ML20043G768

LER 90-008-00:on 900508,Tech Spec Required Shutdown Initiated Due to Primary Coolant Sys Leakage.Caused by High Cycle Fatigue Failure of Weld on Upstream Side of Drain Valve.Drain Valves replaced.W/900608 Ltr

ML20043G768
Person / Time
Site:	South Texas
Issue date:	06/08/1990
From:	Ayala C, Vaughn G HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-008-01, LER-90-8-1, ST-HL-AE-3479, NUDOCS 9006210108
Download: ML20043G768 (5)

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t 7The Light; companySouth Team Project Electric Generating

. Houston Lig ting A Power Station P.O. Box 289 Wadsworth, Texas 77483 '

7 June 8, 1990 ST-HL AE 3479 File No.: G26 10CFR50.73 U.'S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC' 20555 South Texas Project Electric Generating' Station Unit 2 '

Docket No. STN 50-499 Licensee Event Report 90-008 Regarding Technical Specification Required Shutdown Due to Primary Coolant System Leakage 7 Pursuant to 10CFR50.73, Houston Lighting & Power Company (HL&P) submits v the attached Licensee Event Report (LER 90-008) regarding Technical  !

Specification required shutdown-due to primary coolant system leakage. This. i event did not have any adverso impact on the health and safety of the public, t

If you should have any questions on this matter, please contact Mr. C. A. Ayala at (512) 972-8628 or myself at; (512) 972-7921.

G. E. Vaughn y i Vice President '

Nuclear Generation

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Attachment:

LER 90 008 (South Texas, Unit 2)

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'A1/LER008U2.L01 A Subsidiary of flouston Industries incorporated

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' , . Homton Lighting & Power Company File No.: 026

. South Texas Project Electric Generating Station Page 2 cc:

Regional Administrator, Region IV Rufus S. Scott I Nuclear' Regulatory Commission Associate General Counsel  ;

611 Ryan Plaza Drive, Suite 1000 Houston Lighting & Power Company  ;

Arlington,-TX 76011 P. O. Box 61867 i Houston, TX 77208 }

George Dick, Project Manager [i U.S. Nuclear Regulatory Commission INPO Washington, DC 20555 Records Center l 1100 Circle 75 Parkway  ;

J. 1. Tapia Atlanta, GA 30339 3064 j Senior Resident Inspector  !

Dr. Joseph M. Hendrie c/o U. S. Nuclear Regulatory Commission 50 Bellport Lane  !

P. O. Box 910 Be11 port, NY 11713 -j Bay City, TX 77414 -j D. K. Lacker >

J. R. Newman, Esquire Bureau of Radiation Control l Newman & Holtzinger, P.C. Texas Department of Health  ;

1615 L Street, N.W. 1100 West 49th Street l Washington, DC 20036 Austin, TX 78704 ,

D. E. Ward /R. P. Verret  !

Central Power & Light Company P. O. Box 2121 <

Corpus Christi, TX 78403 1 J. C. Lanier "

Director of Generation City of Austin Electric Utility 721 Barton Springs Road ,

Austin, TX 78704  !

R. J. Costello/M. T. Hardt --

City Public Service Board i P. O. Box 1771 l

-San Antonio, TX 78296 ,

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8 ACILITV NAMG nl DOCEST NUMStR 13) Pau. (3 South Texas, Unit 2 o is i o g o l o l 4l 919 1 loFl 0 l 3 ItTLE tot A Technical Specification Required Shutdown Due to Primary Coolant System Leakage-EVENT DAYS (Si LtR NUMSER (6) htPDAT DAf t 176 OTHER f ACILITitS INVOLVlD 101 MONTH DAY VSAR YtAR ENn Z*,$ MONT H DAY YEAR ' AcetaTv h Auts DOCEtT NvuotRise o isto lo g o s i i

