05000335/LER-2006-001
| Docket Numbersequential R Bi Eftnu Nenasmonth Day Year Year Month Day Year | |
| Event date: | |
|---|---|
| Report date: | |
| 3352006001R00 - NRC Website | |
Description of the Event
While performing a surveillance run of the turbine driven 1C AFW pump [EIIS:BA:PP] on January 5, 2006, with St. Lucie Unit 1 in Mode 1 at 100 percent power, the mechanical overspeed trip mechanism [EIIS:BA:12] actuated and tripped the turbine. The trip mechanism is designed to trip the turbine when the speed exceeds the setpoint of 4500 rpm. This mechanism is a backup to the governor speed control and the electrical overspeed trip device. The mechanical overspeed mechanism must also remain latched during automatic starts in order for the AFW to perform its design basis functions.
The AFW pump was running stable, at a rated speed of 3600 rpm just prior to the trip.
Operators reported there was no noticeable increase in speed nor was the electrical overspeed trip coil actuated. Following the trip it was observed that the mechanical trip tappet nut was in the tripped position, which indicated a potential problem with the mechanical overspeed trip mechanism.
In response to this event a root cause team was convened to determine why the trip occurred, corrective actions to prevent recurrence and generic implications.
Cause of the Event
The Root Cause Team concluded the turbine trip was a result of the AFW turbine not being properly latched following a previous surveillance on the AFW turbine completed on December 16, 2005.
A latent procedural weakness existed such that clear direction for re-latching and confirming that the turbine overspeed mechanism was properly latched was not provided with the surveillance procedure. In addition, binding of the tappet nut in the head bracket bore enabled the improper latching of the overspeed mechanism. Prior to the event (December 16, 2005) the AFW turbine was latched following performance of a surveillance (Auxiliary Feedwater Periodic Test, 1-0700050, Data Sheet C). It is postulated that following the turbine trip of December 16, 2005 in accordance with the surveillance procedure, the head lever and emergency trip rod had not been sufficiently retracted to permit the tappet to fully drop to the seated position.
Binding of the tappet nut occurred which allowed the overspeed mechanism to remain improperly latched, while giving the appearance of being fully engaged. This condition remained unnoticed until the unexpected trip.
The overspeed trip alarm not being optimally adjusted to alert operators to a improperly latched condition and insufficient training also contributed to the event.
Analysis of the Event
This event is reportable under 10 CFR 50.73(a)2)(i)(B) as "any operation or condition which was prohibited by the plant's Technical Specifications..." because the trip mechanism was improperly reset for 20 days, exceeding the TS AOT of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
Investigation of the event revealed that without clear guidance on how to reset the overspeed mechanism, Operations only performed a cursory check to ensure the mechanical overspeed mechanism was securely latched.
Inspection of the overspeed mechanism found that the tappet nut was binding in the bore of the head bracket. Based on further reviews, differences in manufacturing of St. Lucie Unit 1 05000335 the tappet nut may have caused the tappet nut to bind. However, further troubleshooting determined that binding of the tappet nut would not have prevented the correct or proper latching of the mechanism if the pump was required after the trip.
Although Unit 1 and Unit 2 are of the same design, the only degraded condition that is related to the current event for Unit 2 was an AFW pump failure to trip due to a binding tappet nut during testing. Corrective actions are being tracked in the site corrective action program. All identified corrective actions for Unit 1 will be applicable to Unit 2.
Analysis of Safety Significance Given the availability of the other two AFW pumps and the ability of the operators to correct the tripped condition within analytical bounds and time frames, the identified pump failure had minimal impact on the health and safety of the public. The assumed fault exposure time is 20 days based on the time since the last successful start. It is concluded that the identified failure mode would not have prevented the operator from re-latching the mechanical overspeed trip device and starting the pump. The estimated non-recovery probability is 3.47E-02. The change in risk over the 20-day exposure time is approximately 4.33E-08. A change in risk of less than 1E-06 is considered not risk significant.
Corrective Actions
The proposed corrective actions and supporting actions listed below are entered into the site corrective action program. Any changes to the proposed actions will be managed under the commitment management change program.
1. Corrective actions have been taken to address the procedural deficiencies identified in latching and replacement of the tappet nut (Complete).
2. The tappet and nut have been sent to the manufacturer for failure analysis. Upon receipt and review of analysis, determine any additional corrective actions that would be necessary (7/21/06).
3. Adjustments have been made to the overspeed trip switch to provide annunciation if the overspeed mechanism is not properly latched (Complete).
Similar Events A review of the condition report database since 1995 was performed to identify similar events. Although there were multiple instances of AFW turbine trips due to actual overspeed found, there was no history of similar events, i.e., where improper latching of overspeed trip mechanism resulted in inadvertent trips or functional failures related to the overspeed trip mechanism on either unit.
Failed Components
None