ML20245E695
| ML20245E695 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 08/03/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20245E692 | List: |
| References | |
| NUDOCS 8908110329 | |
| Download: ML20245E695 (7) | |
Text
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NUCLEAR REGULATORY COMMISSION 5 l WASHINGTON. D. C. 20555
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ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO INCLUSION OF AN ALTERNATE REPAIR METHOD IN A PREVIOUSLY GRANTED REQUEST FOR RELIEF FROM ASME, SECTION XI CODE REPAIR REQUIREMENTS TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR POWER PLANT, UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328
1.0 INTRODUCTION
Sequoyah has experienced microbiologically induced corrosion (MIC) in the butt ,
welds of stainless (austenitic) steel piping in the essential raw cooling (ERCW) system. The MIC attack initiated at the inside surface of the welds and degradation occurred in the form of voids or pits that became larger and deeper, and eventually developed into a through-wall leak. The leakage was small and was characterized as drip.s or' moist areas around butt welds. The voids can grow in size and number to the extent that the criteria for the structural reserve factor is not maintained.
The Tennessee Valley Authority (TVA) developed a MIC program for Sequoyah based upon the data developed through inspection. The MIC program and the engineering evaluation which formed the basis for the program are described in TVA's letter dated January 20, 1988 (Reference 1). This program was reviewed and accepted (with three additional requirements) in the staff's Safety Evaluation Report forwarded by letter dated March 31, 1988 (Reference 2). It was noted that if leakage should occur in the ERCW piping, the requirements of the American Society for Mechanical Engineers Boiler and Pressure Vessel Code,Section XI (Code) apply, and relief from the Code will be required in accordance with 10 CFR 50.55a for the interim period until permanent Code repairs are made.
By letter dated April 4, 1988 (Reference 3) and clarification letters dated !
April 4, 1988 and May 4, 1988 (Reference 4 and 5), TVA submitted a request for '
relief from the requirements of IWA-5250(a)(2). In lieu of repairing leaks in aCCordance with IWA-4000, TVA requested that if the weld is considered struc-turally sound, the leakage is insignificant, and the safe shutdown equipment is protected from leakage, the leak will be scheduled for repair at the next available outage. The staff in a letter dated May 11, 1988 (Reference 6) granted the request for relief provided the three additional requirements in the staff's letter dated March 31,1988 (Reference 2) are incorporated in the MIC program.
8908110329 890003 -
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r TVA in their letter of October 28, 1988 (Reference 7): (1) addressed the three additional requirements contained in the staff's May 11, 1988 letter, (2) pro-vided a description of TVA's evaluation process if the leakage criteria are exceeded, and (3) presented TVA's contingency plan for performing a temporary repair with a welded structural sleeve when MIC damage has encroached upon the structural reserve margin. A telephone conference on June 12, 1989 was held to clarify the meaning of the 5/8-inch margin and the application of the margin in the case of the sleeve repair option.
2.0 EVALUATION 2.1 MIC Program The NRC letter of May 11, 1988 accepted TVA's program for monitoring and evaluating MIC damage in Sequoyah's essential raw water cooling (ERCW) system piping welds with the following additional requirements:
- 1. The frequency of direct visual inspection of weld specific leaks is increased to monthly, and performed as scheduled.
- 2. At the same time leakage is detected, an assessment is made to ensure the acceptance criterion is not exceeded prior to the scheduled outage or repair.
- 3. The leakage from a leaking weld does not exceed 0.5 gpm and the total leakage from all welds does not exceed 2.0 gpm. The licensee will establish a threshold below which leakage is considered immeasurable for the purpose of calculating toe combined leakage.
For Items 1 and 3, the licensee's Preventive Maintenan'ce (PM) program at Sequoyah was modified (PM's 3082-067 and 3083-067) to require direct visual inspection of identified leaks by monthly walkdown inspections. These procedures establish a threshold leakage rate for recordkeeping purposes of more than 20 drops a minute.
For Item 2, a measurement has been made of the worst case of MIC damage growth in nine months, extrapolated to one year. This growth margin has been sub-tracted from the maximum length of MIC damage allowable for structural integ-rity with reserve margin. This approach will provide for an adequate length of structurally sound weld metal to be present until the next outage when a permanent repair is to be made.
TVA has established procedures in the event that the measured leakage from any weld exceeds 0.5 gal / min or the leakage total for all welds exceeds 2.0 gal / min.
