05000374/LER-2022-003-01, Manual Scram Due to Lsophase Bus Duct Fire Followed by 2A RPS Normal Power Supply Trip

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Manual Scram Due to Lsophase Bus Duct Fire Followed by 2A RPS Normal Power Supply Trip
ML23040A104
Person / Time
Site: LaSalle Constellation icon.png
Issue date: 02/09/2023
From: Van-Fleet J
Constellation Energy Generation
To:
Office of Nuclear Reactor Regulation, Document Control Desk
References
RA22-055 LER 22-003-01
Download: ML23040A104 (1)
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LER-2022-003, Manual Scram Due to Lsophase Bus Duct Fire Followed by 2A RPS Normal Power Supply Trip
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
10 CFR 50.73(a)(2)
10 CFR 50.73(a)(2)(x)
10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor
10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat
10 CFR 50.73(a)(2)(1)
10 CFR 50.73(a)(2)(v), Loss of Safety Function
10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
10 CFR 50.73(a)(2)(viii)(A)
10 CFR 50.73(a)(2)(viii)(B)
10 CFR 50.73(a)(2)(iii)
10 CFR 50.73(a)(2)(ix)(A)
3742022003R01 - NRC Website
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text

Constellation~

RA22-055 February 9, 2022 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 LaSalle County Station, Unit 2 Renewed Facility Operating License No. NPF-18 NRC Docket No 50-374 LaSalle County Station 2601 North 21 Road Marseilles. IL 61341 815*415-2000 Telephone 10 CFR 50.73

Subject:

Licensee Event Report 2022-003-01, Supplement to Manual Scram due to lsophase Bus Duct Fire and 2A RPS Normal Power Supply Trip.

In accordance with 10 CFR 50.73(a)(2)(iv)(A), Constellation Energy Generation, LLEC (CEG) is submitting Licensee Event Report (LER) Number 2022-003-01 for LaSalle County Station, Unit 2.

There are no regulatory commitments in this letter. Should you have any questions concerning this report, please contact Mr. Dan Mearhoff, Regulatory Assurance Manager, at (815) 415-2800.

Respectfully, frYll1~

John Van Fleet Jr Plant Manager LaSalle County Station

Enclosure:

Licensee Event Report cc:

Regional Administrator - NRC Region Ill NRC Senior Resident Inspector - LaSalle County Station

NRC FORM366 U.S. NUCLEAR REGULA TORY COMMISSION APPROVED BY 0MB: NO. 3160-0104 EXPIRES: 08/31/2023 (08-2020)

Estima!ed burden par response to co~ly wilt, this mandatory collection request 80 hDUB. Repof1ed

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LICENSEE EVENT REPORT (LER) lessons leilnOd are in:aponltad inl> the licensing p,coess and fad bacl< lo induslJy. Send commena regarding burden estimale I> the FOIA. libnuy, and lnbmalon Colloctions Br.wich (T-o A10M), U.S.

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(See Page 3 for required number of digits/characters for each block)

Nuclear Regulaay Comrrission, Washing1on, DC 20555--0001, 0(

by e-mai I>

(See NUREG-1022, R.3 for instruction and guidance for completing this form lnfocollects.Resourte@ruc.gov, and the OMS nMOWer at 0MB Office of ~formation and Regulaby http ://www.nrc.gov/reading-rm/doc-collections/nuregs/staff /sr1022/r3D Affairs, (3150-0104), Alln: Desk ait on subnjssion@omb.eop.goy. The NRC may not conduct <<

sponsor, and a person is 001 required ID respond lo, a colectilxl ol infonnalion unless f1a documen1 requesting<< requiing Ole IXlllecion d"isplays a currenlly vald 0MB control number.

1. Faclllty Name
2. Docket Number 3.Page LaSalle County Station, Unit 2 05000 -374 1 OF 3
4. Tltle Manual Scram due to lsophase Bus Duct Fire followed by 2A RPS Normal Power Supply Trip
6. Event Date
6. LER Number
7. Report Date
8. Other Facllltles Involved Sequential Revision Facility Name Docket Number Month Day Year Year Number No.

