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| Start date | Reporting criterion | Title | Event description | System | LER | |
|---|---|---|---|---|---|---|
| ENS 40847 | 22 May 2004 09:29:00 | 10 CFR 50.73(a)(1), Submit an LER | Invalid Actuation of Division 1 Primary Containment Isolation Due to Loss of a Rps Bus | The following information was received from the licensee via facsimile: On May 22, 2004, at approximately 4:29 am EDT, while the plant was operating at 100% power, the normal power supply to the Division 1 Reactor Protection System (RPS) bus was lost. This caused the actuation of the Division 1 primary containment isolation logic circuit. This was an event that resulted in the actuation of a general containment isolation signal affecting more than one system. However, as this event meets the definition of an invalid actuation (i.e., not a response to an actual plant parameter exceeding a trip setpoint), this notification is being made in accordance with 10CFR50.73(a)(1) in lieu of a Licensee Event Report. Operators implemented the applicable response procedures, and shifted Division 1 RPS to its alternate power supply. The containment isolation signal was reset, and systems were restored to normal alignment. Reactor power was not affected by this event. All safety related equipment controlled by the affected circuits responded to the loss of Division 1 RPS as required, with one exception. The Division 1 control building emergency filtration train should have automatically initiated, but did not. Subsequent troubleshooting discovered a failed relay in its control circuitry. This defect would not have prevented the Division 2 control building emergency filtration train from performing its safety function. It was determined that a relay coil in the control circuitry for the Division 1 RPS motor-generator set failed. This caused the main output contactor on the MG set to open, interrupting power to the bus. Repairs were completed on June 3, 2004, and the Division 1 RPS bus was then shifted back to its normal power supply. The licensee has notified the NRC Resident Inspector of this notification. | Reactor Protection System Primary containment | |
| ENS 43012 | 29 September 2006 18:33:00 | 10 CFR 50.73(a)(1), Submit an LER | Invalid Specific System Actuation | On September 29, 2006, at 1:33 pm CDT, a safety-related 120VAC distribution panel was inadvertently de-energized during a planned shift of its power supply. The plant was operating at 100 percent power at the time. The loss of power resulted in the automatic closure of primary containment isolation valves in the reactor water cleanup and the suppression pool cooling/cleanup systems. This event is being reported in accordance with 10CFR50.73(a)(1) as an invalid actuation of the containment isolation valves affecting more than one system. This event occurred during a planned shift of the inverters that supply the distribution panel. This shift was being performed to support post-maintenance testing of one of the inverters. The operators performing the shift were utilizing the appropriate procedure. At a certain point in the procedure, the off-going inverter was deenergized, and the operators discussed the expected equipment response. Following that discussion, a step was erroneously performed out of sequence, resulting in the loss of power to the 120VAC distribution panel. The primary containment isolation signal was actuated as designed, and the appropriate valves responded correctly. Reactor power was not affected by the containment isolation signal. The pertinent response procedures were implemented, and actions were taken to restore the distribution panel to service. This was completed at 5:31 pm CDT that day. The licensee will notify the NRC Resident Inspector. | Primary containment Reactor Water Cleanup | |
| ENS 44278 | 23 April 2008 20:56:00 | 10 CFR 50.73(a)(1), Submit an LER | 60-Day Optional Notification of Invalid Specified System Actuation | On April 23, 2008, at 3:56 p.m. CDT, power was lost on the reactor protection system (RPS) bus 'B'. The plant was operating at 100 percent power at the time. The loss of RPS bus 'B' interrupted power to the containment isolation actuation circuitry. This caused the Division 2 containment isolation valves in various balance-of-plant systems to automatically close, as designed. Plant operators implemented recovery procedures to shift RPS 'B' to its alternate power supply, and restore the systems affected by the isolation to service. The containment isolation valves were confirmed to have actuated as required. Plant (generating) capacity was not interrupted by the containment isolation signal. The investigation of this event did not conclusively determine the cause of the loss of RPS bus 'B'. At the time of the event, scheduled maintenance was in progress to replace a relay which had contacts in the RPS 'B' circuitry. It is postulated that during this work, the technician inadvertently contacted part of the relay case while re-terminating the wiring, causing a short-circuit which blew a fuse in the circuit. This caused the output breaker on the RPS 'B' motor-generator set to trip, which de-energized the bus. This event is being reported in accordance with 10CFR50.73(a)(2)(iv)(A) as an invalid actuation of the Division 2 containment isolation actuation system. The loss of RPS 'B' also caused a half-scram signal to be generated. The licensee has notified the NRC Resident Inspector. | Reactor Protection System | |
