05000313/LER-2025-001, Automatic Reactor Trip Due to Fault on the B Main Phase Transformer
| ML25324A399 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 11/20/2025 |
| From: | Toben J Entergy Operations |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 1CAN2025-01000 LER 2025-001-00 | |
| Download: ML25324A399 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3132025001R00 - NRC Website | |
text
~ entergy 1CAN2025-01000 November 20, 2025 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Joshua J. Toben Manager Regulatory & Emergency Planning 4 79-858-3135 10 CFR 50.73
Subject:
Licensee Event Report 50-313/2025-001-00, Automatic Reactor Trip Due to Fault on the 'B' Main Phase Transformer Arkansas Nuclear One - Unit 1 NRC Docket No. 50-313 Renewed Facility Operating License No. DPR-51 Entergy Operations, Inc. (Entergy) submits the enclosed Licensee Event Report (LEA) 50-313/2025-001-00 for Arkansas Nuclear One, Unit 1. This event is reportable in accordance with 1 O CFR 50.73(a)(2)(iv)(A) as any event or condition that resulted in manual or automatic actuation of the Reactor Protection System (RPS). The LEA describes an automatic reactor trip due to a fault on the 'B' Main Phase Transformer, which occurred on September 24, 2025.
This letter contains no new commitments.
Should you have any questions concerning this issue, please contact Joshua J. Toben, Manager, Regulatory & Emergency Planning, at 479-858-3135.
Enclosure: Licensee Event Report 50-313/2025-001-00 Entergy Operations, Inc., 1448 SR 333, Russellville, AR 72802
1CAN2025-01000 Page 2 of 2 cc:
NRC Region IV Regional Administrator NRC Senior Resident Inspector - Arkansas Nuclear One NRC Project Manager - Arkansas Nuclear One Designated State Official - Arkansas
Enclosure 1CAN2025-01000 Licensee Event Report 50-313/2025-001-00
Abstract
On September 24, 2025 at 134 7 Central Daylight Time (CDT), Arkansas Nuclear One, Unit 1 (ANO-1) automatically tripped on Reactor Protection System (RPS) actuation due to a fault on the 'B' Main Phase Transformer.
The cause of this event was the actuation of the Transformer Deluge System. The Transformer Deluge System was actuated for approximately six minutes before the main generator output breakers opened and RPS tripped the reactor. Subsequently, a Transformer Deluge System manual actuation pull station was found open with the tamper seal broken which had caused the deluge valve to open. Following the reactor trip, the 'B' Main Phase Transformer High Voltage Bushing was cosmetically reconditioned, inspected, tested and found to be satisfactory and the Surge Arrestor was replaced. The Transformer Deluge System manual actuation pull station tamper seal was replaced prior to plant startup on September 29, 2025.
There were no consequences to the general safety of the public, nuclear safety, industrial safety or radiological safety. There was not a radiological release resulting from this event.
PLANT STATUS 050 052
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00313 NUMBER NO.
I YEAR SEQUENTIAL REV a-I 001 1-G Arkansas Nuclear One, Unit 1 (ANO-1) was operating at 89.7 percent Rated Thermal Power (RTP) due to planned coast down in preparation for starting refueling outage 1 R32.
EVENT DESCRIPTION
On September 24, 2025 at 1347 Central Daylight Time (CDT), ANO-1 automatically tripped on Reactor Protection System (RPS) [JC] actuation due to a fault on the 'B' Main Phase Transformer [XFMR]. The Reactor Trip Emergency Operating Procedure (EOP) was entered, all control rods fully inserted and the plant was stabilized in MODE 3 (Hot Standby). There was not a radiological release resulting from this event.
This event was reported under 10 CFR 50.72(b )(2)(iv)(B) for RPS actuation and 10 CFR 50. 72(b )(3)(iv)(A) for the Specified System Actuation.
This report is made pursuant to 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of the RPS.
SAFETY ASSESSMENT
RPS tripped the reactor as designed on a "Main Turbine Trip" signal when the 'B' Main Phase Transformer fault occurred causing the main generator output breakers to open. Following the reactor trip, the Main Feedwater System (MFW) [SJ]
supplied feedwater to both Steam Generators (SG) [AE]. This allowed for the MFW System to provide normal post trip heat removal via the SGs and did not result in consequences to safety of the general public, nuclear safety, industrial safety or radiological safety. The safety significance of this event is determined to be low.
EVENT CAUSE(S)
On September 24, 2025 at 1341 CDT the ANO-1 Control Room received a Fire Alarm and Firewater Flow Alarm for the
'B' Main Phase Transformer due to an actuation of the Transformer Deluge System. Both Fire Water Pumps automatically started as designed to meet Fire Water System flow demand. At 1344 CDT the ANO-1 Control Room received a 'B' Main Phase Transformer trouble alarm. Observation and security camera footage confirmed that the 'B' Main Phase Transformer experienced an arc flash around the High Voltage Bushing and Surge Arrestor. At 1347 CDT both main output breakers opened resulting in a Main Turbine trip and subsequent reactor trip. Later it was identified that a Transformer Deluge System manual actuation pull station had opened due to a failure of the external tamper seal and an improperly sized internal glass break rod. The tamper seal failure was attributed to degradation due to sun and weather exposure over time.
In addition to the direct cause, Transformer Deluge System actuation, additional casual factors of this reactor trip event include a failure to maintain design configuration and ineffective corrective actions.
CORRECTIVE ACTIONS
050 052
- 2. DOCKET NUMBER
- 3. LER NUMBER I
00313 NUMBER NO.
I YEAR SEQUENTIAL REV a-I 001 1-G This event was entered into the station's corrective action program.
Following the automatic reactor trip, ANO-1 remained in MODE 3 until the 'B' Main Phase Transformer High Voltage Bushing was cosmetically reconditioned, inspected, tested and found to be satisfactory and the Surge Arrestor was replaced.
Corrective actions were taken during refueling outage 1 R32 to replace the damaged 'B' Main Phase Transformer High Voltage Bushing. Additionally, the ANO-1 Transformer Deluge System manual actuation pull stations were replaced with an improved design.
Corrective actions have been issued to address the failure to maintain design configuration. The ineffective corrective actions causal factor was determined to be a latent issue that has been corrected.
PREVIOUS SIMILAR EVENTS
A review of previous ANO operating experience identified several similar issues related to Fire Water System manual actuation pull station deficiencies. Specifically, a similar condition occurred on September 23, 2025 at ANO-2 where the
'A' Main Phase Transformer Deluge System actuated due to a failed tamper seal on the manual actuation pull station.
NOTE: Energy Industry Identification System (EIIS) codes are identified in the text as [XX). Page 3
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