05000259/LER-2016-004, Regarding Incorrect Tap Settings for 480V Shutdown Board Transformers Results in Inoperability of Associated 480V Shutdown Boards
| ML16342C540 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 12/07/2016 |
| From: | Bono S Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 16-004-00 | |
| Download: ML16342C540 (9) | |
| Event date: | |
|---|---|
| Report date: | |
| 2592016004R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 December 7, 2016 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001
Subject:
Browns Ferry Nuclear Plant, Unit 1 Renewed Facility Operating License No. DPR-33 NRC Docket No. 50-259 Licensee Event Report 50-259/2016-004-00 10 CFR 50.73 The enclosed Licensee Event Report provides details of the incorrect tap settings for the 480V Shutdown Board Transformers and associated 480 shutdown boards that resulted in inoperability of several transformers. This resulted in a condition prohibited by Technical Specifications and a Safety System Functional Failure. The Tennessee Valley Authority is submitting this report in accordance with 10 CFR 50. 73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications, and 10 CFR 50. 73(a)(2)(v)(D), as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
There will be a supplement to this Licensee Event Report due to ongoing evaluation.
There are no new regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact M. W. Oliver, Acting Nuclear Site Licensing Manager, at (256) 729-2636.
Enclosure: Licensee Event Report 50-259/2016-004 Incorrect Tap Settings for 480V Shutdown Board Transformers Results in lnoperability of Associated 480V Shutdown Boards cc (w/ Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant
ENCLOSURE Browns Ferry Nuclear Plant Unit 1 Licensee Event Report 50-259/2016-004-00 Incorrect Tap Settings for 480V Shutdown Board Transformers Results in lnoperability of Associated 480V Shutdown Boards See Enclosed I.
Plant Operating Conditions Before the Event
At the time of discovery, Browns Ferry Nuclear Plant (BFN), Unit 1, was in a planned refueling outage in Mode 5 at 0 percent rated thermal power.
II.
Description of Event
A. Event Summary:
On October 8, 2016, during the performance of preventive maintenance of the transformer winding turns ratio test on the 480V Shutdown Board transformer 1-XFA-231-TS 1 B (TS1 B)[XFMR], it was discovered that the transformer was set on the incorrect tap [TTC]
(4160/480V instead of 3952/480V). After TS1 B was found to be on the incorrect tap, additional inspections were performed on the 480V Shutdown Board transformer 1-XFA-231-TS1A (TS1A). TS1A was also discovered to be set on the incorrect tap (4160/480V instead of 3952/480V). The taps on 480V Shutdown Board transformers TS2A, TS2B, and THB were inspected and found to be correct. After discovery, TS 1 A was set on the correct tap setting on October 12, 2016, and TS1B was set on the correct tap setting on October 9, 2016.
The tap settings for the 480V Shutdown Board Transformers TS 1 A and TS 1 B were not set as specified on the applicable drawings and as established in the applicable calculation when installed. The potential consequences of the referenced transformer taps being improperly set would be lower than the minimum required voltages at the electrically downstream buses and equipment under degraded voltage conditions.
B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event:
No inoperable systems, structures, or components contributed to the event.
00
C. Dates and approximate times of occurrences
November 18, 2004 TS 1 A winding tap incorrectly set December 15, 2004 TS1 B winding tap incorrectly set May 21, 2007 Unit 1 entered Mode 2 October 8, 2016 Incorrect tap setting for transformer TS1 B discovered 0049 CST October 9, 2016 October 10, 2016 2038 CST October 12, 2016 TS 1 B set to the correct tap Incorrect tap setting for transformer TS1A discovered TS 1 A set to the correct tap
- 3. LER NUMBER SEQUENTIAL NUMBER 004 D. Manufacturer and model number (or other identification) of each component that failed during the event:
No component failures were identified that occurred during the event.
E. Other systems or secondary functions affected
There were no other systems or secondary functions affected.
F. Method of discovery of each component or system failure or procedural error
REV NO.
00 This condition was discovered during preventive maintenance. It was determined that the TS 1 A and TS1 B tap settings do not agree with the applicable drawing and calculation.
G. The failure mode, mechanism, and effect of each failed component, if known:
There were no components that failed during the event.
H. Operator actions
There were no operator actions performed in response to this event.
I.
Automatically and manually initiated safety system responses
There were no safety system responses initiated in response to this event.
Ill.
