ML20137D049

Forwards Relief Requests NDE-34 & 36 for Leaking Welds for Units 1 & 2,respectively & Basis for Relief Requests Re Svc Water Sys Leaks

ML20137D049
Person / Time
Site:	North Anna
Issue date:	03/19/1997
From:	Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
97-123, GL-90-05, GL-90-5, NUDOCS 9703250246
Download: ML20137D049 (13)

Text

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J March 19, 1997 United States Nuclear Regulatory Commission Serial No.97-123 Attention: Document Control Desk NL&OS/ETS R1 Washington, D.C. 20555 Docket Nos. 50-338 50-339 License Nos. NPF-4 NPF-7 i Gentlemen:

ylBGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 A_SME SECTION )rl RELIEF REQUEST NDE-34 AND 36 SERVICE WATER SYSTEM LEAKS On February 10,1997 during a system walkdown, three locations with evidence of j possible previous leakage i.e., stains, were identified in three ASME Class 3 Service i 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 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 February 10,1997 through March 18,1997, when the last weld was repaired. Relief Requests NDE-34 and 36 for the leaking welds for Units 1 and 2, respectively, 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 Letter 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 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 laboratory 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 program included walkdowns of the

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l affected welds to identify and quantify any leakage. No significant changes were noted 4 in the condition of the affected piping during this period.

j This relief request has been reviewed and approved by the Station Nuclear Safety and Operating Committee.

if you have any additional questions conceming this request, please contact us.

Very truly yours, I

4 R. F. Saunders

! Vice President - Nuclear Engineering and Services

Attachment Commitments made in this letter

None cc: U. S. Nuclear Regulatory Commission Region ll 101 Marietta Street, N. W.

Suite 2900 Atlanta, Georgia 30323 NRC Senior Resident inspector i North Anna Power Station l l

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ASME Section XI Relief Requests NDE-34 and 36 1

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North Anna Power Station Units 1 and 2 Virginia Electric and Power Company I

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Virginia Electric & Power Company North Anna Power Station Units 1 and 2 Second 10 Year Interval Request for Relief Number NDE-34 (Unit 1)

Request for Relief Number NDE-36 (Unit 2)

I. IDENTIFICATION OF COMPONENTS Mark / Weld # Line# Drawin9# Joint Unit 1 6 2"-WS-C82-153A-Q3 11715-CBM-78G-2 SHT. 1 SW 11715-WS-2C82A 19W 4"-WS-57-163-Q3 11715-CBM-78C-2 SHT. 2 BW 11715-WS-16D Unit 2 base metal 2"-WS-776-163-Q3 11715-CBM-78C-2 SHT. 2 pipe adjacent to weld 56 (a) The above welds are Class 3, moderate energy piping in the Service Water (SW) system and; (b) Provide cooling water from the service water to instrument air compressors, and charging pump lube oil coolers for both units and return service water back to the return headers. Normal flow is 20 to 100 gpm at an operating pressure of 100 psig. The design pressure is 150 psig and design temperature is 150 F.

(c) Joint type - butt weld (BW) and socket weld (SW).

II. IMPRACTICABLE CODE REQUIREMENTS The above welds had external evidence of through-wall leakage, 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 detected during the conduct of a system pressure test, the requirements of IWA-5250 of the 1983 Edition and Summer 1983 Addenda and the 1986 Edition are applicable to Unit 1 and Unit 2, respectively. The specific Code requirement for which relief is requested is IWA-5250 (a) (2) for the 1983 Edition and Summer 1983 Addenda and IWA-5250 (a) (3) for the 1986 Edition.

Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 1 of 6

t f 'IWA-5250 Corrective Measures:

l (a) The source of leakage d6sacted during the conduct of a  :

i system pressure test shall be located and evaluated by  !

p 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-l 7000, respectively."

l Articles IWA-4000 and IWD-4000 of ASME Section XI Code repair requirements would require removal of the flaw and subsequent weld repair.

Virginia Electric and Power Company decided to replace each i weld having evidence of through-wall leakage in a planned ,

evolution to reduce the number of action statements and 4 service water system manipula Mons. '

III. BASIS FOR RELIEF REQUEST  !

This relief request is submitted in accordance with NRC i Generic Letter 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 Soecific Considerations Scone The scape consists of the welds identified in Section I with evidence of possible through-wall leaks in the shared Service Water System for North Anna Power Station Unit 1 and Unit 2.

The material of the piping is stainless steel ASME SA-312 type 316L with 316 weld filler metal for pipe class 163 and ASME SA-312 type 304 with 304 filler metal for pipe class 153A.

