05000333/LER-2024-002-01, Nuclear Power Plant, Reactor Protection System Electric Power Monitoring System Trip Caused Primary Containment Isolation
| ML24327A097 | |
| Person / Time | |
|---|---|
| Site: | FitzPatrick  |
| Issue date: | 07/08/2024 |
| From: | Sterio A Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| JAFP-24-0060 LER 2024-002-01 | |
| Download: ML24327A097 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3332024002R01 - NRC Website | |
text
Constellation.
JAFP-24-0060 November 22, 2024 United States Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555-0001 James A. FitzPatrick NPP P.O. Box 110 Lycoming, NY 13093 Alexander Sterio Site Vice President-JAF James A. FitzPatrick Nuclear Power Plant Renewed Facility Operating License No. DPR-059 NRC Docket No. 50-333
Subject:
Dear Sir or Madam:
LER: 2024-002-01, Reactor Protection System Electric Power Monitoring System Trip Caused Primary Containment Isolation This report is being submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A).
There are no new regulatory commitments contained in this report.
Questions concerning this report may be addressed to Mr. Mark Hawes, Regulatory Assurance, at (315) 349-6659.
Sincerely,
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Alexander Sterio Site Vice President ADS/MH
Enclosure:
LER: 2024-002-01, Reactor Protection System Electric Power Monitoring System Trip Caused Primary Containment Isolation cc:
USNRC, Region I Administrator USNRC, Project Manager USNRC, Resident Inspector INPO Records Center (IRIS)
Abstract
On July 8, 2024, at 1927, James A. FitzPatrick Nuclear Power Plant (JAF) received annunciators and indication of a half-scram on the B side Reactor Protection System (RPS). The cause was determined to be an overvoltage condition that tripped the Electrical Protection Assembly (EPA) for the B RPS Motor Generator (MG) Set. This event caused invalid primary containment isolation signals in multiple systems, reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). The voltage adjustment potentiometer had intermittent high resistance along the full span of the potentiometer; which can cause the voltage regulator to drive up voltage resulting in the EPA tripping on overvoltage. The cause was a degraded potentiometer due to component age and construction. Corrective actions replaced the voltage adjusted potentiometer and preventative maintenance practices were changed to replace aged potentiometers in use or in stock at a higher frequency.
(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
(See Page 2 for required number of digits/characters for each block)
LICENSEE EVENT REPORT (LER)
=
Background===
Reactor Protection System (RPS) [EIIS identifier: JE] Electric Power Monitoring System is provided to isolate the RPS bus from the motor generator (MG) set or an alternate power supply in the event of overvoltage, undervoltage, or underfrequency. This system protects the loads connected to the RPS bus against unacceptable voltage and frequency conditions and forms an important part of the primary success path of the essential safety circuits. Some of the essential equipment powered from the RPS buses includes the RPS logic, scram pilot valve solenoids, and various valve isolation logic.
RPS electric power monitoring assembly will detect any abnormal high or low voltage or low frequency condition in the outputs of the two MG sets or the alternate power supply and will de-energize its respective RPS bus, thereby causing all safety functions normally powered by this bus to de-energize. (Safety functions powered by the RPS buses deenergize to actuate.)
Two redundant Class IE circuit breakers are connected in series between each RPS bus and its MG set, and between each RPS bus and its alternate power supply. Each of these circuit breakers has an associated independent set of Class IE overvoltage, undervoltage, and under frequency sensing logic. Together, a circuit breaker and its sensing logic constitute an electric power monitoring assembly. If the output of the inservice MG set or alternate power supply exceeds predetermined limits of overvoltage, undervoltage, or underfrequency, a trip coil driven by this logic circuitry opens the circuit breaker, which removes the associated power supply from service.
Event Description
On July 8, 2024, at 1927, with James A. FitzPatrick Nuclear Power Plant (JAF) at 100% power, annunciators were received with indication of a half-scram on B side RPS. AOP-60, Loss of RPS Bus B Power, was entered and the cause was determined to be a trip of the B RPS MG set due to an overvoltage condition.
The direct cause of the output breaker tripping was due to an Electric Power Monitoring System overvoltage trip on 71EPA-RPS1B1G, Electrical Protection Assembly (EPA).
The condition was corrected when the RPS Bus was placed on the alternate power supply.