| ENS 44347 | 2 June 2008 11:24:00 | 10 CFR 50.73(a)(1), Submit an LER | Invalid System Actuation of Division 1 Containment Isolation Valves | On June 2, 2008 at 6:24 a.m. CDT power was lost on the reactor protection system (RPS) bus 'A'. The plant was operating at 100 percent power at the time. The loss of RPS bus 'A' interrupted power to containment isolation actuation circuitry. This caused the Division 1 containment isolation valves in various balance of plant systems to automatically close, as designed. Plant operators implemented recovery procedures to shift RPS 'A' to its alternate power supply, and restore the systems affected by the isolation. The containment isolation valves were confirmed to have actuated as required. Plant capacity was not interrupted by the containment isolation signal. As their primary power source, each RPS bus is supplied by its own motor generator set. The investigation of this event found that two resistors on a circuit card in the voltage regulator on the 'A' RPS motor-generator set failed. This had the effect of causing the generator voltage to fluctuate significantly, resulting in a automatic trip of the generator output breaker and a loss of power on the bus. This event is being reported in accordance with 10CFR50.73(a)(2)(iv)(A) as an invalid actuation of the Division I containment isolation actuation system." The licensee informed the NRC Resident Inspector. | Reactor Protection System | |
| ENS 44517 | 1 August 2008 00:30:00 | 10 CFR 50.73(a)(1), Submit an LER | Invalid Actuation of Containment Isolation Valves Due to a Power Failure | On July 31, 2008, at 7:30 p.m. CDT, power was lost on the reactor protection system (RPS) bus A. The plant was operating at 100 percent power at the time. The loss of RPS bus A interrupted power to the containment isolation actuation circuitry. This caused the Division 1 containment isolation valves in various balance-of-plant systems to automatically close, as designed. Plant operators implemented recovery procedures to shift RPS A to its alternate power supply, and restore the systems affected by the isolation to service. The containment isolation valves were confirmed to have actuated as required. Plant capacity was not interrupted by the containment isolation signal. The normal power supply for each RPS bus is a dedicated motor-generator set. The output of that generator is routed to the bus via two Electrical Protection Assembly (EPA) breakers arranged in series. Each EPA breaker has an internal power supply supporting the various trip functions designed into the breaker. Troubleshooting activities determined that the power supply in the upstream EPA breaker between the motor-generator set and the A RPS bus had failed, causing the undervoltage function of that EPA to actuate. The cause of the failure of the power supply has not been determined. This event is being reported in accordance with 10CFR50.73(a)(1) and 10CFR50.73(a)(2)(iv)(A) as an invalid actuation of the division 1 containment isolation actuation system. The licensee notified the NRC Resident Inspector. | Reactor Protection System | |
| ENS 46844 | 20 March 2011 12:24:00 | 10 CFR 50.73(a)(1), Submit an LER | "a" Train Reactor Protection System Unexpectedly De-Energized Resulting in a Half Scram Signal | On March 20, 2011, at approximately 7:24 a.m. CDT, the 'A' reactor protection system (RPS) bus was unexpectedly de-energized. Plant systems responded as designed, resulting in a half-scram signal and the actuation of the Division 1 primary containment isolation sub-system. The Division 1 primary containment isolation valves closed in several balance-of-plant systems. After confirming that no valid RPS trip signal had occurred, operators executed the appropriate procedures to return the affected systems to the proper configuration, and to reset the half-scram signal. Plant capacity was not affected by this event. This event is being reported in accordance with 10CFR50.73(a)(1) as an automatic actuation of the Division 1 primary containment isolation valves in multiple systems resulting from an invalid signal. Each of the two redundant distribution buses in the RPS system is normally powered by its own motor-generator (MG) set. The investigation of this event determined that the output breaker of the 'A' MG set had tripped open. The most likely cause for this trip was a malfunction of the over-voltage relay circuit card in the MG set. Although the troubleshooting was inconclusive, there apparently was a momentary transient in the output of the trip setpoint adjustment potentiometer on the card, likely caused by oxidation on the contacting surface between the wiper arm and the windings. The over-voltage relay card on the 'A' MG set was replaced, and the unit was restored to service. Preventative maintenance tasks are being revised to periodically cycle all potentiometers in the MG set control circuits to wipe oxidation from the windings. The Licensee has notified the NRC Resident Inspector. | Reactor Protection System Primary containment | |
