ML20236R828

Safety Evaluation Approving Proposed Alternative (one-time Temporary non-Code Repair) Pursuant to 10CFR50.55a(a)(3) (II)

ML20236R828
Person / Time
Site:	Peach Bottom
Issue date:	07/15/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20236R821	List: ML20236R819 ML20236R828
References
NUDOCS 9807220302
Download: ML20236R828 (4)

Text

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i s* NUCLEAR REGULATORY COMMISSION o y WA8HtNOToN, D.C. 20666-4001 l k . . . . . /*

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR RELIEF FROM ASME CODE REQUIREMENTS FOR TEMPORARY j NON-CODE REPAIR TO HIGH PRESSURE COOLANT INJECTION SYSTEM l PECO ENERGY COMPANY i PEACH BOTTOM ATOMIC POWER STATION. UNIT 2 DOCKET NO. 50-277

1.0 INTRODUCTION

lt is required by 10 CFR 50.55a(g) that nuclear power facility piping and components meet the ,

applicable requirements of Section XI of the American Society of Mechanical Engineers 2 (ASME) Boiler and Prrssure Vessel Code (the Code).Section XI of the Code specifies Code-acceptable repair methods for flaws that exceed Code-acceptable limits in piping that is in-service. A Code repair is required to restore the structuralintegrity of flawed Code piping, independent of the operational mode of the plant when the flawis detected. However, the implementation of required Code 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, often requiring a shutdown of the nuclear power plant.

Alternatives to the Code requirements may be used by nuclear power plant licensees when authorized by the Commission if the proposed alternatives to the requirements are such that they are shown to provide an acceptable levd of quality and safety [10 CFR 50.55a(a)(3)(i)), 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 facility (10 CFR 50.55a(g)(5)(iii)). Pursuant to 10 CFR 50.55a(g)(6)(i), the Commission may grant such relief and may impose such alternative requirements as it determines is authorized by law and will not endanger life or property or the common defense and security and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirement were imposed on the facility.

2.0 LICENSEE'S RELIEF REQUEST By letters dated June 17 ant June 23,1998, PECO Energy Company (PECO Energy, or, the licensee) requested relief from the provisions of the ASME Code in order to perform a temporary non-Code repair of a smallleak in a 1-inch drain line off of the Peach Bottom Atomic Power Station, Unit 2, high pressure coolant injection (HPCI) system. The leak was through a short crack in a socket weld connection where a 1-inch drain line connects to a half coupling attached to the 16-inch pump suction piping of the HPCI system. The leak rate was estimated to be approximately 19 drops per minute. The materials of construction are carbon steel; i

Enclosure 9807220302 980715 PDR P

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l ASTM A-106 grade B pipe and A-105 fittings. The request for relief was due to the inability to isolate the affected component to perform a Code repair without a unit shutdown. A permanent repair / replacement will be performed during the upcoming Unit 2 outage, currently scheduled for October 1998.

2.1 CODE REQUIREMENT l

1 l Article IWA-4000 of the 1980 Edition through 1981 Addendum of Section XI of the ASME Code  ;

requires removal of the flaw and a subsequent weld repair.

l 2.2 PROPOSED ALTERNATIVE AND BASIS FOR RELIEF 1

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l In place of a unit shutdown and the Code repair, the licensee proposed a temporary non-Code repair consisting of a split sleeve, fabricated from a 2-inch A-106 pipe, that would be fitted over the cracked socket weld and welded in place, thereby forming a new, leak-tight, pressure boundary.

l The proposed repair was engineered to withstand all design operating stresses at the location, i

and the analysis took no credit for any remaining load bearing capability of the flawed component. The proposed duration of use for this repair would be until the next refueling l outage for the unit, presently scheduled to start in October 1998.

3.0 DISCUSSION The appropriateness of a non-Code repair proposalis dependent upon the evaluation of several factors. Those factors include:

(1) Operating conditions of the system being repaired, (2) Nature of the flaw, flaw size, and growth rate,

(3) Materials and methods used for the repair, and (4) Risk of proposed repair method versus other options.

