05000325/LER-2023-001, Loss of Redundant Power Supply Operation Causes Turbine Trip and Subsequent Reactor Scram

Loss of Redundant Power Supply Operation Causes Turbine Trip and Subsequent Reactor Scram

ML23166A081
Person / Time
Site:	Brunswick
Issue date:	06/15/2023
From:	Krakuszeski J Duke Energy Progress
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
RA-23-0116 LER 2023-001-00
Download: ML23166A081 (1)

LER-2023-001, Loss of Redundant Power Supply Operation Causes Turbine Trip and Subsequent Reactor Scram

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation
3252023001R00 - NRC Website

text

U.S. Nuclear Regulatory Commission Page 2 of 2

cc (with enclosure):

Ms. Laura Dudes, NRC Regional Administrator, Region II Mr. Luke Haeg, NRC Project Manager Mr. Gale Smith, NRC Senior Resident Inspector

Abstract

At 01:48 Eastern Daylight Time (EDT) on April 20, 2023, with Unit 1 in Mode 1 at approximately 100% power, an automatic reactor scram occurred due to a turbine trip. All control rods inserted as expected. Turbine bypass valves did not open on the scram; Safety Relief Valves (SRVs) opened automatically to control reactor pressure. Reactor Pressure reached approximately 1100 psig on the scram, exceeding the Reactor Protection System (RPS) setpoint. Reactor water level reached low level 1 following the scram resulting in automatic actuation of Primary Containment Isolation System (PCIS) Group 2, 6, and 8 isolation valves, by design.

Operations responded and stabilized the plant. Operations transitioned pressure control from SRVs to main steam line drains to the condenser. Following the scram, reactor water level was maintained via the condensate and feedwater systems.

The turbine trip and subsequent inoperative turbine bypass valves resulted from a complete loss of 24 Vdc power to the Turbine Protection system. The power supplies were replaced prior to restart.

There was no impact on the health and safety of the public or plant personnel. This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) due to valid actuation of the RPS and PCIS.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

Background

Initial Conditions

At the time of the event, Unit 1 was in Mode 1 (i.e., Power Operation), at approximately 100 percent rated thermal power.

Reportability Criteria

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) because it involved actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B). Specifically, the Reactor Protection System [JC] and Primary Containment Isolation System [JM] actuated during this event.

The NRC was notified of this event per 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50. 72(b)(3)(iv)(A) via Event Notification 56478 at 05:24 Eastern Daylight Time (EDT) on April 20, 2023.

Event Description

At 01:48 Eastern Daylight Time (EDT) on April 20, 2023, with Unit 1 in Mode 1 at approximately 100% power, an automatic reactor scram occurred due to a turbine trip. All control rods inserted as expected.

The turbine bypass valves [JI] did not open on the scram; main steam [SB] Safety Relief Valves (SRVs) 1C, 1F, 1G, 1K, and 1L cycled open automatically to control reactor pressure. Reactor pressure reached approximately 1100 psig on the scram; exceeding the 1060 psig Reactor Protection System (RPS) setpoint.

Reactor water level reached the low level 1 (LL1) setpoint following the scram. Per design, the LL1 signal resulted in Primary Containment Isolation System Group 2 (i.e., floor and equipment drain isolation valves), Group 6 (i.e., monitoring and sampling isolation valves) and Group 8 (i.e., shutdown cooling isolation valves) isolations.

Operations responded and stabilized the plant. Operations manually cycled open SRV 1B then transitioned pressure control from SRVs to the main steam line drains [SB] to the condenser [SG]. Following the scram, reactor water level was maintained via the condensate [SD] and feedwater [SJ] systems.

Event Cause

The direct cause of the turbine trip and subsequent inoperative turbine bypass valves was a complete loss of 24 Vdc power to the Turbine Protection system [JJ]. When 24 Vdc power was lost, Turbine Protection system data inputs for required pressure and flow transmitters were lost. With the loss of monitoring capability, the Turbine Protection system responded as designed by initiating a turbine trip to protect the turbine and inhibiting the opening of the turbine bypass valves to protect the condenser.

24 Vdc power is supplied to the Turbine Protection system via redundant power supplies [RJX]. Due to inadequacies in the vendor design, a unique combination of internal component failures on one power supply impacted the operation of the redundant power supply. As a result, both power supplies failed/shutdown within a short timeframe of one another resulting in a total loss of 24 Vdc power.

Prior to this event the power supplies had operated successfully with no indication of deficiencies for approximately five years.

Safety Assessment

There was no adverse impact on the health and safety of the public or plant personnel. The safety significance of this event is minimal. The condenser remained available for pressure control. The condensate and feedwater systems remained available for level control.

Corrective Actions

Prior to restart from this event, the 24 Vdc Turbine Protection system power supplies were replaced.

The site is planning to implement changes to the current 24Vdc power supply functionality that will prevent a power supply internal failure from impacting the redundant power supply.

Any changes to corrective actions or completion schedules w ill be made in accordance with the sites corrective action program.

Previous Similar Events

No previous similar events have occurred within the past three years in which inadequate vendor design resulted in a turbine trip and reactor scram.

Commitments

No regulatory commitments are contained in this report.