Cause of the event
A. The cause of each component or system failure or personnel error, if known:
The applicable drawings which specified transformers TS 1 A and TS 1 B tap settings were incorrectly listed in the implementation Work Orders (WOs) as "documentation only" drawings and were classified as Category 3 drawings. As a result during the installation of these transformers, there was not a step in the WOs to set the taps on the correct tap.
Since there was no specific step in the WOs to change the settings for TS 1 A and TS 1 B to match the applicable drawing, the transformers were left on the same tap setting (4160/480V) as they were shipped from the factory.
B. The cause(s) and circumstances for each human performance related root cause:
There were no human performance related root causes.
IV.
Analysis of the event
The Tennessee Valley Authority (TVA) is submitting this report in accordance with 10 CFR 50. 73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications, and 10 CFR 50. 73(a)(2)(v)(D), as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
Technical Specification (TS) 3.8.7 requires that the Unit 1 480V Shutdown Boards and the Unit 1 Reactor Motor Operated Valve (RMOV) Boards 1A and 1 B be Operable. The required action for Condition B for the 480V shutdown board is to restore the board to Operable status within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and 12 days from discovery of the failure to meet the Limiting Condition for Operation (LCO). If the required action and associated completion time of Condition B is not met, the Unit must be in Mode 3 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and Mode 4 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. TS Bases Section B 3.8. 7 states the Operability of the AC and DC electrical power distribution subsystems is consistent with the initial assumptions of the accident analyses and is based upon meeting the design basis of the unit.
This includes maintaining distribution systems Operable during accident conditions in the event of:
a) An assumed loss of all offsite power or all onsite AC electrical power sources; and b) A postulated worst case single failure.
The Specified Safety Function affected by the condition is the ability of the Unit 1 480V Shutdown Boards 1A and 1 B to receive proper voltage from the 4kV Shutdown Board supply via transformers TS1A and TS1 B, respectively. The transformer tap changer settings for TS1A and TS 1 B ensure proper voltage is available to 480V Shutdown Board 1 A and 1 B respectively during all required operating conditions and accident scenarios. Proper voltage availability on the AC and DC buses ensures that the required power is readily available for motive as well as control functions for critical system loads connected to these buses.
00 V.
Assessment of Safety Consequences
With a qualified offsite power source, operation at the degraded voltage dropout setting is considered an abnormal operating condition that is expected to be a short term duration until the load tap changer attempts to take action to increase the voltage to above the reset setting. The licensing and/or design basis requirement as per the "4.16kV and 480V Bus Load and Voltage Drop Calculation" is that connected loads not become damaged or become unavailable due to protective device actuation as a result of operating at a degraded voltage. Any voltage below the reset value of 3983V is degraded, and operation is expected to be a short duration. If the voltage at the degraded voltage level did not recover within 5.6 seconds, the Emergency Diesel Generator (EDG)[DG] associated with the 4kV Shutdown Board would start and carry the loads, therefore, the offsite power source would not be considered Operable as per the requirements of 10 CFR Part 50, Appendix A, General Design Criteria (GDC) 17. Considering that the Diesel Generators would have started and carried the loads if the voltage at the degraded voltage level dropped to 3900V and did not recover within 5.6 seconds, and that the safety-significant loads would have operated with a degraded voltage condition during a LOCA then initial core cooling would have been established and the condition is considered to not significantly degrade plant safety. This condition is not significant with respect to the health and safety of the public A Probabilistic Risk Assessment analysis was performed and concluded that this condition had no impact on the Loss of Coolant Accident (LOCA)/loss-of-offsite power design basis event. The only impact this condition had was on LOCA events that involved a degraded voltage event where the voltage did not degrade to the point to transfer the safety related boards to the EDGs, and was not high enough to assure the functionality of all safety related loads. Additional evaluation is necessary to determine the impact of this condition on other components that are supported by the 480V Shutdown Boards. A review of the voltages on the 4.16kV system demonstrated no instances where the voltages dropped below 4.16kV since 2013. Additionally, the probability of a degraded voltage concurrent with a postulated design basis accident is considered very low.
Therefore, this non-conforming condition is not considered to be risk significant.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
No components or systems failed due to this event.
B. For events that occurred when the reactor was shut down, availability of safety-related systems or components:
This event occurred while the reactor was shut down. Due to the condition existing when operability was required since 2007, past operability was assessed.
