ML20217D643

Safety Evaluation Granting Request for Approval to Repair Flaws in Accordance W/Gl 90-05 for ASME Class 3 SSW Piping for Pilgrim

ML20217D643
Person / Time
Site:	Pilgrim
Issue date:	10/01/1997
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20217D629	List: ML20217D623 ML20217D643
References
GL-90-95, NUDOCS 9710060048
Download: ML20217D643 (5)

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%* UNITED STATES

. n NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 30005 4001

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1AFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR APPROVAL TO REPAIR FLAWS IN-ACCORDANCE WITH GENERIC LETTER 90-05 FOR-AMERICAN SOCIETY OF MECHANICAL ENGINEERS CLASS 3 SALT SERVICE WATER PIPING BOSTON EDISON COMPANY PILGRIM NUCLEAR POWER "qlQH DOCKET NO. 50-293

1.0 INTRODUCTION

10 CFR 50.55a(g) requires nuclear power facility piping and components to meet the applicable requirements of Section XI of the American Society of Mechanical Engineers (ASME) Boiier and Pressure Vessel Code (hereafter referred to as the Code).Section XI of the Code specifies Code-acceptable repair methods for flaws that exceed Code acceptance limits in piping that is in-service. A Code repair is required to restore the structural integrity of flawed Code piping, independent of the operational mode of the plant when the flaw is detected. Those repairs not in compliance with Section XI of the Code are non-Code repairs. However, the implementation of required Code (weld) repairs to ASME Code Class 1, 2 or 3 systems is often impractical for nuclear licensees since the repairs normally require an isolation.of the system requiring the repair, and often a shutdown of the nuclear power plant.

Alternatives to Code requirements may be used by nuclear licensees when authorized by the Director of the Office of Nuclear Reactor Regulation if the proposed alternatives to the requirements are such that they are shown to provide an acceptable level of quality and safety in lieu of the Code requirements (10 CFR 50.55a(a)(3)(1)), or if compliance with the Code requirements would result in-hardship or unusual difficulty without a compensating increase in the level of quality and safety (10 CFR 50.55a(a)(3)(ii)).

A licensee may also submit requests for relief from certain Code requirements when a licensee has determined that conformance with certain Code requirements is impractical for its fr.cility (10 CFR 50.55a(g)(5)(Ht)). Pursuant to 10 CFR 50.55a(g)(6)(i), the Commission will evaluate o d erminations of impracticality, may grant relief and may impose alternative requirements as it determines is authorized by law.

Generic Letter (GL) 90-05, entitled " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2 and 3 Piping," and dated June 15, 1990, provides guidance for the staff in evaluating relief requests submitted by licensees for temporary non-Code repairs of Code Class 3 piping. The staff uses the guidance in GL 90-05-as its criteria for making its safety evaluation of relief requests for temporary non-Code repairs of Code Class 3 piping.

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2.0 BACKGROUND

In a letter dated July 7,1997, Boston Edison Company (hereafter referred to as the licensee) reported to the NRC that degradation has been identified on a piping spool piece. The spool piece is associated with Pilgrim Nuclear Power Station's (PNPC) salt service water (SSW) system. The SSW system is a moderate energy system and provides the ultimate heat sink for containment heat removal. The piping degradation involved leakage and pitting and is located on a rubber lined carbon steel pipe. The pipe is 18 inches outer diameter, ASTM A53 Grace B material having nominal thickness of 0.312 inches.

The licensee requested under the provisions of 10 CFR 50.55a(g)(6)(i) a relief from the ASME Code,Section XI requirements to perform Code repair or replace the degraded piping. The relief is sought until the next refueling outage which is scheduled to take place in the spring of 1999. At that time, the licensee will replace the degraded piping. The licensee based its request for relief on the results of a "through-wall flaw" evaluation that was performed by the licensee in accordance with the guidelines and acceptance criteria contained in GL 90-05.

3.0 LICENSEE'S REUJF REOUEST 3.1 Comoonents for Which Relief is Reauested The affected piping is riassified as ASME Code Class 3, moderate energy piping and is a part of the SSW system. The piping spool that has through-wall leaks is located immediately downstream of MO-3806 butterfly valve and downstream of the reactor building closed cooling water heat exchanger. The leaks are adjacent to the pipe slip-on flange that mates with the valve. The line is designed to take 100 psi pressure. However, the line is open ended and there is usually a small vacuum in the pipe at this location related to the changing

' tides.

3.2 Section XI Edition for the Pilarim Plant 1980 Edition of the ASME Code,Section XI including Winter 1980 Addenda.

3.3 ASME Section XI Code Reauirement The ASME Code Section XI requires that repairs or replacements of ASME Code Class components be performed in accordance with rules found in Articles IWA-4000 or IWA-7000, respectively. The intent of these rules serve to provide an acceptable means of restoring the structural integrity of a degraded Code Class system back to the original design requirements.

3.4 Content of the Relief Reauest Relief is sought from performing a Code repair'or replacement of the SSW system piping per the requirements of Article IWA-4000 or IWA-7000, respectively. Relief is being sought until the next refueling outage which is scheduled to take place in spring of 1999. The relief is being sought because

-3 Performing a Code repair during plant operation was-determined to be impracticable. The licensee will-perform a permanent Code repair for the affected piping during the next scheduled outage.

