ML20136B467
| ML20136B467 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 03/03/1997 |
| From: | Saunders R VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 97-093, 97-93, GL-90-05, GL-90-5, NUDOCS 9703100294 | |
| Download: ML20136B467 (13) | |
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Waisir Esmernic ann Powen Cmtl'ANY Ricissioso, Vuu;iN A 2326:
l March 3, 1997 1
1 United States Nuclear Regulatory Commission Serial No.97-093 Attention: Document Control Desk NL&OS/ETS R0 Washington, D.C. 20555 Docket No.
50-338 License No. NPF-4 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNIT 1 ASME SECTION XI RELIEF REQUEST NDE-33 SERVICE WATER SYSTEM LEAKS i
On January 8,1997 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 i
implemented for the three affected service water lines. The three welds were repaired j
in accordance with ASME Code requirements by January 23,1997. 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 January 8,1997 through January 23,1997. Relief Request NDE-33 for the leaking welds and the basis for the relief request are provided in Attachment 1 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 as part of a recurring system visual inspection which involves the stainless steel piping associated with the Service Water System. 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 the leaking indications were determined to be microbiologial influenced corrosion (MIC).
The condition of the Service Water System was monitored during the period corresponding with the relief request. The monitoring program included walkdowns of the affected welds to monitor and quantify any leakage. No significant changes were noted in the condition of the affected piping during this period.
WY 9703100294 970303 I
PDR ADOCK 05000338 O
M I
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This relief request has been reviewed and approved by the Station Nuclear Safety and i
Operating Committee, i
If you have any additional questions concerning this request, please contact us.
Very truly yours, R. F. Saunders Vice President - Nuclear Engineering and Services j
Attachment j
Commitments made in this letter:
None i
i cc:
U. S. Nuclear Regulatory Commission Region ll 101 Marietta Street, N. W.
i Suite 2900 Atlanta, Georgia 30323 Mr. R. D. McWhorter NRC Senior Resident inspector j
North Anna Power Station i
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North Anna Power Station Unit 1 ASME Section XI Relief Request NDE-33 i
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Virginia Electric and Power Company North Anna Power Station Unit 1 4
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Virginia Electric & Power Company North Anna Power Station Unit 1 Second 10 Year Interval Request for Relief Number NDE-33 I.
IDENTIFICATION OF COMPONENTS Mark / Weld #
Line#
Drawing #
Joint 68 4"-WS-F63-163-Q3 11715-CBM-78A-2 SHT. 4 BW 11715-WS-2D87B 97 3"-WS-74-163-Q3 11715-CBM-78G-2 SHT. 2 BW i
11715-WS-1074A 1
2"-WS-C81-153A-Q3 11715-CBM-78G-2 SHT. 1 SW l
(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, control room chillers, and charging pump lube oil coolers for i
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 i
150*F.
l (c)
Joint type -
BW butt welded and SW socket welded.
II. IMPRACTICABLE CODE REQUIREMENTS The above welds had external. evidence of through-wall leakage, i.e.
stains. Virginia Electric and Power Company decided to a
proceed under the assumption that all of the above welds i
contain through-wall flaws. Although this evidence of leakage j
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 i
relief is requested is IWA-5250 (a) (2).
i "IWA-5250 Corrective Measures:
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(a) The source of leakage detected during the conduct of a i
system pressure test shall be located and evaluated by the Owner for corrective measures as follows:...
i (2) repairs or replacements of components shall be 1
performed in accordance with IWA-4000 or IWA-7000, respectively."
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Relief Request NDE-33 Page 1 of 6 1
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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 all welds having evidence of through-wall leakage in a planned 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 following information and justification are provided in accordance with the guidelines of Part B and C of Enclosure 1 to GL 90-05.
Scoce. Limitations and Specific Considerations Scoce The scope consists of the welds listed above in piping with evidence of possible through-wall leaks in the shared Service Water System in North Anna Power Station Unit 1 and Unit. 2.
The material of the piping is stainless steel ASME SA-312 type 316L with 316 or inconel weld tiller metal for pipe class 163 and ASME SA-312 type 316 filler metal for pipe class 153A.
