ML20086L233
ML20086L233 | |
Person / Time | |
---|---|
Site: | Sequoyah ![]() |
Issue date: | 12/31/1991 |
From: | Goetcheus D TENNESSEE VALLEY AUTHORITY |
To: | |
Shared Package | |
ML20086L191 | List: |
References | |
NUDOCS 9112160106 | |
Download: ML20086L233 (75) | |
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Enclosure WO3 911206 001 SEQUOYAH NUCLEAR PLANT UNIT 1 CYCLE 5 REFUELING OUTAGE OCTOBER - DECEMBER 1991 STEAM GENERATOR TUBE PLUGGING REPORT (AS REQUIRED BY TECHNICAL SPECIFICATIONS SECTION 4.4.5.5.a) RESULTS OF STEAM GENERATOR TUBE INSERVICE INSPECTION (AS REQUIRED EY TECHNICAL SPECIFICATIONS SECTION 4.4.5.5.b) [. RESULTS.OF C-3 STEAM GENERATOR. TUBE INSPECTIONS AND
-RESULTS OF INVESTIGATIONS CONDUCTED TO DETERMINE l- CAUSE OF TUBE DEGRADATION AND CORRECTIVE MEASURES p- (AS REQUIRED BY TECHNICAL SPECIFICATIONS SECTION 4.4.5.5.c)
I j 1-l PREPARED BY: D. F. Goetcheus NSSS/SG/TG Programs Marsger Tennessee Valley Authority Chattanooga, Tennessee December 1991 9112160106 911209 7 PDR ADOCK 050 R. ,
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-IhBLE OF CONTENTS EAGE INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . , . . 1 .'/G TUBE INSERVICE INSPECTION SCOPE AND RESULTS . . . . . . . . . . . 2 ULTRASONIC TESTING (UT) 0F LARGEST RPC CIRCUMFERENTIAL INDICATIONS . . 21 CIRCUMfERENTIAL CRACK GROWTH. RATE . . . . . . . . . . . . . . . . . . 26 DAMAGE MECHANISM ASSESSMENT . . . . . . . . . . . . . . . . . . . . . 27 STEAM GENERATOR REPAIRS - PLUGGING AND STABILIZATION . . . . . . . . . 39 CORRECTIVE ACTIONS TAKEN TO PREVENT A RECURRENCE OF PWSCC RELATED TO TUBESHEET EXPANSION .-. . . . . . . . . . . . . . . . . 47 ENHANCED PRIMARY-TO-SECONDARY LEAK MONITORING . . . . . . . . . . . . 48 10 CFR 50.59 SAFETY EVALUATION / REGULATORY GUIDE 1.121 ANALYSIS . . . . 49 ,
CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 ATTACHMENTS I
SUMMARY
OF STEAM GENERATOR EDDY CURRENT INSPECTION . . . . 3 l- !- 2 STEAM GENERATOR INSPECTION SCOPE, SAMPLE CLASSIFICATION THRU AND-SERVICE-INDUCED WALL LOSS INDICATIONS . . . . . . . . . 12 4 6-1 PLUGGED TUBES . . . . . . .. . . . . . . . . . - . . . . . . 45 THRU 6-2 7 WESTINGHOUSE ELECTRIC CORPORATION NUCLEAR SAFETY. . . . . . 51 (- EVALUATION CHECKLIST SECL-91-431 2I68q
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IETAQDE.TIDH During the scheduled Seguoynh Nuclear Plant (SON) Unit.1 Cycle 5 (U1C5)
. refueling outage'after 1,820 effective full power days (EFPD) of operation, l inservice inspections were conducted in all four. steam generators (S/Gs). The results of the inspections were classified as C-2 for S/Gs 1, 2, and 3. S/G 4 was classified as'C-3 due to the detection of primary water stress corrosion cracking (PWSCC) at the explosively expanded (WEXTEX) top-of-tubesheet (TTS) expansion zone region of the tubes.
This report meets the. reporting requirement of SON Technical Specifications section 4.4.5.F.a for reporting S/G tube plugging, section 4.4.5.5.b to report the results of S/G inservice inspection (ISI), and section 4.4.5.5.c to provide information on investigations conducted to determine the cause of SON unit 1-S/G tube degradation including details of nondestructive examinations, results of examinations, and corrective measures to prevent recurrence. The Nuclear Regulatory Commission (NRC) will be provided details of an enhanced primary-to-secondary leakage monitoring program being implemented at SON. A safety evaluation was conducted under contract by Westinghouse Electric Corporation (M) for circumferential1y oriented PWSCC of WEXTEX transitions and is included as an attachment to this report.
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S/G TUBE INSIRVICE INSPECTIQLSCQPE AND RESULTS For the SON S/G tube ISI, initial samples of approximately 30 percent of the tubes in each S/G were eddy current tested (ECT) full length with bobbin coil exams, and rotating pancake coil (RPC) exams of WEXTEX (explosive) expansion transitions were initially conducted on approximately 20 percent of the tubes in each S/G. Due to the detection of expansion transition PWSCC, the RPC examination scope was expanded to 100 percent. Attachment 1 summarizes the SQN UIC5 ECT ISI exams and summarizes the results of exams conducted. Attachment 2, S/G Inspection Scope, Sample Classification, and Service-Induced Wall Loss Indications for the SON UIC5 Outage, provides a summary of the tube damage detected and a characterization of the damage morphology. Detailed information on all defects identified during this inspection are tabulated in Attachment 3, SQN 1 Cycle'5 Indications List, and provides detailed information on ECT probes used, defect circumferential or axial extent, percent through-wall, location and damage characterization. Also
. included is prior examination history of defects.
The most significant S/G tube degradation detected was TTS PWSCC. RPC exams detected in S/G 2 two tubes with maximum circumferential extents of 238' (R12 C38) and 206* (R12 C24). In S/G 4, R13 C41 had a circumferential indication of 206' and R14 C39 had the largest circumferential ind! cation detected 256*. Included are the terrain plots of the four previously identified four tubes with the most important circumferential TTS PWSCC indications. A frequency versus circumferential angular extent histogram of all circumferential TTS PWSCC indications at SON for the cycle 5 inspection is enclosed as attachment 4 and clearly indicates the majority of crack 9 to be smaller in angular extent. In summary, the SQN S/Gs are in excellent condition with 0.7, 0.8, 1.4, and 1.3 percent tubes plugged in S/G 1 through 4, respectively. -Of the 76 tubes plugged, 68 were plugged due to PWSCC, 3 were plugged due to loose parts damage, and 5 were plugged for-all other damage mechanisms. Of the tube degradation detected, other than PHSCC, AVB wear affected 12 tubes, cold leg thinning affected 3 tubes, and TTS outside diameter stress corrosion cracking (ODSCC) in the sludge pile area affected 2 tubes. l l 2168q l
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. ATTACID4ENT 1 12/05/91 SEQUOYAH NUCLEAR PLANT UNIT 1 CYCLE 5 l
SUMMARY
OF STEAM GEHERATOR EDDY CURRENT INSPECTION i l ELEXhM TYEE S/Q 1 S/G 2 S/G 3 Sf_Q_i Full-Length Bobbin Coil 1,069 1,072 1,316 1,315 Partial-Length Bobbin Coil 114 96 82 109 Support Plate RPC 102 107 100 101 Top-of-Tubesheet RPC 3,377 3,377 3,362 3,368 U-Bend RPC 0 0 42 41 TOTAL EXAMS 4,692 4,652 4,902 4,934 TOTAL TUBES EXAMINED 3,377 3,377 3,362 3,368
-IliDICATIONS (TUBESl i Defects (140% Wall Loss)
PWSCC HTS Circ. 4 15 10 23 PWSCC HTS Axial 5 0 10 1 AVB Wear 0 0 0 1 Cold. Leg Thinning 0 0 1 0 Loose Part 3 0 0 0 ODSCC 0 2 0 0 Degradations (120% and <40% Wall Loss) PWSCC U-Bend 0 0 1 0 AVB Near 2 3 4 2 Cold Leg Thinning 0 0 -0 1 Loose Part 3 0 G 0 FLBD Wear 3 0 0 0 Imperfections (<20% Wall Loss) AVB Wear- 0 0 1 0 Cold Leg Thinning 1 0 0 0 FLBD Wear 3 0 0 0 2168g.
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ATIACHMENT 2-1 ST O fAl:fiUCLEJ.R PLANT UNIT 1 CYCLE 5 STUJi GENERATOR 1 INSPECTION $ COPE, %MPLE CLASSIFICATION AliD SIRVh'/-!HDUCEO VALL !OS5 IN0! CAT 10NS
.?
St.G 13W CQL SAbf1F 19 1.0C1.TIOf3 CMVAqlidlEUL4 M$QtOTa0N EXAM SCOPE (1U31$. TESTED)
! Initia) Sample 1 '1 1 0 30 V'S+17.9 FLLO V3AR Non lion., red 690 HTS RPC ,
1 1 5 0 22 HTS +17.8 FLB1 WE/,1 lEne Ratquired 102 H01 RPC 1 1 90 0 21 HTS +e7.5 FLDD WEAR Now Required 1069 F/L BC 1- 1. 91 0 4 HTS +17.3 FL60 VEAR hone Required 144 P/L BC 1 1 ' 92 0 10 HTS +17.0 FLO3 WI.'R None Uequired 1 1 98 0 15 HTS +17.6 FLau WEAR Nor.e 'l qui red 1 7 74 0 COI HTh 0.0 PWTCC CIR'. . Plug Tube 1 16 71 0 30 HTS 41.73 ODSCC None Roqutred i 23 4; O C5 16010 AVB wCAll None Kequired
! 1 28' 46 0 20 AV2+0.00 AVB WEAR None Faquired 22 AV3+11.00 AVD ViAR None Required 1 32 16 0 13 401+J.00 C/L THINNING None flaquired i 41 32 0 34 HT$+0 19 LOOSE PART ilone Required 1 42 33 0 2S HTS +0.22 LOOSE PART None Required j ? 43 .43 0 8tl 11T5+0.28 -LOOSE PART Plug Tube Steam Generator _,_i,, Inspection irntial sample results have been classified as Category _C-L.
First Expai.ston Samplo 1 3 57 1 SAI HTS-0.14 PWSCC AXIAL Plug Tube 1036 HTS RPC 1 12 54 i SAI HTS-0.13 P1 SCC AXIAL - Plug Tube 1 12 56 1 COI NTS-0.16 PWSCC CIRC. Plug Tube 1 '13 61 1 COI HTS-0.H PWSCC CIRC. Plug Tube 1 13' 68 - 1 CO2 HTS-0.09 PWSCC CIRC. Plug 1 abe 1 2F 58 1 SA1 HTS-0.11 PWSCC AXIAL Plug Tube 1 40 33 - 1 26 HTS +0.2 LOOSE PART None Required 1 41 33 1 45 HTS +0.25 . LOOSE par Plug Tube-1 43 32- 1 62- HTS +0.27 LOOSE PART Plug Tube Steam Generator. 1 Inspects 2 41st expansion results have been classified as Category AL,
- Second Expansion Sample No indications found.
Steam Generator 1 Inspection hd expansion results have ,c .. 473 HTS RPC classified as Category _C-L. Third Expansion Sample 1 13 7 3 _.SAI HTS +0. O PWSCC AXIAL Plug Tute 1178 HIS RPC 1 14 7 3 SAI HTS +0. 3 PWSCC AXIAL Flug Tube Steam Generator 1 Inspection 3ru expansion result 1 have been classified as Category _C-L. 2168q
_; ATTACHMENT 2-2 SEQUOYAH NUCLEAR PLANT UNii 1 CYCLE 5 STEAM GENERATOR 2 INSPECTION SCOPE, SAMPLE CLASSIFICATION AND SERVICE-INDUCE 0 WALL LOSS INDICATIONS 1/.G Egg (QL 1A3PLI IND LOCATION CHARACTERIZATION Ei$QLQ110N E! LAM SCOPE (11 tali __11$T[Q1 Initial Sample 2 12 ?S 0 COI HTS-0.07 PWSCC CIRC. Plug Tube 690 HTS RPC 2 13 21 0 COI HTS-0.18 PWSCC CIRC. Plug Tube 107 H01 RPC 2 14 19 0 COI HTS-0.09 PWSCC CIRC. Plug Tube 1072 F/L BC 2 16 48 0 SAI HTS +0.49 00 SCC Plug Tube 96 P/L BC 2 18 46 0 SA! HTS +0.d8 ODSCC Plug Tube 2- 19 66 0 19 HTS +1.35 00 SCC None Required 2 20 61 0 37 HTS +1.31 00$00 None Required 2 21 34 0 33 HTS +1.62- 00 SCC None Required 2- 22 31 0 38 HTS +1.86 00$CC None Required k 23 52 0 12 HTS +1.95 00 SCC None Required 2 24' 31 0- COI HTS-0.14- PWSCC CIRC. Plug Tube 2 27 41 0 10 HT5+1.48 ODSCC None Required 11 HTS +0.95 ODSCC None Required 2- 30 - 48 0 COI HTS-0.05 PWSCC CIRC. Plug Tube 2 33 49 0 33 AV2+0.00 AVB WEAR None Required 26- AV3+0.0C AVB WEAR None Required 2 38 -47 0 22 AV3+0.00- AVB WEAR None Required Steam Generator . 2 Inspection initial sample results have been classified as Category 1-L. First Expansion Sample 2 7 74 1 C01 HTS-0.24 PWSCC CIRC. Plug Tube 1024 HTS RPC 2 11 46 1 ;COI HTS-0.12 PWSCC CIRC. Plug Tube 2 12 24- 1 COI HTS-0.19 PWSCC CIRC. Plug Tube 2 12 38 1 COI HTS-0.09 PWSCC CIRC. Plug Tube 2 13 34 1 COI HTS-0.04 PWSCC CIRC. Plug Tube 2 27 44 ' -1 COI HTS-0.10 PWSCC CIRC. Plug Tube
~ Steam Generator 2 inspection 1st espansion results have been classified as Category _f-L. - Second Expansion Sample 2 13 20 2 COI HTS-0.05 PWSCC CIRC. Plug Tube 1663 HTS RPC 2 17 - 24 2 COI HTS-0.Ob PWSCC CIRC. Plug Tube 2 '17 32 2 COI HTS-0.19 PWSCC CIRC. Plug Tube 2 39 23 2 COI- HTS-0.29 PWSCC CIRC. Plug Tube Steam Generator 2 Inspection 1st expansion results have been classified as Category ._C-L.