3.5 Basis for Relief -

Request for relief has been submitted and alternatives to the Code requirements have been proposed by the licensee. The NRC staff reviewed the proposed alternatives for compliance with the provisions of 10 CFR 50.55a(a)(3)(ii). The licensee has evaluated the leak in accordance with the guidance provided in GL 90-05. Based upon the evaluation, it was established that the piping is degraded but it is operable. The leaking piping also-satisfies the criteria for-non-Code repair as described in GL 90-05 and performing permanent repairs in accordance with the ASME Code during plant operation would constitute an undue burden (create undue hardship) upon the licensee since the repairs would have necessitated a unit shutdown.

3.6 Licensee's Alternative Proaram The licensee has proposed a temporary Code repair to stop the leak and maintain the structural integrity of the piping until the piping is replaced during an outage of sufficient duration. The temporary repair will consist of fillet-welding a cover plate to the pipe at the leak location. - The welding procedures and the welders will be qualified using the guidance provided in ASME Code Case N-562. A mock-up rubber lined spool piece will be used for the qualification of the procedure in order to ascertain that the welding process used in production will not adversely affect the integrity of the rubber  ;

lining. - After completion of the repair, the cover plate will be  !

ultrasonically examined periodically until the pipe is replaced in the next refueling outage. The cover plate is acceptable for 100 psi which represents a very conservative value because-the pressure at the repair location ranges from a-slight vacuum to a slight positive pressure. In addition, plant operators will visually monitor for changes to the pipe's leakage rate once per shift during operator tours until permanent ASME Code repair is completed.

Further, weekly monitoring (ultrasonic testing) of the degraded pipe will continue until test results show the test frequency can be changed. However, the maximum allowed frequency will be once every 3 months.

4.0 STAFF EVALUATION 4.1 Doerability Determination. Root Cause Analysis and Structural Mnteority Evaluation The licensee determined that a pipe spool located on the SSW system has "through-wall flaws."' All flaws were analyzed in accordance with the position-stated in GL 90-05 and were found to be within the stress criteria allowable flaw size. The licensee performed an operability determination of the SSW system in the "as found" condition and the system was determined to be operable. The system was constructed in accordance with the requirements of ASME Code, Class 3.

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The. preliminary root cause of the through-wall leaks was attributed to l

i delamination of aging rubber pipe lining due to localized high flow velocities resulting from throttling of the butterfly valve which is located immediately i

, upstream. Rubber lined piping flaws experience accelerated erosion -and -

' corrosion where the rubber lining has delaminated. Where the lining remains '

-_ intact, the pipe remains at its nominal full wall thickness. Hence,_the wall *

, thinning is local to the areas where lining has delaminated. This conclusion ,

was also confirmed by the results of the ultrasonic examination of five  !

additional pipe locations'which identified no other type of operationally ,

caused defects. The licensee ovaluated the structural integrity of the piping l using the guidance of GL 90-05. Based upon the evaluation, it was determined

that the integrity of the piping would be maintained and that the flawed '

piping satisfied the criteria of GL 90-05.

j 4.2 Auamented Inspection To assess the-overall degradation of the SSW system augmented ultrasonic F examinations were performed on five additional locations. The locations that ,

L were exhmined are similar locations of the other reactor building component cooling water and turbine building component cooling water heat exchanger

outlet valves. All augmented inspection results at these locations found i

values greater than the manufacturers minimum pipe wall thickness.

4.3 Pronosed Temoorary Non-Code Renair and Monitorina Provisions l The licensee has proposed a temporary non-Code repair to stop the leak and maintain the structural integrity of the pi)ing until the piping is replaced during an outage of sufficient duration. Tie licensee will install.a fillet-

welded cover plate to the pipe at the leak location. After completion of the i repair, the cover plate will be ultrasonically examined periodically until the

{ pipe is replaced in the next refueling outage. The cover ) late is acceptable

' for 100 psi. This is acceptable because the pressure at tie repair _ location

is well below that value. It ranges from a slight vacuum to a-slight positive

-pressure. In addition, plant operators will visually monitor for changes to

.the pipe's leakage rate once per shift during operator tours until permanent 4

ASME Code repair is completed. Further, weekly monitoring (ultrasonic-L testing) of the degraded-pipe will cont'nue until-test results show the test '

frequency can be changed. However, the maximum allowed frequency will be once-every 3 months.

4.4 Staff Evaluation The staff has determined that the licensee's flaw evaluation has been consistent with the guidelines and acceptance criteria of GL 90-05. The 1 staff, therefore, finds the licensee's structural integrity and operability assessments to be acceptable. The licensee will-weld a patch plate over the degraded area and thus the leak will be fixed. During the period of plant operation and until a permanent Code repair is accomplished, the repair area will be monitored by plant personnel. In addition, the licensee has evaluated i the temporary repair and determined that the SSW is operable.

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5.0 CONCLUSION

The staff firds that performance of an immediate Code renair during plant operation is impractical and would have constituted an undue burden (create undue hardship) u)on the licensee since the repair would have necessitated a unit shutdown. Stutting the unit down is not in the best interest of plant safety, given the magnitude of the flaw and the licensee's alternative program. The staff, therefore, grants licensee's request for relief from performing the Code repair, pursuant to 10 CFR 50.55a(g)(6)(1), and finds that implementation of the licensee's alternative program is authorized by law and will not etidanger life or proparty or the common defense and security, and is otherwise in the public interest, giving due consideration to the burden upon the licensee and facility that could have resulted if the Code requirements were imposed on the facility. The alternative program is authorized.

Principal Contributor: G. Georgiev Date: October 1, 1997

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