05000255/LER-2008-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No | |
| Event date: | 01-13-2008 |
|---|---|
| Report date: | 0-0-3010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2552008001R00 - NRC Website | |
tripped unexpectedly. Operators entered the loss of main feedwater procedure and appropriately carried out the procedurally directed action for loss of a main feed pump at >80% power, which is to manually trip the reactor [RCT;AB]. As expected, the auxiliary feedwater system [BA] started automatically to recover steam generator [SG;AB] level.
The event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in an actuation of both the reactor protection system [JC] and the auxiliary feedwater system.
CAUSE OF THE EVENT
The immediate cause of the main feedwater pump trip was low lube oil pressure. The low lube oil pressure resulted from the loss of the main feedwater pump's shaft-driven lube oil pump, which occurred when its drive coupling became disengaged.
The drive coupling is a vertically oriented sleeve, which mates upper and lower drive hubs. The inner surface of the sleeve is machined with splines to mesh with teeth that are machined on the outside surface of the drive hubs. The upper hub is secured to its shaft via an interference fit.
The lower hub is not an interference fit, and was previously secured by a key with a projection, which served to maintain the hub in vertical position by preventing the hub from sliding down its shaft past the projection.
Investigation revealed that the installed key for the lower hub did not have the desired projection.
At an unknown time in the past, the lower hub key was either replaced with one that had no projection, or the original projection on the key was removed. Pump component descriptions and drawings did not show the desired shape of the key, which allowed an incorrect key for this application to be installed. Without the correct key, the lower hub was able to slide down the shaft, making only partial engagement with the sleeve.
The partial tooth engagement between the lower hub and sleeve was not sufficient to withstand the shaft torque, which ultimately resulted in stripping the splines on the coupling sleeve and decoupling the lube oil pump's shaft drive. The partial engagement configuration existed from October 30, 2007, when the lube oil pump was installed and the main feedwater pump was placed in service, until its failure on January 13, 2008.
lower hub to prevent the hub from sliding down the shaft and losing engagement with the coupling sleeve.
The feedwater pump work instructions will be upgraded to include detailed vendor drawings and information.
SAFETY SIGNIFICANCE
The event is considered to be of very low safety significance. All safety systems functioned as expected.
PREVIOUS SIMILAR EVENTS
None