ML20210K256
| ML20210K256 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 08/08/1997 |
| From: | Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 97-453, GL-90-05, GL-90-5, NUDOCS 9708190201 | |
| Download: ML20210K256 (13) | |
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Vim;tNI A El.ECTRIC ANil POWER COMi%NY l Riemiosin, Vima N A 232<,i i
August 8, 1997 United States Nuclear Regulatory Commission Serial No.97-453 Attention: Document Control Desk NL&OS/ETS R0 Washington, D.C. 20555 Docket Nos. 50-338 License Nos. NPF-4 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNIT 1 ASME SECTION XI RELIEF REQUEST NDE-42 SERVICE WATER SYSTEM LEAKS On June 25,1997 during a system walkdown, three locations with evidence of possible previous leakage i.e., stains, were identified in three ASME Class 3 Service Water lines in North Anna Unit 1. In order to reduce the number of entries into action statements and service water manipulations, a repair plan was developed and implemented for the three 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 June 25,1997 through July 9,1997, when the last weld was repaired. Relief Request NDE-42 for the leaking welds for Unit 1, and the basis for the relief requasts 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 Letter 90-05, " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2, and 3 Piping." The evaluation determined the operability 3 and continued safe operation of the examined service water lines until the necessaiy ASME Code repairs could be made. The three 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 welds. The three indications of possible leakage were in the welds or the adjacent base material. Based on subsequent laooratory assessments of two of the repaired leaking indications, the cause of leakage was determined to be microbiological influenced corrosion (MIC).
The condition of the Service Water System was monitored during the period corresponding to the relief request. The monitoring prograrn included walkdowns of the '
affected welds to identify and quantify any leakage. No significant changes were noted in the condition of the affected piping during this period, i I
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This relief request has been reviewed and approved by the Station Nuclear Safety and Operating Committee.
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L- If you have any additional questions concerning this request, please contact us.
Very truly yours, k
R. F. Saunders Vice President - Nuclear Engineering and Services
) Attachment Commitments made in this letter:
None cc: U. S. Nuclear Regulatory Commission Region II Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, GeorDi a 30303 Mr. M. J., Morgan NRC Senior Resident inspector North Anna Power Station
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r ASME Section XI Relief Request NDE-42 i
s-North Anna Power Station Unit 1 Virginia Electric and Power Company
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Virginia Electric & Power Company North-Anna Power Station Unit 1 Second110 Year Interval Request for Relief-Number _NDE-42 I. IDENTIFICATION OF COMPONENTS Mark / Weld # Line# Drawino# Joint
.. 9W 2"-WS-84-163-Q3 11715-CBM-78C-2-SHT. 2-SW 11715-WS-1087E-
-16W 4"-WS-46-163-Q3 11715-CBM-78C-2 SHT. 2 BW 11715-WS-19F 84 4"-WS-57-163-Q3 11715-CBM-78C-2 SHT. 2 BW 11715-WS-16E' '
19W 3"-WS-75-163-Q3 11715-CBM-78C-2 SHT. 2 BW 11715-WS-1075A (a) The above welds are Class 3, moderate energy
-piping in the Service Water 'SW) system; (b) Line 4"-WS-46-163-Q3 provides cooling water to the Unit'l charging pump lube oil coolers and instrument air compressors. Line 2"-WS 163-Q3 is the return-from the-1-IA-E-1A heat exchanger. Lines 4"-WS-57-163-Q3 is the re-turn from instrument air compressors and char-ging pump lube oil coolers. Line 3"-WS-75-163-Q3 is the return line from the charging pump lube oil coolers. The nominal operating pres-sure and temperature is 75 psig and 95'F, respectively; and
-(c) Joint type - butt weld'(BW), and socket weld '
-(SW) .
II. CODE REQUIREMENTS The above welds had external evidence of through-wall leak-age, i.e., active leaks or stains. VirginiaLElectric and Power Company decided to proceed under'the assumption that each of the above welds contain through-wall-flaws. Al-though 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 Edition and Summer 1983 Addenda is applicable.
"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) repairs or replacements of components shall be Relief Request NDE-42 Unit 1 Page 1 of 6 l
l 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 REQUIREMENT FROM 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) .
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.
