05000389/LER-2013-003
| St. Lucie Unit 2 | |
| Event date: | 06-10-2013 |
|---|---|
| Report date: | 08-08-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3892013003R00 - NRC Website | |
Description of the Event
On June 10, 2013, St. Lucie Unit 2 was in Mode 1 at 100% reactor power. During the monthly surveillance run of the 2A Emergency Diesel Generator (EDG), the EDG did not start when the local idle start pushbutton was depressed. This was accompanied by Control Room and local alarms. The 2A EDG was subsequently declared inoperable.
Troubleshooting revealed an electrical short circuit caused by an uninsulated butt splice which caused a fuse in the EDG control circuit to clear. The butt splice was at a diode and arced across the coil mounting plate for the priming pump contactor.
Cause
The cause that prevented the 2A EDG from being locally started during the monthly 2A EDG surveillance run was an uninsulated butt splice that shorted to the fuel priming pump contactor coil mounting plate (magnet frame), which caused the fuse to clear due to an actual overcurrent condition. The uninsulated butt splice was a legacy condition from the original installation of the 2A EDG.
Analysis of the Event
The EDGs are an emergency power source. Upon a loss of the normal power sources, the tie breakers between the normal and emergency buses will automatically open and the EDG will automatically start and begin supplying power directly to the emergency buses. Similarly, in the event of a loss of offsite power (LOOP) in conjunction with an emergency signal, the EDG will restore power to the emergency bus within 10 seconds and all required loads to mitigate the accident condition will be loaded onto the EDG in a sequential fashion (3 second time loading intervals).
The uninsulated butt-splice configuration of the fuel priming pump diode resulted in spurious intermittent contact between energized components on the 2A EDG control circuit leading to a short circuit that caused the fuse to clear on March 13, 2013, and again on June 10, 2013.
Troubleshooting for the March 13, 2013 event found the cleared fuse. The symptoms were not repeatable following replacement of the fuse. This and operating experience for the fuse led the station to determine that the fuse had intermittently failed. Subsequently, the 2A EDG was started successfully four more times before it failed to start on June 10, 2013.
The fuel priming pump contactor energizes the 2A EDG Fuel Pumps upon actuation and the subject diode is installed across the contactor coil to suppress inductive kick following deenergization of contactor. The proximity of the diode's uninsulated butt-splice to the contactor coil plate (magnet frame) afforded the opportunity for a short to occur between these components when the contactor was electrically operated. The originally installed contactor was manufactured such that the coil mounting plate was electrically floating, not at any voltage potential. Since 2008 the replacement parts have had a new design that replaced a nonconductive part with a metallic part and caused the coil mounting plate to be at line voltage potential (i.e., in this application 125V DC positive). The most recent contactor replacement was installed on September 7, 2012.
The remaining EDGs were inspected to verify that they did not have this legacy issue of an uninsulated butt-splice in a similar configuration.
Safety Significance
Between the previous event on March 13, 2013, and the recent event on June 10, 2013, the 2A EDG had been successfully started four times. The 2A EDG was returned to service on June 11, 2013, following replacement of the defective component.
From the probabilistic risk assessment perspective, the 2A EDG was available to perform its design function during the exposure window with the exception of the actual failure times in March and June of 2013. The failure rate resulted in an increase of baseline risk of less than 1E-06.
This condition unknowingly existed for greater than the allowable Technical Specification action time of 14 days per Technical Specification 3.8.1.1.b and is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). During the period of 2A EDG inoperability, the opposite train 2B EDG was unavailable during three routine surveillances on March 25, April 22 and May 28. Had this intermittent failure occurred while the 2B EDG was unavailable during one of these three monthly surveillances, this condition could have prevented the fulfillment of a safety function. Therefore, this event is also reportable pursuant to 10 CFR 50.73(a)(2)(v)(A) & (D). This event had no effect on the health and safety of the public. During this period the lA EDG, 1B EDG and station blackout crosstie were available to supply Unit 2 with emergency AC power if required.
Corrective Actions
1. To correct the problem, the diode with the uninsulated butt-splice was replaced, and the new diode has been insulated.
Similar Events A search of the St. Lucie corrective action database for three years was performed and identified no issues that were related to the faults identified with this report.
Failed Component(s) Fuel Priming Pump Contactor Manufacturer The contactor was manufactured by Square-D Company, now a subsidiary of Schneider Electric.
Engine Systems, Inc. is the Emergency Diesel Generator supplier.