05000327/LER-2024-001-01, Reactor Trip Due to a Turbine Trip
| ML25099A196 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 04/09/2025 |
| From: | Marshall T Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LER 2024-001-01 | |
| Download: ML25099A196 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 3272024001R01 - NRC Website | |
text
Sequoyah Nuclear Plant, Post Office Box 2000, Soddy Daisy, Tennessee 37384 April 9, 2025 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 50-327/2024-001-01, Reactor Trip due to a Turbine Trip
Reference:
TVA letter submitted to NRC dated October 17, 2024, Licensee Event Report 50-327/2024-001-00, Reactor Trip due to a Turbine Trip The enclosed licensee event report has been revised with supplemental information concerning an automatic reactor trip due to an electrical trouble turbine trip. This revised report reflects the results of the causal analysis and associated corrective actions. This event was previously reported in accordance with 10 CFR 50.73(a)(2)(iv), as an event that resulted in an automatic actuation of the reactor protection system and the auxiliary feedwater system. Changes to the Reference report are indicated by revision bars on the right-side margin of the page.
There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr. Rick Medina, Site Compliance Manager, at (423) 843-8129 or rmedina4@tva.gov.
Respectfully, Thomas Marshall Site Vice President Sequoyah Nuclear Plant Enclosure: Licensee Event Report 50-327/2024-001-01 cc:
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Sequoyah Nuclear Plant Chris Rice, Plant Manager for Tom Marshall
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 was determined to be the loose adjacent stator pole #3 movement acting on the stator pole #4 connections and elevated seismic vibration levels from degradation of the exciter foundation. Corrective actions to prevent recurrence included replacing the exciter, regrouting the exciter base plate (replacing the exciter and regrouting the base plate occurred after the subsequent Unit 1 trip on September 29, 2024), revising preventive maintenance instructions, and establishing seismic vibration limits and the associated actions to be taken if those limits are exceeded. Page 5 of 5 B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future The exciter base plate was regrouted and the hold down bolts were repaired. The exciter associated with this trip was replaced with a model that has shorter and stiffer pole leads.
Replacing the exciter and regrouting the base plate occurred after the subsequent Unit 1 trip on September 29, 2024.
Preventive maintenance (PM) instructions were revised to include:
obtaining as-found torque value on frame hold down bolts each time exciter is moved; checking for substrate voids using vibration ping testing of the soleplate; performing sounding-only checks for concrete hold-down bolts; adding exciter pole tightness checks; and reviewing high speed camera footage obtained during startup to identify any abnormal stator pole movement.
Seismic vibration limits and the associated actions to be taken if those limits are exceeded have been established.
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 not related to the cause of the Unit 1 event.
VIII. Additional Information
There is no additional information.
IX.
Commitments
There are no new commitments.