05000298/LER-2007-007

LER-2007-007, Cooper Nuclear Station 05000298 1 of 4

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
2982007007R00 - NRC Website

PLANT STATUS

Cooper Nuclear Station (CNS) was in Mode 1 at 100 percent, steady state power at the time of this event.

BACKGROUND

The Control Room Emergency Filter (CREF) System [EIISS:JH] provides a radiologically controlled environment from which control room personnel can safely operate the plant following a design basis accident. The CREF System is a single train, standby safety system consisting of a filter train [EllS:FLT], a supply fan [EIIS:FAN], an emergency booster fan, and an exhaust booster fan.

Upon receipt of an initiation signal, the CREF System automatically switches to the emergency bypass mode of operation. A system of dampers [El IS:DMP] isolates the control room. The emergency booster fan starts and draws air in from the outside where it is also routed through the filter system before entering the control room air conditioning ductwork [EIIS:DUCT]. The exhaust booster fan discharges the return air downstream of the control room envelope to the suction of the main control room air conditioning unit [EIIS:ACU]. This method maintains the control room at positive pressure to prevent infiltration of contaminated air from surrounding buildings and the outside atmosphere.

The control room exhaust booster fan is normally in operation at all times. It is driven by an electric motor [EIIS:MO] and is capable of exhausting the required air flow from the control room.

EVENT DESCRIPTION

On December 3, 2007, the plant was in Mode 1, in day one of a seven-day shutdown action statement of Limiting Condition for Operation (LCO) 3.7.4. The CREF System was inoperable for scheduled maintenance. While performing maintenance on the control room exhaust booster fan, a required support feature for the CREF System, the motor terminal box was found loose on the fan motor. The cover was removed to tighten the terminal box and the leads for one phase of the fan motor were found degraded. Although the control room exhaust booster fan had been in service prior to being secured for preventive maintenance and had no previous failures, it was determined at 20:51 the physical condition of the fan motor leads may not support the required safety function of the CREF System.

The control room exhaust booster fan motor was replaced and post-work testing completed. As part of the extent of condition, the motor leads on the control room emergency booster fan motor were also inspected. Those leads and motor were found in good condition. The CREF System was returned to operable status at 17:48 on December 4, 2007, and LCO 3.7.4, Condition A was exited.

Further analysis was performed and concluded the probable cause of this condition was the continuous operation of the control room exhaust booster fan motor with a motor lead conductor that had damaged wire strands. The strands were likely broken during original installation of the wire lugs by the vendor in 1974. The damaged strands created a localized high resistance spot in the conductor where heat was generated. In turn, the heat caused the failure of additional, adjacent strands during the 34-year service life of the fan motor.

BASIS FOR REPORT

This is reportable under 10 CFR 50.73(a)(2)(v)(D) as a condition which could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.

SAFETY SIGNIFICANCE

This event is not considered to be risk significant. The CREF System is not credited in the CNS internal events probabilistic risk assessment model. Therefore, the impact on core damage probability as a result of the unavailability of the CREF System is determined to be below levels established for risk significance.

This event is considered a safety system functional failure as defined in Nuclear Energy Institute 99-02 Revision 5, Regulatory Assessment Performance Indicator Guideline.

CAUSE

The preventive maintenance plan for the control room exhaust booster fan motor was inadequate. The preventive maintenance plan required a visual inspection of the motor and lubrication of the motor bearings; however, there was no regular maintenance program established to ensure reliability of the motor.

CORRECTIVE ACTION

A corrective action to prevent recurrence of this condition is being tracked in the CNS corrective action program to implement a routine preventive maintenance plan to ensure reliability of the control room exhaust booster fan. Additionally, a corrective action has been initiated to implement a similar preventive maintenance plan for the control room emergency booster fan.

PREVIOUS EVENTS

There have been no related reportable events.