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This report is being made pursuant to 10CF … This report is being made pursuant to 10CFR50.72(b)(3)(ii)(B). At 20:46 on 02/05/05 while reviewing Operation Experience from another Nuclear Site, it was discovered that the design of the AC Auxiliary Power System incorporates a common circuit which could result in a bus lockout preventing the re-energization of both unit one safeguards buses from either onsite or offsite power sources due to a single failure in the common portion of the circuitry. This common circuit includes various metering circuits and is also connected to overcurrent devices that feed breaker lockout relays. Should a failure occur on these common circuits, all breakers supplying power to these buses would be opened, locked out, and prevented from reclosure onto the buses.</br>Due to the loss of the ability to accommodate a single failure, one offsite power source ( CT11 ) has been declared inoperable and Technical Specification 3.8.1 Condition A, entered. Corrective actions are in progress to isolate the common circuit and eliminate the single point vulnerability.</br>The licensee notified the NRC Resident Inspector.</br>See similar events #41362 (Crystal River), #41366 (LaSalle), #41369 (Quad Cities) ,#41370 (Dresden) and #41377 (Monticello).</br> * * * RETRACTION FROM S. SEILHYMER TO S. ROTTON AT 1140 ON 6/13/06 * * * </br>The event reported on February 6, 2005 (4160 volt relaying and metering single failure vulnerability, NRC Event Number 41377) is hereby retracted. Subsequent analysis has concluded that a common mode failure resulting in a simultaneous lockout of both safeguards 4160 volt Bus 15 and Bus 16 due to the common metering circuit was not a credible event in the as-found condition. The analysis showed that the required magnitude of a fire induced fault is much greater than the available fault current in any of the associated circuits. The analysis also showed that dual bus lockout due to an open circuit under non-fire induced conditions is not credible since the imbalance current will be less than the trip setpoint of the affected relays.</br>Thus, although the plant had been in an unanalyzed condition, that condition has now been shown to have not significantly degraded plant safety and is, therefore, not reportable under the requirements of 10 CFR 50.72(b)(3)(ii)(B). Additionally, NMC will be submitting a letter to cancel the associated LER 1-05-01.</br>The licensee notified the NRC Resident Inspector. Notified R3DO (Louden).sident Inspector. Notified R3DO (Louden).
02:46:00, 6 February 2005 +
41,377 +
00:43:00, 6 February 2005 +
02:46:00, 6 February 2005 +
This report is being made pursuant to 10CF … This report is being made pursuant to 10CFR50.72(b)(3)(ii)(B). At 20:46 on 02/05/05 while reviewing Operation Experience from another Nuclear Site, it was discovered that the design of the AC Auxiliary Power System incorporates a common circuit which could result in a bus lockout preventing the re-energization of both unit one safeguards buses from either onsite or offsite power sources due to a single failure in the common portion of the circuitry. This common circuit includes various metering circuits and is also connected to overcurrent devices that feed breaker lockout relays. Should a failure occur on these common circuits, all breakers supplying power to these buses would be opened, locked out, and prevented from reclosure onto the buses.</br>Due to the loss of the ability to accommodate a single failure, one offsite power source ( CT11 ) has been declared inoperable and Technical Specification 3.8.1 Condition A, entered. Corrective actions are in progress to isolate the common circuit and eliminate the single point vulnerability.</br>The licensee notified the NRC Resident Inspector.</br>See similar events #41362 (Crystal River), #41366 (LaSalle), #41369 (Quad Cities) ,#41370 (Dresden) and #41377 (Monticello).</br> * * * RETRACTION FROM S. SEILHYMER TO S. ROTTON AT 1140 ON 6/13/06 * * * </br>The event reported on February 6, 2005 (4160 volt relaying and metering single failure vulnerability, NRC Event Number 41377) is hereby retracted. Subsequent analysis has concluded that a common mode failure resulting in a simultaneous lockout of both safeguards 4160 volt Bus 15 and Bus 16 due to the common metering circuit was not a credible event in the as-found condition. The analysis showed that the required magnitude of a fire induced fault is much greater than the available fault current in any of the associated circuits. The analysis also showed that dual bus lockout due to an open circuit under non-fire induced conditions is not credible since the imbalance current will be less than the trip setpoint of the affected relays.</br>Thus, although the plant had been in an unanalyzed condition, that condition has now been shown to have not significantly degraded plant safety and is, therefore, not reportable under the requirements of 10 CFR 50.72(b)(3)(ii)(B). Additionally, NMC will be submitting a letter to cancel the associated LER 1-05-01.</br>The licensee notified the NRC Resident Inspector. Notified R3DO (Louden).sident Inspector. Notified R3DO (Louden).
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