| 06-22-2005 | On December 10, 2004, at 1:17 p.m. CST, an automatic reactor scram occurred while the plant was operating at 100 percent power. The scram was the indirect result of the failure of a 120 volt AC uninterruptible power supply (UPS) on a non-safety related instrument bus. The UPS failed due to a shorted capacitor on an internal circuit board. The loss of the instrument bus resulted in the downshift of the reactor recirculation pumps to slow speed and the lockup of the main feedwater regulating valves. The decrease in coolant flow caused a flow-biased simulated thermal power signal to be sensed in the average power range monitoring system, which actuated the reactor protection system (RPS). The reactor core isolation cooling (RCIC) system was manually started in response to the failure of the feedwater regulating system. RCIC subsequently shut down as designed due to a high reactor water level signal. While it was idle, an alarm actuated indicating the presence of water in the RCIC turbine exhaust drain trap. A conservative decision was made to leave RCIC out of service and to start the high pressure core spray (HPCS) system as needed for reactor water level control. The reactor scram occurred as designed, and reactor water level was maintained above the low-low alarm setpoint. This event is being reported in accordance with 10CFR50.73(a)(2)(IV)(A) as a condition that resulted in the actuation of the RPS, RCIC, and HPCS systems. This event was of minimal safety significance. |
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| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 12-10-2004 |
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| Report date: | 06-22-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(IV)(A) |
| 4582004005R01 - NRC Website | |