05000259/LER-2024-002, Reactor Scram Due to Generator Step-Up Transformer Failure

Reactor Scram Due to Generator Step-Up Transformer Failure

ML24176A102
Person / Time
Site:	Browns Ferry
Issue date:	06/24/2024
From:	Sivaraman M Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
LER 2024-002-00
Download: ML24176A102 (1)

LER-2024-002, Reactor Scram Due to Generator Step-Up Transformer Failure

Event date:
Report date:
2592024002R00 - NRC Website

text

Post Office Box 2000, Decatur, Alabama 35609-2000

June 24, 2024 10 CFR 50.73

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001

Browns Ferry Nuclear Plant, Unit 1 Renewed Facility Operating License No. DPR-33 NRC Docket No. 50-2 59

Subject: Licensee Event Report 50-259/202 4-002 -0 0 - Reactor Scram due to Generator Step-Up Transformer Failure

The enclosed Licensee Event Report provides details of the Reactor S cram due to Generator Step-Up Transformer Failure on Browns Ferry Nuclear Plant Unit 1. The Tennessee Valley Authority is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(i v)(A), as an automatic actuation of the Reactor Protection System,

Primary Containment Isolation System, the High-Pressure Coolant Injection System, and the Reactor Core Isolation Cooling System.

There are no new regulatory commitments contained in this letter. Shoul d you have any questions concerning this submittal, please contact David J. Renn, Site Licensing Manager, at (256) 729-2636.

Respectfully,

Manu Sivaraman BFN Site Vice President

Enclosure: Licensee Event Report 50-2 59/2024 -002 Reactor Scram due to Generator Step-Up Transformer Failure

Cc (w/ Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant

ENCLOSURE

Browns Ferry Nuclear Plant Unit 1

Licensee Event Report 50-259/2024-002-00

Reactor Scram due to Generator Step-Up Transformer Failure

See Enclosed

Abstract

On April 24, 2024, at 2215 Central Daylight Savings Time, while Unit 1 was at 100 percent rated thermal power, Browns Ferry Nuclear Plant Unit 1 e xperienced an automatic reactor scram due to a fault within the 1B Main Transformer. All plant equipment responded as expected, and Unit 1 was transitioned to Mode 4.

The cause of the transformer failure is currently under investigation. The root cause of the transformer failure will not be known until a forensic tear down is complete.

This event i s being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an automatic actuation of the Reactor Protection System, Primary Containment Isolation System, the High-Pressure Coolant Injection Syst em, and the Reactor Core Isolation Cool ing System.

NRCF ORM 366AU.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per respons e to complywith this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Repo rted lessons learned are incorporated intothe licensing process and fed back to industry. Send comments regarding bur den estima te to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

LICENS EE EVENT REPORT (LER) Nuclear Regulatory Commission, Was hington, DC 20555-0001, or by e -mail to Infocollects.Resource@ nrc.gov, and the OMB reviewer at: OM B Office of Informat ion and Regulatory CONTINUATION SHEET Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Wa shington, DC 20503; e-mail: o ira_s ubmission@omb.eop.gov. The NRC may not conduct or (See NUREG-1022, R.3 for instr uction and guidance f or completing this form sponsor, and a person is not required to respond to, a c ollection of inf ormation unless the document http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NA ME CKET NUMR 3. LEER 050 2 SEQUENTIAL REV YEAR NUMBER NO.

Bro wns Ferry Nuclear Plant, Unit 1 00259 052 2024 - 002 - 00

NARRATIV E I. Plant Operating Conditi ons before the Eve nt

At the time of discovery of this event on April 24, 2024, Browns Ferry Nuclear Plant ( BFN) Unit 1 was in Mode 1 at approximately 100 percent Rated Thermal Power (RTP).

II. Descr iption of Event

A. Event Summary

On April 24, 2024, at 2215 Central Daylight Savings Time (CDT), while Unit 1 was at 100 percen t RTP, Browns Ferry Nuclear Plant (BFN) Unit 1 experienced an automatic reactor scram f rom a turbine control valve (TCV) [XCV] fast closure signal due to a fault within the 1B Main Transformer

[XFMR]. All plant equipment responded as expe cted, and Browns Ferry Unit 1 was trans itioned to Mode 4.

Primary Containment Isolation Systems (PCIS) [JM] Groups 2, 3, 6, and 8 isolation signal reactor water level (RWL) L evel 3 (+2) was received. Upon receipt of th is signal, all components actuated as required. Followi ng the reactor scram, due to reactor water level reaching Level 2

(-45), reactor recirculation pumps tripped as expected and both High Pressure Coolant Injection (HPCI)[BJ] and Reactor Core Isolation Cooling (RCIC) [BN] initiation signals were received, and both systems initiated as designed. All safety systems operated as expected. At no time was public health and safety at risk.

