05000456/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
| ML21230A030 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 08/18/2021 |
| From: | Exelon Generation Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML21230A028 | List: |
| References | |
| BW210054 LER 2021-002-00 | |
| Download: ML21230A030 (3) | |
| 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) |
| 4562021002R00 - NRC Website | |
text
Abstract
On June 21, 2021 at 0051 hours5.902778e-4 days <br />0.0142 hours <br />8.43254e-5 weeks <br />1.94055e-5 months <br />, Unit 1 experienced an automatic reactor trip due to a generator lockout relay actuation and subsequent turbine trip and reactor trip.
The cause of the event was determined to be due to a lightning strike within the unit switchyard exceeding the capabilities of the switchyard grounding system, which resulted in induced voltage on the 125 Volt DC load rejection circuit for the Unit 1 generator output breakers. Planned corrective actions are to conduct a study of the grounding system and implement appropriate approved upgrades to the station and switchyard grounding grid to reduce the impact of future lightning strikes.
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) 1 O CFR 50. 73(a)(2)(iv)(B)(1) for the "Reactor protection system (RPS) including: reactor scram or reactor trip," and
- 2) 1 O 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 June 21, 2021 MODE: 1 Unit 1 Reactor Coolant System (RCS) [AB]:
YEAR 05000456 2021 Reactor Power: 100 percent SEQUENTIAL NUMBER 002 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 June 21, 2021 at 0051 CDT Braidwood Unit 1 experienced an automatic reactor trip due to a generator lockout relay actuation and subsequent turbine trip and reactor trip.
Both trains of auxiliary feedwater [BA] started automatically following the reactor trip to maintain steam generator water levels. All systems responded as expected with the exception of a failure of the source range nuclear instruments to automatically re-energize following the reactor trip. Both source range nuclear instruments were manually energized in accordance with station procedures.
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 55320 made on June 21, 2021.
C. Cause of Event
The cause of unit trip from the generator load rejection was determined to be due to a lightning strike within the unit switchyard exceeding the capabilities of the switchyard grounding system [FC], which resulted in induced voltage on the 125 Volt DC load rejection circuit for the Unit 1 generator output breakers.
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 conduct a study of the grounding system and implement appropriate approved upgrades to the station and switchyard grounding grid to reduce the impact of future lightning strikes.
F.
Previous Occurrences
SEQUENTIAL NUMBER 002 REV NO.
00 No previous, similar Licensee Event Reports were identified at the Braidwood Station in the past three years.
G. Component Failure Data
Manufacturer N/A Nomenclature NIA N/A Mfg. Part Number N/A Page _3_ of _3_