05000373/LER-2024-004, Emergency Diesel Generator Auto-Start and Load Due to Loss of Station Auxiliary Transformer
| ML24171A003 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 06/19/2024 |
| From: | Casey Smith Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| RA24-021 LER 2024-004-00 | |
| Download: ML24171A003 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3732024004R00 - NRC Website | |
text
Constellation RA24-021 June 19, 2024 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 I 1,
,J LaSalle County Station, Unit 1 Renewed Facility Operating License No. NPF-11 NRC Docket No. 50-373 LaSalle County Station 2601 North 21 " Road Marseilles. IL 61341 815-415-2000 Telephone 10 CFR 50.73
Subject:
Licensee Event Report 2024-004-00, Emergency Diesel Generator Auto-Start and Load due to Loss of Station Auxiliary Transformer In accordance with 10 CFR 50.73(a)(2)(iv)(A), Constellation Energy Generation, LLC (CEG) is submitting Licensee Event Report (LER) Number 2024-004-00 for LaSalle County Station, Unit 1.
There are no regulatory commitments contained in this letter.
Should you have any questions concerning this report, please contact Ms. Laura Ekern, Regulatory Assurance Manager, at (815) 415-2800.
Christopher J. Smith Plant Manager LaSalle County Station Enclosure: Licensee Event Report 2024-004-00 cc:
Regional Administrator-NRC Region Ill NRC Senior Resident Inspector - LaSalle County Station
Abstract
On April 20, 2024, with LaSalle Unit 1 operating at 100 percent power and gas circuit breaker (GCB) 11-13 open to support transmission system operator testing of power line L0104 relays, oil circuit breaker (OCB) 1-13 tripped open on differential current resulting in the loss of the Unit 1 station auxiliary transformer (SAT). Bus 141 Y (Division 1 ), normally supplied by the Unit 1 SAT, failed to fast transfer to the Unit 1 unit auxiliary transformer (UAT), resulting in the Division 1 emergency diesel generator (EOG) to automatically start and load onto the 141Y bus, causing a load shed of the 141Y bus and momentary loss of power to the 141 Y bus. The loss of the Unit 1 SAT also caused the Division 3 EDG to automatically start and load, as designed, onto bus 143 (Division 3).
The load shed on bus 141Y caused power to be lost to the Unit 1 "A" (1A) reactor protection system (RPS) bus, resulting in a "A" RPS half-scram condition and closure of isolation valves in multiple systems. All actuations and isolations performed as designed. During the fast transfer of Division 2 from the Unit 1 SAT to UAT, the Unit 1 reactor building ventilation system (VR)
'B' supply isolation damper and "A' exhaust isolation damper went shut unexpectedly causing a loss of Unit 1 VR.
These events are reportable in accordance with 10 CFR 50. 73(a)(2)(iv)(A).
Plant and System Identification
- 2. DOCKET NUMBER YEAR 00373 2024 -
- 3. LER NUMBER SEQUENTIAL NUMBER 004 REV NO.
00 LaSalle Country Station Unit 1 is a General Electric Boiling Water Reactor with 3546 Megawatts Thermal Rated Core Power. The affected system was the alternating current (AC) power system. The affected component is 345 kV oil circuit breaker (OCB) 1-13, the mechanism operated contacts (MOC) 15 and 16 of air circuit breaker (ACB) 1412, and the TSC switch contacts on ACS 1415.
