05000391/LER-2016-008

LER-2016-008, Reactor Trip Resulting from Failure of 2B Main Bank Transformer

Watts Bar Nuclear Plant, Unit 2
Event date:	08-30-2016
Report date:	10-28-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3912016008R00 - NRC Website

LER 16-008-00 for Watts Bar, Unit 2, Regarding Reactor Trip Resulting from Failure of 2B Main Bank Transformer

ML16302A296
Person / Time
Site:	Watts Bar
Issue date:	10/28/2016
From:	Simmons P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 16-008-00
Download: ML16302A296 (7)

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 internet 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,

I. PLANT OPERATING CONDITIONS BEFORE THE EVENT

Watts Bar Nuclear Plant (WBN) Unit 2 was in Mode 1 at 98 percent Rated Thermal Power (RTP) .

II. DESCRIPTION OF EVENT

A. Event Summary On August 30, 2016, at 2110 Eastern Daylight Time (EDT), the WBN Unit 2 reactor tripped on turbine trip as a result of an electrical fault. All control rods fully inserted and no safety or relief valves lifted. The Auxiliary Feedwater (AFW) system {EIIS:BA} actuated as designed.

The electrical fault was caused by an internal fault on the low voltage side of the 2B Main Bank Transformer (MBT){EIIS:XFMR} which resulted in a fire. The MBTs are part of the main generator output power system {El IS:EL}. The electrical fault was cleared by the 2B MBT sudden pressure and phase differential relays. Automatic fire suppression for the 2B MBT operated as expected and a fire fighting team was established by the fire brigade with assistance from local fire departments. The fire was extinguished at 2230 EDT.

This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a safety system actuation.

B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable systems contributed to the event.

C. Dates and Approximate Times of Occurrences Date Time (EDT) Event 8/30/16 2110 Unit 2 Reactor trip due to Turbine Trip (electrical fault). Concurrently receive fire alarms and fire pumps start.

2113 2B MBT reported engulfed in flames. Operations transitions to 2-ES-01, Reactor Trip Response.

2120 Notification of Unusual Event (NOUE) declared.

2122 Offsite assistance requested for fire fighting 2130 State of Tennessee Notified 2149 NRC notified of NOUE. Operations transitions to 2-GO-5, Unit Shutdown from 30 percent Reactor Power to Hot Standby.

2230 Fire extinguished 2314 Completed reactor shutdown. Unit stabilized in Mode 3.

2342 Exit from NOUE D. Manufacturer and Model Number of Components that Failed During the Event The 2B MBT is a General Electric Single Phase Power Transformer rated for 22.5kV/500kV service, Serial Number M-100714.

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 internet 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.

3. LER NUMBER

2016 - 00

E. Other Systems or Secondary Functions Affected

No other safety systems were affected by this event.

F. Method of discovery of each Component or System Failure or Procedural Error The failure of the 2B MBT became readily apparent based on field observation and protective relay actuation.

G. Failure Mode and Effect of Each Failed Component The 2B MBT failed as a result of an internal fault on the low voltage side of the transformer. No indications of transformer degradation (e.g. temperatures, bushing oil levels, dissolved gas levels) were present prior to the failure.

H. Operator Actions

Following the reactor trip, operations moved promptly through the emergency procedures and stabilized the plant. A senior reactor operator was established as the incident commander and directed the response to the transformer fire.

I. Automatically and Manually Initiated Safety System Responses All safety systems operated as expected. The reactor protection system and AFW system automatically actuated as designed.

III. CAUSE OF THE EVENT

A. The cause of each component or system failure or personnel error, if known.

This event was the result of an internal fault on the low voltage side of a large power transformer.

No indications of transformer degradation (e.g. temperatures, bushing oil levels, dissolved gas levels) were present prior to the failure.

A root cause evaluation (RCE) is in progress. The draft RCE indicates that the most likely cause was inadequate clearance between the X3 bus and the X1 flex braid as a result of either a latent design issue or initial installation error.

B. The cause(s) and circumstances for each human performance related root cause.

No human performance root cause is applicable to this event.

IV. ANALYSIS OF THE EVENT

Under normal operating conditions, the main generators supply electrical power through isolated-phase buses to three single phase main step-up transformers, which provide power to the switchyard and to serve on-site power loads through the unit station service transformers. The MBTs are 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 internet 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.

3. LER NUMBER

2016 - 00 provided with a deluge system in the event of their failure. The fault on the 2B MBT resulted in a turbine trip and resultant reactor trip, and concurrently resulted in a transformer fire with actuation of the were uncomplicated, with all safety related equipment operating as expected.

V. ASSESSMENT OF SAFETY CONSEQUENCES

The loss of the 2B MBT led to a reactor trip and a fire in the plant switchyard. The response to the Unit trip was uncomplicated with the exception of the transformer fire. The fire was extinguished in a little over one hour with the assistance of local firefighters. The probabilistic risk analysis of this event indicates that while a moderate increase in the potential for a Loss of Offsite Power (LOOP) occurred, the core damage increase did not significantly exceed annual baseline values.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event No safety systems failed during this event.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Not applicable.

C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service Not applicable.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority (WA) Corrective Action Program and is being tracked under condition report (CR) 1208823.

A. Immediate Corrective Actions

The WBN spare MBT was prepared and set up to replace the failed 2B MBT. A root cause evaluation was initiated.

B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future Internal inspections of the 2A, 2C and Spare MBT were performed to confirm adequate clearances were present in these transformers. TVA is in the process of procuring replacement transformers for WBN for long term reliability. As a result of a subsequent event documented in CR 1225886, MBTs where clearances were minimal.

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 internet 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.

3. LER NUMBER

2016 - 00 008

VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE

No previous large transformer failures have occurred at the Watts Bar site.

VIII. ADDITIONAL INFORMATION

If the final root cause for this event is significantly different than what is described in this LER, the LER will be supplemented.

IX. COMMITMENTS

None.