Limitations Based on radiographic examinations and laboratory examinations of removed portions of piping from replacements, Microbiological Influence Corrosion (MIC) was determined to be the cause of the flaw. 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 the period of operation from the identification of a through-wall flaw until repair is accomplished. To the extent practical, the repair will be accomplished in accordance with j the guidance of NRC Generic Letter 90-05. This period extended from identification of the first leaking weld on February 10, t 1997 to the repair of each weld suspected of having through-wall flaws was completed. The flaws at weld 6 and adjacent to weld - 56 were repaired by February 20, 1997. Weld 19 was l Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 2 of 6

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' repaired by March 18, 1997.

Specific Considerations System interactions, i.e. , consequences of flooding and spray on equipment were evaluated. The flaws were located on the piping such that potential through-wall leakage would not affect plant equipment.

The structural integrity of butt weld 19W 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 i 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. Butt weld 19W on line 4"-WS-57-163-Q3 was analyzed and found acceptable. The radiography for the area adjacent to weld 56 on line 2"-WS-776-163-Q3 showed the possibility of MIC for 77% of the circumference. This length l of MIC exceeded the established allowable limit and the weld was declared inoperable at that time. The line was isolated l and the weld was repaired prior to returning the line to service. No additional structural integrity analysis was performed on this flaw.

Radiography of socket weld 6 was not attempted since radiographs of socket welds do not yield meaningful results.

Since the flaws cannot effectively be characterized for socket welds, a 3/4" hole was postulated. The socket weld was replaced nine (9) days after the evidence of leakage was detected, before a complete structural integrity analysis could be completed. The socket weld was monitored for leakage weekly until replaced.

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

o Weekly visual monitoring of through-wall flaws and leakage.

o Radiographic examination of butt weld 19W and the area adjacent to socket weld 56 was performed. The area adjacent to socket weld 56 was replaced on February 19, 1997. Weld 19W was replaced on March 18, 1997.

Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 3 of 6

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Generic Letter 90-05 allows two options for temporary non-code l

repairs of Class 3 piping in moderate energy systems, (1) non-welded repairs, e.nd (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 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.

1 Evaluation l Flaw Detection Durino Plant ODeration and ImDracticality Determination .

1 The subject welds were identified as having evidence of through-wall leakage during a Service Water System walkdown i conducted on February 10, 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. Removing portions of the Service Water System which may have leaked in the past prior to performing a structural analysis could reduce the margin of safety by unnecessarily 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.

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. Additionally, an examination performed by a Virginia Electric and Power Company staff metallurgist of a piping segment removed for repair confirmed the presence of MIC.

Flaw Evaluation Flaw evaluation for butt weld 19W and the additional sample of welds subject to augmented inspection 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, the structural assessment considered each indication to be through wall. Areas identified by radiography as incomplete fusion or incomplete penetration were analyzed Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 4 of 6

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i by conservatively assuming a twenty five percent (25%) reduced pipe wall thickness all around the cross-section.  ;

The radiography for the area adjacent to weld 56 on line 2"-

WS-776-163-Q3 showed the possibility of MIC for 77% of the i i-circumference. This length'of MIC exceeded the established l allowable limit and the weld was declared inoperable at that  ;

i time. The line was isolated and the weld was repaired prior to l returning the line to service. No additional structural  !

! integrity analysis was performed. l The analyses detenmined butt weld 19W in line 4"-WS-57-163-Q3 {

and butt welds subjected to augmented examination were capable  :

of maintaining their structural integrity until they. were  ;

repaired. This is based on the results from the analysis:

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

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

factor of safety of 4.0 against a ductile rupture for the j most limiting case analyzed. '

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

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. Therefore, a brittle fracture was unlikely.

One weld, 1W on line 4"-WS-57-163-03, radiographed as part of the augmented inspection did exceed the allowable stress intensity by twelve percent (12%) against a built in safety margin of forty percent (40%) added in the calculation of stress intensity. The radiograph of this i weld showed incomplete penetration for two' inches (2") i out of a total circumferential length of 14.137", but was J conservatively analyzed with a seventy five percent (75%)

wall thickness all around the weld. Additionally, a very '

conservative through-wall flaw evaluation method was performed for a thin wall degradation situation. This l weld has not leaked in the past. Therefore, a failure by ]

brittle fracture is unlikely to occur. '

IV. AUGMENTED INSPECTION ,

l 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 u l examined using radiography. All welds in this sample group had evidence of MIC but no evidence of through-wall leakage. Any indication of MIC was treated as a through-wall defect and i

Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 5 of 6

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' analyzed for structural. stability as described in Attachment  !

