ML20211D267
| ML20211D267 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 09/19/1997 |
| From: | Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 97-530, GL-90-05, GL-90-5, NUDOCS 9709290087 | |
| Download: ML20211D267 (13) | |
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Vinoisir Et.i:cTune ANi> Pows:n CmiswNv Hamimin. VinmNir 2326: j September 19, 1997 United States Nuclear Regulatory Commission Serial No.97-530 Attention: Document Control Desk NL&OS/ETS R1 Washington, D.C. 20555 Docket Nos. 50-338 50-339 License Nos. NPF-4 NPF.7 Gentlemen:
YlBGMALELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 and 2 ASME_SELCTION XLBEMEE_BEQLIESTS NDE-43 and 38 SERVICE WATER _ SYSTEM. LEAKS On August 6,1997 during a system walkdown, four locations with evidence of possible previous leakage i.e., stains, were identified in four ASME Class 3 Service Water lines in North Anna Units 1 and 2. In order to reduce the number of entries into action statements and service water manipulations, a repair plan was developed and implemented for the affected service water lines. Pursuant to 10 CFR 50.55a(g)(6)(i),
Virginia Electric and Power Company requests relief of ASME Code requirements, paragraph IWA-5250(a)(2) for the period of August 6,1997 until weld 69 on line 4"-WS-F64-163 03 is replaced. Weld 69 will be replaced no later than the next Unit 1 refueling outage scheduled for October 18,1998. Relief Requests NDE-43 (Unit 1) and NDE-38 (Unit 2) for the leaking welds, and the basis for the relief requests are provided in the attachment to this letter.
Where meaningful results could be obtained, the areas of leakage were examined by radiography and an evaluation was performed for continued operation in accordance with the Generic Lt.,tter 90-05, ' Guidance for Performing Temporary Non Code Repair of ASME Code Class 1,2, and 3 Piping." The evaluation determined the operability and continued safe operation of the examined service water lines until the necessary
- ASME Code repairs could be made. The leaking locations were identified during a recurring system visual inspection which involves all of the stainless steel piping associated with the service water system. Additionally, in accordance with GL 90-05, radiographic assessment was performed on an additional sample of five welds. One of these five welds failed the structural integrity evaluation. Therefore, an additional radiographic assessment was performed on an additional sample of five welds. All of these welds were found acceptable by radiography and structural integrity analysis.
The indications of possible leakage were in the welds or the adjacent base material. ,
Based on subsequent laboratory assessments of the repaired leaking indications, and q
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previous. replacements the cause of leakage was determined to be microbiological influenced corrosion (MIC).
The condition of the Service Water System will be monitored during the period corresponding to the relief request. The monitoring program will include walkdowns of the affected weld 69 on line 4" WS F64163 03 to identify and quantify any leakage. If the weld is not replaced within ninety (90) days from the walkdown date of August 6, 1997 it will be radiographed to determine its structural integrity. l This relief request has been reviewed and approved by the Station Nuclear Safety and Operating Committee.
If you have any additional questions concerning this request, please contact us.
Very truly yours, 024A&
R. F. Saunders Vice President - Nuclear Engineering and Services Attachments Commitments made in this letter:
- 1. Radiograph weld 69 on line 4" WS F64 -163 03 if not replaced by November 4, 1997.
cc: U. S. Nuclear Regulatory Commission Region 11 Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303 Mr. M. J. Morgan NRC Senior Resident inspector North Anna Power Station-l i
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i ASME Section XI Relief Requests NDE- 43 and NDE 38 North Anna Power Station Units 1 and 2 Virginia Electric and Power Company
' Virginia Electric & Fower Company North Anna Power Station Units 1 and 2 Second 10 Year Interval Request for Relief Number NDE-43 (Unit 1)
Request for Relief Number NDE-38 (Unit 2)
I. IDENTIFICATION OF COMPONENTS Mark / Weld # Line# Drawing # Joint 42A 2"-WS-80-163-Q3 11715-CBM-78C-2 SH. 2 SW 11715-WS-1078C 10 2"-WS-948-153A-03 11715-CBM-78G-2 SH. 2 SW 12050-WS-2948A 36 2"-WS-954-153A-03 11715-CBM-78G-2 SH. 2 SW 12050-WS-2954A 90 4"-WS-46-163-03 11715-CBM-78C-2 SH. 2 BW 11715-WS-19F 3W 4"-WS-56 163-Q3 11715-CBM-78C-2 SH. 2 BW 11715-WS-16F 4W 4"-WS-56-163-03 11715-CBM-78C-2 SH. 2 BW 11715-WS-18F 69 4"-WS-F64-163-03 11715-CBM-78A-2 SH. 1 BW 11715-WS-2D88B (a) The above welds are Class 3, moderate energy piping in the Service Water (SW) system; (b) Line 2"-WS-80-163-03 is the supply to the Unit 1 instrument air (1-IA-E-1C) heat exchanger.
