05000280/LER-2010-003
| Docket Ni Imfiersequential Rev | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2802010003R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On June 8, 2010 at 0948 hours0.011 days <br />0.263 hours <br />0.00157 weeks <br />3.60714e-4 months <br />, with Units 1 and 2 at 100% power, Unit 1 120 VAC Vital Bus 1-III [EIIS-EF-BU] was lost when the Uninterruptible Power Supply (UPS) [EllS-EF-UJX] VB 1A-2 static switch swapped from the inverter to the Regulating Line Conditioner (RLC) [El IS-EF-90] (the alternate AC source), which was tagged out for on-going maintenance. The static switch from the inverter [El IS-EF-ASU] swapped to the RLC because a worker dropped a lead during the maintenance. As a result, the Main Feedwater Pump Recirculation Valve A [El IS-SJ-XCV] opened and Main Feedwater Regulating Valves (MFRV) [EIIS-SJ-FCV] B and C aligned to automatic-hold mode of operation. MFRV A, which also should have aligned to automatic-hold mode, closed due to an equipment malfunction. With this reduction in main feedwater flow, a Unit 1 automatic reactor trip occurred at 0949 hours0.011 days <br />0.264 hours <br />0.00157 weeks <br />3.610945e-4 months <br /> due to a feed flow/steam flow mismatch in conjunction with low steam generator (SG) [EllS- BA-SG] level.
Both Motor Driven Auxiliary Feedwater Pumps [EllS-BA-P] and the Turbine Driven Auxiliary Feedwater Pump automatically started, as expected.
At 0950 hours0.011 days <br />0.264 hours <br />0.00157 weeks <br />3.61475e-4 months <br />, Safety Injection (SI) [El IS:BQ] initiated due to high steam flow in conjunction with the low Reactor Coolant'System (RCS) [EIIS-BA] average temperature. The false indication of high steam flow signal, caused by the loss of Vital Bus 1-III instrumentation, satisfied the required logic for high steam flow for all three steam lines. The loss of the Vital Bus also caused the loss of one channel of RCS temperature input into the SI logic. An additional low RCS temperature signal completed the logic for SI. During the operation of the main steam dump valves, the RCS momentarily cooled below the average no load temperature of 547°F and the SI setpoint for RCS temperature (543°F), completing the coincidence for the high steam flow with low RCS average temperature automatic SI logic. The high steam flow with RCS Tave matrix also closed the Main Steam Trip Valves [El IS-SB-TCV] and opened the Refueling Water Storage Tank [El IS-BE-TK] cross-tie valves. The SI actuation resulted in automatic start of Emergency Diesel Generators (EDG) [El IS-EK-DG] #1 and #3; however, the EDGs were not required since off-site power remained operable. The Component Cooling return trip valve [EllS-CC-ISV] for Reactor Cooling Pump (RCP) 1A also closed due to the loss of the Vital Bus and RCP 1A [EllS-AB-P] was procedurally secured.
In addition, numerous field inputs to the Plant Computer System (PCS) [EllS-ID] were lost and resulted in non-functionality of the Safety Parameter Display System (SPDS) [EllS-ID]. The PCS, Main Control Room (MCR) annunciators [ENS-NA-ANN], and sufficient MCR instrumentation remained functional to monitor critical safety functions.