2168q
- - ___ . _ - _ _ ._- _ _ _ _ ~
.~_ ____.m . __. _ - - . _ _ _ _ _ _ _ _ _ _ . . . _ _ - . . . _ _
ATTACHMENT 2-3 SEQUOYAH NUCLEAR PLANT UNIT 1 CYCLE 5 STEAM GENERATOR 3 INSPECTION SCOPE, SAMPLE CLAShlFICATION AND SERVICE-INDUCED WALL LOS$ IN0! CATIONS SIG EQL! CQL 16M1.1 IN,Q LQfAUM CHARACTERI_ZAU M EISDWUQ3 EXAM SCOPE (TUDES TESTED.1 Initial Sample 3 1 1 0 22 HTS +17.5 .FLB0 WEAR None Required 730 HTS RPC 3 1 67 0 SAI H07+10.2 PWSCC AXIAL Plug Tube 100 H01 RPC 3 8 74 0 COI HTS-0.11 WSCC CIRC. Plug Tube 42 U/8 RPC 3 10 63 0 COI HTS-0.00 PWSCC CIRC. Plug Tube 1106 F/L DC 3 17 48 0 COI HTS-0.12 PWSCC CIRC. Plug Tube 82 P/L DC 3 19 22 0 18 AV3+0.00 AVB WEAR None Required 3 22 64 0 22 AV2+0.00 AVB WEAR None Requi.'ed 24 AV3+0.00 AVB WEAR None Required 3 22 67 0 33 AV240.00 AVB WEAR None Required 31 AV3t0.00 AVB WEAR None Required 3 34 16. 0 52 C01+0.00 C/L THINNING Plug Tube 3 38 55 0 28 AV3+0.00 AVD WEAR None Required 3 38 64 0 19 AV1+0.00 AVB WEAR None Required 27 AV2+0.00 AV?l WEAR None Required 19 AV3+0.00 AVP WEAR None Required Steam Generator 3 Inspection initial sample results hoe t.een classified as Category C-2 . First Expansion Sample l 3 8- 65 1 SAI HTS-0.65 PWSCC AXIAL Plug Tube 1021 HTS RPC l 3 9 44 1 SAI HTS-0.14 PWSCC AXIAL Plug Tube 210 F/L BC E 3- -9 54 1 CCI HTS-0.11 PWSCC CIRC. Plug Tube 3 10 62 1 COI HTS-0.15 PWSCC CIRC. Plug Tube
'3 10 68 1 COI HTS +0.00 PWSCC CIRC. Plug Tube 3 10 69 1 COI HTf+0.00 PWSCC CIRC. Plug Tube 3- 16 25 1 SAI HTS-0.49 PWSCC AXIAL Plug Tube 3 16 54 1 SAI HTS-0.51 PWSCC AXIAL Plus Tube l, 3 16 66 1 COI HTS-0.10 " SCC CIRC. Plug Tube l' 3 17 46 1 COI HTS-0.15 7.i'.C CIRC. Plug Tube 1
- l. Steam Generator 3 Inspection 1st expansion sample results have been L classified as Category C-2 .
Second Expansion. Sample 3- 9 84 2 SAI HTS-0.93 PWSCC AXIAL Plug' Tube 1611 HTS RPC 3 11 84 2 SAI HTS-0.36 PWSCC AXIAL- Plug Tube 3- 15 76 2 COI HTS-0.21 PWSCC CIRC. Plug Tube 3 24 68 2 SAI HTS-0.29 PWSCC AXIAL Plug Tube l 3 26 16 2 SAI HTS-0.20 PWSCC AXIAL Plug Tube 3 34 66 2 SAI HTS-1,33 PWSCC AXIAL Plug Tube 3 40 28 2 SAI HTS-2,10 PWSCC AXIAL Plug Tube Steam Generator 3 Inspection 2nd expansion sample results have been classified as Category _C-L. 2168q
- .. .- - . - . - ~ - - . . - - -~ - -. - - . _ - -
4 g ATTACHMENY 2-4 SEQUOYAH NUCLEAR PLANT UNIT 1 CYCLE 5 STEAM GENERATOR 4 INSPECTION SCOPE. SAMPLE CLASSIFICATION AND SERVICE-INDUCE 0 WALL LOSS INDICATIONS
$20 EW COL $MfP(i IN_Q [QCATION .
CHARACTERIZATION ELSQLVIl0B E2 LAM SCOPE (TVBES TESTED)
. Initial Sample 4 1 1 0 26_ CTS,+17.57 FLB0 WEAR None Required 729 HTS RPC 4 3 86 0 25 CO2+0.00, C/L THINNING None Required 101 H01 RPC -4 9 34 0 COI HTS-0.04 PWSCC CIRC. Plug Tube 41 U/D RPC 4 11 34 0 COI HTS-0.07 PWSCC CIRC, Plug Tube 1128 F/L BC 4 13 25 0 SAI HTS-2.09 PWSCC AXIAL Plug Tube 92 P/L BC 4 13 38 0 COI HTS-0.14 PWSCC CIRC. Plug Tube 4 15 39 0 COI H15-0.05 PWSCC CIRC. Plug Tube 4 15 73 -0 19 C01+16.28 HFG, FLAW None Required 4 16 41 0 COI HTS-0.09 PWSCC CIRC, Plug Tube 4 17 42 0 COI HTS-0.05 PWSCC CIRC. Plug Tube 4 19 43- 0 COI HTS-0.11 PWSCC CIRC. Plug Tube 4 20 34 0 COI HTS-0.06 PWSCC CIRC. Plug Tube 4 20 45 0 COI HTS-0.15 PWSCC CIRC. Plug Tube 4 23 .58 0' COI HTS-0.07 .PWSCC CIRC, Plug Tube 4 29 - 31 0 31 AV240.00 AVB WEAR None Required .30- AV3+0.00 AVB WEAR None Required 4- 29 - 38 0 COI HTS-0.11 PWSCC CIRC. Plug Tube 4 34 48' O 28 AV1+0.00 AVB WEAR -
30 AV2+0.00 AVB WEAR - 45 AV3+0.00 AVB. WEAR Plug Tube Steam Generator 4 Inspection initial sample results have been classified as Category C-3 . First Expansion Sample 4- '11 31 1 COI _ HTS-0.07 PWSCC CIRC, Plug Tube 2639 H15 RPC
- 4. 11 33 1 COI HTS-0.07 PWSCC CIRC. Plug Tube 187 F/L BC 4 12 42 1 COI HTS-0.09. .PwSCC CIRC. Plug Tube 17 P/L BC 4 13 ' 41 1 C0! HTS-0.07 PWSCC CIRC. Plug Tube-
~4 14 '38 1 COI HTS-0.06 PWSCC CIRC. Plug Tube 4 14 39 1' 'COI HIS-0.12 PWSCC CIRC. Plug Tube -
4 14 45 1 COI HTS-0.13 PWSCC CIRC. Plug Tube 4 14 46 - 1 COI HIS-0.06 PWSCC CIRC. Plug Tube L 4 15 31 1 COI hfS-0.10' PWSCC CIRC. Plug Tube 4 : 15 1 -COI HTS-0.03 .PWSCC CIRC, Plug Tube I
'4 15- 47 1 COI HTS-0.09 PWSCC CIRC. Plug Tube 4 18 - 47 1- COI HTS-0.'O PWSCC CIRC, Plug Tubo 4 35 43- 1 36 AV23 0.00 AVB WEAR None Required -21 AV3+0.00 AVB WEAR None Required' 4 37 -. 40 -1 10- C04+6.54 -HFG. FLAW Nono Required L
E Steam Generator _4__ Inspection 2nd expansion results have been classified as Category _C-2_ l l 2168q c.
__ . _ _ _ _ . . . . _m _ - .._ _ __ . _ _ - . _ _ __ _ _ . . . I . . j l 12/02/91 ATTACHMF.NT 3-1 . SON 1 CYCLE 5 INDICAtl0NS S/G RDW COL SAMPLE PRO [XT IND LOCATION CHARACTERllAllDN RE SOLUTION COMMENIS a===================================================================================================================== 1 1 1 0 BC - 30 His+17.9 TLBD WE AR CVC4 16% , 1 1 5 0 BC - 22 His+17.8 ft60 WEAR CYC4 - NDD (BC), 6/88 10% (BC) l 1 1 90 0 BC - 21 His+17.5 fLBD WCAR CYC4 2!% 1 1 91 0 BC - 4 HIS+17.3 TLBD WLAR CYC4 - NDO (PC), 6/88 9% (CC) 1 1 92 0 BC - 10 HT5+17.0 TLED WEAR CYC4 - 111 (BC) !
- 1. 1 94 0 BC - 15 HIS*17,6 ftBD WEAR CYC4 - 12% (BC) ]
1 2 44 0 BC - 12 H02+3.23 Mrc FLAW CYC4 - 9% (BC) 1 7 24 0 RPC 74 COI H1$+0.0 PWSCC CIRC. PLUCCED CYC3 - NDD (BC) 1 9 69 0 BC - 25 H01+33.34 MFC TLAW CYC4 - 21% (BC) 1 16 71 0 BC + 30 MIS *1.73 DDSCC CYC5+NDD (RPC), CYC4-38% fBC) 1 22 7 0 BC 23 C15+46,9 mfg FLAW CYC4 28% (BC) 1 22 9 0 BC - 9 C01*13.30 mfg TLAW CYC3 - <20% (BC) 1 23.45 0 BC - 25 Av3+0.00 AVB WE AR CYC3 - NDD (BC)
-1 28 46 0 BC -
20 AV2+0.00 AVB WEAR CYC4 - 9% (BC) BC - 22 AV3+0.00 AVB WEAR CYC4 14% (BC) 1 32 16 0 DC - 13 C01+0.00 C/L THINNING CYC4 - NDD (BC) 1 41 32 0 RPC 56 34 His*0.19 LOOSE PARI CYC4 NDD (BC), CYC3 - NDD (LC) 1 42 33 0 RPC 68 25 His+0.22 100SE PART CYC4 NDD (SC), 8806 - NDD (SC)
-1 42 56 0 BC 10 Cis+33.4 MfC FLAW CYC4 - 17% (BC) 1 43 33 0 RPC 61 E6 HIS*0.28 LOOSE PARI PLUGCED CYC4 - kDD (SC), 6806 NDD (BC) 't 3 57 1 RPC 0.28 SAI HTS 0.14 PWSCC AXIAL PLUGCED CYC4 NDD (BC) 1 12 54 1 RPC 0.34 SA! his-0.13 PVSCC AX1AL PLUGGED CYC3 - NDD (BC) 1 12 56 1 RPC 160 col HTS-0,16 PWSCC CIRC, PLUG /STABILI2ED CYC4 - ADD (SC) 1 13 61 1 RPC 60 C0! HTS-0.11 PVSCC CIRC. PLUGGED CYC3 - NDO (BC) 1 13 68 1 RPC 73 Col His-0.09 PWSCC CIRC, PLUGGED CYC3 - NDD (BC) 1 29 58 1 RPC 0.25 SAI HIS 0.11 PWSCC AXIAL PLUGGED 8701 - NDD (BC) 1 40 33 1 RPC 52 26 His+0.2 LOOSE PART CYC4 - ND (BC) 1 41 33 1 RPG 68- 45 hiS+D,25 LOOSE PARI PLUGGED CYC3 - m. (BC) 'l' 43 32 1 RPC 59 62 bis +0.27 LOOSE PART PLUGGED CYC4 - hDo (BC) 1 13 7 3 RPC 0.07 sal HTS +0.10 PWSCC AXIAL . PLUGGED CYC3 - NDD (SC) 1 14 7 3 RPC 0.20 SAI h1S+0.13 PWSCC AXIAL PLUGGED CYC4 - NDD (BC) =====au====s=============================================================e=42=========================rs==============
2 2 24 0 BC - 10 CTS +27.81 Mfc rLAW CYC4 - NDD (BC),CYC3 - 7% (BC) 2 6 2 0 BC - 8 C04+24.19 MFC TLAW CYC4 - API (SC),CYC3 - <20% (BC) 2 6 70 0 BC - 10 HIS+1.12 00 SCC CYC5 - NDD (RPC), CYC4 IRI (BC) 2 11 10 0 00 - 8 H01+24.2 MfC TLAW CYC4 8% (BC), CYC3 - 4% (BC) 16 Hu3+3.27 - mfg (LAW CYC4 - 11% (SC), CYC3 - 12% (BC) 2 12 35 0 RPC 79 Col HTS-0.07 PWStc CIRC. PLUGGED CYC4 - hDD (80) 2 13 21 0 RPC 183 Col MIS 0.18 PWSCC CIkt. PtuG/SIA91LIZED CYC4 hDD (BC) 2 14 19 0 Ent 70 col MIS-0.09 PuSCC CIRC. Pl uGGED CYC4 - IRI (BC HTS +1.70) 2 16 48 0 RPC 0.4 SAI HIS+0.49 ,005CC FLUGCED CYC5-?0%(BC), CYC4-30%(BC) 001(RPC) 2 18 46 0 RPC 0.3/ Sil FIS+0.48 005CC PLUGGED CYC5 54% (BC),CYC4-26%,CYC3-17%(BC) 2 19 66 0 BC - 19 HIS+1,35 005CC CYC5 NDD (RPC), CYC4 22% (BC) 2 20 61' O BC 37 HTS *l.31 095CC CYC5 - NDD (RPC), CYC4 - 37% (BC) 2 21 34 0 BC + 33 815+1.62 00 SCC CYC5 - NDD (RPC), CYC4 - 25% (BC) 2 22 31 0 BC 38 HIS*1.86 ODSCC CYC5 - NDO (RPC), CYC4 36% ,(BC) 2 23 52 0 BC 12 HIS*1.95 00 SCC CYC5 - NDD (RPC), CYL4 - 35% (GC) 2 24 31 0 RPC 72 Col HIS 0.14 PWSCC CIPC. PLUGCfD CVC4 - NDD (BC) 2 27 41 0 BC . - 10 HTS +1.48 ODSCC CYC5 NDD (RPC), CYC4 - 1RI (BC) BC - 11 HIS+0.95 00 SCC CYC5 NDD (RPC). CYC4 - IRI (nC)
- 16
.. . . - .. . . _ _ _ ~ . . . . . . - - .-- - _ _ _ . - _ . - _ _ . . -- . . - . .--. - ~ . * .