Line 2" WS-948-153A-Q3 is the supply to the Unit 2 charging pump gear box cooler (2-CH-E-1C1) and Unit 2 seal coolers 2-CH-E-1C2A and 2-CH-E-1C28. Line 2"-WS-954-153A-Q3 is the return from the Unit 2 charging pump gear box cooler (2-CH-E-1C1) and Unit 2 seal coolers 2-CH-E-1C2A and 2-CH-E-1C2B. Line 4"-WS-46-163-03 provides cooling water to the Unit 1 charg-ing pump lube oil coolers and instrument air compressors. Line 4"-WS-56-163-Q3 is the return from the Unit 1 charging pump lube oil coolers and instrument air compressors. Line 4"-WS-F64-163-Q3 is the return from the Unit 1 air conditioning condenser. The nominal oper-ating pressure and temperature is 75 psig and 95'F, respectively; and (c) Joint type - butt weld (BW), and socket weld (SW).
Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 1 of 6
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l II. CODE REQUIREMENTS l
The above welds had external evidence of through-wall leak-age, i.e., active leaks or stains. Virginia Electric and Power Company decided to proceed under the assumption that each of the above welds contain through-wall flaws. Al-though Tis evidence of leakage was not detected during the conduct of a system pressure test, it is being treated as such, and the requirements of IWA-5250 of the 1983 Edition and Summer 1983 Addenda is applicable to Unit 1. The re-quirements of IWA-5250 of the 1986 Edition is applicable to Unit 2.
"IWA-5250 Corrective Measures:
(a) The source of leakage detected during the conduct of a system pressure test shall be located and evaluated by the Owner for corrective measures as follows:...
(2 or 3) repairs or replacements of components shall be performed in accordance with IWA-4000 or IWA-7000, respectively."
Articles IWA 4000 and IWD-4000 of ASME Section XI Code repair requirements would require removal of the flaw and subsequent weld repair.
III. CODE REQUIREMEN' ROM WHICH RELIEF IS REQUESTED Relief is requested from performing the above Code required repair of the above identified welds until the effected piping system can be taken out of service. The specific Code requirement for which relief is requested is the 1983 Edition and Summer 1983 Addenda, IWA- 5250 (a) (2 ) for Unit 1, and the 1986 Edition, IWA- 52 50 (a) (3 ) for Unit 2.
IV. BASIS FOR RELIEF REQUEST This relief request is submitted in accordance with NRC Generic Letter 90-05 (GL 90-05) , " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping." The following information and justification are provided in accordance with the guidelines of Part B and C of Enclosure 1 to GL 90-05.
Scope. Limitations and Specific Considerations SXQP_e The scope consists of the welds identified in Section I with evidence of possible through-wall leaks in the rervice water system for North Anna Power Station Units 1 and .2. The material of the piping is stainless steel ASME SA-312 type 316L for welde 42A, 90, 3W, 4W and 69. The material of the piping for weld 10 is ASME SA-376 type 304. The material of the piping for weld 36 is ASME SA-312 type 304.
Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 2 of 6
Limitationn Based on radiographic examinations and laboratory examina-tions of removed portions of piping f_om previous replace-ments, Microbiological Influence Corrosion (MIC) was deter-mined to be the cause of the flaws. Addicionally, laborato-ry examinations of weld 94 shows evidence of MIC. Radio-graphs of welds 90, 3W, and 4W show indidations of MIC.
The MIC induced flaws or.iginated on the inner diameter of the pipe and were detected during plant operation. The intent of this request is to obtain relief for the period of opcs ^ ion from the identification of a through-wall flaw untia repair was accomplished. To the extent practical, the repair waa accomplished in accordance with the guidance of NPS Generic Letter 90-05. This period extends from identi-fication of the first leaking weld on August 6, 1997 to the repeir of each weld suspected of having thr ugh-wall f1-s-is completed. All identified welds suspectti cf having through-wall flaws were repaired by August 20, 1997, except weld 29 Specific Considerations System interactions, i.e., consequences of flooding and spray on equipment were evaluated. The identified flaws were located op the piping such that potential through-wall leakage would not affect plant equipment.
