ML20197H994
| ML20197H994 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 12/18/1997 |
| From: | Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 97-714, GL-90-05, GL-90-5, NUDOCS 9801020049 | |
| Download: ML20197H994 (13) | |
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VMilN14 E,l. ire Hic ann Pows:n CosseuNv Riensimo, %wssNir 23261 December 18, 1997 i
United Statos Nuclear Regulatory Commission Sorial No.97-714 Attention: Document Control Desk NL&OS/ETS R0 Washington, D.C. 20555 Docket No.
50 338 License No.
NPF 4 1
Goritlemen:
MRQlNIA_ELEGIBlO AND PQMER COMPANY NORTH ANNA POMEB_STAflQMMNilt ASMERCIlmLXLRELIEE.BEQUESIMQE 46.
SERYlCE WATER SYSTG.LEAES On October 21, 1997 during a system walkdown, three locations with evidence of possible previous !aakage 1.0., stains, were identified in ASME Class 3 Service Water linos. In North Anna Unit 1.
In order to reduce the number of entries into action statomonts and Service water maalpulations, a repair plan was developed and im 'amented for the affected service water lines. Pursuant to 10 CFR 50.55a(g)(3)(i),
VW.la Eloctric and Power Company requests relief of ASME Code requirements, paragraph IWA 5250(a)(2) for the period of October 21, 1997 through November 4, 1997, when the last wold was repaired. Relief Roquest NDE 46 for the leaking wolds, and tho basis for the relief requests are provided in the attachmont to this letter.
Where meaningful results could be obtained, the treas of leakage were examined by j
radiography and an evaluation was performed for continuod operation in accordance with the Gonoric Letter 90 05, ' Guidance for Performing Temporary Non Code Repair I
of ASME Codo Class 1,2, and 3 Piping.' The ovaluation Jetermined the operability and continued safe operation of the examined service water lines until the necessary l
ASML Codo repairs could be made. The leaking locations were identified during a pO recurring system visual inspection which involves all of the ASME Secton XI stainless 1
stool pip'.g associated with the service water system. Additionally, in accordance with
- GL 90-05, radiographic assessment was performed on an additional sample of five wolds. Two of these five welds failed the structural integMy evaluation. Therefore, an additional radiographic-assessment was performed on an additional sample of five welds. Two of these five welds failed the structural integrity evaluation. As before, an l'
additional radiographic assessment was performed on an additional sample of five welds. All of those welds woro found acceptable by radiography and structural integrity analysis. The indications of possible leakage were in the welds or the adjacent base l
material.
Based on previous laboratory assessments of the repaired leaking l.
Indications, the cause..of leakage was believed to be microbiological influenced corrosion (MIC)..
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9901020049-97121 PDR ADDCK O 3G
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The condition of the Servico Water System was monitored during the period corresponding to the reliof request. The rnonitoririg program included walkdowns of the affected wolds to identify and quantify any leakago. No significant changos woro noted in the condition of the affected piping during this po, lod.
This rollof request has been reviewed and approved oy the Station Nuclear Safety and Operating Committoo.
If you have any additional questions concerning this request, please contact us.
Very truly yours, R. F. Saunders Vico President - Nuclear Enginooring and Services Attachment Commitments mado in this lottor:
Nono cc:
U. S. Nuclear Regulatory Commission Region II Atlanta Federal Contor 61 Forsyth St., SW, Suito 23T85 Atlanta, Georgia 30303 Mr. M. J. Morgan NRC Sonior Resident inspector North Anna Power Station
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i ASME Section XI Rollef Requests NDE-46 i
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i North Anna Power Station Unit 1 Virginia Electric and Power Company i
l Virginia Electric & Power Company North Anna Power Station Unit 1 Second 10 Year Interval Request for Rollef Number NDE-46 1.
IDENTIFICATION OF COMPONENTS i
Mark / Weld No.
Line No.
Drawing No.