| ENS 50774 | 6 December 2014 16:12:00 | 10 CFR 50.73(a)(1), Submit an LER | 60 Day Optional Report - Half Scram and a Division 2 Primary Containment Isolation Signal | On December 6, 2014, at approximately 1012 CST, while the plant was operating at 100 percent power, the Division 2 reactor protection system (RPS) bus de-energized unexpectedly. This resulted in a half-scram and a Division 2 primary containment isolation signal. Operators executed the appropriate abnormal operating procedures to begin an orderly restoration of the affected systems. Atmospheric pressure in the primary containment momentarily reached the high-pressure alarm setpoint, necessitating entry into the emergency operating procedure for that condition. Automatic isolation valves in the following systems closed as designed: - reactor plant component cooling water - drywell unit cooler water supply - reactor building floor and equipment drains - reactor building HVAC chilled water supply - containment airlock seal air supply - reactor recirculation system flow control valve hydraulics - main steam line drains - reactor water cleanup - auxiliary building and annulus HVAC systems These engineered safety systems actuated as designed: - standby gas control filter trains - fuel building filter trains - control building filter trains The event occurred approximately 25 hours after the Division 2 RPS motor-generator (MG) was aligned to the bus following replacement of the voltage regulator. Following the event, the MG set was found running with its output breaker tripped. A failure analysis determined that the spike suppressor and the field flash card were potential sources of the MG breaker trip. The spike suppressor was replaced. Inspection of the field flash card found a strand of wire from one of the attached leads nearly touching a trace on the circuit board. Testing determined that the wire strand was the most likely cause for the breaker trip. With no spare card readily available, the wire strand was removed and the field flash card was re-installed. Other cards were inspected, and no similar conditions were found. The MG set was load tested for 30 hours, and was placed in service on December 17(, 2014). Additionally, it is suspected that there is an intermittent failure occurring in the field flash card. A design change will be developed to correct that condition. The licensee has notified the NRC Resident Inspector. | Reactor Protection System Primary containment HVAC Reactor Water Cleanup Main Steam Line | |
| ENS 51003 | 24 February 2015 22:02:00 | 10 CFR 50.73(a)(1), Submit an LER | 60-Day Optional Telephone Notification of Invalid System Actuation Due to Motor Generator Output Breaker Trip | On February 24, 2015, at approximately 1702 CDT, while the plant was in cold shutdown, power was lost on the Division 1 reactor protection system (RPS) bus. This event resulted in the automatic closure of the Division 1 primary containment isolation valves in the residual heat removal (RHR) and reactor water cleanup systems. Additionally, the primary containment atmospheric monitoring system automatically actuated, and ventilation systems in the fuel building, auxiliary building, and control building shifted to emergency mode. The closure of the isolation valves in the residual heat removal system caused an automatic trip of the 'A' RHR pump, which had been in the shutdown cooling alignment. The equipment response to the isolation signal was as expected. This event is being reported in accordance with 10 CFR 50.73(a)(1) as an invalid actuation of the Division 1 primary containment isolation system. The isolation was promptly diagnosed as having resulted from a trip of the output breaker of the RPS motor generator (MG) set 'A,' and not from a valid signal. Operators implemented the appropriate response procedures to align power to the bus via the alternate source, and began restoring the affected systems. The 'A' RHR pump was re-started within twelve minutes, during which time coolant temperature increased approximately seven degrees to a maximum of approximately 100F. Other affected systems were restored over the next few hours. The causal analysis concluded that the MG set output breaker