ML19327B215

LER 89-022-00:on 890919,overtemp Delta Temp Turbine Runback Occurred,Resulting in Reactor Trip.Caused by Turbine Setpoint Design Intolerance of Small Temp Deviations.Runback Feature Disabled & Analysis performed.W/891019 Ltr

ML19327B215
Person / Time
Site:	South Texas
Issue date:	10/19/1989
From:	Ayala C, Chewning R HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-022, LER-89-22, ST-HL-AE-3267, NUDOCS 8910270113
Download: ML19327B215 (5)

Text

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The Light llouston, Texas 77001 (713) 228 9211 Houism ilghting Ac Power . - _. - _P.O. Ilox 1700 October 19, 1989 ST-HL- AE- 326 7 File No.: G26 10CTR50.73 i

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 89-022 Regarding a Reactor Trip Due to Actuation of the Overtemperature Delta Temperature Turbine Runback Circuit Pursuant to 10CFR50.73 Houston Lighting & Power (HL&P) i.ubmits the attached Licensee Event Report 89-022 regarding a reactor trip due to actuation of the overtemperature delta temperature turbine runback circuit.

,, 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.

R. W. Chewning /

Vice President Nuclear Operations RWC/BEM/eg Attachment LER 89-022, South Texas, Unit 2

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L Regional Administrator, Region IV Rufus S. Scott Nuclear Regulatory Commission Associate General Counsel (

611 Ryan Plaza Drive, Suite 1000 Houston Lighting & Power Company l L. Arlington; TX 76011 P. O. Box 1700,' .

I l' Houston, TX 77001 l g George Dick, Project Manager .

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! U. S. Nuclear Regulatory Commission INPO  ;

L Washington, DC 20555 Records' Center  !

L 1100 Circle 75 Parkway l l' Jack E. Bess Atlanta, GA 30339-3064 l Senior hesident Inspector-Unit 1 c/o U. S. Nuclear Regulatory Commission Dr. Joseph M. Hendrie P. O. Box 910 50 Be11 port Lane Bay City, TX 77414 Bellport NY 11713 J. I. Tapia D. K. Lacker f Senior Resident Inspector-Ussit 2 Bureau of Radiation Control  !

c/o U. S. Nuclear Regulatory Commission Texas Department of Health  !

P. O. Box 910 1100 West 49th' Street ,

Bay City. TX 77414 Austin, TX 78756-3189-l J. R. Newman, Esquire ';

Newman & Holtzinger, P.C. j 1615 L Street, N.W.  !

Washington, DC 20036 R. L. Range /R P. Verret

. Central Power & Light Company l P. O. Box 2121 Corpus Christi, TX 78403 R. John Miner (2 copies)  ;

Chief Operating Officer City of Austin Electric Utility  ;

L 721 Barton Springs Road -l Austin, TX 78704 R. J. Costello/M. T. Hardt  !

City Public Service Board [

l- P. O. Box 1771

San Antonio, TX 78296 [

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On September 19, 1989. Unit 2 was in Mode 1 at 100 percent power. At 1237 hours0.0143 days <br />0.344 hours <br />0.00205 weeks <br />4.706785e-4 months <br /> during the cross calibration of the incore and excore nuclear instrumentation, an Overtemperature Delta Temperature (OTDT) turbine runback occurred resulting in an OTDT reactor trip. The turbine tripped on the reactor trip and an auxiliary feedwater actuation occurred on low steam generator level. The Main Steam Isolation Velves were closed to prevent excessive > Reactor Coolant System cooldown. The Engineered Safety Features l functioned as designed. The cause of this event was that the turbine runback

, setpoint design did not provide sufficient operating margin to allow for small devintions in RCS average temperature. The OTDT runback feature has been l disabled and analysis is being performed to determine if it can be restored without causing unwanted plant transients.

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DESCRIPTION OF OCCURRENCE: i i

On September 19, 1989, Unit 2 was in Mode 1 at 100% percent power. Cross  !

calibration of the incore and excore nuclear instrumentation was being i performed. Control room operators were using manual control rod motion with )

boration and dilution to control neutron flux distribution and Reactor Coolant i System temperature. During the test, Tave increased above the Tref value by  ;

approximately one degree. The increase in Tave caused a decrease in the l Overtemperature Delta Temperature (OTDT) setpoint and initiated a 200 Hegawatt j turbine runback. It was later determined that the installed runback time  !

delay setting was 10.4 seconds which was in excess of the design setting of l 1.5 seconds. The result was a further increase in Tave and subsequent ]

decrease in the OTDT reactor trip setpoint resulting in a reactor trip. The j turbine tripped from the automatic reactor trip signal. A feedwater isolation 1 occurred on low Tave and an auxiliary feedwater actuation occurred as a result l of a low steam generator water level signal. Control room operators verified ]

the reactor trip, turbine trip, power to the Engineered Safety Features busses ,

and the auxiliary feedwater actuation. The Main Steam Isolation Valves were ,

closed to limit the Reactor Coolant System cooldown. The plant safety systems l responded as designed. The NRC was notified pursuant to 10CFR50.72 at 1409 '

hours on September 19, 1989.

CAUSE OF EVENT:

The cause of this event was that the turbine runback setpoint design did not provide sufficient operating margin to allow for small deviations in Tave. A )

contributing factor was that the amount of turbine runback was set in excess of the values provided in design documents.

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, ANALYSIS OF EVENT: l t

Reactor trip and Engineered Safety Features actuation is reportable pursuant to 10CFR50.73(a)(2)(iv). The plant was bree4ht to a stable condition in Mode ,

3 with no unexpected post trip transients.

The OTDT reactor trip provides core protection against Departure from Nucleate Boiling (DNB). The setpoint is automatically reduced upon an increase in coolant temperature and is dyn6mically compensated. The turbine runback is a e non safety-related feature designed to be used in conjunction with automatic ,

rod position control to anticipate high Tave and reduce turbine and reactor power sufficiently to prevent a reactor trip. However, as designed, the ,

runback setpoint did not allow sufficient operating margin to prevent unwanted runbacks due to small temperature increases. During this event, the OTDT turbine runback was initiated when Tave was one degree above the Rated Thermal Power value and increasing. The turbine runback further increased Tave sufficiently to receive the OTDT reactor trip.

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f Analysis was performed of this event which showed that the incorrect setting  !

of the turbine runback time delay circuit did not directly cause the trip. l This feature was set prior to initial startups however, due to the nature of  ;

its function, proper operation could not be verified. Therefore, the error in the setting was not detected. l CORRECTIVE ACTION:

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

1. The OTDT turbine runback has been disabled on both units. Further analysis is being performed to determine if it can be restored without causing unwanted reactor transients or trips.

2. A review vill be performed of a representative sample of similar non safety-related functions which could not be verified by test to ensure that their settings are in accordance with design. This action will be completed by April 30, 1990. Based on this review, further corrective actions will be identified and scheduled at that time.  ;

ADDITIONAL INFORMATION:

There have been no previous events reported regarding reactor trips due to f actuation of the OTDT turbine runback feature.

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