An electrical calculation software (ETAP) was performed with the TS1A and TS1 B taps set on 4160/480V conclude that the voltage at the 480V Shutdown Bus 1 A would be 431 V and the voltage at the 480V Shutdown Board 1 B would be 433V, which is less than the required 440V. Because the 480V Shutdown Boards have a low voltage at the 480V Bus, this results in the downstream 480V Motor Control Centers and loads being supplied at a 00 2016 004 low voltage. The 480V board bus voltages would be as follows: 480V RMOV Board 1A would be at 430V, 480V RMOV Board 1 B would be at 431 V, 480V RMOV Board 1 C would be at 432V, and 480V Control Bay Vent Board A would be at 428V with 432V being the minimum required bus voltage at the boards specified in the applicable calculation. The valves on 480V RMOV Board 1 C would not have been available for the required 90% of rated voltage (414V for 460V rated valve motors) according to the ETAP calculation.
Analyses demonstrated that with a degraded voltage condition of 3983V at the 4kV Shutdown Board, voltage at the 480V Shutdown Boards 1A and 1 Band the 480V RMOV Boards would have been less than the minimum required voltages specified in the applicable calculation. Operation at the degraded voltage dropout setting and at the degraded voltage reset set point is considered an abnormal operating condition that is normally expected to be a short term duration until the Load Tap Changer takes action to increase the voltage to above the reset setting. The only requirement is that connected loads not become damaged or become unavailable due to protective device actuation as a result of operating at a degraded voltage. Any voltage below the reset value of 3983 Volts is degraded, and operation is expected to be a short duration. If the voltage at the degraded voltage level dropped to 3900V and did not recover within 5.6 seconds due to degraded offsite power source, the EOG associated with the 4kV Shutdown Board would start and carry the loads, and this challenges the operability of the offsite power circuit per requirements of 10 CFR Part 50, Appendix A, GDC 17.
C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from the discovery of the failure until the train was returned to service:
REV NO.
00 The time frame for the condition is from November 18, 2004, for TS 1 A and December 15, 2004, for TS 1 B for Modes 1, 2, and 3 until the condition was corrected. However, the inoperability was recordable beginning in 2007 when Unit 1 entered the mode of applicability. The condition was corrected when transformer TS1Awas set on the correct tap on October 12, 2016, and transformer TS1 B was set on the correct tap on October 9, 2016.
VI.
Corrective Actions
Corrective Actions (CA) are being managed by TVA's Corrective Action Program under Condition Report (CR) 1221273.
The CAs described below address this condition:
- 1. The transformer taps for transformers TS 1 A and TS 1 B have been set in accordance with the applicable drawing, which specify the correct tap settings.
- 2. Verify that the 4160/480V transformer tap settings are correct in accordance with the applicable drawings.
- 3. Revise the classification for the applicable drawing from Category 3 to Category 2.
VII.
Previous Similar Events
SEQUENTIAL NUMBER 004 REV NO.
00 A review for CRs was performed for similar events. The following instance was discovered in regards to transformer tap settings.
CR 03-001022-000 - Two Unit 3 transformer tap settings, TUSS 3A and THB, were found to disagree with the value used in the ETAP database, the applicable calculation, and drawing. The immediate corrective action was to perform calculations to verify that system voltages were acceptable. The corrective action for the CR was to revise the applicable calculation to support the correct tap settings for TUSS 3A and THB.
The actions for CR 03-001022-000 would not have prevented this condition.
VIII. Additional Information
Analyses demonstrate that with a degraded voltage condition of 3983V at the 4kV Shutdown Board, voltage at the 480V Shutdown Boards 1 A and 1 B and the 480V RMOV Boards would have been less than the minimum required voltages specified in the applicable calculation. Operation at the degraded voltage dropout setting and at the degraded voltage reset set point is considered an abnormal operating condition that is normally expected to be a short term duration until the Load Tap Changer takes action to increase the voltage to above the reset setting. The only requirement is that connected loads not become damaged or become unavailable due to protective device actuation as a result of operating at a degraded voltage. Any voltage below the reset value of 3983V is degraded and operation is a short duration. If the voltage at the degraded voltage level did not recover within 5.6 seconds, the EOG associated with the 4kV Shutdown Board would start and carry the loads. The offsite power system is designed to be independent of the EDGs to supply power to safety related equipment during a LOCA; therefore, the 480V Shutdown Boards were not considered to have been able to perform their specified safety function. In conclusion, this event is considered to be a Safety System Functional Failure in accordance with NUREG-1022.
There will be a supplement to this Licensee Event Report due to ongoing evaluation.
IX.
COMMITMENTS
There are no new commitments. Page _7_ of _ 7_