05000374/LER-2021-001, High Pressure Core Spray Inoperable Due to Water Leg Pump Breaker Cubicle Motor Contactor
| ML21049A048 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 02/18/2021 |
| From: | Hansett P Exelon Generation Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RA21-009 LER 2021-001-00 | |
| Download: ML21049A048 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3742021001R00 - NRC Website | |
text
Exelon Generation LaSalle County Station 2601 North 21 " Road Marseilles, IL 61341 815-415-2000 Telephone www.exeloncorp.com 10 CFR 50.73 RA21-009 February 18, 2021 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Subject:
LaSalle County Station, Unit 2 Renewed Facility Operating License No. NPF-18 NRC Docket No. 50-374 Licensee Event Report 2021-001-00, High Pressure Core Spray Inoperable due to Water Leg Pump Breaker Trip In accordance with 10 CFR 50.73(a)(2)(v)(D), Exelon Generation Company, LLC (EGC) is submitting Licensee Event Report (LER) Number 2021-001-00 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. Daniel Mearhoff, Regulatory Assurance Manager, at (815) 415-2800.
Respectfully, P/~
Phil Hansett Plant Manager LaSalle County Station
Enclosure:
Licensee Event Report cc:
Regional Administrator - NRC Region Ill NRC Senior Resident Inspector-LaSalle County Station
Abstract
On December 23, 2020, the Unit 2 high pressure core spray (HPCS) water leg pump (WLP) tripped off due to a fault within its breaker cubicle. The HPCS WLP is a support component required to maintain the HPCS system operable. With the loss of the WLP, the HPCS system was declared inoperable. The unit entered applicable technical specification required actions to immediately verify that the reactor core isolation cooling (RCIC) system was operable and to restore the HPCS system to operable within 14 days.
This condition was previously reported under emergency notification system (ENS) 55046 as a single-train system inoperability that could have prevented the fulfillment of a safety function used to mitigate an accident. The condition is being reported under 1 OCFR 50. 73(a)(2)(v)(D) as an event or condition that could have prevented fulfillment of a safety function. The system was restored to operable following replacement of the affected HPCS WLP breaker cubicle control power transformer and motor contactor. The cause was age-related failure of the breaker cubicle motor contactor.
Plant and System Identification
LaSalle Country Station Unit 2 is a General Electric Boiling Water Reactor with 3546 Megawatts Thermal Rated Core Power. The affected system was the Division 3 high-pressure core spray (HPCS). In accident situations, the HPCS system provides cooling to the reactor core to prevent excessive fuel cladding temperatures following a break in the nuclear system piping.
Condition Prior to Event
Unit(s):
2 Date:
December 23, 2020 Time:
06:53 CST Reactor Mode(s):
1 Mode(s) Name: Power Operation Power Level:
100 percent
Description
00 On December 23, 2020, the main control room operating crew received an alarm for the Unit 2 HPCS water leg pump (WLP) having tripped off, followed shortly by a ground alarm for the associated electrical circuitry that cleared with 15 seconds. Field operators subsequently reported an acrid order in the Division 3 switchgear room and subsequently found the breaker for the HPCS WLP in the closed position but the breaker's cubicle was warm to the touch. The operating shift crew removed the HPCS WLP from service, declared the HPCS system inoperable, and entered applicable technical specifications {TS) 3.5.1 required action (RA) 8.1 to immediately verify by administrative means that the reactor core isolation cooling (RCIC) system was operable, and TS 3.5.1 RA 8.2 to restore the HPCS system to operable within 14 days. The operating shift established robust barriers to administratively protect the RCIC system following discovery through the subsequent restoration of the HPCS system inoperability period. The Unit 2 HPCS system was restored to operable the same day at 20:18 CST, after replacement of the WLP breaker cubicle control power transformer and motor contactor and post-maintenance testing.
Cause
Initial investigation identified that the WLP motor starter (2AP79E-2C) contactor and control power transformer were damaged. The cause was age-related failure of the contactor. The failure analysis found the motor starter damage had resulted from a heating event that began inside the contactor coil which eventually led to an open circuit in the coil. The increased current flow impacted the control power transformer, which overheated and failed.
Reportability and Safety Analysis
The HPCS system is part of the Division 3 emergency core cooling system (ECCS). In accident situations, HPCS provides cooling to the reactor core to prevent excessive fuel cladding temperatures following a break in the nuclear system piping. A secondary purpose of HPCS is to supply cooling water to the reactor pressure vessel as a backup to the RCIC system if the main condenser and feedwater were unavailable. The HPCS WLP is a support system required to maintain the HPCS system operable. With the loss of the HPCS WLP, HPCS was declared inoperable and the operating shift entered applicable TS RA statements.
This condition could have prevented the HPCS system, a single train safety system, from performing its design function. Therefore, this condition is reportable in accordance with 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are required to mitigate the
- 3. LER NUMBER SEQUENTIAL NUMBER 001 REV NO.
00 consequences of an accident. The event was previously communicated to the NRC under emergency notification system (ENS) 55046 in accordance with 10CFR 50.72(b)(3)(v)(D).
This condition was a safety system functional failure (SSFF) as defined in accordance with NEI 99-02, "Regulatory Assessment Performance Indicator Guideline." The support system WLP could have prevented the HPCS system from performing its design function.
Corrective Actions
Immediate corrective actions taken in response to the condition were:
Declared the Unit 2 HPCS system inoperable on December 23, 2020 at 06:53 CST Completed maintenance and post-maintenance testing at 19:48 CST Declared Unit 2 HPCS operable and exited associated TS 3.5.1 required actions at 20:18 CST Performed failure analysis for the failed components to identify any additional actions The extent of condition for potential similar failure mechanisms was determined to exist among the seven continuously energized contactors in each of the Unit 1 and Unit 2 Division 3 (HPCS) motor control centers (MMCs),
a total of 14 contactors. A corrective action assignment was made to evaluate further for contactor replacements.
Previous Occurrences
There have been no similar events involving contactor faults for safety related systems within the previous three-year period. However, during that period there has been a previous event involving a breaker in the Unit 1 HPCS system MCC bus bar and another event involving a unit crosstie breaker.
373-2018-002-00, Damaged Bus Bar Identified Potentially Affecting High Pressure Core Spray System Station personnel identified damage of the bus bar at the breaker that supplies the Unit 1 HPCS diesel generator auxiliaries. The degradation of the bus bar may have prevented the Division 3 DC battery charger from performing its function, which could have prevented the HPCS system from performing its design safety function. The condition of the MCC bus bar was the result of a degraded connection between the "A" phase bus bar and the "A" phase bus clip (stab) associated with the breaker cubicle 28. The apparent cause of the degraded bus bar to clip connection was due to relaxation of the bucket clip over time.
374-2019-001-00, Safety Related Electrical Bus Undervoltage Results in Valid Actuation of the Common Emergency Diesel Generator While shut down for a scheduled refueling outage, Unit 2 Division 1 safety-related electrical bus (241 Y) tripped during surveillance testing. The trip of the electrical bus resulted in a valid undervoltage actuation signal to the common emergency diesel generator, causing it to start and load to bus. The cause was attributed to the crosstie breaker having a degraded lug on the terminal between its Unit 2 A-phase current transformer and the Unit 1 breaker differential current relay.
Component Failure Data
Device: 2AP79E-2C, HPCS WLP MCC Manufacturer: General Electric [G080]
Component Type: Contactor [CNTR]
Model: CR206CO Page_3_ of _3_