroughwall axial crack length e caeds 0.24 inch. Test results have shown that the average burst pressure of 270' throughwall circumferential cracks with intersecting 0.24 inch long axial cracks f alls on the burst curve for s!ngle ;
circumferential cracks. Since the circurnferential cracks of interest occur near the surface of support plates, the axial crack length in the support plate crevice need not be considered due to den..ag within the TSPs. Only the axial crack length osside of the tube support plate will affect the bu st pressure and this crack length must be greater than 0.24 inch before any significant degradation in burst pressure is observed.
With respect to fatigue crack growth under turbulence and fluidelastic vibration loadings, the lim!!ing smallligament case is usefulc As a circumferentialcrack approaches an axial crack, the stress intensity factor w111 increase. This effect can be bounded by considering the axial crack as another circumferential crack. Hence, frorn previous results, if the ligament b] tween a circumferential crack and an axial crack is greater thar 25', the cracks can be considered non intaracting in terms of a fatigue crack growth threshold.
Based on the above, the presence of an RPC measured ligament implies a >30* ligament which permits the axial and circumferential cracks to be evaluated separately as non intoracting cracks for either tube burst or fatigue considerations. Thus, the RPC ligament precludes the need for more detailed mixed mode cracking evaluations.
L i
5S
t 5.6 Acceptable Single Crack Slees for Tube Vibration The analysis methods and models used for the tube vibration assessments for Noith Anna Unit 1 have been presented in Section 7 of WCAP 13034. This section presents the limiting circumferential crack sizes for the WEXTEX expansion transitions and the TSPs with respect to vibration induced crack propagation.
The tube vibration assessment given in WCAP.13034 was performed for each tube location in the SG. The analyses determir*.ed the uniformly throughwall crack angle at which a circumferential corrocion crack muld potentially propagate by tube vibracon.
Minimum crack angles for propagation were developed for both turbulent and fluidetat'ic Induced tube vibration. For TSP intersections, crack angles for propagation were obtained for a throughwall crack only (TW-oniv) and for a throughwall crack with a 50% deep crack in the remaining ligament (TW+50%). Tabte 5 2 summarizes the minimum crack angles for initiating propagation for conservative unifortnly throughwall cracks and for the expected crack morphology. The expected crack morphology for WEXTEX PWSCC cracks is segmented cracks (short, deep cracks separated by uncorroded ligaments). For ODSCC at TSPs, it is expected that less than 60% of an RPC measured crack angle would be throughwall. It can be seen from Table r 5 2 that the potential for crack propagation is minimal for the expected crack morphology. WEXTEX segmented cracks at all tubs locations and TSP ODSCO indications in central regions would not propagate even for 360* RPC crack angles. Only TSP circumferential cracks at the bottom of the first TSP at the periphery of the tube bundle would propagate for the expected morphology at RPC angles of [ )b.c. It should be -
noted that the circumferential cracks at the upper edge of the top TSP were not evaluated, as circumferentist Indications have not been found above the fifth TSP, although cd TSN were 100% inspected.
The tube vibration assessment for tubes with circumferential cracks near the WEXTEX
- transition region can be summarized for sensitivity assessments by dividing the
- tubesheet region into zones with varying degrees of susceptibility to vibration-induced crack propagation. Figure 5 5 shows a tubesheet map with the various zones identified.
The zones are described as follows:
5C
t Zone 1 Tubes with potential for initiation of vibration. induced crack propagation for throughwall crack angles of [ Jb,c, P
Zone 2 Tubes with potential for initiation of vibration-induced crack propagation for throughwall crack angles of [ )b,c, Zone 3 Tubes with potential for inillation of vibrallorhinduced crack propagation for throughwall crack angles of ( )D.C.
Zone 4 Tubec for which initiation of vibration induied crack propagation requires throughwall crack angles of [ Jb,c; Zone 4 corresponds to the low flow, sludge deposition area.
Zone 1 contains 620 tubes, Zune 2 contains 344 tubes, Zone 3 contains 940 tubes, and Zone 4 contains 1484 tubes. The minimum angles for crack propagation apply to only a few limiting tubes in each zone, For the most limiting peripheral tube locations in Zone 1, for a crack at the WEXTEX transition, the initiation of turbulence driven propagation occurs for a throughwall crack angle of [ ]b,c;in this condition, the tube r4 mains stable and turbulence driven propagation occurs until the crack angle reaches [ }b.c, which is the crack
, ngle associated with fluidelastic instability. The limiting total growth time curing the
.urbulence propagation phase is [ )b,c. Once the crack has reached [ ]b,c, the tube vibrates in a fluidelastic manner and fluidolastic crack propagation is initiated.
The additional fluidelastic driven growth time to tube rupture is calculated to be [
]b,c. The crack angle vs. time plot showing the turbulence and fluidelastic driven growth times is provided in Figure 5 6. For Zone 1 with crack angles <169*, all crack propagation would be initiated by turbulence induced vibration. Thus, Zone 1 defines all tubes which would initiate vibration below the crack angle for fluidolastic crack
- propagation.
~
[ For the limiting tube in Zone 2 with a crack at the WFXTEX transition, the initiation of turbulence driv.m oropagation occurs for a throughwall crack angle of [ ]b,c;in this condition, the tube remains stable and turbulence driven propagation occurs until the crack angle reaches the critical angle for fluidelastic vibration, after which crack 5-7 ,
propagation is driven by fluidelastic instability. For the limiting tube in Zone 3 with a crack at the WEXTEX transition, the initiation of turbulence driven propagation occurs for a throughwall crack angle of [ )b,c;in this condition, the tube remains stable and turbulence driven propagation occurs until the crack angle reaches the crack angle l required for fluidelastic instability. For the limiting tube in the central sludge deposition arca (Zone 4) with a crack at the WEXTEX tr u.t.i La, ihe initiation of turbulence driven propagation will not occur unless the throughwall crad angle exceedt
[ )D C; however, a tube in Zone 4 vibrating in a fluidelastic manner would i experience vibration induced crack propagation if the throughwall crack angle exceeds i
[ Jb,c. Zone 4 was found to include >90% of the WEXTEX indications observed during the 1992 mid cycle inspections. ,
For circumfesntial cracks at the TSPs, only cracks at the bottom edge of the first TSP have significant potential for vibration induced propagation up to the largest angle of 244' throughwall that was evaluated. Above the first TSP, cross flow velocities and tube vibration amplitudw are sufficiently low as to be insufficient to cause tube vibr' ion induced crack propa.Ttion. The TSP circumferential cracks have an ODSCC crack morphology. The RPC threshold for detecting ODSCC circumferential cracks is about 50% depth. For the tube vibration assessment of ODSCC cracks at the TSPs, the portion of the tube apparently free from degradation is conservatively assumed to have 50%
throughwall degradation.
