ML20138G301

Requests Relief of ASME Code Requirements,Paragraph IWA-5250(a)(2) for Period of 970319 Through End of Upcoming Unit 1 Refueling Outage in Order to Reduce Number of Svc Water Sys Manipulations

ML20138G301
Person / Time
Site:	North Anna
Issue date:	04/29/1997
From:	Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
97-235, NUDOCS 9705060255
Download: ML20138G301 (12)

Text

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. Waisia EuccTRIC ANI) POWIC H COM PA N Y Ru:im ONI), YIRGINIA 2,Wil April 29, 1997 l

United States Nuclear Regulatory Commission Serial No.97-235 Attention: Document Control Desk NL&OS/ETS R1

, Washington, D.C. 20555 Docket No. 50-338 License No. NPF-4 Gentlemen:

i VIRGINIA ELECTRIC AND POWER COMPANY

, NORTH ANNA POWER STATION UNIT 1 ASME SECTION XI RELIEF REQUEST SERVICE WATER SYSTEM LEAKS On March 19,1997 during a system walkdown, five locations with evidence of possible previous leakage i.e., stains, were identified on one ASME Class 3 service water line in North Anna Unit 1. In order to reduce the number of Service Water System manipulations, a repair plan was developed and implemented for the affected service water line. 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

March 19,1997 through the end of the upcoming Unit 1 refueling outage, currently scheduled to end June 12,1997. Relief Request NDE-36 for the feaking welds for Unit 1 and the basis for the relief request is provided in the attachment to this letter.

Each of the five locations was 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 confirmed the operability and continued safe operation of the examined service water line until the necessary ASME Code repairs could be made.

The five 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 five indications of possible leakage were in the welds or the adjacent base material. Based on radiography and the previous laboratory

assessments of repaired leaking indications, the cause of leakage was determined to be microbiological influenced corrosion (MIC).

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9705060255 970429

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The condition of the Service Water System will be monitored during the period corresponding to the relief request. The monitoring program includes walkdowns of the affected welds to identify and quantiiy any leakage.

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, 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, Georgia 30323 NRC Senior Resident inspector North Anna Power Station i

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ASME Section XI Relief Request NDE-36 4

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Virginia Electric & Power Company l North Anna Power Station Unit 1  !

! Second 10 Year Interval 5 Request for Relief Number NDE-36 I I. IDENTIFICATION OF COMPONENTS  !

Mark / Weld # Line# Drawina# Joint ,

Unit 1 l 13 4"-WS-F63-153-03 11715-CBM-78A-2 SHT. 1 BW '

l 11715-WS-2D87A L 16 -4"-WS-F63-163-Q3 11715-CBM-78A-2 SHT. 1 BW 11715-WS-2D87A l

! 61 4"-WS-F63-163-03 11715-CBM-78A-2 SHT. 1 BW  !

11715-WS-2D87B 64 4"-WS-F63-163-Q3 11715-CBM-78A-2 SHT. 1 BW l 11715-WS-2D87B '

66 4"-WS-F63-163-Q3 11715-CBM-78A-2 SHT. 1 BW 1 l 11715-WS-2D87B i (a) The above welds are Class 3, moderate energy piping in the Service Water (SW) system and; (b) Provides cooling water to the Unit 1 control room chillers. Normal flow is 237 gpm at an operating pressure of 80 psig and an operating temperature of 959F. The design pressure is 150 psig and design temperature is 125'F.

(c) Joint type - butt weld (BW).

II. IMPRACTICABLE CODE REQUIREMENTS The above welds had external evidence of through-wall leakage, I i.e., stains. Virginia Electric and Power Company decided to proceed under the assumption that:each of the above welds contain through-wall flaws. Although this evidence of leakage was not detect.ed during the conduct of a system pressure test, the requirements of IWA-5250 of the 1983 Edition and Summer l 1983 Addenda. The specific Code requirement for which relief i 1

is requested is IWA-5250 (a) (2) . i l i l

l "IWA-5250 Corrective Measures:  !

j (a) The source of leakage detected during the conduct of a l system pressure test shall be located and evaluated by  ;

} the Owner for corrective measures as follows:... l

! (2) repairs .or replacements of components shall be i performed in accordance with IWA-4000 or IWA-7000, j respectively."

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Relief Request NDE-36 Unit 1 Page 1 of 5 l

Articles IWA-4000 and IWD-4000 of ASME Section XI Code repair .

requirements would require removal of the flaw and subsequent l weld repair, l

Virginia Electric and Power Company decided to replace each weld having evidence of through-wall leakage in a planned i evolution to reduce the number of action statements and '

service water system manipulations.

