05000265/LER-2024-002, 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

ML24204A105
Person / Time
Site:	Quad Cities
Issue date:	07/22/2024
From:	Hild D Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
SVP-24-046 LER 2024-002-00
Download: ML24204A105 (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
2652024002R00 - NRC Website

text

Constellation.,

SVP-24-046 10 CFR 50.73

July 22, 2024

U.S. Nuclear Regulatory Commission ATTN: 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

Subject: Licensee Event Report 265/2024-002-00 "Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure"

Enclosed is Licensee Event Report 265/2024-002-00 "Turbine Trip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure," for Quad Cities Nuclear Power Station, Unit 2.

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.

Doug Hild Site Vice President Quad Cities Nuclear Power Station

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1. Fac ility Nam e ~ 050 2. Docket Number 3. Page Quad C ities Nuclear Power Station, Unit 2 052 00265 1 OF 4

4. Title Turbine T rip and Automatic Scram due to Digital EHC Power Supply Intermittent Failure

5. Eve nt Date 6. LER Number 7. Report Date 8. Other Facilities Involved Mo nth Day Year Year Number No. Month Day Year 050 Sequential Revision Facility Name Do cket Number

05 23 2024 2024 002 00 07 22 2024 Facility - - 052 Name Dock et Number

9. Operating Mode 110. Power Level 1 - Power Operation 038

No rgJ Yes (If yes, complete 15. Expected Subm ission Date) 11 14 2024 16. Abstract (Limit to 1326 spaces, i.e., approximately 13 single-spaced typewritten lines)

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

General E lectric - 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 Event Date : May 23, 2024 Event Time : 2223 CST

Reactor Mode: 1 Mode Name : Power Operation 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 w ith 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 Co n trol (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 tu rb ine trip due to a 100% actual speed compared to a 0 % speed demand, which is larger than the 10% setpoint.

Wh ile 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

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 soleno ids 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 inpuUoutput modules for channel <R1 >.

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

Followup:

1. Finalize causal analysis.

E. PREVIOUS OCCURRENCES

The station ev e nts database, LERs, and INPO Industry Reporting Information System (IRIS) were reviewed for similar events a t Quad Cities Nuclear Power Station in the last three years. A similar turbine trip and automat ic reactor scram occu rred 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 th is current Licensee Event Report was caused by a fault on the dig ital EHC system while pursu ing a repair on a tu rbine co ntro l valve.

No other relevant events were identified during the previous 3-yea r 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