05000370/LER-2025-001, Condition Prohibited by Technical Specifications Due to Emergency Diesel Ventilation Damper Malfunction
| ML25307A321 | |
| Person / Time | |
|---|---|
| Site: | Mcguire |
| Issue date: | 11/03/2025 |
| From: | Rollins T Duke Energy Carolinas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| RA-25-0258, 02544243 LER 2025-001-00 | |
| Download: ML25307A321 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3702025001R00 - NRC Website | |
text
J_-, DUKE
~ ENERGY Serial No: RA-25-0258 November 3, 2025 U.S. Nuclear Regulatory Commission Washington, D.C. 20555 ATTENTION: Document Control Desk
Subject:
Duke Energy Carolinas, LLC McGuire Nuclear Station, Unit 1 Docket No. 05000370 Renewed License No. NPF-17 Licensee Event Report 2025-001-00 Nuclear Condition Report Number 02544243 Travis Rollins Manager Nuclear Support Services Duke Energy MG01VP 112700 Hagers Ferry Road Huntersville, NC 28078 o: 980.875.4646 Travis.Rollins@duke-energy.com 10 CFR 50.73 Pursuant to 10 CFR 50.73 Section (a)(2)(i)(B), attached is Unit 2 Licensee Event Report (LER) 2023-001-00, regarding a condition prohibited by Technical Specifications due to an Emergency Diesel Ventilation damper malfunction.
This event is considered to have no significance with respect to the health and safety of the public. There are no regulatory commitments contained in this LER.
If questions arise regarding this LER, please contact Jeff Sanders at 980-875-4680.
Sincerely, Travis B. Rollins Duke Energy McGuire Nuclear Station Manager, Nuclear Support Services Attachment
U.S. Nuclear Regulatory Commission RA-25-0258 Page2 cc:
Julio F. Lara Administrator Region II (Acting)
U.S. Nuclear Regulatory Commission Marquis One Plaza 245 Peachtree Center Avenue NE Suite 1200, 30303-1257 J. Klos Project Manager (McGuire)
U.S. Nuclear Regulatory Commission Mail Stop O-9-E3 11555 Rockville Pike Rockville, MD 20852 Chris Safouri NRC Senior Resident Inspector McGuire Nuclear Station
Abstract
On February 12, 2025, with Unit 2 in Mode 1 and operating at approximately 100 percent power, the 2A Diesel Generator (DG) was operable with the 2A2 Emergency Diesel Ventilation (VD) fan in service and the 2A1 fan secured for maintenance.
While in this configuration, technicians discovered the 2A1 Outside Air Backdraft Damper (OABD) stuck open allowing recirculation flow through the idle fan. Under certain conditions, a stuck open OABD could cause the DG room to exceed required temperature limits. Subsequent vendor analysis of this configuration determined the 2A DG was unknowingly inoperable for approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> on February 16, 2025. During this 9-hour period, Surveillance Requirement (SR) 3.8.1.1, Verify correct breaker alignment and indicated power availability for each offsite circuit, was not performed within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> nor was Unit 2 placed in Mode 3 within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> as required by Technical Specifications.
All eight OABDs on both units were inspected. Identified conditions were resolved or scheduled for resolution. A causal analysis and corrective action plan for the event is currently in progress. The initial investigation revealed OABD bearings in poor condition, leading to increased rotational resistance when damper blades try to close. This event had a very low impact on plant risk and there was no impact to the health and safety of the public.
50.73(a)(2)(i)(B)
X 50.73(a)(2)(v)(D)
BACKGROUND The following information is provided to assist readers in understanding the event described in this LER.
Applicable Energy Industry Identification System [EIIS] and component codes are enclosed within brackets.
Onsite Power System [EA, EB]
An Offsite Power System and an Onsite Power System are provided to supply electric power to each unit's auxiliaries for normal operation and to each unit's Protection Systems and Engineered Safety Features (ESF) during abnormal and accident conditions. The Onsite Power System is divided into redundant load groups (A train and B train) so that the loss of any one group does not prevent the minimum safety functions from being performed. Each train has connections to two preferred offsite power sources and a single Diesel Generator (DG) [EK], which is the standby power source for each 4160 Volt ESF bus. Each DG starts automatically on a Safety Injection (SI) signal (i.e., low pressurizer pressure or high containment pressure) or on an ESF bus degraded voltage or undervoltage signal. After the DG has started, it will automatically tie to its respective bus after offsite power is tripped as a consequence of ESF bus undervoltage or degraded voltage, independent of or coincident with an SI signal. When the DG is tied to the ESF bus, loads are then sequentially connected to its respective ESF bus by the automatic load sequencer. In the event of a loss of preferred power, the ESF electrical loads are automatically connected to the DGs in sufficient time to provide for safe reactor shutdown and to mitigate the consequences of a Design Basis Accident such as a Loss of Coolant Accident (LOCA).
Diesel Building Ventilation System [VJ]
The Diesel Building Ventilation System (VD) consists of both normal and emergency ventilation systems. The normal ventilation system maintains diesel room temperature when the engine is not operating, while the emergency ventilation system maintains room temperature while the engine is operating. Outside air is used to cool the building. Each diesel building is provided normal ventilation through a 100 percent capacity ventilation fan which draws air from the outside air plenum through building louvers. Upon DG start, the emergency ventilation fans are actuated, while the normal ventilation fan is deenergized and isolated. The emergency ventilation system for each diesel generator room consists of two nominal 50 percent capacity emergency ventilation fans, ductwork, and diffusers arranged to maintain room temperature below required limits by supplying outside air. Exhaust air is discharged through relief backdraft dampers. One emergency ventilation fan is thermostatically controlled while the other emergency ventilation fan operates continuously. Below a certain outdoor air temperature, a single emergency ventilation fan, duct, and diffusers are capable of maintaining Diesel Building temperature below required operability limits.
