05000327/LER-2024-001, Reactor Trip Due to a Turbine Trip

Reactor Trip Due to a Turbine Trip

ML24295A020
Person / Time
Site:	Sequoyah
Issue date:	10/17/2024
From:	Marshall T Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
LER 2024-001-00
Download: ML24295A020 (1)

LER-2024-001, Reactor Trip Due to a Turbine Trip

Event date:
Report date:
3272024001R00 - NRC Website

text

Sequoyah Nuclear Plant, Post Office Box 2000, Soddy Daisy, Tennessee 37384

October17, 2024 10 CFR 50.73

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

Sequoyah Nuclear Plant, Unit 1 Renewed Facility Operating License No. DPR-77 NRC Docket No. 50-327

Subject:Licensee Event Report 50-327/2024-001-00, Reactor Trip due to a Turbine Trip

The enclosed licensee event report provides details concerningan automatic reactor trip on an electrical trouble turbine trip. This report is being submitted in accordance with 10CFR50.73(a)(2)(iv),as an event that resulted in an automatic actuation of the reactor protection system and the auxiliary feedwater system. A supplement to this LER will be submitted following completion of the associated root cause evaluation.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr.RickMedina, Site Licensing Manager, at (423)843-8129or rmedina4@tva.gov.

Respectfully,

Thomas Marshall Site Vice President Sequoyah Nuclear Plant

Enclosure: Licensee Event Report 50-327/2024-001-00 cc:NRC Regional Administrator -Region II NRC Senior Resident Inspector -Sequoyah Nuclear Plant

Abstract

On August 23, 2024, at 1219 eastern daylight time (EDT), SQN Unit 1 experienced an automatic reactor trip due to a turbine trip. The turbine trip first out had Generator Breaker Trip Turbine Trip and Turbine Trip alarms flashing. It was determined that the generator exciter experienced a loss of excitation due to an open circuit caused by a failure of a stator pole #4 winding.

Operators performed the appropriate actions in response to the reactor trip. All plant safety systems responded as designed. This event did not adversely affect the health and safety of plant personnel or the general public.

The root cause for this event is still under investigation. When the final investigation is complete a supplement will be provided with the identified cause and associated corrective action(s).

I. Plant Operating Conditions before the Event

At the time of the event, Sequoyah Nuclear Plant (SQN) Unit 1 was in Mode 1 at approximately 100 percent rated thermal power.

A. Event Summary

On August 23, 2024, at 1219 eastern daylight time (EDT), SQN Unit 1 experienced an automatic reactor [EIIS: RCT] trip due to a turbine [EIIS: TRB] trip.The turbine trip first out had Generator Breaker Trip Turbine Trip and Turbine Trip alarms flashing. It was determined that the generator exciter [EIIS: EXC] experienced a loss of excitation due to an open circuit caused by a failure of a stator pole #4 winding.

Troubleshooting identified the failure mechanism to be mechanical fatigue failure of a stator pole #4 winding due to excessive vibration. The stator pole was refurbished and reinstalled to fix the open circuit. B locking was installed on stator pole winding connectors to mitigate vibrations.

Loose anchors were reset in their respective grout well utilizing epoxy grout and the anchors were torqued to design specification. Damaged ceiling resistors and failed diodes were replaced.

Regression testing was performed on the automatic voltage regulator.

All plant safety systems responded as expected.

The event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in an automatic actuation of the Reactor Protection System [EIIS: JC] and the Auxiliary Feedwater (AFW) System [EIIS: BA].

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

No inoperable structures, components, or systems contributed to this event.

C. Dates and approximate times of occurrences

Date/Time (EDT) Description

08/23/24, 1219 SQN Unit 1 experienced an automatic reactor trip due to a turbine trip. The unit entered Mode 3.

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

The root cause for this event is still under investigation. When the final investigation is complete, a supplement will be provided.

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

Main control room (MCR) alarms and annunciators provided indication to the operators during the reactor trip.

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

Troubleshooting identified the failure mechanism to be mechanical fatigue failure of a stator pole #4 winding due to excessive vibration.

H. Operator actions

MCR operators responded to the reactor trip, as required, and then transitioned to post-trip response procedures.

I. Automatically and manually initiated safety system responses

The reactor protection system, including feedwater isolation and AFW start, responded to the trip, as designed.

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

The root cause for this event is still under investigation. When the final investigation is complete, a supplement will be provided.

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

The root cause for this event is still under investigation. When the final investigation is complete, a supplement will be provided.

I V. Analysis of the event

The plant safety system responses during and after the reactor trip were bounded by the responses described in the Updated Final Safety Analysis Report. Therefore, this event did not adversely affect the health and safety of plant personnel or the general public.

V. Assessment of Safety Consequences

There were no actual safety consequences as a result of this event.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event

None.

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 event did not occur when the reactor was shut down.

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

There was no failure that rendered a train of a safety system inoperable.

VI. Corrective Actions

This event was entered into the Tennessee Valley Authority Corrective Action Program under condition report number 1953128.

A. Immediate Corrective Actions

Troubleshooting identified the failure mechanism to be mechanical fatigue failure of a stator pole #4 winding due to excessive vibration. The stator pole was refurbished and reinstalled to fix the open circuit. Blocking was installed on stator pole winding connectors to mitigate vibrations.

Loose anchors were reset in their respective grout well utilizing epoxy grout and the anchors were torqued to design specification. Damaged ceiling resistors and failed diodes were replaced.

Regression testing was performed on the automatic voltage regulator.

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

The root cause for this event is still under investigation. When the final investigation is complete, a supplement will be provided.

VII. Previous Similar Events at the Same Site

LER 50- 328/2024-001- 00 was submitted for a reactor trip due to an electrical trouble turbine trip.

The cause of the Unit 2 event is under investigation.

VIII. Additional Information

There is no additional information.

IX. Commitments

There are no new commitments.