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| Start date | Reporting criterion | Title | Event description | System | LER | |
|---|---|---|---|---|---|---|
| ENS 51168 | 21 June 2015 20:25:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Manual Secondary Containment Isolation | In response to a trip of Unit 2 reactor enclosure HVAC (Heating, Ventilation and Air Conditioning), subsequent loss of reactor enclosure delta-p and rising reactor enclosure room temperatures, a Unit 2 manual secondary containment isolation was initiated per station procedures. This manual isolation also resulted in an isolation signal to containment atmosphere control (CAC) system valves and primary containment instrument gas (PCIG) system valves. System responses were as expected. Unit 2 secondary containment delta-p and room temperatures were restored via the standby gas treatment system (SGTS), and Unit 2 secondary containment integrity remains intact and operable. Investigation of the trip of Unit 2 reactor enclosure HVAC is ongoing. This is being reported under 50.72(b)(3)(iv)(A) for containment isolation signal affecting containment isolation valves in more than one system. The licensee informed the NRC Resident Inspector. | Secondary containment Primary containment HVAC Standby Gas Treatment System | |
| ENS 50981 | 13 April 2015 06:20:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Manual Reactor Scram Initiated | Following a pre-planned scram for entry into the U2 Refueling Outage, the 'B' Reactor Protection system (RPS) was unable to be reset due to a deficiency with the '2H' Intermediate Range Power Monitor. Due to this failure to reset RPS, a manual full scram was initiated as required by plant procedures. The licensee notified the NRC Resident Inspector. | Reactor Protection System | |
| ENS 48936 | 17 April 2013 01:42:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Specified System Actuation During Turbine Stop Valve Logic Testing While Shutdown | During outage main turbine stop valve RPS logic surveillance testing, an invalid RPS actuation occurred due to an error in executing main turbine surveillance testing procedures. A Turbine Stop Valve closure RPS signal occurred due to an error in the restoration sequence of restoring the RPS bypass signal and a subsequent manual trip of the main turbine. This resulted in a full scram and a trip of both reactor recirculation pumps. The site post-scram response procedure was entered, which required that the mode switch be placed in the locked SHUTDOWN position. This caused an expected but valid RPS actuation. No control rod motion occurred due to all control rods were inserted at the time of the invalid RPS actuation and subsequent valid RPS actuation. The license has notified the NRC Resident Inspector. | Main Turbine Reactor Recirculation Pump Control Rod | 05000353/LER-2013-001 |
| ENS 48121 | 19 July 2012 19:37:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Reactor Protection Signal Actuation Due to Changing Mode Switch Position During Surveillance Testing | During surveillance testing, the reactor mode switch was moved from the locked shutdown position and placed in the refuel position without the minimum required operable source range monitors per Limerick Generating Station Technical Specifications. Upon discovery of the issue, the reactor mode switch was returned to the locked shutdown position causing an expected but valid RPS actuation. No control rod motion occurred because all control rods were already inserted at the time of the event. The licensee has notified the NRC Resident Inspector. | Control Rod | 05000352/LER-2012-006 |
| ENS 47888 | 2 May 2012 11:24:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Manual Initiation of Secondary Containment Isolation | In response to a trip of Unit 1 reactor enclosure (secondary containment) HVAC and subsequent loss of reactor enclosure delta-p, a Unit 1 manual secondary containment isolation was initiated per station procedures. This manual initiation also resulted in an isolation signal to containment atmosphere control (CAC) system valves and primary containment instrument gas (PCIG) system valves. System responses were as expected. Unit 1 secondary containment delta-p was restored via standby gas treatment system (SGTS), and Unit 1 secondary containment integrity remains intact and operable. Investigation of the trip of Unit 1 reactor enclosure HVAC is ongoing. This is being reported under 50.72(b)(3)(iv) for containment isolation signal affecting containment isolation valves in more than one system. The licensee notified the NRC Resident Inspector. | Secondary containment Primary containment HVAC Standby Gas Treatment System | 05000352/LER-2012-003 |
