05000353/LER-2012-001
| Limerick Generating Station, Unit 2 | |
| Event date: | 05-10-2012 |
|---|---|
| Report date: | 07-09-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3532012001R01 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 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 Thursday, May 10, 2012, Unit 2 was operating at 100% power.
Engineering System Manager performance monitoring on the redundant reactivity control system (RRCS) (EIIS:JC) was in progress. The System Manager identified that the Unit 2 plant monitoring system (PMS) computer point (S044) for RRCS Division 2 channel B reactor pressure vessel (RPV) pressure was indicating approximately 20 psig lower than the other three Unit 2 RPV pressure channels.
Further investigation identified that the daily surveillance test (ST-6-107-590-2) channel check taken at the RRCS auxiliary equipment room panel display (20-C635) also recorded a 20 psig deviation of the pressure transmitter (EIIS:PT) PT-042-2N403F reading for channel 2B as compared to the other three RRCS channels (1A, 1B and 2A).
All four channel readings noted in the daily surveillance test were within the test acceptable range (990 - 1100 psig). Based on analysis of setpoint, as left calibration information and current readings, Engineering was concerned that the 2B channel of RRCS may not function at the trip setpoint of less than or equal to 1149 psig.� Engineering notified Operations shift management of the deficiency. The Unit 2 Division 1 RRCS functions were not affected by this condition.
At 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br />, shift management declared the degraded channel inoperable and Technical Specification (TS) 3.3.4 ATWS Recirculation Pump Trip System Instrumentation, action "b" was entered. Action "b" requires placing the inoperable channel in the tripped condition within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
The investigation used computer trending and identified that degraded performance of the analog trip module (ATM) card was evident following refueling outage 2R11 (April 2011). The investigation also identified that the card performance was further degraded following the failure of an RRCS auctioneered power supply which occurred on September 8, 2011. The degraded channel was later identified as inoperable during the initial performance of an RRCS performance monitoring activity on May 10, 2012. A new ATM card was installed, calibrated and tested satisfactorily. The degraded card had been in-service since January 31, 2007.
A review of instrument operating history identified that the channel was inoperable from September 8, 2011 to May 10, 2012; therefore, the channel was inoperable for approximately 8 months which exceeded the TS allowed outage time.
This event involved a condition prohibited by Technical Specifications. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There was no actual safety consequence associated with this event.
The potential safety consequences of this event were minimal. The redundant Unit 2 Division 1 RRCS trip system was not affected by this degraded channel. The Unit 2 Division 1 trip system was operable during the affected period. The Unit 2 Division 2 trip system remained capable of actuation at a reactor pressure approximately 20 psig higher than its required setpoint.
With the 2B channel inoperable the Unit 2 Division 2 trip system may not have actuated on a high reactor pressure condition. However, the Unit 2 Division 1 trip system would have actuated to mitigate a reactor high pressure event. The operable Division 1 trip system remained capable of actuating the alternate rod insertion, reactor recirculation pump trip, feedwater runback, and standby liquid control system initiation as designed.
Cause of the Event
Division 2 RRCS system was a premature failure of the reactor pressure ATM card. The ATM card failure was most likely due to a failure of the U1-amplifier sub-component on the card.
Corrective Action Completed The degraded ATM card was replaced, calibrated and tested successfully.
The Daily Surveillance Log/OPCONS 1,2,3 RRCS channel check has been revised to ensure unacceptable RRCS channel signal drift will be identified and evaluated as required.
1 Previous Similar Occurrences There were no recent previous similar occurrences of RRCS inoperability that resulted in a condition prohibited by TS.
Component Data:
System Redundant Reactivity Control System PS-X-M1-20136 Reactor Pressure Analog Trip Module Manufacturer General Electric Company Model number 152D8036G004