View source for 05000410/LER-2024-002-01, Supplement to NMP2 Licensee Event Report 2024-002-00, Automatic Reactor Scram on Turbine Trip Due to Failed Breaker

{{Adams
| number = ML25009A027
| issue date = 01/10/2025
| title = Supplement to NMP2 Licensee Event Report 2024-002-00, Automatic Reactor Scram on Turbine Trip Due to Failed Breaker
| author name = Crawford C
| author affiliation = Constellation Energy Generation, LLC
| addressee name = 
| addressee affiliation = NRC/NRR, NRC/Document Control Desk
| docket = 05000410
| license number = NPF-069
| contact person = 
| case reference number = NMP2L2896
| document report number = LER 2024-002-01
| document type = Letter, Licensee Event Report (LER)
| page count = 1
}}
{{LER
| Title = Supplement to NMP2 Licensee Event Report 2024-002-00, Automatic Reactor Scram on Turbine Trip Due to Failed Breaker
| Plant = 
| Reporting criterion = 10 CFR 50.73(a)(2)(iv)(A)
| Power level = 
| Mode = 
| Docket = 05000410
| LER year = 2024
| LER number = 2
| LER revision = 1
| Event date = 
| Report date = 
| ENS = 
| abstract = 
}}

=text=
{{#Wiki_filter:10 CFR 50.73 NMP2L2896 January 10, 2025 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Nine Mile Point Nuclear Station, Unit 2 Renewed Facility Operating License No. NPF-69 Docket No. 50-410 

==Subject:==
Supplement to NMP2 Licensee Event Report 2024-002, Automatic Reactor Scram on Turbine Trip due to Failed Breaker In accordance with the reporting requirements contained in 10 CFR 50.73(a)(2)(iv)(A), please find enclosed NMP2 Licensee Event Report (LER) 2024-002, Automatic Reactor Scram on Turbine Trip due to Failed Breaker.
There are no regulatory commitments contained in this letter.
Should you have any questions regarding the information in this submittal, please contact Matt Rossi, Site Regulatory Assurance Manager, at (315) 349-7012.
Respectfully, Carl J. Crawford Plant Manager, Nine Mile Point Nuclear Station CJC/MLR 

==Enclosure:==
NMP2 Licensee Event Report 2024-002, Automatic Reactor Scram on Turbine Trip due to Failed Breaker cc:
NRC Regional Administrator, Region I NRC Resident Inspector NRC Project Manager Crawford, Carl Digitally signed by Crawford, Carl Date: 2025.01.09 07:29:55 -05'00'

Enclosure NMP2 Licensee Event Report 2024-002, Automatic Reactor Scram on Turbine Trip due to Failed Breaker Nine Mile Point Nuclear Station, Unit 2 Renewed Facility Operating License No. NPF-69 

=Abstract=
Nine Mile Point Unit 2

410 4
Supplement to LER 2024-002-00, Automatic Reactor Scram on Turbine Trip due to failed breaker 09 23 2024 2024 002 01 01 09 2025 Mode 1, Power Operation 100%

Matt Rossi, Site Regulatory Assurance Manager (315)-349-7012 X
TL BKR G080 Yes X
TL BD GE Yes

On 9/23/2024 failure of the Main Generator Field Circuit Breaker (41M-2EXSX01) [EXC] lead to a Generator Field Loss of Excitation [TL]. This resulted in a Main Generator Lockout which caused a Turbine Trip [TA] and automatic anticipatory RPS actuation [JC] at 07:20. High resistance across Main Generator Field Circuit Breaker (41M-2EXSX01) caused a large voltage drop across the breaker main contacts. The Auto Voltage Regulator (AVR)
[90] control system responded to the lower excitation through the feedback and increased output to compensate. The increased output resulted in increased voltage drop, heat at the contacts, a subsequent fire, and Loss of Field Excitation. The resulting Turbine Trip and Turbine Stop and Control Valve closure caused an automatic anticipatory RPS actuation.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). Page of
: 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
052 050 The increased current further increased temperature which also increased resistance on the A (-) phase breaker contacts. The feedback loop between resistance increase, generator field voltage drop, AVR current control, and breaker contact temperature, continually increased temperature and degraded the breaker contacts. The AVR continued to respond until it hit a max current limit at which point it could not improve generator field voltage although field voltage continued to degrade. This resulted in the loss of the generator field. This tripped the 40-1-2SPGZ02 relay based on settings calculated in EC-167 which takes into account generator reactive capability. This in turn initiated the 86-1-2SPGZ01 relay which triggered the generator lockout sequence and resulted in the turbine trip and SCRAM. 

==IV. CORRECTIVE ACTIONS==
A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:
During the N2F47 forced outage the station replaced the 41M-2EXSX01 Main Generator Field Breaker and cubicle (panel) assembly (W/O C93993280).
B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:
Preventive Maintenance program changes for the breaker have been implemented with thermography performed every two weeks and monitoring of generator field voltage and current.
The Corrective Action to Prevent Recurrence is to modify the generator field circuity to remove the generator field breaker.
This will remove the component and cubicle and prevent a similar failure mechanism. 

==V. ADDITIONAL INFORMATION==
A. FAILED COMPONENTS:
Main Generator Field Circuit Breaker (41M-2EXSX01)
B. PREVIOUS LERs ON SIMILAR EVENTS:
None 4
4 Nine Mile Point 410 2024 002 00
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