1. All augmented weld locations were found structurally ,

acceptable. '

V. ALTERNATE PROVISIONS As an alternative to performing Code repairs in accordance l 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 service i until a scheduled code repair, unless the structural integrity l has been determined to be unacceptable. This alternate i provision applies to the subject welds, weld 6 on line 2"-WS- 1 C82-153A-03, weld 19W on line 4 "-WS-57-163-Q3, and line 2"-WS-  !

776-163-Q3 adjacent to weld 56, from identification of the  :

first leaking weld on February 10, 1997 to the repair of each l weld suspected of having a through-wall flaw is completed. ,

Piping adjacent to weld 56 was replaced on February 19, 1997.  !

Weld 6 was replaced on February 20, 1997. Weld 19W was -

replaced on March 18, 1997. The through-wall flaws which have been replaced were subject to monitoring and met the criteria for flooding and spraying consequences and for structural ,

integrity as described above.  ;

i The proposed alternative stated above ensures that the overall '

levels of plant quality and safety will not be compromised.  ;

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Attachment:

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1. Flaw Evaluation Methods and Results

References:

1. USAS B31.1 Power Piping 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" l

Relief Request NDE-34 Unit 1, NDE-36 Unit 2 Page 6 of 6

Attachment 1 Flaw Evaluation Methods and Results Introduction Butt welds identified by radiography as having MIC were analyzed i for structural integrity by three methods, area reinforcement, '

limit load analysis, and linear elastic fracture mechanics i evaluation.

Area Reinforcement haalvsis j The area reinforcement analysis is used to determine if adequate reinforcing exists such that ductile tearing would not occur. The  ;

guidel.ines of ANSI B31.1 paragraph 104.3. (d) 2 (reference 1) are i 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 )

3 Where to is the code minimum wall, and di is the outside diameter The Code reinforcement area required 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 j 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-34 Unit 1, NDE-36 Unit 2 Page 1 of 4 l

Attachment 1

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 - (2cos (#) -sin (0) ) in-lbf or - flow stress = 0.5 (S +y S;,) , psi Sy = yield stress, psi Su = ultimate stress, psi R. = mean radius of the pipe (inches)

II (Rl=P) +F 2 4o;R;t 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 length 2 R, MR = Resultant Moment from the above mentioned loading conditions MR =/MY' +MZ' + T' MY = Bending Moment MZ = Bending Moment  :

T = Torsion l

The calculated factor of safety is, FS = _Mm_

(MR)

The minimum factor of safety of 1.4 is required to be qualified for continued operation, j A summary of the results is listed in Table 1.

Zyacture 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-34 Unit 1, NDE-36 Unit 2 Page 2 of 4 Attachment 1

A thr'ough-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 c = 135 I

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

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

i Kr, = [ ad- ( w R, 0 ) "] Fb The applied stress intensity factor for internal pressure, K p, isi found by:  !

I Kp i = a ,- ( w R 0 ) *** F , j The applied stress intensity factor for axial tension, K r is found I i by:

K r = at- ( x R. 0 ) * *

• Fs i

The stress intensity factor for residual stresses, Kra is found by:

Kg = S - ( x R. 0 ) ' *

• F t i  ;

l Total applied stress intensity Kr includes a 1.4 safety factor and is calculated by:

Kr - 1. 4 - (Krs + K r + K r) + Kra i i The allowable stress intensity factor is taken from Generic Letter 90-05.

Kai, = 135 ksiVin for stainless steel.

Stress Intensity Factor Ratio is defined as:

SR = Er_--

Kur, 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-34 Unit 1, NDE-36 Unit 2 Page 3 of 4 Attachment 1

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l Table 1

SUMMARY

OF FLAW EVALUATION RESULTS FOR SERVICE WATER WELDS Allowable Bending Bending Resutting Limit Factor Applied. '

Atlouable Crack Axial Torsion Moment Moment Moment Load M. of K, .

Ke Weld Nos. Line Nos. Lenoth in Load Ibs T ft-lbs MY ft-tbs MZ ft-lbs MR ft-lbs ft-lbf Safety' ksf/in ksiA n 1W 4"-WS-57-163-Q3- 0.625 208 170 685 376 800 1.522x10' 19.03 29.16 '135 Ketes:

1. Limit load factor of safety is Attouable Limit Load /Resulting Moment.

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Relief Request NDE-34 Unit 1, NDE-36 Unit 2 'Page 4 of 4 Attachment 1 [

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