05000298/LER-2008-003
| I Telephone Number (Include Area Code) | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2982008003R00 - 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 [EIIS: JH] provides a protected environment from which control room personnel can safely operate the plant following an uncontrolled release of radioactivity, hazardous chemicals, or smoke. The CREF system is a single train, high efficiency air filtration system for emergency treatment of outside supply air and a control room envelope boundary that limits the inleakage of unfiltered air. The system consists of a filter train [EIIS: FLT], fans [EIIS: FAN], and the associated ductwork [EIIS: DUCT], valves [EIIS: V], dampers [EIIS: DMP], doors [EIIS:
DR], barriers, and instrumentation.
The CREF system is a standby system. Upon receipt of an initiation signal, the CREF system automatically switches to emergency bypass mode of operation to minimize infiltration of contaminated air into the control room envelope. The control room envelope boundary is the combination of walls, floor, roof, ducting, doors, penetrations, and equipment that physically form the control room envelope. The control room envelope boundary provides protection from smoke and hazardous chemicals to the occupants by manual isolation.
The CREF system is operable when the control room envelope boundary is maintained such that leakage of unfiltered air into the envelope will not exceed the inleakage assumed in the licensing basis analysis of design basis accident radiological consequences, and during hazardous chemical and smoke events. The control room envelope boundary is allowed to be opened intermittently under administrative controls as long as the boundary such as doors, hatches, etc., can be quickly closed and restored to design condition.
EVENT DESCRIPTION
On November 30, 2008, at approximately 01:29 Central Standard Time (CST), a security officer on routine patrol of the control building passed through door H200 twice.
The officer confirmed the door was closed and latched each time he approached the door, but was unaware whether the door closed and latched after he passed through the second time. At 01:58 CST, a station operator found door H200 partially open. The condition was immediately reported to the control room.
Door H200 is a fire and control room envelope boundary door. With this door not fully closed and latched, CNS lost reasonable assurance that the CREF system would fulfill its safety function. Operations declared the CREF system inoperable at 01:58 CST and entered Technical Specification Limiting Condition for Operation (LCO) 3.7.4 Condition B. Operations personnel inspected the door and found a metal plate on the hinge edge of the door protruding from the bottom. At 02:14 CST on November 30, 2008, personnel closed door H200 and CNS exited LCO 3.7.4 Condition B. A watch was established to ensure proper door closure after passage. Door H200 was open not more than 29 minutes before discovery of the condition and not more than 45 minutes before the CREF system was restored to operable.
Upon further inspection, maintenance personnel found the plate to have originated from the inside of the door between the hinge edge and bottom edge support channel. They identified that the spot welds between the bottom edge support channel and the door skin were cracked. Maintenance lifted the door skin and removed two metal plates, one of which was protruding and obstructing the door. At 16:48 CST on November 30, 2008, maintenance completed repair of the door skin-to-channel welds and restored door H200 to fully functional service.
CNS determined through subsequent discussions with the original equipment manufacturer, Overly Manufacturing, that the metal plates were spot weld calibration test pieces inadvertently left in the door during assembly, prior to original plant construction (i.e., prior to 1974). This material had fallen through the gap between the hinge edge and door channel and obstructed door H200 from closing. As part of the extent of condition, personnel walked down and operated all fire, control room envelope boundary, and secondary containment doors to determine if any foreign material existed inside, inspected door skin to door edge channel welds, and entered discrepancies into CNS' corrective action program. Additionally, CNS communicated expectations for control of doors and self-checking door closure to personnel.
CNS later concluded as part of the root cause investigation, that specific guidance for employees to self-check closure was not contained in procedures. Site procedures discuss that fire and boundary doors should be in a closed position; however, these procedures are geared toward definition of the doors for maintenance and testing and do not specifically provide guidance on responsibilities when passing through a door.
Also, review of site computer based training showed training on aspects of door use and expectations for ensuring they are properly closed, but the amount of discussion is limited.
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
Door H200 is designated as a fire protection system and control room envelope boundary door. The CREF system is not credited in the CNS internal or external events probabilistic risk assessment model for determining core damage frequency or large early release frequency. As such, the inoperability of the CREF system due to the partial opening of door H200 has no impact on the internal or external events calculated risk. Additionally, the partial opening of door H200 was determined to have a minimal impact on the risk associated with a fire at CNS. Therefore, this event has negligible safety significance.
This event is a safety system functional failure as defined in Nuclear Energy Institute 99-02 Revision 5, Regulatory Assessment Performance Indicator Guideline.
CAUSE
The root cause of the event was that the expectation to self-check door closure was not communicated and reinforced. The mechanistic failure was caused by material inadvertently left in the door during fabrication.
CORRECTIVE ACTION
CNS will develop and implement a change management plan that addresses expectations for site personnel to self-check door closure. As an interim action, CNS will perform weekly field observations on site personnel use of self-checking for door closure.
Additionally, CNS will revise applicable door inspection procedures to include inspection of the accessible bottom and top door channels for spot weld separation.
PREVIOUS EVENTS
There have been no related reportable events.