05000461/LER-2023-002-01, Reactor Core Isolation Cooling Inoperable Prior to Mode Change

Reactor Core Isolation Cooling Inoperable Prior to Mode Change

ML24270A035
Person / Time
Site:	Clinton
Issue date:	09/24/2024
From:	Plumey N Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation
References
U-604815 LER 2023-002-01
Download: ML24270A035 (1)

LER-2023-002, Reactor Core Isolation Cooling Inoperable Prior to Mode Change

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4612023002R01 - NRC Website

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Facility Operating License No. NPF-62 NRC Docket No. 50-461 Subject: Licensee Event Report 2023-002-01 10 CFR 50.73 Enclosed is Licensee Event Report (LER) 2023-002-01: Reactor Core Isolation Cooling Inoperable Prior to Mode Change, Revision 1. This report is being submitted in accordance with the requirements of 10 CFR 50.73.

There are no regulatory commitments contained in this report.

Should you have any questions concerning this report, please contact Mr. David Livingston, Regulatory Assurance Manager, at (217) 937-2800.

Respectfully, Q_y_~

Norha Z. Plumey Site Vice President Clinton Power Station Attachment:

Licensee Event Report 2023-002-01 cc:

Regional Administrator - Region Ill NRC Senior Resident Inspector - Clinton Powe*r Station Office of Nuclear Facility Safety - Illinois Emergency Management Agency

Abstract

On October 10, 2023, at 23:02 CST, the Reactor Core Isolation Cooling (RCIC) system tripped during performance of Surveillance Requirement (SR) 3.5.3.3. This SR is performed when RCIC steam supply pressure is at the higher operating ranges of the system to verify that the RCIC pump can develop the required flow rate against a system head corresponding to reactor pressure. The system was restored to operable on October 16, 2023, at 02:51 CST following replacement and calibration of the Electric Governor-Remote (EG-R). Engineering personnel suspected that the RCIC system trip was due to a faulty EG-R; however, subsequent failure analysis identified no significant issues with the EG-R. On December 19, 2023, Engineering personnel determined that work performed on August 28, 2023 affected functionality of the EG-R and caused RCIC to be inoperable per Technical Specifications (TS) 3.5.3, "RCIC System."

This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS 3.0.4. Specifically, on October 9, 2023, at 05: 14 CST, during unit startup following a refueling outage, the Applicability for TS 3.5.3 was entered (i.e.,

Mode 2 with reactor steam dome pressure greater than 150 psig) with the RCIC system inoperable. In addition, the unit was taken to Mode 1 on October 10, 2023.

CONDITION PRIOR TO THE EVENT Unit: 1 Mode: 1 Event Date: December 19, 2023 Mode Name: Power Operation EVENT DESCRIPTION

2. DOCKET NUMBER 00461 Event Time: 0800 CST Reactor Power: 100 YEAR 2023

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

01 On October 8, 2023, Operations commenced reactor startup following a refueling outage. On October 10, 2023, at 23:02 CST, the Reactor Core Isolation Cooling (RCIC) system tripped on low suction pressure during performance of Surveillance Requirement (SR) 3.5.3.3 and the RCIC system was declared inoperable. This SR is performed when RCIC steam supply pressure is at the higher operating ranges of the system to verify that the RCIC pump can develop the required flow rate against a system head corresponding to reactor pressure. Troubleshooting activities commenced to determine the cause of the RCIC system trip. As part of the troubleshooting activities, the RCIC Electric Governor - Remote (EG-R) was replaced and calibrated. Following replacement of the EG-R, SR 3.5.3.3 was re-performed satisfactorily and the RCIC system was declared operable at 02:51 CST on October 16, 2023.

Based on the successful re-performance of SR 3.5.3.3 following replacement of the EG-R, Engineering personnel suspected that the RCIC system trip on low suction pressure was due to a faulty EG-R. The suspect EG-R was sent offsite for failure analysis. Results from the failure analysis were documented in a failure analysis report that was issued on November 17, 2023. The report stated that the EG-R showed no signs of degradation and that the sharp, rapid changes in the EG-R output that were observed on October 10, 2023, could not be replicated.