The structural integrity of the butt welds was evaluated based un radjographic examination results, the required design loading conditions, including dead weight, pressure, thermal expansion and seismic loads. The methods used in the structural int 3grity analysis consisted of an area rein--
forcement, fracture muchanics, and limit load analysis.
Each indication was considered to be through-wall due to the inability of either radiography or ultrasonics to determine indication depth. A summary of the flaw evaluation is pro-vided in Attachment 1. all welds were analyzed and found acceptable, except weld 90.
Radiography of socket weld 42A on line 2"-WS-80-163-Q3, weld 10 on line 2 ' WS-948-153A-Q3, and weld 36 on line 2"-
WS 954-153A-Q3 were not attempted because radiograpns of socket welds do not yield meaningful results. Additionally, flaws cannot be characterized for socket welds. Therefore, complat3 structural integrity analysis was not performed.
Lines 2"-WS-80-163-Q3, 2"-WS-948-153A-Q3, and 2"-WS-954-153A-03 were retaoved from service and the socket welds were replaced by August 20, 1997, iourteen (14) days after the evidence of leakage was detected. Weld 90 on line 4"-WS 163-03 was removed f rom se vice af ter the weld was radiogra-phed. Because of the inability of both RT and UT to give reliable through-wall depth for MIC indicatiors, all MIC Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 3 of 6
indications was considered through-wall. This conservative essumption caused the weld (weld 90) to fail the assessment requirements of GL 90-05. The weld (weld 90) was replaced on August 14, 1997 one (1) day after the weld was radiograp-hed and removed from service. Welds 3W and 4W on line 4"-
WS-56-163-Q3 showed evidence of acceptable MIC as determined by radiography and structural integrity analysis. However, these welds were replaced as a conservative measure an August 22, 1997 one (1) day utter the welds were radiograph-ed and removed from service.
The structural integrity for each weld identified with evidence of through-wall leakage (and remaining in service) was monitored weekly by visual monitoring of through-wall flaws to determine any degradation of structural integrity.
Generic Letter 90-05 allows two options for temporary non-code repairs of Class 3 piping in moderate energy systems, (1) non-welded repairs, and (2) leaving the piping as-is if there is no leakage and the flaw is found acceptable by the "through-wall tlaw" approach discussed in Section C.3.a.
... The temporary non-code repair approach selected was to leave the welds as they were found, subject to monitoring and y{g.l.' meeting the criteria for consequences and for structural integrity as described above until replaced.
Evaluation Flaw Detection Durina Plant Oneration and Impracticality Determination The subject welds were identified as having evidence of a through-wall leakage during a Service Water System walkdown conducted on August 6, 1997, when both Units were operating.
During the past several months Virginia Electric a nd Power Company has been monitoring, evaluating, and replacing through-wall leaks in the Service Water System caused by MIC. hemoving portions of the Service Water System, prior to performing a structural integrity analysis, due to MIC can unnecessarily reduce the margin of safety by isolating portions of the Service Water System that are structurally sound and capable of performing their intended safety func-tion. Therefore, performing Code repairs immediately was considered impractical for welds 42A, 10, 36, and 69.
Root.Ctuse Determination and Flaw Characterization The Service Water System at North Anna Power Station has previously experienced MIC. The radiograph examinations of the service water welds with indicatJ7ns of through-wall leaks revealed small voids surrounded b, exfoliation, which is typical of MIC. No other type of operationally caused Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 4 of 6
. - . . .-=
l defects were identified by the radiographs.
Flaw Evaluation Flaw evaluation for the welds was performed as described in Attachment 1. The flaws were evaluated by three types of analyses, area reinforcement, limit load analysis, and fracture mechanics evaluation using the guidance from NRC Generic Letter 90-05. Because of the inability for either radiography or ultrasonic techniques to determine the extent of wa?.1 degradation, at the identified location, the struc-tural assessment considered each indication to be through-wall.
The anrlv'.as determined that weld 69 is capable of maintain-ing i.e structural integrity until it is repaired. The follo*ing is based on the results from the analyses:
- 1. Ductile tearing will not occur at the flaw locations when the piping is subjected to the design pressure from the area reinforcement calculation.