tripped due to an overly conservative setpoint on the overvoltage trip relay. The low trip setpoint was a latent condition that had existed since the output voltage was raised in 1988 at the recommendation of the vendor, but at which time the trip setpoint was not changed. To correct this condition, the MG overvoltage trip setpoint was raised to restore adequate operating margin to the normal MG output voltage. At the time of the event, the plant was in MODE 5 with the reactor cavity flooded to greater than 23 feet above the vessel flange. The shutdown cooling system was promptly restored to service. This event was of minimal safety significance to the health and safety of employees and the public. The licensee has notified the NRC Resident Inspector. | Reactor Protection System Primary Containment Isolation System Primary containment Shutdown Cooling Residual Heat Removal Reactor Water Cleanup | |
| ENS 51784 | 10 January 2016 08:43:00 | 10 CFR 50.73(a)(1), Submit an LER | Invalid Actuation of the Primary Containment Isolation Logic | On January 10, 2016, at 0243 CST, with the plant in cold shutdown, the primary containment isolation logic was actuated as the result of an invalid signal. This condition occurred while operators were installing electrical jumpers designed to bypass certain isolation signals for the suction valves in the residual heat removal (RHR) system that comprise the shutdown cooling flow path. These jumpers are installed under procedural guidance for the purposes of increasing the reliability of the shutdown cooling loop by disabling isolation signals that are not required to be operable in certain plant operating modes. Although it could not be proven, it appears that inadvertent contact with an energized circuit occurred during the jumper installation, causing a fuse to blow, de-energizing part of the primary isolation logic. This caused the automatic closure of Division 1 suction and return valves in the shutdown cooling loop, as well as valves connecting the reactor plant sampling systems to the RHR system. The main control room crew implemented recovery procedures to restore shutdown cooling to service at 0401 CST, prior to exceeding any temperature limits. This event resulted from the failure to maintain corrective actions in place that were develop after a similar event in 1994. Additionally, the operators were not using the type of jumpers required by the procedure, which likely contributed to the blown fuse. The RHR system operating procedure has been revised to require that the potentially affected valves in the shutdown cooling loop will be de-energized during jumper installation to eliminate the possibility of inadvertent isolation. This is being reported in accordance with 10 CFR 50.73(a)(1) as an invalid actuation of the primary containment isolation logic. During this event, the RCS temperature increased from approximately 130 to 190 degree F. The licensee will notify the NRC Resident Inspector. | Primary containment Shutdown Cooling Residual Heat Removal | |
| ENS 54121 | 30 April 2019 11:50:00 | 10 CFR 50.73(a)(1), Submit an LER | 60-Day Optional Telephonic Notification Due to Invalid Actuation of a General Containment Isolation Signal | This 60-day telephone notification is being made in accordance with the reporting requirements specified by 10 CFR 50.73(a)(1) and 10 CFR 50.73(a)(2)(iv)(A) to describe an invalid actuation of a general containment isolation signal affecting multiple systems. On April 30, 2019, at approximately 0650 CDT, a level 2 containment isolation signal was introduced when a fuse for the Nuclear Steam Supply Shutoff System was removed for a maintenance clearance. The level 2 containment isolation signal caused a trip of the Division I DC bus back-up charger, leaving only the Division I battery to carry the DC bus. At 0707 CDT the bus was de-energized when another unrelated clearance opened the battery supply breaker to the DC bus causing another containment isolation signal. This event did not affect Shutdown Cooling or any other protected Safety Related Equipment. The containment isolation signals caused an isolation of the systems listed below. All components that were not removed from service, gagged in position, already in the expected position due to plant conditions, or de-energized due to plant condition performed as designed. Containment Isolation valves for the following systems isolated as expected: Drywell and Containment Floor Drains, Drywell and Containment Equipment Drains, Condensate Makeup, Fire Protection Water, Service Air, Instrument Air, Reactor Water Cleanup, Spent Fuel Cooling and Cleanup, Reactor Plant Component Cooling Water, Chilled Water, Reactor Recirculation, Main Steam Drains, Reactor Building Ventilation, and Fuel Building Ventilation. The licensee notified the NRC Resident Inspector. | Shutdown Cooling Reactor Building Ventilation Reactor Water Cleanup Main Steam |