12/02/91 ATTACllMENT 3-2 SQN 1 CYCtt 5 INDICAll0NS S/G ROW COL SANPLE PRO (Xi IND LOCAll0N CHARACTER 12All0N RESOLUTIDN COMMENIS swe=a am anens ees= =========sa ss 3 = ==,ss= = == se 33 ==,3 ===== saws ... s ss e s s es as e s.a ssa s es sa ee sse se s s as==a me sas seses. 2 30 48 0 RPC 68 COI HTS 0.05 PWSCC CIRC. PLUGGED CYC5 . NDO (BC), CYC3 18% (BC)
'2 33 49 0 BC -
33 Av2+0.00 AVB WEAR CYC4 . <20% (BC) BC . 26 AV3*0.00 AVB WEAR CYC4 - NDO (BC) 2 35 27 0 BC . 15 HIS+16.8 MrG FLAW CYC4 + 27% (BC) 2 38 47 0 BC - 22 AV3+0.00 AVB WEAR , CYC3 NDO (BC) , '( 2 7 74 1 RPC 70 Col H15 0.24 PVSCC CIRC. PLUGGED CYC4 . NDO (BC) 2 11 46 1 RPC 96 COI HIS 0.12 PVSCC CIRC. PLUG / STABILIZED CYC4 . NDO (BC)
-2 .12 24 1 RPC 206 C01 HIS 0.19 PWSCC CIRC. PLUG / STABILIZED- CYC4 . NDD (BC) 2 12 38 1 RPC. 238 Col HIS 0.09 PWSCC CIRC. PLUG /SIABill2ED CYC4 - NDD (BC) 2 13 34 1 RPC 187 COI HTS 0.04 PWSCC CIRC. PLUG / STABILIZED CYL4 . NDD (BC) 2 27 44- 1 RPC 68. COI HIS 0.10 PWSCC CIRC. PLUGGED CYC4 NDD (RPC) 2 13 20 2 RPC .186 Col His-0.05 PWSCC CIRC, PLUG /SIABilllED CYC3 NDD (BC) 2 17- 24 2 RPC 88 COI His-0.05 PWSCC CIRC. Plug /STABill2ED CYC3 - NDD (BC) 2 17 32 2 RPC 170 Col His 0.19 PWSCC CIRC. PLUG / STABILIZED CYC4 NDD (BC) 2 39~ 23 2 RPC 63 C01 His-0.29 PWSCC CIRC. PLUG / STABILIZED CYC4 NDD (SC) .==sessseas 3, sm z a ,3 = = s s = = = = = r n w.m u s = = = i r = = =, e = = ,3 ====3s=====s===rams:ssve======s===r=4=sesas============zs==sasses 3 1 1 0 BC .
22 HTS *17.5 FLBD WEAR CYC4 26% (BC) 3 1 67 0 URPC 0.6 SAI H07+10.2 PVSCC AX1AL PLUGGED CYC4 . IPI (URPC) 3 8 74 0 RPC 160 COI'HIS-0.11 PWSCC CIRC. PLUG /STABill2ED CYC3 NDD (BC) /
-3 10 63 0 RPC 80 COI HIS-0.00 PVSCC CIRC. PLUGCED CYC3 - NDD (BC) 3 17 48 0 RPC 143 Col His-0.12 PWSCC CIRC. PLUG /STABIL17ED CYC4 NDD (BC) 3 19 22- 0 BC + 18 AV3*0.00 AVB WEAR CYC4 - 22% (DC) 3 21 9 0 BC -
18 H04*44.18 Mrc FLAW CYC4 - 27% (BC) 3 22 64 0 BC - 22 AV2*0.00 AVB WEAR CYC4
- 15% (BC) 24 AV3+0.00 AVB WEAR CYC4 18% (BC) 3 22 67 0 BC . 33 AV2+0,00 AVB VEAR CYC3
- NDD (6C) 31 AV3*0.00 AVB WEAR CYC3 + NDO (BC)
'3 34 16 - 0 BC -
52 C01+0.00 C/L THINN!kG PLUGGED CYC4 - NDD (BC)
=3 33 55 0 BC -
28 AV3+0.00 AVP WEAR CYC3 - NDD (BC)
~3 33= 64 0 BC -
19 AV1+0.00 AVB WEAR CYC4 - 12% (BC) 27- AV2+0.00 AVB WEAR CYC4 - 23% (BC) 19 AV3+0.00 AVB WEAR . CYC4 - 12% (BC)
............ .... ....... ....... .............. ..... ....... .. ........ .. 4.... . ................... .. . ...
3- 8 65 1 RPC 0.27 SAI HIS-0.65 PWSCC AX1AL PLUGGED CYC3 - NDO-(BC) 3 9 44 - .1 RPC 0.49 SAI HTS 0.14 PWSCC AX1AL PLUGGED CYC3 - NDD (BC) 3 9 54 1 RPC 115 COI HIS-0.11 PVSCC CIRC. PLUG /SIABILIZED CYC4 - NDD (BC) 3 10 62 1 RPC 83 COI HIS 0.15 PWSCC CIRC; PLUG /SIABILIZED - CYC4 - NDD (RPC) 3 10 -68 1 RPC 109 C01- hts *0.00 PWSCC ClRC. PLLIG/SI ABIt ! 7ED CYC3 - NDD (BC)
'3 10 69 1 RPC 73 C0! HTS +0.00 PWSCC CIRC. PLUGGED CYC3 - NDO (BC) 3.-16 25 1 RPC 0.23 sal HIS 0.49 PWSCC AXIAL PLUGGED CYC3 NDD (BC) 3 16 54 1 RPC D.26 SAI His 0.51 PWSCC AXIAL PLUGGED CYC4 - 0.25" (RPC), CYC3 - NDD (BC)
- 3. 16 66 1 PPC 100 Col HIS-0.10 PWSCC CIRC. PLUG /SIARill2[D CYC4 NDO (RPC) 3 17 46 1 RPC 69 C01. HIS*0.15 PWSCC CIRC. PLUGGED 8806 NDD (RPC)
.3 9 84 2 RPC 0.36 SAI HIS 0.93 PWSCC AXtAL PLUGGLD CYC3 - NDD (BC) .3 11 84 2 RPC 0.3 SA! HTS *0.36 PWSCC AXIAL PLUGGED CYC3 NDD (DC) 3-'15 76 2 RPC 74 Col HIS 0.21 PWSCC CIRC. PLUGGED CYC3 - NDO (BC) 3 24 68 2 RPC 0.43 SAI HTS 0.29 PWSCC AxjAL PLUGGED CYC4 - NDD (BC) 3 26 16 2 RPC 0.2 SAf His 0.20 PWSCC' AXIAL PLUGGED CYC4 NDD (BC) 3 34 66 2 RPC 0.32 SAI HIS*l.33 PWSCC AXtAL PLUGGED CYC'. - NDO (BC) y w---, g
12/02/91 ATTACllMENT 3-3 s SON 1 CYCLE $ INDICAi!ONS S/G RDW COL SAMPLE PRO EXT IND LOCAT ION CHARACTER 12AtlON RESOLUTION CUMMENTS as sa ...a ss e e s s e s = = = == = s e a s s e n e s s e s s e s s e s s s s s s s s s = = s 3 = == s s e s = = = = = s s= = = = s a m e s = = s s e s s e s = = = = = = = = s s = = = = = = s == = = = = = = = = = = s s s = 3 40 28 2 RPC 0.25 SAI N15 2.10 PWSCC AXIAL FLUGGED CYC4 - NDD (SC) a s s a s s a s s ss s s = = = s s= = = = s s 3 = s as s s s = = s = 3 = = = s s e s s s s s s == = = = = = = a s s = = = = = s s = = = = = = = == = = = = s s a n s s 3 s = = = = == s s = = s s 3 = = = = = s a s a s s = = = = 4 1 1 0 BC - 26 CTS =17.57 mfg FLAW CYC4 - 30% (BC) l 4 3 86 0 DC - 25 CO2+0.00 C/L ININNING CYC3 - NDO (BC) 4 9 34 0 RPC 64 Col HIS-0.04 PVSCC CIRC. PL UGGED CYC3 NDD (8C) 4 11 34 0 RPC 79 C0! Hi s'-0.07 PVSCC CIRC. PLUGGLD CYC3 - NDD (BC) 4 13 26 0 RPC 0.86 SAI HIS-2.09 PWSCC AxlAL PLUGGED CYC3 NDD (DC) 4 13 38 0 RPC 106 Col HIS 0.14 PVSCC CIRC. PLUG /STABill2ED CYC3 - NDD (BC) 4 15 39 0 RPC 93 Col HIS 0.05 PWSCC CIRC. Plug /SIABill2ED CYC4 - NDD (BC) 4.15 73 0 BC - 19 C01+16.28 Mf G ILAW CYC4 - 20% (BC), CYC3 17% (BC) 4 16 '41 0 RPC 106 Col His-0.09 PWSCC CIRC. PLtlG/ S T Ap t L l 2E D CYC4 - NDD (BC) 4 17 42 0 RPC 96 001 HIS 0.05 PVSCC CIRC. PtuG/STABitl2ED CYC4 - 83 DEGs CIR EXI (RPC) 4 19 43 0 RPC 80 001 HIS-0.11 PWSCC CIRC. PLUGGED CYC4 - NDO (RPC) 4 20 34 0 RPC 71 C0! HIS 0.06 PWSCC CIRw. PLUGGFD CYC4 - NDD (RPC) 4 20 45 0 RPC 110 Col HIS-0.15 PWSCC CIRC. PLUG /SIABILIZED CYC4 - 105 DEGs CIR EX1 (RPC) 4 23 58 0 RPC 78 COI HIS-0.07 PVSCC CIRC. PLUGGLD CYC4 57 DEGs CIR EXT (RPC) 4 29 31 0 BC - 31 AV2*0.00 AVB WEAR CYC4 14% (BC) ) BC - 30 AV3+0.00 AVB WE AR CYC4 NDO (BC) I 4 29 38 0 RPC 72 C01 H15 0.11 PVSCC CIRC. PLUGGED CYC4 NDD (BC) 4 34 48 0 BC - 28 Av1+0.00 AVB WEAR PL Ur.GE D CYC3 - NDD (BC) GC - 30 Av240.00 AVB WEAR CYC3 NDO (BC) SC - 45 Av3 0.00 AVB VCAR CYC3 - NDD (DC) 4 11 31 1 RPC 65 Col HTS 0.07 PWSCC CIRC. PLUGGED CYC4 - NDO (RPC) 4 . 11 33 1 RPC 86 C0! HIS-0.07 PWSCC CIRC, PLUG /STABill2ED CYC3 NDD (UC) 4 12 42 1 RPC 98 Col HIS 0.09 PWSCC CIRC. PLUG /SIABill2ED CYC3 + WDD (BC) 4 13 41 1 RPC 206 Col HTS 0.07 PVSCC CIRC. PLUG /STABill2ED CYC3 - NDD (BC) 4 14 33 1 RPC 130 Col HTS 0.06 PWSCC CIRC. PLUG /SIABill2ED CYC3 - NDO (BC) 4 14 39 1 RPC 256 C01 HIS-0.12 PWSCC CIRC. PLUC/ST ABill2ED CYC3 - NDD (BC) 4 14 45 1 kPC 118 col HTS 0.13 PWSCC CIRC. PLUG /STABIL12ED LYC4 - NDD (BC) 4 14 46 1 RPC 163 C01 HfS 0.06 PWSCC CIRC. PLUG /STASill2ED 8806 NDD (SC) 4 15 31 1 RPC 96 COI HIS 0.10 PWSCC CIRC. PLUG /STABIL12ED CYC4 NDD (BC) 4 15 33 1 RPC $8 COI HIS 0.03 PWSCC CIRC. PLUGGED CVC3 - NDD (BC) l 4 15 47 1 RPC 128 COI HIS 0.09 PVSCC CIRC. PLUG / STABILIZED CYC4 NDD (RPC) 4 18 47 1 RPC 86 COI Fis-0,10 PWSCC CIRC. PLUG /STABIL12LD CYC3 NDO (BC) 4 35 43 1 CC - 36 AV2+0.00 AVB WEAR CYC3 NDD (RC) 21 AV3 0.00 AvB WEAR CYC3 NDD (BC) 4 37 40 . 1 BC - 10 C04*6.54 mfg FLAW CYC5 NDD (RC) t I i I
a N O t 11/27/91 ATTACl! MENT 3-4 SON 1 CYCLE $ INDICAll0NS S/G OOW COL SAMPLE PRO CXi IND LOCAllON CHARACIERIZA110N RE SOLUTIC#4
.................~.........=w............ .............................essansse===.
