05000445/LER-1991-001, :on 910103,control Room Air Conditioning ESF Inadvertently Actuated When Current Surge Exceeded Capacity of Power Supply Output Fuse.Caused by Equipment Sensitivity. Bulb Replacement Training Held

:on 910103,control Room Air Conditioning ESF Inadvertently Actuated When Current Surge Exceeded Capacity of Power Supply Output Fuse.Caused by Equipment Sensitivity. Bulb Replacement Training Held

ML20066L083
Person / Time
Site:	Comanche Peak
Issue date:	02/04/1991
From:	William Cahill, Hope T TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-001, LER-91-1, TXX-91009, NUDOCS 9102060264
Download: ML20066L083 (7)

LER-1991-001, on 910103,control Room Air Conditioning ESF Inadvertently Actuated When Current Surge Exceeded Capacity of Power Supply Output Fuse.Caused by Equipment Sensitivity. Bulb Replacement Training Held

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
4451991001R00 - NRC Website

text

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Sa w ramasse4N OC Log # TXX 91009 i

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February 3, 1991 4

W. J. Cahm f.sewitvr he (*resulent U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C.

20555

SUBJECT:

COMANCHE PEAK STEAM ELECTRIC STATION DOCKET NO. S0 445 MANUAL OR AUTOMATIC ACTUATION OF ANY ENGINEERED SAFETY-FEATURE LICENSEE EVENT REPORT 91 001 00 Gentlemen:

Enclosed is Licensee Event Report 91 001 00 for Comanche Peak Steam Electric Station Unit 1, " Inadvertent Actuation of Control Room Air Conditioning Engineered Safety Feature Caused by Sensitivity of Radiation Monitoring Device

- to Overcurrent Conditions."

Sincerely,

[

e William J. Cahill, Jr.

JAA/daj Enclosure c - Mr. R. D. Martin, Region IV

- ResidentInspectors,CPSES(3) 9102060264 910204 MM i

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, wie.a e., n, i.ne.i nei On January 3,1991, at approximately 2348 CST, an Auxiliary Operator was attempting to change a burned out bulb on the local microprocessor associated with one of the radiation monitors in the Control Room air conditioning air intake. When the bulb was unscrewed, a short piece of the bulb's loop filament fell across the two terminal posts inside the bulb. The momentary current surge exceeded the capacity of the power supply output fuse, resulting in a loss of power to the monitor The Control Room air conditioning system automatically realigned to the emergency recirculation mode. The cause of the event was determined to be equipment sensitivity to overcurrent conditions. Corrective actions included training and administrative controls over bulb replacement in monitors with automatic ESF functions.

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DESCRIPTION OF THE REPORTAJ.LE EVENT A.

REPORTABLE EVENT CLASSIFICATION

An event or condition that resulted in an automatic actuation of any Engineered Safety Feature (ESF).

B.

PLANT OPERATING CONDITIONS PRIOR TO THE EVENT On January 3,1991, at approximately 2348 CST, Comanche Peak Steam Electric Station (CPSES) Unit 1 was in Mode 1, Power Operations, with reactor power at approximately 96 percent.

C.

STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT The status light (Ells:(IL)(IL)) was burned out on the locally mounted RM 80 microprocessor (Ells:(DCC)(ll.)) for the radiation monitor (Ells:(MON)(IL)) in the Control Room air intake duct (Ells:(DUCT)(VI)).

D.

NARRATIVE

SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES On January 3,1991, just prior to the event, an Auxiliary Operator (utility, non-licensed) was performing normal rounds in the Control Room Heating, Ventilation, and Air Conditioning (HVAC) equipment area. The Auxiliary Operator observed a de-energized light bulb on the local microprocessor associated with one of the radiation monitors in the Control Room air intake duct. The light Indicates the monitor is in a normal operating condition. The Auxillary Operator suspected that the bulb had burned out. At approximately 2348 CST the Auxiliary Operator unscrewed the bumed out bulb, whereupon the bulb flashed, the monitor deenergized, and the Control Room HVAC system automatically realigned to the emergency recirculation mode.

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. - ~ w n us.a,n Control Room personnel responded in accordance with the applicable abnormal operating procedure, placing the unit in a configuration allowed by the associated Technical Specifications. On January 4 at approximately 0217 CST, the Nuclear Regulatory Commission was notified of the event via the Emergency Notification System line in accordance with 10CFR50.72.