Condition Prior to Event
Unit(s): 1 Date: April 20, 2024 Time: 07:04 CDT Reactor Mode(s): 1 Mode(s) Name: Power Operation Power Level: 100 percent
Description
Emergency Diesel Generator (EOG) Actuation and Load On April 20, 2024, with gas service breaker (GCB) 11-13 open to support transmission system operator (TSO) testing of power line L0104 relays, OCB 1-13 tripped open on differential current resulting in the loss of the Unit 1 station auxiliary transformer (SAT). Bus 141Y (engineered safety feature (ESF) Division 1), which is normally supplied by the Unit 1 SAT, failed to fast transfer to the Unit 1 unit auxiliary transformer (UAT), resulting in the Division 1 EOG to automatically start and load onto the 141 Y bus, causing a load shed of the 141 Y bus and a momentary loss of power to the 141 Y bus. The loss of the Unit 1 SAT also caused the Division 3 EOG to automatically start and load, as designed, onto bus 143 (ESF Division 3), which is normally supplied by the Unit 1 SAT. Bus 142Y (ESF Division 2) fast transferred to the Unit 1 UAT, as expected, resulting in no requirement for the Division 2 EOG to automatically start. The automatic starting of the Division 1 and Division 3 EDGs is considered a valid actuation due to the loss of power to ESF buses.
During the fast transfer of ESF Division 2 from the Unit 1 SAT to UAT, the Unit 1 reactor building ventilation system (VR)
'B' supply isolation damper and "A' exhaust isolation damper went shut unexpectedly causing a loss of Unit 1 VR. This resulted in a main steam tunnel high temperature condition which was bypassed per station procedure. The dampers were reopened, and Unit 1 VR was restored.
Reactor Protection System (RPS) Unit 1 "A" Half-Scram As a result of the load shed on bus 141Y, power was lost to the Unit 1 "A" (1A) RPS bus, resulting in a "A" RPS half-scram condition and closure of isolation valves in multiple systems. The condition was considered an invalid actuation because it was the result of losing power to the bus and not the result of a valid input signal to the RPS or intentional manual action. All actuations and isolations performed as designed.
The loss of the 1A RPS bus resulted in isolations of the reactor water cleanup system, main steam line drains, reactor coolant system sampling, instrument nitrogen system, reactor recirculation hydraulic power units, and containment monitoring system. The loss of the 1A RPS bus also resulted in the loss of power to portions of the average power range monitor, local power range monitor, oscillation power range monitor, and control rod block systems; as well as portions of the radiation monitoring for the main steam lines, reactor building vent, and fuel pool vent. Appropriate actuations and primary containment isolations occurred as designed.
After the loss of power to the 1A RPS bus, the applicable Technical Specification (TS) timeclocks and actions were entered. Power was restored to the 1A bus by transferring the bus to the alternate power supply in accordance with plant
- 2. DOCKET NUMBER
- 3. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO.
00373 2024 -
004 00 procedures. The half-scram and isolations were reset, and the equipment was returned to operable status. All applicable TS timeclocks and actions were exited.
Cause
EDG Actuation and Load The cause of OCB 1-13 tripping open is under investigation through the corrective action program. A supplemental LER will be submitted once the cause has been finalized.
Troubleshooting determined the cause of the fast transfer failure of bus 141 Y from the Unit 1 SAT to the Unit 1 UAT was increased resistance across MOC 15 and 16 of ACS 1412, and the TSC switch contacts were open on ACB 1415, Unit 1 UA T feed breaker to bus 141 Y.
RPS Unit 1A Half-Scram The cause of the RPS Unit 1 A loss of power and half scram was the momentary loss of power from the 141Y bus due to the load shed caused by the fast transfer failure. The loss of power from the 141Y bus resulted in a loss of power to the 1A RPS motor-generator set that normally powers the 1A RPS.
Reportability and Safety Analysis
EOG Actuation and Load The loss of the Unit 1 SAT and actuation of the Division 1 and Division 3 emergency diesel generators was initially reported as an 8-hour notification (ENS 57083) in accordance with 10 CFR 50. 72(b)(3)(iv)(A) for a valid actuation of the emergency AC electrical power systems. This event is also required to reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that results in the valid actuation of the emergency AC electrical power systems.
RPS Unit 1 A Half-Scram The loss of the Unit 1A RPS bus and associated RPS actuation is considered an invalid actuation because it was not the result of a valid signal and not an intentional manual action. The loss of the RPS bus resulted in general containment isolation signals that actuated containment isolation valves in more than one system or multiple main steam isolation valves while the unit was critical. Therefore, the loss of the RPS bus is reportable in accordance with 10 CFR
- 50. 73(a)(2)(iv)(A).
