05000529/LER-2024-001, Valid Specified System Actuations of Unit 2 Train B Emergency Diesel Generator and Train B Auxiliary Feedwater

Valid Specified System Actuations of Unit 2 Train B Emergency Diesel Generator and Train B Auxiliary Feedwater

ML24144A277
Person / Time
Site:	Palo Verde
Issue date:	05/23/2024
From:	Horton T Arizona Public Service Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
102-08806-TAH/CWD LER 2024-001-00
Download: ML24144A277 (1)

LER-2024-001, Valid Specified System Actuations of Unit 2 Train B Emergency Diesel Generator and Train B Auxiliary Feedwater

Event date:
Report date:
5292024001R00 - NRC Website

text

10 CFR 50.73

Todd A. Horton Senior Vice President Nuclear Regulatory & Oversight

Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel: 623.393.6418 102-08806-TAH/CWD May 23, 2024

U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject: Palo Verde Nuclear Generating Station Unit 2 Docket No. STN 50-529 /Renewed License No. NPF-51 Licensee Event Report 2024-001-00

Enclosed, please find the Licensee Event Report (LER) 50-529/2024-001-00 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER reports an event in which Unit 2 had valid specified system actuations of the Unit 2 Train B Emergency Diesel Generator (EDG) and Train B Auxiliary Feedwater (AF).

In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the Nuclear Regulatory Commission (NRC) Regional Office, NRC Region IV, and the Senior Resident Inspector.

No new commitments are being made to the NRC by this letter. Should you need further information regarding this submittal, please contact Matthew Cox, Department Leader, Nuclear Regulatory Affairs, at (623) 393-5753.

Sincerely, Horton, Todd Digitally signed by Horton, Todd (Z10098)

(Z10098) Date: 2024.05.23 14:03:14 -07'00' TAH/CWD/cr

Enclosure: Unit 2 Licensee Event Report 2024-001-00 cc: J. Monninger NRC Region IV Regional Administrator W. T. Orders NRC NRR Project Manager for PVNGS L. N. Merker NRC Senior Resident Inspector for PVNGS

A member of the STARS Alliance, LLC

Callaway

• Diablo Canyon

• Palo Verde

• Wolf Creek ENCLOSURE

Unit 2 Licensee Event Report 2024-001-00

Abstract

At 16:34 Mountain Standard Time (MST) on March 24, 2024, a valid actuation of the B Train Emergency Diesel Generator 

(EDG) for the Palo Verde Nuclear Generating Station (PVNGS) Unit 2 occurred due to an undervoltage condition on the 

respective 4.16 kilovolt (kV) Class 1E bus. The B Train EDG, the B Train essential spray pond pump, and th e B Train

Auxiliary Feedwater (AF) pump started.

The loss of power to the Class 1E bus and resulting component actuations were the result of the Unit 2 Engineered Safety

Features (ESF) Transformer supply breaker opening. Unit 2 entered Technical Specification (TS) Limiting Condition for 

Operation (LCO) 3.8.1, Condition A, for one required offsite circuit inoperable. In addition, the unit also entered TS LCO

3.8.9, Condition A, for the loss of the Class 1E bus, which was exited when the EDG started and restored power to the bus. 

TS LCO 3.8.1, Condition A, was exited when the bXVKLQJ was repaired and normal power supply was restored on March 28, 

2024.

All times are Mountain Standard Time and approximate unless otherwise indicated.

1. REPORTING REQUIREMENT(S)

This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to report a valid automatic actuation of the circuitry that starts the Emergency Diesel Generator (EDG) (EIIS: EK) for Unit 2 following an undervoltage condition on the affected B Train Class 1E bus on March 24, 2024. The event included actuations of the associated essential spray pond (SP) pumps (EIIS: BS) and the Unit 2 B Train Auxiliary Feedwater (AF) pump (EIIS: BA).

This event was reported to the NRC on March 24, 2024 via the event notification system (EN 57047).

2. INITIAL PLANT CONDITIONS

PVNGS Unit 2 began and remained in MODE 1, 100% power for the duration of the event. There were no structures, systems, or components (SSCs) that were inoperable at the start of the event and contributed to the event.

