05000443/LER-2005-008
| Docket Numbersequentialmonth Day Year Year Rev Month Day Year N/Anumber No. 05000 | |
| Event date: | 11-30-2005 |
|---|---|
| Report date: | 12-28-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4432005008R00 - NRC Website | |
I. Description of Event
On November 29, 2005 at 0323 with the station in Mode 1 and at 100% power, Inverter 1F [EE, INVT] failed, and associated vital power panel 1F automatically transferred to its maintenance power supply.
Subsequent investigation, in consultation with the vendor, Elgar, determined that the most probable cause was a card failure in the A-2 leg of the inverter. While AC vital panel 1F remained energized from its maintenance power supply, Inverter 1F was not available to energize power panel 1F while connected to DC Bus 11B [EE,BU] within the 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> required by TS 3.8.3.1. action b. During troubleshooting it was determined that an additional 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> would be necessary to complete the repairs to inverter 1F, perform post-maintenance testing, and restore the inverter to operable status. Compliance with the 24-hour allowed outage time (AOT) would have forced a shutdown of Seabrook Station prior to completing restoration of the inverter. Such a shutdown would have introduced an unnecessary plant transient with potential safety consequences and operational risks that are not commensurate with a concern for public health and safety.
At 0220 on November 30, 2005, prior to the expiration of the 24-hour allowed outage time, the NRC granted a Notice of Enforcement Discretion that extended the allowed outage time by 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br />. As a result, Seabrook Station operation continued at 100% power for the duration that the inverter was inoperable. To expedite the repair and restore the Inverter to an operable status, three circuit cards were simultaneously removed, replaced and the connections cleaned and verified. The inverter was restored to operable status at 1522 on November 30, 2005. No adverse consequences resulted from this event.
II. Cause of Event
The cause of the inoperability of the inverter was a circuit card failure. Troubleshooting, repair and testing of the inverter could not be completed within the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Allowed Outage Time (AOT) specified for the inoperable inverter, resulting in the request for and granting of Enforcement Discretion.
Ill. Analysis of Event On November 29, 2005 at 0323 with the station in Mode 1 and at 100% power, Inverter 1F failed, and associated vital power panel 1F automatically transferred to its maintenance power supply. This maintenance supply is powered by the Train B Emergency Diesel Generator (EDG), so the inverter 1F failure only represented a marginal loss of redundant supply (i.e., DC power input) to the associated loads, but did not fail any equipment. During operation with an inoperable, out-of-service vital instrument bus inverter, the associated balance-of-plant (BOP) instrument bus is energized from its maintenance source through a safety-related static transfer switch. The source of power for the maintenance supply is a nonsafety-related motor control center (MCC), which relies on the EDG for back-up power. Should a loss of off-site power (LOOP) occur while an instrument bus is aligned to its maintenance source, the instrument bus would have remain de-energized for approximately 10 seconds until the EDG started and energized the maintenance supply. In order for the instrument bus to remain de-energized, the EDG would have to fail or the MCC that provides the maintenance power source would have to fail to energize. While Inverter 1F was inoperable the redundant instrument bus inverter was available, operable and aligned to a DC power supply. The Supplemental Emergency Power System (SEPS) was also available and operable to supply power to the bus in the event of a station blackout.
This event is of regulatory significance because it met the reporting criteria of 10 CFR 50.73 (a)(2)(i)(B) for a condition prohibited by the TS. The event was reported to the NRC on November 30, 2005 at 1714 (event # 421784) in accordance with the Facility Operating License Condition 2.G as a violation of the TS.
No consequences resulted from this event and, therefore, the event had no adverse impact on the plant or on the health and safety of the public. This condition did not involve a Safety System Functional Failure and no inoperable structures, systems, or components contributed to this event.
IV.Corrective Action Inverter 1F was repaired and returned to service on November 30, 2005 at 1522.
On December 6, 2005 FPL Energy Seabrook submitted License Amendment Request (LAR) 05-11 to extend the allowed outage time detailed in TS 3.8.3.1. action b for Inverters 1E and 1F from 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to seven days.
V. Additional Information
At the time of this report it is unknown what caused the failure of the circuit card to perform its intended function. As part of the equipment failure analysis, the circuit cards removed from Inverter 1F have been sent to Elgar for analysis. A Root Cause of the event is also being performed.
The Energy Industry Identification System (EIIS) codes are included in this LER in the following format: [EIIS system identifier, HIS component identifier].
Similar Events Seabrook Station has had no similar events involving inverter failure.