Safety System Functional Failure Review The system equipment responses do not result in a safety system functional failure (SSFF) as defined in accordance with NEI 99-02, "Regulatory Assessment Performance Indicator Guideline." The EOG and RPS safety systems performed their as-designed safety functions.
Corrective Actions
050 0
052 Corrective actions taken in response to the conditions were:
- 2. DOCKET NUMBER YEAR 00373 2024 Entered the required TS timeclocks and actions and notified the TSO to secure all testing.
- 3. LER NUMBER SEQUENTIAL NUMBER 004 Transferred the 1 A RPS bus to the alternate power supply, reset the half-scram, and exited applicable TS timeclocks and actions.
Closed OCB 1-13 and reenergized the Unit 1 SAT REV NO.
00 Transferred buses 141Y, 142Y, and 143 to the Unit 1 SAT; shutdown the Division 1 and Division 3 EDGs; and exited applicable TS timeclocks and actions.
Transferred the 1A RPS bus back to the normal power supply.
Racked out and then racked in ACB 1415 to close the TSC switch contacts.
Cleaned MOC 15 and 16 on ACB 1412 to decrease the resistance back to acceptable levels.
Develop electrical maintenance procedure to check all 4 kV and 6 kV switchgear breaker contacts to confirm alignment for fast transfer logic.
Update the normal startup procedure to incorporate new electrical maintenance procedure during bus alignment.
The investigation for this event is still ongoing. A supplemental LER will be submitted once the investigation is complete, and the corrective actions will be updated as needed.
Previous Occurrences
There have been no previous occurrences of an EOG actuation in the past 3 years.
There have been two occurrences of RPS half-scrams in the past 3 years that were attributed to a loss of power.
Unit 1 LER 373-2024-002-00 On March 25, 2024, LaSalle Unit 1 was operating at 100 percent power, when the 18 RPS MG set electric power monitoring assembly (EPMA) output breaker tripped due to an underfrequency condition, which resulted in the loss of the 1 B RPS bus. The loss of power to this bus created a half-scram condition and initiated closure of isolation valves in multiple systems. The condition was an invalid actuation because it was not the result of actual plant conditions and not an intentional manual action. The cause of the MG set EPMA output breaker trip was the EPMA trip card for the output breaker being out of range high for the underfrequency setpoint which caused the trip on underfrequency. As a result, the EPMA trip card was replaced, and the system was restored back to its original configuration.
Unit 2 LER 374-2021-002-00 On May 31, 2021, LaSalle Unit 2 was operating at 100 percent power, when the 2A RPS motor generator (MG) set output breaker tripped due to an overload condition, which resulted in a loss of 2A RPS bus. Loss of power to this bus created a half-scram and initiated closure of outboard isolation valves in multiple systems. The condition was an invalid actuation because it was not the result of actual plant conditions and not an intentional manual action. The cause of the issue was a degraded voltage adjustment potentiometer (device 1 P) on the 2A RPS MG set, which was causing intermittent resistance readings. The intermittent resistance caused the output voltage of the RPS MG set to fluctuate. As a result, the degraded potentiometer and the voltage regulator were replaced as part of the trouble-shooting efforts, and the system was restored back to its original configuration.
Component Failure Data
Manufacturer: Westinghouse Electric Company Device: Oil Circuit Breaker, 345 kV 050 052 Component ID: 0SYOCB1-13, 345 kV Oil Circuit Breaker 1-13 Manufacturer: ITE Circuit Breaker Limited Device: Air Circuit Breaker, 4160 V
- 2. DOCKET NUMBER YEAR 00373 2024
- 3. LER NUMBER SEQUENTIAL NUMBER 004 REV NO.
00 Component ID: ACB 1412 Bus Feed from SAT 1AP91 E (TR 142)
Manufacturer: ITE Circuit Breaker Limited Device: Air Circuit Breaker, 4160 V Component ID: ACB 1415 1AP04E / 1AP03E (141Y / 141X) Bus Tie Page_5_ of _5_