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| Start date | Reporting criterion | Title | Event description | System | LER | |
|---|---|---|---|---|---|---|
| ENS 55888 | 7 May 2022 03:10:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Automatic Actuation of Emergency Feedwater | The following information was provided by the licensee via fax: At 2310 EDT on May 6, 2022, with Unit 3 in Mode 3, an actuation of the Emergency Feedwater (EFW) System occurred while entering a planned refueling outage. The reason for the EFW auto-start was a loss of all Main Feedwater (MFDW) Pumps which occurred when the 3A MFDW Pump tripped on steam generator (SG) overfill protection due to high level in the 3B SG. The high level in the 3B SG occurred when a Startup Feedwater Control Valve (3FDW-44) malfunctioned, resulting in excessive feedwater flow to the 3B SG. Investigation and repairs are in progress. Units 1 and 2 were not affected. This event is being reported as an 8-hr non-emergency notification in accordance with 10 CFR 50.72(b)(3)(iv)(A) as a valid actuation of the EFW system. There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified. | Steam Generator Feedwater | |
| ENS 55750 | 22 February 2022 03:07:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Manual Reactor Trip | The following information was provided by the licensee via fax or email: At 2207 (EST) on 2/21/2022 with Unit 2 in Mode 1 at 68 percent power, the reactor was manually tripped due to lowering water level in the 2A Steam Generator. The trip was not complex with all systems responding normally post-trip. Operators responded and stabilized the plant. Decay heat is being removed by discharging steam to the main condenser using the turbine bypass valves. Units 1 and 3 were not affected. Due to the Reactor Protection System actuation while critical, this event is being reported as a four-hour, non- emergency notification per 10 CFR 50.72(b)(2)(iv)(B). There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified."
The following information was provided by the licensee via phone and email: On 2/21/2022, Unit 2 was in Mode 1 increasing reactor power following startup from a forced outage. At 2205 (EST) with Unit 2 at 68 percent power, a feedwater control valve failed to properly control feedwater flow to the 2A Steam Generator and the Integrated Control System initiated an automatic runback. At 2207 (EST), the reactor was manually tripped from 39 percent power due to lowering water level in the 2A Steam Generator. Immediately following the manual reactor trip, an actuation of the Emergency Feedwater System (EFW) occurred. The 2A and 2B Motor Driven Emergency Feedwater (MDEFW) pumps automatically started as designed when the 'low steam generator level' signal was received for the 2A Steam Generator. The trip was not complex with all systems responding normally post-trip. Operators responded and stabilized the plant. Decay heat was removed by discharging steam to the main condenser using the turbine bypass valves. Units 1 and 3 were not affected. Unit 2 was restarted on 2/27/2022 following repairs. Due to the Reactor Protection System actuation while critical, this event was reported on 2/22/2022 as a four-hour, non-emergency notification per 10 CFR 50.72(b)(2)(iv)(B). Following further evaluation, it was determined that a valid EFW actuation occurred, therefore this event is now also being reported as a late 8-hour non-emergency notification of a valid actuation of the EFW system in accordance with 10 CFR 50.72(b)(3)(iv)(A). There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified. Notified the R2DO (Miller). | Steam Generator Feedwater Reactor Protection System Main Condenser | |
| ENS 55800 | 13 February 2022 21:25:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Emergency Feedwater System Actuation | The following information was provided by the licensee via email and phone: At 1625 (EST) on 2/13/2022, with Unit 2 in Mode 3 at 0 percent power and plant heat up to normal operating temperature in progress, an actuation of the Emergency Feedwater System (EFW) occurred. The reason for the EFW auto-start was lowering water level in the 2A and 2B Steam Generators due to failure of the 2A Main Feedwater Pump to respond as required to maintain Steam Generator water level as Steam Generator pressure increased during plant heat up. The 2A and 2B Motor Driven Emergency Feedwater (MDEFW) pumps automatically started as designed when the 'low steam generator level' signal was received for the 2A and 2B Steam Generators. Following further evaluation, it was determined that a valid EFW actuation occurred, therefore this event is being reported as a late 8-hour non-emergency notification of a valid actuation of the EFW system in accordance with 10 CFR 50.72(b)(3)(iv)(A). There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified. | Steam Generator Feedwater | |