The tube vibration assessment for tubes with circumferential cracks at the TSPs utilizes the same zones with varying degrees of suscep!'bility to crack propagation as described for the WEXTEX region. Figure 5 5 shows the various zones used. Although the tubes within each zone are unchanged from the WEXTEX tube vibration assessment, the limiting crack angles for the onset of vibration induced propagation are different than in the WEXTEX tube vibration assessment. The zones for the TSPs are defined as fo'.ows:
Zone 1 Tubes with potential for initiation of vibration induced crack propagation for throughwall crack angles 2122' Zone 2 Tubes with potential for initiation of vibration-induced crack
[ propagation for throughwall crack angles 2170*
5-8 t
_.__ _.-._.._ _ _._ __.-._ _ _ ~ _ _ . _ _ . _ - _ ..__ _ _.. _
i ZonJ 3 Tubes with potential for initiation of vibration induced crad i propagation for throughwall crack angles 2230' Zone 4 Tubes for which initiation of vibration induced crack propagation requires throughwall crack angles of greater than the 244' minimum throughwall crack analyzed for crack propagation, For the most limiting peripheral tube locations in Zone 1, for a circumferential crack at the bottom sf the first TSP with 50% throughwall degradatbn assumed in the region apparently free from degradation (TW+50%), turbulence driven propagation initiates at a throughwall crack angle of [ )b,c;in this condition, ihe tube remalns stable and turbulence driven propagation occurs until the throughwall crack angle reaches the 244' crack angle to sociated with fluidelastic Instability. Figure 5 7 shows the time history of crack propagation for the limiting peripheral tube. If the most limiting Zone 1 tube location is assumed to have a throughwall crack with no degradation in the ,
remaining ligament, the limiting crack angle for the onset of vibration induced crack propagation increases to [ b J,c.
For the limiting tube in Zone 2 with a circumf6:etlat creck at the TSPs, the initiation of turbulence driven propagation occurs at [ ]b,c (TW+50%); in this condition, the tube remains stable and turbulence driven propagation occurs until the crack angle reaches the point of fluidelastic instability [ )b,c, For the limiting tube in Zone 3 with a crack at the TSPs, the initiation of turbulence driven propagation occurs for a
! throughwall crack angle of [ )b,c (TW+50%). For the limiting tube in the central sludge deposition area (Zone 4) with a circumferential crack at the TSPs, the initiation of vibration induced crack propagation will not occur unless the throughwall crack angle cyceeds the [ )bc maximum crack angle analyzed; this is true for both the -
TW only and TW+50% conditions.
f l
5-9
. . - _ . - . - . ._- - . -. - . . - . - - . ~ . . . - - .
. . - . - - - - - _ - - _ ~ . . . . . - . - _ . . . . _ _
Table 51 Sin 'e Circumferential and Axial Crack Tube Burst Capabilhy Summary CRACK HORPHOLOGY 34 P N .O. APSLB l
CIRCUMFERENTIAL CRACK ANGLES SEGMENTED CRACKS SINGLE CRACK SINGLE CRACK WITH 50%
DEEP. CRACK -- -
AXIAL CRACK LENGTHS - INCH SEGMENTED CRACK $
SINGLE CRACKS 1
l l
5 10
- . _ . . . ._ _ - - _ _ _ _ _ _ _ . _ _ _ _ _ . . = _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . . _ _ _ _ _ _ _ _ _ _ _ _ _ . _ . _ _ _
Table 5 2 ACCEPTABLE RPC CRACK ANGLES FOR TUBE VIBRATION WEXTEX PWSCC TSP ODSCC(l)
TW ONLY TW+ 50%(2)
CONSERVATIVE TW CRACKS 0 PERIPHERAL REGION _
b,c
- TURBULENCE FLUIDELASTIC 0 CENTRAL REGION
- TURBULENCE FLUIDELASTIC -
EXPECTED CRACK HORPHOLOGY 0 PWSCC SEGMENTED CRACKS - - b.c
- TURBULENCE FLUIDELASTIC _ _
0 ODSCC HODEL
- PERIPHERAL; ,_
b,c TURBULENCE FLUIDELASTIC
- CENTRAL
- TURBULENCE FLUIDELASTIC -
NOTES:
- 1. LIMITING CRACK ANGLES AT TSPS APPLICABLE TO BOTTOM OF 1ST TSP ONLY. HIGHER ELEVATIONS TO
_ BOTTOM b pF TOP TSP REQUIRE TW CRACK ANGLES
, )EVALUATEDFORCRACKPROPAGATIONDUEi.*
VIBRATION. _
- 2. THROUGHWALL CRACK -
] a.c
- 3. HOST LIMITING PERIPHERAL TUBE LOCATIONS
- 4. ODSCCH0DELASSUMES[
]ac 1A72026:022692 5 11
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l t
Figure 51 Burst Test Et/M Slit Geometries and Test Results for MCis (Cross sectional views; shaded areas are remaining ligaments) b,c I
soep
- l 5 12
Figure 5 2 Burst Pressure vs. Total Circumferential Crack Angle, -
Multiple Cracks Compared with Single Crack Case t
BURST PRESSURE VERSUS TOTAL CRACK ANGLE '
, 7/8X0.050. INCH TUBING VITH TSP RESTRAINT _ ,
W M
W 5
.m W
W W
CL.
5 ca L
1 l
l 5 13
.._ _ ._ _ - ~ - _ . . ._._ _. _ _._. _ . . _ _ . . _ . . -
Figure 5 3 Fatigue Test EDM Slit Geometries (Cross sectional views; shaded areas are remaining ligaments)
I r
110' wr 180* of Wall Remaining 70' ,
70' i
135' 35' 50% of Wall Remaining i
_ 35' two 70' Ligaments 35' 35' i
3-35' Ligaments Remaining i
5 14
Figure 5 4 Measured Fatiguo Crack Growth Rate Vs. Stress Intensity Range Conventional and Ugament Geometries
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6.0 TUBE INTEGRITY ASSESSMENT l
6.1 Crack Distributions for Second Half of Cunent Fuel Cyc'e )
The second half of the current fuel cycle is planned for 252 EFPD compared to 254 EFPD for the first half of the cycle. Thus,if inspection methods and operating conditions (Thot) were the same for both hafves of the cycle, the last (mid cycle,2/92) inspection results on number and size of Indications would be typical of that expectud at the end of the currant cycle. However, the operating interval for the second half of the fuel cycle (henceforth, current operating cycla) is expected to result in both fewer and smaller indications than found at the last inspection (Section 3.0). The following two co' crations lead to the reduced crack severity ant!cipated for the current operating cycle:
o As described in Section 4.0, the reduced power and end of cycle coastdown i planned for the current operating intervallead to an approximately 7%
reduction in the number of newly initiated cracks and in the growth rates of existing cracks undetected at the last inspection, o As described in Section 3.11, the ' inspection transient" introduced by the first 100% RPC inspection at the TSP intersections would lead to a reduction in the number and size of indications at the end of the next operating interva!.
