05000265/LER-2024-002-01, Turbine Trip and Automatic Scram Due to Digital EHC Power Supply Intermittent Failure

Turbine Trip and Automatic Scram Due to Digital EHC Power Supply Intermittent Failure

ML24304A856
Person / Time
Site:	Quad Cities
Issue date:	10/30/2024
From:	Hild D Constellation Energy Generation
To:	Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation, Document Control Desk
References
SVP-24-063 LER 2024-002-01
Download: ML24304A856 (1)

LER-2024-002, Turbine Trip and Automatic Scram Due to Digital EHC Power Supply Intermittent Failure

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2652024002R01 - NRC Website

text

Constellation.

SVP-24-063 October 30, 2024 U.S. Nuclear Regulatory Commission A TIN: Document Control Desk Washington, D.C. 20555 Quad Cities Nuclear Power Station, Unit 2 Renewed Facility Operating License No. DPR-30 NRC Docket No. 50-265 10 CFR 50.73 Subject:

Licensee Event Report 265/2024-002-01 "Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure" Supplement Reference:

Letter, D Hild to Document Control Desk - Licensee Event Report 265/2024-002-00 "Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure," July 22, 2024 (ML24204A105)

Enclosed is Licensee Event Report 265/2024-002-01 "Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure," for Quad Cities Nuclear Power Station, Unit 2.

This letter is a supplement based on the completed causal evaluation. The updated information is denoted by revision bars located in the left-hand margin.

This report is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A) for an event or condition that resulted in manual or automatic actuation of the reactor protection system including a reactor scram, and for containment isolation signals affecting more than one system.

There are no regulatory commitments contained in this letter.

Should you have any questions concerning this report, please contact Conner Bealer at 779-231-6207.

Respectfully, 4t-#:JJ Doug Hild Site Vice President Quad Cities Nuclear Power Station cc:

Regional Administrator - NRC Region Ill NRC Senior Resident Inspector - Quad Cities Nuclear Power Station

Abstract

On May 23, 2024, at 2223 CST, Quad Cities Unit 2 had an automatic scram from 38 percent power due to a trip of the main turbine. A Digital Electro-Hydraulic Control (DEHC) erroneous Speed Difference Trip signal initiated a Turbine Trip signal. The Unit 2 Main Turbine tripped as designed and generated an automatic reactor scram. All control rods inserted and the scram was uncomplicated. Containment isolation valves actuated closed in multiple systems on valid Group II and Group Ill signals as a result of low reactor water level.

The cause of the DEHC trip signal was intermittent channel <R1 > core power supply failure, resulting in an erroneous speed demand value being generated. The power supply and several DEHC cards.were replaced with new components.

This report is being submitted per 10 CFR 50. 73(a)(2)(iv)(A) for an event that resulted in manual or automatic actuation of the reactor protection system including a reactor scram, and containment isolation signals affecting more than one system.

PLANT AND SYSTEM IDENTIFICATION

2. DOCKET NUMBER YEAR 00265 2024

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

01 General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in text as [XX].

EVENT IDENTIFICATION Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure CONDITION PRIOR TO EVENT Unit: 2 Reactor Mode: 1 Event Date: May 23, 2024 Mode Name: Power Operation Event Time: 2223 CST Power Level: 38 percent No structures, systems, or components that were inoperable at the start of the event contributed to the event.

A. DESCRIPTION OF EVENT At 2223 CST on May 23, 2024, with Quad Cities Unit 2 at 38 percent power, the reactor automatically scrammed due to a turbine [TRB] trip signal resulting in turbine stop valve closure, creating a valid reactor protection system signal. Reactor vessel level reached the low-level set-point following the scram, resulting in valid Group II and Group Ill containment actuation signals. The trip was not complex with all systems responding as expected post-trip. This report is being submitted per 10 CFR 50. 73(a)(2)(iv)(A) for an event that resulted in manual or automatic actuation of the reactor protection system including a reactor scram, and containment isolation signals affecting more than one system.

Prior to the scram, the unit was reducing power for planned transformer maintenance. A turbine speed versus speed demand comparison generated a Speed Difference Trip and turbine trip signal due to an erroneous speed demand signal.

B. CAUSES OF EVENT The cause of the event is an intermittent power supply failure that was not recognized as a single point vulnerability due to the diversity of the system. The turbine trip signal was generated by Digital Electro-Hydraulic Control (DEHC) [TG] turbine speed and speed demand mismatch logic. An intermittent failure of the channel

<R1 > core power supply [RJX] resulted in an erroneous speed demand value being generated. DEHC initiated a turbine trip due to a 100% actual speed compared to a 0% speed demand, which is larger than the 10% setpoint.

While there was known degradation of this power supply, it was not recognized as a single point vulnerability to DEHC due to system redundancies.

C. SAFETY ANALYSIS SYSTEM DESIGN 050 052

2. DOCKET NUMBER YEAR 00265 2024

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

01 Sections 7.2 and 10.4 of the Updated Final Safety Analysis Report (UFSAR) describe how the EHC system feeds into a turbine trip and reactor scram. "The electrohydraulic control (EHC) system compares generator stator current to the high pressure turbine exhaust (crossaround) pressure and operates these valves upon a mismatch indicative of a turbine generator load rejection (see Section 10.4). These pressure switches on each fast-acting solenoid provide signals to both RPS trip systems. The logic is a one-out-two-twice arrangement so that operation of any solenoid causes a single system trip, and the operation of one or more solenoids in each trip system initiates a scram."

SAFETY ANALYSIS There were no safety consequences as a result of this event. The operators performed actions in accordance with procedures and training. An automatic scram occurred without complications due to a turbine trip caused by digital EHC channel <R 1 > core power supply intermittent failure. All expected Engineering Safety Feature (ESF) actuations occurred as designed to bring the reactor to a safe shutdown condition. The event was within the analysis of the UFSAR Chapter 15, UFSAR Chapter 6, and there were no radioactive releases.

Unit 1 was not affected by the Unit 2 scram.

This is not a Safety System Functional Failure per NEI 99-02, Revision 7.

D. CORRECTIVE ACTIONS Immediate:

1.

DEHC Components were replaced: Power Supply and various input/output modules for channel <R 1 >.

2.

Changed Speed Difference Trip setpoint from 10% to 110%.

Followup:

1.

Change Speed Difference Trip setpoint for Unit 1 DEHC equivalent to the change implemented on Unit 2.

2.

Increase frequency of preventative maintenance tasks for periodic replacement of <R1> power supplies.

E. PREVIOUS OCCURRENCES The station events database, LERs, and INPO Industry Reporting Information System (IRIS) were reviewed for similar events at Quad Cities Nuclear Power Station in the last three years. A similar turbine trip and automatic reactor scram occurred on August 11, 2023 where a fault in the Essential Service System (ESS) [UJX] caused a feedwater heater (FWH) [HX] system transient. The event in this current Licensee Event Report was caused by a fault on the digital EHC system while pursuing a repair on a turbine control valve.

No other relevant events were identified during the previous 3-year history.

F. COMPONENT FAILURE DATA System: Electrohydraulic Control (EHC)

Component: GE Mark VI Rack Power Supply Manufacturer: General Electric Nomenclature: Power Supply Model Number: IS2020RKPSG3A 050 052

2. DOCKET NUMBER

3. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO.

00265 2024 -

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