05000327/LER-2025-001-01, Manual Reactor Trip Due to Reboot of the Distributed Control System and Lowering Steam Generator Levels
| ML25295A001 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  (DPR-077) |
| Issue date: | 10/22/2025 |
| From: | Michael K Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LER 2025-001-01 | |
| Download: ML25295A001 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(b)(3)(iv)(A) |
| 3272025001R01 - NRC Website | |
text
Sequoyah Nuclear Plant, Post Office Box 2000, Soddy Daisy, Tennessee 37384 October 22, 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 50-327/2025-001-01, Manual Reactor Trip due to Reboot of the Distributed Control System and Lowering Steam Generator Levels
Reference:
TVA Letter submitted to NRC dated August 21, 2025, Licensee Event Report 50-327/2025-001-00, Manual Reactor Trip due to Reboot of the Distributed Control System and Lowering Steam Generator Levels.
The enclosed licensee event report has been revised with supplemental information concerning a manual reactor trip due to a reboot of the Distributed Controls System and lowering steam generator water levels. This revised report reflects the results of the causal analysis and the associated corrective action. This event was previously reported in accordance with 10 CFR 50.73(b)(3)(iv)(A) as an event that resulted in actuation of the Reactor Protection System and 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, Kevin Michael Site Vice President Sequoyah Nuclear Plant Enclosure: Licensee Event Report 50-327/2025-001-01 cc:
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Sequoyah Nuclear Plant Michael, Kevin Mckinley Digitally signed by Michael, Kevin Mckinley Date: 2025.10.21 09:47:00
- - 04'00'
Abstract
On June 24, 2025, at 0014 eastern daylight time (EDT), SQN main control room operators manually tripped Unit 1 due to an apparent failure of the Distributed Control System. Dual Control Processor (CP), S110CP, rebooted resulting in a loss of secondary plant control. Unit 1 was manually tripped due to lowering steam generator water levels. 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 event was caused by automatic CP reboots. The CP reboots were caused by the CPs experiencing multiple overrun failures. When this happened, the CP was forced to do a memory dump and automatically perform a dual CP reboot. This resulted in a loss of associated component control for approximately five minutes. Troubleshooting identified that the cause of the reboot failure was a flaw inherent in the CP firmware. To correct the issue, the firmware was updated for all components that had the affected firmware version installed.
Page 5 of 5 B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future The firmware was updated for all components that had the affected firmware version installed.
VII. Previous Similar Events at the Same Site
There were no previous similar events at SQN occurring within the last three years.
VIII. Additional Information
There is no additional information.
IX.
Commitments
There are no new commitments.