05000454/LER-2014-003
| Byron Station, Unit 1 | |
| Event date: | 03-15-2014 |
|---|---|
| Report date: | 05-14-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4542014003R00 - NRC Website | |
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A. Plant Condition Prior to Event Event Date/Time: March 15, 2014 / 1102 hours0.0128 days <br />0.306 hours <br />0.00182 weeks <br />4.19311e-4 months <br /> CST Unit 1 - Mode 6 - Refueling, Reactor Power 0 percent Reactor Coolant System: Ambient Conditions
B. Description of Event
On March15, 2014, at 1102 hours0.0128 days <br />0.306 hours <br />0.00182 weeks <br />4.19311e-4 months <br />, Byron Station Unit 1 experienced a Loss of Off-site Power (LOOP) event.
The event occurred during refuel outage B1R19 with Unit 1 in Mode 6 during reactor fuel offload activities.
Both Unit 1 Emergency Diesel Generators (DGs) auto-started and re-energized the safety related buses as designed. Coincident to the LOOP, Operational Analysis Department (OAD) technicians were performing relay calibrations for the Station Auxiliary Transformer (SAT) buses in accordance with approved work instructions. During the activity, the plant received a SAT differential relay actuation that initiated a trip and lockout of the Unit 1 SAT feed breakers, thereby, resulting in a Unit 1 LOOP with subsequent DG auto-start.
OAD personnel observed that the SAT differential relay trip occurred during the installation of the over- current relay. A potential faulty test switch caused a charge to build up in the energized-open circuit. An electrical discharge occurred when the over-current relay was inserted, resulting in a trip of a differential relay and lockout of the SAT feed breaker.
C. Cause of Event
The cause of the LOOP with subsequent DG auto-start was indeterminate. The most probable cause was a combination of equipment failures involving a faulty test switch that caused a charge to build up in the energized-open circuit. There was a subsequent electrical discharge when an over-current relay was inserted in the circuit, thereby tripping one of the differential relays and locking out the SAT feed breaker.
D. Safety Significance
This event is not considered an event or condition that could have prevented fulfillment of a safety function.
A Risk Management deterministic review/judgment concluded that there were no actual safety consequences to this event. For potential accident conditions, it is normally assumed that a LOOP occurs and that the DGs supply the safety related buses. This assumption bounds the event that occurred. The 1A and 1B DGs started and performed their safety function as designed. The Unit 1 safety related buses could have been powered by their respective DGs or the Unit 2 safety related buses, which were also capable of being powered by either their offsite sources or their respective DGs.
2. DOCKET S. LER NUMBER 1 3. PAGE 1. FACILITY NAME
E. Corrective Actions
Troubleshooting was performed and offsite power was restored to Byron Unit 1 safety related Buses 141 and 142 on March 15, 2014 at 2033 and 2115 hours0.0245 days <br />0.588 hours <br />0.0035 weeks <br />8.047575e-4 months <br /> respectively. The Diesel Generators were shutdown per approved procedures following restoration of normal safety related bus configuration.
F. Previous Occurrences
1. Licensee Event Report (LER) 455-2012-001-00, "Unit 2 Loss of Normal Offsite Power and Reactor Trip and Unit 1 Loss of Normal Offsite Power Due to Failure of System Auxiliary Transformer Inverted Insulators,' dated March 30, 2012. This LER involved an actuation of the Emergency DGs following loss of offsite power when switchyard porcelain insulators failed.
2. Licensee Event Report (LER) 455-2012-001-00, "Unit 2 Manual Reactor Trip During power Ascension Due to Steam Generator Level Approaching Turbine Trip Setpoint Caused by an Overly Complex Startup Procedure, " dated April 6, 2012. This LER involved a Unit 2 manual reactor trip and Auxiliary Feedwater System actuation.
3. Licensee Event Report (LER) 455-2013-001-00, "Unit 2 Manual Reactor Trip Due to Loss of Main Generator Stator Cooling Water," dated may 17, 2013. This LER involved the actuation of the reactor protection system following manual trip of the Unit 2 reactor.
4. Licensee Event Report (LER) 454-2014-001-00, "OA Essential Service Water (SX) Makeup Pump Unexpected Auto Start during OB SX Pump Monthly Surveillance, dated March 24, 2014. This LER involved the auto-actuation of the OA SX Make-up Pump while lowering water level in the OB SX Cooling Tower basin.
A review of these LERs concluded that the causes and corrective actions taken would not have been expected to prevent this event.