| ENS 46906 | 30 May 2011 15:50:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Specified System Actuation - Manual Reactor Scram Due to Dual Recirc Pump Trip During Startup | Limerick Unit 2 was 'manually scrammed' from 0% power on 5/30/11 at 1150 hours in accordance with plant procedure OT-112 'Recirculation Pump Trip', when both the '2A' and '2B' Recirculation Pumps tripped. The cause of the pump trip is currently under investigation at this time. At the time of the 'manual' SCRAM: The Reactor Mode Switch was in 'Startup' Mode 2. All control rods were inserted to 'full-in' position (00). Reactor Shutdown was in-progress in accordance with Plant procedure GP-3 'Normal Plant Shutdown'. No primary or secondary containment isolations occurred. The plant is currently in HOT SHUTDOWN maintaining normal Reactor level with Control Rod Drive (CRD) hydraulic and Reactor Water Cleanup (RWCU) systems. Licensee is investigating a potential issue with the relay logic associated with the scram bypass feature. Offsite power circuits and emergency diesel generators are operable and available. There was no increase in plant risk associated with this event. The NRC Resident Inspector has been notified. | Secondary containment Emergency Diesel Generator Reactor Water Cleanup Control Rod | 05000353/LER-2011-005 |
| ENS 44948 | 31 March 2009 22:50:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Valid Diesel Start Signal Received During Maintenance Testing of Diesel | A valid actuation of the D23 Emergency Diesel Generator bus undervoltage minimum actuation logic occurred following manual operator action to mitigate a bus overvoltage condition during emergency diesel generator post maintenance testing . Due to the output breaker being opened, the D23 EDG experienced a undervoltage start signal. EDG undervoltage actuation instrumentation actuated, however, the EDG was in PTL (Pull-to-Lock) EDG shutdown. The event was caused by a failure of the emergency diesel generator voltage regulator most likely due to an intermittent failure of the #1 rectifier bank. The licensee stated that the D23 EDG was powering an isolated safety related bus for a post-maintenance test when the overvoltage condition occurred. The operator opened the EDG output breaker and placed the EDG in PTL. With the bus de-energized, a valid start signal was sent to EDG D23. However, EDG D23 did not actually start because it was in PTL. No safety related equipment was lost when the bus was de-energized because the bus had been unloaded prior to the test. The licensee has notified the NRC Resident Inspector. | Emergency Diesel Generator | |
| ENS 43898 | 12 January 2008 14:04:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Valid Specified System Actuation During Edg Governor Testing | An overvoltage condition occurred on the D23 Safeguards 4kv bus during planned system outage window EDG governor testing. The EDG output breaker was immediately opened and the engine secured per test direction. Due to the output breaker being opened, the D23 bus then experienced an undervoltage condition. Bus voltage monitoring instrumentation automatically initiated automatic closure of the 201-D23 Bus Offsite Supply Feed Breaker after a 1 second time delay and re-energized the D23 Safeguards Bus per design. The remaining three 4kv buses and Emergency Diesel Generators were unaffected and remain operable. The voltage regulation circuit is being investigated at this time to assist in determining the cause of the overvoltage condition. The EDG was out of service and inoperable during the testing. The EDG governor was replaced during the outage window and was being fine tuned when the event occurred. The licensee is currently in day 6 of a 30 day LCO (3.8.1.1). The licensee notified the NRC Resident Inspector. | Emergency Diesel Generator | |
