ML20005E247

LER 89-028-00:on 891130,control Room Ventilation Sys Actuated to Recirculation Mode as Result of Spurious Signal. Caused by Memory Error Causing Microprocessor to Energize Relay Incorrectly.Troubleshooting planned.W/891229 Ltr

ML20005E247
Person / Time
Site:	South Texas
Issue date:	12/29/1989
From:	Ayala C, Vaughn G HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-028, LER-89-28, ST-HL-AE-3317, NUDOCS 9001040157
Download: ML20005E247 (5)

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s , The Light comayany P.O. Ilox 1700 llouston, 'lixar 77001 (713) 228 9211

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December 29, 1989-ST-HL-AE-3317 File No.: G26 10CFR50,73-U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC .20555 I

South. Texas Project Electric Generating Station l Unit.2 Docket No. STN 50-499 Licensee Event Report 89-028 Regarding i Control Room Ventilation Actuation to Recirculation Mode Due to a Spurious Sigt.al From A Toxic Gas Analyzer Pursuant to 10CFR50.73, Houston Lighting & Power (HL&P) submits the attached Licensee Event Report (LER No.89-028) regarding a control room ventilation actuation to the recirculation mode due to a spurious signal-from a-toxic gas analyzer. The safety systems performed as designed and the 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.

w. / A i G. . n Vice President Nuclear Operati s GEV/BEM/nl

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a Regional Administrator,I_ Region IV- Rufus S.' Scott W Nuclear' Regulatory Commission Associate General' Counsel 611 Ryan Plaza Drive,= Suite 1000- Houston Lighting & Power Company Arlington, TX; 76011 P. O.-Box 1700 Houston, TX 77001

-George Dick,1 Project Manager EU. S. Nuclear Regulatory Commission INPO

- Washington, DC 20555 Records Center 1100 Circle 75 Parkway J. I. Tapia Atlanta, GA 30339-3064 Senior Resident. Inspector-c/o U. S. Nuclear Regulatory Commission ' Dr. Joseph M. Hendrie -

. P, O. Box'910~ 50 Bellport Lane Bay City,'TX 77414 Bellport, NY 11713

= J. R. Newman, Esquire 'D. K. Lacker Newman & Holtzinger, P.C. Bureau of^ Radiation Control 1615 L Street, N.W. Texas Department of Health Washington,EDC -20036 1100 West 49th Street.

Austin, TX 78756-3189 R.1L.: Range /R. P. Verret Central Power & Light ~ Company

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-Corpus Christi, TX 78403 >

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- Director'of Generations City of Austin Electric Utility 721 Barton Springs-Road Austin,'TX 78704 R. J. Costello/M. T. Hardt

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~~~} vis ne P . . o re uxtecreo suswss10n certs T} NO  ; l l A =1RACT m,,,,.. . o .. . ,.. . ,,N r..,-., ,,. m ,n i on November 30, 1989, Unit 2 was in Mode 5. At 1411 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.368855e-4 months <br />, the control room ventilation system actuated to the recirculation mode as a result of a spurious signal from a toxic gas annlyzer. The redundant analyzer did not

. actuate. The cause of the signal, has been attributed to a memory error which caused the mircoprocessor to incorrectly energize the high-concentration relay. The most likely cause of this condition is the fluctuations in the AC power supplied to the analyzer. Additional troubleshooting is planned to attempt to locate the problem. Corrective actions will be established based on the results of the troubleshooting activities, s

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On November 30, 1989, Unit 2 was in Mode 5. At 1411 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.368855e-4 months <br />, the control room ventilation system actuated to the recirculation mode as a result of a spurious signal from a toxic gas analyzer (XE-9325). The redundant analyzer did not actuate. Plant Operations personnel verified that the control room damper lineup was correct for the recirculation mode of operation. The NRC was notified at 1515 hours0.0175 days <br />0.421 hours <br />0.0025 weeks <br />5.764575e-4 months <br /> on November 30, 1989.

The toxic gas analyzers perform a spectroscopic examination of a sample of air from the control room inlet air plenum. The concentration of potentially toxic gases is calculated by a microprocessor and compared to acceptable '

values which are stored in memory. The microprocessor actuates the high ccncentration relay which initiates the protective action if one of these values is exceeded. Toxic gas concentrations detected during this event were not sufficient to initiate in actuation. This distinguishes this event from previous actuations which occurred when toxic gas analyzers actually indicated detection of high concentrations.

A calibration was performed on the analyzer which confirmed that it was working properly. The manufacturer's representative assisted in_the investigation; however, no specific cause for the spurious actuation was identified. It is-believed that a memory error in the microprocessor may have caused the actuation relay to close without the detection of an unacceptable concentration of toxic gas. The analyzer memory was cleared, tested, and reloaded by the manufacturer's representative. The analyzer was returned to service on December 8, 1989.,

As a result of previous toubleshooting activity, the 120 Volt AC power supply from the Inverter to the Unit I toxic gas analyzer (XE-9325) was monitored.

Voltage readings were recorded indicating fluctuation in the'120 Volt AC input lines to the analfzer. The inverter power output did not show these fluctuations. Similar problems have been noted in the Unit 2 toxic gas analyzer (XE-9325). These voltage fluctuations are not in conformance with the manufacturer's requirements. Similar fluctuations were not detected in the power supplied to the redundant analyzer (XE-9326) in either unit.

CAUSE OF OCCURRENCE:

The ESF actuation was caused by a spurious signal from a single toxic gas analyzer. The cause of the signal has been attributed to a memory error which caused the microprocessor to incorrectly energize the high-concentration reley. The most likely cause of this condition is the fluctuations in the AC power supplied to the analyzer which is not in conformance with the manufacturer's requirements, i

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.. n .es wame w asuwim ANALYSIS OF EVENT:

There were no adverse radiological or safety consequences as a result of this control' room ventilation actuation to recirculation mode since actuated ESF equipment operated as required.

While any unnecessary challenge to an Engineered Safety Feature is undesirable, actuation of the control room ventilation system to recirculation mode represcats a minimal hazard since it could not cause, worsen, nor prevent mitigation of an accident.

CORRECTIVE ACTION:

Additional troubleshooting is planned to attempt to locate the apparent defect in the power supply-circuit or-components. Troubleshooting will be completed and necessary corrective actions will be established by March 2, 1990. Both STPEGS units will be addressed as appropriate.

ADDITIONAL INFORMATION:

Several other events involving the toxic gas analyzers have been reported and numerous corrective actions have been implemented to improve the reliability of the Foxboro Miran 981 Analyzers.

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