Due to the relatively low growth rates found for circumferential cracks at both the TSPs and WEXTEX transitions, newly initiated indications are not a significant concem for tube integrity in the current operating cycle. More important are the indications already present, but undetected, and their growth over the operating cycle. Thus the influence of the reduced power conditions on crack growth is more important to the tube integrity assessment than their influence on crack initiation.
The 7% reduction in crack growth from the reduced power conditions would apply to both growth in depth and in crack angle. However, only crack angle is measured with confidence by the RPC inspection. Thus the growth redu; tion is applied only to the crack angle. The largest crack angle growth rates can be expected to result in the limiting >
crack angles. This is particularly relevant following an inspection transient, such as 6-1
_ , . _ _ _.___.__a_-_
M first 100% RPC inspection at the TSPs in 1992, for which the large crack angles
$. ar. be expected to have been detected. From Table 3-10, the largest crack growth rates
) found at TSPs were 39' and 45' for the last completed cycle. A 7% reduction in towth
?
rate would reduce growth by about 3' and therefcre 'ead to an expected reduction in the largest crack angte from 240' (Sec' ion 3.4) to about 237*. This correction is small y and can be applied for all Indications for the expected EOC crack dir' lbution.
hk' ,
[ -
..ispection 1ransient", as discussed in Section 3.11, applies to the TSP indications
., ' J at North Anna-1 in tnat a 100% RPC inspection was performed for the first time, ne first inspection with more sensitive probes or EC nnalysis guidelines tends to be
- wed by both fewer and smaller indications at the subsequent inspection. This alies for the expected end-of-c;ycle (EOC) indications at the TSPs being evaluated in g this report. In Section 3.11 (Table 3-14),it is shown that the RPC inspection ;
transient resulted !r :$ reduction in the numte r of indications by a factor of 7 for the North Anna Unit 1 vvtiXTEX indications. From Tables 311,3-12 ano 314, the reductions 'or other tubesheet expansion indications were factors of 28,6, and 2. No direcby comparahle data is available for ODGCC at TSPs. It is reasonable to expect that the nerv.ber of 7 SP indications at the end of the current cycle will be at least a factor 1.5 lower thar : m e' b . last inspection. Tnis conclusion is applied in Sectbn 7 for the es pected SLB isk rate at EOC conditier , ,
b The WEXTEX inspection transients also have been fourid to result in smaller crack angias :
at tile subsequent inspection. For example, the mar! mum WEXTEX indication found at the first North Anna-1 RPC inspection was about 247* while the subsequent inspection found a maximum angle c! 199* (R25C36, S/G B). The 199* followed a 9 month cycle.
The growth of this indiertion over the 9 mond, cycle was 24' Adding an additional 24' growth to the 199' indication yinids an estimated 223' indication for an 18 month cycle.
Thus the RPC inspection transient has resulted in a reduction of ~24' in the maximum WEXTEX indication. For Plant L and North Anna Unit 2, the reductions in maximum circumferential crack ar.gles bvween the first and subsequent RPC inspections were 105' and 110'. Thus it is reasonable to expect that the 10C% RPC irspection at the TSPs will result in a reduction of the maximum crack angte of at least 20' at the EOC 1
_a compared to the last inspection. This corresponds to about an 8% reduction in crack angles auch as for the 240* largest indication found at ihe TSPs.
62
I Based on the above discussion, lite expected EOC crack angles at TSP intersections are assumed to be reduced by 8% due to the RPC inspection transient and by about 3* due to the reduced power conditions. For example, the largest TSP crack angle of 240* at the last inspection would be expected to be (240' x 0.92) 3' = 218' at the next EOC.
Table 6-1 summarizes the above influence of reduced temperatures and the TSP inspection translent on the expected number of indications and crack angles for the current North Anna 1 opnating cycle. The expected maximum crack angles are used for the tube integrity assessment given in the following sections.
1 Prior to the tast outage, axial indications outside the TSP were detected by bobbin coil I inspection and confirmed by RPC. The 100% RPC inspection increased the number of detected axialindications from 126 to 286 or close to a factor of 2.2 increase. Thus it is reasonable to apply the factor of 1.5 reduction in number of indications for the TSP circurnferentialindications to the axialindications. The maximum crack length found in i the 1992 inspection was shorter than found in tne 1991 inspection, which was likely !
due to the shorter opere'"/; cycle. For conservatism in the tube integrity and SLB leak rate analysis, the effect or reduced temperatures and the RPC inspection transient at ignored for axial crack lengths. The axialiengths found in the 1992 inspection are apphed for the end of the current cycle.
6.2 WEXTEX Tube Burst Assessment The largest RPC circumferential, single crack angle found in the last outage was 199*
(Set Figure 61). From Table 6-1, the hwer temperature conditions for the current cycle would be expected to reduce crack growth and result in a crack angle of about 196*.
In either case, the largest RPC crack angle found in the last cycle or oxpected for the current tyycle is less than the acceptable 226* through wall crack angh ' hat meets 3AP tube burst capability.
The MCI burst test results given in Section 5.1 and evaluated in Section 5.5 show inat p MCis separated by ligaments of 30-35' or more result in burst pressures exceeding
( that for an SCI having the same total crack '.ngle. As noted in Section 3.6.1, the most limiting MCI found in the last inspection was tube R24C33 in S/G A (See Figure 6-2.)
This indication had RPC crack angles of 149* and 85* separated by a measured 12*
63
lignment. After adjusting for RPC resolution (See Section 3.8), the minimum ligame-*
- would be 42*. Thus the smallest ligament exceeds the 35' minimum ligament to support axeptable MCI burst capat,luty. Ths total RFC crack angle of 234' (174' adjusted for RPC rasolution) ls less than the kB6' developed in Section 5.5 for 3AP burst capability.
