Due to failure of an instrument power inverter (NN11) which supplies vital 120Vac bus NN01, Callaway Plant entered L.C.O. 3.8.7, Electrical Power Systems, Inverters - Operating, Condition A at 0721 on October 20, 2003. The inverter supplying the bus is required to meet the LCO. Since the initiation of the inverter failure, the vital instrument bus has been energized from alternate power supply transformers in accordance with plant design and procedures. It is not an acceptable alternative to meet the LCO OPERABILITY requirement.
Details of the impact on the plant due to this failure are detailed in Corrective Actions Program Report (C.A.R.) 200307636. In summary, the bus voltage was lost momentarily, then restored by operation of the inverter static transfer switch. The failure and subsequent restoration of power to AC Vital Bus caused some control systems responses that were corrected by the control room staff. After the event was stabilized, power remained at 100%.
The LCO Action is to restore the inverter to OPERABLE status within twenty four (24) hours, otherwise be in MODE 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> AND be in Mode 5 in thirty six (36) hours. The twenty four hour period ends at 0721 October 21, 2003. In advance of the action time, a controlled plant shutdown to Mode Three (3) was commenced at 0100 on October 21, 2003 at a rate of ten (10) percent per hour. At the time of this report, maintenance efforts have determined that the problem is internal to the inverter Static Transfer Switch, therefore the reduction of power will continue.
No safety system actuations resulted from the control power transfer. The licensee has notified the NRC Resident Inspector.
