05000318/LER-2023-004-01, Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer

Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer

ML24107B099
Person / Time
Site:	Calvert Cliffs
Issue date:	04/12/2024
From:	Moodie P Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
LER 2023-004-01
Download: ML24107B099 (1)

LER-2023-004, Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer

Event date:
Report date:
3182023004R01 - NRC Website

text

Constellation© April 12, 2024 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 Subject:

Calvert Cliffs Nuclear Power Plant, Unit No. 2 Renewed Facility Operating License Nos. DPR-69 NRC Docket No. 50-318 Licensee Event Report 2023-004, Revision 01 Peter F. Moodie Plant Manager Calvert Cliffs Nuclear Power Plant 1650 Calvert Cliffs Parkway Lusby. MD 20657 410-495-5205 Office 603-548-7775 Mobile www.constellation.com peter.moodie@constellat1on.com 10 CFR 50.73 Automatic Reactor Trip from Reactor Protection System Actuation due to Loss of Unit Service Transformer The attached report is being sent to you as required by 10 CFR 50. 73.

There are no regulatory commitments contained in this correspondence.

Should you have questions regarding this report, please contact Mr. Larry D. Smith at (410) 495-5219.

Respectfully, Peter F. Moodie Plant Manager PFM/MJF/aj Attachment:

LER 318-2023-004, Rev 01 cc:

NRC Project Manager, Calvert Cliffs NRC Regional Administrator, Region I NRC Resident Inspector, Calvert Cliffs S. Seaman, DNR

Abstract

On November 16, 2023, the Unit 2 reactor automatically tripped due to a Reactor Protection System actuation based on an undervoltage condition. Specifically, the U-4000-22 Unit Service Transformer was lost due to a ground fault signal which resulted in the de-energization of the 22, 23, and 24 4KV buses. This resulted in a loss of both Control Element Drive Mechanism Motor Generator Sets, causing the reactor trip bus undervoltage condition. The Main Feedwater pumps tripped; therefore, Auxiliary Feedwater was manually initiated to supply both Steam Generators. The 2B Emergency Diesel Generator automatically started and restored its bus. Heat removal remained via the normal turbine bypass valves to the main condenser. The cause of the U-4000-22 Unit Service Transformer ground fault signal occurrence was a spurious signal that actuated the ground fault relay for the high side feeder breaker for U-4000-22 due to electromagnetic interference. All components associated with the suspected ground fault circuitry were replaced prior to placing the transformer back into service. Independent digital relays monitoring the affected transformer and associated Voltage Regulator (2H2103REG) at the time of the event and subsequent equipment testing showed no indication of an actual ground fault condition on the 13KV system at the time.

(See Page 2 for required number of digits/characters for each block)

(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) Page 4 of 4 for data monitoring and retrieval for electrical quantities for all U-4000 transformers protective relay schemes.

Additional corrective actions include:

Correcting the cable installation at the Unit 1 and Unit 2 site self-power breakers, 1BKR252-1106 and 2BKR252-2106 respectively, to reduce the likelihood of corona discharges from occurring.

Installation of new insulated ground sensor cables on the sites five remaining 4KV USTs.

G. PREVIOUS OCCURRENCES A review of Calvert Cliffs events was performed. A similar event occurred on Unit 2 on November 7, 2023, as reported in LER 318-2023-002 wherein the U-4000-22 UST was lost and Unit 2 scrammed due to a short to ground based on an open-phase detection current transformer (CT) secondary wire that inappropriately contacted an energized 13KV bus bar internal to the voltage regulator, 2H2103REG. It is suspected that the U-4000-22 UST ground sensor cables armor jacket was damaged, making it more vulnerable to effects of EMI, during the Ground Potential Rise (GPR) from the November 7, 2023 ground event and loss of U-4000-22.

H. COMPONENT FAILURE DATA Component IEEE 803 IEEE805 FUNCTION ID SYSTEM ID Voltage Regulator Ground Protection Circuit XCT EA