05000443/LER-2025-001, Erratic Potentiometer Caused Upper Detector Flux Deviation Which Exceeded Technical Specification Allowed Outage Time
| ML25258A204 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 09/15/2025 |
| From: | Mack K NextEra Energy Seabrook |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| LER 2025-01-00 | |
| Download: ML25258A204 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4432025001R00 - NRC Website | |
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ENERGY~
SEABROOK U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 Re:
Seabrook Station Docket No. 50-443 Renewed Facility Operating License No. NPF-86 Reportable Event: 2025-01-00 Date of Event: May 31, 2025 September 15, 2025 L-2025-156 10 CFR 50.73 Erratic Potentiometer Caused Upper Detector Flux Deviation Which Exceeded Technical Specification Allowed Outage Time The attached Licensee Event Report (LER), Seabrook Station LER 2025-01-00, Erratic Potentiometer Caused Upper Detector Flux Deviation Which Exceeded Technical Specification Allowed Outage Time is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications.
This event did not have an adverse effect on the health and safety of the public.
Should you have any questions regarding this submission, please contact Maribel Valdez, Fleet Licensing Manager, at 561-904-5164.
This letter contains no new or modified regulatory commitments.
Sincerely, Director, Licensing and Regulatory Compliance Attachment cc:
USNRC, Regional Administrator, Region I USNRC, Project Manager, Seabrook Station Nuclear Plant USNRC, Senior Resident Inspector, Seabrook Station Nuclear Plant NextEra Energy Seabrook, LLC PO Box 300, Seabrook, NH 03874
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LICENSEE EVENT REPORT (LER)
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- 1. Facility Name 00 050
- 2. Docket Number
- 3. Page Seabrook Station D
052 00443 1 OF 3
- 4. Title Erratic Potentiometer Caused Upper Detector Flux Deviation Which Exceeded Technical Specification Allowed Outage Time
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Facilities Involved Month Day Year Year Sequential Revision Month Day Year Facility Name D
Docket Number Number No.
050 05 31 2025 2025 -
01 -
00 9
15 2025 Facility Name D
Docket Number 052
- 9. Operating Mode 110. Power Level 1
100
)
Abstract
On 5/31/2025, while in Mode 1, Control Room Operators received an Upper Flux Deviation alarm associated with Nuclear Instrumentation (NI) power range Channel 1. Initial troubleshooting incorrectly concluded that the Flux Deviation Alarm was due to drift in the isolation amplifier, and did not impact the safety related function. The isolation amplifier was replaced. However, the Upper Flux Deviation alarm remained intermittent and recurred 4 more times between 6/6/2025 and 7/15/2025. On 7/15/2025 troubleshooting determined that an erratic potentiometer caused upper detector signal drift. This erratic potentiometer affected the safety related Overtemperature Delta Temperature (OTDT) protection channel setpoint. Given the intermittent Upper Flux Deviation alarm there is firm evidence that Channel 1 OTDT function was inoperable, and not bypassed as required by Technical Specifications (TS), from 5/31/2025 to 7/15/2025. Corrective actions included wiping the potentiometer per vendor recommendations and restoring operability of the Channel 1 OTDT function. Additionally, On 6/19/2025, the Channel 4 OTDT function was declared inoperable due to a failed circuit card in Channel 4 protection cabinet. The failed circuit card was replaced, and Channel 4 returned to service. This resulted in less than the required 3 channels operable for 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />. This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by TS.
Description of Event
I
- 2. DOCKET NUMBER
- 3. LER NUMBER IB SEQUENTIAL REV NUMBER NO.
00443
- - I I-~
01 On 5/31/2025, while in Mode 1, Control Room Operators received an Upper Detection Flux Deviation alarm associated with the NI power range channel 1 [EIIS: JC]. Channel 1 is one of four identical power range channels. The power range detectors are uncompensated ionization chambers that consist of two separate neutron sensitive sections, designated A and B. The A section monitors the upper half of the core; the B section monitors the lower half of the core. Each section transmits a current signal directly to the Nuclear Instrumentation System (NIS) panel in the Main Control Room. The signals from the A and B detectors are processed both individually and together in the NIS panel. The detectors signal is combined to calculate the total NI Power. The A and B signals also individually provide a bias input to the OTDT setpoints. Isolated outputs from the NIS cabinets provide input to the station computer system. The Flux Deviation Alarms are generated by the station computer system. Initial troubleshooting concluded that the Flux Deviation Alarm was the result of drift in the isolation amplifier associated with the A detector, to the station computer. The isolation amplifier was replaced, however the Upper Flux Deviation alarm occurred on 6/6/2025. The alarm re-occurred on 6/9/2025, 6/18/2025 and 7/8/2025.
On 7/15/2025, troubleshooting determined that the cause of the upper detector signal drift was an erratic gain potentiometer, located upstream of the isolation amplifier to the station computer. At this point it was concluded that the erratic gain potentiometer affected the upper flux input to the OTDT protection channel. The impact of the error was sufficient to conclude that the Channel 1 OTDT function [EIIS: JC] was in fact inoperable from 5/31/2025, through 7/15/2025, which does not meet the requirements of Seabrook TS Table 3.3-1 Action 6A. Therefore, 7/15/2025 is the discovery date when the Condition Prohibited by TS became known.
Additionally, on 6/19/2025, a separate failure of Channel 4 protection cabinet resulted in the Channel 4 OTDT function becoming inoperable from 6/19/2025, through 6/20/2025. The duration of this condition was approximately 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />.
Therefore, Channel 4 was inoperable while Channel 1 should have been declared inoperable. This resulted in less than the required 3 channels operable. This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by TS.
This event did not result in a Safety System Functional Failure.
Cause of Event
The cause of the event was an erratic gain potentiometer in the Channel 1 Upper Flux signal, [EIIS: JC], which affects the input to the Channel 1 OTDT set point calculation. This was not immediately recognized as the cause of the channel drift; therefore, it was not recognized that the input to the OTDT function was adversely impacted by the condition. The erratic potentiometer was not corrected, nor was the channel placed in the trip condition prior to expiration of the allowed outage time in accordance with TS. During the period Channel 1 was inoperable, the Channel 4 protection cabinet experienced a power supply failure, which resulted in a failure of a circuit card in the Channel 4 OTDT protection circuit. This resulted in two channels being inoperable at the same time, which is a condition prohibited by TS.
Safety Significance
- 2. DOCKET NUMBER
- 3. LER NUMBER I ""
[ 202s I-I SEQUENTIAL REV NUMBER NO.
01 1-0 I
00443 The safety significance was low as redundant protection channels remained available. No loss of safety function occurred. Specifically, when Channel 1 was out of service, the other 3 protection channels were available to perform their safety function. During the brief period when both the Channel 1 and Channel 4 OTDT channels were not operable, Channels 2 and 3 were available to complete the fulfillment of the safety function if required.
This event did not result in a Safety System Functional Failure.
This event is being reported pursuant to 10 CFR 50. 73(a)(2)(i)(B), Operation or Condition Prohibited by TS.
Corrective Action
Immediate corrective actions included wiping of the erratic potentiometer in accordance with vendor recommendations, and restoration of the Channel 1 OTDT function to operable status. The failed circuit card in the Channel 4 OTDT circuit was replaced and the channel was returned to service.
An extent of condition was performed on the remaining NI channels. The remaining potentiometers on the other channels were tested satisfactorily. The knowledge and Operating Experience for this event will be captured in site procedures.
Similar Events
A review of events over the past 5 years before 5/31/2025 did not identify any similar events with the same cause as this event. Page 3
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