Limitations Basec' an radiographic examinations and laboratory examinations of removed portions of piping from replacements, Microbiological Influence Corrosion (MIC) was determined to be the cause. 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 could be accomplished without a forced shutdown by following the guidance of NRC Generic Letter 90-05 to the extent practical. This period extended from identification of the first leaking weld on January 8, 1997 to the repair of each weld suspected of having through-wall flaws was completed on January 23, 1997.
Soecific 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 l
affect plant equipment.
Relief Request NDE-33 Page 2 of 6
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j The structural integrity of the butt welds were evaluated based on radioc;raphic 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 1.
reinforcement, fracture mechanics, and limit load analyses.
Each indication was considered to be through-wall due to the 2
inability of either radiography or ultrasonics to determine indication depth. A summary of the flaw evaluation is provided in Attachment
- 1. Butt weld 68'on line 4"-WS-F68-163-Q3 was j
analyzed and found acceptable. The radiography for butt weld 97 on line 3"-WS-74-163-Q3 indicated the possibility of MIC i
over 89% of the circumference. This length of MIC exceeded the established allowable limit and the weld was declared in Jerable at that time. The line was isolated and the weld was repaired prior to returning the line to service. A detailed structural integrit.y analysis was not performed. Meld 4
i 97 was removed and cut open for analysis.
The analysis determined that in addition to MIC the weld contained construction
- defects, lack of fusion and a
poor root
)
condition.
1 Radiography of socket weld 1 was not attempted since past radiographs of socket welds did not yield meaningful results.
l Since flaws cannot effectively be characterized for socket
- welds, a 3/4" hole was postulated.
The socket weld was s
replaced nine (9) days after the evidence of leakage was j
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 all butt weld locations was performed. This examination was not repeated since the subject welds were replaced before the next three (3) month RT surveillance was due. The socket weld was not radiographed.
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.
i Relief Request NDE-33 Page 3 of 6
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Evaluation Flaw Detection Durina Plant Oneration Determination
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and Imoracticality
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The subject welds were identified as having evidence of through-wall leakage during a Service Water System walkdown when both Units were operating. During the past several months Virginia Electric and Power Company has been monitoring,
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evaluating, and replacing through-wall leaks in the Service Water System for attack 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 unnecessarily.
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 through-wall leakts revealed small i
voids surrounded by exfoliation, which is typical of MIC. No other type of operationally caused defects were identified by the radiographs. Additionally, examinations performed by a Virginia Electric and Power Company staff metallurgist of a sample of the piping segments removed to repair the leaking welds confirmed the presence of MIC. The analysis of weld 97 on line 3 "-WS-74 -163 -03 determined that in addition to MIC the weld contained construction defects, lack of fusion and a poor root condition.
Flaw Evaluation Flaw evaluation for butt welds subjected to radiographic examination 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.
i Because of the inability for either radiography or ultrasonic techniques to determine the extent of wall degradation, the structural assessment considert.1 each indication to be through wall.
Additionally, radiography is not conclusive in differentiating purely MIC conditions from conditions such as slag and incomplete root penetration, which have been aggravated by corrosion during service.
Therefore, these areas were also considered fully degraded for the structural assessments. The radiograph of weld 97 on line 3"-WS-74-163-03 i
showed evidence of degradation from MIC for 89% of the circumference. This weld was declared inoperable and was not analyzed.
l Relief Request NDE-33 Page 4 of 6 j
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The analyses determined butt weld 68 and butt welds subjected to augmented examination in line 4"-WS-F63-163-Q3 were capable of maintaining their structural integrity until they were i
repaired.
This is based on the results from the analysis:
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Ductile tearing will not occur at the flaw locations when j
the piping is subjected to the design pressure.
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The limit load analysis shows that there was at least a factor of safety of 1.6 against a ductile rupture for the i
most limiting case analyzed.
3.
A linear elastic fracture mechanics analysis shows that 4
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.
IV.
AUGMENTED INSPECTION i
i To assess the overall degradation of the Service Water System j
augmented radiographic examinations were performed. Af ter the j
initial through-wall flaws were identified five (5) additional i
locations on lines having the same size and function were j
examined using radiography. This sample group identified two (2) welds, 60 and 71 on line 4"-WS-F63-163-Q3, degraded by MIC. 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 structurally acceptable.
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 unless the structural integrity has been i
determined to be unacceptable.