NOTE: 1. EXT COLUMN IS CIRCUMIERENil AL EXTENT (DEGREES) FOR COI INDICA 110NS AND IN AXIAL INCHES FOR sal's
- 2. FLB0 FLOW LANE GLOCKING DEVICE ' *
- 3. IRI - NONRELEVENT INDICA 110N
- 4. CSI CRACKED SUPPORT PLATE INDICA 110N
- 5. Col - CIRCUMFERENilALLY ORtENTED INDICAllON 6, SAI - SINGLE Aylal INDICAiltal
- 7. RPC - ROTATING PANCAKE COIL EODY CURRENT TROBE
- 8. BC - B000!N COIL EDDY CURRENT PROBE
- 9. NDD - NO DETECTABLE DEFECT
- 10. AV3
- ANil V! brail 0N BAR
- 11. PVSCC PRIMARY WATER STRESS CORRosl0N CRACKING
- 12. CYC3 - 8/85 INSPEC110N
- 13. 8701 - 1/87 INSPECilON
- 14. 8806 - 6/88 INSPECil0N
- 15. CYC4 - 4/90 INSPECTION
- 16. MAN FLAW - MANUFACTURihG FL AW i
- l9 -
SQN U1C5 CIRCUMFERENTIAL PWSCC CUMUIATIVE HISTOGRAM 20 19 - l 18 - - - - - - - - - - - f 17 - - j 16 - 15 -------- 1 ! 14 - 13 - g i 12 h N 2 E 2; 11 _ 5 i El E D 10 ------------------- " Or *
- l. ~4 9 -
! g A 8 - 7 - 6 - - 5 -
- 4 --------
3 - 2 - -------------------- 1 - O , i , , , , l 30 70 110 150 190 '230 270 310 350 a l P RANGE +10/-9 DEGREES
ETRASONIC TESTltlG (UT) OF LARGEST RPC CitCUi&ERENTI AL IMDlCAT10ES UT examinations to resolve the actual circumferential extents and crack morphology of TTS PWSCC detected in tubes R13 C41 and R14 C39 in S/G 4 provided the following results: RPC (ECT) UT EQw C.91 Man Cire. Erdent Characterigation 14 39 256* 2 cracks - 123' and 82* separated by a 25' ligament. 205* total circumferential ex+s-Two areas of 100% t'.n ough-wall involvement, 70* and 30*. 13 41 206* 3 cracks - 25', 80', and 70* separated by ligaments of 25' and 40*. 175* total circumferential extent. A single area of 100% through-wall involvement of 50'. Attached are the UT C-Scan and Waterfall plots for tubes R14 C39 and R13 C41 that illustrate the multiple crack networks that could not be distinguished by RPC exams. ECT RPC exams over colled the circumferential angular extent by 3 y 48' and 31' illustrating conservatism associated with RPC ECT angular extents, s 21689
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CIECUREBENTIALCRACJLGR0HIlLRATI To determine a citcumferential crack growth rate, a review of past ECT exams for all tubes identified at the SON Unit. 1 Cycle 5 outage with PWSCC was initiated. This comparison was made with the 1991 knowledge t. hat a defect was ! present and then to determine if this defect may have besn present in previous exams. Below is comparative data for 11 tubes with circumferential indications previously examined with RPC during prior outages. The average , circumferential crack growt.h from cycle 4 to cycle 5 was 63.9' wlth a minimum . of 5' and a maximum of 128*. L WEXTEX EXPANSION TRANSITION CIRCUMTERENTIAL CRACK GROWTit RATE DASED ON RPC DATA
- Ci rcumf e rent.lal Cycle 4 C4-C5 Extent RPC Results Growth SLG E9w Colmito Z_o.nn (Digreen.L IDngrnell IDegr.casi 2 27 44 4 68 NDD 68 3 10 62 4 83 NDD 83 3 16 66 4 100 NDD 100 ,
3 17 46 4 69 NDD 69 4 17 42 4 96 83 13 4 19 4J 4 80 HDD 80 4 20 34 4 71 NDD 71 4 20 45 4 110 105 5 4 23 58 4 78 57 21 4 11 31 4 65 NDD 65 4 15 47 4 128 NDD 123 Growth Average C4-C5 63.9* AX1AL CRACK GROWTH RATE BASED ON RPC DATA
- Axial Cycle 4 C4-C5 Extent RPC Results Growth SIG Rnw Column Zann Ilachell __Linchnal__ Linchen L 3 16 54 4 0.26 0.25 DJ1 i
l Growth Average C4-C5 0.01 inches l
- Based on November 1991 Reanalysis of Sequoyah Unit 1 Cycle 4 RPC Data I
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NEh0I EECitA111EdlSSESSME1C The damage snechanism occurring in the Alloy 600 SON unit 1 S/Gs at the TTS expansion transition internal surf ace is PWSCC. Alloy 600 PWSCC is influenced by tensile stress, operational temperature, and a susceptible metallurgical structure it. the presence of normal primary coolant water. The metallurgical condition of all tubes with PWSCC detected at WEX'AeX expansion transitions was evaluated by the snechanical properties (tensile, yield, percent elongation, and Rockwell 13 hardness) relative to the average of all tubes in the S/G. Generally, higher carbon contents in Alloy 600 imply a higher propensity to susceptibility to PWSCC. Attachment 5 tabulates SON Unit 1 tubes with PWSCC, heat number, chemical analysis, and the mechanical proporties. Comparison of CarignlontgnLani_Me.nhaniradropcities _ Carton _Innallt. _Xield _ Elongation IIArdnen (%) (rtSLL ._(Isil (%) ( DL_ Average _oLa ' c_E013. S /G2 _Q&i _ 102 E _5h m . EAD EL hYttAge of tubta_Mith_lTSCC _.J2DL.__1A22LD_._bh16 &21. Ufu1L Tubes with TTS expansion zone PWSCC in the SON unit 1 S/Gs do not have significantly difforent mechanical properties than the average tubes in the SON S/Gs. An assessment ot' the location of PWSCC cracks was obtained by evaluating the distribution, by zones, of cracks (axial and circumferential) within the S/Gs and is listed below (see tubesheet maps attached): 1!umbat_oLCrackn_in_Znnes Inne 1 zone l Zone 3 lone _4 Iqtakin_1LG_ s/G AnlaL _Cirs. _Arial C're. _ Axial. . Circ. _ Axial _C11 a _Axint _CJrc< 1 2 3 4 5 4 2 1 3 0 11 0 15 3 1 1 4 1 4 9 10 10 4 1 _ _2 3 1 ._1L_ 19151 _ 3 1 __1 0 4 4 8 47 ._16.,_ __52_ Total / Kore 4 1 8 55 68 _ Ettgent 5.9% 1.5% 11.8% 80.9% Results of the SON Unit 1 Cycle 4 examinations show zones 3 and 4 contained 92.7 percent of all axial and circumferential expansion tone PWSCC and contained 98 percent of all circumferential defe ns. Zones 3 and 4, being located in the center section of the S/G, would sperate at a higher temperature and be insulated by the sludge pl.'.e further raising the tube temperatures and driving the kinetics of PWSCC. 2168q
__ m.__ . . . . - . _ . _ . . _ _ _ _ . _ _ _ . _ _ . _ _ _ . _ _ , _ _ _ _ . . _ . _ _ _ - _. Internal diameter tubesheet expansion transition profiles of tubes with PWSCC detected were obtained from bobbin coil eddy current data utilizing ZETEC tube profiling software. Evaluations of the expansion transition profiling plots identified the majority of the circumferential PWSCC detected was associated l with oversired tubesheet hole conditions that produced transitions with changes in diameter of greater than 15 mils and/or irregularly shaped (sharp or uneven) transitions. Tubesheet expansion transition diametrical changes and tubenheet expansion conditions obtained in the review of the available ' expansion zone profile data are tabulated below. Attached are examples of tube profile data. Tuhenheet Irliguhr__ Conditions _Ivaluation Tubes with Oversized Percent of Tubos Tubes with PFSCr* IIoles (011) and/or with Irregular _S/d and Profili Data._ _lrl.eguinL JKEnD112n5_ Co.ndlilons 2 - 0 11 2 of 3 ., 1 3 2 tqtal 63% 7-OH , 2._ irregular 9 of 15 2 15 8 total 60% 2 - 0 11 4 irregular 6 of 8 3 8 5 total 751 , 8 - 0 11 L,1rlagular 11 of 11 ; 4 11 11 tgj;al .. . .100% Based on the NDE characterizations of the TTS expansion transition conditions, chemistry, and metallurgical condition of the Alloy 600 tubes, it was concluded the damage mechanism is FWSCC originating in the WEXTEX expansion transitions, is located 93 percent of the time in zones 3 and 4, occurs with a < higher probability in tubes with oversized diamotor-tubesheet hole conditions or irregular tubesheet expansion conditions. E 1 Y 2168q a , , . .. _ . , . ..m.__,,__.~._ .,m,.,.___._,.,_.,
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ELG_EIEAIFS, _ELVEIRG_M2_EIMlLTIAT10ll Repairs convrcted during t h) UICS outage included mechanical plugging with Alloy 690 U-6esigned plugs and stabilization of tubes with circumferential PWSCC at the TTS to preclude f uture adjacent tube damage associated with flow-induced vibration of the plugged tubes. Attachment 6 lists all defective tubes plugged and stabilized during the SON UIC5 outage. Tubes were stabilized wit.h 7-f oot stainless steel flexible stabilizers providing support from the tubesheet to above the first tube support plate. The criteria used for stab 111 tang circumferential TTS indications (refer to attached tubesheet map with sono identification) were: Zones 1 and 2 - Stabl? tre all tubes with circumf erential PWSCC detected. Zones 3 and 4 - Stabilize if crack angle is greater .han 80 degrees. Below la the current status of SON Unit 1 S/G plugging as of the cycle 5 outage. Also included are toboshe9t rnaps f or S/Gs 1-4 with all plugged tubes indicated. SEQUOYAH NUCLEAR PLANT UNIT 1 CYCLE 5 STEAM GENERATOR PLUGGING STATUS ELUGOIMG_SIATUS 1/G 1 S/G 2 S/G 3 S/G 4 Previously Plugged Tubes 11 11 26 20 Plugged Cycle 5 Damage Mechanism PHSCC HTS Cire. 4 15 10 23 PWSCC HTC Axlal 5 0 10 1 PWSCC U-Bend (pre-existing) 0 0 1 0 AVB Near 0 0 0 1 Cold Leg Thinning 0 0 1 0 Loose Part 3 0 0 0 ODSCC 0 2 0 0 l TUDES PLUGGED CYCLE 5 12 17 22 25 TOTAL TUBES PLUGGED 23 28 48 45 2168q
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ATTACID4ENT 6-1 SEQUOYA!! NUCLEAR PLANT UNIT 1 CYCLE i OUTAGE EktlEGID_1VDIS PERCENT j SLG ROM COL LOCAT10H DEnimDAROH CilARACIER12AT10H REEatan0H ] 1 1 3 57 !!TS-0.14 SAI PWSCC AXIA,L PLUG TUDE . 3 7 24 IITS+0.0 COI PWSCC CIRC. PLUG TUDE 1 12 54 !!TS-0.13 Fi I PWSCC AXIAL PLUG TUDE 1 12 56 IITS-0.16 COI PWSCC CIRC. PLUG TUDE* 1 13 7 IITS 4 0.10 SAI PWSCC AXIAL PLUG TUDE 1 13 61 IITS + 0.11- COI PWSCC CIRC. PLUG TUDE 1 13 68 IITS-0.09 COI PWSCC CIRC. PLUG TUDE 1 14 7 !!TS+0.13 SAI PWSCC AXIAL PLUG TUDE 1 29 58 IITS-0.11 SAI PWSCC AXIAL PLUG TUDE 1 41 33 IITS+0.25 45 LOOCE PART PLUG TUDE , 1 43 32 11T5 + 0. 27 62 1,00SE PART PLUG TUDE 1 43 33 IITS+0.28 86 LOOSE PART PLUG TUDE 2 7 74 !!TS-0.24 COI PWSCC CIRC. PLUG TUDE 2 11 46 IITS-0.12 COI PWSCC CIRC. PLUG TUDE* 2 12 24 11T5-0.19 COI PWSCC CIRC. PLUC TUDE* 2 12 35 IITS-0. 07 COI PWSCC CIRC. PLUG TUDE 4 2 12 1A !!TS-0. 09 COI PWSCC CIRC. PLUG TUDE* 2 13 29 IITS-0.05 COI PWSCC CIRC. PLUG TUDE* 2 13 21 IITS-0.18 COI PWSCC CIRC. PLUG TUDE* 2 13 $4 117 S - 0 . 0 4 COI PWSCC CIRC. PLUG TUDE* 2 14 10 !!TS-0. 0 9 COI PWSCC CIRC, PLUG TUDE 2 16 ,48 IITS 4 0. 73 CAI ODSCC PLUG TUDE 2 17 24 lits-0. 0 8 CCI PWSCC CIRC. PLUG TUDE* 2 17 32 IITS-L .19 COI PWSCC CIRC. PLUG TUDE* 2 18 46 IITS+0.48 SAI ODSCC PLUG TUDE 2 24 31 IITS -0.14 COI PWSCC CIRC. PLUG TUDE 2 27. 44 IITS-0.10 COI PWSCC CIRC. PT,UG TUDE 2 30 48 !!TS -0. 0 5 COI PWSCC CIRC. Pluti TUDE 2 39 2? IITS-0.29 COI PWSCC CIRC. PLUG TUDE*
- llot leg stabilizer installed (IITE - 1101) 2168q
. . ATTACHMENT 6-2 SEQUOYAll NUCLEAR PLANT UNIT 1 CYCLE 5 OUTAGE P1DGGED_IURES I
PERCENT SLQ R011 CE LOCAIl0H DEGRADAT10H CHARACIEE1ZATIOli RESOLUT10li 3 1 67 1107 + 10. 2 SAI PWSCC AXIAL PLUG TUDE 3 8 65 !!TS-0. 6 5 SAI PWSCC AXIAL PLUG TUBE 3 8 74 IITS-0.11 COI PHSCC CIRC. PLUG TUDE* 3 9 44 IITS-0.14 SAI PWSCC AXIAL PLUG TUBE 3 9 54 IITS-0.11 COI PWSCC CIRC. PLUG TUDE* 3 9 84 IITS-0.93 SAI PWSCC AXIAL PLUG TUDE 3 10 62 IITS-0.15 COI PWSCC CIRC. PLUG TUBE
- 3 10 63 IITS-0.09 COI PWSCC CIRC. PLUG MIBE 3 10 68 HTS 40.00 COI PWSCC CIRC. PLUG TUDE*
3 10 69 !!TS+0.00 COI PWSCC CIRC. PLUG TUDE 3 11 84 IITS-0. 3 6 SAI PWSCC AXIAL PLUG TUDE 3 15 76 HTS-0.21 COI PWSCC CIRC. PLUG TUBE 3 16 25 IITS -0. 4 9 SAI PWSCC AXIAL PLUG TUBE 3 16 54 !!TS -0. 51 SAI PWSCC AXIAL PLUG TUDE 3 16 66 IITS-0.10 COI PWSCC CIRC. PLUG TUBE
- 3 17 46 IITS-0.15 COI PWSCC CIRC. PLUG TUBE 3 17 48 IITS+0.08 COI PWSCC CIRC. PLUG TUBE
- 3 24 68 IITS-0.29 SAI PWSCC AXIAL PLUG TUDE 3 26 16 IITS-0. 2 0 SAI PWSCC AXIAL PLUG TUBE 3 34 16 C0140.00 52 C/L TilINNING PLUG TUDE 3 34 66 1175-1.33 SAI PWSCC AXIAL PLUG TUDE 3 40 28 IITS- 2 .10 SAI PWSCC AXIAL PLUG TUBE 4 9 34 IITS-0. 04 COI PWSCC CIRC. PLUG TUBE 4 11 31 IITS-0. 07 COI PHSCC CIRC. PLUG TUDE 4 11 33 11TS-0. 07 COI PWSCC CIRC. PLUG TUDE*
4 11 34 HTS-0.07 COI PHSCC CIRC. PLUG TUDE 4 12 42 IITS-0.09 COI PWSCC CIRC. PLUG TUBE
- 4 13 26 IITS -2 . 0 9 SAI PWSCC AXIAL PLflG TUDE 4 13 38 lITS-0.14 COI PWSCC CIRC. PLUG TUDE*
4 13 41 IITS-0.07 COI PWSCC CIRC. PLUG TUBE 4 4 14 38 HTS-0.06 COI PWSCC CIRC. PLUG TUBE
- 4 14 39 11TS - 0.12 COI PWSCC CIRC. PLUG TUBE
- 4 14 45 IITS-0.13 COI PWSCC CIRC. PLUG TUBE 6 4 14 46 IITS-0.06 COI PWSCC CIRC. PLUG TUDE*
4 15 31 IITS-0.10 COI PWSCC CIRC. PLUG TUBE
- 4 15 33 IITS -0. 03 COI PWSCC CIRC. PLUG TUDE 4 15 39 IITS-0.05 COI PWSCC CIRC. PLUG TUBE
- 4 15 47 IITS -0. 0 9 COI PWSCC CIRC. PLUG TUDEa 4 16 41 !!TS-0. 09 COI PWSCC CIRC. PLUG TUBE
- 4 17 42 !!TS-0. 0 5 COI PWSCC CIRC. PLUG TUDE*
4 18 47 !!TS-0.10 COI PWSCC CIRC. PLUG TUBE
- 4 19 43 IITS-0.11 COI PHSCC CIRC. PLUG TUBE 4 20 34 IITS-0.06 COI PWSCC CIRC. PLUG TUDE 4 20 45 IITS-0.15 COI PWSCC CIRC. PLUG TUBE
- 4 23 58 HTS-0.07 COI PWSCC CIRC. PLUG TUDE 4 29 38 IITS -0.11 COI PWSCC CIRC. PLUG TUDE 4 34 48 Av3+0.00 45 AVB WEAR PLUG TUDE
- Ilot leg stabilizer installed (IITE - 1101) 2168q
CORBECUYE_ACUQNS TAKEN TO PREVENT A RECURREHEE_QT PWSCC RELATEDJQ_IURESilfET
. EXEAHS10H .
