05000270/LER-2024-001, Common Cause Inoperability of Both Trains of Control Room Ventilation System Outside Air Booster Fans Due to Supply Breaker Wiring Deficiency Resulted in a Condition That Have Prevented Fulfillment
| ML24354A312 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 12/19/2024 |
| From: | Denise Wilson Duke Energy Carolinas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| RA-24-0305 LER 2024-001-00 | |
| Download: ML24354A312 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2702024001R00 - NRC Website | |
text
David Wilson Nuclear Support Services Manager Oconee Nuclear Station Duke Energy ON01VP l 7800 Rochester Hwy Seneca, SC 29672 o. 864.873.3451 f: 864.873.3411 David.Wilson2@duke-energy.com RA-24-0305 December 19, 2024 10 CFR 50.73 Attn: Document Control Desk U. S. Nuclear Regulatory Commission 11555 Rockville Pike Rockville, MD 20852-2746 Duke Energy Carolinas, LLC Oconee Nuclear Station Unit 2 Docket Number: 50-270 Renewed Operating Licenses: DPR-47
Subject:
Licensee Event Report 270/2024-001, Revision 00 - Common Cause Inoperability of Both Trains of Control Room Ventilation System Outside Air Booster Fans due to Supply Breaker Wiring Deficiency Resulted in a Condition that Could Have Prevented Fulfillment of a Safety Function Licensee Event Report 270/2024-001, Revision 00, is being submitted pursuant to the requirements of 10 CFR 50.73 to provide notification of the subject event.
There are no regulatory commitments associated with this LER.
There are no unresolved corrective actions necessary to restore compliance with NRC requirements.
If there are questions, or further information is needed, contact Brandy Campbell, Regulatory Affairs, at (864) 873-3436.
Sincerely, David A. Wilson Manager, Nuclear Support Services Oconee Nuclear Station Enclosure: Licensee Event Report 270/2024-001 Rev.00
RA-24-0305 December 19, 2024 Page 2 cc (w/Enclosure):
Ms. Laura Dudes, Administrator, Region II U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Mr. Shawn Williams, Project Manager U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop O-08B1A Rockville, MD 20852-2738 Mr. Nick Smalley NRC Senior Resident Inspector Oconee Nuclear Station
Abstract
On October 28, 2024, at 0533 the normal supply breaker to Motor Control Center (MCC) 1XR tripped open. The alternate supply to the MCC was out of service for preventative maintenance. This resulted in loss of power to inlet dampers for both Unit 1&2 A and B Control Room Ventilation System (CRVS) Outside Air Booster Fans (OABF) and Unit 1&2 A CRVS OABF. Unit 2 entered Technical Specification (TS) Limiting Condition of Operation (LCO) for inoperability of both Unit 1&2 CRVS OABF Trains. The Unit 1&2 B CRVS OABF was returned to operable status at 1059 after the trains inlet damper was manually opened in accordance with the normal operating procedure. The loss of power to MCC 1XR was due to a legacy wiring issue in the MCC supply breaker interlock circuit. The Unit 1&2 CRVS OABF trains were restored following repairs to the MCC supply breaker interlock circuit and further corrective actions are planned to revise applicable MCC feeder breaker maintenance procedures to prevent similar issues.
This event was reported on October 28, 2024, in Event Notification 57401, under 10 CFR 50.72(b)(3)(v)(D) and is also reportable under 10 CFR 50.73 (a)(2)(v)(D) and 10 CFR 50.73 (a)(2)(vii).
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
BACKGROUND The Control Room Ventilation System (CRVS) Outside Air Booster Fan (OABF) trains [VI] are designed to limit in-leakage of radioactive effluents or toxic gas by pressurizing the Control Room Envelope [NA] with filtered air. The CRVS OABF trains contain outside air booster fans that are started manually and dampers that are automatically positioned with start of the associated outside air booster fan. The control room for Units 1 and 2 has two outside air booster fans. Both CRVS OABF trains are required to be operable per Technical Specification (TS) 3.7.9 in MODES 1, 2, 3, 4, 5, 6, and during movement of recently irradiated fuel on any Unit.
The Unit 1&2 A CRVS OABF is powered from Unit 1 MCC 1XR and the Unit 1&2 B CRVS OABF is powered from Unit 2 MCC 2XR. The motor-operated inlet dampers for both Unit 1&2 CRVS OABF trains are powered from Unit 1 panel board 1KG, which is powered from MCC 1XR. MCC 1XR is provided with redundant power supplies from Unit 1 load centers 1X6 and 1X7 and automatic power-seeking logic and an interlock to prevent the MCC from being simultaneously powered from both power supplies.