| ENS 55612 | 27 November 2021 10:19:00 | 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Automatic Actuation of the Emergency AC Electrical Power System | At 0519 EST on November 27, 2021, with Unit 2 in Mode 5 at zero percent power, an actuation of the Emergency AC Electrical Power System occurred. The reason for the Emergency AC Electrical Power System auto-start was a lockout of the CT-2 transformer; causing a temporary loss of AC power to the main feeder bus. The Keowee Hydroelectric Units 1 and 2 automatically started as designed when a main feeder bus undervoltage signal was received. This event is being reported in accordance with 10 CFR 50.72(b)(3)(iv)(A) as an event that results in a valid actuation of the Emergency AC Electrical Power System. Additionally, the temporary loss of AC power resulted in a loss of Decay Heat Removal (DHR) that was restored upon power restoration to the main feeder bus. Therefore, this condition is also being reported as an eight-hour, non-emergency notification per 10 CFR 50.72(b)(3)(v) for an event or condition that could have prevented fulfillment of a safety function. There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified. The loss of the CT-2 transformer is under investigation. Main feeder bus power was restored within a minute so no plant heat up occurred as a result of the loss of the decay heat removal system. | Decay Heat Removal | |
| ENS 53677 | 19 October 2018 04:00:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Turbine Steam Seal Header Pressure Malfunction | On 10/19/18 at 2202 EDT, at 19 (percent) Reactor power, a malfunction of (the) Turbine Steam Seal Header pressure control caused a loss of Condenser vacuum, resulting in an automatic trip of the Main Turbine and a manual reactor trip (RPS Actuation). Just prior to the reactor trip, Emergency Feedwater was manually initiated to mitigate the potential loss of Main Feedwater. Condenser vacuum was recovered after the reactor trip and Main Feedwater remained in operation. Due to the RPS actuation while critical, this event is being reported as a 4-hour non-emergency per 10CFR50.72(b)(2). Also, due to the manual initiation of Emergency Feedwater, this event is also being reported as an 8-hour non-emergency per 10CFR50.72(b)(3). Following the reactor trip, all systems responded as expected with no complications. Emergency feedwater was secured at 2300. Unit 1 is in Mode 3 and stable, continuing to cooldown for a refueling outage. The NRC Resident Inspector has been notified. | Feedwater Main Turbine | |
| ENS 51268 | 27 July 2015 13:56:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Valid Actuation of Unit 2 Emergency Feedwater System During Startup | At approximately 0956 EDT on July 27, 2015, Oconee Nuclear Station Unit 2 experienced a valid actuation of the Emergency Feedwater System (EFW). At the time of the event, Unit 2 was in Mode 1 at approximately 17% power and increasing with preparations in progress for placing the main turbine on line during a unit startup. The (EFW) actuation was due to a low level on the 2B steam generator, which resulted from failure of 2B Main Feedwater Block Valve 2FDW-40 to automatically open upon demand. All systems operated as expected with no problems observed. Unit 2 is currently stable at approximately 16% power while troubleshooting valve 2FDW-40 (and the 2B Steam Generator level stable at the normal operating level). Units 1 and 3 were unaffected and remain on line and stable at 100% power. Public health and safety were not impacted by this event. This event is being reported as an 8 hour non-emergency in accordance with 10 CPR 50.72(b)(3)(iv) for a valid actuation of the Emergency Feedwater System. The NRC Resident Inspector has been notified. Corrective Action: Troubleshooting of valve 2FDW-40 is on-going. | Steam Generator Feedwater Main Turbine | 05000270/LER-2015-001 |