When these leakage rates are exceeded, the preventative maintenance procedure requires that a condition adverse to quality report (CAQR) be issued and the NRC site inspector be notified. The CAQR program will cause this event to be addressed systematically. Not only will the integrity of the welds be adoressed, but also the significance of the event in terms of operability of the system, potential of damage to other safety-related components, root cause determina-tion, further investigation of MIC damage rates and mechanism, and generic implications of the damage will be addressed.
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I I The potentially affected welds are located in headers for which the 2.0 gal / min leakage rate would constitute only a minor flow reduction. Because the source of the ERCW is the infinite heat sink, leakage due to MIC damage is unlikely to affect the operability of the system.
2.2 Structural Sleeve Repair The original granting of relief from the repair requirements of the Code wds l based upon the collection and drainage of leakage near safe shutdown equipment.
l However, MIC damage could also degrade the weld structurally beyond the i established criteria. TVA has developed a designed stainless steel sleeve weld l repair as a contingency repair for structurally degraded welds. The sleeve would be applied only when X-ray examination of a degraded weld will have a reserve factor at the weld calculated to be less than 1.0 before the next refueling outage and other repair / replacement methods are not feasible for continued operation of the unit. The sleeve repair is a stop-gap measure which will be removed for a permanent repair during the next refueling outage.
The sleeve repair will temporarily provide structural integrity and eliminate leakage at the weld joints to which it is applied. TVA has reviewed the potential impact of a structural sleeve on the ERCW piping analysis. The
- current rigorous and alternate analysis at a proposed sleeve location will be reviewed on a case-by-case basis to ensure that the analysis is not impacted by the sleeve installation. Generic studies have been completed showing that the sleeve will not affect the seismic analysis of the piping system because the number of sleeves per pipe span will be limited, and other basic engineering properties will not be affected by the application of the sleeves. TVA has also retained the option of perfonning a permanent Code repair if it is to its advantage to do so.
To satisfy the Code requirement for weld repair without relief would require shutdown of the facility and repair of the leaking pipe weld or a pipe weld structurally deficient because nf MIC damage. This is an unnecessary operating cycle on the facility and an unnecessary burden on TVA. The sleeve repair method precludes an unnecessary unscheduled shutdown and provides adequate assurance of leak integrity and structural integrity of stainless steel weld joints in the ERCW system which have severe MIC attack damage, until the unit shuts down in its next refueling outage where permanent Code approved repairs are made.
2.3 Code Relief In its letter dated May 11, 1988, the staff granted relief from the Code Section XI, IWA-5250(a)(2) which requires, "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: repairs or replacements of components shall be performed in accordance with IWA-4000 or IWA-7000, respec-tively." The basis for this relief was TVA's submittal dated January 20, 1988 on the MIC Program, which does not require imediate repairs of leaking com- ,
ponents as does the Code.
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As discussed in Sections 2.1 and 2.2 above, the three additional requirements in the staff's letter dated May 11, 1988 have been acceptebly implemented and the structural sleeve weld repair is an acceptable method to temporarily repair the MIC damaged pipe until the unit enters its next refueling outage where permanent Code approved repairs would be made.
Therefore, the basis for granting the relief in the staff's letter dated May 11, 1988 has not changed. The Code repair requirements of MIC damage (leaks and structural integrity loss) within stated limits of ERCW piping welds while the plant is in its operating cycle are impractical and that, pursuant to 10 CFR 50.55a(a)(3)(1), the MIC program and the temporary weld method will provide an acceptable level of quality and safety. The staff also concludes that the temporary weld repair method will not endanger life or property, or the common defense and' security and is otherwise in the public interest considering the burden that could result upon TVA if the Code requirements were imposed upon Sequoyah.
3.0 CONCLUSION
The staff has. determined (1) that the licensee has acceptably incorporated the three additional requirements stipulated in our granting of the request for .
relief by our letter of May 11,1988,(2) that TVA's evaluation process in the event that the leakage criteria are exceeded is a conservative and rpnstructive approach to address MIC damage in excess of the design basis for tb program, and (3) TVA's contingency plan for installing structural sleeves as a temporary repair is acceptable within the original relief granted to TVA. The staff concludes that the repair requirements of the Code are impractical to perform on ERCW piping system welds with MIC damage at Sequoyah Units 1 and 2 during power operations of the plant. The structural sleeve repair will not endanger life or property, or the common defense and security, and is otherwise in the public interest considering the burden that could result if the Code require-ments were imposed on the facility.
