ML13333A384

Discusses Actions Taken to Alleviate Cracked Snubber Problem.Cracked Welds Were Repaired W/Reactor at Cold Shutdown & Feedwater Regulating Valve Has Been Readjusted

ML13333A384
Person / Time
Site:	San Onofre
Issue date:	06/19/1979
From:	Johari Moore Southern California Edison Co
To:	Engelken R NRC/IE, NRC/RGN-V
Shared Package
ML13333A383	List: ML13333A381 ML13333A384
References
NUDOCS 7907100603
Download: ML13333A384 (3)

Text

Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 JACK B. MOORE TELEPHONE VIC9 P.mS-K June 19, 1979 213-572-2292 U. S. Nuclear Regulatory Commission Region V Office of Inspection and Enforcement Suite 202, Walnut Creek Plaza 1990 North California Boulevard Walnut Creek, California 94596 Attention: Mr. R. H. Engelken, Director Docket No. 50-206 San Onofre Unit 1

References:

1) SCE (J. M. Curran) letter to.NRC Region V (R. H. Engelken) dated June 5, 1979

2) SCE (J. B. Moore) letter to NRC Region V (R. H. Engelken) dated November 6, 1978 Reference 1 provided prompt notification of a failed shock suppressor found during snubber functional testing conducted in conjunction with the current maintenance outage which began June 1, 1979.

This letter and the attached Licensee Event Report provide follow-up information submitted in accordance with the provisions of-6.9.2.a(9) of Appendix A to our Provi sional Operating License No. DPR-13.

Reference 2 reported the results of snubber functional testing con ducted during the last refueling outage. During that inspection, nine degraded snubbers were identified. In six of these, the degradation con sisted of damaged threads on the piston rod and rod locking nut resulting in partial disengagement of the piston from the piston rod. The degraded condition was attributed to operating transients includipg water hammer that occurred early in plant life. The other three snvbber failures were attributed to normal degradation from extended service life. Two exhibited excessive wear on the piston hydraulic seal, and one had a loosened connec tion on the line between the snubber body and hydraulic reservoir. None of these nine degraded snubbers had previously undergone functional testing.

All safety related snubbers, with the exception of the three located on the steam generators were functionally tested during that outage.

'a'907 10O 0 02

U. S. Nuclear Regulatory Commission Page 2 As further discussed in Reference 2, while removing snubbers for functional testing, cracks were noted on the "stitch" welds of four snubber attachments.

These welds provide a backup means of restraining slippage of a clamp along the axis of the pipe by acting as a shear block to restrain potential clamp motion.

During that outage, the accessible portions of all safety related snubbers which are similarly attached were visually inspected. Cracking of the weld buildup was attributed to their inferior quality in comparison to the other welds in-.

spected and to incidents of feedwater line water hammer.

Based upon information presented to your office in Reference 2 and upon subsequent discussions with members of your staff, the following actions had been scheduled for the first cold shutdown following the 1978 refueling outage:

1. A visual examination of all previously cracked snubber attachment stitch welds.

2. A functional test of each of the degraded snubbers discussed above, in addition to the representative sample required for routine sur veillance.

On June 2, 1979, San Onofre Unit 1 was brought to the cold shutdown condi tions in order to perform various maintenance activities. In connection with that shutdown, the above described snubber inspection program was performed with the following results:

Stitch Weld Visual Examinations Stitch welds associated with four feedwater line snubber attachments and which were observed to be cracked during the 1978 refueling outage were visually examined. The result of this examination was that these stitch welds were found to be in good condition with no observable cracks.

Snubber Functional Testing The initial functional testing program included twenty-two snubbers, nine that had previously been found in degraded condition, and thirteen others that were subject to a similar operating environment. Following the functional test, each snubber was partially disassembled to allow an inspection of the piston rod and locking nut for tightness and indication of thread damage. Snubber 1-SW-393-1 on the feedwater supply line to steam generator A was found to have a sheared locking nut. As a result, all snubbers on the feedwater lines inside containment that had not been scheduled for examination were added to the program.. None of these seven additional snubbers showed signs of degradation.

The plant operating history since the last refueling outage was reviewed to identify any abnormal condition in the feedwater system that could have caused the snubber failure.

It was noted that significant water hammers in

U. S. Nuclear Regulatory Comission page 3 the feedwater system were detected following a unit trip from full power on May 14, 1979. The cause of the water hammer is under investigation but is believed to be associated with improper adjustment of set points for closing feedwater regulating valves following a unit trip. The controls for these valves are intended to position the valves following a unit trip to allow approximately 5 percent feedwater flow. However, at the time of the May 14, 1979 unit trip, the control set points were such that the valves were po sitioned to their fully closed positions, resulting.in a momentary interrup tion of feedwater flow following the trip. The water hammers observed fol lowing the unit trip-is believed to be associated with the sudden termina tion and subsequent restoration of feedwater flow.

Snubber 1-SW-393-1 was repaired and returned to service. The feedwater regulating valve control circuits have been readjusted such that feedwater flow is maintained following a unit trip.

The progression of this problem will be monitored during the next re fueling outage through routine snubber surveillance testing. In addition, snubber 1-SW-393-1 will be disassembled and inspected for indication of damage to the piston rod threads. Should feedwater system water hammer be detected during future plant operations, an investigation will be conducted to determine the severity of the problem and the consequences of the occur rence with regard to snubber operability.

Very truly yours, (J. B. Moore)

/dl7b Attachment cc: Director, Office of Inspection and Enforcement (30)

Director, Office of Management Information & Program Control (3) bcc: NARC Members OSRC Members E. J. Bresnahan C. R. Kocher/J. A. Beoletto a-2101 Moody/R. W. Krieger D. R. Pigott (Chickering & Gregory)

F. P. Riley G. T. McLandrich EDM Files