05000266/LER-2011-001
| Point Beach Nuclear Plant Unit 1 | |
| Event date: | 11-27-2011 |
|---|---|
| Report date: | 01-25-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vi)(A) 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2662011001R00 - NRC Website | |
Description of the Event:
On 11/27/2011 restoration of the 1X-03 high voltage station auxiliary transformer [XFMR] was in progress. The 13.8kV buses [BU] were connected and powered from the 2X-03 high voltage station auxiliary transformer and were cross-connected. On 11/27/2011 at 0226 CDT, during the restoration process, the H-02 to H-01 bus tie breaker was opened to separate the high voltage station auxiliary transformers. Upon opening the breaker an undervoltage condition occurred on the Unit 1, 4.16kV safeguard buses 1A-05, and 1A-06. As a result of the undervoltage condition, the four emergency diesel generators [DGs] started and the G-01 and G-03 EDGs loaded onto buses 1A-05 and 1A-06, respectively. The 2X-03 high voltage station auxiliary transformer remained energized throughout the event. An Unusual Event (UE) was declared at 0238 CDT based upon the interruption of offsite power to the essential buses. The UE was subsequently terminated at 0700 CDT on maintained offsite power throughout the event.
The diesel start sequence resulted in a separation of the vital buses from off-site power as designed. The EDGs performed as expected, fast starting and loading onto safeguards buses, 1A-05, and 1A-06, respectively. The plant safety systems functioned as expected. A review of plant process computer data confirmed that power to the required residual heat removal pump [P] was restored and the pump restarted within 46 seconds. There was a negligible increase in RHR suction temperature during the 46-second period to restart the required pump following sequencing of the EDGs.
After assessing the cause of the loss of offsite power and confirming that the redundant circuit for off site power remained available, offsite power was restored to the Unit 1 safeguards buses by synchronizing the running EDGs to the grid and closing the alternate feed from offsite power. Offsite power remained available to the affected unit (Unit 1) via this redundant path (2X03), as well as from the opposite unit's low voltage station auxiliary transformer (2X04 to 4160/480V buses to Unit 1) throughout the event.
At the time of the event, Unit 1 was in MODE 5 with the primary system intact, the pressurizer was water-solid, and reactor coolant system temperature and pressure were approximately 115°F and 75 psig, respectively.
Residual heat removal, component cooling, and service water systems were in service providing decay heat removal, and the secondary side of the steam generators were available to provide decay heat removal if necessary. The loss of offsite power did not constitute a safety system functional failure.
Cause of the Event:
The specific cause of this event was the failure of all three phases of the 1F89-112, high side circuit switcher, to the 1X-03 high voltage station auxiliary transformer. This resulted in the degraded voltage condition to safeguards buses 1A05 and 1A06. Binding of the 1F89-112 circuit switcher "A" phase operating mechanism prevented proper resetting (closure) of the load contacts when the circuit switcher was opened. This condition subsequently resulted in failure of the circuit switcher when the next closure operation was performed. The root cause of the event was that the circuit switcher preventive maintenance (PM) program did not include manufacturer recommendations.
Analysis of the Event
The 13.8 kV system boundaries include the high voltage station auxiliary transformer (1/2X-03) up to the high side connection of the low voltage station auxiliary transformers (1/2X-04), and various 480 V transformers.
The 13.8 kV system supplies offsite power to the station via the 4.16 kV and 480 V systems. The 13.8 kV and is normally served by high voltage station auxiliary transformer 1X-03. The H-02 bus supplies power to low voltage station auxiliary transformer 1X-04. Similarly, the H-03 bus supplies Unit 2 and is normally served by high voltage station auxiliary transformer 2X-03. The H-03 bus supplies power to the low voltage station auxiliary transformer 2X-04. The units can be interconnected to alternate supplies by arranging bus tie breakers to connect H-02 to H-01 and H-03 to H-01.
Safety Analysis
The failure of the circuit switcher occurred during switchyard realignment and resulted in a low voltage condition which started the four standby emergency diesel generators. However, the fault (a high resistance connection) did not cause a lockout of 1X03, the associated switchyard component. As a result, the automatic transfer to close in the redundant offsite power supply in the switchyard was not initiated, and the GO1 and G03 emergency diesel generators proceeded to automatically load to the Unit 1 safety system buses once they had reached operating voltage and frequency.
An assessment confirmed that the redundant circuit for offsite power remained available. Offsite power was restored to the Unit 1 safeguards buses by synchronizing the running EDGs to the grid and closing the alternate feed from offsite power. Offsite power remained available to the affected unit from this redundant path, as well as from the opposite unit's low voltage station auxiliary transformer throughout the event.
Corrective Actions
Station procedures have been revised to check local circuit switcher indicators for proper configuration prior to and following operation. Additional corrective actions associated with the PMs are being tracked to completion in the station's corrective action program.
Similar Events None
Component Failure Data
Manufacturer Description Model Number S & C Circuit Switcher Type G, 3 Gap, Center-Break