EVENT DESCRIPTION
On October 28, 2024, Unit 1 was in NO MODE for a planned refueling outage. Units 2 and 3 were in MODE 1 at approximately 100% power and no movement of recently irradiated fuel was in progress on any Unit. Load center 1X6 was out of service for 6-year load center preventative maintenance (PM), which also removed the alternate feeder for motor control center (MCC)[MCC] 1XR from service (breaker 1X6-6C). No structures, systems or components were out of service on Unit 2 at the time of this event that contributed to this event. Unit 3 was not affected by this event.
At time 0533, the normal supply breaker for MCC 1XR (breaker 1X7-4D from load center 1X7) tripped during performance of secondary disconnect inspection and maintenance on breaker cubicle 1X6-6C. This resulted in a loss of power for MCC 1XR, which resulted in loss of power to the Unit 1&2 A CRVS OABF and loss of power to downstream panelboard 1KG. Loss of power to panelboard 1KG resulted in loss of power to the normally closed motor-operated inlet dampers for the Unit 1&2 A and B CRVS OABF trains.
Operations entered Technical Specification (TS) Limiting Condition of Operation (LCO) for inoperability of both trains of Unit 1&2 CRVS OABF trains. The inlet damper to the Unit 1&2 B CRVS OABF train was failed open in accordance with the normal operating procedure to restore that train to service. With one CRVS train restored to operable, TS 3.7.9 Condition F was exited at 1059.
While the Unit 1&2 Control Room Ventilation System Booster Fans trains were in standby at the time of the event, the loss of power to MCC 1XR rendered both trains of Control Room Outside Air Booster Fans being unable to supply filtered makeup air to the main control room should a design basis event occur.
Reportability
This event was reported to NRC on October 28, 2024, in Event Notification 57401, as an 8-hour non-emergency notification under 10CFR 50.72(b)(3)(v)(D) - Event or Condition that Could Have Prevented Fulfillment of a Safety Function for Accident Mitigation. The event is also reportable under 10CFR 50.73(a)(2)(v)(D) - Event or Condition that Could Have Prevented Fulfillment of a Safety Function for Accident Mitigation. Also, since the failure resulted in independent portions of more than one train in the same system to become inoperable, this is reportable under and 10 CFR 50.73(a)(2)(vii) - Common Cause Inoperability of Independent Trains or Channels.
CAUSAL FACTORS The direct cause of this event was due to a legacy internal wiring issue in breaker cubicle 1X6-6C. The wiring is in an interlock circuit which caused the MCC 1XR normal feeder breaker to trip open. When the secondary disconnect was manipulated for inspection and replacement, this allowed the interlock circuit to be inadvertently completed, resulting in the normal feeder breaker tripping open.
A lugged termination on the backside of the secondary disconnect was oriented such that the lug and conductor was physically tensioned across the lugged termination of another conductor on the secondary disconnect. During removal of a portion of the 1X6-6C secondary disconnect, this legacy wiring issue resulted in the inadvertent completion of the MCC 1XR feeder breaker interlock circuit when some exposed conductor strands contacted. Actuation of the interlock caused breaker 1X7-4D to trip open, resulting in the loss power to both motor operated inlet dampers for the Unit 1&2 CRVS OABF trains. The wiring deficiency was on the back side of the secondary disconnect and was not visible prior to manipulation of the secondary disconnect.
CORRECTIVE ACTIONS
Completed
- 1. Repaired legacy wiring issue on secondary disconnect in 1X6-6C cubicle.
- 2. Returned both Unit 1&2 CRVS OABF trains to operable by restoring MCC 1XR to normal alignment with normal operating procedures.
Planned
- 1. Revise MCC secondary disconnect maintenance procedure to prevent unintended actuation of the feeder breaker interlock circuit.
- 2. Develop and implement an Engineering Change to separate the power supplies for the motor operated inlet dampers for the Unit 1&2 CRVS OABF trains.
SAFETY ANALYSIS
The loss of 1XR event had no impact on the health and safety of the public and a negligible impact to core damage risk. The CRVS has been screened from inclusion in the Oconee PRA based on low-risk significance. The function of the CRVS OABF trains is to pressurize the main control rooms for accident conditions using filtered outside air to prevent the in-leakage of radioactive effluents, smoke, or toxic gases from adjacent buildings or the outside atmosphere, and thus protects control room personnel. These fans do not provide any core cooling function or support any other core cooling systems, and they do not serve to prevent or reduce dose consequences to the public. The unavailability of both OABF trains was also very short given that the loss of 1XR was annunciated and actions taken to manually open the Unit 1&2 B train outside air damper to restore availability of Unit 1&2 B CRVS Outside Air Booster Fan.
These factors taken together support the conclusion that this event had no impact on the health and safety of the public and a negligible impact to overall plant risk.
ADDITIONAL INFORMATION
A review of Duke Energy's Corrective Action Program did not identify any Oconee LERs or additional events in the last three years that involve the same underlying concerns or reasons as this event.
This event is considered INPO IRIS Reportable. There were no releases of radioactive materials, radiation exposures, or personnel injuries associated with this event. Page 4 of 4