05000282/LER-2025-003, Nuclear Instrumentation Over-Temperature Delta Temperature Function Inoperable Due to Defective Potentiometers
| ML25296A198 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 10/23/2025 |
| From: | Currier B Northern States Power Company, Minnesota, Xcel Energy |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-PI-25-042 LER 2025-003-00 | |
| Download: ML25296A198 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2822025003R00 - NRC Website | |
text
Xcel Energy 1717 Wakonade Drive Welch, MN 55089 October 23, 2025 L-PI-25-042 10CFR 50.73 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Prairie Island Nuclear Generating Plant, Unit 1 Docket No. 50-282 Renewed Facility Operating License No. DPR-42 Prairie Island Nuclear Generating Plant Unit 1 Licensee Event Report 2025-003-00 Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter NSPM), hereby submits Licensee Event Report (LER) 50-282/2025-003-00 per 10 CFR 50.73(a)(2)(i)(B).
Summary of Commitments This letter makes no new commitments and no revisions to existing commitments.
Bryan Qyrrier Plant Manager, Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc: Administrator, Region III, USNRC Project Manager, Prairie Island, USNRC Resident Inspector, Prairie Island, USNRC State of Minnesota Connected
- Committed
- Trustworthy
- Safe
ENCLOSURE PRAIRIE ISLAND NUCLEAR GENERATING PLANT LICENSEE EVENT REPORT 50-282/2025-003-00 3 pages follow
Abstract
On July 22, 2025, at 12:23, Prairie Island Nuclear Generating Plant (PINGP) Unit 1, entered Mode 2, which is the Mode of Applicability for the Technical Specification (TS) Limiting Condition for Operation (LCO) 3.3.1 Over-temperature Delta Temperature (OTdT) function. On July 23, 2025, the upper Power Range Nuclear Instrumentation 1NI-41C, red channel, was found to be providing incorrect indications compared to the other three channels in the system. Troubleshooting revealed the red channel was inoperable due to defective potentiometers R304 and R310. This impacted multiple downstream functions, including Quadrant Power Tilt Ratio, Axial Flux Difference, and OTdT penalty calculations. The condition of inoperability of the red channel was not known upon entering Mode 2, and the required TS LCO actions to place this channel in trip were not taken. The condition is reportable under 10 CFR 50.73(a)(2)(i)(B). On July 30, 2025, both potentiometers R304 and R310 were replaced, and the red channel OTdT function of 1NI-41C was declared operable.
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027
Plant Operating Conditions Prior to the Event:
Unit 1 - Mode 2, 0 Percent Power.
EVENT DESCRIPTION
On July 22, 2025, at 12:23, Prairie Island Nuclear Generating Plant (PINGP) Unit 1 entered Mode 2 while restarting the reactor following a recent trip that was reported under LER 2025-001-00, Accession number ML25251A231. On July 23, 2025, the upper Power Range Nuclear Instrumentation 1NI-41C, Percentage Differential Flux Rate Channel 1N41, red channel, displayed a value inconsistent with the other three Power Range (PR) channels [IG], 1N42 - white, 1N43 - blue, and 1N44 - yellow. During troubleshooting of 1NI-41C, it was discovered that the Over-temperature Delta Temperature (OTdT) function associated with the red channel was inoperable due to the incorrect indication. OTdT function is part of the Reactor trip logic within the Reactor Protection System (RPS)[JD]. Further investigation traced the problem to inside 1N41B, Power Range 1N41 Chassis B, specifically to the coarse gain potentiometer R304 (Westinghouse 2384A53H09) and the fine gain potentiometer R310 (Westinghouse 2384A53H03), which impact multiple downstream functions, including:
- - Quadrant Power Tilt Ratio (QPTR) calculation on Emergency Response Computer System (ERCS).
- - Axial Flux Difference (AFD) board indication and ERCS calculation.
- - OTdT penalty calculation.
Dates and Approximate Times of Occurrences
- July 22, 2025 at 12:23 [Event Date]: Unit 1 entered Mode 2 for startup. This was later determined to be the time when the inoperability of the red channel began.
- July 23, 2025 at 00:23: Unit 1 should have entered Mode 3 based on the red channel OTdT function being inoperable 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> prior.
- July 23, 2025 at 12:24: Issue formally discovered; Technical Specification (TS) Limiting Condition for Operation (LCO) 3.3.1 Conditions A and E" entered.
- July 23, 2025 at 18:24: Risk Informed Completion Time was implemented for TS LCO 3.3.1 required action E.1.
- July 30, 2025: Potentiometers R304 and R310 were replaced, restoring red channel OTdT function of 1NI-41C.
- July 30, 2025 at 02:28: TS LCO 3.3.1 Conditions A and E were exited.
- August 26, 2025 [Discovery Date]: A Past Operability Review was completed and concluded that the condition of inoperability of the OTdT function associated with the red channel began when Unit 1 entered Mode 2, on July 22, 2025 at 12:23.
The condition is reportable under 10 CFR 50.73(a)(2)(i)(B) as an Operation or Condition Prohibited by Technical Specifications. Specifically:
- - With one channel inoperable, entry into TS LCO 3.3.1 Condition E is required.
- - TS LCO 3.3.1 Condition E requires placing the channel in TRIP within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
- - If Condition E completion is not met, TS LCO 3.3.1 Condition W requires the plant to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
- - The inoperability of this channel existed for greater than the 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> allowed by TS LCO 3.3.1 Conditions E and W.
- 1. FACILITY NAME [3
050
- 2. DOCKET NUMBER
- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
Prairie Island Nuclear Generating Plant, Unit 1 052
- - 282 2025 003 00 COMPONENT INFORMATION The OTdT functions are as follows:
- 1. Trip the reactor to protect the core against Departure from Nucleate Boiling (DNB) for any combination of power, pressure, temperature, and axial core power distribution.
- 2. Provide a turbine runback at a specified level below the trip point. Rod withdrawal block accompanies the turbine runback, and the runback follows a preprogrammed function until core Delta Temperature is less than the runback limit.
ASSESSMENT OF SAFETY CONSEQUENCES
There were no actual or potential nuclear, radiological, industrial, or environmental impacts. The RPS remained capable of performing its safety function due to redundancy in the 2-out-of-4 trip logic that ensured protection was maintained. The abnormal indication was only noted on the OTdT function of the red channel, the other 3 OTdT channels tracked together.
This is not a condition that could have prevented fulfillment of a safety function. The health and safety of the public and site personnel were not impacted during this event.
There were no components or systems that were inoperable at the start of the event that contributed to the event.
CAUSE OF THE EVENT
The direct cause was defective potentiometers R304 and R310.
CORRECTIVE ACTIONS
Potentiometers R304 and R310 were replaced on July 30, 2025.
An extent of condition review concluded that other channels do not exhibit similar deficiencies.
PREVIOUS SIMILAR EVENTS
No previous similar events have been reported to the NRC by PINGP in the prior 3 years.
ADDITIONAL INFORMATION
All times are in Central Daylight Time.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX],Page 3 of 3