III. BASIS FOR RELIEF REQUEST .

This relief request is submitted in accordance with NRC Generic' Letter 90-05, " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping." The j following information and justification are provided in  ;

l accordance with the guidelines of Part B and C of Enclosure 1 l to GL 90-05.

l Scone. Limitations and Soecific Considerations  !

Scoce The scope consists of the welds identified in Section I with evidence of possible through-wall leaks in the shared Service l Water System for North Anna Power Station Unit 1. The material l of the piping is stainless steel ASME SA-312 type 316L with i 316 weld filler metal.

Limitations Based on radiographic examinations and laboratory examinations of removed portions of piping from previous replacements,

Microbiological Influence Corrosion (MIC) was determined to be the cause of the flaws. The MIC induced flaws originated on the inner diameter of the pipe and were detected during plant '

operation. The intent of this request is to obtain relief for j the period of operation from the identification of a through-wall flaw until repair is accomplished. To the extent j practical, the repair will be accomplished in accordance with l

the guidance of NRC Generic Letter 90-05. This period extends from identification of the first leaking weld on March 19, '

1997 to the repair of each weld suspected of having through-wall flaws is completed. All identified welds suspected of having through-wall flaws will be repaired by the end of the Unit 1 1997 Refueling Outage, scheduled to end on June 12, 1997.

Soecific 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 i not affect plant equipment.

i Relief Request NDE-36 Unit 1 Page 2 of 5 l

The structural integrity of the welds was evaluated based on radiographic examination results for the required design loading conditions, including dead weight, pressure, thermal expansion and seismic (DBE) loads. The methods used in the structural integrity analysis consisted of an area reinforcement, 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 provided  ;

in Attachment 1. All welds were analyzed and found acceptable. ]

The structural integrity for each weld identified with evidence of through-wall leakage was monitored by the  !

following methods: l l

o Weekly visual monitoring of through-wall flaws and !

leakage. '

o Radiographic examination.

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 integrity as described above.

Evaluation Flaw Detection' Durino Plant Operation and Impracticality Determination The subject welds were identified as having evidence of through-wall leakage during a Service Water System walkdown conducted on March 19, 1997, when both Units were operating.

During the past several months Virginia Electric and Power Company has been monitoring, evaluating, and replacing  ;

through-wall leaks in the Service Water System caused by MIC.

Prior to performing a structural analysis,' removing portions of the Service Water System which may have leaked in the past 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 function. Therefore, performing Code repairs immediately was considered impractical.

Relief Request NDE-36 Unit 1 Page 3 of 5

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1 t Root Cause Determination and Flaw Characterization l' 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 i

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  !

i Flaw evaluation for the welds was performed as described in- t

! Attachment 1. The flaws were evaluated by three types of '

I analyses, area reinforcement, limit load analysis, and -

j fracture mechanics evaluation using the guidance f rom NRC Generic - Letter 90-05. Because of the inability for either radiography or ultrasonic techniques to determine the extent  ;

of wall degradation, the structural assessment considered each i

indication to be through wall.

The analyses determined all welds were capable of maintaining their structural integrity until they were repaired. This is based on the results from the analyses:

i j 1. Ductile tearing will not occur at the flaw locations when the piping is subjected to the design pressure.

] 2. The limit load analysis shows that there was at least a

• l factor of safety of 5 against a ductile rupture for the  ;

j most limiting case analyzed. '

1 l' 3. For the subject welds a linear elastic fracture mechanics  !

!' analysis shows that the applied stress intensity factor i 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 2

unlikely to occur.

I IV. AUGMENTED INSPECTION To assess the overall degradation of the Service Water System augmented radiographic examinations were performed. Af ter the  ;

initial through-wall flaws were identified, five (5) l

additional locations on lines having the same function were I examined using radiography. Three (3) out of the five (5) 3 welds had evidence of MIC but no evidence of through-wall leakage. Any indication of MIC was treated as a through-wall defect and analyzed for structural stability as described in Attachment 1. All augmented weld locations were found 1 . structurally acceptable.

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Relief Request NDE-36 Unit 1 Page 4 of 5 1

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V. 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 March 19, 1997 to the repair of each weld suspected of having a through-wall flaw is completed. All through-wall flaws will be repaired by l the end of the Unit 1 1997 Refueling Outage, scheduled to end ,

on June 12, 1997. The through-wall flaws must meet the .

criteria for flooding and spraying consequences and for l structural integrity and will be visually monitored as  !

described above until Code repaired. '

The proposed alternative stated above ensures that the overall levele of plant quality and safety will not be compromised.