The staff concludes that the inclusion of TVA's proposed contingent method of repair to the previously granted relief from Code weld repair requirements of Section XI in the staff's letter of May 11, 1988 is acceptable and no new relief from the Code need be granted.
4.0 REFERENCES
Subject:
Microbiologically Induced Corrosion (MIC) Program, dated
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January 20, 1988.
- 2. Letter from G. Zech (NRC) to S. A. White (TVA),
Subject:
Safety Evaluation of Microbiologically Induced Corrosion (MIC) Program, dated March 31, 1988.
Subject:
Request for Relief from the American Society of Mechanical Engineers (ASME)Section XI Code as it relates to SQN's Microbiologically Induced Corrosion (MIC) Program, dated April 4, 1988.
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Letter from R. Gridley (TVA) to NRC, Sequoyah Nuclear Plant (SQN) Units 1 4.
and 2.
Subject:
Microbiologically Induced Corrosion (MIC) Program -
Supplemental Response, dated April 4, 1988.
Subject:
Clarification of SQN's Microbiologically Induced (MIC)
Program, dated May 4, 1988.
- 6. Letter from R. A.. Hermann (NRC) to S. A. White (TVA),
Subject:
Request for-Relief from ASME Boiler and Pressure Vessel Code,Section XI, Inservice Inspection Program, dated May 11, 1988.
Subject:
Additional Retails to TVA's Microbiologically Induced Corrosion (MIC)
Program Requiring NRC Approval, dated.0ctober 28, 1988.
Principal Contributor: D. Smith Dated: August 3, 1989 m
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will-not endanger life or property', or the common defense and security and is-otherwise.in the public interest.
' Accordingly, pursuant to 10 CFR 50.55a(g)(6)(1) of the Commission's 'regula-tions, .TVA selected portions of later editions and addenda of the ASME Code as provided in 10 CFR 50.E5a(g)(4). These selections'are documented in.TVA's:
letter dated January 15, 1988 and evaluated in the enclosed staff safety l: . evaluation for the SPT. program for Browns Ferry. . Units 1. 2 and 3.
Sincerely, Suzanne C. Black, Assistant Director for Projects TVA Projects Division Office of Nuclear Reactor Regulation
Enclosure:
Safety Evaluation cc w/ enclosure:
See-next page Distribution Docket File NRC PDR ,
Local PDR ADSP Reading D. Crutchfield B. D. Liaw S. Black R. Pierson L. Watson W. S. Little M. Simms-G. Gears D. Moran T. Daniels N. Markisohn J. Rutberg S. Varga B. Grimes E. Jordan ACRS(10)
GPA/CA BFN Rdg. File
- NRR:TVA/BC : : :
OFC :NRR:TVA/LA :NRR:TVA/PM 3JVA:AD/P NAME :MSimms ' *DM :SBlack : DTerao (' :
t nava e d..e dame... oW# .rao e / #22 7/10/ag : :
.- ' Code requirements, authorized by law and will not endanger life or property, or the common defense and security and is otherwise in the public interest.
Accordingly, pursuant to 10 CFR 50.55a(g)(6)(1) of the Commission's regula-tions, TVA is authorized to use selected portions of later editions and addenda of the ASME Code as provided in 10 CFR 50.55a(g)(4). These t : lections are documented in TVA's letter dated January 15, 1988 and evaluated in the enclosed staff safety evaluation for the SPT program for Browns Ferry, Units 1, 2 and 3.
Sincerely, Suzanne C. Black Assistant Director for Projects TVA Projects Division Office of Nuclear Reactor Regulation
Enclosure:
Safety Evaluation cc w/ enclosure:
See next page Distribution Docket File NRC PDR Local PDR ADSP Reading D. Crutchfield B. D. Liaw S. Black R. Pierson L. Watson
. W. S. Little l M. Simms G. Gears D. Moran T. Daniels N. Markisohn J. Rutberg l S. Varga B. Grimes E. Jordan ACRS(10) ,
GPA/CA l BFN Rdg. File
- SEE PREVIOUS CONCURRENCE I sl.
OFC :hRR:TVA/LA* :NRR:TVA/FM* g {p# TVA: AD/P NRR:TVA/BC*
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^ NAME :MSimms :DMoran:as : ()f y SBlack :DTerao : : :
DATE :7/7/39, :7/7/89 : 'f/M /89 / /89 :7/10/89 : :
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