ML20043A781

LER 90-005-00:on 900414,simultaneous Reactor & Turbine Trip Occurred Due to Failure of Main Turbine Electrohydraulic Control Line.Caused by Fatigue Stress of Weld.Valve Modified to Add anti-swirl Baffles & anti-rotation pins.W/900515 Ltr

ML20043A781
Person / Time
Site:	South Texas
Issue date:	05/15/1990
From:	Ayala C, Vaughn G HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-005-01, LER-90-5-1, ST-HL-AE-3462, NUDOCS 9005230116
Download: ML20043A781 (5)

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The Light com any South Teams Project Electric Generating Station F. O. Boa 289 Wadsworth. Teams F7483 Houston Lig ng & Power May 15, 1990 ST.HL AE 3462 File No.: G26 10CFR50.73 l U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 South Texas Project Electric Cenerating Station Unit 2 Docket No. STN 50 499 Licensee Event Report 90-005 Regarding a Reactor Trip Due to Failure of a Main Turbine Electrohydraulic Control Line Pursunnt to 10CFR50.73,llouston Lighting & Power Company (llL&P) submits the attached Licensee Event Report (LER 90 005) regarding a reactor trip due to failure of a main turbine electrohydraulic control line. This event did not have any adverse impact on the health and safety of the public.

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.EN 4

c. E. vaughn Vico President-Nuclear Generation BEM/n1

Attachment:

LER 90-005 (South Texas, Unit 2) 9005230116 900513 PDR 5

ADOCK 05000499 /

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A Subsidiary of Houston Industries incorporraed ,

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ST-HL AE- 3462 Houston Lighting & Power Company File No. : G26 South Texas Project Electric Generating Station Page 2 cc:

Regional Administrator, Region IV Rufus S. Scott Nuclear Regulatory Commission Associate General Counsel 611 Ryan Plaza Drive, Suite 1000 Houston Lighting & Power Company Arlington, TX 76011 P. O. Box 61867 Houston, TX 77208 Ceorge Dick, Project Manager U.S. Nuclear Regulatory Commission INPO Washington, DC 20555 Records Center 1100 circle 75 Parkway J. 1. Tapia Atlanta, CA 30339 3064 Senior Resident inspector c/o U. S. Nuclear Regulatory Dr. Joseph M. Hendrie Commission- 50 Be11 port Lane P. O. Box 910 Be11 port, NY '11713 Bay City TX 77414 D. K. Lacker J. R. Newman, Esquire Bureau of Radiation Control 1 Newman & Holtzinger, F,J. Texas Department of-Health 1615 L Street, N.W. 1100 West 49th Street Washington, DC 20036 Austin, TX 78704 D. E. Ward /R. P. Verret Central Power & Light Company P. O. Box 2121 Cerpus Christi, TX 78403 J. C. Lanier Director of Ceneration City of Austin Electric Utility 721 Barton Springs Road Austin, TX 78704 R. J. Costello/M. T. Hardt '

City Public Service Board P. O. Box 1771 San Antonio, TX 78296

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r l On April 14,1990, Unit 2 was in Mode 1 at 100 percent power. At 1704 hours0.0197 days <br />0.473 hours <br />0.00282 weeks <br />6.48372e-4 months <br />.

a simultaneous reactor and turbine trip occurred on low turbine electrohydraulic control (EHC) fluid pressure. The plant was brought to a stable shutdown in Mode 3 with no unexpected post-trip transients. The cause of this event was failure of the DiC supply line to a main turbine amernor valve. The line failed due to fatigue stress of a weld caused by governor valve inducea vibration. The valve vibration was caused by valve plug l rotation. Interim corrective action has been taken to repair the Dic supply piping and add additional supports. The valve used to throttle flow during full power operation will be modified to add anti swirl baffles and anti rotation pins on the plug to stem connection during the firct refueling  ;

outage.

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On April 14, 1990, Unit 2 was iri Mode 1 at 100 percent power. At 1704 hours0.0197 days <br />0.473 hours <br />0.00282 weeks <br />6.48372e-4 months <br />, a simultaneous reactor and turbine trip occurred on low turbine electrohydraulic centrol (EHC) fluid pressure. Workers in the area of the main turbine governor valves reported the rupture of an EHC line to one of the governor valves. A feedwater isolation occurred on low Reactor Coolant System average temperature and an Auxiliary Feedwater System actuation occurred on low steam generator level as expected. The main steam isolation valves were closed to limit the cooldown and the steam generator power operated relief valves (PORV) were used to control temperature. The plant was stabilized in Mode 3.

Investigation of this event revealed that the weld on the EHC supply line to the governor valve had failed at the tube side toe of the weld where it connected to a tee in the supply header. The weld was cut out, rewelded and dye penetrant examined for integrity. At the time, it was believed that the failure had resulted from recent maintenance activities on the governor valve.

On April 25, 1990, following restart of the unit, dye penetrant inspection of the new weld showed a new linear indication. Dye penetrant inspections of other welds of EHC piping on Unit 2 were then performed and additional linear indications were found. Metallurgical examination of the original failure site indicated fatigue failure with origins at multiple locationa around the l circumference of the pipe. Observation of the Unit 2 EHC piping during plant

! operation indicated that significant vibration occurred concurrent with vibration of the governor valve. The cyclic stress from the governor. valve-induced vibration could have resulted in the earlier fatigue failure.

l Temporary supports were added which greatly reduced the vibration amplitude, i

Inspections of Unit 1 EHC piping did not reveal any cracksi

, CAUSE OF EVENT:

i The cause of the reactor trip was a loss of EHC pressure due to a supply line l failure. The line failed due to fatigue stress of a weld caused by governor

valve induced vibration.

1e The cause of the governor valve vibration has been determined to be valve plug rotation, l

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noe w antaw nn ANALYSIS OF EVENT:

Unplanned Reactor Trip System actuation is reportable pursuant to 10CFR50.73(a)(2)(iv). The plant was brought to a stable shutdown in Mode 3.-

(' There were no unexpected post trip transients.  ;

CORRECTIVE ACTION:

The new linear indications found on the EHC supply piping have been evaluated l and repaired as required.

In the interim, to compensate for the cause of the vibration and limit its .

effects the following actions are being taken: -

1. As a conservative measure, additional supports have been added to the EHC lines on Unit 2 to dampen any unusual vibration.

2. The governor valve control logic has been changed'such that.the valve which vibrated during operation is now maintained fully open at full load and another valve is modulated to-vary turbine load.

The valve used to throttle flow during full power operation will be modified [

to add anti swirl baffles and anti rotation pins on the plug to stem connection during the first refueling outage. The remaining valves will be modified as schedule permits, i ADDITIONAL INFORMATION: i l

There have been no previous events reported regarding reactor trips due to .!

failed EllC piping, i 1

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