05000461/LER-2025-001, MSIV B21-F022D Limit Switch Failed Testing
| ML25321A460 | |
| Person / Time | |
|---|---|
| Site: | Clinton  |
| Issue date: | 11/12/2025 |
| From: | Alexander Sterio Constellation Energy Generation |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| U-604856 LER 2025-001-00 | |
| Download: ML25321A460 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| 4612025001R00 - NRC Website | |
text
Constellation Clinton Power Station 8401 Power Road Clinton IL 61727 U-604856 November 12, 2025 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Clinton Power Station, Unit 1 Facility Operating License No. NPF-62 NRC Docket No. 50-461 Subject: Licensee Event Report 2025-001-00 10 CFR 50.73 Enclosed is Licensee Event Report (LER) 2025-001-00: MSIV B21-F022D Limit Switch Failed Testing. 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. Derek Hillinger, Regulatory Assurance Manager, at (217) 937-2800.
Respectfully, OL....r~a~
Alexander Sterio Acting Site Vice President Clinton Power Station
Attachment:
Licensee Event Report 2025-001-00 cc:
Regional Administrator - Region Ill NRC Senior Resident Inspector - Clinton Power Station Office of Nuclear Facility Safety - Illinois Emergency Management Agency
Abstract
During performance of "RPS Main Steam line Isolation 821 -F022D Channel Calibration", Main Steam Line 'D' Inboard Isolation Valve 1821 -F022D failed to generate a Reactor Protection System (RPS) trip when it was closed, indicating that its Full Open Limit Switch (LS) failed. This was a failure of Surveillance Requirement (SR) 3.3.1.1.13, "Perform CHANNEL CALIBRATION" for Technical Specification (TS) 3.3.1.1 RPS Instrumentation, function 6 (MS IV-Closure). This TS function was not required in the current Mode of Operation when it was discovered, but it was a Condition Prohibited by Technical Specifications.
CONDITION PRIOR TO THE EVENT
- 2. DOCKET NUMBER YEAR 00461 2025
- 3. LER NUMBER SEQUENTIAL NUMBER 001 REV NO.
00 Unit: 1 Mode: 5 Event Date: September 13, 2025 Mode Name: Refueling Event Time: 03:30 CST Reactor Power: 0
EVENT DESCRIPTION
On March 10, 2024, during performance of Main Steam Line Isolation Valve (MSIV) Channel Functional testing (SR 3.3.1.1.9) for TS Table 3.3.1.1-1 function 6 MSIV-Closure, the Main Steam Line 'D' Inboard Isolation Valve 1B21-F022D Limit Switch #5 (LS-5), the Division 4 channel, failed to issue a trip signal to the Reactor Protection System (RPS).
On March 30, 2024, a temporary configuration change was implemented to connect Limit Switch #3 (LS-3), a spare limit switch installed on the 1B21-F022D MSIV, into the Division 4 RPS logic in lieu of the normally connected LS-5. After connecting LS-3 into the Division 4 RPS logic, post maintenance testing including performance of a channel functional test was conducted to restore the 1 B21-F022D MS IV-Closure TS function to an operable status.
On February 13, 2025, the NRC Comprehensive Engineering Team Inspection team challenged that post maintenance testing methodology chosen by the station for declaring 1 B21-F022D MSIV-Closure TS function operable credit for performance of a channel calibration relied on historical data and engineering changes with unquantified uncertainties instead of the prescribed physical calibration procedure. They raised concerns that this approach may not fully satisfy the technical specification requirements for performance of a channel calibration after modifying the reactor protection system input. As a result, this issue was placed in the Clinton Corrective Action Program (CAP).
As part of the shift review and operability determination process, the Shift Manager invoked SR 3.0.3 for this condition. SR 3.0.3 allowed for continued operability based on a risk evaluation and management plan and the reasonable expectation that the channel calibration surveillance will be met when performed at the first reasonable opportunity. The next reasonable opportunity was determined to be during the fall 2025 Refueling Outage (C1 R22) prior to removal of temporary configuration change and repair/replacement of 1 B21-F022D LS-5.
On September 13, 2025, when performing Main Steam line Isolation Valve B21-F022D Channel Calibration (SR 3.3.1.1.13), 1 B21-F022D LS-3 failed to change state. No as-found measurement of the setpoint was obtained. It is concluded that the as-found calibration failed, resulting in failure to meet SR 3.3.1.13. Per guidance of NUREG-1022, the switch must be considered inoperable longer than allowed by LCO 3.3.1.1 Condition G and is reportable in accordance with 10 CFR 50. 73(a)(2)(i)(B).
CAUSE OF THE EVENT
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Additional failure analysis is being performed to determine the cause of the limit switch failure. A supplemental LER will be sent when the analysis is completed.
SAFETY CONSEQUENCES
- 2. DOCKET NUMBER
- 3. LER NUMBER SEQUENTIAL REV YEAR NUMBER NO.
00461 2025 -
001 00 There were no safety consequences as a result of this event. The failure of LS-3 to actuate only affected the 1 B21-F022D MSIV-Closure TS function. MSIV closure signals are initiated from position switches located on each of the eight MS IVs.
Each MSIV has one position switch. The logic for the Main Steam Isolation Valve-Closure Function is arranged such that either the inboard or outboard valve on two or more of the Main Steam Lines (MS Ls) must close in order for a scram to occur. During the time that 1 B21-F022D MS IV-Closure TS function was inoperable, all seven other position switches were operable and would have fulfilled the specified safety function of ensuring a scram occurs prior in anticipation of the complete loss of the normal heat sink and subsequent over-pressurization transient.
CORRECTIVE ACTIONS
The temporary configuration change was removed, normal plant configuration restored with LS-5 providing the 1 B21 -
F022D MSIV-Closure TS function, and Main Steam line Isolation Valve B21-F022D Channel Calibration procedure (SR 3.3.1.1.13) performed on the as-left configuration.
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_