Month Day Year NA NA 09 26 2022 2022 003 01 02 03 2023 Facility Name Docket Number NA NA

9. Operating Mode 1

110. Power Level 100 percent

11. This Report Is Submitted Pursuant to the Requirements of 10 CFR §: (Check all that apply)
ff~ R ""it'IO

,'* D 20.2203(a)(2)(vi)

D 50.36(c)(2)

[8J 50.73(a)(2)(1v)(A)

D 50.73(a)(2)(x)

  • 20.2201(b)

D 20.2203(a)(3)(i)

D 50.46(a)(3)(11)

D 50.73(a)(2)(v)(A)

/ ' 10,CFR Part 73
  • 20.2201(d)

D 20.2203(a)(3)(ii)

  • 50.69(g)

D 50.73(a)(2)(v)(B)

D 73.71(a)(4)

D 20.2203(a)(1)

  • 20.2203(8)(4)
  • 50.73(a)(2)(1)(A)

D 50.73(a)(2)(v)(C)

  • 73.71(a)(S)

D 20.2203(a)(2)(i) 1,.: 10 ~'::~'"'i't 21

  • 50.73(a)(2)(1)(B)

D 50.73(a)(2)(v)(D)

D 73.77(a)(1)(i)

D 20.2203(a)(2)(ii)

D 21.2(c)

  • 50.73(a)(2){i)(C)

D 50.73(a)(2)(vii)

D 73.77(a)(2){i)

D 20.2203(a)(2)(iii) 10 C~~ Part IO

  • 50.73(a)(2)(ii)(A)

D 50.73(a)(2)(viii)(A)

D 73.77(a)(2)(ii)

D 20.2203(a)(2)(iv)

  • 50.36(c)(1)(i)(A)
  • 50.73(a)(2)(il)(B)

D 50.73(a)(2)(viii)(B)

D 20.2203(a)(2)(v)

  • 50.36(c)(1)(ii)(A)

D 50.73(a)(2)(iii)

D 50.73(a)(2)(ix)(A)

  • OTHER (Specify here, in abstract, or NRC 366A).
12. Licensee Contact for this LER Licensee Contact

'Phone Number (Include area code)

CJ Smith, Operations Director (815) 415-2200 Cause System Component Manufacturer Reportable to IRIS t

Cause

System Component Manufacturer Reportable to IRIS X

EF RHE 0026 y

~

D MP GBU X100 y

14. Supplemental Report Expected Month Day Year 0 No I ~ Yes (If yes, complete 15. Expected Submission Date)
16. Expected Submlaalon Date 04 03 2023
6. Abstract (Limit to 1560 spaces, i.e.. approximately 15 single-spaced typewritten lines)

At 0238 CDT on September 26, 2022 with Unit 2 in Mode 1 at 100 percent power, a manual scram was initiated due to a reported fire on the isophase bus duct. The scram was uncomplicated with all systems responding normally with the exception of a loss of 2A reactor protection system (RPS) normal power supply. The 2A RPS normal power supply output breaker tripped on over voltage. Operations successfully transferred 2A RPS power to the alternate power supply. The fire was reported extinguished at 0240 CDT on September 26, 2022.

The cause of the 2A RPS loss of normal power was a degraded voltage adjust potentiometer (POT) on the output breaker. The most likely cause of the isophase bus duct fire is written guidance that governs the work did not provide the detailed instructions needed to perform disassembly and reassembly of the Unit 2 lsophase Bus Duct (IPBD) bolted connections with high precision.

Plant and System Identification

LaSalle County Station Unit 2 is a General Electric Boiling Water Reactor with 3546 Megawatts Thermal Rated Core Power.

The affected systems were the Main Power system (MP) and the Reactor Protection System (RPS).

01 The MP system provides safe and reliable distribution of electrical power from the Main Generator to the off-site transmission network.

RPS initiates a rapid insertion of all the control rods (scram) when monitored variables exceed their pre-established limits, neutron flux instrumentation becomes inoperable, or a manual scram signal is inserted by the operator. In addition, the setpoints, power sources, and controls and instrumentation are arranged in such a manner as to preclude spurious scrams.

