05000282/LER-2025-001, Reactor Trip and auto-start Actuation of Auxiliary Feedwater Due to Bus 11 Undervoltage Relay Issues
| ML25251A231 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 09/08/2025 |
| From: | Currier B Northern States Power Company, Minnesota, Xcel Energy |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-Pl-25-037 LER 2025-001-00 | |
| Download: ML25251A231 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2822025001R00 - NRC Website | |
text
(},, Xcel Energy*
September 08, 2025 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 1717 Wakonade Drive Welch, MN 55089 L-Pl-25-037 10 CFR 50.73 Prairie Island Nuclear Generating Plant Unit 1 Licensee Event Report 2025-001-00 Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), hereby submits Licensee Event Report (LER) 50-282/2025-001-00 per 10 CFR 50.73(a)(2)(iv)(A).
Summary of Commitments This :r,_
r mmitments and no revisions to existing commitments.
Bryan c-OTr Plant MaUer, Prairie Island Nuclear Generating Plant Northern States Power Company - Minnesota Enclosure cc:
Administrator, Region Ill, 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 50-282/2025-001-00 3 pages follow
Abstract
At 09:59 on July 16, 2025, with Prairie Island Nuclear Generating Plant Unit 1 operating at 100 percent power, Unit 1 experienced an automatic actuation of the Reactor Protection System (RPS) with a trip of the Unit 1 reactor. The trip occurred due to an equipment issue while performing a surveillance procedure to test the operability of the 4kv Bus 11 /12 undervoltage and underfrequency relays. Operators responded to the trip and stabilized the plant in accordance with approved procedures and safely placed the plant in Mode 3. All systems responded normally post trip.
Following the Unit 1 trip, the undervoltage relays 27B/B11 and 27A/B12 were replaced, and the new relays were tested and found satisfactory. The cause of the event is currently under evaluation and will be provided later, along with corrective actions, as a supplement to this LER.
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) due to actuation of both RPS and Pressurized Water Reactor Auxiliary Feedwater System.
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 04/30/2027
EVENT DESCRIPTION
At 09:59 on July 16, 2025, Prairie Island Nuclear Generating Plant (PINGP) Unit 1 was in Mode 1 and operating at 100 percent power. The Unit 1 reactor automatically tripped due to an equipment issue while performing Surveillance Procedure (SP) 1857, 4kv Bus 11/12 undervoltage and underfrequency relay test. This trip occurred due to multiple undervoltage relay actuations when the knife switch 97A/B11 was opened to de-energize and actuate 27A/B11, BUS 11 UNDERVOLTAGE RELAY TRAIN A. At this same time, 27B/B11, BUS 11 UNDERVOLTAGE RELAY TRAIN B, and 27A/
B12, BUS 12 UNDERVOLTAGE RELAY TRAIN A, unexpectedly actuated as well. This resulted in an automatic actuation of the Reactor Protection System (RPS) [JD] and Pressurized Water Reactor (PWR) Auxiliary Feedwater (AFW) System [BA],
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) due to both RPS and PWR AFW actuation.
Unit 2 was not affected during this event and remained at 100 percent power.
EVENT ANALYSIS
The performance of SP 1857, Step 8.1.5 removes Potential Transformer input from 27A/B11, 81A/B11, BUS 11 UNDERFREQUENCY RELAY TRAIN A, [81] and 27R/B11, BUS 11 UNDERVOLTAGE RELAY, [27] by opening knife switch 97A/B11. The expected response from 27A/B11 would de-energize relays 1-27A/B11-XA, BUS 11 VOLTAGE SWITCH TRAIN A TRIP RELAY, and 1-27A/B11-XB, BUS 11 VOLTAGE SWITCH TRAIN B TRIP RELAY, in the RPS.
These relays drive computer point V0320D which can be observed on the Emergency Response Computer System (ERCS) and is a input to Bus 11 and Bus 12 Undervoltage Reactor Trip. The circuit logic requires one channel from Bus 11 and one channel from Bus 12 to detect an undervoltage condition to initiate a reactor trip. SP 1857 is written such that only one channel is tested at a time to prevent a reactor trip as the scram signal is generated from a two-out-of-four (2/4) coincidence logic.
At 09:59 on July 16, 2025, RPS relays for 27A/B11 de-energized from opening the 97A/B11 switch and 27B/B11 and 27A/
B12 de-energized momentarily making up the reactor trip logic.
ASSESSMENT OF SAFETY CONSEQUENCES
All systems responded normally post trip. Decay heat was removed by discharging steam using the Condenser steam dump system [SG] and the Main Feedwater System [SJ]. Unit 2 was not affected.
The health and safety of the public and site personnel were not impacted during this event.
CAUSE OF THE EVENT
The cause of the Unit 1 reactor trip was due to multiple undervoltage relay actuations during performance of SP 1857.
The direct cause of this event is being investigated and will be provided in the supplement to this LER.
CORRECTIVE ACTIONS
The station is currently investigating the cause(s) of the event. Additional corrective actions will be provided in the supplement to this LER.
Following the trip of Unit 1, PINGP conducted an extensive troubleshooting plan to confirm the cause of the reactor trip and the failure.
The undervoltage relays 27B/B11 and 27A/B12 were replaced, and the new relays were tested and found satisfactory.
Unit 1 was placed back online at 00:59 on July 23, 2025.
PREVIOUS SIMILAR EVENTS
There were no previous similar events in the past 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
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