Thus the WEXTEX MCis found in the last intpection exceed 3AP burst capability and
'epresent an upper bound on indications expected !n the current cycle, 6.3 TSP Circumferentiat indication Tube Burs' Assessment Two TSP circumferential indications were identified in the last outage with RPC angles exceeding the 3AP burst capability of 226'. The associated RPC angles were 240' and 239' as shuwn in Figures 6 3 and 6-4. As discussed in Section 61, indications at the ond of the current cycle wouid be less than the last cycle as a mnsequence of the 100%
RPC Inspection transient and the reduced temperature conditions of the current operating cycle. As developed in Table 61, the expected maximum TSP crack ar.gle at the EOC would be 218' corresponding to a 240' crack in the prior cycle. Thus the limiting crack angle at '.he currer. IOC would be less than 226' and satisfy 3AP burst capability, it can be noted that even the 240* indications found at the last outage would meet 3AP burst capability. The 226* angle for 3AP capability is based on a umformly through wall crack. From Figures A-3 and 6-4, it is seen that the high amplitude responses for the indications correspond to angles of about 134' and 96* for the two indications. The deepest part of the cracks correspond to the largest amplitudes. Thus, even if the indications l'.ad through wall penetration, the through wall angle would be <60% of the RPC crack angle. Thus these indications would meet 3AP tube burst capability.
The MCI burst test results (Sections 5.1 and 5.5) for through wall cracks p'us a 50%
deep crack in the remaining ligaments show that MCis separated by ligaments of 70*
, meet 3AP tube burst capability. The maximum sum of crack angles in this case would be 220', or less than the 226' for 3AP burst capability. From Table 3 6, the limiting TSP MCI was at TSP 3H in Tube R2C41 (See Figure 6-5). The RPC measuredligaments for this indication were 66' and 41' which, after inc adjustment for RPC resolution developed in Sections 3.8 and 5.5, would exceed the 70* ligaments required fcr 3AP tube burst capability. In addition, the MCI crack cngles would ba educed by about 30* for 6-4
. __ _ _ - . _ _ _ - _ . _ _ __. m _ _
regions for all steam generators during the February 1992 inspections. Zone 4 contains 90% of the WEXTEX circumferentialindications.
The throughwall crack angles required for the initiation of turbulence and fluidelastic driven crack propagation are given in Section 5.6. The WEXTEX region circumferer.tlal cracks at North Anna Unit 1 are comprised of PWSCC segmented cracks. Crack propagation due to turbulent or fluidelastic vibration does not occur for the expected segmented PWSCC crack morphology, even for crack network angles of 360'.
Nevertheless, the crack angles observed during the 1992 mid cycle inspection are compared to the acceptable crack angles from Section 5.6 below.
Only one WEXTEX circumferential indication was observed in Zone 1, in R25C8 of SG B, with an arc length of 142' measured in the field; laboratory reanalysis of the field data produced a measured are length of 157* The throughwall extent cf this crack was less taan the minimum [ ]b c necessary for turbulence driven crack prcpagation in Zone 1; furthermora, the calculated throughwall angle necessary for turbulent crack propagation at the location of R25C8 is [ )b,c. No circumferentialindications were observed in Zone 2. Two WEXTEX circumferentialindications were observed in Zone 3; both had RPC angles less than the [ )b.c ar:gle necessary for initiation of turbulence driven propagation (the maximum crack angle was 118'in R21C23 of SG A). The max; mum crack angle in Zone 4 was 199', in R25C35 of SG B. All WEXTEX region circumferentialindications in Zone 4 had RPC crack angles less than the [ ]b,c minimum through wall angle required for crack propagation. Therefore, it is concluded that no WEXTEX indications are subject to crack propagation due to tube vibration.
G.7 TSP Tube Vibration Assessment The tube vibration assessment for tubes with circumferential cracks at the TSPs is performed by dividing the tubesheet region into zones with varying degrees of susceptibility to crack propagation caused by tube vibration, as descnbed in Section 5.6.
Figure o-5 shows a tubesheet map with various zones identified. The zones for the TSP tube vibration assessment are the same as the zones used for the WEXTEX assessroent; however, the acceptable crack angles for T"o circumferemial cracks are somewhat different, as summarized in Section 5.6. 1escribed in Section 5.6, only the l circumferential cracks at the bottom edge of the first TSP have significant potential for 6-7
l vibration induced fatigue. Therefore, the following discussion addresses only the indications in the first TSP region.
Figure 6-7 shows the distribution of circumferentialindications at the bottom edge of
]
the first TSP in all SGs. Of the 31 circumferential indications observed at the bottom edge of the first TSP,7 indications were within Zone 1. The longest,167' in R45C50 of 1
SG A, had well defined, deep crack ar'gles of ~93*; this is less than the [ ]b,c l
through wall angle required for vibration induced crack propagation for the most t liraiting Zone 1 tube location and the [ l b,c required angle for the R45C50 tube -
- ocation. The remaining six circumferential indications, all had crack angles of 104' or j less. All three of the circumferentialindic ns in Zone 2 were less than the [ }b,c 'l minimum throughwall crack angle necessar) for vibration induced propagation. All six ;
of the circumferentialindications in Zone 3 were less than the [ ]b.c minimum taroughwall crack angle necessary for vibration induced propagation. All fifteen of the l
circumferentialindications in Zone 4 were less than the 244' rnaximum throughwall I
- crack angle analyzed for vibration induced propagation. Only the 31 indications at the bottom edge of the first TSP were potentially susceptible to vibration induced propagation. Therefore, nona of the circumferentialindications at the TSPs are expected to experience tube vibration. . - 2d crack propagation, j - 6.8 MCl and Mixed Mode Tube Vibration Assessment All of the MCis in the WEXTEX region wero observed in Zone 4, where the minimum throughwall crack angle necessary for vibration induced crack propaga@n was 275*.
Accounting for the segmented crack morphology in this reg!on, the circumferential cracks would be required to reach 360' for crack propagation. None of the MCis observed in the February 1992 inspections were su'ficiently long to reach these limiting crack angles. In addition, there were no occurrences of mixed mode cracking observed in the WEXTEX region.
As described previously, only cirumferential cracks at the bottom edge of the first TSP were found to have the potenti.31 for vibration induced propagation during the 1SP tube vibration assessment. There were no occurrences of MCis or mixed mode cracking at the bottom edge of the fWt TSP.
68
. _ - - . _ -- _ _ _ - - -. -- -~ -
ths current cycle due to the RPC inspection transient and reduced temperature conditions.
In summary, the current EOC indications can be expected to meet 3AP tube burst capability. Indications found in the last inspection meet 3AP capability and the affects of the RPC inspection transient and reduced temperatures would further increase the tube burst margins.