McGuire Technical Specification (TS) 3.8.1, AC Sources -- Operating Limiting Condition for Operation (LCO) 3.8.1.b requires two operable DGs capable of supplying the Onsite Essential Auxiliary Power Systems in Modes 1, 2, 3, and 4. With one 3.8.1.b DG inoperable (CONDITION B),
LCO 3.8.1.b is not met and the associated remedial Required Actions (B.1 through B.4.1 or B.4.2) and Required Action B.5, Restore DG to OPERABLE status, must be met prior to expiration of their associated Completion Times (CT). Required Action B.2, Perform SR 3.8.1.1 for the required offsite circuit(s), has a 1-hour CT and Required Action B.5 has a 72-hour CT. If the Required Actions and associated CTs of B.2, B.3, B.4.1, B.4.2, or B.5 are not met (CONDITION I), then Required Action I.1 requires the unit to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Required Action I.2 requires the unit to be in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> (unless the LCO is met or is no longer applicable prior to expiration of the specified CTs).
EVENT DESCRIPTION
On February 12th, 2025, while performing electrical testing on 2A1 Emergency Ventilation Fan (VD) motor, test data indicated irregular phase resistance. While investigating the cause, it was discovered the motor was windmilling in the reverse direction due to a stuck open 2A1 Outside Air Backdraft Damper (OABD), located at the inlet to the 2A1 Emergency Ventilation Fan. When the 2A1 OABD was closed by hand, 2A1 VD fan motor electrical testing was completed with satisfactory results. The design configuration for the 2A1 OABD is normally closed when the 2A1 VD fan is not in service. The OABDs are passive, gravity assisted dampers that should automatically close when forward air flow stops. The OABDs function to prevent diesel building room ventilation air from recirculating when the normal ventilation fan is in service and when only one emergency fan is in operation.
Further, Duke Energy engaged an external vendor who performed analytical modeling of the 2A DG room where the 2A1 OABD was stuck open concurrent with one emergency ventilation fan in operation. This model replicated conditions during the period when the above ventilation configuration may have adversely affected the capability of the 2A DG to perform its specified safety function. A final report was provided to Duke Energy September 3, 2025, which concluded the 2A DG average room temperature would have exceeded required operability temperature limits for approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> on February 16, 2025, should a diesel generator demand have occurred.
REPORTABILITY DETERMINATION A configuration existed for approximately 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br /> where 2A VD would not have provided sufficient cooling to maintain DG operability. During that 9-hour period, SR 3.8.1.1 was not performed within the required 1-hour CT.
Further, in consequence of the failure to perform SR 3.8.1.1 in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, Unit 2 was not placed in Mode 3 within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> as required by Required Action I.1. The failure to complete Required Actions prior to the expiration of the specified CTs is reportable as an operation or condition prohibited by TS.
CAUSAL FACTORS A causal analysis is currently in progress to identify the causes and contributors that led to the 2A1 VD OABD failure to close as designed when its respective VD fan was secured. Further, an extent of condition review was performed to evaluate all eight VD trains on both units, equipment design, station and industry Operating Experience (OE), existing maintenance strategy, performance testing, technical basis for preventative maintenance, and risk.
Mechanically, the initial investigation revealed OABD bearings in poor condition, leading to increased rotational resistance when damper blades try to close. Three additional OABDs were discovered stuck open when inspected per the extent of condition plan. Each damper required corrective maintenance to the bearings, restoring antifriction capabilities. The stuck open OABDs were evaluated for reportability. Only the condition associated with the 2A1 OABD was determined to be reportable as an operation or condition prohibited by Technical Specifications.
CORRECTIVE ACTIONS
Corrective actions completed or planned for this event include the following:
- 1. Upon discovery, the stuck open 2A1 VD OABD was inspected and repaired.
- 2. An extent of condition review was performed. All OABDs on both units were inspected, and no functionality concerns were identified.
SAFETY ANALYSIS
This event did not result in a plant transient, radiological release, or other challenge to normal plant operation to cause any impact to the public health and safety. In a loss of offsite power event, the stuck-open OABD would be of no consequence to Diesel Room temperatures unless there was a coincident failure or unavailability of the associated VD fan. McGuire Units 1 and 2 also have considerable defense-in-depth for AC power besides having two DG trains. The Emergency Supplemental Power System (ESPS) provides a means to restore AC power to the 2A Essential Bus (2ETA) if the 2A DG failed to run. Similarly, another recovery action is the ability to align offsite power from Unit 1 if available via the Shared Auxiliary Transformer SATA. The Standby Shutdown Facility (SSF) also provides an alternative means of maintaining safe and stable conditions with the SSF Diesel Generator powering the Standby Makeup Pump for reactor coolant pump (NCP) seal cooling and instrumentation and controls for operation of Turbine-Driven Auxiliary Feedwater Pump. The final level of safety is the ability to use FLEX equipment and strategies to provide alternative core cooling to prevent core damage.
Further, had SR 3.8.1.1 been performed over the period of the 2A DG inoperability, it would have been met. With these factors taken together, the event had a very low impact on plant risk and there was no impact to the health and safety of the public.
ADDITIONAL INFORMATION
Following completion of the causal analysis, a supplement to this Licensee Event Report (LER) will be issued. A review of McGuire LERs issued in the last three years was performed. No similar LERs were identified. Page 5 of 5