ML24107B099

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Automatic Reactor Trip from Reactor Protection System Actuation Due to Loss of Unit Service Transformer
ML24107B099
Person / Time
Site: Calvert Cliffs Constellation icon.png
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)
v • d • e


Text

Peter F. Moodie Plant Manager

Calvert Cliffs Nuclear Power Plant 1650 Calvert Cliffs Parkway Constellation © Lusby. MD 20657

410 -495 -5205 Office 603-548-7775 Mobile www.constellation.com

peter.moodie @constellat1on.com

10 CFR 50.73 April 12, 2024

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

Calvert Cliffs Nuclear Power Plant, Unit No. 2 Renewed Facility Operating License Nos. DPR-69 NRC Docket No. 50-318

Subject:

Licensee Event Report 2023-004, Revision 01 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 Resident Inspector, Calvert Cliffs NRC Regional Administrator, Region I S. Seaman, DNR NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150 -0104 EXPIRES: 0 4/30/2027 (04- 02-2024) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden LICENSEE EVENT REPORT (LER) estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S. Nuclear Regulatory (See Page 2 for required number of digits/characters for each block) Commission, Washington, DC 20555-0001, or by email to Infocollects.Resource@nrc.gov, and the OMB reviewer at: OMB Office of Information and Regulatory Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory (See NUREG-1022, R.3 for instruction and guidance for completing this form Commission, 725 17th Street NW, Washington, DC 20503. The NRC may not conduct or sponsor, and a person is http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) not required to respond to, a collection of information unless the document requesting or requiring the collection displays a currently valid OMB control number.

1. Facility Name 050 2. Docket Number 3. Page Calvert Cliffs Nuclear Power Plant, Unit 2 052 05000318 1 OF 4
4. Title Automatic Reactor Trip from Reactor Protection System Actuation due to Loss of Unit Service Transformer
5. Event Date 6. LER Number 7. Report Date 8. Other Facilities Involved

Sequential Revision Facility Name Docket Number Month Day Year Year Number No. Month Day Year 050

Facility Name Docket Number 11 16 2023 2023 - 004 - 01 04 12 2024 052

9. Operating Mode 10. Power Level
11. This Report is Submitted Pursuant to the Requirements of 10 CFR §: (Check all that apply) 10 CFR Part 20 20.2203(a)(2)(vi) 10 CFR Part 50 50.73(a)(2)(ii)(A) 50.73(a)(2)(viii)(A) 73. 1200(a)

20.2201(b) 20.2203(a)(3)(i) 50.36(c)(1)(i)(A) 50.73(a)(2)(ii)(B) 50.73(a)(2)(viii)(B) 73.1200(b)

20.2201(d) 20.2203(a)(3)(ii) 50.36(c)(1)(ii)(A) 50.73(a)(2)(iii) 50.73(a)(2)(ix)(A) 7 3.1200(c)

20.2203(a)(1) 20.2203(a)(4) 50.36(c)(2) 50.73(a)(2)(iv)(A) 50.73(a)(2)(x) 73.1200(d)

20.2203(a)(2)(i) 10 CFR Part 21 50.46(a)(3)(ii) 50.73(a)(2)(v)(A) 10 CFR Part 73 73.1200(e)

20.2203(a)(2)(ii) 21.2(c) 50.69(g) 50.73(a)(2)(v)(B) 73.77(a)(1) 73.1200(f)

20.2203(a)(2)(iii) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(C) 73.77(a)(2)(i) 73.1200(g)

20.2203(a)(2)(iv) 50.73(a)(2)(i)(B) 50.73(a)(2)(v)(D) 73.77(a)(2)(ii) 73.1200(h)

20.2203(a)(2)(v) 50.73(a)(2)(i)(C) 50.73(a)(2)(vii)

OTHER (Specify here, in abstract, or NRC 366A).

12. Licensee Contact for this LER Licensee Contact Phone Number (Include area code)

Michael J. Fick, Principal Regulatory Engineer 410- 495-6714

13. Complete One Line for each Component Failure Described in this Report

Cause System Component Manufacturer Reportable to IRIS Cause System Component Manufacturer Reportable to IRIS

B EA XCT X999 Y

14. Supplemental Report Expected Month Day Year
15. Expected Submission Date No Yes ( If yes, complete 15. Expected Submission Date)
16. Abstract (Limit to 1326 spaces, i.e., approximately 13 single-spaced typewritten lines)

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.

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments LICENSEE EVENT REPORT (LER) regarding burden estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by email to CONTINUATION SHEET Infocollects.Resource@nrc.gov, and the OMB reviewer at: OMB Office of Information and Regulatory Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, (See NUREG-1022, R.3 for instruction and guidance for completing this form Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) respond to, a collection of information unless the document requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NAME 2. DOCKET NUMBER 3. LER NUMBER 050 YEAR SEQUENTIAL REV NUMBER NO.

Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 052 2023 - 004 - 01

NARRATIVE PLANT AND SYSTEM IDENTIFICATION

Calvert Cliffs Nuclear Power Plant, Unit 2, is a Combustion Engineering Pressurized Water Reactor with a licensed maximum power level of 2737 megawatts thermal. The Energy Industry Identification System code used in the text is identified as [EA].

A. CONDITION PRIOR TO EVENT

Unit: 2 Date: November 16, 2023 Power level: 100 Mode: Unit 2 was in Mode 1 when the condition was discovered.