Efforts continued to determine the cause of the RCIC system low suction pressure trip. On December 19, 2023, a review of Plant Process Computer data revealed an anomaly to the Electric Governor - Magnetic Pickup (EG-M) output that occurred on August 28, 2023. Further investigation determined that on August 28, 2023, with Unit 1 in Mode 1 at 100%

power, work associated with a plant modification on the transient test system disturbed electrical connections for the EG-M. Transient Test is a high-speed data acquisition system which monitors and records analog and digital signals from various plant systems. The EG-M is the flow control system component that transmits an electrical signal to the EG-R proportional to the error signal developed between turbine speed demand and actual turbine speed. When the electrical connections were disturbed, EG-M output fell to 0% scale, indicating a loss of functionality of that component. This was unrecognized by station personnel at the time of occurrence.

Further investigation has also found that between September 11, 2023 and October 8, 2023, through various maintenance activities performed on the RCIC system, there is a potential for air to have been entrained in the lubricating oil system. Air entrainment in the lubricating oil control subsystem can cause governor speed and ramp control issues.

On December 19, 2023, at approximately 08:00 CST, it was determined that the work performed on August 28, 2023, affected functionality of the EG-M and potentially caused the RCIC system to be inoperable per Technical Specifications Section 3.5.3, "RCIC System." Condition A of TS 3.5.3 applies when the RCIC system is inoperable, with a Required Action to restore the RCIC system to operable status within 14 days. The Completion Time for this Required Action was not exceeded because the Applicability for TS 3.5.3 was exited prior to the expiration of the 14-day Completion Time to support the unit shutdown for a planned refueling outage.

However, this event is reportable in accordance with 1 O CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS 3.0.4.

Specifically, on October 9, 2023, at 05: 14 CST, during unit startup following the refueling outage, the Applicability for TS 3.5.3 was entered (i.e., Mode 2 with reactor steam dome pressure greater than 150 psig) with TS 3.5.3 not met because the RCIC system was inoperable. In addition, the unit was taken to Mode 1 on October 10, 2023.

CAUSE OF THE EVENT The probable cause of the RCIC Turbine Trip was either:

2. DOCKET NUMBER YEAR 00461 2023

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

01

1.

An electrical disturbance on the EG-M resulting from the Transient Test system modification that resulted in a loss of control output to the EG-R

2.

Outage activities (system cooldown and tagout blocking) that resulted in air formation in the RCIC lubricating oil system SAFETY CONSEQUENCES The condition described in this LER is reportable under 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by the plant's TS, due to entering the Applicability for TS 3.5.3 with the RCIC system inoperable. There were no actual safety consequences associated with the event. The RCIC system is not an engineered safety feature and no credit is taken in the safety analysis for its operation. Furthermore, RCIC does not meet the threshold for a failure of a single train safety system since it is not included in the safety analysis. High Pressure Core Spray (HPCS) system remained operable and therefore able to perform its safety function during the entire period that RCIC was inoperable. There were no actual demands for either RCIC or HPCS during this period.

CORRECTIVE ACTIONS The EG-R was replaced and the replacement unit calibrated in accordance with the Electric Power Research Institute (EPRI) Terry Turbine Maintenance Guide.

Update maintenance procedures to expand direction to fill and vent lube oil subsystem following all forms of maintenance Update maintenance procedures to expand direction to calibrate the EG-M following any work that could result in even minor changes to the RCIC governor electrical circuit Implement BWR Owners Group recommendation for a modification to insert a 7 second time delay on RCIC low suction pressure.

PREVIOUS OCCURENCES A review of Licensee Event Reports for the past five years identified no previous similar occurrences at Clinton Power Station. Page _3_ of _3_