r
- During SON UIC5 outage (from October 24 to November 23, 1991), all unplugged tubes were shotpeoned to prevent PWSCC in the explosively expanded tubesheet from the tube end,to two inches above the TTS. The shotpeening was implemented by H. The-expansion region inturnal turface stresses were changed l from 40-45 kai tensile stresses to 60 kai compressive stresses with an c(fective surface depth of 4 to 7 mils Anto the internal surface. The change of stress state to compressive will provide margin against initiation of i future PWSCC. Preexisting cracks with depths greater than 4 mils will not be [
ameliorated by shotpooning and will have a reasonable probability of progressing to detectable flaws in future fuel cycles. Based on SON unft 1 ' 5.18 effective. full power years (EFPY) of operation and industry experience, it is projected that less than 5 percent of the tubes-in SQN unit I will be affected by expansion transition PWSCC. Industry experience with shotpoening of hard-rolled expansson transition plants with equivslent EFPY of operation have not had the positive Affect identified for HEXTEX explosive expansion ' transition S/Gs due to the 20 to 25 kai higher tensilre residual stress associated with hard-rolled transitions (see Attachment 7). Extensive ' shotpoon qualification testing for HEXTEX (explosive) expansion transitions by H and the WEXTEX Owners Group verlfled shotpeenJng effectiveness in preventing PWSCC and established the field process perameters used at SON. Specific information on SON WEXTEX expansion ahetpeen procesa qualification testing can be provided to the NRC if requested. TVA's SQN unit 1 S/Gs are the first explosively expanded tubesheet tranpitton S/Gs in the world to be shotpoened. TVA has taken a very aggressive approacc. of qualification testing and implementation of shotpcening to prevent long-term S/G tube degradatlon of the SON S/Gs due to PWSCC in the tubasheet expansion region. l f 2168q l s c .
-. -- . . ~ . . - . - . _ . - - _ - - - - - . . - . ~ . - - . . . . . - - - - -- . - - -
l t ENHANCED PRIMARY-TO-SECONDARY LEAK MONITORING l A program to monitor and trend primary-to-secondary leakage is currently in -! effect at SQN and meets the requirements of NRC Bulletin 98-02 for rapidly ! propagating fatigue cracks. Based upon recent industry events described in NRC Informacion Notice 91-43 and UIC5 examination results, ef forts are under ; way to develop an enhanced primary-to-secondary leak monitoring program that is suitable for detecting, tre.. ding, and averting a rapidly propagating fatigue failure that occurs over an approximate 90-minute interval. The SQN , methodology is based on similar programs developed at Maine Yankee and Salem l' and includes the following elementst , 1) The blowdown and condenser vacuum exhaust radiation monitors will be used as on-line monitors by operators in the control room without reliance on-grab samples.
- 2) The alarm setpoints will be set at levels _to-ensure prompt alert of increasing leakage.
- 3) Trending wil1~ be requirvd at intervals suitable to ensure rapidly -
propagating leakage is detected and monitored such that shutdown occurs prior to tube rupture.
- 4) A primary-to-secondary leakage limit of 128 gallons per day will be i implemented.
Since both units at SQN have experienced tube leakage that.has been monitored by chemical sampling, it.is possible to accurately correlate radiation monitor counts per minute to gallons per day leakage by retrieving historical data from these previous leakage events. Thus, when implemented, this program wil1~
- provide for the early detection and response to rapidly propagating leak rates -more rapid than would be predicted on the basis of NRC Bulletin 38-02.
Implementation of-this program is expected to occur soon after the restart - from the' current Unit i refueling outage. k 2168q *
-. . _ . _ . . - -....- - .- . .-..- - - -- ., .- - - ..- . ~ - -
o 1.Q CFR 50.59 SAFETY EVALUAIION/ REG %ATQRY CUIDE 1.12.l_Al% LYSIS A safety evaluation in accordance with the criteria of 10 CFR 50.59 and in accordance with the guidelines of Regulatory Guide 1.121 was prepared by if to evaluate the safety significance of continued operation. ).f Safety Evaluation , SECC-91-431 is attached. Following a 100-percent eddy cutrent examination of the hot leg TTS region, the safety evaluation focused on an evaluation of the largest undetected crack that could exist (based.upon industry accepted ediy current detection thresholds) and the growth of the crack over a fuel cycle (based upon SQN specific growth rate data). Applying the criteria of Regulatory Guide 1.121, the following margins of safety were determined to exiet based upon worst-case crack growth ratest ZRAD_1 Znut:_2 ZDatL.2 KDntL_4 Limiting crack 135' 169' 238' 238' Detection threshold -40' -40' -40* -40' Growth allowance _ _fd * -63*. -118' -12.A' Safety margin 32' 66* 70' 70* It is therefore concluded that no tubes are expected to exceed the criteria of Regulatory Guide 1.121 over the next fuel cycle of operation. In the unlikely event that a crack propagates during operation resulting in primary-to-secondary leakage, the 128 gpd leakage limit, in conjunction with the enhanced primary-to-secondary leak monitoring program, will ensure a safe, orderly shutdown of the plant prior to the crack exceeding structural limits. l \ i S
- 49.-
21689
..u..-._,,_.____._.__.__,
! CONCLUSIONS
- The NDE testing completed on the SQN Unit 1 S/Gs of approximately 30 percent full length bobbin coil ErT exams. 100 percent RPC exams of the hot leg tubesheet expansion transitions, and plugging of defective tubes met the technical specification and ASME Section XI Code requirement for ISIst therefore each SQN Unit 1 S/G has been demonstrated operable.
- A detailed investigation of the S/G TTS tube degradation detected during the Unit 1 Cycle 5 outage identified top-of-tubesheet expansion zone PWSCC of the Alloy 600 explosively expanded tubes as the damage mechanism.
- The expansion transition PWSCC occurred in Alloy 600 tubes with average mechanical properties not significantly different than a'1 heats of .
material used in the SQN Unit i S/Gs. Tubesheet expansion abnormalities such as larger diameter tubesheet holes or irregular profiles exhibit a general correlation to the incidence of PWSCC.
- The first-time implementation of shotpeening explosively expanded tubesheet expansions is a conservative and proactive corrective action taken by TVA to prevent a reoccurrence of expansion transition PWSCC in the SQN Unit 1 5/Gs.
- A safety evaluation utilizing the criteria of 10CFR50.54 and in acecrdance with the guidelines of Regulatory Guide 1.121 was conducted by Westinghouse Electric :orporation. In addition, tne primary-to-secondary leakage limit will be administratively lowered to 128 OPD in conjunction with the implementation of an enhanced primary-to-secondary leak monitoring program to provide for the early detection and response to rapidly propogating leak rates, more rapid than would be predicted on the basis of NRC Bulletin 38-02.
2168q
t ATTACHMENT 7 SECL-91-431 Page 1 of 23 Customer Reference No(s). N/A Westinghouse Reference No(s). N/A WESTINGHOUSE NUCLEAR SAFETY EVALVATION CHECK LIST
- 1) NUCLEAR PLANT (S)_SE0V0YAH UNIT 1
- 2) CHECK LIST APPLICABLE 10 :CIRCUMFERENTIALLY ORIENTED PRIMARY WATER STRESS CORROSION CRACKING WEXTEX REGION i
- 3) The written safety evaluation of the revised procedure, design change or modification required by 10CFR50.59 has been prepared to the extent required and is attached. If a safety evaluation is not required or is incomplete for any reason, explain on Page 2.
( Parts A and B of this Safety Evaluation Check List are to be completed only on the basis of the safety evaluation performed. CHECK LIST - PART A
.(3.1) Yes No.J_A change to the plant as described in the FSAR?
(3.2) Yes No X A change to procedures as described in the FSAR? (3.3) Yes__No X A test or experiment not described in the FSAR? (3.4) Yes No X A change to the plant technical specifications (Appendix A to the Operating License)?
- 4) CHECK LIST - PART B (Justification for Part D answers must be included on Page 2.)
(.' '.) Yes
. NojL Will the probability of an accident previously evaluated in the FSAR be increased?
(4.2) Yes No X Will the consequences of an accident previously evaluated in the FSAR be increased? (4.3) Yes No X May the possibility of an accident which is different _ than any already evaluated in the FSAR be created? O (4.4) Yes_ _No X Will the probability of a malfunction of equipment important to safety previously evaluated in the FSAR be increased? (4,5) Yes No X Will the consequences of a malfunction of equipment important to safety previously evaluated in the FSAR be increased? (4.6) Yes No X May the possibility of a malfunction of equipment important to safety different than any already evaluated in the FSAR be created? (4.7) Yes NoJL, Will the margin of safety as defined in the bases to any technical specification be reduced?
.o b SECL-91-431 Page 2 of 23
- ',i si
- ?p) be, *f tr4 answers to any of the above questions are a known, indicate v"" r 3) REM /0K4 and explain below.
