05000395/LER-2006-003
| Virgil C. Summer Nuclear Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3952006003R00 - NRC Website | |
PLANT IDENTIFICATION
Westinghouse — Pressurized Water Reactor
EQUIPMENT IDENTIFICATION
'B' Emergency Diesel Generator (DG) 'B' Emergency Feedwater Pump (BA) 'B' Charging Pump (BQ) Undervoltage Relays (27) 'B' Emergency Bus (EK) Supplemental Instrument Air Compressor (IA) 'B' Hydrogen Analyzer (BB)
IDENTIFICATION OF EVENT
While performing Surveillance Test Procedure STP-125.021, "Periodic Testing of The Alternate AC Power Supply," the 'B' Diesel Generator (DG) started on undervoltage as the 'B' Emergency Feedwater Pump was started. This is a reportable event under 10 CFR 50.72(b)(3)(iv)(A) and 10 CFR 50.73(a)(2)(iv)(A).
EVENT DATE
11/09/2006
REPORT DATE
01/08/2007
CONDITIONS PRIOR TO EVENT
Mode 5, Refueling
DESCRIPTION OF EVENT
VCSNS was shutdown, post refueling, in Mode 5, prior to plant restart. At 1444 on 11/09/06, after initial installation of the Alternate AC Power Supply, post modification testing was being performed in accordance with STP-125.021, "Periodic Testing of The Alternate AC Power Supply." The stated purpose of STP-125.021 is to demonstrate the capability of the Alternate AC Power Supply to supply one train of ESF equipment. During the test the 'B' DG was in the standby mode and bus power was being supplied by the Alternate AC Power Supply. As individual loads were manually loaded on the 1DB bus, bus voltage momentarily decreased as each load was started. When the 'B' Emergency Feedwater Pump was started, an undervoltage condition occurred on the bus that was sufficient to pick up the undervoltage (UV) relays, (27-1DB) starting the 'B' DG. The 'B' Engineered Safety Features Actuation System load sequencer was de-energized prior to the test as required by STP-125.021 and, therefore, the DG output breaker did not close onto the 1DB bus. Undervoltage conditions were reached on other pump starts also. Voltage on the 1DB bus was restored by the Alternate AC Power Supply after each loading sequence, all loads were successfully started, and the test was completed satisfactorily. After the DG started it was allowed to continue running for the remainder of the surveillance test, approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 20 minutes.
CAUSE OF EVENT
The procedure in use, STP-125.021, placed the DG in standby mode during the performance of the test. This mode enabled the DG to start on a valid start signal. As the loads were manually sequenced onto the 1DB bus, the pump starting current would draw the voltage down on the bus. Starting currents for the large pump loads were sufficient to lower the voltage enough to energize the UV relays, thus starting the DG on low voltage, when the Emergency Feedwater Pump was started.
Aligning the DG to prevent an inadvertent start during performance of the test would have prevented it from starting when low voltage conditions occurred.
ANALYSIS OF EVENT
While the plant was in Mode 5 with decay heat being removed by 'A' train components, STP-125.021 was performed using 'B' train in order to validate the function of the Alternate AC Power Supply. As individual large loads were loaded on the bus the voltage was observed to decrease momentarily during motor starts. However, Design Engineering has determined that the bus voltages were sufficient for operation of the bus. The Alternate AC Power Supply was functional. It met all its test requirements. The 'B' DG was operable and it auto-started on low voltage, according to design. No load breakers or MCC based contactors opened due to low voltage during the performance of the test. The Supplemental Instrument Air Compressor tripped during the low voltage dip from the Charging Pump start, and the B train Hydrogen Analyzer trouble alarm came in. Both of these components have local controllers and are not supplied by MCC based contactors. Although both were affected by the low voltage on the bus they were not a contributing factor in the UV start of the DG.
CORRECTIVE ACTIONS
Condition Evaluation Report C-06-4031 assigned an action to revise STP-125.021 "Periodic Testing of the Alternate AC Power Supply" to align the DG to prevent an inadvertent start during testing.
PRIOR OCCURRENCES
This was the first use of STP-125.021 after installation of the Alternate AC Power Supply during RF16. There has been no other occurrence of a DG start on undervoltage while the emergency bus was supplied by the Alternate AC Power Supply.