05000263/LER-2013-002
| Monticello Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2632013002R00 - NRC Website | |
EVENT DESCRIPTION
On May 24, 2013, prior to the event, the Monticello Nuclear Generating Plant (MNGP) was in Mode 5 at approximately 0% power. Operations was performing 4kV load testing on the new 2R Transformer when they were unable to observe red-light indication for a 4kV breaker [BRK] either in the control room or locally. Lack of red-light indication with the breaker closed indicates that the breaker cannot be electrically tripped.
Operators took action to trip the breaker manually using the mechanical trip button as directed by procedures.
As a result, at 0334 MNPG experienced a loss of power to the Bus-15 (Division 1-4kV Essential Bus) [BU] which initiated an Essential Bus Transfer of Bus-15 and automatic start of 12-Emergency Diesel Generator (EDG) [DG]. 11-EDG was in pull-to-lock as directed by the test plan and therefore did not start. At the time, all credited safety systems were lined up to Bus-16 (Division 2-4kV Essential Bus) which was not affected by the event, as a defense-in-depth measure. Bus-15 was automatically re-powered from the 1AR-Reserve Transformer [XFMR] as designed. During the event all systems responded as expected to the Essential Bus Transfer.
EVENT ANALYSIS
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) System Actuation for the valid actuation of the 12-EDG. This event was not considered a safety system functional failure.
SAFETY SIGNIFICANCE
Bus-15 automatically transferred to the 1AR-Reserve Transformer and 12-EDG auto started but did not load as expected due to the availability of normal off-site power to Bus-16. All safety systems credited with decay heat removal were aligned to Bus-16 during the event which was not affected. During the evolution all critical safety functions remained available and responded as expected to the Essential Bus Transfer.
CAUSE
During the MNGP refueling outage, extensive modifications to the electrical distribution system were performed which resulted in the need to remove five normally installed circuit breakers from the Lower 4kV Room. The number of breakers that required temporary storage exceeded normal storage capacity in the 4kV rooms; the remaining breakers were stored outside the 4kV rooms out of the path of travel and with foreign material exclusion covers. Troubleshooting immediately following the event determined the cause of the lack of red light indication was a damaged secondary disconnect pin. The damage misaligned the pin so that it could not make contact with the secondary disconnect rail.
The apparent cause of the damage to the breaker, which ultimately resulted in an Essential Bus transfer, was inadequate protection provided for the breaker during temporary storage.
CORRECTIVE ACTION
A spare breaker was successfully tested and installed. Other breakers in temporary storage were inspected for potential damage. The long term corrective action is to revise the operations manual to provide additional guidance regarding the appropriate storage of breakers and to specify items requiring inspection prior to installation into switchgear cubicles.
2. DOCKET
05000-263
PREVIOUS SIMILAR EVENTS
There were no previous similar licensee event reports in the past three years.
ADDITIONAL INFORMATION
Energy industry identification system (EMS) codes are identified in the text within brackets [xx].