Jak11 18W 3'-WS 75163 03 11715 CBM 78G 2 SHT,2 BW 11715 WS 1075A 99 3* WS 76163 03 11715 CBM 78G 2 SHT. 2 BW i
11715 WS 1076A 96 4" WS 57163 03 11715 CBM 780 2 SHT. 2 BW 11715 WS-16E (a)
The above welds are Class 3, moderate energy piping in the Service Water (SW) system; (b)
Line 4* WS 57163 03 is the return from instrument air compressors and charging pump lube oil coolers. Lines 3" WS 75163 03 and 3' WS76-163 03 are retum lines from the charging pump lube oil coolers. The nominal operating pressure and temperature is 75 psig and 950F, res, actively; and (c)
Joint Typa - butt weld (BW).
II.
CODE REQUIREMENTS The above welds had extemal evidence of through wall leakage, i.e., active Inks or stains. Virginia Electric and Power Company decided to proceed under tne assumption that each of the above welds contain through wall flaws.
Although this 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 Er', )n and Summer 1983 Addenda are applicable.
"lWA 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) repairs or replacements of components shall be peiformed-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.
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CODE REQUIREMENT FROM WHICH RELIEF IS REQUESTED Rolief is requestod from performing the above Code requirod repair of the above identiflod wolds until the affected piping system can be taken out of servico. The spocific Code requiremont for which relief is requested is the 1983 Edition and Summer 1983 Addenda, IWA 5250(a)(2).
IV. BASIS FOR RELIEF REQUEST This rollef request is submitted in accordanco with NRC Generic Letter 90-05 (GL 90 05), " Guidance for Performing Tomporary Non Code Repair of ASME Codo Class 1,2, and 3 Piping." The following information and justification are provided in accordance with the guidelinos of Part B and C of Enclosure 1 to GL 90 05.
Scopo. Limitdcun and SpecifktQonsiderations SCORQ Tho scopo consists of the wolds identified in Section I with evidence of possible through wall loaks in the Servico Water System for North Anna Power Station Unit 1. The material of the piping is stainloss stool ASME SA 312 type 316L.
LIIDilallona Based on radiographic examinations and laboratory examinations of removed portions of piping from previous replacements, Microbiological influonco Corrosion (MIC) was dolormined to be the cause of the flaws. The MIC induced flaws originated on the innor diamotor of the pipe and were detected during plant operation. Tho intent of this request is to obtain roliof for the period of operation from identification of a through wall flaw until repair was accomplished. To the extent practical, the repair was accomplishod in accordance with the guidance of NRC Generb Lottor 90-05. This porlod extends from identification of the first loaking wold on October 21,1997 to when the repair of each wold suspected of having through wall flaws was completed. All identified welds suspected of l
having through wall flaws woro ropalrod by November 4,1997.
Specific Considerations System interactions, l.o., consequences of flooding and spray on equipment woro ovaluatod. The identified flaws woro located on the piping such that potential through wallloakage would not affact plant equipment.
The structuralintegrity of the butt wolds were ovaluated based on radiographic examination results, the required design loading conditions, including dead weight, pressure, thermal expansion and seismic loads. The methods used in Pago 2 of 10
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the structural integrity analysis consisted of an area reinforcement, fracture mechanics, and limit load analysos.
Each indication was considered to be j
through wall due to the inability of either radiography or ultrasonics to determine indication depth. A summary of the flaw ovaluation is provided in Attachment 1.
Wold OS was analyzed and found acceptable. Wolds 18W and 99 failed the structural integrity evaluation because the actual flaw longth was greator than the analyzed flaw longth. The analyzed flaw length was bounded by the 15%
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circumforontial length as the maximum through wall flaw lengths permitted by NRC GL 90-05.
'Noirt 99 on lino 3"-WS 76163 03 was removed from service after the weld was radiographod.
Because of the inability of both RT and UT to give reliable through wall depth for MIC indications, all MIC indications was considered through wall.
This conservative assumption caused the wold to fall the assossment requiromonts of GL 90 05. The wold was replaced on October 28, 1997 ono (1) day after & wold was radiographod and removed from service.
Wold 18W on line 3"4n %t03 03 was removed from service after the weld was radiographod. Becauco uf the inability of both RT and UT to give reliable through wall depth for MIC indications, all MIC indications was considered through-wall.
This conservative assumption caused the wold to fall the assessmont requiremonts of GL 90 05. The wold was replaced on November 4, 1997 one (1) day after tho wold was radiographod and removed from service.
Wold 96 on lino 4" WS 57163-03 was removed from service after the wold was radiographed.