The HPCI system is an ASME Code Class 2 system. As such, temporary repairs are considered by the staff on a case-by-case basis. Generally, the staff finds that repair attempts l

on operating high energy systems carry a degree of risk to plant personnel and systems. The HPCI system is designed for 150 psig at 300 F. Maximum operating conditions (accident

!. conditions) are 70 psig at 170 *F. Normal operating conditions are 7 psig at 100 'F. This low normal operating pressure and temperature thereby poses no significant hazard to personnel performing a repair attempt during normal operation.

l The defect was characterized as a small crack at the toe of the socket weld on the pipe side of the weld. The apparent cause of the flaw was believed to be by vibration-induced fatigue.

Although no vibration of the line is noted under normal plant operating conditions, minor vibration of the line has been noted with the HPCI system in operation. The potential for flaw growth during installation of the proposed repair is negligible. Failure analysis of the flaw will be performed following removal of the flawed component during the upcoming refueling outage.

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During inspection of the flaw, the licensee noted that the weld profile at the location of the flaw was deficient, evidently, an artifact of original construction. Over a short length, the weld size

. was smaller than the adjacent fillet profile. This undersized weld portion, which contained the location of the crack, evidently acted as a local stress riser, further enhancing the potential for development of a small fatigue crack. l The proposed repair consists of a 2-inch, A-106, grade B schedule 160 pipe, which will be split and placed over the flawed 1-inch pipe weld to encapsulate it. The two halves will be reweided and the ends will be welded, respectively, to the coupling and the 1 inch root valve. The welds have been sized for system design pressure rating and stresses. All welding will be performed using existing station procedures qualified in accordance with the Code.  !

The metallu,rgical implications of water-backed welding were investigated. Previously, extensive testing was performed at the Electric Power Research Institute's (EPni) nondestructive' examination center to support other work related to welding carbon steel pipes ,

with water backing. Microstructural analyses were performed on many carbon steel coupons to I determine hardensbility. These coupons were welded in a water-backed environment. The hardenability of thin sections when welding with water backing was not significantly greater than

' when welding without water backing.. Thus, the licensee concluded that welding in the proposed configuration would not be materially different metallurgically from a normal Code l repair with the system drained.

To preclude trapping water or moisture inside the annulus between the repair and original pipe (during welding), a drain hole, threaded for a pipe plug, was incorporated into the split sleeve.

Two full size mock-ups were constructed to verify the methods, access for welding and acceptability of the ressulting repair. One mock-up was water filled to simulate the effects of the 'l water backing. The mock up welds were successfully completed. l Prior to performing any welding, the leak will be temporarily stopped with epoxy sealant or sealant with tape backing. No welding will be performed with leaking occurring. After welding, a magnetic particle or liquid penetrant exam and a pressure test (through the drain hole) will be i

. performed.

i The consequences of a failed repair were considered. It was postulated that the entire 1-inch drain line broke off during the repair attempt. The licensee determined that the postulated unisolable leak could be more than adequately compensated for by any one of four independent sources of makeup water. Therefore, the HPCI system would be available for sufficient time to permit an orderly unit shutdown. Additionally, plugs and hose clamps would be staged at the repair area in the remote event of a pipe break in the drain line.  ;

Since the original flaw would be encapsulated by the repair, no periodic monitoring is possible or necessary, i i

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The licensee considered but rejected a temporary repair consisting of an engineered mechanical clamp as described in Code Case N-523-1. Normally, this is the NRC staff's preferred method for similar temporary repairs. However, the licensee advised that application to the specific location was not readily available.

Augmented inspections of the system were performed. One additional smallindication was i found at the coupling to a 16-inch pipe weld. After evaluation, this flaw was removed by a light grinding of the surface.

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4.0 CONCLUSION

The NRC staff finds that the proposed alternative (a one-time temporary non-Code repair) is authorized pu.suant to 10 CFR 50.55a(a)(3)(ii). Installation of the above described temporary i non-Code repair will provide assurance of continued integrity of the piping and ensure that the i system adequately performs its safety function until the permanent repair / replacement is l installed during the next refueling outage for the unit. Therefore, compliance with the Code (requiring shutdown of the unit) would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

Principal Contributor: Geoff Hornseth Date: July 15, 1998 l

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