05000454/LER-2015-002
| Byron Station, Unit 1 | |
| Event date: | 03-03-2015 |
|---|---|
| Report date: | 05-01-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4542015002R00 - NRC Website | |
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A. Plant Condition Prior to Event Event Date/Time: March 3, 2015 / 1101 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.189305e-4 months <br /> CST Unit 1 - Mode 1 - Power 100 percent Reactor Coolant System [AB]: Normal operating temperature and pressure. There was no inoperable equipment that contributed to this event.
B. Description of Event
At 1101 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.189305e-4 months <br /> on March 3, 2015, Byron Station Unit 1 tripped from full power due to a phase to phase fault on the 1 E Main Power Transformer (MPT) between electrical Phase A and Phase B. Before this trip, the Byron area had experienced severe weather. At the time of the event, the temperature was approximately 30 degrees F; however, temperatures had been around 23 degrees F hours earlier. An ice formation on the Phase B Bus bar that is located directly above the high voltage bushings dropped and caused a phase to phase fault between Phase A and B. No equipment malfunctions existed that complicated the post reactor trip response and all plant systems responded to the event as designed.
The 1 E MPT protective relays actuated to trip the transformer as designed. Following the trip, an inspection of the associated MPT revealed that the top of the Phase A bushing was charred and the top of the Phase B bushing was charred with two holes in it. Additionally, a portion of the corona ring for both Phase A and B had melted. An approximate five foot section of ice was found on the ground below.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) for any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B).
C. Cause of Event
The cause of this event was due to latent design vulnerability that existed in the configuration of the Bus bar relative to the MPT bushings. The Bus bar location directly above and parallel to the MPT bushings created a condition that resulted in a phase to phase short when ice from the Bus bar fell across the bushings. The Bus bar is six inches in diameter and located approximately twenty-three feet above the top of the bushings.
D. Safety Significance
This event is not considered an event or condition that could have prevented fulfillment of a safety function. The results of the risk assessment of this event indicate that the Unit 1 trip was not a risk significant event. The equipment operated as designed to take the plant to a safe condition.
At the time of 1 E MPT phase to phase trip, Unit 1 and Unit 2 online risk was green. All of the mitigating systems responsible for responding to an accident scenario were available to perform their function. All of the fission product barriers (cladding, reactor coolant system, containment) were in no danger of compromise throughout the event. In addition, the event was within the assumptions of the probabilistic risk assessment (PRA) model, with no anomalous PRA equipment occurrences affecting core damage frequency. No Emergency Action Level threshold in accordance with the Emergency Plan was met and no emergency classification was declared during the event.
E. Corrective Actions
The 1 E MPT high voltage and neutral bushings were replaced during the forced outage. Following successful testing of the 1 E MPT, Byron Unit 1 was restarted.
The corrective action to prevent recurrence is to implement a design solution to eliminate the vulnerability associated with the Bus bar being located directly above the MPT high voltage bushings and reduce the potential for a phase to phase fault from an ice fall.
Extent of Condition — All of the large outside transformers at Byron Station were reviewed and only the MPTs on both units have a vulnerability to a phase to phase fault due to ice falling from the Bus bar. The extent of condition associated with ice falling with the potential to cause a unit trip or derate is limited to the MPTs and select switchyard equipment.
F. Previous Occurrences
Byron Station Unit 1, February 17, 1986 at 0809 hours0.00936 days <br />0.225 hours <br />0.00134 weeks <br />3.078245e-4 months <br /> (LER 454-86-008-00). A reactor trip occurred due to a Phase A to Phase B fault on Bus 4 in the switchyard due to ice falling from a transmission line tower that caused Unit 1 to trip. Historical weather data was obtained that indicates the temperature was approximately 30 degrees F at the time of the event. A potential transformer was damaged in the switchyard and replacement was required to restore operation.