| ENS 46159 | 7 August 2010 18:51:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Manual Reactor Trip After Reactor Coolant Pumps High Vibration | At 1451 on 8/7/2010, Oconee Unit 1 initiated a manual reactor trip from approximately 17% power due to indicated vibrations on 1A1 and 1A2 reactor coolant pumps (RCP) reaching the high vibration trip criteria. All systems responded normally following the reactor trip. Unit 1 is currently stable in MODE 3. An investigation is in progress to determine the cause of the elevated reactor coolant pump vibrations. All control rods fully inserted on the trip. Decay heat is being removed via the turbine bypass valves to the main condenser. Steam generator water level is being maintained with main feedwater. There is no evidence that the PORVs or safety valves lifted. The plant is in its normal shutdown electrical lineup. There were no indications on the loose parts monitor except for the RCP high vibration. There was no affect on units 2 or 3. The licensee notified the NRC Resident Inspector. | Steam Generator Feedwater Main Condenser Control Rod | |
| ENS 44638 | 7 November 2008 13:34:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Automatic Reactor Trip | At 0834 on 11-7-08, Oconee Nuclear Station (ONS) Unit 3 experienced an automatic reactor trip due to a reactor protective systems (RPS) actuation. ONS Unit 3 post trip parameters are normal. Main Feedwater remained in service following the event and Emergency Feedwater was not required and remained available. Electrical power automatically transferred to the Startup power source from the switchyard and emergency AC power sources were not required and remained available. The cause of the RPS actuation and automatic reactor trip are under investigation. ONS Unit 1 remained at 100% power with no issues following the ONS Unit 3 reactor trip. ONS Unit 2 remained in No-Mode. No other safety systems have actuated or exhibited abnormal behavior. Therefore, the safety significance of this condition is low. All control rods fully inserted during this event. The unit is removing decay heat to the main condenser. No primary relief valves lifted and Main Steam relief valves cycled following the trip. There are no Steam Generator tube leaks. The licensee notified the NRC Resident Inspector. | Steam Generator Feedwater Main Condenser Control Rod Main Steam | |
| ENS 44109 | 31 March 2008 17:52:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Automatic Reactor Trip on Turbine Trip During Maintenance Activities | Event: At 1352 hours, Unit 2 experienced a Reactor Trip due to a turbine trip. The indicated cause was low condenser vacuum. The exact cause is still under investigation but is believed to be related to a maintenance procedure in progress on the condenser vacuum instrumentation. Post-trip response was normal. Auxiliary power transferred to the start-up source (switchyard) as expected. Main Feedwater was not affected so there was no demand for Emergency Feedwater. A second High Pressure Injection pump was manually started per procedure to maintain Pressurizer level indication on scale. This is a routine action to compensate for post-trip RCS temperature and volume changes. In an apparently unrelated event, Keowee Hydro Units (KHU) 1 and 2 were shutdown from commercial operation at approximately 1425 hours. During the shutdown, the KHU 1 output breaker failed to open as expected and KHU 1 was manually locked out. The lockout removed both the Overhead and Underground Power Paths from service, making on-site emergency power unavailable to all three Oconee units. Per Tech Specs, a gas turbine unit at Lee Steam Station was started and used to energize the Oconee Standby Bus at 1518 hours. Initial Safety Significance: There is little or no safety significance to the Unit trip. The subsequent KHU 1 lockout removed both the Overhead and Underground Power Paths from service, making on-site emergency power unavailable until the Lee gas turbine was aligned. There was no demand for the Keowee emergency power function during either event. Corrective Action(s): Investigations are in progress as to the cause of the Reactor Trip and the KHU 1 lock out. Alignment of KHU 2 to the underground is currently on hold, pending evaluation of the problem which led to the KHU 1 lockout. All control rods fully inserted with decay heat being removed via the turbine bypass valves. The licensee notified the NRC Resident Inspector.