3. EVENT DESCRIPTION

On March 24, 2024 at 16:34, a valid loss of power (LOP) actuation occurred due to an undervoltage condition on the Unit 2 B Train Class 1E 4.16kV bus. The undervoltage condition resulted from the opening of the B Train ESF Transformer supply breaker. The B Train EDG started and loaded as designed. The EDG start was accompanied by the start of the associated Unit 2 B Train SP pump and the Unit 2 B Train AF pump, as designed for the undervoltage condition. The Unit 2 B Train AF pump was not needed for steam generator level control and no AF valves repositioned. The Unit 2 B Train AF pump did not supply feedwater to the steam generators. Unit 2 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, Condition A for one required offsite circuit inoperable and TS LCO 3.8.9, Condition A for the loss of the Class 1E bus. Unit 2 exited TS LCO 3.8.9 after the EDG started and restored power to the respective Class 1E bus (EIIS: EB).

The NRC was notified by the Emergency Notification System (ENS) report 57047 on March 24, 2024 at 21:48 (00:48 EST) in accordance with 10 CFR 50.72(b)(3)(iv)(A). The NRC residents were notified at the time of the event.

Unit 2 exited TS LCO 3.8.1, Condition A on March 28, 2024 at 01:04 after the bXVKLQJ was repaired and normal alignment restored.

Unit 1 and 3 were unaffected by the event. Unit 2 A Train 4.16kV Class 1E (EIIS: EB) and all Non-Class 1E (EIIS: EA) buses were unaffected by the event.

4. ASSESSMENT OF SAFETY CONSEQUENCES

There were no inoperable structures, systems or components that contributed to this event. The B Train EDG responded as designed to the undervoltage condition on the respective safety bus. The B Train SP and AF pumps actuated as designed for the loss of power.

This event did not result in any challenges to the fission product barriers or result in the release of radioactive materials. This event did not prevent the fulfillment of a safety function as described by 10 CFR 50.73(a)(2)(v), nor did it result in a safety system functional failure.

5. CAUSE OF THE EVENT

The direct cause was a line-to-ground fault on the B phase bushing in the B Train Class 1E 4.16kv switchgear causing an opening of the supply breaker to the ESF transformer, resulting in a loss of power to the B Train Class 1E 4.16kv bus. The apparent cause was water intrusion into the B Train 4.16kV Switchgear cubicle as a result of multiple cracks on the B phase bushings mounting flange.

Walkdowns of the switchgear identified multiple protective relays actuated. Review of Digital Fault Recorder (DFR) data confirmed the line-to-ground fault. Internal inspection of the switchgear revealed water stain residue that tracked from the enclosures ceiling to the bushings electrical taping. External inspection of the switchgear indicated water discoloration and bowing around the roof area of the B phase bushing.

6. CORRECTIVE ACTIONS

The immediate corrective action was to repair the faulted bushing and restore normal power alignment.

A corrective action (CA) is in progress to fix the roof bowing around the faulted bushing. Inspections for all remaining external switchgear bushings on all three units are in progress.

The existing bus connections will be changed from an open-air connection to an enclosed bus connection type as part of scope expansion of an existing ESF Transformer replacement modification. An enclosed bus will help prevent further water intrusion events. This is planned for all three units.

7. PREVIOUS OCCURENCES

On August 14, 2022, a valid actuation of the EDGs for PVNGS Unit 1 B Train and Unit 3 A Train occurred due to an undervoltage condition on their respective 4.16kV Class 1E buses. The loss of power to the Unit 1 and Unit 3 Class 1E buses and resulting component actuations were the result of a startup transformer tripping offline. The cause of the event was attributed to water intrusion into the 13.8kV switchgear cubicle as the result of a heavy rainstorm and high winds, in combination with degraded weatherstripping and conductor insulating boots that are not waterproof. This was reported under LER 1-2022-001-00.

The corrective actions from the previous LER event would not have prevented the event which is the subject of this report.

There have been other events at PVNGS involving water intrusion, but these have not directly caused a similar event as discussed in this report.