The event resulted in invalid system actuations of "B" side half scram and Primary Containment Isolation System (PCIS) [JM] isolations. The following systems isolated as a result of the loss of B RPS bus: Reactor Water Cleanup [CE], Reactor Building ventilation [NG], B Containment Atmosphere Dilution [BB], Torus Vent and Purge, Drywell Equipment and Floor Drain Sumps [WK], B Drywell Containment Atmospheric Monitors [IP], Recirculation System Sample Line, and Main Steam Line Drains [SB]. B Standby Gas Treatment System [BH] started as designed. These system actuations were not initiated by signals in response to actual plant conditions or parameters satisfying the requirements for initiation of the system.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). U.S. NUCLEAR REGULATORY COMMISSION (04-02-2024)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027
- 1. FACILITY NAME 050
- 2. DOCKET NUMBER
- 3. LER NUMBER James A. FitzPatrick Nuclear Power Plant 05000333 YEAR SEQUENTIAL NUMBER REV NO.
052 2024
- - 002
- - 01 Page 3 of 4 (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
Event Analysis
Following the trip on July 8, 2024, at 19:33, troubleshooting included replacing the voltage adjustment potentiometer 1P and the voltage regulator. The replacement parts were monitored for stability and the parts which were removed were sent to PowerLabs for analysis.
PowerLabs electrical resistance testing of the potentiometer 1P identified spurious resistance due to dead spots. The dead spots were found throughout the potentiometer range but primarily at both ends of travel.
High resistance on the potentiometer caused the voltage regulator to drive up voltage, resulting in a high voltage output trip of the EPA.
It was determined that component age may have contributed to the dead spots found in the potentiometer range. Prior to installation in May 2023, the potentiometer had been stored in the warehouse for the past 10 years. As part of corrective actions from the May 2023 event, Supply restocked the potentiometers with newly constructed units. Therefore, the currently installed potentiometer is a newly constructed unit and is not affected by an aged condition.
In addition, PowerLabs identified solder drippings affecting 9 windings of the potentiometer, which provides evidence that there was a manufacturing issue with the potentiometer. This solder decreases the resistance across the potentiometer, which did not cause the event, but it provides evidence that the potentiometers construction may have caused the premature failure.
Cause
The cause of the event was a degraded Voltage Adjustment Potentiometer (1P) due to component age and construction.
Similar Events
JAF Licensee Event Report LER-2023-002-01, JAFP-23-0045 dated August 14, 2023, reported the same overvoltage trip on 71EPA-RPS1B1G which resulted in similar system actuations in April and May 2023. The investigation into the potentiometer for this event identified a similar erratic resistant reading across the range of travel. The cause was attributed to corrosion buildup in the voltage adjustment potentiometer.
The potentiometer replacement from that event is the subject component of this LER. Maintenance practices were changed to use an analog multimeter to detect and wipe corrosion buildup. However, component aging was not considered a cause. The potentiometers subject to both LER-2023-002-01 and LER-2024-002-01 were approximately aged 2007 and 2011, respectively. U.S. NUCLEAR REGULATORY COMMISSION (04-02-2024)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027
- 1. FACILITY NAME 050
- 2. DOCKET NUMBER
- 3. LER NUMBER James A. FitzPatrick Nuclear Power Plant 05000333 YEAR SEQUENTIAL NUMBER REV NO.
052 2024
- - 002
- - 01 Page 4 of 4 (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
Corrective Actions
Completed Actions RPS Bus was placed on the alternate power supply after the event.
Replaced degraded voltage adjustment potentiometer 1P component with a newly constructed unit.
Applied a shelf-life of 6 years for voltage adjustment potentiometers RHS500E and current stock older than 6 years was removed.
Planned Actions Update preventative maintenance for RPS potentiometers to have a 4 year replacement and not rely on wiping the potentiometer to resolve age related failure.
FAILED COMPONENT IDENTIFICATION:
Manufacturer:
Ohmite Manufacturer Model Number:
RHS500E Manufacturer Code:
O026 Component Code:
EC FitzPatrick Component ID:
Voltage adjustment potentiometer 1P is a subcomponent to 71RP-1B(GEN)
Safety Significance
There were no actual nuclear consequences. RPS Electric Power Monitoring System responded as designed to an overvoltage condition to protect the electrical power systems. Then system actuations responded to the loss of power condition as expected without any issue. There was no loss of any safety functions during this event.
References Issue Report - IR 04785661, AOP-60 entry due to B side half scram, dated July 8, 2024 LER Report - LER: 2023-002-01, Reactor Protection System Electric Power Monitoring System Trip Caused Primary Containment Isolation, JAFP-23-0045, dated August 14, 2023 PowerLabs Report - JAF-60990, Failure Analysis of a Voltage Regulator Potentiometer, dated June 19, 2023 PowerLabs Report - JAF-53293, Failure Analysis of: Voltage Regulator, dated August 26, 2024