05000498/LER-2025-001, Two Inoperable Emergency Core Cooling System Trains Resulting in Potential Loss of Safety Function
| ML25097A252 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 04/07/2025 |
| From: | Tomlinson J South Texas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 35717060 LER 2025-001-00 | |
| Download: ML25097A252 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4982025001R00 - NRC Website | |
text
April 7, 2025 NOC-AE-25004093 10 CFR 50.73 STI: 35717060 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 1 Docket No. STN 50-498 Licensee Event Report 2025-001-00 Two Inoperable Emergency Core Cooling System Trains Resulting in Potential Loss of Safety Function Pursuant to reporting requirements in 10 CFR 50.73(a)(2)(v)(D), STP Nuclear Operating Company (STPNOC) hereby submits the attached Licensee Event Report 2025-001-00 for South Texas Project Unit 1.
The event did not have an adverse effect on the health and safety of the public.
There are no commitments in this submittal.
If there are any questions regarding this submittal, please contact Chris Warren at (361) 972-7293 or me at (361) 972-8945.
Jason R. Tomlinson Site Vice President Attachment: Unit 1 LER 2025-001-00, Two Inoperable Emergency Core Cooling System Trains Resulting in Potential Loss of Safety Function cc:
Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511 Jason Tomlinson Digitally signed by Jason Tomlinson Date: 2025.04.07 13:07:13 -05'00'
NOC-AE-25004093 Attachment Attachment Unit 1 LER 2025-001-00 Unit 1 LER 2025-001-00, Two Inoperable Emergency Core Cooling System Resulting in Potential Loss of Safety Function
Abstract
South Texas Project Unit 1
00498 4
Two Inoperable Emergency Core Cooling System Trains Resulting in Potential Loss of Safety Function 02 19 2025 2025 001 00 04 07 2025 1
100
Chris Warren, Licensing Engineer 361-972-7293 BQ SMV A415 Y
06 19 2025 On February 19, 2025, while in MODE 1 and with Train 'C' Emergency Core Cooling System (ECCS) inoperable for planned maintenance, a leak was identified downstream of a normally closed sample valve of ECCS Train 'B'. Operations declared Train
'B' ECCS inoperable. STP Engineering evaluated the material condition and determined that the leak rate did not exceed administrative leak rate limits for the ECCS and recommended the system be considered OPERABLE with the leaks present.
Prior to STP Engineerings recommendation, Operations placed the High Head Safety Injection pump 1B handswitch in "Pull-to-Lock" as a compensatory measure for Technical Specification 3.7.7 action "e.1", rendering ECCS Train 'B' inoperable. This resulted in a condition that could have prevented fulfillment of safety functions of the ECCS system, where less than the required number of ECCS trains were OPERABLE from 0749 to 1405 on February 19, 2025. Operations declared ECCS Train 'B' OPERABLE following corrective maintenance and post-maintenance testing to stop a tee fitting leak. A causal investigation is scheduled to determine causes of the event and any additional corrective actions and will be provided in a supplement to this report.
Summary of the Event Timeline (Note: All times listed are Central Standard Time) 02/18/2025, 0400 - Operations declared Essential Cooling Water Pump 1C inoperable and non-functional for planned maintenance. This inoperability has a cascading effect resulting in the inoperability of ECCS and Control Room Envelope (CRE) HVAC Train 'C'.
02/18/2025, 1207 - Operations declared Essential Cooling Water Train 'C' OPERABLE following planned maintenance.
Essential Chilled Water System Train 'C' remains inoperable for planned maintenance, resulting in continued inoperability of ECCS and CRE HVAC Train 'C'.
02/19/2025, 0324 - Operations declared ECCC and CRE HVAC Train 'B' inoperable due to system leakage exceeding the amount allowed by surveillance procedures. A tee fitting downstream of valve SI-0126B, which is a normally closed sample valve, was found with a leak rate of approximately 3.73 milliliters per minute prior to starting High Head Safety Injection (HHSI) Pump 1B. The leak progressed to two pencil-sized streams from either side of the tee fitting once HHSI Pump 1B was started.