The Tennessee Valley Authority (T VA ) is submitting this report in acco rdance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(i v)(A), as an automatic actuation of the Reactor Protection System (RPS) [JC], the Primary Containment Isolation System (PCIS) [JM],

the High-Pressure Coolant Injection (HPCI) [BJ] System, and the Reactor Core Isolation Cooling (RCIC) [BN] System.

B. Status of stru ctures, component s, or systems that were inoperable at the start of the event and that contributed to the event

There were no structure s, systems, or components (SSCs) whose inop erability contributed to this event.

C. Dates and approximate times of oc curre nces

DATE AND APPROXIMATE TIMES OCCURRENCE (time s are Central Time)

April 24, 2024, at Browns Ferry Unit 1 experienced an automatic reactor 2215 CDT scram due to a faul t within the 1B Main Transformer.

April 25, 2024, at U1 Event Notification (EN 57090) was made to the Nuclear 0122 CDT Regulatory Commission (NRC).

May 5, 2024 The Unit 1 & 2 Spare Main Bank Transformer was tested and placed into service.

D. Manufa cturer and model numb er of each compon ent th at failed during the event

The 1B Main Bank Transf ormer (500-22 KV) was made by ABB, part number XV12 089004-B (Serial No. 12089-001).

E. Other systems or secondary functions affected

No other systems or sec ondary functions were affected.

F. Method of discovery of each c omponent or system failure or p rocedural error

On April 24, 2024, at 2215 CDT, the Browns Ferry 1B Main Bank Transformer experienced an internal fault. Unit1, operating at 100% power at the time of the transformer failure, received an automatic reactor scram following transformer protective relay actuation. All plant equipment responded as expected and Browns Ferry Unit 1 was transitioned to Mode 4.

Initial engineering walkdowns revealed that the transformer tank itself did not exper ience any structural damage and there w as no collateral damage to adjacent structures.

G. The failure mode, mechanism, and effect of each failed component

The primary mode of failure cannot be identified until the transformer is removed and an in-depth inspection of the internals is performed. Initial internal inspection reveals that the fault was most likely to have originated in the left winding assembly and core limb. NRCF ORM 366AU.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per respons e to complywith this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Repo rted lessons learned are incorporated intothe licensing process and fed back to industry. Send comments regarding bur den estima te to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

LICENS EE EVENT REPORT (LER) Nuclear Regulatory Commission, Was hington, DC 20555-0001, or by e -mail to Infocollects.Resource@ nrc.gov, and the OMB reviewer at: OM B Office of Informat ion and Regulatory CONTINUATION SHEET Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Wa shington, DC 20503; e-mail: o ira_s ubmission@omb.eop.gov. The NRC may not conduct or (See NUREG-1022, R.3 for instr uction and guidance f or completing this form sponsor, and a person is not required to respond to, a c ollection of inf ormation unless the document http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NA ME CKET NUMR 3. LEER 050 2 SEQUENTIAL REV YEAR NUMBER NO.

Bro wns Ferry Nuclear Plant, Unit 1 00259 052 2024 - 002 - 00

NARRATIV E

H. Operator actions

Operations personnel stabilized the plant following the reac tor and turbine trip and subsequently initiated a plant cooldown to Mode 4.

I. Automatically and manually in itiated safety system r esponses

PC IS Groups 2, 3, 6, and 8 isolation signals wer e received. Upon receipt of these signals, all components actuated as re quired. Following the reactor scram, both HPCI and RCIC initiation signals were recei ved, and both i nitiated as designed. All safety systems operated as expected.

III. Cause of the event

A. Cause of each compone nt or system failure or personnel error

The primary mode of failure cannot be identifi ed until the transformer is removed, and an in-d epth inspection of the internals is performed. Initial internal inspection reveals that the fault was most likely to have originated in the left winding assembly and core limb.

B. Cause(s) and ci rcumstances for each hum an performa nce related roo t cause

There were no human performanc e related root causes.

I V. Analysis of the event

On April 24, 2024, at 2215 CDT, the Browns Ferry 1B Main Bank Transformer experienced an internal fault, resulting in a loss of the transformer. Unit 1, operating at 100% power at th e time of the transformer failure, received an automatic reactor s cram following plant protective relay actuation. All plant equipment responded as expected and Browns Ferry Unit 1 was transitioned to Mode 4. An NRC Event Notification (EN 57090) was m ade on April 25, 2024, at 0122 C DT.

Initial engineer ing walkdowns revealed that the transformer tank itself did not experienc e any structural damage and there was no collateral damage to adjacent structures.