6.4 TSP AxialIndication Tube Burst Assessment Only the axial crack lengths above and below the edges of the TSPs are considered for the tube burst and leakage evaluations; the extensive denting at the TSPs is considered to prevent axial cracks within the cor. fines of the TSPs from opening. The axial crack lengths were measured by RPC, rather than by Dobbin as in previous outages; EPRI c'ocument NP-6368-L demonstrates that there 6 good agreement c' RPC-measured crack lengths with putted tube data. The axial crack lengths measured by RPC during the 1992 inspections were shorter than crack lenoths measured in previous bobbin
' inspections, likely due to the reduced operating cycle time and to reduced lead in and lead out effects for RPC compared to the bobbin coil. During the 1992 mid-cycle inspection program, the longest axial crack length o"' side the TSPs was 0.49 inch; this was less than the axial crack lengths of up to -0.9' observed in prior outages.
As described in Section 5.3, the exposed axlat crack length resulting in burst at 3AP is
[ )b,c and the length for burst at APSLB is [ ]b." based on the Eelgian burst equation. However, examination of pulled tubes at North Anna Unit 1 has shown " e presence cf small ligaments in the axial crack networks (WCAP-12349).
Accounting for the segmented crack burst capability as described in Section 5.3, the longest macrocrack with elastic ligaments to meet 3AP is [ )b,c and macrocracks up to [ )Ec in length moet APSLB-As described in Section 6.1, the effect of reduced temperatures and the RPC inspection transient are ignored for axial crack lengths, and the axiallengths found in the 1992 inspection are applied for the end of the current cycle for conservatism. For exposed lengths of axial cracks above or below the TSP edges, the maximum expected length is 0.49 inch. On a segmented crack basis, this is well below both the [ ]b,c length 65
for 3AP burst capability and the [ ]b,c length required for APSLBburst capabill:y. Therefore, the current EOC axialindications can be expected to meet botha e 3AP and AP SLB burst requirements.
6.5 Potential TSP Mixed Mode Assessment Of the 469 tubes plugged for axial and circumferentialind: cations at TSPs, there wem 14 occurrences of circumferential and axialindications at the same TSP. Table 3-8 summarizes the locations, circumferential crack angles and ligament angles of all potential m!xed mode indications. Seven of these occurrences involved axial cracks ehher inside the TSP or on the opposite edge from the circumferential cracks; these occurrences are not a concern for mixed modo considerations. The other seven
, occurrences involved axialindications at the same edge of the TSPs as the circumfer.mlindications.
From Table 3-8, all TSP indications with axial and circumferential indications at the same edge of the TSP bcv3 measured RPC ligaments. Tube R30C67 has the smallest RPC measured ligament of 5', which implies (Sections 3.8 and 5.5) a minimum ligament of >35' when adjusted for RPC resolution of deep crack indications. This ligament size is also obtained using the derived ligament angle of Table 3-8.
As developed in Section 5.5, the presence of an RPC ligament permits both tube burst and fatigue evaluations to be performed based on separate, non-interacting cracks. Thus, none of the TSP indications of Table 3 b quire a more detailed mixed mode evaluation.
The axial and circumferentialindications given in TEble 3 8 meet 3AP burst capability when evaluated as non-interacting indications.
6.6 WEXTEX Tube Vibration Assessment The tube vibration assessment for tubes with circumferential cracks near the WEXTEX transition region is performed by dividing the tubesheet region into zones with varying decrees of susceptibility to crack propagation caused by tube vibration, as described in Section 5.6. Figure 5-5 shows a tubesheet map with various zones identified. Figure 6-6 shows the locations of all 31 circumferential indicat;ons found in the WEXTEX I
66
Table 6-1 Expected Reductions in Circumferential Crack Angles for Current Cycle Causative Factor TSP Cire. Indications WEXTEX Cire. Indication _s Nojnd. Crack Anotes No.Ind. Crack Anales Reduced Temp. & Ignored 3' ignored 3' Power Conditions RPC Inspection S6%* 8% Not Applicat,te Transient influence on C rent Ooeratino Cveig Mar, Crack Angle ---
240* ---
199*
Last Cycle Resulting Expected --
218' --
196' Maximum Crack Angle in Current Cycle No. Indications 212 --
31 --
Last Cycle Expected No. Ind. 140 --
31 - - -
in Current Cycle Also applied for TSP axial indications.
69
.~ . . - - - .- _
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distributions for the hot leg WEXTEX and TSP regions are conservatively projected to be the same as those found duritig the 100% RPC inspections of those regions performed in January - February 1992. This assumpticn is based on similar lengths of operation (254 EFPD for the first half of the current cycle vs. 252 EFPD for the second half of the cycle). Changes in operating ccaditions (Section 4.0) and inspection transient considerations (Section 3.11) dictato ii.at the number and size of indications at the end of the current fuel cycle are expected a be less than tSose found at the end of the first half of the fuel cycle. Therefore, use of the crack distributions shown in Tables 7-4 and 7-5 for the limiting SG is judged to p ovide a conservative, bounding leak rate.
7.2 Leak Rate f.nalysis Methods 7.2.1 Leaka0e Modol for Axial Cracks An analytical model hw been developed for predicting primary-to-secondary leak rates through cracked steam generator tubes. Tho model is based on [
ja.c. The model predictions are shown to exhibit acceptable agreement with laboratory and pulled tube test data. The leakage mode' for axial cracks has been described in detailin Section 6.3 of WCAP-13034. A comparison of the crack model ter.:!!I with experimental results is provided below.
The crack data base consists of field (pulled tubes) and Westinghouse laboratory formed stress corrosion cracks. Crack model results are compared with the test data in Figures 7-1 and 7-2 for normal plant operation and steam line break conditions, respectively.
The crack model results are also compared to other experimental data (labeled crack data base in the figures). As indicated in the figures, good agreement between prediction and measurement is shown. Data scatter is attributed to crack geometry parameters which are difficult to define with any degree of precision. Crack geometry parameters which affect flow are [ ja,c, An error analysis of the measurements versus predictions was performed to obtain the standaid dev:ation for uncertainty analysis. The standard deviation of the prediction is 7-3
[ jb,c for normaloperation and [ }b,c for stearn line break for the log log
- comparisons of Figures 7-1 and 7 2. For an uncertainty factor, t , the predicted leak rate would be factored by [ }b,c for normal operation and by
[ ]b,e for steam line break.