Because of the inability of both RT and UT to givo rollable through wall depth for MIC indications, all MIC indications was considered through wall. However, this conservativo assumption did not cause the weld to fall the assessmont requirements of GL 90 05. However, the wold was replaced for ease of construction on October 28,1997 one (1) day after the wold was radiographod and romoved from service.
The structural integrity for each wold identified with evidence of through-wall leakago (and romalning in service) was monitored weekly by visual monitoring of through wall flaws to determino any degradation of structuralintegrity.
Genoric Lottor 90 05 allows two options for temporary non-code repairs of Class 3 piping in moderate energy systems, (1) non wolded repairs, and (2) leaving the piping as is if there is no leakage and the flaw is found acceptable by the
'through wall flaw" 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 meeting the criteria for consequences and for structural integrity as described abovo until replaced.
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a Evaluation Flaw Detection Durina Plant Ooeration and imoracticality Determination The subject wolds were identified as having evidence of through wall leakage during a Servico Water System walkdown conducted on October 21,1997, when both Units were operating. During the past soveral months Virginia Electric and Power Company has been monitoring, evalucting, and replacing through. wall leaks in the Servico Water System caused by MIC. Removing portions of the Service Water Systern, prior to performing a structural integrity analysis, due to MIC can unnecessarily reduce the margin of safety by isolating portions of the Servico Water System that are structurally s-)und and capable of performing their intended safety function. Therefore, performing Codo repairs immediately was considorod impractical.
Root Cause Determination and Flaw Characterization The Servico Water System at North Anna Power Station has previously experienced MIC. The radiographic examinations of the service water welds with indications of through wall leaks revealed small volds surrounded by oxfollation, which is typical of MIC. No other type of operationally caused defects were identified by the radiographs.
Flaw Evaluation Flaw evaluation for the welds was performed as described in Attachment 1. The flaws were ovaluated by three types of analyses, area reinforcement, limit load analysis, and fracturo mochanics 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 wall degradation, at the identified location, the structural assessment considered each indication to be through wall.
V.
AUGMENTED INSPECTION To assess the overall degradation of the Service Water System augmented radiographic examinations were performed. After the initial through wall flaws wore identified, fivo (5) additional locations on lines having the same function were examined using radiography. All five (5) welds had evidence of MIC, without showing evidence of through wall leakage, i.e. stains. Welds 1W, 3W and 47 on line 3" WS 76163 03 were found structurally acceptable by radiography and structural integrity evaluation, however welds 1W and SW were replaced for ease of construction. Weld 2W on line 3" WS76-163 03 and weld 95 on line 4" WS57-163 03 failed structural integrity evaluation, and were replaced on October 28,1997 prior to returning the lines to service.
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Because weld 2W on line 3' WS 76163-03 and weld 95 on line 4* WS 57163-i
- 03 failed structural integrity evaluation five (5) additional locations on lines
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having the same function were examined using radiography. All five (5) welds had evidence of MIC, without showing evidence of through wall leakage, i.e.
stains. Welds 18W,72 and 73 on line 3' WS 75163 03 were found structurally acceptable by radiography and structural integrity evaluation, however these j
welds were replaced for ease of construction. Welds 15W and 17W on line 3'-
WS 75163-03 failed structural integrity evaluation, and were replaced on November 4,1997 prior to returning the lines to teorvice.
Because welds 15W and 17W on line 3' WS 75163 03 failed structural integrity -
evaluation five (5) additional locations on lines having the same function were examined using radiography. None of these welds exhibited evidence of MIC on l
the radiographs. Therefore, all of the five welds were found acceptable by j
radiography and structural integrity analysis was r'ot performed.
VI.
ALTERNATE PROVISIONS As an alternative to performing Code repairs in accordance with IWA 5250(a)(2) on through wall flaws in the Service Water System, it is proposed to allow the through wall flaws to remain in service until a scheduled code repair, unless the structural integrity has been determined to be unacceptable. This alternate provision applies to the subject welds from identification of the first leaking weld on October 21,1997 to the repair of each weld suspected of having a through-Wall flaw. All through wall flaws have been repaired by November 4,1997. The through wall flaws were evaluated to assure they met the criteria for flooding and spraying consequences and for structural integrity and were visually monitored as described above until code repaired, t
The proposed attemative stated above ensures that the overall levels of plant quality and safety will not be compromised.
