05000456/LER-2025-001, Over Temperature Delta Temperature Due to Lightning Strike
| ML25269A165 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 09/26/2025 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25269A163 | List: |
| References | |
| BW250042 LER 2025-001-00 | |
| Download: ML25269A165 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(1) 10 CFR 50.73(a)(2)(iv)(B)(6) |
| 4562025001R00 - NRC Website | |
text
Abstract
On July 30, 2025 at 1230, the Braidwood Unit 1 reactor tripped due to an Overtemperature Delta Temperature (OTdT) signal.
The cause of the event was determined to be a -56 kA lightning flash with multiple strokes that impacted the Unit 1 turbine building at approximately 12:30:41. The lightning induced a voltage transient that resulted in actuation of the power supply overvoltage protection circuits within the Unit 1 control rod drive power cabinet(s) which resulted in multiple control rods dropping into the core, causing an OTdT reactor trip.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) for "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section,... "Specifically, for 1) 10 CFR 50.73(a)(2)(iv)(B)(1) for the "Reactor protection system (RPS) including: reactor scram or reactor trip," and 2) 10 CFR 50.73(a)(2)(iv)(B)(6) for the "PWR auxiliary or emergency feedwater system."
A. Plant Operating Conditions Before the Event
Event Date:
Unit: 1 July 30, 2025 MODE: 1 Unit 1 Reactor Coolant System (RCS) [AB):
- 2. DOCKET NUMBER YEAR 05000456 2025 Reactor Power: 100 percent
- 3. LER NUMBER SEQUENTIAL NUMBER 001 Normal operating temperature and pressure REV NO.
00 No structures, systems or components were inoperable at the start of this event that contributed to the event.
B. Description of Event
On July 30, 2025 at 1230, the Braidwood Unit 1 reactor tripped due to a lightning induced transient that resulted in an Overtemperature Delta Temperature (OTdT) signal.
Both trains of auxiliary feedwater [BAJ started automatically following the reactor trip to maintain steam generator water levels. All systems responded as expected following the reactor trip with the exceptions of one steam pressure instrument, one steam generator level instrument, one loop hot leg temperature circuit, and one steam flow instrument which all failed during the lightning storm and subsequent transient.
This event is reportable in accordance with 10 CFR 50. 73(a)(2)(iv)(A) for "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section,... "Specifically, for
- 1) 10 CFR 50.73(a)(2)(iv)(B)(1) for the "Reactor protection system (RPS) including: reactor scram or reactor trip," and
- 2) 10 CFR 50.73(a)(2)(iv)(B)(6) for the "PWR auxiliary or emergency feedwater system" This LER is being submitted in follow-up to ENS 57839 made on July 30, 2025.
C. Cause of Event
The cause of the unit trip was a -56 kA lightning flash with multiple strokes that impacted the Unit 1 turbine building.
The lightning induced a transient that resulted in actuation of the power supply overvoltage protection circuits within the Unit 1 control rod drive power cabinet(s) which resulted in multiple control rods dropping into the core, causing an OTdT reactor trip.
D. Safety Consequences
There were no safety consequences impacting plant or public safety as a result of this event. The reactor trip system responded automatically due to the trip signal received. There was no loss of any function that would have prevented fulfillment of actions necessary to 1) shutdown the reactor and maintain it in a safe shutdown condition, 2) remove residual heat, 3) control the release of radioactive material, or 4) mitigate the consequences of an accident There was no loss of safety function for this event.
E. Corrective Actions
Planned corrective actions are to improve the station's lightning protection system to minimize the likelihood of instrumentation and control transients that could result in a reactor trip and to install upgraded Rod Drive power supplies that provide improved overvoltage protection with a time delay.
F. Previous Occurrences
- 2. DOCKET NUMBER
- 3. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO.
05000456 2025 -
001 00 8/18/2021 - LER 2021-002 - Unit Trip on Generator Load Rejection Due to Lightning Strike Within the Unit Switchyard Exceeding the Capabilities of the Switchyard Grounding System
G. Component Failure Data
Manufacturer Nomenclature N/A N/A Model N/A Mfg. Part Number N/A Page _3_ of _3_