05000352/LER-2012-008
| Limerick Generating Station, Unit 1 | |
| Event date: | 09-13-2012 |
|---|---|
| Report date: | 11-12-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3522012008R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Wednesday, September 12, 2012, Unit 1 main steam isolation valve (MSIV) (EIIS:ISV) isolation instrumentation response time as-found testing (ST-2-041-908-1) for the 1A main steam line (MSL) flow channel was performed. The test failed when the as-found channel response time was greater than the Technical Specification (TS) limit of 0.5 seconds. Four relays (EIIS:RLY) were replaced and the test was performed successfully.
On Thursday, September 13, 2012, the as-found testing (ST-2-041-911 1) for the 1D MSL flow channel was performed. The test failed when the as-found channel response time was greater than the test TS limits. Five relays were replaced and the PMT failed due to a bench testing error. Five additional relays were replaced and the test was performed successfully.
On Thursday, September 20, 2012, the as-found testing (ST-2-041-909 1) for the 1B MSL flow channel was performed successfully. No relays were replaced.
On Wednesday, October 03, 2012, the as-found testing (ST-2-041-910-1) for the 1C MSL flow channel was performed. The test failed when the as-found channel response time was greater than the test TS limits.
Five relays were replaced and the test was performed successfully.
During sequential testing, the as-found tests on the 1A, 1C and 1D MSL flow channels exceeded the TS maximum response time of 0.5 seconds; therefore, it is likely that three channels were inoperable at the same time during plant operation for a period longer than permitted by TS.
This event involved a condition prohibited by TS. This event also involved a common-cause inoperability of independent channels.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B) and 10CFR50.73(a)(2)(vii).
Analysis of the Event
There was no actual safety consequence associated with this event.
The potential safety consequences of this event were minimal. The inboard MSIV closing times were not degraded due to this condition.
Therefore, a MSL break accident would have been mitigated as designed.
UFSAR 15.6.4 Steam System Piping Break Outside Primary Containment describes the analysis of a MSL break. The total integrated mass leaving the reactor pressure vessel (RPV) through the steam line break is 108,785 lb; however, for the radiological consequence evaluation, a total mass of 140,000 lb is assumed. This analysis bounds the as-found response times effect on the main steam line break accident radiological release analysis.
The MSIV isolation logic is one-out-of-two-twice ((1A or 1C) and (1B or 1D)). Each MSL flow element supplies 4 flow transmitters (EITS:FT). Each flow transmitter inputs into each MSIV's closing circuit; therefore, each test records 32 response times. Overall there are 128 response times recorded by the 4 divisional response time tests.
The 1B channel was not degraded; therefore, half of the closing logic (1B or 1D) was not affected. Some of the 1A and some of the 1C response times were slow but not all MSIV closing times were affected. The 4 inboard MSIV closing times were not degraded and and HV-041-1F028D).
Cause of the Event
The response time test failures were caused by a failure to proceduralize the method of replacement relay selection to ensure the fastest contact release times. This caused the overall as-left logic response time to lose margin to the TS limit.
Corrective Action Completed The affected relays in the 1A, 1C and 1D MSL flow channels were replaced.
Corrective Action Planned The relay testing and/or replacement procedure (IC-C-11-04067) will be revised to include a step-by-step method of selecting relays with the shortest contact release times upon de-energization of the relay.
A new stock code will be created for new relays that have been response time tested and found to be acceptable for this application.
Previous Similar Occurrences On April 6, 2011, an apparent cause evaluation (IR 1186147 assignment 02) was completed regarding MSL flow response time test failures that occurred in 2010.
closing stroke times exceeded the 5 second TS limit.
Component data:
System:�Primary Containment Isolation System B21H-K4A NSSS System Steam Line High Flow Relay Manufacturer: Tyco Electronics Model: EGPB004 System: Primary Containment Isolation System B21H-K14A Main Steam Line Isolation Relay Manufacturer: Tyco Electronics Model: EGPI004