I D if the answer '
,, of the above questions in Part (3.4) cannot be
_ . giM j answered in tu .r.4ative, based on written safety evalur. tion, the li9, cha? review .., aid require an application for license amendment as "ggM'
. required by 10CFR50.59 (c) and submitted to the NRC pursuant to
. 10CFR50.59, s REMARKS Nons: The following summarizes the justification based upon the written safety n,'uation (1) for answers given in Part A (3.4) and Part B of this Safety Ev s .stinn Check List: Please refer to the attacced written safety evaluation. Reference documsnt(s): Please see Sect \on 8.0 of this evaluation. FOR FSAR UPDATE Seetion: __Pages: Iabl ee - Figures: , Reason for / Description of Change: Prepared by (Nuclear Safety): b Date:/ [I Coordinated with Engineei l .) a. K. CULLEN Date: - i SjcJnatreonFile) , \
/ - - O in - - -
jL - Date: g/J_'O Nuclear Safety Group Manager: R' 1
-. t b ,E M - -
ij/ s Page 2 of 23 t e
SECL-91-431 Page 3 of 23 SEQUOYAH UNIT 1 SAFETY EVALUATION FOR , CIRCUMFERENTIALLY ORIENTED PRIMARY WATER STRESS CORROSION CRACKING WEXTEX TRANSITIONS
1.0 INTRODUCTION
Page 4 2.0 TUBE DEGRADATION CHARACTERIZATION Page 5 2.1 Eddy Current Examination Results Page 5 ,
-2.2 Expected Crack Morphology Page 5 2,3 Ultrasonic Testing Results Page 5 -3.0 TUBE INTEGRITY EVALVATION Page 6 3.1 Regulatory Basis Page 6 4.0 LIMITING CRACK MORPHOLOGY DETERMINATION Page 7 4.1 Tube Burst Considerations Page 7 4,1.1 Burst Test Results Page 7 4.1.2 Crack Length Discussion Page 9 4.2_ Flow induced Vibration Loadings Page 9 4.2.1_ Normal Operating Condition Results Page 10 -4.0.2 Steam Line Break (SLB) Evaluation Page 10 4.2.3 Loss-of Cooiant Accident (LOCA) Page 11
- 4. 2, 4 Safe Shutdown Earthquake (SSE)- Page 12 4.2.5 Combination of Postulated Accident Page-12 Condition Loadings-5,0 IDENTIFICATION OF LIMITING CRACK MORPHOLOGY Page 12 5.1 Circumferential Crack Growth Rate Discussion Page 13 5.2 Eddy Current Uncertainty N .cussion . Page 14 5.3 Safety Margin Dete(aination _
Page 14 5,4 Leak Sefere Break-(LBB) Con:iderations Page 15 5.5 Expected End Of. Cycle Conditions page 16 6.0~ REMEDIAL MEASURES Page 18 6.1 -Shotpeening Page 18 6.2 Enhanced Leak Manitoring Capabilities Page 18
7.0 CONCLUSION
S- Page IC l S.0- REFERENCES Page'21 APPENDIX A Page 22 APPENDIX B Page 23 Page 3 of 23 i;
i a . SECL-91-431 Page 4 of 23 SEQUOYAH UN!1 1 SAFETY EVALUATION
1.0 INTRODUCTION
During the current outage at the Sequoyah Unit 1 Nuclear Power Plant, a total of f2 steam generator tubes were plugged due to curcumferentially oriented cracking in the WEXTEX transition region, along with a total of 16 tubes plugged due to axially oriented cracking. Rotating Pancake Coil (RPC) probes were used to gather the data for the detection of WEXTEX region cracks. The larg'est circumferential indication as determined by RPC was found to be 256 , in tube R14C39, S/G 4. All hot leg WEXTEX transitions in all four steam generators were inspected using the RPC probe. All locations which exhibited NDE indications in the WEXTEX region have been removed from serv me by plugging. In addition, some locations which exhibited larger circumferential indications by RPt inspection have had stabilizers inserted. This evaluation, completed in accordance with 10 CFR 50.59 criteria, assesses the scfety significance of Cycle 6 operation of Sequoyah Unit I with potential circumferentially oriented cracking occurring at the WEXTEX expansion transition region. Potentially circumferentially cracked tubes remaining in service are those which presently contain degradation below the detection threshold of the RPC probe. Pertaining to tube strength considerations, the expected End of Cycle (E0C) crack morphology has been evaluated against the guidelines of US NRC Regulatory Guidt 1.121, " Bases for Plugging Degraded Steam Generator Tubes", issued for comment, to confirm that tube integrity coniiderations are expected to remain commensurate with these criteria for up to the end of Cycle 6 operation. 2.0 TUBE DEGRADATION CHARACTERIZATION 2.1 Eddy Current Examination Results All WEXTEX transitions in each of the four steam generator hot legs wc' examined using the RPC probe. The following Table 2.1 lists the numbers of tubes identified as having either circumferential or axial indications in the WEXfEX transition areas. TABLE 2.1 S/G Number of Cirt. Ind. Number of Axial Ind, a 1 4 5 2 15 0 3 10 10 4 23 1 TOTAL S2 16 l
l SECL-91-431 Page 5 of 23 2.2 Expected Crack Morphology Based on tubes pulled from another plant, circumferentially oriented PWSCC occurring in the WEXTEX expansion region is best characterized as segmented cracks. Axial and circumferential1y oriented cracking in the WEXTEX expansion transition zone has not been observed on the same tube. 2.3 RPC and Ultrasonic Testing Results Metallographic examination of pulled tubes has shown that RPC may have a tendency to exaggerate the dimensions of the actual circumferential cracks. RPC is often unable to distinguish individual crack segment:. that might be separated by sound ligaments. A more accurate method of characteriting circumferential cracking and crack segments is ultrasonic inspection (UT). In order to determine whether or not some of the larger cracks reported by RPC were actually segmented crack networks, the two tubes witn the largest cracks measured by RPC in 5/G 4, R13C41 and R14C39, were UT inspected. Tube R14C39 had the largest reported crack angle by RPC of all steam generators, 256 , while the RPC measured angle for R13C41 was 2060 . Of the 52 circumferential indications, 31 were less than 100 in 0 extent, 10 were 0 between 100 and 150 , 11 were greater than 150 and 4 of these were U greater than 2000 . The average circumferential crack extent was 1090 . The UT results showed that for R13C41, the total crack network was comprised 0 of three crack segments 25 , 80 and 70 in extent, separated by ligaments of 25 and 40 .3Tube R14C39 was comprised of two crack segments, 123 0 and 82 0
, separated by a 250ligament. Tube R13C41 had a through-wall involvemer,t of 50 degrees as measured by UT, and R14C39 had two areas of through wall involvement, 70 and 30 degrees, separated by a 150 degree ligament (Reference 1).
Axial indications in the WEXTEX transition region would normally be detectable by bobbin coil examination. Of the 16 axial indications detected, 14 were within the transition region ( l.00 inch from the top of the tubesheet). Axial lengths as determined by RPC inspection ranged from 0.07 inch to 0.49 inch for thc . indications in the transition region. Only two of these 14 axial crack tips extended above the top of the tubesheet. The uppermost tips of the cracks for these two locations were 0.10 and 0.13 inch / above the top of the tubesheet. 2.4 Tube Bundle Condition Inferred from NDE Evaluation Program With the exception of the tubes identified above, circumferentially oriented indications were not reported in any other tubes in any of the steam generators. The capabilities of the RPC probe suggest that tube degradation of this sort could be detected after it has proceeded to 50% depth over approximately 40 arc or 100% depth and 25 arc. Therefore, degradation greater'than these limits is not believed to be present in the sequoyah Unit I steam generators. l
l l I
- 3. -
SECL-91-431 Page 6 of 23 l 3.0 TUBE INTEGRITY EVALUATION l 3.1 Regulatory Bases The NRC Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam j Generator Tubes", issued for comment, addresses tubes with through-wall ; cracking. Any through-wall crack morphology that is projected to result in a condition such that the limiting crack morhology is exceeded during an operating interval when a corrosion growth allowance for continued
' degradation and-eddy current uncertainty are considered is unacceptable for continued operation. The Regulatory Guide utilizes safety factors on loads for tube burst and collapse that are consistent with Section III of the ASME Code, its use establishes a reactor coolant pressure boundary that should have an extremely low probability of abnormal leakage, rapidly propagating failure or gross rupture. The required condition of Leak Pefore Break (LBS) satisfies the " defense in depth" approach of RG 1.121.
In summtary, the analytical and loading criteria applicable to tubes with through-wall cracks in thinned and unthinned tubes per RG 1.121 states that:
- 1. Through-wall cracks in minimum thickness tubes should not propagate and result in tube rupture under accident conditions.
- 2. A tube with a single crack of the largest permissible crack length should exhibi, strength' characteristics such that the burst pressure is greater than or equal to 3 times the normal operating pressure differential, and,
- 3. The leakage rate determined for the largest permissible crack should be less than the leakage rate limit specified by the plant technical specification.
The primary stress calculations should also consider all the stresses and tube deformations imposed on the tube bundle during the most adverse of the postulated accident condition loadings. Additionally, all major hydrodynamic and flow induced forces should be considered in the analysis to determine the limiting crack morphology. Regulatory Guida 1.83, " Inservice Inspection of PWR Steam Generator Tubes", describes-a method acceptable to the NRC staff to reduce the probability and consequences'of steam generator tube failur_es through periodic inservice
-inspections for the early detection of steam oenerator tube degradation.
Due to the potential for tube burst due to rapid propagation of a circuinferential indication, only circumferential cracking will be addressed. Axial Indications, even those in the WEXTEX region, are judged to behave like axial-cracking at TSP intersections, and as such are covered by leak before break,1.hus allowing safe shutdown of the plant should detectable leakage occur. Axial cracks, unlike circumferential cracks, have been previously
. judged to not be susceptible to flow induced propagation mechanisms.
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j SECL-91-431 Page 7 of 23 4.0 LIMITING CRACK MORPHOLOGY DETERMINATION 4.1 Tube Burst Considerations Observations of circumferentially oriented PWSCC in steam generator tubing at the WEXTEX transition region require an evaluation predicting the effects of such cracking on tube structural integrity. As a part of the defense in depth approach to structural integrity, burst test results, leak rate analyses and considerations of circumferential crack morphologies are used to develop a LBB position such that leak rate limits during normal operation provide protection against tube burst and, more specifically, a coupled tube burst and steam line break event, which is outside of the design basis of the unit. Detailed discussions of these considerations are presented in the following sections of the report. 4.1.1 Burst Test Results A series of room temperature burst tests have been conducted on tubes containing simulated through wall cracks. Narrow circumferentially oriented slits (less than 0.008 inches wide) were machined through ona wall of nominal 0.875 inch 0.D. by 0.050 inch wall Alloy 600 tubing. Tube slitting was performed using an EDM (electron discharge machining) process. Because of the high toughness of Alloy 600 tubing, tht; failure mode is one of- plastic collapse. Therefore, burst pressure is dominated by the plastic flow response of the tubing. Room temperature tens!1e tests of the heat used for the burst tests gave the following average val"as for yield and ultimate strengths: 0.2% Yield Strength Ultiate Tensile Strength 47.8 ksi 105 ksi
-The average flow strength, based on the average of the yield and ultimate strengths, is-76.4 ksi.
Burst testing was performed I,y supplying pressurized water to a flexible bladder inserted into the tube. A section of thin foil (0.004 inch) was used to cover the through-wall slit in order to prevent premature bladder L rupture due to interaction with the slit edge. The foil did not overlap in the circumferential direction, and thus did not add-any component of stiffness to the tube. Lateral motion of the tube was restricted by a simulated tube support plate, simulating an open crevice at the first support plate. l l
SECL-91-431 Page 8 of 23 Bur.t test results are included in Table 4.1, When tube motion f: unrestricted, comparatively low burst pressures are observed particularly at the larger crack angles. The slit coening at maximum pressure is large and tearing at the slit ends is common. When lateral motion of the tube ends is restricted (by a structure simulating the tube support plate), burst prassures at large crack angles increase dramatically as expected from plastic collapse considerations. Rertricted bending forces the development of another plastic collapse moie which requires higher pressures. When lateral motion is restricted, the slit opening at release of the bladder is reduced as is the incidente of tearing at the slit tip. Table 4.1 Burst Test Re ults lateral Motion Restricted at first TSP Single Circumferential Sl t; Mean Arc Length Hurst Pressure (degrees) (psi) 65 9690 84 8490 116 7450 149 6915 180 6015 212 5715 244 5345 277 4230 (3 N.0. delta P) (is 4155 psi) Multiple Colinear Circumferential Slits (0.30 inch through wall slits, with 0.110.0. by 0.061.3. ligaments) 155 9525 210 7620 265 8820 320 7890 Burst test results have been compared with analytical predictions and a good agreement has been shown. A favorable correlation supports the extrapolation of test data to tube conditions in operation. This is accomplished by adjustment of average flow strength to the temperature of interest and the use of minimum expected v41uas (material lower tolerance limit, LTt. values). An adjustment has beec nade to the room temperature burst pressure data given in Table 4.1 according to the Sequoyah Unit 1 operating temperatures. Adjustment of the crack angle which satisfies three times the normai operating pressure differential is given below in Section 4.1.2. ' A previous evaluation (Reference 3) dc< eloped the limiting through-wall crack angles for the Sequoyah Unit 1 steam generators. The evaluation divided the tubesheet into 4 distinct zones, based upon the forcing condition which produced the nost limiting crack condition. The limiting . crack angles and description of forcing function is provided in Sections I 4.1.2 thru 4.2.5. !
