05000277/LER-2001-001
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(vi) |
| 2772001001R00 - NRC Website | |
Unit Conditions Prior to the Event Both Unit 2 and 3 were in Mode 1 and operating at approximately 100% rated thermal power when the event occurred. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On June 18, 2001 at approximately 0315 hours0.00365 days <br />0.0875 hours <br />5.208333e-4 weeks <br />1.198575e-4 months <br />, the 343SU-E offsite power source (EIIS:FK)experienced an electrical fault causing it to trip. Two emergency 4160 VAC buses on both Units 2 and 3 transferred to their respective alternate power supply (2SU-E offsite power source). As a result, an invalid actuation of primary containment isolation valves in multiple systems occurred as expected for the loss of power.
The invalid actuation of primary containment isolation valves occurred in the following systems on both Units 2 and 3 (EIIS:JM): Reactor Water Cleanup, Drywell Equipment and Floor Drain isolation valves, Nitrogen Supply to the Drywell, and the TIP purge valve. All systems responded as expected for a loss of power and were restored to their normal configuration by the operating crew.
An unplanned, invalid actuation of a specified system meets the reporting requirements of 10CFR50.73(a)(2)(iv).
After the affected emergency buses transferred to the 2SU-E emergency power source, the operating crew entered procedure "Response to a Loss of #343 Off- site Startup Source." The procedure directed the crew to "flag" all "tripped" breaker control switches to their actual position; however it did not address the alternate breaker control switches. This resulted in the normal power supply (tripped) breaker control switches being taken to open and the alternate power supply breaker control switches being left in the open position (with actual breaker position being closed). With both the normal and alternate breaker control switches in the open position, an interlock would have prevented the emergency diesel generator (EIIS:EK) associated with two emergency 4160 VAC buses on each unit from automatically supplying power to their respective buses in the event of a loss of offsite power/LOCA. This condition existed for approximately three hours, when the operating crew took the control switch for the alternate breaker to the closed position.
Peach Bottom has four emergency buses per Unit. As a result of the procedure deficiency, two of four emergency buses per unit would not have automatically received power from their respective emergency diesel generators in the event of a loss of offsite power.
The inability of two emergency diesel generators to automatically provide power to their respective emergency buses could have prevented the fulfillment of a safety function to mitigate the consequences of a Loss of Offsite Power/Loss of Coolant Accident. This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(D) and 10CFR50.73(a)(2)(vii).
Analysis of the Event
Each unit had diesel generators available to two of four emergency buses.
Each emergency bus supplies power to both a RHR and a Core Spray pump. In the event of a Loss Of Offsite Power concurrent with a Design Basis Loss of Coolant Accident, each unit would have had one completely operable loop of both core spray and RHR to supply cooling water flow to the reactor.
Additionally, emergency and abnormal operating procedures would have directed the operating crew to restore power to any buses that were de-energized. Bus restoration could have been performed by matching the breaker switch "flag" on the alternate power supply breakers to actual breaker position. Once the "flag" was matched the diesel generators would have supplied power to their respective buses.
This event resulted in a minimal change in core damage frequency. The condition existed for approximately three hours when the operating crew recognized the discrepancy and properly aligned the breaker control switches.
This event did not impact the initiating event cornerstone or the barrier cornerstone; however, since the emergency diesel generators are mitigating systems, the mitigating system cornerstone was affected. Each unit had two available diesel generators at all times and some credit may be given for the recovery of the inoperable trains based on the ability of the operating crew to restore function in a timely manner.
Cause of the Event
The cause of the loss of 343SU-E offsite power source, which resulted in several invalid specified system actuations, was a raccoon coming in contact with an insulator, which caused a phase-to-ground trip on the bus.
The cause of the loss of safety function of two emergency diesel generators per unit was a latent procedural error, which directed the operator to align the breaker control switches for tripped breakers, only. This created a condition in the diesel generator logic, which would have prevented the diesel generator from automatically supplying power to its respective bus.
This affected two of four emergency buses on each unit.
Corrective Action Completed Further investigation determined that this particular procedural deficiency existed in three different procedures that provide direction for responding to the loss of an offsite power source. All three procedures have been temporarily changed to require that both the tripped breaker and the closed breaker control switches be flagged to match breaker position.
Corrective Actions Planned Permanent changes to the three procedures will be processed.
Previous Similar Occurrences None