05000352/LER-2010-002
| Limerick Generating Station, Unit 1 | |
| Event date: | 12-12-2010 |
|---|---|
| Report date: | 02-10-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 46474 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3522010002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On December 12, 2010, Limerick Unit 1 was operating at 100% power and the High Pressure Coolant Injection (HPCI) Turbine Overspeed Mechanism Operability Check (RT-6-055-330-1) was in progress. At 1042 hours0.0121 days <br />0.289 hours <br />0.00172 weeks <br />3.96481e-4 months <br />, the HPCI (EIIS:BJ) overspeed trip mechanism (EIIS:12) would not return to the reset position when manually tripped. Subsequent efforts to adjust the reset time did not correct the problem. With the auxiliary oil pump operating the overspeed trip mechanism continued to cycle between an intermediate position and the tripped position and the HPCI turbine stop valve remained closed. Prior to securing the auxiliary oil pump and exiting the test procedure a final attempt to manually reset the overspeed trip mechanism was successful. Subsequent restart of the auxiliary oil pump to perform troubleshooting (TRT 10-154) to verify and adjust overspeed trip mechanism hydraulic pressures resulted in satisfactory operation of the HPCI turbine hydraulic system and the overspeed trip mechanism. The failure of the overspeed trip mechanism to reset when manually tripped could not be duplicated during subsequent performances of RT-6-055-330-1.
The licensed operators declared the HPCI system inoperable and unavailable. The operators entered Technical Specification (TS) 3.5.1, ECCS, which requires restoration of the HPCI system within 14 days. HPCI was restored to available status at 2156 hours0.025 days <br />0.599 hours <br />0.00356 weeks <br />8.20358e-4 months <br /> during troubleshooting (TRT-10-154). HPCI was returned to operable status on Monday December 13, 2010 at 1754 hours0.0203 days <br />0.487 hours <br />0.0029 weeks <br />6.67397e-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 (#46474) was completed on Sunday, December 12, 2010 at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-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. The HPCI System was rendered inoperable due to failure of the overspeed trip mechanism to reset when manually tripped for a periodic test; however, all other Emergency Core Cooling Systems and the Reactor Core Isolation Cooling (RCIC) System remained operable during the event.
Troubleshooting activities did not identify any HPCI component failure that caused this problem. Failure of the overspeed trip mechanism to reset when manually tripped could not be duplicated during subsequent performances of the test. Given the intermittent nature of the problem, the most likely cause of the failure of the overspeed trip mechanism to reset during the initial performance of RT-6-055-330-1 is believed to be either sludge or foreign material temporarily plugging the overspeed trip mechanism piston drain port (Port "F") which prevented the oil pressure at Port "C" from decreasing and the trip mechanism reset spring from returning the piston to the reset position. The cause of the foreign material entering the overspeed trip mechanism piston drain port is unknown. Unit 1 HPCI turbine lube oil samples and oil filter performance did not indicate an oil cleanliness issue prior to the event.
Cause of the Event
The most likely cause of the failure of the overspeed trip mechanism to reset during the initial performance of the test was either sludge or foreign material temporarily plugging the overspeed trip mechanism piston drain port.
Corrective Action Completed Troubleshooting efforts were completed which eliminated equipment failure and improper hydraulic circuit pressure settings as the potential cause of the problem and flushed the overspeed trip mechanism.
The periodic test that demonstrates operability of the Unit 1 HPCI overspeed trip mechanism was placed on increased frequency. The test was completed successfully on a weekly frequency for four weeks and is presently on a monthly frequency.
Corrective Action Planned The maintenance procedure that governs HPCI turbine inspections (M-C-756- 001) will be enhanced to provide more specific requirements regarding inspection and cleaning of the overspeed trip mechanism.
The Unit 1 and Unit 2 HPCI overspeed trip mechanisms will be disassembled and inspected.
Previous Similar Occurrences There have been no malfunctions of the HPCI overspeed trip mechanism due to foreign material in the previous five year period.
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