05000348/LER-2012-002
| Joseph M. Farley Nuclear Plant, Unit 1 | |
| Event date: | 02-15-2012 |
|---|---|
| Report date: | 04-09-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3482012002R00 - NRC Website | |
Westinghouse -- Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX]
Description of Event
On February 15, 2012 at 0212, with Unit 1 operating at 100 percent power, 1B containment cooling fan (A Train) [Big failed to start in slow speed during surveillance testing. Subsequent troubleshooting revealed that the failure of the fan to start was due to a malfunction of the 600 volt breaker that provides power to the fan motor slow speed winding. Based upon the investigation performed, it was determined the breaker had been rendered incapable of closing since January 18, 2012. This was the last time the breaker had been closed and the containment cooling fan had operated in slow speed satisfactorily.
Although the 1B containment cooling fan was inoperable since January 18, 2012, the A Train of containment cooling was nonetheless operable due to the fully redundant 1A containment cooling fan being operable and selected for automatic start. The 1B containment cooling fan was selected for automatic start on January 23, 2012. Its selection on that date began the period of inoperability for the A Train of containment cooling. Therefore, the A Train was inoperable for approximately 23 days (January 23, 2012 to February 15, 2012) until the failure was discovered during surveillance testing on February 15, 2012. Upon discovery, the automatic start function was de-selected for the 1B containment cooling fan and re-selected for the 1A containment cooling fan and A Train operability was restored in approximately eight minutes on February 15, 2012 at 02:20. Since the seven day completion time of Technical Specification 3.6.6 Condition C was exceeded, this represents a condition prohibited by Technical Specifications and is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).
Investigation into the breaker failure determined that the cause was due to a failed charging motor cut- out switch. The faulted motor cut-out switch was manufactured by Westinghouse (Model: "Switch — Motor Cut Off' and part number 567F430G06).
Cause of Event
The direct cause of this event was the failure of the breaker's charging motor cut-off switch.
With the breaker open and springs discharged, the charging motor is energized through the motor cut- off switch. Once control power is applied, the motor runs and charges the breaker closing springs.
When the springs are fully charged, the motor cut-off switch actuates to deenergize the charging motor.
Once this charging cycle is finished, the breaker is ready to close.
Upon removing the cover of the motor cut-off switch, it was discovered that an air gap was present between the movable contact and stationary contacts. With no continuity across this normally closed set of contacts, the charging motor could not receive power to recharge the closing springs. The air gap was the result of internal binding within the motor cut-off switch.
NRC FORM MA (10-2010) Although the failure mechanism is known, the events that took place in order to arrive at this condition are not. This failure (internal binding) could not be recreated under existing conditions. The motor cut- off switch operated smoothly and showed no signs of defective components. The extensive number of successful switch cycles across the industry with only two identified possible failures of this type reported indicates that the design has been highly reliable.
Safety Assessment The Containment Cooling system at Farley Nuclear Plant is comprised of two trains, each of sufficient capacity to supply 100 percent of the design cooling requirement. Each train consists of two fully redundant fan units supplied with cooling water from a separate train of service water (SW). Each fan unit has two speeds of operation, high speed for normal operation and slow speed for post-accident operation. Each fan motor has two separate windings (one high speed and one slow speed), that are powered from separate breakers. However, under post-accident conditions, a single fan unit from either train with at least 600 gpm of SW flow provides sufficient cooling capacity to meet post accident heat removal requirements. Air is drawn into the coolers through the fan and discharged to the steam generator compartments, pressurizer compartment, and outside the secondary shield in the lower areas of containment.
During normal operation, up to four fan units are operating in high speed with SW supplied to the cooling coils. The Containment Cooling System is designed to limit the ambient containment air temperature during normal unit operation to less than the limit specified in LCO 3.6.5, "Containment Air Temperature." This temperature limitation ensures that the containment temperature does not exceed the initial temperature conditions assumed for the Design Basis Accidents.
In post-accident operation following an actuation signal, unless a Loss of Site Power (LOSP) signal is present, the Containment Cooling System fans are designed to start automatically in slow speed if not already running. If an LOSP signal is present, only the two fans selected (one per train) will receive an auto-start signal and will start in slow speed. If running in high (normal) speed, the fans automatically shift to slow speed. The fans are operated at the lower speed during accident conditions to prevent motor overload from the higher water content in the containment atmosphere.
During the time period from January 23, 2012 (date at which the 1B containment cooling fan was selected for automatic start) until February 15, 2012 (discovery date of 1 B containment cooling fan failure), the A Train Containment Cooling system was rendered inoperable due to the unknown breaker malfunction. However, the B Train Containment Cooling system remained operable and capable of providing 100 percent of the design post-accident cooling requirement. In addition, the 1A containment cooling fan (A Train) remained available and would have manually started upon demand and was further capable of providing 100 percent of the design post-accident cooling requirement. Therefore, the event did not result in a loss of safety system function. Also, there were no events which required a Containment Cooling system slow speed actuation during the period of unrecognized inoperability.
Based on these considerations, there was no adverse affect on plant safety or on the health and safety of the public.
This event is reportable pursuant to 10 CFR 50.73 (a)(2)(i)(B) for a condition prohibited by Technical Specifications.
Corrective Action The A Train operability was restored in approximately 8 minutes when the 1A containment cooling fan was selected to automatic start function on February 15, 2012.
The failed breaker was replaced with a suitable operable breaker. Surveillance testing was completed satisfactorily on this breaker on February 21, 2012.
Causal analysis was completed and the additional corrective actions identified were entered into the corrective action program (CAP). Work Orders to replace the motor cut-off switches with new style switches that supply safety related loads actuated from Safety Injection and LOSP sequencers were entered into the CAP.
Additional Information
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