05000306/LER-2007-002

LER-2007-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function
3062007002R01 - NRC Website

EVENT DESCRIPTION

On October 8, 2007, the 22 Diesel Room Cooling Fan1 failed to start automatically upon the starting of the Train B Unit 2 emergency diesel generator (D62). The room cooling fan is required for D6 operability (to preclude eventual overheating of equipment in the room). The past operability evaluation concluded that D6 had been inoperable between approximately September 3, 2007 and the date of the event. The September 3 date was an estimate, since procedurally operators are directed to run the fan manually until the room cools off after a diesel run. The past operability evaluation concluded a loose fan switch3 caused the switch to be in an intermediate position (neither in the "Auto" nor in the "Run" position — the two normal positions of the switch), consequently, no contacts were made up preventing the automatic start circuit from working.

EVENT ANALYSIS

Per the guidance of NUREG 1022, equipment failure is assumed to occur at the time of discovery, unless firm evidence is available to demonstrate the equipment failed earlier. In this case, D6 room cooling fan (required to support D6) was inoperable and firm evidence exists that the fan was inoperable from the last time the switch was operated to the date of discovery - approximately September 3, 2007, to October 8, 2007. Technical Specification Limiting Condition of Operation (LCO) 3.8.1, Condition B has four required actions when D6 is inoperable, including restoring the inoperable diesel generator to operable status in 14 days. If LCO Condition B is not met, LCO Condition F requires the affected unit to be in Mode 3 in six hours and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. In this case, firm evidence exists that D6 was inoperable for greater than 14 days without Unit 2 being shutdown as required by Technical Specifications. Thus, this event is reportable per 10 CFR 50.73(a)(2)(i)(B).

Impact on Safety System Functional Failure Performance Indicator No loss of function occurred because the opposite train diesel generator (D5) was not rendered inoperable as a result of this condition (the room cooling fan for D5 was satisfactorily tested for common cause failure as required by Technical Specifications) and, although removed from service during the period of this condition, D5 was removed from service as part of a planned evolution for surveillance testing in accordance with an approved procedure. Consequently, this event is not reportable per 10CFR 50.73(a)(2)(v).

1 EIIS Component Identifier: FAN 2 EIIS Component Identifier: DG 3 EIIS Component Identifier: HS FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) 07 -- 02 -- 1

SAFETY SIGNIFICANCE

Except during required surveillance testing on September 17, 2007, the opposite train diesel generator (D5) remained operable. During the surveillance test, D5 powers its safeguards bus4 (Bus 15), which is paralleled to the grid. D5 is considered inoperable while connected to the offsite grid due to potential single failures, which could result in a consequential failure of D5. However, D5 was inoperable for a short time (3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> 7 minutes) and the surveillance procedure includes instructions to ensure proper response in case offsite power is lost while D5 is running and neither the potential single failure nor the consequential failure of D5 occurred. Therefore, this condition did not affect the health and safety of the public.

CAUSE

NMC conducted a cause evaluation to determine why the fan failed to start. The failure of the fan to start was caused by the switch being in a mid-position (not in "Auto" or in "Run"), which prevented the fan from operating on a D6 start signal as designed. The switch body had rotated in the control panel, which caused the switch electrical contacts to be out of alignment when compared to the position labels on the control panel. This resulted in none of the switch contacts actually being closed. The rotation of the switch body was due to the threaded collar that holds the switch in place becoming loose.

CORRECTIVE ACTION

Immediate:

1. The switch was positioned such that the "Auto" contacts were closed. This allowed the room cooling fan to start and operate as designed.

2. After the D6 engine run, the affected control switch housing was aligned properly and the other panel switches were checked for tightness.

Subsequent:

3. D5 and D6 control switches have been checked and tightened.

Planned:

4. A preventive maintenance activity will be established to periodically check D5 and D6 control panel switches.

5. Complete a design change to install anti-rotation devices on the D5 and D6 control panel switches.

4 EIIS Component Identifier: BU FACILITY NAME (1) � DOCKET NUMBER (2)� LER NUMBER (6� PAGE (3) � 05000306 YEAR 07� 02� 1

PREVIOUS SIMILAR EVENTS

There have been no other reportable events in the past three years that were related to the cooling system that supports emergency diesel generators.