05000352/LER-2024-002, Invalid Actuation Due to a Trip of 1B Reactor Protection System Circuit Breaker
| ML24162A325 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 06/10/2024 |
| From: | Gillin M Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LG-24-066 LER 2024-002-00 | |
| Download: ML24162A325 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| 3522024002R00 - NRC Website | |
text
Constellation
LG-24-066
June 10, 2024
U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Limerick Generating Station, Unit 1 Renewed Facility Operating License No. NPF-39 NRC Docket No. 50-352
Subject: LER 2024-002-00 Invalid Actuation Due to a Trip of 1 B Reactor Protection System Circuit Breaker
In accordance with the requirements of 10 CFR 50.73(a)(2)(iv)(A), Limerick Generating Station hereby submits the enclosed Licensee Event Report.
There are no commitments contained in this letter.
If you have any questions, please contact Jordan Rajan at (610) 718-3400.
Respectfully,
Michael F. Gillin Vice President - Limerick Generating Station Constellation Energy Generation, LLC
cc: Administrator Region I, USNRC USNRC Senior Resident Inspector, Limerick Generating Station
Abstract
On April 11, 2024, Limerick Generating Station (LGS) Unit 1 was in Operational Condition (OPCON) 5, refueling, performing a surveillance test on the 1B Reactor Protection System (RPS) Power Monitoring System.
Following the completion of the testing, when returning the system to the normal plant alignment, an invalid isolation occurred due to a trip of the 1B-Y160 RPS panel.
Troubleshooting on circuit breaker 52-DY24801 found the circuit breaker contacts were not closed even though the handle was in the closed position. The circuit breaker was replaced. This resulted in containment isolation signals that actuated containment isolation valves in more than one system; therefore, the loss of the RPS bus is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A).
Initial Plant Condition
Prior to the event, Unit 1 was in Operational Condition (OPCON) 5 (Refueling) with reactor coolant temperature 99 degrees Fahrenheit with a time to boil of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Event Description
On April 11, 2024, a surveillance test was being performed to test the 1B Reactor Protection System (RPS) [EIIS System JC] Power Monitoring System by simulating undervoltage, overvoltage, and underfrequency signals for the 1B RPS series circuit breakers. During the testing, power is maintained to the 1B RPS system through a maintenance bypass circuit which is parallel to the series breakers.
On April 11, 2024, at 13:48 EST the station experienced a loss of the RPS panel 1B-Y160 which resulted in a 1B RPS half scram signal and isolation of the following systems: Drywell Chilled Water (DWCW) [EIIS System KM], Reactor Enclosure Chilled Water (RECW) [EIIS System KM], Reactor Water Cleanup (RWCU) [EIIS System CE], Primary Containment Instrument Gas (PCIG) [EIIS System LD, and Primary Containment Leak Detector System [EIIS System IJ]. Shutdown Cooling [EIIS System BO] received an isolation signal but did not isolate because the Shutdown Cooling isolation instruments were bypassed in accordance with station procedures. The B train of Standby Gas Treatment System (SGTS) [EIIS System BH] started and was isolated due to drawing the fuel floor to a high negative pressure. The A train of SGTS was started and operated as designed. The cause of the B train of SGTS drawing a high negative pressure was the panel 1B-Y160 supplies power to the refuel floor slide gate damper position. Without this position indication the B train of SGTS will draw excessively from the fuel floor. The plant responded as expected to the loss of the 1B RPS Panel 1B-Y160.
The investigation identified that one of the series circuit breakers, 52-DY24801, appeared to be closed but troubleshooting determined the circuit breaker was open. Additional troubleshooting was performed which cycled the breaker several times to ensure the expected continuity was achieved. The station was able to re-energize the 1B RPS power panel to restore normal ventilation. On April 13, 2024 circuit breaker 52-DY24801 was replaced.
Analysis of the Event
The loss of the RPS bus and associated actuations is considered invalid because it was not the result of a valid signal and not an intentional manual action. The loss of the RPS Panel 1B-Y160 was a result of opening the maintenance bypass switch with one of the series breakers open (52-DY24801). This resulted in containment isolation signals that actuated containment isolation valves in more than one system. Therefore, the loss of the RPS bus is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A).
Safety Consequences
The system response 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 and responded as expected to the loss of the 1B RPS Panel 1B-Y160.
Cause of the Event
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Failure analysis on circuit breaker 52-DY24801 identified a mechanical problem with the circuit breaker closing handle.
The handle failed to reposition to the tripped position during testing of the circuit breaker. Failure of the circuit breaker to close could not be reproduced in the post-event testing.
Corrective Actions Completed
Circuit breaker 52-DY24801 was replaced.
Corrective Actions Planned
Evaluate options to determine if the series circuit breakers are closed prior to opening the maintenance switch during testing.
Previous Similar Occurrences
There have been no previous similar occurrences for an actuation due to a circuit breaker that appeared to be closed, but was open, within the past five years.
Component Data
System: JC (Plant Protection System)
Component: 52 (Circuit Breaker, AC)
Manufacturer: W121 (Westinghouse)