Vil.
IMPLEMENTATION SCHEDULE Repairs of the effected weids were completed by November,4,1997.
Additionally, the repair of weld 69 on line 4*-WS F64163-03 was completed on November 4,-1997. This weld was previously identified in Virginia Electric and Power Company Letter 97 530, dated September 19, 1997, in relief request NDE-43, Unit 1.
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References:
1.
USAS B31.1 Power Piping Code 1967 Edition 2.
EPRI Report NP-6301 D, ' Ductile Fracture Handbook" 3.
Nuclear Regulatory Commission Generic Le'ter 90 05 ' Guidance for Performing Temporary Non Code Repair of ASME Code Class 1,2, and 3 Piping' i
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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.
Atwa Reinforcement Analvils The area reinforcement analysis is used to determine if adequate reinforcing exists such that ductile leadng 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,)
Where t,,, is the code minimum wall, and d, is the outside diameter The Code required reinforcement area is provided by the available material around the flaw in the reinforcing zone.
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. The hole size is based upon 15%
circumferential length limit of NRC GL 90-05.
Limit Load Analy111 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 ulumate strength and yield point stress for the SA312-TP316L stainless steel material at the design temperature of 1500F.
The flawed sections were subjected to deadweight, thermal, and seismic DBE loading.
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The allowable limit load is given by, M, = 2 o, R,,,' t-(2cos(p)-sin (0)) in lbf a, = flow stress = 0.5 (S, + S,), psi S, = yield stress, psi S, = ultimate stress, psi R,,, = mean radius of the pipe (inches)
O n.{Rl P)+ F
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R,,, t R, = intomal radius of the pipe (inches)
P = pressure (psig)
F = axialload (Ibs)
D = Outside diameter (inches) t = pipe thickness (inches) 0 = half angle of the crack (radians) = crack length 2 R, MR = Resultant Moment from the above mentioned loading conditions MR = MMY' + MZ' + T 2
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.
Etautun_MshoalenEnlyallon 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 dograded 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).
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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 K = 135 kslVin was used for the e
material per NRC GL 90-05.
The applied stress intensity factor for bending, K., is found by:
i K = o,-(n.R 0)" F.
The applied stress intensity factor for intemal pressure, K,, is found by:
K, = o,,,-(n R,,, 0)".F,,,
The applied stress intensity factor for axial tension, K is found by:
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K, = o,-(n R,,,0)" F, i
The stress intensity factor for residual stresses, K,, is found by:
K,, = S-(n.R,,,0)" F, Total applied stress intensity factor K, includes a 1.4 safety factor and is calculated by:
K, = 1.4.(K. + K, + K ) + K,,
y The allowable stress intensity factor is taken from Generic Letter 90-05.
Kyt = 135 kslVin for stainless steel.
Stress Intensity Factor Ratio is defined as:
i SR=K.
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3 Kyt The stress intensity factor ratio shall be less than 1.0 for continued operation.
A summary of the results are listed in Table 1.
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Table 1
SUMMARY
OF FLAW EVALUATION RESULTS FOR SERACE WATER WELDS i
l Flaw Max.
Max.
Max.
Aaousable Length Actual Max.
Max.
Bendirg Bendrg Res Jant Limm Factor Arceed Apowat::=
Analyzed Flaw Axial Torsson Moment Moment Moment Load M, of K,
94 We8d Nos.
Lbe Nos.
Lent:h in Lenath h LeadIbs IMt;2 1AY f*ct:s
- AZ ftt:s MR ft-R:s fllf Sid h3OD h&d!D j
18W 3*-WS-75-163C3 tis 5JB375 144 179 488 425 671 6452 9
63 135 99 3"-WS-7616343 1.G5 2.000 122 189 444 333 622 6452 to 62 135 96 4*-WS-57-163C3 2.12 0.1875 372 96 99 343 370 11950 32 63 135 i
Notes:
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- 1. Limit load factor of safety is Allowable Limit LoadMesultant Mon ent.
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- 2. Welds 18W and 99 failed the structural integnty evaluation because the actual flaw length was greater than the analyzed flaw length. The analyzed flaw length was bounded by the 15% circumferential length as maxrnurn thru-wap flaw lengths permrtted by NRC Genenc Letter GL 90-05.
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