B. DESCRIPTION OF EVENT At 0227 on 11/16/2023, the Unit 2 reactor automatically tripped from a Reactor Protection System (RPS) actuation based on a reactor trip bus undervoltage (UV) condition. At that time, a loss of the U-4000- 22 Unit Service Transformer (UST) caused a loss of 22, 23, and 24 4K V buses. This resulted in a loss of both Control Element Drive Mechanism (CEDM) Motor Generator (MG) Sets, causing the reactor trip bus undervoltage condition. The loss of 22 and 23 4K V non-safety related (NSR) buses resulted in a trip of Main Feedwater.

Auxiliary Feedwater was manually initiated to feed both Steam Generators. The 2B Emergency Diesel Generator (EDG) automatically started and restored the 24 4KV safety related (SR) bus. Heat removal remained via the normal turbine bypass valves to the main condenser.

C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES November 16, 2023

  • 0227: Unit 2 reactor automatically tripped (scrammed) on reactor trip bus undervoltage due to a loss of U-4000- 22 UST. The loss of U-4000-22 UST was caused by its feeder breaker tripping open. The loss of U-4000- 22 UST resulted in the following effects:

o Loss of 24 4K V bus automatically started the 2B EDG which loaded onto the 24 4K V SR bus and repowered the bus as designed based on a valid UV signal from ESFAS.

  • 0330: Power was restored to the 22 and 23 4K V buses via their alternate feeder breakers.
  • 0341: 24 4KV bus was powered from its alternate feeder breaker and the 2B EDG was separated from the bus and secured in accordance with normal operating procedures.

NRC FORM 366A (04- 02-202 4) Page 2 of 4 NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments LICENSEE EVENT REPORT (LER) regarding burden estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by email to CONTINUATION SHEET Infocollects.Resource@nrc.gov, and the OMB reviewer at: OMB Office of Information and Regulatory Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, (See NUREG-1022, R.3 for instruction and guidance for completing this form Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) respond to, a collection of information unless the document requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NAME 2. DOCKET NUMBER 3. LER NUMBER 050 YEAR SEQUENTIAL REV NUMBER NO.

Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 052 2023 - 004 - 01

NARRATIVE D. CAUSE OF EVENT

The method of discovery for this event was self -revealing and is documented in the sites Corrective Action Program (CAP) under IR 04718086. The Unit 2 reactor automatically tripped (scrammed) on Reactor Trip Bus Undervoltage due to a loss of the U-4000-22 UST. The root cause evaluation for the loss of the U-4000- 22 UST due to a ground fault signal identified the root cause to be a spurious ground fault signal that actuated the ground fault relay (250G) for 2BKR252-2103 (High Side Feeder for U-4000-22) due to electromagnetic interference (EMI). 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. The presence of high frequency electrical noise from corona discharge in the rear of an adjacent circuit breaker cubicle, 2BKR252-2106, facilitated sustained actuation of the 250G device which has known susceptibility to false operation when subjected to high frequency electrical noise. Contributing causes were identified to be:

  • Component selection of the ground sensor circuit during a legacy modification installation. This includes the ground sensor, metal armored cable inside isolation and bypass switch enclosure, cablings from the switch enclosure to 252-2103 cubicle, the 250G and 262G devices.
  • Cable installation deficiencies during legacy site self-power modifications (related to circuit breaker 2BKR252-2106). The installation of the cable was inadequate in that the cable shield was not carried all the way through the ground CT up to the cable termination and allowed for the occurrence of coronal discharges.

The ground fault signal was detected by the U-4000- 22 USTs protective relaying, tripping the high-side feeder breaker to the Voltage Regulator and the U-4000-22 UST, along with its associated low-side 4K V breakers.

E. SAFETY ANALYSIS

The subject event satisfies the criteria in NUREG-1022, Revision 3, for any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10CFR50.73, paragraph (a)(2)(iv)(B).

Specifically, for this event, the Reactor Protection System, the 2B Emergency Diesel Generator, and the Auxiliary Feedwater System. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A). There were no safety consequences as a result of the event. All safety systems functioned and operated as designed.

F. CORRECTIVE ACTIONS

The ground fault protection circuit was installed with the U-4000-22 Voltage Regulator (2H2103REG) in the mid-1990s. As part of the troubleshooting activities in support of the automatic trip due to the loss of U -4000- 22, the site replaced all components in the suspected ground fault protection circuit, including the ground sensor, ground sensor relay, ground relay timer, lockout relay, and accessible wiring from the ground sensor to the ground sensor relay. The corrective action to preclude repetition i s to design and implement a modification to replace the currently installed 250G relays (GKC style) and associated ground sensor circuit components with microprocessor-based relays and necessary circuit components (e.g., ground sensor, interfacing cabling appropriate for circuit) providing harmonic filtering capabilities, robust resistance to EMI capacity, and the ability

NRC FORM 366A (04- 02-202 4) Page 3 of 4 NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments LICENSEE EVENT REPORT (LER) regarding burden estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

Nuclear Regulatory Commission, Washington, DC 20555- 0001, or by email to CONTINUATION SHEET Infocollects.Resource@nrc.gov, and the OMB reviewer at: OMB Office of Information and Regulatory Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, (See NUREG-1022, R.3 for instruction and guidance for completing this form Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) respond to, a collection of information unless the document requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NAME 2. DOCKET NUMBER 3. LER NUMBER 050 YEAR SEQUENTIAL REV NUMBER NO.

Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 052 2023 - 004 - 01

NARRATIVE 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 sc rammed 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

NRC FORM 366A (04- 02-202 4) Page 4 of 4

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