The licensee is retracting a portion of the original report identified under 10CFR50.72(b)(3)(v)(D) based on the following: Event: UPDATE: At 1352 hours, Unit 2 experienced a Reactor Trip due to a turbine trip. The indicated cause was low condenser vacuum. The exact cause is still under investigation but is believed to be related to a maintenance procedure in progress on the condenser vacuum instrumentation. Post-trip response was normal. Auxiliary power transferred to the start-up source (switchyard) as expected. Main Feedwater was not affected so there was no demand for Emergency Feedwater. A second High Pressure Injection pump was manually started per procedure to maintain Pressurizer level indication on scale. This is a routine action to compensate for post-trip RCS temperature and volume changes. In an unrelated event, Keowee Hydro Units (KHU) 1 and 2 were shutdown from commercial operation at approximately 1425 hours. During the shutdown, the KHU 1 output breaker, ACB-1, failed to open as expected and KHU 1 was manually locked out. The lockout removed KHU 1 and the Overhead Power Path from service due to the failed ACB. Because KHU 1 was the unit aligned to the Underground Power Path, Operations declared that path inoperable also. This condition was initially reported as making on-site emergency power unavailable to all three Oconee units. Per Tech Specs, a gas turbine unit at Lee Steam Station was started and used to energize the Oconee Standby Bus at 1518 hours. In this alignment, the Lee gas turbine provides the on-site emergency power function. A design feature allows a KHU to automatically align to the Underground Path when the unit originally aligned to that path has been locked out, the overhead path is locked out and an emergency start signal exists. Keowee Operations confirmed that during the lockout event the required lockout signals were present on KHU 1 such that KHU 2 would have aligned to the Underground Power Path if an emergency start demand had occurred. As a result, the Underground Power Path remained available during this event and there was no loss of safety function. Therefore the portion of the event related to 10CFR50.72(b)(3)(v)(D) is RETRACTED. The Underground Power Path was administratively inoperable because a surveillance (SR 3.8.1.3) to verify operability of KHU 2 to the underground path, required per TS 3.8.1 Condition C.1, could not be performed. The surveillance procedure utilizes a normal start signal, which was inhibited by the lockout on the overhead path. The surveillance procedure does not include provisions for using an emergency start signal. At 1906 hours, after the overhead lockout had been reset, KHU 2 successfully completed an Operability test aligned to the Underground Power Path. Investigation determined that the failure of ACB-1, the output breaker for KHU 1, was due to a terminal strip sliding link in the trip circuit being in an intermediate position. It was damaged during repair so the entire sliding link block was replaced. The unit was successfully tested connected to each power path and was declared Operable on 4-1-08 at approximately 0600 hours. At 0930 hours on 4-1-08 the standby bus was disconnected from Lee. Initial Safety Significance: There is little or no safety significance to the Unit trip. The subsequent KHU 1 lockout removed the Overhead Power Path from service. The Lee gas turbine was aligned to energize the standby bus. Operations and Engineering subsequently confirmed that on-site emergency power remained available via KHU 2 and the Underground Power Path. There was no demand for the Keowee emergency power function during either event. Corrective Action(s): Investigation as to the cause of the Reactor Trip continues. Unit 2 is now on-line at low power and is in the process of returning to full power. ACB-1, the output breaker for KHU 1, terminal strip sliding link has been repaired/replaced; both KHUs have been tested and declared operable; the Lee gas turbine has been secured. The licensee informed the NRC Resident Inspector. Notified R2DO (Evans). | Feedwater Control Rod | |
| ENS 43169 | 15 February 2007 21:54:00 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Unit 1 and Unit 2 Reactor Trip | At 1654 EST on 2-15-07, Oconee Units 1 & 2 tripped from 100% power. The cause of the trips is under investigation. Unit 1 experienced a partial loss of 4160 volt power (Non-Safety Related) which resulted in a loss of Main Feedwater. Emergency Feedwater started on Unit 1. Unit 2 remained on Main Feedwater. Reactor Coolant Pump power was not affected by the trip on either Unit. Post trip equipment response on Unit 2 was normal. On Unit 1, a second HPI pump auto-started due to a non-safety low seal flow signal (one normally running for normal RCS makeup). Units 1 & 2 have been stabilized in Mode 3. Restoration of 4160 volt loads on Unit 1 is in progress. All Control rods fully inserted on both units. No PORVs or Safety valves lifted during the event for either unit. Both units are removing decay heat by steaming to the Main Condenser via the Turbine Bypass valves. Both units are in a normal shutdown electrical lineup. Unit 3 was not affected by this event. The licensee notified the NRC Resident Inspector. | Feedwater Main Condenser Control Rod | |
