05000244/LER-2011-001
| R.E. Ginna Nuclear Power Plant | |
| Event date: | 08-11-2011 |
|---|---|
| Report date: | 10-04-2011 |
| 2442011001R00 - NRC Website | |
I. DESCRIPTION OF EVENT
A. PRE-EVENT PLANT CONDITIONS:
The reactor was in Operational Mode 1 at 100% power, 2235 psig and 574 degrees F.
B. EVENT:
Both trains of Non-Safety Related 4160V station power are located in the same area of the Turbine Building. During normal plant operation, these busses supply the 480V transformers for the safeguards busses located in separate areas of the Auxiliary Building. A 4160V circuit breaker is on the primary side of the transformer and a 480V circuit breaker is on the secondary side, providing normal power. The control circuit for the 480V normal supply breaker is interlocked with the 4160V breaker to prevent the 480V breaker from closing unless the 4160V breaker is closed, and causes the 480V breaker to trip when the 4160V breaker trips. A control cable passes from the 480V breaker control circuit to the 4160V circuit breaker to form these interlocks. The 480V bus diesel generator supply breaker is also interlocked with the 480V normal supply breaker to prevent the diesel generator from automatically loading onto the bus if the 480V normal supply breaker is closed.
A hypothetical fire in the Turbine Building has the potential to cause a loss of 4160V power to both busses and a short in the control cable to the 480V normal supply breaker. This results in a loss of control power to the 480V breaker because the control power fuses are assumed to open.
If the control power is lost prior to the 4160V power, then the normal supply breaker will not trip on under-voltage and the diesel generator bus supply breaker will be unable to automatically close onto the bus. This condition could affect both trains of safeguards power. Any attempt to close diesel generator supply breakers from the control room or locally without first opening the normal supply breaker would result in back feeding the transformer, potentially tripping the diesel generator output breaker on over current conditions.
The same fire was previously assumed to disable DC control power to the Turbine Driven Auxiliary Feedwater (TDAFW) pump, rending the pump unavailable for decay heat removal. The combination of a Station Blackout and loss of the TDAFW pump is an unanalyzed condition with significant safety impact.
Compensatory measures have been established to provide guidance to operators to check the control power indication for the 480V normal supply breaker prior to attempting to close the diesel generator supply breaker. If the control power is extinguished, direction is provided to locally trip the normal supply breaker prior to closing the diesel generator supply breaker. This guidance was incorporated into equipment restoration procedures.
As part of the NFPA-805 project, a fire risk evaluation will be performed to determine if any further corrective actions are required.
appears to be an inaccurate analysis of interlock effects in the Appendix R Safe Shutdown analysis.
Additional corrective actions will be evaluated upon completion of the NFPA-805 Fire PRA and development of a Fire Risk Evaluation. Compensatory measures will remain in effect until this evaluation is complete.
III ANALYSIS OF THE EVENT:
This event is reportable in accordance with 10 CFR50.73, Licensee Events Report System under item (a)(2)(ii)(B) based on the plant being in an unanalyzed condition that significantly degraded plant safety.
An assessment was performed considering both the safety consequences and implications of this event with the following conclusions:
This hypothetical scenario could have resulted in a Station Blackout with no auxiliary feedwater to provide a secondary heat sink. However, there were no adverse safety consequences that result from the given scenario, as there was no actual fire in the affected area. Administrative controls, availability of fire detection and suppression systems, and a trained on-site fire brigade make it unlikely that a credible fire would result in the circuit failure combinations in the required sequence for the hypothetical scenario to occur. In the event it did occur, the plant has emergency damage mitigation guidelines for beyond design basis fires that would restore DC power to the TDAFW pump, minimize the impact of the event and provided additional time for restoration of power.
Restoration of AC power could be accomplished within minutes of recognizing the cause and could be implemented by operators without additional site support. Since no damage to the diesel generators, their output breakers, or control circuits is postulated to occur as a direct result of the fire, the likelihood of restoration is high.
Compensatory measures include standing orders to raise operator awareness of the issue, identify available indication, and specify required actions. Procedure changes have been implemented for equipment restoration procedures that mitigate the condition. As part of the NFPA-805 Project, a Fire Risk Evaluation will be developed upon completion of the Fire PRA model to determine if other corrective actions are warranted.
This event does not have any impact on NRC performance indicators.