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L on May 8, 1990, Unit 2 was in Mode 1 at 100 percent power. At approximately 0030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />, it was determined that a small pressure boundary leak (approximately 10 ml/ minute) existed on the ASME Class 2 Steam Generator C lower head-drain line. A Notification of Unusual Event was declared and Technical Specification required shutdown was initiated to Mode 5. The cause l of this event was high cycle fatigue failure of the weld on the upstream side 1

of the drain valve. The drain valves on both units' steam generators have

• l been replaced with threaded and welded caps which are not expected to develop high alternating stresses. The permanent design of the drain valve piping is being reviewed to assure no cyclic fatigue stress concerns exist. An evaluation of the high cycle fatigue experienced on the Steam Generator drain -

line will be performed for potential generic implicat.lons.

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0l 0 0l2 OF 0l3 DESCRIPTION OF EVENT: .

On May 8,1990, Unit 2 was in Mode 1 at 100 percent power. At approximately 0030 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />, it was determined that a small Reactor Coolant System (RCS) i pressure boundary leak (approximately 10 al/ minute) existed on the Steam l Generator C lower head drain line. Action A of Technical Specification j 3.4.6.2 requires a plant shutdown in the event of any unisolable pressure- l boundary leakage. A Notification of Unusual Event (NOUE) was declared and a l' shutdown initiated to Mode 3. The NRC was notified of the event at 0045 hours5.208333e-4 days <br />0.0125 hours <br />7.440476e-5 weeks <br />1.71225e-5 months <br /> on May 8, 1990.

On April 22, 1990 the Containment Atmosphere Particulate and Gaseous Radiation  :

Monitors indicated an increase in the containment atmosphere radiation level. 1 An investigation was performed to determine the source of the increased activity. On May 8, 1990 the source of the radiation was determined to be primary coolant leakage from the Steam Generator C lowSr head drain line where it connected to the drain valve. The weld on the upstream side of the valve had cracked. Metallurgical examination of the weld concluded that the crack 1 was due to high cycle fatigue. Ultrasonic examination of the remaining seven  ;

pipe to drain valve welds on both units detected the presence of partial linear indications in three other locations. The drain valve on each steam generator was. replaced with a threaded and welded cap to minimize the mass of unsupported pipe, change the natural frequency substantially and prevent subsequent fatigue failures. Repairs were completed and the plant restarted on May 19, 1990.

CAUSE OF EVENT The cause of this event was high cycle fatigue failure of the pipe to valve weld on the upstream side of the Steam Generator C drain line isolation valve. '!

The forcing function is believed to be vibration induced by Steam Generator '

flow. This, in combination with the slender configuration of the drain with unsupported suspended masses (valve and flange) appears to have caused resonant excitation leading to unexpected high cycle fatigue failure at.the point of severe transition.

ANALYSIS OF EVENT:

Completion of a shutdown required by Technical Specification is reportable pursuant to 10CFR50.73(a)(2)(1)(A). The pressuro boundary leak occurred in a Class 2 line which is downstream of a 3/8" flow restricting orifice. The leakage rate was estimated at 10 ml/ minute. In the event of a complete failure of the weld, the resulting leakage is an analyzed condition which would not exceed the capacity of the charging system. There were no adverse safety _or radiological consequences as a result of this event.

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The following corrective actions are being taken as a result of this event: 3

1. The steam generator valves on both units have been replaced with threaded and welded pipe caps. The associated welds and extensions have been cut out resulting in a configuration with a substantially_

different natural frequency which is not likely to be subject to i

fatigue failure.  !

2. A permanent design for the steam generator drain valves and associated piping will be developed.to assure minimization'of cyclic -;

fatigue stresses. This design is currently scheduled to be completed by September 30, 1990 An implementation schedule will be developed as the design is finalized.

3. An evaluation of the high cycle fatigue experienced on the Steam Generator drain lines will be performed for potential generic implications. -This action will be completed by September 30, 1990.

ADDITIONAL INFORMATION:

There have been no previous events reported regarding high cycle fatigue of RCS. pressure boundary-piping.

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