05000458/LER-2012-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A 05000 | |
| Event date: | 05-24-2012 |
|---|---|
| Report date: | 07-23-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4582012003R00 - NRC Website | |
On May 24, 2012, at 3:40 p.m. CDT, a manual reactor scram was initiated in response to the loss of the running reactor feedwater pump ("P"). The plant was operating at approximately 32% power at the time.
The reactor core isolation cooling system (BN) (RCIC) was manually started to provide high pressure makeup to the reactor. The high pressure core spray (BG) (HPCS) system was manually started during the recovery from the event, but was not aligned to the reactor vessel.
An electrical transient caused by the failure of a lockout relay (**86**) resulted in the main supply breaker (**BKR**) to the "B" 13.8kv switchgear (EA) to trip. Reactor recirculation (AD) pump "B" tripped due to the loss of its power source; the "A" reactor recirculation pump continued to operate in slow speed. The electrical transient also caused a loss of power to all main condenser circulating water pumps (NN) and normal service water pumps (KG), necessitating the manual closure of the main steam isolation valves. The standby service water system (BS) actuated as designed in response to low normal service water pressure.
No reactor safety-relief valves (SRVs) operated automatically as a result of the scram. The operators manually operated selected SRVs for reactor pressure control and for reactor cooldown.
The residual heat removal system (BO) was manually started in the suppression pool cooling mode to support RCIC operation, as well as for control of suppression pool level.
Personnel in the turbine building reported the presence of smoke in the area of the feedwater pumps, but no actual fire was observed.
This event is being reported in accordance with 10CFR50.73(a)(2)(iv)(A) as an actuation of the reactor protection system (JC) and the standby service water system. There were no safety related systems out of service at the time of the event.
INVESTIGATION and CAUSAL ANALYSIS At the time of the event, the "C" reactor feedwater pump was in service. When the operator started the "B" feedwater pump, an electrical fault occurred at the pump motor. The lockout relay on the pump's feeder breaker failed to trip the breaker, and the main supply breaker to the "B" 13.8kV switchgear tripped to clear the fault. This caused the loss of power to the "C" pump, as well as switchgears supplying the circulating water system and the normal service water system.
The inspection of the terminal box on the "B" feedwater pump determined that fault occurred due to an inadequately crimped terminal lug on one of the three current transformers. The motor (**MO**) had been rewound by a vendor in 2008. When the motor was returned, new lugs were investigation found that the lugs were too large for the application. Additionally, the crimping tool used for the installation did not fully compress the lugs, leaving an inadequately bonded connection.
The lockout relay installed on the breaker for the "B" feedwater pump is a General Electric HEA 61. The analysis of this event found that the lockout relay failed to operate as designed due to age-related mechanical binding and a possible coil failure. This condition resulted from an inadequate preventative maintenance program for the relays and a design issue with the trip plate.
Prior to plant restart, similar lockout relays were functionally tested. Additional failed relays were discovered (none in safety-related service), and these were replaced.
CORRECTIVE ACTION TO PREVENT RECURRENCE
The preventative maintenance program for lockout relays is being evaluated to develop appropriate changes. A training needs analysis is being conducted for those departments responsible for development and implementation of the preventative maintenance program.
Thermographic imaging will be performed on other large motors potentially susceptible to this same type of lug failure.
These actions are being tracked in the station's corrective action program.
PREVIOUS OCCURRENCE EVALUATION
No previous scrams have occurred with the same root cause as this event. There was a failure of the same type of lockout relay in February 2011.
SAFETY SIGNIFICANCE
No plant parameters requiring the automatic actuation of the emergency diesel generators or the emergency core cooling systems were exceeded. The RCIC system operated properly in response to the operators' manual control and provided high pressure makeup to the reactor.
Control of reactor pressure was accomplished by manual actuation of selected SRVs. The plant was placed in cold shutdown. The standby service water system operated as designed. This event was of minimal safety significance to the health and safety of the public.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)