05000352/LER-2014-004
| Limerick Generating Station, Unit 1 | |
| Event date: | 03-04-2014 |
|---|---|
| Report date: | 05-05-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 49871 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3522014004R00 - NRC Website | |
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Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Tuesday, March 4, 2014, Limerick Unit 1 was operating in end- of-cycle coast-down at approximately 100% power. At 2334 hours0.027 days <br />0.648 hours <br />0.00386 weeks <br />8.88087e-4 months <br />, a valid manual actuation of the reactor protection system (RPS) (EIIS:JC) was initiated by the operators as directed by the end of cycle steady state operations procedure (GP-5 Appendix 1) due to an unexpected closure of all six main turbine intercept valves (EIIS:V). The manual RPS actuation was performed to prevent turbine (EIIS:TRB) damage due to potential rapid heating resulting from windage losses in the exhaust hood.
The operators entered the procedure for reactor pressure vessel (RPV) control (T-101) and stabilized reactor parameters. The operators verified that all control rods were fully inserted.
Reactor level initially decreased to a minimum of -4 inches and then increased to a maximum of +35 inches on wide range level instrumentation. The +54 inch high-level turbine trip setpoint was not exceeded. The reactor water level of less than +12.5 inches resulted in an isolation signal to the closed Group IIB valves as expected.
Reactor pressure initially was 1040 psig, increased to 1059 psig, and decreased to approximately 950 psig, then stabilized.
Reactor pressure remained less than the lowest safety relief valve (SRV) setpoint of 1170 psig; therefore, no SRVs actuated.
The main steam bypass valves opened as designed to control pressure.
The post-scram troubleshooting identified a degraded EHC 30 VDC house power supply.
A 4-hour NRC ENS notification was required by 10CFR50.72(b)(2)(iv)(B) for an actuation of RPS when the reactor was critical. The ENS notification (#49871) was completed on Wednesday, March 5, 2014, at 0226 EDT. This event involved a manual actuation of RPS. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. The plant equipment performed as designed during the transient with the following exception. The "A" channel of end- of-cycle recirculation pump trip (EOC-RPT) logic failed to actuate; however, the recirculation pump trip safety function was completed by the "B" channel logic. The logic was restored to operable status during the 1R15 refueling outage. The operators effectively stabilized reactor parameters and verified all control rods were fully inserted.
A main turbine trip transient is discussed in UFSAR section 15.2.3. A turbine trip with bypass is an incident of moderate frequency. The function of the IVs is discussed in UFSAR section 10.2.2.3 Protective Valve Functions. Each low pressure turbine is equipped with two combined intermediate valves (CIVs) which function to protect the turbine against overspeed following a turbine trip.
All six main turbine IVs fast closed due to the degraded EHC power supply which actuated the IV overspeed protection logic.
The EHC system uses auctioneered power supplies, but the degraded power supply adversely affected the performance of the partner power supply. This resulted in the IV closures.
The GE MARK 1 EHC control system was replaced by a Westinghouse Ovation Digital Electro-hydraulic Control (DEHC) system during DEHC modification is scheduled for refueling outage 2R13 in April 2015.
Cause of the Event
The cause of the main turbine IVs closure was due to a degraded EHC 30 VDC house power supply.
Corrective Actions Completed The Unit 1 EHC system was replaced with a DEHC system during the subsequent refueling outage 1R15.
The Unit 2 EHC house and permanent magnet generator (PMG) power supplies as-found voltages were verified to be within the calibration procedure limits and the as-left voltages were adjusted to the middle of the procedure acceptable band during the planned maintenance outage 2M49.
Corrective Action Planned The Unit 2 EHC system is scheduled to be replaced with a DEHC system during the next refueling outage 2R13 in April 2015.
Previous Similar Occurrences There was no previous similar occurrence in the last five years of manual RPS actuation due to main turbine IV closure.
Component data:
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Serial number:
IT Main Turbine Instrumentation System JX Power Supply, Electric E/S X-M2-11022B L045 Lambda Electronics Div LMF-28-0VMY-3397-3 F73411