Condition Prior to Event

Unit(s): 2 Reactor Mode(s): 1

Description

Fire and manual scram Date:

Mode(s) Name:

September 26, 2022 Power Operation Time:

0238 CDT Power Level: 100%

Electrical Maintenance personnel identified smoke coming from the general area in the Unit 2 Turbine Building that contains the Unit 2 isolated phase bus duct and notified the Main Control Room (MCR). The Field Supervisor was dispatched and at 0230 CDT they notified the MCR that there was visible sparks, smoke and indications of a fire from the bus duct enclosure.

Operations dispatched the fire brigade and entered abnormal operations procedures for a fire. The unit was manually scrammed at 0238 CDT. The brigade was able to successfully gain control of the fire using dry chem and the fire was reported extinguished at 0240 CDT.

2A RPS Breaker trip Following the Manual Scram, the 2A RPS Breaker normal feed tripped on overvoltage. All Primary Containment isolations occurred as expected following the loss of A RPS power. Operations entered abnormal operating procedures, swapped 2A RPS power to the alternate feed and restored containment isolations.

Cause

The most likely cause of the isophase bus duct fire is written guidance that governs the work did not provide the detailed instructions needed to perform disassembly and reassembly of the Unit 2 lsophase Bus Duct (IPDB) bolted connections with high precision. This resulted in degraded shunt bolted connection resistance causing overheating in the bolted connections.

The overheating resulted in a failure of the IPBD support structure hardware and ignition of the retractable cover neoprene gasket.

The cause of the 2A RPS Breaker trip was a degraded voltage adjust potentiometer.

Reportability and Safety Analysis

Fire and manual scram The fire was extinguished in less than 15 minutes of identification and the reactor was safely shutdown. Offsite power sources and station emergency power sources were available at all times throughout the event. Emergency response personnel acted promptly to extinguish the fire and there was no personnel injury during this event or damage to plant property from the fire other than the bus duct itself.

RPS actuation is reportable under 10 CFR 50.72(b)(2)(iv)(B) as an event or condition that results in the actuation of the RPS when the reactor is critical. An ENS report was made to the NRC at 0639 on September 26, 2022 (EN 56120).

2A RPS Breaker trip The RPS is divided into two trip systems that are physically and electrically independent. The design of this system is such that the toss of power to one of these trip systems neither prevents nor causes a reactor scram. Normal power to RPS buses A and Bis supplied by two motor-generator (MG) sets. Alternate power for either RPS bus is from the Alternate Instrument and RPS Bus Transformer. The loss of an RPS bus and associated RPS actuation was considered an invalid actuation because it was not the result of a valid signal and not an intentional manual action. The RPS bus loss condition resulted in general containment isolation signals that affected containment isolation valves in more than one system or multiple main steam isolation valves while the unit was critical. Therefore, the RPS bus loss condition is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A).

Safety System Functional Failure Review The system equipment responses did not result in a safety system functional failure (SSFF) as defined in accordance with NEI 99-02, "Regulatory Assessment Performance Indicator Guideline." The RPS logic safety function was satisfied by its actuation for designed inputs.

Corrective Actions

Corrective actions taken in response to the conditions were:

Unit 2 Manually scrammed.

Fire brigade dispatched and fire promptly extinguished.

Swap 2A RPS to alternate power supply and restored containment isolations Degraded 2A RPS voltage potentiometer replaced and normal power to 2A RPS restored.

Damaged connections in the isophase bus duct restored and Unit 2 restarted.

01 Actions created to track the replacement of Unit 1 and Unit 2 IPBD bolted enclosure shuts and flexible links with welded flexible connections.

Actions created to improve thermographic monitoring of the IPBD.

Previous Occurrences

LER 37 4-2021-002-01, Reactor Protection System Half Scram due to Motor Generator Set Output Breaker Trip. On May 31, 2021, the 2A RPS normal power supply output breaker tripped on overvoltage. The cause of this event was a degraded POT.

Component Failure Data

Device: 2A RPS Logic MG Set Output Breaker - Voltage Adjustment Potentiometer Component Type: Rheostat / Potentiometer [RHE]

Manufacturer: Ohmite [0026]

Part: RHS500 Device: Bus, lso Phase, Main Power Transformer Component Type: Bus, isolated phase [GBU]

Manufacturer: Commonwealth Edison [X000]

Part: 706232 Page_3_ of _3_

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