ML19324C436

LER 89-026-00:on 891012,control Room Emergency Ventilation Sys Actuation Occurred Due to Momentary False High Radiation Signal from Control Room Radiation Monitor B.Caused by Sensitivity of Thumbwheel switch.W/891108 Ltr

ML19324C436
Person / Time
Site:	Peach Bottom
Issue date:	11/08/1989
From:	Cribbe T, Danni Smith PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-026-01, LER-89-26-1, NUDOCS 8911170023
Download: ML19324C436 (4)

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SUBJECT:

Licensee Event Report Peach Bottom Atomic Power Station - Unit 2 This LER concerns actuation of the Control Room Emergency Ventilation System.

Refetence: Docket No. 50-277 Report Number: 2-89-026 Revision Number: 00 Event Date: 10/12/89 Report Date: 11/08/89 Facility: Peach Bottom Atomic Power Station RD 1 Box 208, Delta, PA 17314 This LER is being submitted pursuant to the requirements of 10 CFR 50.73(a)(2)(iv).

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cc: J. J. Lyash. USNRC Senior Resident inspector W. T. Russell, USNRC, Region !

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..sv. C,,t On October 12. 1989, at 0900, a Control Room Emergency Ventilation actuat' ion occurred. The actuation occurred due to a momentary false high radiation signal from the Control Room Ventilation "B" radiation monitor. The actuation occurred while a technician was restoring the monitor high radiation setpoint to normal range, following performance of a surveillance test, using the thumbwheel setpoint switch.

The thumbwheel apparently generated a spurious mi$ representative setpoint value below normal background radiation level. The cause of this anomaly is attributed to sensitivity of the switch to interruption of contact of the switch contacts when changing switch position. No actual safety consequences occurred as a result of this event. Attempts to recreate the event did not reproduce the actuation. The switch c;ntacts were cleaned as a precaution. The applicable surveillance tests will be revised to include a note of caution regarding thumbwheel movement. There were no previous similar LERs.

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. Requirements of the Report  ;

This report is required per 10 CFR 50.73(a)(2)(iv) because of actuation of the  :

Control Room Emergency Ventilation System which is an Engineered Safety Feature '

System.

l Unit Status at Time of Event i Unit ? was in the Run Mode at 60% power. Unit 3 was in the Shutdown Mode. i Description of Event On October 12, 1989, at 0900 the normal Control Room Ventilation isolated and the

• Control Room Emergency Ventilation System actuated due to a momentary false high  !

radiation signal from the "B" radiation monitor (Ells: MON). A non-utility chemistry ,

technician was restoring the *B" monitor to service in accordance with surveillance '

test ST 7.6.12 " Calibration of the Control Roort Internal Vent Radiation Monitor with ,

a Known Source" at the time of the actuation. The monitor high radiation setpoint l had been temporarily increased in order to prevent actuation during performance of '

-the test. The actuation occurred when the technician was adjusting the setpoint back i to normal at the end of the test. The high radiation signal immediately reset and  :

normal Control Room ventilation was restored.  :

Cause of the Event i The cause of the event was an electrical anomaly created by manipulation of the {

radiation monitor thumbwheel setpoint switch. The thumbwheel switch establishes a digital setpoint in the comparator which is then compared to the indicated value. If i the indicated value exceeds the setpoint a high radiation actuation occurs. In this  !

event the setpoint had been set several decades above the normal setpoint for i performance of the test. While the setpoint was being dialed back down to the normal setpoint range the thumbwheel switch apparently generated a spurious Gisrepresentative binary value below the normal background radiation level resulting in the actuation. The cause of the misrepresentative setpoint has been attributed to  :

sensitivity of the switch to interruption of contact of the switch contacts when i changing switch position. It is speculated that this momentary interruption could occur if the contacts are dirty or if the thumbwheel is turned too slowly or held ,

between contact positions.

l Analysis of the Event No actual safety consequences occurred as a result of this event.  :

Two radiation monitors continuously sample air from the normal Control Room  :

Ventilation supply duct (EIIS: DUCT). If either monitor senses a high radiation '

condition (>/= 400 counts per minute) the normal fresh air supply fans (Ells: FAN) -

will trip and isolate and the emergency fans will start supplying fresh air to the Control Room through the Emergency Cleanup filters (Ells:fLT).

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tsutw - a w .an =acr asmawnn In this event only the "B" radiation monitor indicated a high radiation condition and it was only a momentary signal. The "A" monitor indicated only normal background radiation levels. Therefore, it is concluded that an actual high radiation condition did not exist.

Corrective Actions Instrument.and Control Technicians attempted to recreate the event in order to determine the exact cause. However, manipulation of the thumbwheel switch could not reproduce the actuation. The switch contccts were cleaned as a precaution.

Surveillance tests ST 7.6.12 and ST 7.6.12.1 will be revised to add a note of caution to change the thumbwheel setpoint switch position with a deliberate motion.

Previous Similar Events No previous LERs involving inadvertent actuation of the Control Room Emergency Ventilation System during surveillance testing were identified.

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