| ENS 42576 | 15 May 2006 14:59:00 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation | Momentary Loss of Dhr Due to Automatic Actuation of the Keowee Emergency Power Supply | Event: At 10:59 hours on 5-15-06, while in Mode 6 following completion of refueling activities, Oconee Unit 3 experienced a lockout of CT-3, the transformer for the Startup power source, which was in service at the time. This resulted in a momentary loss of AC power to the unit. Keowee Hydro Station, the Oconee emergency power source received an automatic emergency start signal, started, and closed in to supply power via the Underground Emergency Power path within approximately 40 seconds. Initial Safety Significance: Initial conditions of significant systems: Normal power via backcharge of main transformer was not available. The Fuel Transfer Canal was full and valves open connecting it to the Spent Fuel Pool. Time to core boil was 58 minutes per procedure. The Equipment Hatch was open. The initial loss of power resulted in interruption of Decay Heat Removal (DHR) Cooling, Spent Fuel Pool Cooling, and other support systems. Power was automatically restored and the affected systems returned to service promptly. Therefore there was no safety significance to this event. Reactor Coolant System heated up from approximately 80F to approximately 89.5F during this event. Corrective Action(s): As stated, Keowee started and supplied power automatically. The appropriate Abnormal Procedures were entered to restore power and restart these systems. DHR was restored at 11:13. Actions were initiated to achieve Containment Closure due to the loss of DHR. The Equipment Hatch was closed by 11:40. Backup power is available from Central Switchyard via CT-5. The cause of the initiating transformer lock out is under investigation. The licensee informed the NRC Resident Inspector. | Reactor Coolant System Main Transformer Decay Heat Removal | |
| ENS 41966 | 31 August 2005 18:28:00 | 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)(i), Tech Spec Required Shutdown 10 CFR 50.72(b)(2)(iv)(A), System Actuation - ECCS Discharge | Unit 3 Experienced an Automatic Reactor Trip During Routine Testing | Event: At 1428 hours on 8/31/2005, Oconee Unit 3 tripped. A routine test of the alternate power source for the Control Rod Drive System was in progress when power to the Control Rod Drive system was interrupted, which resulted in a reactor trip. AC power transferred to the Start-up source (switchyard). Normally the Main Steam Header pressure control setpoint is automatically increased for post-trip RCS temperature control. This did not occur. As a result the RCS cooled down to approximately 536F (versus a normal post-trip temperature of approximately 555F), reducing RCS pressure to the actuation setpoint for Engineered Safeguards Channels 1 and 2. This started the High Pressure Injection pumps in ECCS mode, caused partial containment isolation and initiated start-up of both Keowee Hydro Units (emergency power). Because Start-up power was available, Keowee did not supply power but remained in stand-by. At 1133 hours Operators terminated ECCS injection. Initial Safety Significance: Because RCS pressure decreased below normal post-trip levels which resulted in an ECCS actuation, this is considered an abnormal transient. Unit 3 has been stabilized and at this time the actual event is considered to have low safety significance. The exact cause of the loss of power to the Control Rod Drive system is unknown, but is under investigation. It is suspected that that loss also resulted in the failure of the Main Steam Header Pressure to shift to the post-trip Main Steam pressure control setpoint. Corrective Action(s): Operations stabilized Unit 3. A post-trip investigation is in progress, per site procedures and directives. All control rods fully inserted as a result of the reactor trip. No primary or secondary reliefs or PORVs lifted. Pressurizer level decreased off-scale low and was recovered prior to securing the High Pressure Injection pumps (the licensee estimates approximately 3000 gallons was injected). Current RCS temp is 542F (Tave) with RCS pressure in the normal post-trip band. Decay heat is being removed by the Steam Generators to Condenser through the Turbine Bypass Valves. Main Feedwater remained in service during the transient. The licensee informed the NRC Resident Inspector and does not plan a press release at this time.
During this event, the Engineering Safeguards (ES) System was manually bypassed at 14:33 on 8-31-05 to restore both High Pressure Injection (HPI) System trains to a normal lineup following an ES-initiated safety injection. Manually bypassing ES for both trains of HPI required entry into Tech Spec 3.0.3 at 15:33 on 8-31-05. Tech Spec 3.0.3 requires shutdown of Unit 3 to Mode 3 by 03:33 on 9-1-05 and to Mode 4 by 09:33 on 9-1-05. This condition was discovered to apply at 21:15 on 8-31-05. Initial Safety Significance: Units 1 and 2 remain at 100% power with no issues following the Unit 3 ES Actuation and Keowee Hydro Unit emergency start. Unit 3 remains in Mode 3. No other safety systems have actuated or exhibited abnormal behavior. Therefore, the safety significance of this condition is LOW. Corrective Action(s): Restore ES System to Automatic for the HPI System. The licensee reported this under 10 CFR 50.72(b)(2) (i), Technical Specification Shutdown. The licensee will notify the NRC Resident Inspector. R2DO (Lesser) notified. | Steam Generator Feedwater Control Rod Main Steam | 05000287/LER-2005-002 |