Event Number: 47795
“At 0624 EDT on 04/02/2012 the control room was notified by a plant operator that there was a fire in the plant vital battery room area on Unit 2. The fire was located in the lower compartment of inverter 2KUI.
This equipment supplies power to 120V AC non-safety related components. The fire brigade captain was dispatched along with the site fire brigade. When the fire brigade captain arrived the operator was staged near the inverter with a CO2 [carbon dioxide] fire extinguisher. The fire brigade captain instructed the NLO [non-licensed operator] to extinguish the fire with the extinguisher. The fire was immediately extinguished once the CO2 was applied from the extinguisher. At 0633 EDT the fire brigade captain reported to the control room that the fire was extinguished. Upon further investigation it appears that transformer in the lower compartment of the inverter had caught fire.
The fire was confined to the lower compartment of the inverter and did not affect any other components in the area. Due to the loss of the inverter the loads automatically swapped to the alternate AC source as designed and no indications or equipment were lost due to the loss of the inverter.
“At the time of this event Unit 1 was at 100% power and Unit 2 was shutdown in Mode 6 with reactor coolant system drained to 8% level with 2B Residual Heat Removal pump supplying core cooling.
There was no impact to either unit as a result of this event. All equipment required to maintain core cooling, AC and DC power, and reactor coolant inventory on Unit 2 was maintained during this event. [emphasis added]“
The licensee notified the NRC Resident Inspector.
An electrical fire that affects Control and Instrument Power to many plant systems is not trivial. Although, the 2KUI Inverter feeds non-safety related loads, Unit 1 and 2 was impacted by the loss of the Inverter. Unit 1 which was at 100% power is now shutdown for damage assessment, however, Unit 2 is was more vulnerable. This is because it was in a condition known as Reduced Inventory.
Reduced Inventory is where the reactor water level was lowered to approximately the middle of the Hot Leg to facilitate maintenance activities. While in Reduced Inventory, only one Residual Heat Removal Loop is in operation to prevent vortexing. If RHR were lost, then an uncontrolled Heat Up of the fuel in the Reactor could have occurred and lead to a serious condition.
Lastly, it is not apparent how quickly the fire was burning before it was discovered. There was a potential for the fire to destroy other vital plant equipment.
The accompanying drawings are from a similar plant. The Inverter-Vital Battery configuration is shown. The Vital AC system affects Unit 1 and Unit 2.
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