05000482/LER-2008-005
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 04-26-2008 |
|---|---|
| Report date: | 06-23-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4822008005R00 - NRC Website | |
PLANT CONDITIONS PRIOR TO EVENT:
MODE — 6 Power — 000
BACKGROUND:
Selected plant equipment is required for shutting down the plant and maintaining it in a hot standby conditon in the event of a temporary evacuation of the control room due to a fire. Switches and redundant fuses are provided to isolate the equipment from the control room to ensure the equipment can be operated from outside the control room.
The redundant fuses are provided to ensure control power is available after the control room has been isolated.
The Class 1 E electrical equipment room cooler units [EIIS Code: VI-ACU] are included in the post fire safe shutdown design to ensure adequate room cooling for the operating train of safety related electrical equipment. The air conditioning units provide a support function for Class lE electrical equipment required for safe shutdown. If an electrical short occurs on the control room side of the circuit and blows the fuse, the "isolated" portion of the circuit, fed from a fuse in parallel, would allow the air conditioning units to be re-started and continue to provide room cooling.
EVENT DESCRIPTION:
On April 26, 2008, while trouble shooting a loss of the 'B' train Class 1 E electrical equipment room cooler, it was discovered that the redundant fuse used to ensure the cooler can be started after a control room fire was installed in series with the control power transformer secondary side fuse, not in parallel as per design. In the series configuration, if an electrical short occurs on the control room side of the circuit and blows the fuse, the "isolated" portion of the circuit, fed from the redundant fuse, would also lose power. Therefore, the circuit could not be re-energized and the air conditioning unit could not be re-started in the event of postulated damage to the circuit as a result of a control room fire.
The loss of the Class lE electrical equipment room cooler does not directly result in loss of capability to achieve and maintain safe shutdown in the event of a fire. Rather, room heating beyond design limits could reduce the life of electrical components within the electrical equipment rooms.
Qualification data exists to show that some components in the electrical equipment rooms will survive the expected room temperatures and be functional following a loss of room cooling. Data does not exist for each and every component within the electrical equipment rooms, so it is indeteminate if all equipment would be functional following a loss of room cooling. Therefore, this condition resulted in an unanalyzed condition that could potentially affect post-fire safe shutdown equipment availability.
The Wolf Creek Generating Station (WCGS) was in Mode 6 during refueling outage 16, at the time of the discovery.
There were no compensatory measures required since the plant was in Mode 6. The configuration was corrected prior to entering Mode 4.
BASIS FOR REPORTABILITY:
10CFR50 Appendix R Section 1111.7 requires "The safe shutdown equipment and systems for each fire area shall be known to be isolated from associated non-safety circuits in the fire area such that hot shorts, open circuits, or shorts to ground in the associated circuits will not prevent operation of the safe shutdown equipment." Based on this information, WCNOC made an eight hour Emergency Notification System call in accordance with 10 CFR 50.72(b)(3) (ii)(B).
This condition is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(B) for any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
ROOT CAUSE:
The 'B' train Class 1 E electrical equipment room cooler, power and control cabinets, were replaced in the fall of 2003.
The vendor incorrectly wired the line side of the dedicated post fire safe shutdown control power transformer isolation fuse to the load side of the control room supply fuse. This placed both the isolate and remote power supply fuses of the control power transformer in series instead of the required parallel configuration. The termination error rendered the post fire safe shutdown redundant isolation fuse incapable of performing its intended design function to supply independent isolated control power to the circuit in the event of a control circuit fault due to a fire in the control room.
Normal testing of the circuit did not catch the error.
CORRECTIVE ACTIONS:
The wiring for the fuses was corrected to place them in parallel prior to entering Mode 4.
Over the 23 years of plant operation no other redundant fuse incorrect wiring issues are known to have been found.
As a precaution, the additional twenty circuits in the plant with redundant post fire safe shutdown fusing will be examined to verify that the fuses are wired correctly. This will be accomplished prior to plant startup following Refuel Outage 17.
Procedure AP 16E-002, 'Post Maintence Testing Development,' will be revised by December 19, 2008, to ensure equipment with redundant fusing are properly wired.
SAFETY SIGNIFICANCE:
This issue is of low safety significance. Loss of the Class 1 E electrical equipment room cooler units does not automatically cause inoperability of the associated Class 1 E electrical equipment. There is reasonable assurance that safe shutdown would be achieved if a fire occurred in the control room that resulted in a loss of Class 1 E electrical equipment room cooling.
OPERATING EXPERIENCE/PREVIOUS EVENTS:
None.