05000296/LER-2024-002, Breaker Trip Automatically Started an Emergency Diesel Generator

Breaker Trip Automatically Started an Emergency Diesel Generator

ML24115A165
Person / Time
Site:	Browns Ferry
Issue date:	04/24/2024
From:	Sivaraman M Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
LER 2024-002-00
Download: ML24115A165 (1)

LER-2024-002, Breaker Trip Automatically Started an Emergency Diesel Generator

Event date:
Report date:
2962024002R00 - NRC Website

text

TENNESSEE VALLEY AUTHORITY

Post Office Box 2000, Decatur, Alabama 35609-2000

April 24, 2024 10 CFR 50.73

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001

Browns Ferry Nuclear Plant, Unit 3 Renewed Facility Operating License No. DPR-68 NRC Docket No. 50-296

Subject: Licensee Event Report 50-296/2024-002-00

The enclosed Licensee Event Report provides details of a breaker trip which automatically started an Emergency Diesel Generator on Browns Ferry Nuclear Plant, Unit 3. The Tennessee Valley Authority is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A), as an event or condition that resulted in the unplanned, valid, manual or automatic actuation of emergency AC electrical power systems.

There are no new regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact David J. Renn, Nuclear Site Licensing Manager, at (256) 729-2636.

Respectfully,

Manu Sivaraman Site Vice President

Enclosure: Licensee Event Report 50-296/2024-002 Breaker Trip Automatically Started an Emergency Diesel Generator

cc (w/ Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant ENCLOSURE

Browns Ferry Nuclear Plant Unit 3

Licensee Event Report 50-296/2024-002-00 Breaker Trip Automatically Started an Emergency Diesel Generator

See Enclosed

Abstract

On February 24, 2024, at approximately 0219 Central Standard Time (CST), the closing 4kV shutdown board (SDBD) breaker (3-BKR-211-03EB/009) caused the 3EB 4kV SBDB normal feeder breaker (3-BKR-211-03EB/014) to unexpectedly trip open. This trip resulted in a valid 4kV bus under-voltage condition, which caused the 3B emergency diesel generator (EDG) to automatically start and tie to the board.

No components failed during this event, which was ultimately attributed to an intermittent loose connection. Corrective measures included additional monitoring during cycling of board loads, trouble shooting and tightening of all accessible connections. The condition was unable to be repeated.

I. Plant Operating Conditions before the Event

At the time of discovery, Browns Ferry Nuclear Plant (BFN) Unit 3 was in Mode 5 at 0 percent power due Unit 3 Refueling Outage 21 (U3R21).

A. Event Summary

On February 24, 2024, at approximately 0219 Central Standard Time (CST), scheduled work was being performed on 3B 480 V shutdown board (SDBD) during U3R21. During clearance re-alignment, racking in and closing in transformer TS3E normal feeder breaker (3-BKR-211-03EB/009) caused the 3EB 4kV SBDB normal feeder breaker (3-BKR-211-03EB/014) to unexpectedly trip open. This trip resulted in a valid 4kV bus under-voltage condition, which caused the 3B emergency diesel generator (EDG) [DG] [EK] to automatically start and tie to the board. A initial walkdown and visual inspection of 4kV SDBD 3EB was performed, but adverse conditions were observed.

The Tennessee Valley Authority is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A), as an event or condition that resulted in the unplanned, valid, manual or automatic actuation of emergency AC electrical power systems.

B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event

There were no structures, systems, or components (SSCs) whose inoperability contributed to this event.

C. Dates and approximate times of occurrences

DATE AND OCCURRENCE APPROXIMATE TIME February 24, 2024 The racking and closing 4kV SDBD breaker caused the 3EB 4kV 0219 CST SBDB normal feeder breaker to unexpectedly trip open, automatically starting the 3B EDG.

February 24, 2024 Ops personnel notified the NRC, in accordance with 10 CFR 0827 CST 50.72(b)(3)(iv)(A). This was recorded in Event Notification 56990.

February 24, 2024 3B EDG shutdown after restoring normal power to 3EB 4kV 1111 CST shutdown board.

Ops personnel and maintenance electricians completed their troubleshooting the breakers and their cubicles. However, the February 25, 2024 Ops personnel and electricians reported that:

2012 CST No trips, lockouts, or visual damage on either breaker.

All logic was aligned as expected.

No lock out relays were actuated, and no relay flags were present on the board, other than the undervoltage and overvoltage relay flags.

D. Manufacturer and model number of each component that failed during the event

No components failed during this event.

E. Other systems or secondary functions affected

No other systems or secondary functions were affected.

F. Method of discovery of each component or system failure or procedural error

The event was discovered through the unplanned, valid, automatic actuation of the 3B EDG.

G. The failure mode, mechanism, and effect of each failed component

No components failed during this event, which was believed to have been caused by an intermittent loose connection.

H. Operator actions

There were no operator actions associated with this event.

I. Automatically and manually initiated safety system responses

This event deenergized the associated 3EB 4KV SDBD, which automatically started the 3B EDG to re-energize the SDBD.

III. Cause of the event

An investigation found no root cause of this event. No components failed during this event, which was believed to have been caused by an intermittent loose connection, because the investigation was unable to completely refute that possibility.

A. Cause of each component or system failure or personnel error

The causal investigation found no definite reason for the event. However, the possibility that the event was caused by an intermittent loose connection was not fully refuted.

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

No human performance related root cause was identified.

IV. Analysis of the event

The 4kV bus under-voltage condition was a valid 3B EDG signal, which caused the EDG to fulfil its designed safety function by automatically starting and tying to the board.

V. Assessment of Safety Consequences

All safety systems and alarms performed as designed in response to the unplanned, valid, automatic actuation signal. No actual safety consequences or radiological releases resulted from this event. All Technical Specification and Technical Requirements Manual conditions were met throughout this event. All mitigating systems remained functional and capable of fulfilling their required safety functions throughout this event. No fission product barriers were challenged during this event. BFN Unit 3 was not operating at the time, as the event occurred during its biennial refueling outage.

Based on the above, the TVA has concluded that sufficient systems were available to provide the required safety functions needed to protect the health and safety of the public.

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 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

The 3B EDG automatically started, ensuring the continued operation of all affected systems.

C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from discovery of the failure until the train was returned to service

No safety systems were rendered inoperable because of this event.

VI. Corrective Actions

Corrective Actions for this event are being managed by the TVAs corrective action program under Condition Report (CR) 1912331.

A. Immediate Corrective Actions

No equipment failures were observed. The event is believed to have been caused by an intermittent loose connection, which was resolved during the following troubleshooting activities.

B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future

Troubleshooting including inspection and checking tightness of all accessible wiring and connections as well as the development of Adverse Condition Monitoring Plan (ACMP) CR 1912475 created to perform additional monitoring of circuits associated with loads on this board. Loads were cycled under normal plant operation and the operation of these pumps had no effect on boards supply breaker. This resulted in the condition being unable to be re-created. No additional corrective actions are identified or warranted at this time.

VII. Previous Similar Events at the Same Site

A search of LERs from BFN, Units 1, 2, and 3 over the last five years identified no similar events.

VIII. Additional Information

There is no additional information.

IX. Commitments

There are no new commitments.