- 2. The limit load analysis shows that there is enough margin against a ductile rupture for the most limiting case analyzed.
- 3. For the subject welds a linear elastic fracture mechan-ics analysis shows that the applied stress intensity factor at the analyzed flaws is below the allowable stress intensity factor per the guidance of NRC Generic Letter 90-05. Therefcre, a failure by brittle fracture is unlikely to occur.
V. AU3MENTED INSPECTION To assess the overall degradation of the service water system augmented radiographic examinations were performed.
After the initial through-wall flaws were identified, five (5) additional locations on lines having the same function were examined using ' radiography. Two (2) of the five (5) walds had evidence of MIC, (weld 2W on line 4"-WS-56-163-03 and weld 94 on line ia-WS-46-163-03) without showing evi-dence of through-wall leakage, i.e. stains. Weld 2W was found structurally acceptable by radiography and structural integrity evaluation, and not replaced. Weld 94 failed structural integrity evaluation and was replaced on August 14, 1997 cne (1) day after the veld was radiographed and .e-mcved from service. Weld 6W on line 3"-WS-75-163-Q3 was replaced for ease of construction. The remaining two welds which did not show' evidence of MIC on the radiographs were
-not replaced, (welds 68 and 85 on line 4"-WS-F64-163-Q3).
Relief' Request NDE-43 Unit 1, NDE-38 Unit 2 Page 5 of 6
Because weld 94 on line 4"-WS-46-163-Q3 failed structural integrity evaluation five (5) additional locations on lines having the same function were examined using radiography.
All of the five welds were found acceptable by radiography and structural integrity analysis.
VI. ALTERNATE PROVISIONS As an alternative to performing Code repairs in accordance with IWA-5250 (a) (2) for Unit 1 and IWA-5250 (a) (3) for Unit 2 on through-wall flaws in the Service Water System, it is proposed to allow the through-wall flaws to remain in ser-vice until a scheduled code repair, unless the structural integrity has been determined to be unacceptable. This alternate provision applies to the subject welds from iden-tification of the first leaking weld on August 6, 1997 to the repair of each weld suspected of having a through-wall flaw. All through-wall flaws had been repaired by August 20, 1997, except weld 69.
Thn proposed alternative stated above ensures that the overall levels of plant quality and safety will not be compromised.
VII. IMPLEMENTATION SCHEDULE Repairs of the effected welds were completed by August 20, 1997, except for weld 69. Weld 69 was evaluated to assure it met the criteria for flooding and spraying consequences.
A weekly visual inspection will be performed until weld 69 is replaced. If it is not replaced within 90 days (by November 4, 1997) from the walkdown date of August 6, 1997 it will be radiographed to determine its structural integri-ty per Station Procedure O-PT-75.24.
References:
- 1. USAS B31.1 Power Piping Code - 1967 Edition
- 2. EPRI Report NP-5301-D, " Ductile Fracture Handbook"
- 3. Nuclear Regulatory Commission Generic Letter 90-05 " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping"
-Relief Request NDE-43 Unit 1, NDE-38 hit 2 Page 6 of 6
i Attachment 1 Flaw Evaluation Methods and Results Introduction Butt welds identified by radiography as having MIC were analyzed for structural integrity by three methods, area reinforcement, limit load analysis, and linear elastic fracture mechanics evaluation.
Area Reinforcement Analysis The area reinforcement analysis is used to determine if adequate.
reinforcing exista such that ductile tearing would not occur. The guidelines of ANSI B31.1 paragraph 104.3. (d) 2 (reference 1) are used to determine the Code required reinforcing area. The actual reinforcing area is calculated and is checked against the required reinforcement area.
The Code required reinforcement area in square inches is defined as:
- 1. 07 (t.) (d )
i Where to is the code minimum wall, and d is the outside diameteri The Code required reinforcement area is provided by the available material around the flaw in the reinforcing zore.
The results of this analysis determined that for the subject four inch (4") and three inch (3") pipes, a hole size of 2.2" and 1.7" respectively will be contained by the reinforcement provided by the excess material in the near vicinity.
Limit Load Analysig The structural integrity of the piping in the degraded condition was established by calculating the minimum margin of safety based upon a Limit Load Analysis. These methods are documented in EPRI report NP-6301-D (Ductile Fracture Handbook) (reference 2).
