Semantic search
| Entered date | Event description | |
|---|---|---|
| ENS 51995 | 8 June 2016 17:23:00 | At 1526 Eastern Daylight Time on 6/8/2016, a determination was made involving the potential impact of a tornado on the Emergency Diesel Generators (EDGs). The EDGs are required to be operable to provide power to ensure that acceptable fuel design limits, reactor coolant system pressure boundary limits, and containment integrity are not exceeded during abnormal transients. Further, the EDGs are designed with a crankcase pressure trip (setpoint = 1 inch water), which is bypassed during an emergency start. Engineering has determined that a tornado could potentially cause actuation of the crankcase pressure trip due to a low barometric condition. If an emergency start signal has NOT previously occurred, then during a tornado, actuation of the crankcase pressure trip would energize the shutdown relay causing an EDG lockout condition. The EDG lockout condition prevents subsequent EDG starts (normal or emergency) until operators manually reset the lockout condition locally at the EDG. This condition could potentially affect all four EDGs simultaneously. The EDGs are operable but degraded. All EDGs have successfully passed their required surveillances within the appropriate frequency. No severe weather warnings or watches are forecast in the local areas, which could challenge the crankcase pressure trip. This condition places both units in an unanalyzed condition that potentially significantly degrades plant safety, 10 CFR 50.72 (b)(3)(ii)(B). A compensatory measure has been established, that upon notification of a Tornado Warning, the EDGs would be 'emergency started' and run during the time the Tornado Warning was in effect. This action bypasses the crankcase pressure trip function and allows the EDGs to perform their required safety function. The NRC Senior Resident Inspector has been notified. |
| ENS 51982 | 5 June 2016 16:22:00 | On June 5, 2016 at 1227 Eastern Daylight Time (EDT), Watts Bar Nuclear Plant (WBN) Unit 2 was in MODE 1 at approximately 12.5% power when a safety injection actuation occurred, followed by an automatic reactor protection system (RPS) trip. Preliminary data suggests that the #1 high pressure turbine governor valve failed open causing a steam header pressure rate of decrease safety injection (SI) actuation signal. As designed, the safety injection actuation caused both trains of the shared Emergency Gas Treatment System (EGTS) to align to Unit 2, requiring WBN Unit 1 to enter Technical Specification (TS) LCO 3.0.3 at 1227 (EDT). Also as designed, Unit 1 annulus pressure momentarily increased, causing operation personnel to enter LCO 3.6.15 Condition B at 1240 (EDT). At 1242 (EDT), after annulus pressure normalized, operations personnel exited LCO 3.6.15. At 1245 (EDT), Operations personnel secured safety injection and Unit 2 was stabilized in MODE 3 at normal operating pressure and temperature. By 1349 (EDT), Unit 1 Operations personnel had restored both trains of EGTS to standby readiness, and exited TS 3.0.3. No primary safety barriers (RCS, containment and fuel clad) were challenged and no primary or secondary safety or relief valves actuated during the event. The Unit 2 plant trip was uncomplicated and safety equipment operated as expected. Unit 2 is stable in Mode 3 at normal operating temperature and pressure, in normal shutdown power alignment. Unit 1 is stable in Mode 1 at 100% power. The Senior Resident Inspector has been notified of this event. The MSIVs are shut with the steam generators (SG) discharging steam using the atmospheric dump valves. There is no primary to secondary leakage. Motor driven AFW pumps are running to maintain SG levels. RCS pressure remained above ECCS (Emergency Core Cooling System) discharge pressure. |
| ENS 49154 | 28 June 2013 16:34:00 | On June 28, 2013 at 1330 (EDT), Watts Bar Nuclear Plant Unit 1 reactor automatically tripped due to an electrical fault causing a main generator lockout and subsequent turbine trip. The electrical fault generated an 'A' Main Bank Transformer Differential Relay actuation. Suspected cause is due to an offsite fault. This is being reported under 10CFR50.72(b)(2)(iv)(B). Concurrent with the reactor trip the Auxiliary Feedwater system actuated as designed. This is being reported under 10CFR50.72(b)(3)(iv)(A). All Control and Shutdown rods fully inserted. All safety systems responded as designed. The unit is currently stable in Mode 3, with decay heat removal via Auxiliary Feedwater, steam dumps and the main condenser. The station is in a normal shutdown electrical alignment. The NRC Senior Resident has been notified. There were no primary or secondary relief valve actuations during the plant transient. The cause of the electrical fault is being investigated. |
| ENS 48414 | 17 October 2012 05:46:00 | On October 16, 2012, Watts Bar Nuclear Plant Unit 1 experienced a momentary loss of power to the 1B 6.9 KV Shutdown board. This resulted in an emergency start of all 4 Emergency Diesel Generators due to actuation of the designed undervoltage EDG start signal. A manual transfer of the shutdown board from the Normal power supply to the Alternate was in progress to support maintenance. The momentary loss of power lasted approximately 2 seconds, which caused load shed and Diesel Generator start to occur. Core cooling was maintained throughout the transient. This condition is being reported under 10 CFR 50.72 (b)(3) (iv) (A). All safety systems responded as designed. The unit remains stable in Mode 5. The cause for this failure is not yet known. Investigation is in progress. The NRC Senior Resident has been notified. |
| ENS 48238 | 28 August 2012 06:13:00 | On August 28, 2012, Watts Bar Nuclear Plant Unit 1 reactor automatically tripped due to low level in steam generator (SG) #2. The low level resulted when the Main Feedwater Control Valve for SG#2 (1-FCV-3-48) failed closed. This is being reported under 10CFR 50.72 (b)(2) (iv) (B). Concurrent with the reactor trip the Auxiliary Feedwater system actuated as designed. This is being reported under 10CFR 50.72(b)(3) (iv)(A). All Control and Shutdown rods fully inserted. All safety systems responded as designed. The unit is currently stable in Mode 3. The NRC Senior Resident has been notified. Decay heat is being removed to the main condenser via condenser steam dumps. The plant is in its normal shutdown electrical lineup. No steam safety or relief valves lifted during the event. |
| ENS 48204 | 17 August 2012 14:11:00 | Watts Bar Nuclear Plant's (WBN's) containment ventilation isolation systems provide the means of isolating ventilation systems that pass through containment penetrations to confine to the containment any radioactivity that may be released following a design basis event. The containment ventilation system isolates following a manual or automatic safety injection signal, high containment purge exhaust radiation levels, or manual actuation. On 6/18/12 at 2349 EDT, a B train containment ventilation isolation signal was received in the Main Control Room due to an invalid high radiation signal from a containment purge exhaust radiation monitor (1-RM-131). Corrective action replaced the 1-RM-131 ratemeter following the 6/18/12 B train containment ventilation isolation. However, on 7/2/12 at 1252 EDT, a second B train containment ventilation isolation signal was received in the Main Control Room due to another invalid high radiation signal from 1-RM-131. An investigation found that 1-RM-131 was spiking repeatedly due to a defective Power On indicating light socket which affected the 120 VAC power circuit that is common with the 24 volt power supply. 120 VAC signal fluctuations could affect the 24 volt power supply signal to the rate meter causing output spikes. Corrective action replaced the defective Power On indicating light socket on 7/3/12. This event notification is being made in accordance with 10 CFR 50.73(a)(2)(iv)(A) as a 60 day telephone notification of the invalid initiation of a containment isolation signal. The specific system and train that was actuated was B train containment ventilation isolation. The system functioned as designed and the complete train operated. WBN's NRC Resident Inspector has been notified. |