| ENS 43315 | 24 April 2007 06:10:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation 10 CFR 50.72(b)(2)(iv)(A), System Actuation - ECCS Discharge | Automatic Reactor Scram with Eccs Injection on Lowering Reactor Vessel Water Level | Limerick Unit 2 automatically shutdown on low reactor water level (+12.5 inches) at 0210 on 4/24/07. The cause is under investigation at this time. All control rods inserted as required. HPCI and RCIC initiated, injected and restored reactor water level. Lowest reactor water level noted was -73 inches. Primary containment isolation signals on +12.5 inches and -38 inches occurred. All isolations completed as required. The plant is currently in hot shutdown maintaining normal reactor level with feedwater in service. Approximately 16000 gallons was injected in 4 minutes. There was no indication of RCS leakage. The licensee is investigating whether a feedwater runback had occurred. No SRVs lifted with the highest pressure observed at 1050 psi. The main condenser remains in service for decay heat removal. All EDGs are available, if needed. The licensee informed the NRC Resident Inspector and will inform both the State and Local authorities. | Feedwater Primary containment Decay Heat Removal Main Condenser Control Rod | |
| ENS 43231 | 13 March 2007 11:30:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Low Main Condenser Vacuum Isolation Signal Received During Plant Shutdown | It was discovered that a valid Low Main Condenser Vacuum group 1A Nuclear Steam Supply Shutoff System (NS4) isolation for the Main Steam Isolation Valves (MSIV's) had been received while breaking main condenser vacuum while in Mode 4 for planned refueling outage 2R09. The isolation actuation was received as main condenser vacuum lowered below the isolation setpoint during the planned evolution of breaking of main condenser (vacuum). All valves for this isolation signal had previously been closed by approved plant procedures while in Mode 3. The valid isolation signal was caused by not having the isolation signal bypassed as allowed by approved plant procedures. The licensee will notify the NRC Resident Inspector. | Main Steam Isolation Valve Main Condenser | 05000353/LER-2007-002 |
| ENS 42193 | 9 December 2005 14:01:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Trip of a Startup Bus Breaker Results in Start of Emergency Diesel Generators | At 0901 on December 9, 2005, a trip of the 10 Startup Bus Breaker occurred at Limerick Generating Station in the 220 KV substation. This de-energized one of the required offsite sources and the associated 10 Startup Bus and 101 Safeguard Bus, causing a valid actuation signal for the D11, D13, D22 and D24 Diesel Generators. All diesels started and their associated 4 KV buses swapped to the 201 Safeguard Bus as required. The cause of the 10 Startup Bus Breaker trip is under investigation. Units 1 and 2 remain at 100% power. No safety related loads were lost. The diesel generators were still running at the time of the call but, the licensee was shutting down the diesels per procedure. The licensee notified the NRC Resident Inspector. | Emergency Diesel Generator | 05000352/LER-2006-001 |
| ENS 41573 | 6 April 2005 21:55:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Partial Loss of Offsite Power Resulting in Autostart of Emergency Diesel Generators | At 1655 on April 6, 2005, a trip of the 4A/4B Transformer occurred at Limerick Generating Station in the 500 KV Substation. This de-energized one of the required off site sources and the associated 20 Start-up Bus and 201 Safeguard Bus, causing a valid actuation signal for the D12, D14, D21, and D23 Diesel Generators. All Diesels started and their associated 4 KV buses swapped to the 101 Safeguard Bus as required. Cause of the 4A/4B Transformer trip is under investigation. Power reduction on Unit 2 was performed in response to lowering Main Condenser vacuum during power supply transfers. Condition was stabilized and Unit 2 Reactor power restored to 100 %. The cause of the lowering main condenser vacuum was that when the bus transfer occurred, several air valves on Unit 2 cycled which caused the lowering vacuum. Once power transferred, Unit 2 stabilized and was restored to 100% power. Unit 1 main condenser vacuum was not affected since power was not transferred on Unit 1. Additionally, a Group 6A valve isolation signal was generated. This caused the purge valves to receive an isolation signal but the purge valves were not in operation nor were they required to be in operation at the time of the isolation signal. Both unit's Reactor Water Clean-up (RWCU) pumps tripped due to the isolation signal. At the time of this report, the isolation signal was reset, Unit 1 RWCU pump has been reset, and preparations are being made to restart Unit 2 RWCU pump. The only other anomaly is that Unit 2 D22 diesel is in a maintenance outage and is scheduled to be returned to service tomorrow. Currently, both units are in a 72-hour LCO action statement due to loss of one offsite power supply. The licensee will inform the NRC resident inspector. | Emergency Diesel Generator Main Condenser | 05000352/LER-2005-002 |