Attachment:

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1. Flaw Evaluation Methods and Results

References:

1. USAS B31.1 Power Pipirg 1967 Edition I 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" l

Relief Request NDE-36 Unit 1 Page 5 of 5

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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.

&Kg3 Reinforcement Analysis 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 required reinforcement area.

The Code required reinforcement area in square inches is defined as:

1. 07 (t,) (d )

i 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") piping, a four inch (4 ") diameter hole would have acceptable reinforcement.

Limit Load Analysis 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-36 Unit 1 Page 1 of 4 Attachment 1

i The flawed sections were subjected to deadweight, thermal, and seismic DBE loading.

The allowable limit load is given by, M. - 2

• a r* R,*

• t - (2 cos (4 ) - sin ( 0 ) ) in-lbf or = flow stress = 0.5 (S y +S), o psi  !

Sy .= yield stress, psi  :

So = ultimate stress, psi l i

R, = mean radius of the pipe (inches) I t

p =_O + 11 (R$ P) +F

? 2 4o;R;t ,

l Ri = internal radius of the pipe (inches) l' l P = pressure (psig) l F = axial load (lbs)

.D. = Outside diameter (inches) I t = pipe thickness (inches)  ;

i 0 = half angle of the crack (radians) = crack lenath  ;

2 R, 1 l

MR.= Resultant Moment from the above mentioned loading conditions t i 3

MR=/MY'+MZ + T*

MY = Bending Moment MZ = Bending Moment T = Torsion The calculated factor of safety is, FS = _&__

(MR)

The minimum factor of safety of 1.4 is required to be qualified for continued operntion.

A summary of the results is listed in Table 1.

Fracture Mechanics Evaluation l

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 I the degraded condition was established by calculating the stress  !

l intensity factor ratio based upon a Fracture Mechanics evaluation. l

l. This method is documented in EPRI report NP-6301-D (Ductile j Fracture Handbook) (reference 2).

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Relief Request NDE-36 Unit 1 Page 2 of 4 i Attachment 1 l

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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 DDE loadings. For the purpose of this evaluation a generic allowable stress intensity factor of K c = 135 i

ksiVin was used for the material per NRC GL 90-05.

The applied stress intensity factor for bending, K13, is found by:

K 13 = 03 - (x R 0 ) 0. 5 + F3 The applied stress intensity factor for internal pressure, K p, isi found by:

Kpi = a,- ( x R 0 ) F, The applied stress intensity factor f or axial tension, K r is found 3

by:

Kr= at- ( x R. 0 ) F 3 t The stress intensity factor for residual stresses, Kra is found by:

K a = S - ( N R 0 ) .5, pt i

Total applied stress intensity Kr includes a 1.4 safety factor and )

is calculated by: l Kr = 1. 4 - ( K23 i+Kp + iK r) +Kg i The allowable stress intensity factor is taken from Generic Letter 90-05.

Kat. = 135 ksiVin for stainless steel.

Stress Intensity Factor Ratio is defined as:

SR = Er_

Kut The stress intensity factor ratio shall be less than 1.0 for i continued operation. '

A summary of the results are listed in Table 1.

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i Relief Request NDE-36 Unit 1 Page 3 of 4 Attachment 1 ,

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Table 1

SUMMARY

OF FLAW EVALUATION RESULTS FOR SERVICE WATER WELDS Max. Max. Max. Allowable Max. Max. Max. Bending Bending Resulting Limit Factor Applied Allowable Crack Axial Torsion Moment Moment Moment Load M, of K, K,c V2td Nos. Line Nos. Length in Load lbs T ft-lbs MY ft-lbs M2 ft-lbs MR ft-lbs ft-tbf Safety' ksi/in ksi/in Note 2 2.10 534 719 1258 1386 2005 1.199x10' 5.98 79.98 135 Cotes:

1. Limit load f actor of safety is Allowable Limit Load /Resulting Moment.

2. Enveloping cale includes welds identified with through-wall leaks, 13,16, 61, 64, and 66 on line 4"-WS-F63-163-Q3, and augmented welds 76 on line 4"-F63-163-Q3 and 7 on line 4"-WS-F64-163-Q3. ~

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Relief Request NDE-36 Unit 1 Page 4 of 4 Attachment 1 i

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