05000219/LER-2009-005
| Oyster Creek | |
| Event date: | 07-12-2009 |
|---|---|
| Report date: | 02-05-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2192009005R01 - NRC Website | |
Plant Condition Prior to Event Event Date:
July 12, 2009 Event Time: 0135 EDT Unit 1 Mode: Power Operation Power Level: 100%
Description of Event
Note: Energy Industry Identification System (EllS) codes are identified in the following text in brackets as [XX].
On July 12, 2009, a severe electrical storm resulted in multiple lightning strikes on the Q121 transmission line.
The affected line is a feed to the 34.5 kV Oyster Creek substation, which then feeds the Oyster Creek Startup Transformers [XFMR]. The lightning strikes in conjunction with a failure of a transmission line breaker [BKR] to open caused grid disturbances, a main generator [GEN] trip on over-excitation with subsequent automatic reactor scram (Turbine Trip), and a loss of offsite power to the Startup Transformers [XFMR].
When the plant experienced the loss of offsite power to the Startup Transformers, both emergency diesel generators (EDGs) [EK] started and energized their associated safety related buses. A Main Steam Isolation Valve actuation occurred due to loss of offsite power, resulting in Isolation Condenser [BL] and Electromatic Relief Valve [RV] actuations. Two post-scram anomalies were associated with this event. EDG #1 took longer than expected to auto sync to its safety-related bus, and erratic "B" Isolation Condenser level indication.
Unusual Event MU-1 was declared for the loss of power to the Startup Transformers SA and SB for greater than 15 minutes. All declarations and notifications were made correctly and in a timely manner. The Unusual Event was terminated at 0405 on July 12, 2009 after restoration of offsite power.
Analysis of Event
This event is being reported pursuant to:, 10CFR50.73(a)(2)(iv)(A) due to automatic actuation of the reactor protection system [JC], Isolation Condensers, and EDGs; and 10CFR50.73(a)(2)(v)(D) due to loss of offsite power to the Startup Transformers. There were no safety consequences impacting the plant or public safety as a result of this event.
Two post-scram anomalies were noted with EDG #1 output breaker impacting the auto sync time and the "B" Isolation Condenser erratic level indication. All other safety systems, structures, and components operated normally during this event.
Cause of Event
The 34.5 kV, Q-121 transmission line was struck by lightning. The lightning strike broke the carrier/static line, resulting in a three-phase-to-ground fault. The Q-121 line breaker at Oyster Creek failed to open on the line fault, resulting in the Oyster Creek generator feeding the fault until backup line breakers opened and isolated the line. These grid disturbances caused voltage swings and when the backup line breakers eventually isolated the Q-121 fault, switchyard voltage increased rapidly and the Oyster Creek generator tripped on over excitation, The latter resulted in an automatic reactor scram (Turbine Trip).
The turbine-generator trip and resultant reactor scram was caused by a failure of the Q-121 line breaker to isolate the faulted transmission line following damage caused by lightning strike. The cause of the line breaker failure was a seized trip latch roller bearing in the opening mechanism of the breaker, resulting in sluggish opening time of the breaker. The line breaker is owned and maintained by First Energy (transmission system operator).
A root cause evaluation determined that trending of past performance of the breaker in the switchyard was not performed. If the breaker performance had been trended, the slow performance during a lightning strike in June 2009 could have been identified and either corrected or the line isolated.
Corrective Actions
Corrective actions included restoring the EDG #1 to full operable status, flushing the Isolation Condenser level indication lines and repairing the Q-121 line breaker. These actions addressed the anomalies noted above.
The Q-121 line breaker was repaired by First Energy. The trip latch roller bearing and linkage mechanism in the Q121 breaker was adjusted, lubricated, and exercised. Post-maintenance timing results were within specification.
Lessons learned from that repair and the cause determination were placed in Oyster Creek's CAP database.
The Oyster Creek Performance Monitoring Plan for the switchyard was revised to include breaker maintenance and testing data from opening on a fault to facilitate trending of past performance of the switchyard breaker.
Previous Occurrences
There have been no similar Licensee Event Reports submitted due to lightning strikes at Oyster Creek in the last three years. However, there was a lighting strike in June 2009 on the Q-121 transmission line which resulted in a less severe grid disturbance and no plant scram occurred.
Component Failure Data
Component: Transmission Line Breaker Manufacturer: General Electric Cause: Seized trip latch roller bearing in the opening mechanism of the breaker �