1 SECL-91-431 s Page 9 of 23 4.1.2 Crack length Discussion Relative to tube burst strength considerations for the Sequoyah Unit 1 steam generator tubing, the maximum allowable single, through-wall circumferentially oriented crack extent is 238 . This angle of involvement meets all the requirements of normal, upset and accident condition loadings (3 times normal operating pressure differential is controlling). For segmented cracks, the limiting crack angle for tube burst is approximately 297 0 (Reference 3). 4.2 Flow Induced Vibration loadings A Flow Induced Vibration (FIV) evaluation is performed to detemine the acceptability of tubes with various size through-wll cracks. This evaluation includes effects of normal operation and postulateo accident conditions as they apply to plant operation. Using ATH0S thermal-hydraulic model results, an evaluation is performed to determine the fluidelastic response of the tubes for various crack configurations. The tube response depends on the tube modal damping versus amplitude curve. If the slope of this curve is less than or equal 'f to zero, then the tubes may vibrate with large amplitude displacements when the flow velocity excuds the fluidelastic threshold for tube vibration with small amplitude damping. An evaluation is also performed to determine the turbulence rr spon e of a tube for various size cracks, lurbulence response of the tube is dominant in the absence of fluidelastic response. The evaluation also addresses the potential for contact of adjacent tubes. From the results presented below, for tubes within the sludge pile region, it is concluded that crack propagation will not occur due to flow induced vibration loadings in tubes with potential through-wall cracks that are under 2400 in circumference during a postulated steam line break (SLB) 3 event. These conclusions apply for a dented (or fixed) support boundary condition at the first tube support plate elevation, the most limiting boundary condition. The Sequoyah Unit I steam generators show limited denting so the dented support condition is realistic or conservative. Pinned or open crevice conditions result in much larger crack angles for vibration propagation due to the larger tube damping associated with these boundary conditions. Previaus analytical results for another plant with higher flow velocities thar. ' hose which exist in the Sequoyah steam generators were used for the FIV evaluation (Reference 2). A
SECL-91-431 Page 10 of 23 4.2.1 Normal Operating Condition Results 4.2.1.1 Normal Operation Flow Induced Vibration Results Since it is reported that the Sequoyah Unit I steam generators have packed crevices and cases of denting at the first support plate, it is conservatively assumed for the purposes of this evaluation that all tubes are supported (fixed) at that location. Evaluation results show acceptable fluidelastic vibration amplitudes for through-wall cracking up to 275 arc for tubes which are fixed at the first tube support plate and in the sludge pile region. For peripheral tubes in Zone-2, the limiting through-wall crack which still exhibits acceptable fluidelastic vibration amplitudes is 169 , 0 with the tube fixed at the first support plate. The condition of packed crevices (fixed) at the TSP is most limiting. 4.2.1.2 Normal Operation Turbulence Results Also, an evaluation is performed to determine the response of cracted steam generator tubes due to turbulence. Turbulence response occurs for tubes with increasing crack angles up to a flow velocity above which fluidelastic excitation occurs. Turbulence induced response is evaluated to determine the potential for crack growth caused by the corresponding cyclic stresses. For turbulence situations, the limiting through-wall crack angle in the central region is 3000 , while the limiting crack angle in Zone 1 of the periphery is determined to be 1350 (Reference 2). With regard to the potential for contact with and subsequent wear of adjacent tube (s), the maximum amplitude of' flow induced vibration for a Model 51 steam generator tube, with a single, through-wall crack extending up to 244 around the circumference and fixed at the first support plate is 0.057 inches. The gap between adjacent tubes is 0.406 inches; therefore, contact and wear of adjacent tubes as a result of flow induced vibration is not. expected (Reference 3). This deterinination was based on the amplitude for the most limiting tubeg found in the poriphery. Central region tubes with crack-angles-up to 244 will exhibit vibration amplitudes much lower than 0.057 inch. Only circumferential cracks need be evaluated for the potential of crack propagation due to FIV. Bending stresses induced during loading act in the sarre plane at the crack and thus can cause propagation. Axial cracks exist perpend mular to the induced bending stresses and thus are not affected. 4.2.2 Steam Line Break (SIB) Evaluation The steam line break ~ event duration is typically about 2 seconds for loagings in the generator (with a double ended main steam line pipe break, '4.6 ft break area). During this time period the peak velocity across the tubesheet in the sludge pile region is approximately 4 times the normal operation velocity. Even with this velocity increase, the tubes in both the central _ and peripheral regions are not susceptible to significant fluid-elastic vibra' tion because of the short duration of the event, and therefore, no growth of existing circumferenti;l cracks less than or aqual to 2400 in extent would be expected to occur due to an St.B event (Reference 4). It has also been demonstrated that small break area events are less limiting.
A -A.4a .-.c __ o .1_J. .u , aJ_s_.2 __ aE ._u,_ h. , s SECL-91-431 Page 11 of 23-4.2.3 Loss of Coolant Accident (LOCA) LOCA loads are developed as a result of transient flow, and temperature and pressure fluctuations following a postulated main coolant pipe break. As a . result of a LOCA event, tne steam generator tubing is subjected to the following loading mechanisms:
- 1) Primary fluid rarefraction wave loads.
- 2) Steam generator shaking loads due to the coolant loop motion.
- 3) External hydrostatic pressure loads as the primary side blows down to atmospheric presstire,
- 4) Bending stresses resulting from tubesheet bowing due to the secondary-to-primary pressure drop.
- 5) Bending of the tube due to differential thermal expansion between the tubesheet and first tube support plate following the drop in primary fluid temperature.
- 6) Axially induced loads resulting from differential thermal expansion between the tubes and tie rods / spacers resulting from the tuoe being tight in the first TSP, and the reduction in primary fluid temperature.
The rarefaction wave which passes through the tube results primarily in bending stresses in'the tube U-bends at the top TSP. The tube is essentially unaffected at the top of the tubesbeet, During the time the rarefaction wave is passing through the bundle, the primary to secondary pressure differential never exceeds the normal operating value. The LOCA shaking condition results in bending loads due to the shaking of the steam generator caused by the break hydraulics and reactor coolant loop motion, To obtain the LOCA induced hydraulic forcing functions, a dynamic blowdown analysis is performed to obtain the system hydraulic forcing functions assuming an instantaneous (1.0 insec break opening time) double-ended guillotine break at the steam generator outlet nozzle. The hydraulic forcing functions are then applied, along with the displacement time-history of the reactor pressure vessel (obtained from a separate reactor l vessel-blowdow, analysis), to a system structural model, which includes the steam generate", the reactor coolant pump and the primary :1 ping. This analysis yields tae time history displacements of the steam generator at its upper lateral and lower support nodes. These time-history displacements formulate the forcing functions for obtaining the tube stresses due to LOCA shaking of the steam generator, i- Loading mechanisms 3) through 6) above are not an issue since they occur when
- the pressure differential is from the secondary side tc the primary side and t tuoe burst cannot occur, l
SCCL-91-431 Page 12 of 23 4.2.4 Safe Shutdown Earthquake (SSE) Seismic loads are developed as a result of the motion of the ground during an earthquake. Tube stresses for the Sequoyah Unit I steam generators at the top of the tubesheet have been taken from a generic seismic analysis for Series 51 steam generators. The seismic analysis was performed using finite element analysis techniques accounting for the overall response of the steam generator. In computing the tubing response by response spectrum analysis, the analysis is performed usir.g three earthquake components, two horizontal and one vertical. The horizontal spectra are applied simultaneously in any two arbitrary perpendicular directions in the horizontal plane, with the vertical spectrum applied in the vertical direction. The combined seismic response is given by the square root of the sum of the squares of all significant modal contributions, with mod'.fication, if necessary, for closely spaced modes. The resulting tube bending stress is i 4255 psi for the case of a nondegraded tube and 1030 psi for a tube with a postulated 240 through-wall crack at the top of the tubesheet. The resulting bending stresses are not expected to cause propagation of the postulated 240 crack. 4.2.5 Combination of Postulated Accident Condition Loadirigs As required by RG 1.121, Safe Shutdown Earthquake loadings are combined with the SLB and.LOCA transient effects in a conservative manner that maximizes the effects of the combination of loads on tube burst ' capability. Tube stresses which occur during the most limiting accident, LOCA + SSE (combined by the square root of the sum of the squares method), are calculated to be 2930 psi for 2 cycles and 2340 psi for 240 cycles. These stresses are conservative as they are based on a primary loop pipe break. It has been determined that a tube with a 2400 crack will maintain structural integrity during the combined loadings _ of either SLB + SSE or LOCA + SSE. _However, because leak-before-break has been licensed for Sequoyah Unit 1, smaller pipe breaks become. limiting and even lower forces are exerted on the steam generator. LOCA + SSE loads are not expected to result in steam generator tube collapse or deformation at the top of the tubesheet region in the Sequoyah Unit 1 steam generators. 5.0 IDENTIFICATION OF LIMITING CRACK MORPHOLOGY
- In summary, considering all the stresses and tube deformations-imposed on - the tube. bundle during normal, upset, and accident endition ' loadings, the limiting crack morphology occurring at the top of the tubesheet for the central sludgg pile ragion (zones 3 and'4) is a single, through-wall crack exten & g 238 around the circamference of the tube. The limiting criteria for determining the limiting crack morphology is the maintainance of a safety factor of 3 anainst failure for bursting under normal operating pressure differential and flow induced vibration loadings. The limiting crack morphology occurring at the top of the tubesheet for the most limiting peripheral region (Zone 1) is a single crack extending 1350 hvound the circumference of the tube. The criterion for determining the liriting peripheral crack morphology is elimination of the potential for crack propagation due to turbulence excitatien. -s
l ) I SECL F 431 Page 13 of 23 It should also be noted that this limiting crack angle applies te only one location or, due to symmetry, two tubes in the entire tube bundle. For t ie purposes of analyses, the RPC determined crack angles listed above are assumed to be through-wall for the extent of the crack. This is an exceedingly conservative assumption in that RPC measures crack angles at depths greater than about 40-50% and there are no known pulled tube cases of a through-wall crack length equi to or greater than the measured RPC angle (for angles greater than or equal to the detection threshold of about 22%). In addition, UT inspection demonstrated that large angle RPC indications are comprised of segmented cracks. 5.1 Circumferential Crack Growth Rate Discussion Previous RPC eddy current inspections performed in the Sequoyah Unit 1 steam generators showed no WEXTEX region PWSCC initiated circumferential indications prior to the end of Cycle 4. At the end of Cycle 4 operation, I circumferential indication in steam generator number 4, and I axial indication in steam generator number 1 were detected. Approximately 1300 WEXTEX transitions were inspected at the end of Cycle 4 in the Unit I steam generators (Reference 3). A total of 11 tubes identified as containing circumferential indications during the acast recent inspection were previously RPC inspected. Ten of these were inspected at the end of Cycle 4, and I was inspected at the end of Cycle 3. A list of the tubes containing indications at the end of Cycle 5 that were previously RPC inspected is furnished in Appendix B (Rs.ference 4). The maximum apparent growth through the last cycle was found to be 1280 . The average growth rate of these 11 tubes is 64 degrees. The tube inspected in 1988 (E0C 3, NDD call) has an indicated circumferential involvement of 69 . based on the inspection just performed. Previous eddy current 1 da for these 11 tubes were reevaluated during the current outage. Based on this reevaluation, three tubes were determined to exhibit circumferential indications at the end of Cycle i , The growth rates of these 3 tubes for the last cycle was found to be 50, 13 and 210 . A best estimate of the a'ctual growth rate would be 210 ; however, for conservatism, a growth rate of 128 will be used for E0C conditions for central region tubes. Determining growth rates based on proviously called NDD tubes is exceptionally conservative in that a part through-wall crack, existing just below the detection threshold could have existed in the tube at the previous inspection. The occurrence of PWSCC in expansion transitions is generally associated with permanent sludge pile boundaries. Inspection results from other plants with more involved stages of PWSCC at the WEXTEX transitions have shown that the majority of WEXTEX indications occur within sludge pile regions, and based on top of tubesheet flow studies, the sludge pile is contained within Zones 3 and 4. However, an exceptionally small number of WEXTEX indications have been found in peripheral areas. For example, recent inspection results from another plant with a more advanced st;te of degradation indicated a tote af 200 WEXTEX circumferential indications. No indications were detecttf in Zone 1 tubes, and only 1 indication was detected in a Zone 2 tubt (T,eference 2). This plant has a higher
,. . i SECL-91-431 . Page 14 of 23 operating temperature and normal operating pressure difference than Sequoyah Unit 1. Such factors can influence PWSCC. The inspection results from Sequoyah Unit I showed a single tube in the peripheral region -(Zone 1) as containing a circumferential indication. The angle of involvement of the crack in this tube was 630 . Reference 3 defined the regions of the tubesheet according to the limiting angle for turbulence induced vibration, flow induced vibration and non-vibration concern (central) areas. Of the total number of tubes .for all four S/Gs susceptible to turbulence induced vibration, 2456, the most limiting of the areas of the tubesheet, only 1 indication was detected. This is a percentage of tubes cracked in the most limiting area of 0.041%. Due to the low probability of occurrence of crackir.g in this area and the lack of sludge in this area, the actual crack growth of the tube identified in this i.rea (63 ) will be used for E0C conditions. Also, no indications were reported in the 354 Zone 2 tubes, susceptibh to flow induced vibration, in any of the Sequoyah Unit I steam generators. Therefore, for all tubes in which the limiting crack angle is affected by flow forces, I tube out of 3872 tubes, or 0.026% of these tubes we.s found to contain detectable degradation.
