05000277/LER-2024-003, Automatic Reactor Scram Following Main Turbine Trip Due to Degraded Condenser Vacuum

Automatic Reactor Scram Following Main Turbine Trip Due to Degraded Condenser Vacuum

ML24249A109
Person / Time
Site:	Peach Bottom
Issue date:	09/05/2024
From:	Stiltner R Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
ENS 57221, CCN: 24-45 LER 2024-003-00
Download: ML24249A109 (1)

LER-2024-003, Automatic Reactor Scram Following Main Turbine Trip Due to Degraded Condenser Vacuum

Event date:
Report date:
2772024003R00 - NRC Website

text

10 CFR 50.73 CCN: 24-45

September 05, 2024

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

Peach Bottom Atomic Power Station (PBAPS) Unit 2 Subsequent Renewed Facility Operating License No. DPR-44 NRC Docket No. 50-277

Subject: Licensee Event Report (LER) 2024-003-00 Automatic Reactor Scram Following Main Turbine Trip due to Degraded Condenser Vacuum

Reference: ENS 57221

The subject report is being submitted in accordance with 50.73(a)(2)(iv)(A) for actuation of the Reactor Protection System and Containment Isolation signals.

There are no commitments contained in this letter. If you have any questions, please contact the Peach Bottom Regulatory Assurance Engineer, Ms. Amy Huber at (267) 533-7247.

Respectfully,

Ryan C. Stiltner Plant Manager Peach Bottom Atomic Power Station

Enclosure

cc: USNRC, Administrator, Region I USNRC, Senior Resident Inspector W. DeHaas, Commonwealth of Pennsylvania S. Seaman, State of Maryland B. Watkins, PSE&G, Financial Controls and Co-Owner Affairs

Abstract

While conducting power ascension following a maintenance outage, Peach Bottom Unit 2 began experiencing increasing condensate pump discharge chlorides because of a condenser tube leak. A condenser water box was isolated to facilitate repairs, but due to high river temperatures combined with elev ated offgas inleakage, the water box isolation resulted in degrading condenser vacuum. Attempts to restore vacuum were unsuccessful and ultimately required a trip of the main turbine.

Because the Reactor Protection System (RPS) input for Turbine Stop Valve (TSV) and Turbine Control Valve fast (TCV) closure was armed, the turbine valve closure generated a scram signal. Group II and III Primary Containment Isolation Valves isolated because reactor water level reached the isolation setpoint. Reactor water level was maintained with feedwater and pressure was maintained with bypass valves. The scram occurred without complication and all systems responded as expected. Unit 3 was not impacted by this event.

This event is reportable under 10CFR50.73(a)(2)(iv)(A) due to manual or automatic isolation of systems listed in 10CFR50.73(a)(2)(iv)(B) including RPS and containment isolation signals. An ENS notification was made within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the event, reference ENS 57221.

Plant Operating Conditions Before the Event

Prior to the event, on July 10, 2024, Peach Bottom Unit 2 was in Mode 1 conducting power ascension following a maintenance outage. It was identified that condensate pump discharge chloride conductivity was increasing, indicative of a condenser (EIIS: SG) tube leak. To facilitate investigation and repair of leaking tubes, a condenser waterbox was isolated with the reactor at approximately 62% power. Waterbox isolation combined with seasonally high river temperatures and elevated offgas (EIIS: WF) inleakage from a degraded valve in the offgas system resulted in degrading condenser vacuum. Although the waterbox was returned to service, condenser vacuum continued to degrade. The operating crew reduced power per station procedures, but condenser vacuum reached the threshold requiring a manual trip of the main turbine (EIIS: TA).

Event Description

The turbine was manually tripped on July 10, 2024 at 07:28 with the reactor at 25.6% power, which is within the physical capability of bypass valves (EIIS: SO). This is below the power level at which the Reactor Protection System (RPS) input for Turbine Stop Valve (TSV) and Turbine Control Valve (TCV) fast closure (EIIS: TA) is required to be armed per Technical Specifications, but due to conservative instrument setpoints, the input was still armed and the closure of the valves generated a scram signal. The scram occurred without complication. The Reactor Pressure Vessel (RPV) low level setpoint was reached, resulting in isolation of Group II and III Primary Containment Isolation System (PCIS) (EIIS: JM) valves as expected. Water level in the RPV was restored and maintained with Feedwater (EIIS: SJ) and pressure was maintained with bypass valves.

This event is reportable under 10CFR50.73(a)(2)(iv)(A) due to manual or automatic isolation of systems listed in 10CFR50.73(a)(2)(iv)(B) including RPS and containment isolation signals. An ENS notification was made within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the event, reference ENS 57221.

Safety Consequences

The reactor Scram occurred as designed and resulted in no safety consequences. All safety systems responded as expected. Use of emergency core cooling systems for level and pressure control was not required. Unit 3 was not impacted by this event.

Cause and Corrective Actions

The cause of the scram was RPS input from TSV and TC V fast closure due to a manual trip of the Main turbine. Procedures directed tripping the turbine if steam flow was within bypass valve capability but did not warn that the RPS logic may be armed. Actions have been taken to revise procedures from tripping the turbine if within bypass valve capability to if it is known that the Main TCV/TSV closure is bypassed on A or B RPS.

Additionally, the cause of the degraded condenser vacuum condition was investigated. The root cause of this condition is that the station did not recognize and evaluate the cumulative effect of increased offgas air inleakage and isolation of a condenser waterbox with high river temperatures, culminating in a rapid lowering of main condenser vacuum. As a contributor, station models were unable to accurately predict the impact of elevated offgas inleakage on condenser performance. The corrective actions focus on enhancing the station governance for isolating a waterbox.

Previous Similar Events

In February 2004, Peach Bottom manually scrammed due to decreasing condenser vacuum. The cause of the loss of vacuum was inleakage of non-condensable gases from a crack in a feed pump turbine expansion joint.

Ref. LER 2004-002-00, dated 4/30/2004.

In August 2022, Peach Bottom experienced condenser vacuum degradation during a load drop in support of condenser cleaning activities, after isolating condenser wate r boxes. In this case, conditions were stabilized without requiring a trip of the main turbine or a reactor scram. Increased offgas flow was not a factor in this event.