05000443/LER-2005-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnum Ntiber Al No. 05000 | |
| Event date: | 02-22-2005 |
|---|---|
| Report date: | 04-19-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4432005002R00 - NRC Website | |
I. Description of Event
On February 22, 2005 at 0308 while operating at 100% power, a momentary deenergization of the 4,160 volt, train A emergency bus [EB, BU] resulted in an automatic start and loading of the train A emergency diesel generator [EK, DG] and actuation of the emergency feedwater system [BA]. The loss of power occurred while transferring the emergency bus power supply from the unit auxiliary transformer (UAT) [EA, XFMR] to the reserve auxiliary transformer (RAT) to support planned maintenance on the UAT circuit breaker. During the transfer, the circuit breaker [EB, 52] from the RAT attempted but failed to close and initiated opening of the UAT supply to the emergency bus as designed. This condition initiated a loss of power signal for the emergency bus, and the emergency diesel generator (EDG) automatically started and re- energized the bus. In addition, the loss of power signal automatically started the turbine-driven emergency feedwater (EFW) pump [BA, P]. At 0337, the operators shut down the EFW pump.
Approximately nine hours following initiation of the event, power was restored to the emergency bus via the UAT, and the EDG was returned to a standby condition. At 1720 on February 24, following maintenance on the RAT supply breaker that consisted of cleaning and lubricating the racking mechanism, the emergency bus was transferred to the RAT supply.
II. Cause of Event
The cause of this event was a failure of the mechanical interlock in the racking mechanism for the RAT breaker (ABB Brown Boveri model # 5HK350) to properly engage due to inadequate cleaning and lubrication and the accumulation of dirt and hardened grease in the mechanism. The mechanical interlock is a safety feature that prevents closing of the breaker when it is in an intermediate position during a racking evolution. Additionally, the condition went undetected because of inadequate operating procedure guidance and a lack of individual knowledge related to positive latch engagement during racking operations. During the transfer of the emergency bus power supply, the RAT breaker attempted to close and initiated an opening signal to the UAT breaker; however, improper engagement of the interlock prevented the RAT breaker from closing.
III. Analysis of Event
On February 14, 2005, the RAT supply breaker to the train A emergency bus was racked out to support work in the breaker cubicle for an on going design change. The RAT supply breaker was returned to standby service as the alternate supply for the emergency bus on February 15, 2005.
Consistent with station procedures, the work required no post-maintenance testing of the circuit breaker because the maintenance activity involved only the cubicle and not the breaker. At 0308 on February 22, 2005, the RAT supply breaker failed to close, causing a brief loss of power on the emergency bus. The breaker malfunction was classified as a Maintenance Rule Functional Failure.
However, the condition did not result in a Safety System Functional Failure because off-site power remained available to the redundant safety related electrical train.
This event, which resulted in a valid actuation of the EDG and the EFW system, met the reporting criteria of 10 CFR 50.72(b)(3)(iv)(A) and 50.73(a)(2)(iv)(A). An eight-hour report (event #41428) was made at 0620 on February 22, 2005. This event is of regulatory significance because it resulted in the actuation of standby systems provided to mitigate the consequences of an accident.
This event had no adverse impact on the plant or on the health and safety of the public. The plant systems responded to the loss of power condition as designed, and no consequences resulted from this event. No inoperable structures, systems, or components contributed to the event. Following the event, however, TS 3.8.1.1, AC Sources, action a, with a 72-hour allowed outage time, was in effect for a loss of one of the two required AC sources until 1239 on February 22, 2005, when offsite power was restored to the emergency bus. Also, the shutdown of the turbine-driven EFW pump at 0337 rendered the pump inoperable due to the inability to align the pump for standby operation in the presence of a loss of power signal. This is an expected condition since the loss of power signal, which initiates EFW, remains active until offsite power is restored to the emergency bus.
Nonetheless, the pump remained functional and available for use until it was restored to operable status at 1235 on February 22, 2005.
IV. Corrective Action Following the event, station personnel inspected the breakers that were racked to the connect position and open on the six 4,160 volt and 13,800 volt buses. The inspection concluded that the mechanical interlock mechanisms were properly engaged and would not prevent any additional breakers from closing.
The planned corrective actions for this condition include (1) revising the maintenance procedures for the 4,160 volt and 13,800 volt circuit breakers to specifically clean and lubricate the locking lever portion of the racking mechanism, (2) revising the operating procedures that direct breaker racking operations to confirm proper engagement of the mechanical interlock, (3) evaluating the scope and frequency of training on breaker racking operations, and (4) cleaning and inspecting the mechanical interlock mechanism on other breakers susceptible to the same malfunction to address the extent of condition.
Similar Events This event was the first occurrence of a breaker failing to close on demand due to a malfunction of the breaker racking mechanism.