02/19/2025, 0324 - Entered Technical Specification 3.5.2, Action B: With less than two of the required subsystems OPERABLE, within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> restore at least two subsystems to OPERABLE status or apply the requirements of the Configuration Risk Management Program (CRMP), or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in HOT SHUTDOWN within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
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South Texas Project Unit 1 00498 2025 001 00
Timeline (Note: All times listed are Central Standard Time) 02/19/2025, 0749 - Operations placed HHSI Pump 1B Control Room Handswitch in "Pull-to-Lock" to comply with Technical Specification 3.7.7, Action "e.1" (Immediately initiate action to implement mitigating actions for one ore more inoperable Control Room Makeup and Cleanup Filtration Systems).
02/19/2025, 1036 - NRC HOO notified of non-emergency 8-hour reportable event per 10 CFR 50.72(b)(3)(v)(D).
02/19/2025, 1149 - Operations restored HHSI Pump 1B Control Room Handswitch to "Auto" in preparation for post maintenance testing on HHSI Pump 1B sample line leakage. Entered Technical Specification 3.0.6: Equipment removed from service or declared inoperable to comply with ACTIONS may be retured to service under administrative control solely to perform testing required to demonstrate its OPERABILITY or the OPERABILITY of other equipment. This is an exception to Limiting Condition For Operation (LCO) 3.0.2 for the system returned to service under administrative control to perform the testing required to demonstrate OPERABILITY.
02/19/2025, 1405 - All maintenance and Post-Maintenance Testing completed for ECCS Train 'B' HHSI Pump sample line.
ECCS and CRE declared OPERABLE.
02/25/2025, 1558 - Engineering recommends that the CRE be considered OPERABLE during the leak described in Condition Report 25-1707. Engineering Supervisor and the Operations Shift Manager agree with this assessment.
E. Method of Discovery
The event was self-revealing when STP determined that the identified leak should not have resulted in ECCS inoperability.
The event occurred due to a mitigating action performed to meet TS 3.7.7, Action "e.1," prior to receiving Engineering recommendation that the leak did not render the CRE inoperable.
II. Component Failures
A. Failure Mode, Mechanism, and Effects of the Failed Components A causal investigation is planned to determine the failure mode, mechanism, and effects of any failed components that contributed to this event.
B. Cause of Component Failure
A causal investigation is planned to determine the cause of component failures that contributed to this event.
C. Systems or Secondary Functions That Were Affected by Failure of Components with Multiple Functions No other systems or secondary functions were affected.
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- 3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.
052 050
D. Failed Component Information
System: High Pressure Safety Injection System {BQ}
Component: Discharge Sample Valve {SMV}
Manufacturer: Anderson Greenwood {A415}
Model: {G12A15S-3S3S-2R}
III. Analysis of Event
A. Safety System Responses that Occurred No safety system responses occurred because of this event.
B. Duration of Safety System Inoperability
ECCS Trains 'B' and 'C' were inoperable from 0749 to 1405 on 02/19/2025 for a total of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and 16 minutes.
C. Safety Consequences and Implications
The increase in risk due to the inoperability of ECCS Train 'B' represents a very small change in risk and is of very low
safety significance
This assessment considered ECCS Train 'B' inoperable from 0324, when the leak was identified, to 1405 on February 19, 2025, when the leak from the tee fitting downstream of valve SI-0126B was repaired. The assessment resulted in an Incremental Core Damage Probability and Incremental Large Early Release Probability within Region III of Regulatory Guide 1.174. The event did not result in any offsite release of radioactivity or increase of offsite dose rates and there were no personnel injuries or damage to any other safety-related equipment associated with this event. Therefore, there was no adverse effect on the health and safety of the public.
IV. Cause of the Event
A causal investigation is planned. A supplement to this report will include any causes of the event once the investigation is completed.
V. Corrective Actions
The immediate corrective action was completion of corrective maintenance of the leaking fitting and restoration of ECCS Train 'B' to OPERABLE status.
Corrective maintenance for valve SI-0126B is currently scheduled for March 2026, during the next scheduled Unit 1 refueling outage.
A supplement to this report will include any additional corrective actions identified in the casual investigation.
VI. Previous Similar Events
- 1. STP Condition Report 22-7700 - There is active leakage at valve SI-0126B pipe-to-tubing adapter. No drip rate is noted, but the decking below the leakage is wet.
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