7.2.2 Leakage Relationship for Circumferential Cracks The semi-empirical relationship for leakage through circumforential cracks is given by
[ jb,c where O ls the leak rate in gallons per day (gpd), AP is the differential pressure in psi and the crack length is the total crack length in inches. This relationship is benchmarked by selected data points on measured leak rates and the fact that for small l- cracks,[
l L
l s
! jo,c, 7.3 Leak Before Break (LBB) Capability The leak-before breck rationale is to limit the max! mum allowable primary to l - secondary leak rate during normal operation such that the associated crack lengtn is less than the critical crack length corresponding to tube burst during a postulated steam line
. break event. Thus, on the basis of leakage monitoring during normal operation, it is assumed that unstable crack growth leading to tube burst would not occur in the unlikely 7-4
7.0 - St.B WAK RATE ANALYSES The SLB leak rate analyses are performed for two sets of crack distributions. Expsc:ed leak rates are calculated based on expected reductions in the number and sizr of indications at the end of the current fuel cycle as compared to those found at ino end of the first half of the current cycle in February 1992. Bounding !eak rates are calculated <
based on the actual number and size of indications from the Febmary 1992 inspection programi since both hafves of the current fuel cycle are expected to have approximately the same duration in effective full power days, the 1992 crack distributions are judged to produce conservative leak rates based on inspection transient considerations.
7.1 Crack Distributions for SLB Leak Rates For the SLB leak rate analyses, the number and size of indications are required for the WEXTEX and TSP circumferential indications as well as the TSP axial cracks. Tables 71 through 7 3 summarize the crack distributions found in February 1992 at the end of the first ha:f of the current fuel cycle. Table 71 presents the 1992 WEXTEX crck distributions for all three SGs. (Although shown in Table 7-1, axial cracks in or above the WEXTEX region are not considered in the SLB leak rate analyses, since all axial cracks exeeding 40% through wall den. " e plugged before retuming to power and -
throughwall cracks are not expected in one cycle of operation). The TSP 1 circumferential and axial crack distributions for all three SGs are summarized in Tables l 7-2 end 7-3, respectively. _
The SLB leak rates are calet; lated baseo on the crack distributions for the most limiting -
steam generator, Although SG C has the fewest number of tubes with WEXTEX circumferential cracks (9 in SG C vs.12 in SG A and 10 in SG B), SG C has the most tubes with circumferential and axial cracks at the TSP elevations. The calculated leak
[ rates for the TSP circumferential cracks are substantially larger than for the same crack sizes in the WEXTEX ngion due to the occurrence of segmented PWSCC cracks in the WEXTEX region.- Based on the number of tubes with TSP circumferential and axial cracks in each SG, SG C is therefore selected as the most limiting SG for the SLB loak I rate analyses. The numbers and sizes of WEXTEX and TSP circumfercntial cracks for SG C are shown in Table 7-4. The number and sizes of TSP axial cracks for SG C are shown in Table 7 5. The values shown in Table 7-5 are for crack lengths outside of the 7-1 cn.,
.l TSPs; since the North' Anna 1 TSP intersections are dented, only the cracks extending l outside of the TSPs are considered to leak during SLB.
7.1.1_ Crack Distributions for Expected SLB Leak Rates The expected reductions in the number and size of indications at the end of the current fuel cycle, compared to those found I;) February 1992 after the first half of the current l
cycle, were developed in Section 6.1. For the SLB teak rate analyses, the number and size of indications are required for the WEXTEX and TSP circumferential indications as well as the TSP axial cracks. ;
From the evaluations given in Section 6.1, the current EOC indications can be obtained by the following adjustments to the inspection results of Feb:uary 1992:
4 WEXTEX Circumferential Indications
- reduction in crack Engles by 3*
no change in the number of indications TSP Circumferential Indications
- redu: tion in crack angles by both 3* and 8% of the last inspection results reduction in the number of indications by a factor of 0.67 TSP AxialIndications
- no change in crack length i
- reduction in the r. umber of indications by a factor of 0.67.
The above adjustments are applied to the crack distributions from the February 1992 inspection program to obtain the expected EOC SLB leak rate for the second half of the current fuel cycle. Tables 7-6 and 7-7 show the crack distributions used to obtain the expected SLB leak rate for the most limiting SG.
7.1.2 Crack Distributions for Bounding SLB Leak Rates A bounding SLB leak rate is also calculated based on tho actual crack distributions found at the end of the first half of the cront fuel cycle. The envelopinc 500 crack 7-2
event of a limiting accident. Leak before break provides protection against t %Jpled steam line break and tube rupture event which is outside of the ds!gn basis S she plant.
The edministrath,e leak rate limit for normal operation b 50 gpd ptr steam gr n'<ator, The leak before break rationale for a single through wall crack, grawing in an utderly fashion is straightforward. However, typical cracking patterns are more comphx.
Crack networks rather than single isolated cracks appear most ottan. Pulled tubs examination results and laboratory crackea samples provide examplos of crack networks. As discussed above, ligaments of materia! between through wall uacks in a crack network increase tube strength reittive to that of a single through wallcraik of
' the same total length. While ligaments incrcase strength, they also decrease the laak rate relative to the same length for a angle through wall crack.
7.3.1 Tubes with Axial Cracke The leakage model for tubes with axial cracks, described in Section 7.2.1, was appved using North Anna 1 SG conditions to obtain the following predicted leak rates for nemial operation and SLB as a function of crr
- length:
Cra.k Leak Rates Lenath 110, LLQ. SLB (inch) (gpm) (gpd) (gprn}
_ b.c j
. ,J
.Conside ation has been given to the error of the predictions of leak rate compared to measured leak rates. In Section 7.2.1.3, the error is given as [
ja c 7-5
O I
- a,c, During normal operation, the 01 # rate limit is 50 gpd for North Anna Unit 1. Referring to the tabulations abcve and the mean curve of Figure 7 3 of leak rate versus axial creck length, the allowable leak rate of 50 gpd willlimit the through wall axial crack length to
[ ]b,c. Since the through wall 3AP crack length (Section 5.3) is [
]b,c, LBB is assured on a mean leak rete basis. Conidering the prediction error for
=95% certainty, the axial crack length will be no longer than [ ]b,c. At this length, AP SLB ls satisfied with margin since [ )D C is acceptable.
For segmented cracks, the conservetive assumption is that the ligaments are elastic at normal operation AP and the leak is therefore a minimum equal to the sum of the leakage of the individual cracks. The macrocrack length is therefore maximized and strength -
minimized orhis basis. The leak rates for segmented cracks are plotted in Figure 7-4 with the points for the typical .5/1 aspect ratio crack shown as 'he 0.25' segments line.
