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On August 17, 2017, at 1205 Eastern Daylight Time (EDT), the Watts Bar Nuclear Plant (WBN) lost power to the 1B-B 6.9kV Shutdown Board. The loss of power to this safety related bus resulted in an automatic start of the Unit 1 Turbine Driven Auxiliary Feedwater Pump (TDAFWP). Power to the 1B-B Shutdown Board (SDBD) was restored at 1505 EDT on August 17, 2017.

During the loss of power to the 1B-B SDBD, a reduction in containment and control rod drive mechanism cooling occurred. At 1233 EDT, lower containment average temperature exceeded Technical Specification (TS) limits, and TS 3.6.5 Condition A was entered for containment average air temperature not within limits. Lower containment average temperature was restored to within limits at 1525 EDT on August 17. 2017. This is reportable as a potential loss of safety function.

The cause of this event is mechanical vibration while closing a panel drawer resulting in actuation of protective relays that led to a loss of power.

Clearances will require the relays involved in this event to be isolated during drawer movement to prevent a similar occurrence.

On August 23. 2017. Watts Bar Nuclear Plant (WBN) identified that procedures 1-E-1 and 2-E-1, Loss of Reactor or Secondary Coolant, contained steps to manually open 1-FCV-67-458 in the event of a Train A or B power failure.

Opening 1-FCV-67-458 would result in the crosstie of Essential Raw Cooling Water (ERCW) Headers 2A and 1B, which would lead to providing flow to equipment not operating due to the loss of a train of power. On October 6. 2017.

it was determined that for certain time periods, if a design basis accident had occurred on Unit 2 with a loss of offsite power concurrent with a train failure and with 1-FCV-67-458 opened, inadequate ERCW flow would have been available to remove decay heat after transfer to cold leg recirculation. This condition only affected operability of ERCW Train A. This is reportable as a condition prohibited by Technical Specification 3.7.8.

The issue associated with this incorrect procedural step to cross-tie the ERCW trains in 1-E-1 and 2-E-1 was addressed as part of actions to resolve an ERCW design and procedure issue documented in Licensee Event Report (LER) 390-2017-009. This report, while related, identifies an issue that was not addressed in the prior LER. The cause was determined to be the incorrect application of a cross tie requirement associated with 10 CFR 50 Appendix R. Corrective action will be to include engineering in the review of procedures affected by complex design changes.

NRC FORM 355 ;:;4-217' APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020 comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001. or by e-mail to NEOB-10202. (3150-0104), Office of Management and Budget. Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number. the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

On October 30. 2017. at 0942 Eastern Daylight Time (EDT) Watts Bar Nuclear Plant (WBN) operations personnel received a Main Control Room (MCR) alarm for low control room positive pressure. At 0943 EDT, a Control Room Envelope (CRE) door was found ajar and immediately closed. Technical Specification 3.7.10 Control Room Emergency Ventilation System (CREVS) was declared not met for both trains, and Limiting Conditions for Operation (LCO) Condition B was entered for Unit 1 (Mode 1) and Condition G was entered for Unit 2 (Mode 5). At 0945 EDT the alarm cleared, CREVS was declared operable and LCO 3.7.10, Conditions B and G were exited. The loss of the control room envelope is being reported as a loss of safety function needed to mitigate the consequences of an accident.

The cause of this issue is a human performance error in that an individual leaving the control building complex failed to confirm closure of the MCR envelope boundary door. Corrective actions have been generated to develop and install an engineering feature to inform personnel closing the door that it is fully shut and latched.

From March 18, 2016, when Watts Bar Nuclear Plant Unit 2 first entered Mode 4 to April 14, 2016 with the plant in Mode 3, it was determined that a condition prohibited by Technical Specifications (TS) existed. During this time both automatic and manual closure of the containment isolation valves and the sample isolation valves for the Steam Generator Blowdown (SGBD) sampling lines were disabled due to improperly installed electrical jumpers in the valve control circuits. The misplaced jumpers bypassed the Phase A containment isolation signals, the auto/manual start signals for the Auxiliary Feedwater (AFW) pumps, and the control valve seal-in circuits. Containment isolation on a Phase A signal is used to control potential release of radioactive material to the environ in the event of a Design Bases Accident. The AFW pump auto/manual start signals are used to isolate the SGBD sampling lines to preserve steam generator inventory. The seal-in circuits are used to allow the operator to manually position the valves in either the open or closed position from the main control room. This event occurred prior to initial reactor criticality. There was no loss of safety function.

The isolation valves for the SGBD sample lines were returned to service on April 14, 2016. This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B) and 10 CFR 50.73(a)(2)(vii)(B) and (C).

The Unit 2 Turbine Driven Auxiliary Feedwater (TDAFW) pump auto-started upon a planned Reactor Trip at 0154 Eastern Daylight Time (EDT) on May 28, 2016. At 0157 EDT the Reactor Operator noted that TDAFW forward flow to Steam Generators 1 and 3 were approximately 800 gallons per minute, and placed the associated Level Control Valves in the closed position. At approximately 0203 EDT the Main Control Room received Alarm Window 60-A, TDAFW Pump Electrical Overspeed Trip. Operators walked down the TDAFW pump and determined that the turbine had tripped, by confirming that the Trip and Throttle Valve was no longer latched, and declared the TDAFWP inoperable. The equipment was repaired, the TDAFWP was re-tested successfully and returned to service. Technical Specification (TS) Limiting Condition for Operation 3.7.5 was exited on May 30, 2016.

