05000458/LER-2015-002
| River Bend Station - Unit 1 | |
| Event date: | 3-7-2015 |
|---|---|
| Report date: | 05-05-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4582015002R00 - NRC Website | |
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REPORTED CONDITION
On March 7, 2015, at 9:40 p.m., while the plant was in cold shutdown, power from the reserve station service (RSS) line no. 2 to the Division 2 onsite electrical distribution system was lost. The Division 2 diesel generator (EDG) received an automatic start signal due the under-voltage condition on the 4160v bus, but did not start since it was out of service for scheduled maintenance. The Division 2 standby service water (SSW) pumps were operating at the time for scheduled testing, and subsequently shut down when power was lost.
Operators implemented the appropriate response procedures and restored affected systems to service. Re-alignment of Division 1 SSW to carry the affected heat loads was completed by 10:34 p.m. The Division 2 standby switchgear was re-energized from the alternate source at 3:40 a.m. the next morning.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv) as valid actuation of the EDG starting logic.
INVESTIGATION
At the time of the event, work was in progress to install a new modification on reserve station service transformer "D" to comply with the industry initiative on open-phase conditions as described in NRC Bulletin 12-01, Design Vulnerability in Electric Power Systems.
All the work prior to the point at which the RSS tripped had been performed with the transformers de-energized under a clearance order.
On the day prior to the event, the clearance was released and the transformers were energized to start the post-modification test plan.
Before beginning the subsequent steps, the electricians conducted a pre-job brief that included a review of the schematic drawings of the circuitry being tested, as well as a review of roles and responsibilities of all those involved with the work.
Upon arrival at the transformer yard, the electricians performed a job site review to identify any changes in work conditions and other potential hazards. The following conditions were noted:
- All stationary lighting plants had been removed after previous modification work was completed, so the workers were wearing head lamps
- A conduit near the control panel blocked the full opening of the control cabinet door, thereby limiting access
- In close proximity to the electrical terminal being tested, there were abandoned trip circuitry wires where a 2009 modification had removed a sudden-pressure trip relay. The wiring for that relay had been retired in place.
An investigation team was assigned to determine the cause of the event. There were no other work activities in progress, other than verifying the trip circuitry in the control panel on "D" transformer. The team tried to re-create the alarm by resetting the RSS trip relay. The team went through various progressions that included shaking the terminal box, cabinet door, and other structures. Only when the team placed jumpers in contact with the top lugs between adjacent terminal points, where the abandoned trip circuit were near the adjacent terminal, did the RSS trip logic activate. Based on this demonstration, the team concluded that electricians must have made contact with the sudden-pressure trip circuitry wires while working in the cabinet.
CAUSAL ANALYSIS
The apparent cause of this event was inadequate work practices on the part of the electricians, in that they did not take all available precautions prior to performing the voltage check. The workers recognized the adverse conditions, but did not recognize the need to put into place any robust barriers. The electricians' successful past performance of this type of task likely led to overconfidence.
Reviewers of the work package didn't challenge the potential risks or identify a most error-likely task.
CORRECTIVE ACTION TO PREVENT RECURRENCE
A human performance error review was conducted with the contract electricians involved in this event and their supervisors. The contractor held an all-hands stand-down to reinforce human performance expectations.
PREVIOUS OCCURRENCE EVALUATION
No similar events have been reported by River Bend Station in the last 3 years.
SAFETY SIGNIFICANCE
The EDG start logic responded as designed to the loss of power on the Division 2 electrical systems. This event did not involve any interruption of the shutdown cooling function. This event was, thus, of minimal safety significance with respect to the health and safety of the public.