The TVA System Protection and Analysis group performed an event analysis on the 1B Main Bank Transformer trip that shows that the trip came from the transformer differential (187T) relay via 186TX auxiliary relay at 22:15:48 CDT. The 187TF GSU #1 feeder differential rel ay shots indicate that the fault was external to the feeder differential zone. NRCF ORM 366AU.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per respons e to complywith this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Repo rted lessons learned are incorporated intothe licensing process and fed back to industry. Send comments regarding bur den estima te to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

LICENS EE EVENT REPORT (LER) Nuclear Regulatory Commission, Was hington, DC 20555-0001, or by e -mail to Infocollects.Resource@ nrc.gov, and the OMB reviewer at: OM B Office of Informat ion and Regulatory CONTINUATION SHEET Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Wa shington, DC 20503; e-mail: o ira_s ubmission@omb.eop.gov. The NRC may not conduct or (See NUREG-1022, R.3 for instr uction and guidance f or completing this form sponsor, and a person is not required to respond to, a c ollection of inf ormation unless the document http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NA ME CKET NUMR 3. LEER 050 2 SEQUENTIAL REV YEAR NUMBER NO.

Bro wns Ferry Nuclear Plant, Unit 1 00259 052 2024 - 002 - 00

NARRATIV E Review of the pre-event system monitoring information, which includes temperature and oil system parameters, oil samples, and Serveron data, did not indicate any degrading trends prior to failure. Internal inspe ctions of the failed 1B transformer identified damage, w hich precluded near term recovery of the trans former.

Unit 1 and Unit 2 share a spare main bank transformer that physically resides between the two units. The 1/2 Spare Main Bank Transformer was tes ted and placed into service on May 5, 2024, and received increased monitoring for the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of energization.

V. Assessment of Saf ety Consequences

The plant responded as designed, while maintaining defense -in -depth for nucl ear safety. All Nuclear safety systems functions as designed. This event was of very low nuclear safety significance. At no time was the heal th and safety of the public at risk.

A. Availability of systems or components that could have performed the same function as the compo nents and systems that fai led during the event

Generator step-up (GSU) transformers have no alternate line-up or redundant components available while the transformer s are in service. Al l reactor safety mitigating systems performed as expected.

B. For events that occurred when the reactor was shut d own, availabil ity of systems or components needed to shutdown the reactor and mainta in safe shutdown conditions, remove residua l heat, control the release of radioact ive materi al, or mitigate the consequ ences of an accident

This event did not occur when the reactor was shut down.

C. For failure that rendered a tr ain of a safety system inop erable, es timate of the elapsed time from discovery of the failure until the train was returned to servi ce

There were no s afety systems rendered inoperable.

VI. Corre cti ve Actions

Corre ctive Actions are being managed by the TVA corrective action program under condition reports (CRs ) 1926807 and 1926812. NRCF ORM 366AU.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027 (04- 02-2024) Estimated burden per respons e to complywith this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Repo rted lessons learned are incorporated intothe licensing process and fed back to industry. Send comments regarding bur den estima te to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

LICENS EE EVENT REPORT (LER) Nuclear Regulatory Commission, Was hington, DC 20555-0001, or by e -mail to Infocollects.Resource@ nrc.gov, and the OMB reviewer at: OM B Office of Informat ion and Regulatory CONTINUATION SHEET Affairs, (3150- 0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Wa shington, DC 20503; e-mail: o ira_s ubmission@omb.eop.gov. The NRC may not conduct or (See NUREG-1022, R.3 for instr uction and guidance f or completing this form sponsor, and a person is not required to respond to, a c ollection of inf ormation unless the document http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) requesting or requiring the collection displays a currently valid OMB control number.

1. FACILITY NA ME CKET NUMR 3. LEER 050 2 SEQUENTIAL REV YEAR NUMBER NO.

Bro wns Ferry Nuclear Plant, Unit 1 00259 052 2024 - 002 - 00

NARRATIV E A. Immediate Corre ctive Actions

• Engineering devel oped support/refute matrix to identify the cause of the transformer failure. The cause was unable to be determined but was narrowed down to an internal winding fault or core fault.

• Hitachi Energy performed an initial internal inspection on the 1B transfor mer.

• 1A, 1B, and 1C transformer oil samples were sent off for evaluation.

• Unit 1 Generator was inspected to ensure no damage oc curred during the fault.

• Unit 1/2 spare GSU transformer testing was performed to ensure health before placing into service.

• Additional monitoring was placed on the Unit 1/2 spare GSU transformer for the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of energization.

B. Correct ive Acti ons to Prevent Recurrence or to reduc e the probability of similar event s occurring in the future

The failed transform er has been isolated and will be forensically disassembled preventing it from any future service. Any significant findings from the forensic disassembly that result in substantial changes to the corrective action plan will be reported in a revised LER.

VII. Previous Sim ilar Events at the Same Site

A search of LERs from BFN, Units 1, 2, and 3 over the last five years identified no similar events.

VIII. Addition al Informa tion

There is no additional information.

IX. Commitments

There are no new commitments.