ENS 57836

v t e ENS Event
16:00 Jun 6, 2025
Title	Part 21 - Thermal Overload of Voltage Regulators
Event Description	The following information was provided by the licensee via phone and email: This is an initial report notification pursuant to 10 CFR Part 21.21. Cook Nuclear Plant completed an internal evaluation concerning an issue with an emergency diesel generator (EDG) voltage regulator (VR) supplied by Paragon Energy Solutions, LLC. An issue was identified during a surveillance test conducted on April 22, 2025, concerning a thermal overload (TOL) for the VR which spuriously trips causing a condition where the EDG VR can no longer control the generator voltage, resulting in the inability of the EDG to supply automatic onsite emergency AC power. A failure analysis was completed on June 6, 2025, determined that the cause of the spurious trips is associated with a heater dimensional tolerance deficiency lowering the activation threshold of the TOL bimetallic strip and a workmanship deficiency associated with a braided control wire restricting the movement of the TOL pressure bar. Extent of condition examinations were performed and one additional TOL was found to be impacted. The two impacted TOLs have been replaced. A written notification will be provided within 30 days. Affected Plants: Cook Nuclear Power Plant. The NRC Resident has been notified.

ENS 57836 +/-
Where
Cook Michigan (NRC Region 3)
Reporting

10 CFR 21.21(d)(3)(i), Failure to Comply or Defect
Time - Person (Reporting Time:+1269 h52.875 days <br />7.554 weeks <br />1.738 months <br />)
Opened:	Bradford Culwell 13:00 Jul 29, 2025
NRC Officer:	Ian Howard
Last Updated:	Jul 29, 2025
57836 - NRC Website Loading map...
v • d • e

v t e Emergency Notification System
Cook with 10 CFR 21.21(d)(3)(i), Failure to Comply or Defect
WEEKMONTHYEAR2026-01-26T15:58:00 [Table view]
[ENS list for Cook]

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