L For this typical aspect ratic, on a mean predicted leak rate base the 50 gpd limit corresponds to a macrocrack containing [ jb,c, From Section 5.3, the macrocrack length needed for 33P burst capab'lity is less than or equal to [ jb,c, which is less than the leak limit permitted. [ -]b,c, However, LBB is assured for segmented cracks fcr t.PSLB since a [ ]b,c macrocrack would have APSLB burst capability.
7.3.2 Tubes with Circumferential Cracks
- The leak rate model of Section 7.2.2 for tubes with circumferential cracking results in the following predictions of leak rate for North Anna 1 SG normal operating and SLB conditions as a function of arc length:
7-6
l Arc Leak Rates Length LLO. $1,8 S].S (degrees) (gpd) (gpd) (gpm)
~
l b,c With the 50 gpd leak rate limit, the corresponding are length of a s ngle circumferential crack is slightly less than [ ]b,c. Considerable margin is available versus the single crack that lowers tube burst capability to 3aP, [ ]b,c, and to APSLBI l b,c (Section 5.3).
For WEXTEX clicun;ferential indications, the expected crack morphology is segmented cracks. Segmented cracks are also present for ODSCC in the initial stages of crack formation, although the limited pulled tube data indicates that the OOSCC !igaments may
! be lost by corrosion for throughwall cracks. Figure 7-5 shows the leak rates for segmented circumferential cracks. For aspect ratios of 5/1 and 4/1, the 50 ppd leak limit corresponds to 110* and 180*, respectively. Since these angles are less than the l 297* segmented angle for 3AP burst, LBB capability is obtained for segmented.
I circumferential cracks.
Establishing the operating leak limit of 50 gpd based on segmented crack morphologies, l for which axial PWSCC cracks are most limiting for LBB considerations, recognizes the expected crack morphology and provides substantially greator LBB margin than obtained assuming uniformly throughwall cracks. North Anna Unit 1 has had small operating leak rates although measured RPC crack angles are significant. This reflects the presence of ligaments (segmented and remaining wall thickness) that reduce leakage ano increase
- tube burst capability.
l l:
l l
} 7-7
i 7.4 Limiting Crack SLB Leak Rate This section develops estimated leak rates under SLB conditions based on maximum leakage from the limiting crack leaking at 50 gpd under normal operation. Look rates for the EOC crack distributions are developed in Section 7.5. Use of the EOC crack distributions is highly conservative in that the EOC distr;bution would be expected to leak at wellin excess of 50 gpd under normal operation based on application of the leak rate and crack models used in this report.
The potential for leakage from a single crack during SLB is limited by the 50 gpd normal operation leak rate limit. Assuming operation at the 50 gpd limit, it is 95% certain that a single through wall axial crack (a conservative assumption) is shorter than [
]b c (Figure 7 3). It is also a 95% certainty that a [ }b,c axial crack would leak less than [ ]b c based on nominal SLB leakage of [
]b,c for 95% certainty (Section 7.3.1). The assumption of 95% uncertainty on normal operating leakage r9sulting in +95% uncertainty at SLB leakage is extremely conservative. This process leads to an SLB to normal operating leak rate ratio of 275 compared to the nominal ratio of about 10. Extensive testing of leak rates on corrosion cracks in WCAP 13187 has resulted in SLB/N.O. leak rate ratios of 1 to about 80 with 95% of the 33 specimens tested having a ratio of <20. P a ratio of 20, the 50 gpd normal operating leak rate would correspond to a SLB leak rate of about [ l,c, b
Applying the same rationale to the limiting circumferantial crack of a [ ]D.C (see Table 6.1), assumed to be uniformly through wall, leads to an expected leuk rate of about
[' ]b,c. If it is more realistically assumed that 60% of the 218* crack angle is through wall, the SLB leak rate reduces to about [ l,c, b
Overall, the ratio of 20 for SLB to N.O. leak rate rates is more representative of measured leak rates and the limiting crack causing plant shutdown at 50 gpd would result in an SLB leak rate of about [ b l,c, 7-8
7.5 SLB Leak Rates for Crack Distribations 7.5.1 Expected SLB Leak Rates in the very unlikely event that all indications have through wall or near through wall cracks that do not leak during normal operating conditions but open up to leak during a postulated SLB, the expected SLB leakage under this assumption can be obtained based on calculated leakage for the EOC crack distributions described in Section 7.1.1. This calculation utilizes norainal leak rates for the entire distribution on the basis that an
. average leak rate would result for the large number of indications in service, The EOC distributions utilized are the EOC axial crack distribution of Table 7 7 and the circ iferential crack distributions of Table 7 6. The segmemed crack modelis applied for th ojected TSP PWSCC axial cracks and WEXTEX PWSCC circumferential crack distnbutions. For ODSCC circumferential cracks at the TSPs, it is assumed that the through wall crack length is [ ]b,c of the RPC crack angles of Table 7 6 based on the pulled tube examination results for North Anna Unit 1, R11C14 (see WCAP-13034),
The resulting total SLB leak rate is estimated at about [ ]b c including contributions of about[ ]b,c from WEXTEX cracks, [ ]b,c from circumferential cracks at TSPs and about [ ]b,c from axial cracks at the TSPs.
7.5.2 Bounding SLB Leak Rates I-
- _ In order to bound potential leakage in the unlikely event that through wall or near through wall cracks that do not leak during normal operating conditions but may leak
.during a postutateo SLB exist, a SLB leak rate can be obtained based en calculated leakage for the February 1992 crack distributior,3, as discussed in Section 7.1.2. This calculation utilizes nominalleak rates for the entire disiribution, The EOC distributions l
l} utilized are the 1992 axial crack distribution of Table 7 5 and the 1992 i
L circumferential crack distributions of Table 7-4. The segmented crack model is applied
! for the projected TSP PWSCC axial cracks and WEXTEX circuraferential crack
!. distributions. For ODSCC circumferential cracks at the TSPs, it is assumed that the through wall creck length is [ - ]b,c of the RPC crack angiss of Table 74. The resulting total SLB leak rate is estimated at about [ )b,c including contributions l-l of about[ ]b,c from WEXTEX cracks, [ ]b,c from circumferential cracks at TSPs and about [ ]b,c from axial cracks at the TSPs.
l 7-9
l
\
7.6 Conclusions The test and analysis basis for burst capability and leakage of tubes with axial or circumferential cracking provides a Justification of structuralintegrity and LBB per Reg. Guide 1.121, Margin is demonstrated for burst capability during accident pressure loadings (APSLB) as single and segmented cracks are limited by the normal operation leak rate limit of 50 gpd. The 50 gpd leak limit provides confidence that a 3AP burst capability is obtained in ao cases for tubes with circumferential cracking and for single axial cracks. Segmented cracks up to [ ]b,c in length provide 3AP burst capability but segmented cracks limited only by the 50 gpd normal operation leak rate limit may be longer. The strength of segmented encks has conservatively been minimized consistent with a minimum leakage model (elastic ligaments).