5.2 Eddy Current Uncertainty Discussion The primary method of steam generator tubing eddy current inspection is performed via the bobbin coil. Bobbin probe performance is influenced by a number of factors, which include distortion by diametral change in the transition, copper and magnetite deposits and circumferential indications. The use of surface. riding probes, such as the rotating pancake coil probe, eliminates some of the above mentioned interferences. However, the RPC probe is not without a threshold level of detectability. The industry accepted level.for the RPC probe to detect circumferential cracking is approximately 50% through wall penetration over a 400 arc. Fully through wall indications are accepted to have a 250 detection threshold. Based on a recent WEXTEX WOG study, eddy current uncertainty using the RPC probe in determining crack arc length is approximately a 220 arc. j- 5.3 Safety Margin Determination g During the operation of a nuclear power plant, localized degradation of t the steam generator tubing may occur. As part of a preventative prcgram ta detect degraded tubing, inservice inspections are performed using an eddy current technique. Should through-wsll cracks develop in service, the cracks are judged acceptable for continued operation, provided , margin exists between the limiting crack morphology and the sum of the eddy current detection threshold and an operational allowance for continued degradation until the next scheduled inspection, i l The available safety margin in Sequoyah Unit 1 is defined b9re as the difference between the start of cycle allowable minus the sum of the threshold of detection (conservatively assuud to be 40 ), anf an allowance for crack growth (1280 to 63 0deper, ng upon the Zoc.e of the tubesheet). Table 5.1 lists the margins of sPty for each of the 4 tubesheet zenes. In all cases, the projected r.argin of safety is jadged
t SECL-91-431 Page 15 of 23 to be acceptable. No indications are projected to exceed RG 1.121 criteria during Cycle 6 operation of Segooyah Unit 1. No indications are exoected to exceed RG 1.121 criteria during subsequent operation, provided that similar inspection plans are followed during each outage, and that as the RPC eddy current data base grows for Sequoyah, the projected maximum growth rate of 128 per fuel cycle is maintained, or found to be conservative. 5.4 Leak Before Break Consideration The basis for the iaak before break (LOB) rationale is to limit the maximum allonble prima:, to secondary leak rate during normal operation such that the associated crack length through which allowable leakage occurs is less than the critical crack length for tube burst during a postulated SLB event. Thus, steam generator tube integrity is meintained on the basis of leakage monitoring during normal operation and unstable crack growth leading to tube burst is not expected in the unlikely event of a limiting accident. Leak rates through circumferential cracks are available through a combination of experimental and analytical results. Burst test results presented in Section 4.2 were used to compute burst pressure versus total circumfereotial crack angle using LTL properties at operating tamparature. Based upon burst test data , eviously presented, the calculated limiting single crack angles which will withstand the SLB pressure difference of 2650 psi, and 3 times the normal operating pressure differential were determined to be 321 and 238 , respectively. A single through-eall crack of 106 , growing in an orderly fashion is expected to leak at 500 gpd; therefore, leak before break is verified for a single, circumferentially oriented crack (Reference 3). The leak before break rationale for a single through wall crack, growing uniformly around the circumference of a tube is straightforward. ilowever, typical cracking patterns are more complex. Crack networks, rather than single cracks appear most often. Pulled tube examinations and laboratory cracked samples exhibit branched networks. Ligaments of material existing between through-wail cracks in a network increase the tube strength compared to a single through-wall crack of the same total length. Branched crack l networks exhibit lower leak rates than single cracks of equal extent. The l leak rate through a circumferential crack is proportional to the crack j length to the third power, thus a 1060 crack will leak at approximately S00 gpd and a 2 crack network, with equal 050 cracks will leak at approximately 98 gpd (Reference 3). Top of the tubesheet circumferential cracking patterns can be estimated from cracking patterns from pulled tubes and laboratory samples. For example, primary water stress corrosion cracking patterns point, to arrays of cracks with individual crack aspect ratios (ratio of crack length to crack depth of penetration) ranging from 4 to I to 6 to 1. The largest 00 crack l extent measured by UT in the Segt.oyah Unit 1 steam generators was 700. If a crack network develops, with as many as eight cracks, 35 in , I circumferential extent and exhibiting typical aspect ratios, the expected leakage still would be approximately 128 gpd, at a pretcure differential af 1385 psi (Reference 3). Such a segmented crack would have a tntal l N %. - +-
.. _.- -. . -. - - - -. .-- -- - - - - - . ~ . -
SECL-91-431 Page 16 of 23
-angle of involvement of 29)0 with 20 mil (3 degrees) ligaments separating the individual cracks. The burst pressure of such a crack would be at least 4155 psi, or 3 times the normal operating pressure di f ferential . The leak rat 9 of such a crack at SLB conditions would be approximately 340 gpd. Therafore, the 128 gpd administrative leak rate limit currently in place should be continued.
A single, through-wall crack such as might occur due to the loss of a li? ament betaeen two 35 cracks (73 degrees through-wall extent) would be expected to leak at approximately 140 gpd. A circumferential track of this size is less than the crack angle necessary for flow induced vibration propagation in all regio,'s of the bundle. 5.5 Expected End of Cycle Conditions The expected EOC condition of the tubes with regard to c' eumferentially oriented PWSCC is derived by summing the RPC detection threshold and expected growth rate. Since the intent of this evaluation is to compare the expected E0C conditions with structural limits, NDE uncertainty need not be included. Indications which could challenge tube structural integrity are readily detectable, and would be :dentified at the next subsequent inspection. For tubes in tubesheet zones 3 and 4, the expected E0C-6 condition is 168 , and for tubes in zones 1 and 2, the expected E0C-6 condition is 1030 , as indicated by Table 5.1. TABLE 5.1 MARGINS OF SAFETY IN SEQUOYAH UNIT 1 STEAM GENERATORS FOR , RPC INSPECTION AT THE WEXTEX TRANSITION ZONE 1 ZONE 2 ZONE 3 ZONE 4 LIMITING CRACK 135 0 169 0 238 0 238 0 DETECT. THRESH 0LD -40 0 -40 0 -40 0 -40 0 GROWTH AllCWANCE -63 -63 -128 -128 0 0 E0C-6 CIRC. EXTENT 103 103 1680 168 MARGIN OF SAFETY 32 0 66 73 0 70 0 NOTE: 1) A BREAKDOWN OF THE TUBESHEET ACCORDING 10' ZONES IS GIVEN IN APPENDIX A.
- 2) E0C-6 CONDITIONS REPRESENT COMB 1NED CONDITION CRACKS, i.e.,
CRACKS WITH 100% THROUGH-WALL. EXTENT LESS THAN ESTIMATED E0C-6 VALUE.
SECL-91-431 Page 17 of 23 Based upon the pulled tube data from another plant and the UT inspection results from tubes R13C41 and R14C39 from Sequoyah Unit 1, the ::xtent of throJgh-wall penetration is expected to be less than the measured RPC angle. Therefore, the margin values listed in Table 5.1 are conservative since the E0C-6 maximum conditions represent inside diameter circumferential involvement, as opposed to through-wall involvement based on tbc structural evaluation. Table 5.2 below, summarizes the through-wall circumferential crack limits developed in the above sections. Table 5.2 Structural Limits For Circumferential Indications Within the WEXTEX Transition Sequoyah Unit 1 Steam Generators Tube Burst Vibratica Propagation Circumferential Central Peripheral (All Zones) (Zones 3 & 4) Zone 1 Zone'2 Structural Limits Throughwall Cracks 238 o Turbulence Induced >300 0 135 ---- Vibration o Fluidelastic Induced 275 --- 169 Vibration o LOCA + SSE 240 240 0 240 0 o SLB 240 240 0 240 0 , Segmented Cracks (l) 297 0 360 0 360 360 0 Allowance for Growth 100 100 63 63 Notes:
- l. Use of the segmented crack limits requires verification of ligaments by inspection such as UT or limited crack depth and supported by structural analysis.
- 2. Limited ultresonic impection data suggests that burst strengths of circumferentially cracked Sequoyah Unit I tubes will be bounded by single crack and segmented crack models listed above.
SECL-91-431 Page 18 of 23 3 6.0 REMEDIAL MEASURES 6.1 Shotpeening During the Cycle 5 outage, shotpeening of the WEXTEX expansion transitions was performed at Sequoyah Unit 1 The shotpeening process is the most widely applied technique for increasing the service longevity of tubes subject to PWSCC in the expansion transitions. The shotpeening process imparts a compressive stress to the 1.0. surface of the tube. The compressive stress in effect " masks" the tensile strained 1.0. surface of the tube and minimizes the potential for the occurrence of PWSCC. 6.2 Enhanced leak Monitoring Capabilities The ability of the plant operators to adequately and accurately monitor any primary to secondary leakage will serve as an early warning system to the presence of through wall cracking in the transition. Establishment of a conservative leakage rate limit of 128 gpd will provide leak before break capability for the detection of leakage for a crack considering scgmented cracks as the basis for leakage. "o further mitigate the consequences of the potential presence of circumfr:ential cracks, Tennessee Valley Authority is implementing leakage monitoring practices for the early detection and response to rapidly propagating leak rates, more rapid than would be predicted on the basis of NRC Bulletin 88-02. 6.3 Stabilization TVA has adopted a conservative approach regarding stabilization of tubes with circumferentially oriented indications. All locat?ons in which the circmnferential extent as measured by RPC was greater tnan 80 degrees have Deen stabilized. In addition, the peripheral indication (Zone 1 indication, R39C23, S/G 2) has been stabilized. The arguements in Section 4.2.1.2 have shown that there is no potential for tube to tube interaction, regardless of tubesheet location, assuming the tube has suffered a complete circumierential severance. 7.0 CONCL81S10NS This evaluation has been completed addressing the safety significance of Cycle 6 operation of the Sequoyah Unit 1 steam generators with potential circumferential cracks in the hot leg WEXTEX expansion transition regions. In light of the above, it is concluded that Cycle 6 operation of the Sequoyah Unit 1 steam generators does not represent an unreviewed safety question based on the following justification. l l
SECL-91-431 Page 19 of 23
- 1. Will the probability of occurrence of an accident previously evaluated in the FSAR be increased?
No. Based upon the results of the RPC sampling, and tube repair philosophies adopted by TVA, no tubes in the Sequoyah Unit I steam generators are expected to exceed the tube structural limits developed using the guidance of RG 1.121. In the unlikely event that an existing indication, currently below the threshold of detection of the RPC probe, could propagate resulting in primary to secondary leakage, the 128 gpd administrative leakage rate limit will help to ensure that a safe shutdown of the unit occurs prior to the crack exceeding structural limits.
- 2. Will the consequence of an accident previously evaluated in the FSAR be increased?
No. The worst case consequence of continued operation would be primary to secondary leakage which would be bounded by the steam generator tube rupture ar.M yses performed in the FSAR. In the unlikely event tb a crack may initiate and propagate towards the limiting 2380arc ia % sludge pile region 2f the Sequoyah Unit I steam generators, and I d arc in the periphery during the next cycle, the 128 gpd leakage limit in a steam generator and leakage trending capability beyond the requirements of NRC Bulletin 88-02 implemented by TVA would allow for safe shutdown of the unit prior to crack growth exceeding this limit.
- 3. May the possibility of an accident of a different type than already-evaluated in the FSAR be created?
No. As discussed in the response to Question 1, steam generator tube integrity is expected to be maintained during the upcoming cycles during all plant conditions. Although unlikely, in the event that a through-wall, circumferential1y oriented crack may develop, the aktnistrative primary to secondary leak rate of 128 gpd and leakage tt 1 ding capability beyond the requirements of NRC Bulletin 88-02 shoald minimize the potential for an unanalyzed accident, such as a multiple tube rupture or a coupled steam line break, and tube rupture event.
- 4. Will the probability of a malfunction of equipment important to safety previously evaluated in the FSAR be increased?
No. By the performance of 100% RPC inspection of the WEXTEX transitions, steam generator tubc integrity and leaktightness are expected to be maintained. The overall safety and functional requirements of the steam generator tube bundles are not adversely affected. The steam ger.arator tube bundles are expected to maintain, within recommended margins, loads during normal operation and postulated accident conditions without loss of safety function.
SECL-91-431 Page 20 of 23
- 5. Will the consequences of a malfunction of equipment important to safety previously evaluated in the FSAR be increased?
Nn. The worst case consequences that could occur during subsequent plant operation is primary-to-secondary leakage during normal operating and plant transient conditions. It is expected that leakage will remain within acceptable limits during all plant conditions during the upcoming cycle for the Sequoyah Unit I steam generators.
- 6. May the possibility of a malfunction of equipment important to safety different than that already evaluated in the FSAR be created?
No. As discussed in response to Questions 1, 3 and 4, the Sequoyah Unit I steani generators will continue to meet individual tube integrity and tube bundle leaktightness requirements.
- 7. Will the margin of safety as defined in the basis of any technical specification be reduced?
No. Operation with potential circumferential1y oriented cracks in Ne WEXTEX transition region of the Sequoyah Unit 1 steam generator tubug is expected to be in compliance with RG 1.121 criteria during the upcoming cycle. Steam senerator tube integrity has been maintained at Sequoyah through the implementation of a 100% RPC inspection of the WEXTEX transitions, and subsequent plugging of all locations which were determined to contain a crack, either axial or circumferent tal. Expected End Of Cycle ccnditions of postulated pact-through-wall circumferential cracks which may currently exist at a level below the detection threshold of the RPC probe are expected to provide adequate margin at the End of Cycle 6. In addition, the implementation of a conservative primary to secondary leak rate limit of 128 gpd (500 ypd allowable leakage is the present technical specit e atton limit) imparting the capability to react to leakage trer.ds which are beyond the requirements of NRC Bulle+in 88-02. Therefore, operation of the Sequoyah Unit 1 Nuclear Pl. int with potential ; circumferential indications within the steam generator tuning WEXTEX expansion regions is not expected to adversC y affect steam generator tube integrity and does not represent an unreviewed safety question in accordance with 10 CFR 50.69 criteria. 1 i m_-______._.____ _ _ _ _ _ . _ _ _ _ _ _ . _ . _ . _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ . _ _ _- j
l l L.*
- SECL-91-431 Page 21 of 23 8,0 REFERENCES
- 1. NSD-REG-1104, Rev. 1, "Results of Ultrasonic inspections-Sequoyah Unit 1", November 25, 1991.
- 2. WCAP-13034, " North Anna Unit 1 Steam Generator Operating Cycle Evaluation", July 1991
- 3. SECL 91-309, "RPC Inspection Plan for WEXTEX Transition Regions",
9/10/91
- 4. IVA Letter WO3 911126 001, 'Sequoyah Nuclear Plant Steam Generator ,
Eddy Current Test Results", November 27, 1991 d e _ _ _ m_
.I e SECL-91-431 Page 22 of 23 APPENDIX A TUBESHEET ZONES FOR WEXTEX INDICATIONS SEQUOYAH UNIT 1 e
1 4
J o SECL-91-431 Page 23 of 23 APPENDIX 8 SEQUOYAH UNIT 1 INSPECTION RESULTS TUBES WITH CIRCUMFERENTIAL PWSCC AT THE WEXTEX TRANSITION INSPECTED AT PREVIOUS OUTAGES TUBE S/G E0C-5 CIRC. EXTENT E0C-4 CIRC EXTENT APPARENT GROWTH R10C62 3 83 0 NDD 83 0 0 0 R16C66 3 100 NDD 100 R17C46- 3 69 0 NDD
- 690 RilC31 4 650 NDD 65 0 R15C47 4 128 0 NDD 128 0 R17C42 4 96 0 83 0 13 0
R19C43 4 80 0 NDD 80 0 0 R20C34 4 71 NDD 71 R20C45 4 110 105 0 S C 0 R23C58 4 780 57 210 0 *' AVERAGE APPARENT GROWTH 64 (*): INSPECTED AT E0C-3, 06/88 (**): ASSUMES THAT BEGINNING 0F CYCtE 5 CRACKING LEVEL WAS 00 CIRCUMFERENTIAL EXTENT AND 0% PLHETRATION THROUGH-WALL
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