Scone. Limitations and Specific Considerations Scope The scope consists of the welds identified in Section I with evidence of possible through-wall leaks in the Service Water System for North Anna Power Station Unit 1. The material of the piping, for all welds except 19W, is stainless steel ASME SA-312 type 316L with 316 weld filler metal. The mate-rial of the piping for weld 19W is ASME SA-312 type 316L with inconnel filler metal.
Limitations Based on radiographic examinations and laboratory examina-tions of removed portions of piping from previous replace-ments, Microbiological Influence Corrosion (MIC) was deter-mined to be the cause of the flaws. Additionally, laborato-ry examinations of welds 9W and 19W shows evidence of MIC.
Welds 16W and 84 have not yet been subjected to laboratory examinations, however MIC was seen on radiographs of these welds. The MIC induced flaws originated on the inner diame-ter of the pipe and were detected during plant operation.
~ The intent of this request is to obtain relief for the period of operation from the identification of a through-wall flaw until repair was accomplished. To the extent practical, the repair was accomplished in accordance with the guidance of NRC Generic Letter 90-05. This period extends from identification of the first leaking weld on Relief Request NDE-42 Unit 1 Page 2 of 6
I June 25, 1997 to the repair of each weld suspected of having
- through-wall flaws was completed. All identified welds
-suspected of having through-wall flaws were repaired by July 9, 1997.
Specific Considerations System interactions, i.e., consequences of flooding and spray on equipment were evaluated. The identified flaws were located on the piping such that potential through-wall leakage would not affect plant equipment.
The structural integrity of the butt welds was evaluated based on radiographic-examination results, the required design loading conditions, including dead weight, pressure, thermal expansion and seismic loads.- The methods used in the structural integrity analysis consisted of an area rein-forcement, fracture mechanics, and limit load analyses.
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 welds 84, and 9W.
Radiography of socket weld 9W on line 2"-WS-84-163-Q3 was
.not attempted because radiographs of socket welds do not yield meaningful results. Additionally, flaws cannot be characterized for socket welds. Therefore, complete struc-tural integrity analysis was not performed. Line 2"-WS 163-Q3 was removed from service upon detection of leakage.
The socket weld was replaced two (2) days after the evidence of leakage was detected. Weld 84 on line 2"-WS-57-163-Q3 was removed from service after the weld was radiographed.
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 weld to fail the assessment requirements of GL 90-05. The weld was replaced on July 3, 1997'two (2) days after the weld was radiographed 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 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 Eclief Request NDE-42 Unit 1 Page 3 of 6
integrity as described above until replaced.
Evaluation Flaw Detection Durino Plant ODeration and Impracticality Determination The subject welds were identified as having evidence of through-wall leakage during a Service Water System walkdown conducted on June 25, 1997, when both Units were operating.
During the past several months Virginia Electric and Power Company has been monitoring, evrf.uTting , and replacing through-wall leaks in the Service ifater System caused by MIC. Removing 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 16W and'19W.
Root Cause 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 indications of through-wall leaks revealed small voids surrounded by exfoliation, 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 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 wall degradation, at the identified location, the struc-tural assessment considered each indication to be through wall.
The analyses determined that welds 16W and 19W were capable of maintaining their structural integrity until they were repaired. -The following 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.
Relief Request NDE-42 Unit 1 Page 4 of 6 L
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- 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 Etress intensity factor at the analyzed flaws is below the allowable stress intensity factor per the guidance of NRC Generic Letter 90-05. Therefore, a failure by brittle fracture is unlikely to occur.
V. AUGMENTED 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. Four (4) of the five (5) welds had evidence of MIC, without showing evidence of through-wall leakage, i.e. stains. All augmented weld locations were found structurally acceptable by structural integrity evaluation. However, all five (5) of the welds selected for augmented inspection were replaced. The one weld which did not show evidence of MIC on the radiographs was replaced because it is located on a reducer on the opposite side of a weld containing MIC.
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 unaccept-able. This alternate provision applies to the subject welds from identification of the first leaking weld on June 25, 1997 to the repair of each weld suspected of having a through-wall flaw. All through-wall flaws have been re-paired by July 9, 1997. The through-wall flaws were evalu-ated to assure they met the criteria for flooding and spray-ing consequences and for structural integrity and were visu-ally monitored as described above until code repaired.
