05000352/LER-2008-003
| Limerick Generating Station, Unit 1 | |
| Event date: | 11-02-2008 |
|---|---|
| Report date: | 12-30-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3522008003R00 - NRC Website | |
Unit Conditions Prior to the Event (Power Operation) at approximately 100% power.� Unit 1 was in Operational Condition (OPCON) 1 There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Sunday, November 2, 2008, at 1550 hours0.0179 days <br />0.431 hours <br />0.00256 weeks <br />5.89775e-4 months <br />, a licensed operator observed frequent low-level oscillations on the high pressure coolant injection (HPCI) (EIIS:BJ) system flow indication (EIIS:FI) in the main control room. The system was not in operation at the time. As a result of the observed flow oscillations the HPCI system was declared inoperable. An investigation determined that oscillations were due to the combined effects of a flow loop square root converter and flow transmitter (EIIS:FT). Replacement of the square root converter and recalibration of the flow transmitter corrected the low-level oscillations in flow indication. HPCI was declared operable on Tuesday, November 4, 2008, at 1725 hours0.02 days <br />0.479 hours <br />0.00285 weeks <br />6.563625e-4 months <br />; however, shortly following the post maintenance turbine run, intermittent, brief momentary pulsing of several plant computer points associated with the turbine flow and speed control instrumentation were observed.
On Thursday, November 6, 2008, at 0840 hours0.00972 days <br />0.233 hours <br />0.00139 weeks <br />3.1962e-4 months <br />, HPCI was declared inoperable to install recording equipment for troubleshooting to identify the cause of the intermittent momentary turbine flow and speed control instrumentation signal pulses. The troubleshooting identified that the inverter (EIIS:JX) that powers the turbine control system instruments and components was the cause of the momentary turbine flow and speed control instrumentation signal pulses.
The degraded inverter was replaced and a post maintenance test was completed satisfactorily. HPCI was declared operable on Friday, November 7, 2008, at 0308 hours0.00356 days <br />0.0856 hours <br />5.092593e-4 weeks <br />1.17194e-4 months <br />.
This event resulted in a condition that at the time of discovery could have prevented the fulfillment of the safety function of the HPCI system which is reportable per the requirements of 10CFR50.72(b)(3)(v)(D) and 10CFR50.73(a)(2)(v)(D). An 8-hour NRC ENS notification (#44623) was completed on Sunday, November 2, 2008 at 2034 hours0.0235 days <br />0.565 hours <br />0.00336 weeks <br />7.73937e-4 months <br />. This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(D).
Analysis of the Event
There were no actual safety consequences associated with this event. HPCI was rendered inoperable due to a degraded instrument power supply. However, the reactor core isolation cooling (RCIC) system remained operable during the event.
An inverter supplies power to the HPCI system flow instrumentation and turbine speed control system. The input to the inverter is 125 VDC and the inverter output is 120 VAC. The inverter provides the 120 VAC input for two power supplies, which output 24 VDC to the HPCI flow control loop and 48 VDC to the HPCI electronic governor module (EGM) and ramp generator signal converter (RGSC).
An investigation determined that the oscillations were due to the combined effects of a degraded flow loop square root converter and less than optimal calibration of the flow transmitter. The high zero flow signal from the flow transmitter intermittently exceeded the low flow cutoff setting on the square rooter causing the flow indication to indicate flow intermittently. This problem was independent of the intermittent, momentary tripping of DC/AC inverter.
Cause of the Event
The cause of the HPCI system inoperability was degradation of the inverter that powers the flow control loop and EGM/RGSC instrument power supplies.
Corrective Action Completed The degraded inverter was replaced and the post maintenance test was successful.
Corrective Action Planned Failure analysis will be performed on the degraded inverter.
Previous Similar Occurrences There were two previous similar occurrences of an emergency core cooling system (ECCS) instrument power supply inverter failure that resulted in unplanned HPCI inoperability in the last three years. Unit 1 HPCI was rendered inoperable but available on October 20, 2007 following a failure of the Division 4 Trip Unit Inverter Power Supply (Ref: IR 686796 and AR A1636192). Unit 1 HPCI was also rendered inoperable but available on January 29, 2008 following a failure of the Division 2 Trip Unit Inverter Power Supply (Ref: IR 728581 and AR A1648054). The prior corrective actions for the previous events did not prevent recurrence of this inverter failure since it was not possible to identify a common cause for the failures.
Component data:
System: BJ( (High Pressure Coolant Injection System) Component: TRB (Turbine) Component Number: 10-S211 Manufacturer: T147 Terry Steam Turbine Co Model Number: A-3-DBC-187 Equipment: Square Root Converter Component ID: FY-055-1K601 Manufacturer: B045 ABB Automation Inc.
Model Number: 750020AAAE1 Equipment: Inverter 1000VA Component ID: E/S X-M1-11007 GE Number: E41-K603 Manufacturer: 255N Nuclear Logistics, Inc Stock Code: 111-02282 Old Model Number: NLI-INV1000-125-1175 Old Serial number: 072001-AS-26 New Model Number: NLI-072034-CSI-K-5-A New Serial number: 21036-001-00002