The limit load analysis of the postulated flawed sections were performed with a material flow stress representing the midpoint of the ultimate strength and yield point stress for the SA312-TP316L stainless steel material at the design temperature of 150*F.
Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 1 of 4 a
The flawed sections were subjected to deadweight, thermal, and seiomic DBE loading.
The allowable limit load is_given by, M - 2 at R[ t- (2cos(#)-sin (0)) in-lbf or = flow-stress = 0.5 (S y +S), psi Sy = yield stress, psi S, = ultimate stress, psi R. = mean radius of the pipe (inches)
- p. 8 + II'(Rl P1 +F 2- 4 of R;t R i = internal radius of the pipe (inches)
P = pressure. (psig)
F = axial load (lbs)
D = Outside diameter (inches) t = pipe thickness (inches) 0 = half angle of the crack (radians) = crack lenath 2 R, MR = Resultant Moment from the above mentioned loading conditions MR']MY* +MZ a + T*
MY = Bending Moment MZ = Bending Moment T = Torsion The calculated factor of safety is, FS = _M,_
(MR)
The minimum factor of safety of 1.4 is required to be qualified for continued operation.
A summary of the results is listed in Table 1.
Practure Mechanics Evaluation A linear elastic fracture mechanics analysis was performed for circumferential through-wall crack using the guidance provided in NRC Generic Letter 90-05. The structural integrity of the piping in the degraded condition was established by calculating the stress intensity factor ratio based upon a Fracture Mechanics evaluation.
This method is documented in EPRI report NP-6301-D (Ductile Fracture Handbook) (reference 2).
Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 2 of 4
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A through-wall circumferential crack was postulated for every area containing MIC. The cracks were subjected to a design pressure loading of 150 psig in addition to the deadweight, normal operating thermal and seismic DBE loadings. For the purpose of this evaluation a generic allowable stress intensity factor of Kre = 135 kaiVin was used for the material per NRC GL 90-05.
The applied stress intensity factor for bending, Kre , is found by:
Ki e = 0 3- ( 7 R. 0 ) O . 5 F3 The applied stress intensity factor for internal pressure, Kp, is i found by:
Kr= 0, - ( w R. 0 ) " ?,
i The applied stress intensity f actor for axial tension, K ir is found by:
K it =
at - ( w R. 0 ) F t The stress intensity factor for residual stresses, Kra is found by:
Kg i = S - (n R 0) F t Total applied stress intensity Kr includes a 1.4 safety f actor and is calculated by:
K7 -
1.4-(Krs + Kr i + KIT) +Ka i The allowable stress intensity factor is taken from Generic Letter 90-05.
Kat, = 135 kaiVin for stainless steel.
Stress Intensity Factor Ratio is defined as:
S R = L_ ,
Kat The stress intensity factor ratio shall be less than 1.0 for continued operation.
A summary of the results are listed in Table 1.
Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 3 of 4
Table 1 ~
SUMMARY
OF FLAW EVALUATION RESULTS FOR SERVICE WATER WELDS Max. Max. Max. Attowable f1 w Listi t Factor Applied
- Allowsble tength Actual Max. Max. Berding Bending Resotant Moment Moment Load M. of E, Ec Anatyred Flaw Axiat Torsion Moment Safety' ksi/in Length in Length in Load tbs T ft-lbs MY ft-lbs MZ ft-tbs MR ft-lbs ft-lbf _ksi/in Weld Nos. Line Nos. - - - - -
90 4"-Ws-46-163-03 2.12 3.0625 Note 2 - - -
4"-WS-46-163-03 2.12 3.375 Note 2 - -
67.91 135 94 160 617 472 793.141 11950 15.066 4"-Ws-56-163-03 2.12 0.25 450 67.5 135 3W 27 421 182 459.45 9259 20.152 4=-WS-56-163-03 2.12 0.4375 100 67.095 135 4W 630 301 742.297 11940 16.039 60 4"-WS-F64-163-03 2.12 0.750 596 252 Notes:
- 1. Limit load factor of safety is Attowable Limit Load / Resultant Mornent. The anatyred flaw tength was bounded
- 2. Weld 90 and 94 failed the structural integrity evaluation because the actual T.aw tecath was greater than the anatyred flaw tength.
by the 15% circunf erential length as maxinun thru-wat t flew tengths permitted by NRC Generic Letter st 90-05.
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Relief Request NDE-43 Unit 1, NDE-38 Unit 2 Page 4 of 4
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