The plant conditions at the time of the event were Unit 2 in Mode 3 at Normal Operating Temperature/Normal Operating Pressure following manual reactor trip from Mode 1. The reactor trip was unrelated to this event.

On June 29 2016, a past operability evaluation concluded that the TDAFWP had been inoperable from March 30 through May 30, 2016. This is reportable as a condition prohibited by TS.

On June 20, 2016, the 2B Main Feedwater Pump (MFP) tripped on a loss of vacuum in the 2B MFP turbine condenser, resulting in a loss of normal feed, and the subsequent trip of the main turbine. While operators were reducing power to within the capacity of Auxiliary Feedwater (AFW), the reactor tripped at 1540 Eastern Daylight Time (EDT) on Steam Generator Water Level (SGWL) Lo Lo in Steam Generator No.4. SG water level lowered rapidly due to shrink from the relatively cold AFW following the trip.

All control and shutdown rods fully inserted. All safety systems responded as designed. The trip response was uncomplicated.

The trip was caused by a human performance error during the drain down of the 2A MFP turbine condenser which resulted in a loss of vacuum on the 2B MFP turbine.

On August 23, 2016, at 1356 Eastern Daylight Time (EDT), during power ascension testing, Watts Bar Nuclear Plant (WBN) Unit 2 reactor was manually tripped due to a loss of main feedwater. Concurrent with the reactor trip, the Auxiliary Feedwater system actuated as designed. All control and shutdown rods fully inserted. All safety systems responded as designed.

The loss of main feedwater was due to a leak on a hydraulic fitting associated with the Main Feedwater Pump Turbine High Pressure Governor valve, resulting in the valve going partially closed with reactor power at 48 percent. With the governor valve partially closed, feedwater flow was reduced such that the unit needed to be manually tripped.

Subsequent investigation determined the leak to be caused by the installation of incompatible fittings associated with the governor valve that occurred during plant construction.

upper containment airlock inboard door was found not closed while the outboard airlock door was open. This created a containment bypass with leakage potentially greater than allowed by the Technical Specifications. The operator immediately identified that the pressure equalizing valve for the inner door was not fully closed when the outer door of the airlock was opened. The outer door was promptly shut to isolate the airlock. The inner door was then cycled which closed the equalizing valve. The total time that a containment bypass was present is estimated to be five minutes.

The equalizing valve did not seat properly due to a damaged part in the valve closing mechanism. The airlock remains functional, and an operations caution order was put in place related to use of this air lock. The airlock will be repaired prior to Unit 2 returning to Mode 4.

NRC I ORM 366 M6-2016; APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internal e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

On March 23, 2017, at 0014 Eastern Daylight Time (EDT), Watts Bar Nuclear Plant Unit 2 experienced an unplanned trip condition of both Turbine Driven Main Feed Pumps (TDMFPs) following a loss of Main Condenser Vacuum. The trip of both TDMFPs caused an automatic start of both Motor Driven Auxiliary Feed Water Pumps and the Turbine Driven Auxiliary Feed Water Pump as designed.

The plant was performing a normal startup, and had just synchronized the main generator to the grid. Subsequent to the event, the plant was transitioned to Mode 3 by inserting all control rods with a manual trip. All plant safety systems operated as expected.

The loss of condenser vacuum was the result of a significant breach of the Unit 2 main condenser - B zone. This failure is attributed to the main condenser neck support structural design being inadequate to maintain integrity within specification. Repairs to the condenser will be completed prior to Unit 2 returning to service.

On November 26, 2017. at 1225 Eastern Standard Time (EST), the Watts Bar Nuclear Plant (WBN) Unit 2 experienced an unplanned Emergency Core Cooling System (ECCS) discharge to the Unit 2 Reactor Coolant System (RCS) while de-pressurized. in Mode 5. with the Pressurizer vented to the Pressurizer Relief Tank.

ECCS injection via the Boron Injection flow path occurred during planned Safety Injection system Engineered Safety Features Actuation System (ESFAS) testing. The Boron Injection flow path should have been isolated and should not have resulted in any injection flow to the Unit 2 RCS. The condition was promptly corrected by operator actions based on observed plant conditions.

The cause of this event is that an Operator improperly used a Caution Order to determine the configuration of the breaker for the Boron Injection Tank outlet valve. Correct Component Verification was not utilized as required. and the current position of the breaker in the field was not validated to support testing.

Corrective actions for this event include revising procedures to ensure the breakers associated with the boron injection flow path will be tagged open during ESFAS testing and that lessons learned related to this event are communicated to operating crews. An evaluation on the use of Caution Orders for off normal equipment positions will be performed .

NRC FORM 330604-2O'