The proposed alternative stated above ensures that the overall levels of plant quality and safety will not be conpromised.
VII. IMPLEMENTATION SCHEDULE Repairs of the effected welds were completed by July, 9, 1997.
Relief Request NDE-42 Unit 1 Page 5 of 6 1
References:
l' . fJSNS B31.1 Power Piping Code - 1967 Edition
- 2. EPRI Report NP-6301-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" 5
Relief Request NDE-42 Unit 1 Page 6 of 6
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, , 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, kraa Reinforcement in=1vais The area reinforcement analysis is used to determine if adequate reinforcing exists 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 re-quired reinforcement area.
The Code required reinforcement area in square inches is defined as:
1.07 (t ) (d )
i 1
Where t, is the code minimum wall, and d is the outside diameter i
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.
LLait Load in=1valg !
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 1 strength-and yield point stress for the SA312-TP316L stainless steel material at the design temperature of 150*F.
Relief Request NDE-36 Unit 1 Page 1 of 4 Attachment 1
r The flawed sections were subjected to deadweight, thermal, and seism $c DBE loading.
The allowable limit load is given by, M. = 2 ar R[*ta (2cos (S) -sin (0) ) in-lbf or = flow stress = 0.5 (S y +S), psi
'S y = yield stress, psi So = ultinate stress, psi R. = mean radius of the pipe (inches) p _6_ + Il- ( R *P) + F 2 4orR;c Ri = 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 lenoth 2 R, MR = Resultant Moment from the above mentioned loading conditions MR =/I"Y' +M2 ' + 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.
Fracture 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-36 Unit 1 Page 2 of 4 Attachment 1
A through-wall circumferential-crack was postulated for every
- a,rea'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 Kc = 135 ksiVin was used for the material per NRC GL i
90-05.
-The applied stress intensity factor for bending, Kre,- is found by:-
K n = 03 - (w R 0) '80.5 F3 i
The applied stress intensity factor for internal pressure, Kzp, is found by:
Kpi
= a ,- ( x R. 0 ) ** F.
The applied stress intensity factor for axial' tension, K it 18 found by:
K it =
as- ( x R. 0 ) * *
- Fs The stress intensity factor for residual stresses, Kra is found by:
Kra = S - ( n R. 0 ) F s Total applied stress intensity Kr includes a 1.4 safety factor-and is calculated by:
Kr = 1. 4 - (K23 + Kr + Kr) + Ka i i The allowable stress. intensity factor is taken from Generic Letter 90-05.
Km = 135 ksiVin for stainless steel.
Stress Intensity Factor Ratio is defined as:
SR = Er._.
Km The stress intensity factor ratio shall be lens than 1.0 for continued operation.
A summary of the results are listed in Table 1.
Relief Request NDE-36 Unit 1 Page 3 of 4 Attac'iment 1
.s 8-Table 1. -
. SUM 4ARY OF FLAW EVALUATION RESULTS FOR SERVICE WATER WELDS
' Flaw . Max. Max. Max. Attowable -
Length Actual Max. Max. Bending Bending. .Resulting Limit' Factor _ Applied Attomable:
Analyzed Flaw . Anfat. Torsion- Moment ' Moment Moment load M. of K, ' . Kg .
Weld hos. Line Nos. teneth in Lenoth in' Load Ibs T f t-tbs MY ft-tbs MZ ft-tbs Ma ft-tbs ft-tbf~ safety' ksi/in ksi/in -
16W- 4"-US-46-163-Q3 2.12 1.9 184 0- 236 212 317 11960 '37.71 62.24 .135 84 4"-WS-57 163-03 2.12 .3.75 Note 2 - - - - - -- - -
19W 3"-WS-75-163-03 1.66 .200 64 30 37 76 89.69 6439 71.79 ; 52.65 135 Cotes:
- 1. Limit load factor of safety is Allowable Limit Load /Resulting Moment. ..
- 2. Weld 84 failed the structural integrity evaluation because the actual flew tength was greater than ti analyzed flaw tength. The anatyred flaw tength was bounded by the 15% circumferential length as sexisue thru-wall flaw tengths permitted by NRC Ceneric Letter GL 90-05.
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l Relief Request NDE-36 Unit 1 Page 4 of 4-Attachment 1 i.
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