This alternate provision applies to the subject welds for the period from January 8, 1997 through January 23, 1997. The through-wall flaws that remained in service were subject to monitoring and met the criteria for flooding and spraying consequences and for structural integrity as described above.
Operation in this mode continued until the subject welds were replaced. Each subject weld was replaced by January 17, 1997.
The proposed alternative stated above ensured that the overall levels of plant quality and safety were not compromised.
Relief Request NDE-33 Page 5 of 6
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Attachment:
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- 1. Flaw Evaluation Methods and Results i
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References:
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USAS B31.1 Power Piping 1967 Edition 2.
EPRI Report NP-6301-D, " Ductile Fracture Handbook" i
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- 3. Nuclear Regulatory Commission Generic Letter 90-05 " Guidance for
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Performing Temporary Non-Code Repair of ASME Code Class 1,
2, and 3 Piping" i
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l Relief Request NDE-33 Page 6 of 6 i
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 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 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
- piping, a two (2) inch 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 t he 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 1500F.
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I Relief Request NDE-33 Page 1 of 4
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i The flawed sections were subjected to deadweight, thermal, and seismic DBE loading.
The allowable limit load is given by, M. - 2 at R,* t - (2cos ($) -sin (0) ) in-lbf
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or
- flow stress = 0.5 (S
+S) psi l
y S, = yield stress psi j
S, = ultimate stress psi R.
= mean radius of the pipe (inches) i l
_ O, Il (Rl P) +F i
~2 4a R;t f
l R
internal radius of the pipe (inches)
=
i pressure (psig) l P
=
axial load (lbs)
F
=
Outside diameter (inches)
D
=
pipe thickness (inches) l i
-t
=
O = half angle of the crack (radians) = crack lenoth i
2 R, l
l MR = Resultant Moment from the above mentioned loading conditions 1
l MR=/MY +MZ + T 2
2 2
j
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MY = Bending Moment MZ = Bending Moment T
= Torsion The calculated factor of safety is, I
FS = _&_
(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.
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Th f3 method is documented in EPRI report NP-6301-D (Ductile Fracture Handbook) (reference 2).
Relief Request NDE-33 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 additiori to the deadweight, normal operating thermal and seismic DBE loadings.
For the purpose of this evaluation a generic allowable stress intensity factor of Ke - 135 2
ksiVin was used for the material per NRC GL 90-05.
The applied stress intensity factor for bending, Krs, is found by:
13
[ ad - ( x R, 0 ) DF K
=
b The applied stress intensity factor for internal pressure, K p, is i
found by:
o,- ( x R 0) F,
Kp
=
i The applied stress intensity factor for axial tension, K r is found i
by:
Kr= at- (x R 0) F i
t The stress intent iey factor for residual stresses, Kya is found by:
Kya = S - (x R 0) Ft Total applied stress intensity Kr includes a 1.4 safety factor and is calculated by:
Kr = 1. 4 - ( K13 +Kp + K r) + Kya i
i The allowable stress intensity factor is taken from Generic Letter 90-05.
135 ksiVin for stainless steel.
Kat.
=
Stress Intensity Factor Ratio is defined as:
SR = Er_.
- Ent, 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-33 Page 3 of 4
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.i Table 1 I
SUMMARY
OF FLAW EVALUATION RESULTS FOR SERVICE WATER WELDS
[
Allowable Bending Bending Resulting Limit Factor Applied Alt d e i
Crack Axial Torsion Moment Moment Moment Load M.
of K
K y
c Wald Nos.'
line Nos.
Lenoth in Load lbs f ft-lbs MY ft-lbs MZ ft-lbs MR ft-lbs ft-lbf Safety' kst/in ksi/in
- 68 4"-WS-F63-163-Q3 2.375 109 4089 2822 3%9 6359 1.14x10' 1.7 127.17 135 Enveloping Cate 1 1.0 440 2974 3665 7450 8819 1.44x10*
1.6 89.91 135
,i Notes:
- 1. Enveloping Calc.1 includes welds 60 and 71 on line 4"-WS '63-163-03.
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- 2. Limit load factor of safety is Allowable Limit Load /Resu' ting Moment.
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I Relief Request NDE-33 Page 4 of 4 f
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