05000388/LER-2021-003-01, Automatic Reactor Scram Due to Main Turbine Trip Caused by Inadvertent Trip of the Main Generator Output Breakers

Automatic Reactor Scram Due to Main Turbine Trip Caused by Inadvertent Trip of the Main Generator Output Breakers

ML22083A168
Person / Time
Site:	Susquehanna  (NPF-022)
Issue date:	03/24/2022
From:	Cimorelli K Susquehanna, Talen Energy
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
PLA-7993 LER 2021-003-01
Download: ML22083A168 (4)

LER-2021-003, Automatic Reactor Scram Due to Main Turbine Trip Caused by Inadvertent Trip of the Main Generator Output Breakers

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
3882021003R01 - NRC Website

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March 24, 2022 Kevin Cimorelli Site Vice President Attn: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Susquehanna Nuclear, LLC 769 Salem Boulevard Berwick, PA 18603 Tel. 570.542.3795 Fax 570.542.1504 Kevin.Cimorelli@TalenEnergy.com SUSQUEHANNA STEAM ELECTRIC STATION LICENSEE EVENT REPORT 50-388/2021-003-0l UNIT 2 LICENSE NO. NPF-22 PLA-7993 TALEN~

ENERGY 10 CFR 50.73 Docket No. 50-388 Attached is Licensee Event Report (LER) 50-388/2021-003-0l. The LER reports an event involving an automatic scram due to a main turbine trip. The condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in an automatic actuation of the Reactor Protection System (including a reactor scram).

There were no actual consequences to the health and safety of the public as a result of this event.

This letter contains no new or revised regulatory commitments.

Attachment: LER 50-388/2021-003-01 Copy:

NRC Region I Ms. A. Klett, NRC Project Manager Mr. C. Highley, NRC Senior Resident Inspector Mr. M. Shields, PA DEP/BRP

Abstract

On October 11, 2021, at approximately 13:21, Susquehanna Steam Electric Station Unit 2 reactor automatically scrammed due to a main turbine trip. Both divisions of the Reactor Protection System (RPS) actuated and all control rods inserted. This event was reported by Event Notification 55514 in accordance with 10 CFR 50.72(b)(2)(iv)(A),

(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in automatic actuation of a system listed in 10 CFR 50.73(a)(2)(iv)(B).

The event was caused by the 500kV switchyard primary plant interface relay inadvertently tripping open the main generator output breakers due to a transient voltage resulting from a ground applied on the primary switchyard battery system. This led to a load reject trip of the main turbine that resulted in an automatic scram.

Key corrective actions included removing the ground from the 125 VDC primary switchyard battery system, implementing an increased time delay for trip signals to the primary plant interface relay, and lifting redundant shield grounds on the station side of the relay. Additionally, primary plant interface inputs will be hardened, as appropriate.

There were no actual consequences to the health and safety of the public as a result of this event.

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CONDITIONS PRIOR TO EVENT

Unit 1 - Mode 1, approximately 100 percent Rated Thermal Power Unit 2 - Mode 1, approximately 95 percent Rated Thermal Power There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.

EVENT DESCRIPTION

On October 11, 2021, at approximately 13:21, Susquehanna Steam Electric Station Unit 2 reactor automatically scrammed due to a main turbine [EIIS System/Component Code: TA/TRB] trip. The Unit 2 Control Room received indication of a main turbine trip with both divisions of the Reactor Protection System (RPS) [EIIS System Code: JC] actuated and all control rods inserted. The turbine bypass valves [EIIS System/Component Code: JI/PCV] opened automatically to control reactor pressure and subsequently remained open causing the reactor to depressurize; this event is reported in Licensee Event Report 50-388/2021-004. Operations manually closed the Main Steam Isolation Valves (MSIVs) [EIIS System/Component: SB/ISV] to stop reactor depressurization. The High Pressure Coolant Injection (HPCI) [EIIS System Code: BJ] and Reactor Core Isolation Cooling (RCIC) [EIIS System Code: BN]

systems were manually initiated to control reactor water level. Operations subsequently maintained reactor water at the normal operating band using RCIC and reactor pressure was controlled with HPCI in pressure control mode and the main steam line drains. The Reactor Recirculation Pumps [EIIS System/Component Code: AD/P] tripped on End of Cycle Recirculation Pump Trip.

This event was reported by Event Notification 55514 in accordance with 10 CFR 50.72(b)(2)(iv)(A),

(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in automatic actuation of a system listed in 10 CFR 50.73(a)(2)(iv)(B).

CAUSE OF EVENT

The direct cause of the event was an applied ground by the switchyard [EIIS System: FK] owner contractor on the primary switchyard battery system which caused transient voltage at the relay [EIIS Component: RLY] input. The apparent cause was the switchyard owner interface relay design and settings were not in alignment with the vendor / original equipment manufacturer best practices leading to inadequate margin to prevent transient voltages from unintentionally causing a trip signal.

ANALYSIS/SAFETY SIGNIFICANCE

During the reactor scram off-site power remained available to power all safety related shutdown equipment. All control rods inserted as designed and all safety related equipment operated as designed.

There was no loss of function that prevented the safe shutdown of the reactor and to maintain it in a safe shutdown condition. All safety systems were available to mitigate the consequences of an accident. The condition described herein did not result in a safety system functional failure. Accordingly, this event will not be counted as a safety system functional failure in the Reactor Oversight Process Performance Indicators There were no actual consequences to the health and safety of the public as a result of this event.

CORRECTIVE ACTIONS

Key corrective actions included the following:

1. Removed the ground from the positive terminal of the 125 VDC primary switchyard battery system.

2. Implemented an increased time delay for trip signals to the primary plant interface relay to provide additional protection from inadvertent actuation due to electrical noise.

3. Lifted redundant shield grounds on the station side of the primary plant interface relay for the Unit 2 automatic and manual lockout devices.

4. Harden the inputs, as deemed appropriate, to the primary plant interface relay against electrical noise and/or capacitive coupling onto cables.

COMPONENT FAILURE INFORMATION

Component Identification U2P Relay Component Name - SEL-2411 Programmable Module Manufacturer - Schweitzer Engineering Laboratories (SEL)

PREVIOUS OCCURRENCES

None.