05000353/LER-2009-001
| Limerick Generating Station, Unit 2 | |
| Event date: | 03-31-2009 |
|---|---|
| Report date: | 06-01-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3532009001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 5 (Refueling) at approximately 99 degrees Fahrenheit reactor coolant temperature and zero psig reactor pressure conducting refueling operations. Testing was being performed on D23 Emergency Diesel Generator (EDG) (EIIS:DG) (EIIS:EK). There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Tuesday, March 31, 2009, refueling outage activities were in progress on Unit 2 including the 24-month "D23 Diesel Generator 4 kV Safeguard Loss of Power LSF/SAA and Outage Testing" (ST-6-092-117-2). At 1858 hours0.0215 days <br />0.516 hours <br />0.00307 weeks <br />7.06969e-4 months <br /> the test automatically started D23 EDG, which automatically energized the 4kV bus as expected. Following D23 EDG output breaker closure, the 2C Residual Heat Removal (RHR) pump, 2C Core Spray pump and OC Emergency Service Water (ESW) pumps automatically started as expected. Bus voltage exceeded the range of the voltmeter (5050 volts). The Plant Reactor Operator opened the D23 EDG output breaker and then secured the D23 EDG engine.
The EDG continued to run for a brief period as per design after the EDG output breaker was manually opened until the operator secured the EDG engine. It was during this time period that the automatic start logic was initiated and therefore, with the diesel already running, no actual EDG start occurred. The off-site source feeder breakers did not automatically close because the test had defeated the automatic closure.
The bus undervoltage condition actuated the D23 Division 3 4kV Safeguard Bus undervoltage relay, which initiated the automatic start logic (0.5 second time-delay) for D23 EDG followed by an actuation of the automatic closure logic (1.0 second time-delay) to the D23-101 and D23-201 offsite source feeder breakers.
The investigation determined that the failure of the emergency diesel generator voltage regulator (EIIS:RG) was due to an intermittent failure of the #1 rectifier bank. The #2 rectifier bank was placed in service and the #1 rectifier bank was removed from service. D23 EDG testing was successfully completed and D23 EDG was restored to operable status on April 2, 2009.
This event resulted in a valid actuation of emergency A.C.
electrical power. The 8-hour ENS notification required by 10CFR50.72(b)(3)(iv)(A) was performed on Wednesday, April 1, 2009 at 0010 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> (#44948). Since the EDG automatic start minimum actuation logic for the bus undervoltage condition actuated, this event is being reported as an automatic actuation of emergency A.C. electrical power. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a) (2) (iv) (A).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The 4kV safeguard bus was removed from service to conduct refueling outage testing and was inoperable and unavailable during the testing.
The engineering evaluation of the overvoltage condition determined that the 4kV bus was not adversely affected by the short duration overvoltage condition. Instruments powered from the bus were walked down to ensure no undetected failures occurred.
Cause of the Event
The actuation was caused by a failure of the D23 EDG voltage regulator #1 rectifier bank.
Corrective Action Completed The D23 EDG voltage regulator was swapped to the #2 rectifier bank and the D23 EDG testing was successfully completed.
Corrective Action Planned The #1 rectifier is scheduled for replacement.
Testing will be performed on the degraded rectifier to determine if the cause of the rectifier failure was the same as the cause identified in a previous D23 EDG rectifier vendor failure analysis. The cause of the previous failure was due to excessive forward voltage drop across the flyback diode. The vendor failure analysis was conducted following the D23 EDG overvoltage event that occurred on January 12, 2008.
Previous Similar Occurrences On January 12, 2008 there was a similar failure of D23 EDG voltage regulator rectifier #1 that resulted in a bus overvoltage condition. This event was reported in LER 2-08 001. The corrective actions from this event were not applied to this most recent event since this rectifier was already installed at the time of CA development.
On July 26, 1997 there was a similar failure of D21 EDG voltage regulator rectifier that resulted in a bus overvoltage condition. This event was reported in LER 2-97 008. The vendor tested the degraded D21 EDG rectifier but no cause for the failure was identified and the rectifier was returned to stock as a spare. The need for tightness checks of the silicon controlled rectifier (SCR) bolted connections was not known at the time of vendor rework. The rectifier was later installed in D23 EDG and resulted in the event being reported in this LER when it failed the post maintenance test resulting in the bus overvoltage condition.
Component data:
Equipment: D23 Emergency Diesel Generator Component Number: 2C-G501 Manufacturer: 384C Fairbanks Morse Engine Division Model Number: 00884777 Equipment: D23 Emergency Diesel Generator Voltage Regulator Component Number: 2C-G502 Manufacturer: Basler Electric Company Model Number: SER-CB Part Number: 9119803100 (Rectifier Bank)