05000499/LER-2025-002, Unit 2, Automatic Reactor Trip Due to Main Transformer Lockout
| ML26020A038 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 01/20/2026 |
| From: | Stephens E South Texas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NOC-AE-26004137, STI:35825865 LER 2025-002-00 | |
| Download: ML26020A038 (0) | |
| Event date: | |
|---|---|
| Report date: | |
| 4992025002R00 - NRC Website | |
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January 20,2026 NOC-AE-26004137 10 cFR 50.73 STI:35825865 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 2 Docket No. STN 50-499 Licensee Event Report 2025-002-00 Automatic Reactor Trio Due to Main Transformer Lockout Pursuant to reporting requirements in 10 CFR 50.73(aX2)(iv)(A), STP Nuclear Operating Company (STPNOC) hereby submits the attached Licensee Event Report 2025-O02-00 for South Texas Project Unit 2. Once completed, a supplement to this LER will be provided.
The event did not have an adverse effect on the health and safety of the public.
There are no commitments in this submittal lf there are any questions regarding this submittal, please contact Zachary Dibbern at (361) 972-4336 or me at (361) 972-4687.
Eric VP of Site Services Attachment: Unit 2 LER 2025-002-00, Automatic Reactor Trip Due to Main Transformer Lockout cc:
Regional Administrator, Region lV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 7601 1-4511
NOC-AE-26004137 Attachment Attachment Unit 2 LER 2025-002-00 Unit 2 LER 2025-002-00, Automatic Reactor Trip Due to Main Transformer Lockout
Abstract
South Texas Project, Unit 2 n
00499 4
Automatic Reactor Trip Due to Main Transformer Lockout 11 21 2025 2025 002 00 01 19 2026 1
100 4
Zachary Dibbern, Licensing Engineer 361-972-4336 EL RLY ABB Y
4 03 05 2026 On November 21, 2025, while in MODE 1, Unit 2 experienced an automatic reactor trip and turbine trip due to a Main Transformer lockout, which resulted in a loss of two offsite power sources. Standby Diesel Generators (SBDGs) 21 and 23 automatically started within the required time frame and all Engineered Safety Feature (ESF) equipment responded as designed.
Immediate corrective actions included restoration of offsite power sources and starting of Reactor Coolant Pumps (RCP) 2A and 2D. A causal investigation is planned to determine causes of the event and any additional corrective actions, and these will be provided in a supplement to this report.
Summary of the Event Timeline (Note: All times listed are Central Standard Time) 11/21/2025, 1208 - Unit 2 Turbine Trip and Reactor Trip due to Main Transformer lockout. All four RCPs tripped. Unit 2 entered MODE 3. Entered Technical Specification (TS) 3.8.1.1, Action e: With two of the required offsite A.C. circuits inoperable, within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> restore at least one of the inoperable offsite sources to OPERABLE status or apply the requirements of the Configuration Risk Management Program (CRMP), or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Entered TS 3.4.1.2, action a: With less than the above required reactor coolant loops OPERABLE, restore the required loops to OPERABLE status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or be in HOT SHUTDOWN within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
11/21/2025, 1208 - SBDGs 21 and 23 actuated due to loss of offsite power to two of three ESF buses. An AFW actuation occurred with all four AFW pumps starting and aligning to their associated Steam Generators.
11/21/2025, 1414 - Reactor Coolant Pump 2D started. Exited TS 3.4.1.2, action a.
11/21/2025, 1421 - SBDG 21 secured.
11/21/2025, 1438 - Reactor Coolant Pump '2A' started for plant stabilization.
11/21/2025, 1439 - SBDG 23 secured.
11/21/2025, 1553 - Completed NRC 4-hour Non-Emergency Notification under 10 CFR 50.72(b)(2)(iv)(B) for a Reactor Trip from 100% power (RPS actuation). Event Notification 58054.
11/21/2025, 2002 - AFW pump 23 secured.
11/21/2025, 2003 - AFW pump 22 secured.
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South Texas Project, Unit 2 00499 2025 002 00 nPage of
- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
052 050 C. Safety Consequences and Implications All nuclear safety systems for Unit 2 responded as expected without any complications. There were no inoperable safety systems that contributed to the event and no safety systems were rendered inoperable as a result of the event. There were no actual consequences to the health and safety of the public as a result of this event.
IV. Cause of the Event
A causal investigation is planned. A supplement to this report will include any causes of the event once the investigation is completed.
V. Corrective Actions
Immediate Corrective Actions included:
- 1. Restoration of normal offsite power. SBDGs secured, A and C buses supplied from the Unit Auxiliary Transformer, and B ESF bus is being powered by standby transformers.
- 2. Restoration of RCPs 2A and 2D for plant stabilization.
A supplement to this report will include any additional corrective actions identified in the casual investigation.
VI. Previous Similar Events
- 1. Unit 2 Licensee Event Reports 2024-001-00 and 2024-001-01, Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators (ML24184C083, ML24242A163) 4 4
South Texas Project, Unit 2 00499 2025 002 00 n