The potential SLB leak rate is expected to be limited by the 50 gpd operating leak limit to about 0.7 gpm based on SLB leak rates of about a factor of 20 (at -95% cumulative probability) higher than normal operating leak rates. Very consewatively, the potential SLB leak rate would be bounded by approx:mately [ ]b.c for a single through wall axial crack. In the very unlikely e.<ent that ai! indications have through wall or near through wall non leaking cracks during ricmial operating conditions and that these cracks all open up 'cr SLB leakage, the,'iLB leak rate can be conservatively estimated for the distribution of RPC crack angles and lengths at EOC conditions. For the February 1992 mid-cycle inspection crack distributions, the bounding leak rate would be approximatoiy ' )b c The expected SLB leak rate calculated from the projected EOC crack distributions which account for expected reductions in the number and size of indications at the end of the current fuel cycle as compare <1 to those from the mid-cycle inspection would be approximatley [ )b,c, 10 f
- --T "-
e
-- - y _ - , , , . .-
, r -. 7
- c
-m , .
.y fLV; , -Table 71 y
North Anna Unit 1 February 1992 WEXTEX Inspection Reruits -
. Number of Tubes ,
- Tvom of Indicatinn . g _g ;g 1 Sin 0l e Axial (sal)-l 3 0- 2 l
Multiple Adal(mal)- 0 0 0 i
ca,. Single Circumferential(SCI)- '10 9. 7 -l l
1 Multiple Circumistential(MCI) 2 1 2
'T Note: There were no tubes with mixad mode degradation identified in the WEXTEX .
transitions, t
e
{.',-
y 4' C 4
-T
/
\
s k'
i
,,;; 7 11 s '%g ,
.' ,I:..
' i'/ # ^ ;, r...- , , . . , . .- _ . , . . , , , . . . , . , . . . . _ , . _ , . , , , .
Table 7-2 North Anna Unit 1 Tubes With TSP Circumferential Cracks February 1992 S3A SGB S3C Elevation SCt/ MCI SCIM^t SCl! MCI 1H 25/3 22/0 27/0 2H 15/0 33/3 42'3 3H 2/0 7/2 19/2 4H 1/0 0,0 1/0 SH 1/0 0/0 1/0 6H 0/0 0/0 C/0 7H 0/0 O!O O!O Total 46/3 62/5 92/5 SCI - Single Circumferential Indications MCI - Muttiple Circumforential Indications 7-12
Table 7 3 North Anna Unit 1 Tubes With TSP Axial Cracks February 1992 l
l SGA SGB SGC Elevation sal'M.Al sal /MAI SAf/MAI 1H 3&O 53/4 37/S 2H 24/2 14/3 41/1 3H 7/0 6/0 5/1 4H 5/0 5/0 8'O SH 21/0 5/0 1/0
-6H 3/0 1/0 2/0 7H 0/0 0/0 0/0 Tota! 95/2 80/7 94/7 l
-i 1
SAI- Single AxialIndications l
-l mal - Multiple Axial Indications l
l I
i 7-13 l l
J Table 7 4 Steam Generator C WEXTEX and TSP Circumferer,tial Cracks February 1992 Crack
- Number of Observations Angle WCXTEX ISEs (deg.)
45 0 0 55 1 4 65 1 12 75 4 28 85 2 27 95 1 32 105 0 23 115 0 6 125 0 10 135 0 2 145 0 4 155 1 2 165 1 3 175 0 1 185 0 1 195 0 0 205 0 1 215 0 0 225 0 0 235 0 1 245 0 0 Total 11 157 Mid-point of crack angle range; for example,55' is mid-point of 51*-60' range.
I 7-14
, - - - r----,
Tablo 7 5 Steam Generator C TSP Axial Cracks February 1992 Crack
- Lenoth Number of Observations (inch) 0.05 25 0.15 25 0.25 20 0.35 9 0.45 6 0.55 0 0.65 0
. 0.75 0 0.85 0 0.95 0 Total 85 Mid-point of crack length range; for example,0.15 is mid point of 0.11-0.20 range.
7-15
'l Table 7 6 j Steam Generator C WEXTEX and TSP Circumferential Cracks -
Expected Distribution at EOC l
l Crack
- Number of Observations A021e WEXTEX TSPs )
(deg.) !
l 45 1 6 '
55 1 14 65 - 4 26 l l 75 2 22 85 1 17 l 95 0 6 105 0 4 l
115 0 3 l 125 0 3 ;
135 0 2 145 1 1 l
155 1 1 165 0 0 175 0 1 185 0 0 195 0 0 205 0 1 215 0 0-225 0 0 235 0 0 245- 0 0 Total 11 107 l
- Mid-point of crack angle range; for example,55' is mid-point of 51*-60* range.
7-16
. . , . . . = . ~. .. . - - - . - _- . .. . - . - - . - . - - . . .. . . - . . _
]
l l
Table 7 7 Steam Generator C TSP Axial Cracks Expected Distribution at EOC Crad' Lengib Number of Observations (inch) 0.05 17 0.15 17 025 14 0.35 6
-0.45 4 0.55 0 0.65 0 0.75 0 O.85 0 0.95 0 Total 58
- Mid point of crack length range; for example. 0.15 is mid-point of 0.11-0.20 range.
l-L i
i 7-17 l-l
Figure 71 Measured vs Predicted Leak Rate (Normal Plant Operation) b,C-
-w
~
i am.m 7-18
Figure 7-2 Measured vs. Predicted Leak Rate (Steam Line Break Conditions) i
~
b,c l uu 4 -
4-eutuuuum d
7-19
Figure 7 3
- Leak Rate Versus x aA i l Crack Length '
i
. name
~ - b,c -
O hp.
- eens '
1 i
7 20
Figure 7 4 Leak Rate Versus Axial Crack Length ;
(Leak Rate With Elastic Ligaments) '
s- b,C s
9
.W.
7-21
- - . - - - .. ~ - -. - . . - -. ... - - . . . -
Figure 7 5 Leak Rate Versus Circumferential Crack Angle b,C e
De M
7-22 l
l 1
.__ ___