A work request was initiated to troubleshoot the cause of loss of power to th6 monitor. Initial investigation revealed that the power supply output fuse (Ells:(FU)(IL)) had blown during bulb removal resulting in loss of power to the monitor. The normally energized high alarm relay (Ells:(74)(IL))inside the microprocessor doenergized causing an input to the Control Room HVAC control logic and realignment of the system to the emergency rocirculation mode. The realignment is a design feature of the system and the expected result of a loss of power to the monitor.

E.

THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE OR PROCEDURAL OR PERSONNEL ERROR The Control Room HVAC system realignment was annunciated by several alarms in the Control Room. The blown power supply output fuse was discovered during troubleshooting shortly after the event. The cause of the blown power supply output

- fuse was determined during engineerin0 evaluation several days later.

II, COMPONENT OR SYSTEM FAILURES A.

FAILED COMPONENT INFORMATION

Component description: Lamp, screw base,125V Manufacturer: Dialco Manufacturer's part number: 656/7 B.

FAILURE MODE, MECHANISM, AND EFFECT OF EACH FAILED COMPONENT The capacity of the radiation monitor power supply output fuse was exceeded, causing the fuse to blow and deenergize the monitor.

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CAUSE OF EACH COMPONENT OR SYSTEM FAILURE The status light for the radiation monitor contains a single loop filament connected to two terminai posts and supported by several thin wire stalks. As the burned out light bulb was beinti unscrewed from its socket, a section of the filament fell across the two terminal posts. The decreased filament length rest..ied in a reduced electrical resistance and a momentary increase in current which exceeded the capacity of the power supply output fuse.

D.

SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS WITH MULTIPLE FUNCTIONS There were no other systems or secondar/ functions affected by the event.

Ill.

ANALYSIS OF THE EVENI

- A..

SAFETY SYSTEM RESPONSES THAT OCCURRED Upon de energization of the affected radiation monitor the Control Room HVAC system automatically realigned to the emergency recirculation mode; all associated dampers (Ells:(DMP)(VI)) and fans (Ells:(FAN)(VI)) responded as designed.

B.

D'URATION OF SAFETY SYSTEM TRAIN INOPERABILITY There were no safety systems rendered inoperable as a result of this event.

C.

SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT The engineered safety feature associated with the Control Room HVAC system is the ability of the system to automatically realign into the emergency recirculation mode in response to a loss of offsite power, a safety injection, or a high radiation condition at any one of four radiation monitors located in the Control Room air intake ducts. Loss of power to any one of the four radiation monitors in the Control Room air intake also results in automatic realignment of the system into the emergency recirculation mode. This design feature is intended to preclude unidentified loss of ESF function in the event of a loss of power to the monitor, and is not considered an Engineered Safety Feature actuation signal, but rather the effect of a component failure, l

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.. w a m.,on During the system realignment occurring on January 3, all equipment functioned as designed the operating makeup air supply fan shut down, the Control Room exhaust fan and the kitchen and toilet exhaust fans shut down, the emergency pressur!zation units started, the emergency filtration units started, and all associated dampers positioned as required. The successful realignment demonstrated that the system would have performed its intended function if the actuation had been in response to one of the accident conditions for which it was designed, it is concluded that the event did not adversely affect the safe operation of CPSES Unit 1 or the health and safety of the public.

IV.

CAUSE OF THE EVENT

The root cause of the event was determined to be equipment design which failed to anticipate the conditions encountered. The radiation monitor control power circuit has a relatively high sensitivity to overcurrent; however, no circuit design feature exists to prevent de energization of the monitor as a result of a current surge ci the type leading to this event.

V.

CORRECTIVE ACTIONS

A.

IMMEDIATE Control Room personnel responded to the event in accordance with the abnormal operating procedure, placing the system in a configuration required to comply with the applicable Action requirement of CPSES Unit 1 Technical Specifications. A work request was initiated to identify and correct the source of the problem.

B. - ACTIONS TAKEN PREVENT RECURRENCE Root Cause: Equipment sensitivity to overcurrent

Corrective Action

Because of the low probability of recurrence of this event, no immediate design changes are currently planned. However, a Lessons Learned memo and a Shift Order were issued with interim precautions to be taken while changing light bulbs in radiation monitors with automatic ESF control functions. The interim precautions require that the system operating procedure for the digital

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4 VI.

EREVIOUS SIMILAR EVENTS 1

LER 90 007 00 described an event in which the Control Room HVAC system automatically realigned to the emergency recirculation mode as a result of a loss of power to one of the radiation monitors in the Control Room air intake. However, the details of that event and the resultant corrective actions are sufficiently different from the details of this event to conclude that the previous corrective actions could not be expected to y

prevent the actuation desctlbed in this report, i

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