05000389/LER-2015-002
| St. Lucie Unit 2 | |
| Event date: | 09-17-2015 |
|---|---|
| Report date: | 11-16-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3892015002R00 - NRC Website | |
Description of the Event
On September 17, 2015, at 1222 hours0.0141 days <br />0.339 hours <br />0.00202 weeks <br />4.64971e-4 months <br />, with Unit 2 in Mode 5 at the beginning of a refueling outage, an electrical fault on the 2A 6.9 kV bus resulted in the loss of the 2A startup transformer (SUT) and its associated non-safety related 2A2 and safety-related 2A3 buses. At the time of the event, only the 2B emergency diesel generator (EDG) was required to be in service. The 2A EDG had been removed from service for scheduled maintenance, and did not start. The loss of the 2A SUT actuated the under-voltage relays that would have started the 2A EDG.
Additionally, the 2A train of shutdown cooling (SDC) was de-energized; the 2B (protected) train of SDC was not affected by the event and remained in service to remove decay heat. The 2A shutdown cooling train was restored and made available on September 19, 2015 at 0030. The 2B EDG and 2B SUT were not affected by the event.
During this event, Unit 1 experienced a loss of the lA startup transformer. There was no other effect on Unit 1 operation, as its associated non-safety and safety- related buses remained powered by the auxiliary transformer. The lA startup transformer was returned to service on September 18, 2015 at 2103.
Cause
The actuation of the 2A EDG was a direct result of the electrical fault on the 2A 6.9 kV bus. The root cause of the electrical fault was that the protective boots for a bus bar bolted connection, at a vertical riser section, were not installed properly from initial plant construction (legacy human performance error). Over time, contaminates built up on a boot between the "B" and "C" phases, creating a fault path.
Analysis of the Event
On 9/17/15, a differential relay trip of the lA/2A SUTs occurred separating the "A" train from off-site power. A phase-to-phase fault occurred on the 2A SUT 6.9kV non- segregated bus, between the "B" and "C" phases, that ionized the air in the vicinity of the fault and allowed a phase-to-phase-to-phase event. The initiator of the event was a legacy human performance issue where the protective boots were left off of the bolted connections and placed in between the phases. This allowed the collection of corrosion products over the years to create a path across one of the boots between the "B" and "C" phases.
Safety Significance
This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in automatic actuation of an EDG. The EDG had been previously removed from service and prohibited from starting; however, the actuation logic was satisfied when the undervoltage relays changed state.
The safety significance is minimal because of the preplanned defense-in-depth, the limited cause of the event (improper location of boot) and the prompt actions by station staff, which included restoration of the 2A train of SDC.
With no complications and all systems responding as designed, the associated risk impact is considered very small. Given the response of the plant and the insignificant risk, the health and safety of the public were not affected by this event.
Corrective Actions
Immediate corrective actions were extensive inspections and repair of the remaining vertical portions of this bus. These inspections included visual inspection, checking bus joint bolt torque, replacing insulating boots and megge-r testing.
The follow-up corrective actions listed below have been entered into the site corrective action program. Any changes to the action will be managed under the corrective action program.
1. Perform internal visual inspections of remaining vertical sections of non- segregated buses to ensure that bolted connections on risers have properly installed protective boots.
2. Perform internal inspections on the external buses as part of planned preventative maintenance.
Failed Component(s) Rectangular Nonsegregated Bus Manufacturer Cutler-Hammer