05000298/LER-2013-001
| Cooper Nuclear Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49486 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2982013001R00 - NRC Website | |
Ammeter circuit could result in a secondary fire due to multiple fire induced faults", it was determined that CNS is susceptible to the same condition.
The condition in the OE is as follows: "The wiring design for the ammeters contains a shunt in the current flow from each direct current (DC) battery or charger. Bolted on the shunt bar are two IEEE 383 qualified leads to a current meter in the main control room (MCR). The small difference in voltage between the two taps on the shunt is enough to deflect the current gauge in the MCR when current flows from the battery or charger through the shunt. The ammeter wiring attached to the shunt does not have fuses. It is postulated that a fire could cause one of these ammeter wires to short to ground at the same time the fire causes another DC wire from the opposite polarity on the same battery also short to ground. This would cause a ground loop through the unfused ammeter cable. With enough current going through the cable, the potential exists that the cable could self-heat to the point of causing a secondary fire in the electrical tray at some point along the path of the cable (including the Control Room) or possibly heat up to the point of causing damage to adjacent cables that may be required for safe shutdown.
CNS has cables [CBL] that run from shunts located in the DC switchgear [SWGR] rooms 1A and 1B to ammeters [II] on bench board C in the Control Room. The conductors do not have over-current protection to limit fault current.
In a postulated event, a fire in the area of the shunt conductor's route could cause one of the ammeter wires to short to the ground plane. Simultaneously, the event could cause another DC wire from the opposite polarity on the same battery [BTRY] to short to the ground plane. This would cause a ground loop through the unprotected ammeter wire. Since this circuit is not protected, this event could result in excessive current flow in the ammeter wiring to the point of causing a secondary fire in a separate fire area. This could potentially cause the loss of the ability to conduct a safe shutdown as required by 10 CFR 50, Appendix R.
BASIS FOR REPORT
This event is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as a condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
Event Notification 49486 was made to the Nuclear Regulatory Commission on October 30, 2013.
SAFETY SIGNIFICANCE
This event is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(B) as a condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety in that the self-heating on the overloaded ammeter wiring could cause damage to adjacent cables/equipment that may be required for safe shutdown following a fire in the Control Room or Cable Spreading Room.
There were no actual consequences to report as this identifies an unanalyzed condition with no occurrence of an event. An extent of condition review determined that this issue applies to the 125 and 250 VDC ammeter circuit wiring.
CAUSE
The cause of the unfused ammeter circuits is that the original design criteria had not factored in the potential of the multiple shorts to ground failure mode and therefore, did not require overcurrent protection for remote shunt fed ammeter circuits.
CORRECTIVE ACTION
Compensatory fire watch measures have been implemented until an analysis is performed demonstrating that remote circuits can meet fire protection requirements without fuses. If an analysis cannot demonstrate meeting fire protection requirements with the existing circuit design, then a modification to correct the remote ammeter circuits will be implemented.
PREVIOUS EVENTS
On July 13, 2010, during the review of the Safe Shutdown Analysis Report, station personnel discovered that although the plant credits Containment Overpressure (COP), the Appendix R analysis did not ensure COP is maintained. The cause of the event was approval oversight did not detect that unanalyzed Appendix R issues were being deferred and not promptly addressed.
The event was reported under Licensee Event Report 2010-002-00, Appendix R Containment Overpressure Credit, dated September 7, 2010.