05000443/LER-2024-001, Condition Prohibited by Technical Specifications - Transformer Bushing Failure - Offsite AC Sources

Condition Prohibited by Technical Specifications - Transformer Bushing Failure - Offsite AC Sources

ML24110A087
Person / Time
Site:	Seabrook
Issue date:	04/19/2024
From:	Rasmus P NextEra Energy Seabrook
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
L-2024-058 LER 2024-001-00
Download: ML24110A087 (1)

LER-2024-001, Condition Prohibited by Technical Specifications - Transformer Bushing Failure - Offsite AC Sources

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4432024001R00 - NRC Website

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NEXTera~

ENERGY~

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Attention: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Seabrook Station Docket No. 50-443 Reportable Event: 2024-01-00 Date of Event: March 1, 2024

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SEABROOK April 19, 2024 L-2024-058 10 CFR 50.73 Condition Prohibited by Technical Specifications - Transformer Bushing Failure --

Offsite AC Sources The attached Licensee Event Report 2024-01 is being submitted pursuant to the requirements of 10 CFR 50.73 to provide notification of the subject event.

Should you have any questions regarding this submission, please contact Mr. Kenneth Mack, Fleet Licensing Manager, at 561-904-3635.

Respectfully,

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/VJ~

Paul Rasmus General Manager, Regulatory Affairs Enclosed: Attachment 1 cc:

Seabrook Station NRC Senior Resident Inspector Seabrook Station NRC Project Manager NextEra Energy Seabrook, LLC PO Box 300, Seabrook, NH 0387 4

Abstract

At 0542 on 3/1/24 (Mode 1; 100% Power), Seabrook Station experienced a fault at the 345kV 'B' phase high voltage bushing on Reserve Auxiliary Transformer (RAT), ED-X-3-B [EA; XFMR]. The fault resulted in the opening of switchyard breakers 52 and 695, isolating one of the two offsite power supplies to the station. At the time of the failure, the transformer was in a standby unloaded condition and was not powering any station buses. Both RATs were isolated from the switchyard by the opening of these two breakers. The bushing failure resulted in contamination of the transformer internals with porcelain fragments and internal damage. Therefore, the 3B RAT was replaced with the on-site spare transformer.

This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as the maintenance exceeded the allowed outage time (AOT) of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> under TS 3.8.1.1 "AC Sources" under a Notice of Enforcement Discretion.

Description of Event

At 0542 on 3/1 /24, during normal operation of the plant (Mode 1; 100% power), the control room received alarms associated with 345 kV breakers: 695 'C' and 52 'K'. The actuation of these breakers isolated both Reserve Auxiliary Transformers (RATs) [EA; XFMR] 3A and 38. The switchyard was walked down, and it was identified that the '8' RAT had two local alarms present: sudden pressure relay and low oil level. The direct cause of the issue was a failure of the '8' phase high voltage bushing on the 38 RAT. This resulted in an unplanned entry into the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> AOT for Technical Specification 3.8.1.1.a, loss of a single offsite power source. The bushing failure resulted in contamination of the transformer internals with porcelain fragments and internal damage requiring the transformer to be replaced. The NRC granted a Notice of Enforcement Discretion (NOED) on 3/4/24, allowing the station to remain online an additional 5 days after the AOT expired. During the period of enforcement discretion, and to facilitate on line replacement of the 38 RAT, the NRC approved an Emergency License Amendment Request (LAR) granting a one-time extension of the Technical Specification AOT allowing the unit to remain online until 3/31/24. The emergency LAR was approved on 3/8/2024 and this LER is written for the condition prohibited by technical specifications that existed between the expiration of the original AOT on 3/4/2024 and the approval of the emergency LAR on 3/8/2024. The 38 RAT was replaced with the on-site spare transformer within this extended time frame and returned to service on 3/25/24. There were no Systems, Structures, or Components that were inoperable at the start of the event that contributed to the event

Cause of Event

The key cause was the failure of the 'B' phase of the 345kV high voltage transformer bushing.

Safety Significance

This event did not prevent any safety systems from performing their safety related functions. Failure of the 38 RAT renders one of the two required physically independent circuits between the offsite transmission network and the onsite Class 1 E Distribution System inoperable. At the time of the failure the transformer was in a standby unloaded condition and was not powering any station buses. Additionally, safety related buses 5 and 6 remained powered by the Unit Auxiliary Transformers (UAT) [EA; XFMR] and remained capable of being powered from the Emergency Diesel Generators (EOG) [DG; EK]. This Licensee Event Report is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications. This event did not result in a Safety System Functional Failure.

Corrective Actions

The site submitted a request for Enforcement Discretion (ML24064A077) and subsequently submitted a License Amendment (ML24064A247) to the NRC to extend the allowed out of service time to complete the repair. The affected transformer has been replaced with the on-site spare. The failed bushing will be sent offsite for forensic analysis.

Similar Events

A review was conducted over the past 20 years for Seabrook, and it was concluded that there were no similar events that pertain to this incident. Page 3

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