05000391/LER-2016-008, 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) | |
| Event date: | |
|---|---|
| Report date: | |
| 3912016008R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 October 28,2016 10 cFR 50.73 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001
Subject:
Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391 Licensee Event Report 391/2016-008-00, Reactor Trip Resulting from Failure oI2t3 Main Bank Transformer This submittal provides Licensee Event Report (LER) 39112016-008-00. This LER provides details concerning a recent event where the failure of the 28 Main Bank Transformer resulted in a fire and a reactor trip. This report is being submitted in accordance with 10 CFR 50.73(aX2)(iv)(A).
There are no regulatory commitments contained in this letter. Please direct any questions concerning this matter to Gordon Arent, WBN Licensing Director, at (423) 365-2004.
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Site Vice President Watts Bar Nuclear Plant Enclosure cc: see Pag e 2
U.S. Nuclear Regulatory Commission Page2 October 28,2016 cc (Enclosure):
NRC Regional Administrator - Region ll NRC Senior Resident lnspector - Watts Bar Nuclear Plant
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION (1 1-2015)
LICENSEE EVENT REPORT (LER)
APPROVED BY OMB: NO.3150-0104 EXPIRES: 10/31/2018
, the NRC may not conduct or sponsor, and a person is not required to respond to, the inbrmation collection,
- 1. FACILITY NAME Watts Bar Nuclear Plant, Unit 2
- 2. DOCKET NUMBER 0500039 1
- 3. PAGE 10F5
- 4. TITLE Reactor Trip Resulting from Failure of 28 Main Bank Transformer
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTHI DAY ! YEAR YEAR I ttt'r?J$L REV NO.
MONTH I DAY YEAR FACrLrry NAME I
DOCKET NUMBER N/A lN/A 08 30 I 2016 2016 -008
- - 00 10 28 2016 FACILITY NAME I
DOCKET NUMBER N/A Inn
- 9. OPERATING MODE t I. THIS REPORT lS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR $: (Check all that apply) 1 tr 20.2201(b) tr zo.22os(aX3)(i) tr 50.73(aX2Xii)(A) tr 50.73(aX2XviiiXA) tr 2o.2zo1(d) tr 2o.z2o3(aX3)(ii) tl 50.73(aX2XiiXB) n 50.73(aX2XviiiXB) tr 2o.22os(a)(1) tr zo.zzos(aX4) tr 50.73(ax2xiii) tr 50.73(aX2XixXA) tr 2o.z2o3(a)(2)(i) tr 50.36(cxl XIXA) tr s0.73(aX2XivXA) tr s0.73(aX2Xx)
- 10. POWER LEVEL 98 tr zo.z2os(aX2Xii) tr 50.36(cxl XiiXA) tr 50.73(aX2XvXA) n rcl1(aX4) tr zo.22os(aX2Xiii) tr s0.36(c)(2) tr 50.73(aX2XvXB) tr rclt(aXs) n 2o.2zo3(a)(2Xiv) tr 50.46(ax3xii) n 50.73(aX2XvXc) tr Ts.tt(a)(1) tr 2o.z2o3(aX2Xv) tr 50.73(aX2XiXA) tr 50.73(a)(2XvXD) tr ft.tt(ax2xi) tr 2o,zzos(aX2Xvi) tr 50.73(aX2XiXB) tr s0.73(aX2Xvii) n fi,Tt(ax2xii) tr 50.73(aX2XiXc) tr OTHER Specify in Abstract below or in
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 28 MBT failed as a result of an internal fault on the low voltage side of the transformer. No indications of transformer degradation (e.9. 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.
l. Automatically and Manually lnitiated Safety System Responses Allsafety 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.9. 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
provided with a deluge system in the event of their failure. The fault on the 28 MBT resulted in a turbine trip and resultant reactor trip, and concurrently resulted in a transformer fire with actuation of the associated transforme/s deluge system. With the exception of the fire, the trip and shutdown of Unit 2 were uncomplicated, with all safety related equipment operating as expected.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The loss of the 28 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 (TVA) Corrective Action Program and is being tracked under condition report (CR) 1208823.
A. lmmediate Corrective Actions The WBN spare MBT was prepared and set up to replace the failed 28 MBT. A root cause evaluation was initiated.
B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future lnternal 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, insulating spacers have been installed between certain flex braids and bus work on the Unit 2 MBTs where clearances were minimal.
VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE
No previous large transformer failures have occurred at the Wafts Bar site.
VIII. ADDITIONAL INFORMATION
lf 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.