05000530/LER-2024-002, Invalid Specified System Actuation of Train B Emergency Diesel Generator

Invalid Specified System Actuation of Train B Emergency Diesel Generator

ML24177A322
Person / Time
Site:	Palo Verde
Issue date:	06/25/2024
From:	Harbor C Arizona Public Service Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
102-08821-CDH/CWD LER 2024-002-00
Download: ML24177A322 (1)

LER-2024-002, Invalid Specified System Actuation of Train B Emergency Diesel Generator

Event date:
Report date:
5302024002R00 - NRC Website

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Abstract

On 27 April 2024, at 10:18 Mountain Standard Time (MST), during clearance restoration for the Unit 3 Train B Spray Pond (SP) system, an invalid actuation of the the Unit 3 Train B Emergency Diesel Generator (EDG) occurred. The EDG inadvertently started when the EDG hand switch was taken to REMOTE as part of the restoration activities. The start of the Unit 3 Train B EDG resulted in an automatic actuation of the Unit 3 Train B SP pump, as designed. The automatic start of the EDG was caused by a Loss of Power (LOP) signal that had not been reset as part of previous clearance work. There was no loss of voltage on any required bus. No Technical Specifications (TS) were entered due to Unit 3 being in Mode 6 with the Train B equipment not being necessary to meet TS or safe shutdown requirements. Corrective actions include restoring the system lineups and implementing procedure changes to minimize recurrence. The NRC residents were notified at the time of the event.

No similar events have occurred at PVNGS in the last three years.

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 an an automatic actuation of the Emergency Diesel Generator (EDG) (EIIS:EK) and the subsequent automatic actuation of the associated Essential Spray Pond Pump (EIIS:BS) for Unit 3 during clearance restoration activities on 27 April 2024. The NRC residents were notified at the time of the event.

2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S), AND COMPONENT(S)

The safety related equipment for Unit 3 is powered by one of two load groups (Train A and Train B). Either of the associated trains can provide power for safe plant shutdown. Each alternating current (AC) train includes one Class 1E 4.16 kilovolt (kV) bus (EIIS: EB). The preferred and alternate power sources for each load group is offsite 525 kV AC power (EIIS: EK) and is supplied via the 13.8 kV secondary windings from two of the three startup transformers (EIIS: EA) to six 13.8 kV intermediate buses. Unit 3 receives 13.8 kV power from two of the intermediate buses. Class 1E 4.16 kV safety load group power is provided from the associated intermediate bus engineered safety feature (ESF) transformers.

The standby power supply for each safety load group consists of one EDG (EIIS: EK), including its auxiliary and fuel systems. The standby power supply functions as a source of AC power for safe plant shutdown in the event of loss of preferred power and for post-accident operation of ESF loads. Actuation of the associated SP pump is a designed response to the EDG start to provide required cooling for the EDG.

3. INITIAL PLANT CONDITIONS

Unit 3 was in MODE 6, 0% power, during refueling outage 3R24 at the time of the event. Reactor coolant system (RCS) temperature was 85 degrees F and the reactor pressure was 0 psig (open to atmosphere). There were no inoperable structures, systems, or components at the time that contributed to this event.

Unit 1 and Unit 2 were not affected by the event.

4. EVENT DESCRIPTION

During clearance activities for the Unit 3 Train B SP system, the Unit 3 Train B Balance of Plant - Engineered Safety Features Actuation System (BOP-ESFAS) was de-energized as part of planned work. This resulted in a Loss of Power (LOP) signal sent to the Unit 3 Train B EDG control cabinet. During restoration activities for the Unit 3 Train B BOP-ESFAS, this circuitry was not reset. There were no alarms or indications to indicate this signal was present. On 27 April 2024 at 10:18, when the Unit 3 Train B EDG was taken to the REMOTE position, the EDG control cabinet initiated an emergency start of the Train B EDG. There was no loss of voltage on any required bus. The Unit 3 Train B EDG emergency start resulted in a start of the Unit 3 Train B SP pump, as designed. The Control Room took action to secure and reset the Train B EDG. No TS entries were required due to Unit 3 being in Mode 6 with the Train B equipment not required to meet TS or safe shutdown requirements.

5. ASSESSMENT OF SAFETY CONSEQUENCES

There were no inoperable structures, systems, or components that contributed to this event. The Unit 3 Train B EDG responded as designed to the invalid LOP signal. The Unit 3 B Train SP pump actuated to support the running EDG.

This event did not result in any challenges to the fissio n 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).

6. CAUSE OF THE EVENT

The direct cause of the invalid emergency start of the Unit 3 Train B EDG was the presence of an invalid LOP signal that had not been reset after de-energization of Train B BOP-ESFAS.

The apparent cause of the invalid emergency start of the Unit 3 Train B EDG was that the relevant procedure did not appropriately identify the need to reset the Unit 3 Train B EDG circuitry prior to placing the EDG Mode Control Switch in REMOTE.

7. CORRECTIVE ACTION(S):

Immediate corrective actions were the securing of the EDG followed by the reset and restoration of the BOP-ESFAS signal, the Unit 3 Train B EDG, and the Unit 3 Train B SP pump.

Planned corrective actions include updates to the associated procedure to incorporate cautions for EDG start signals, instructions to utilize equipment status tags, and modification to steps to ensure proper reset of the system prior to return to service. This will identify the need to reset the EDG circuitry using the appropriate procedure section to prevent an automatic emergency start when the EDG is restored.

8. PREVIOUS SIMILAR EVENTS

No similar events have occurred in the prior three years.