05000498/LER-2024-007-01, Supplement to Condition Prohibited by Technical Specifications

Supplement to Condition Prohibited by Technical Specifications

ML25163A305
Person / Time
Site:	South Texas
Issue date:	06/12/2025
From:	Tomlinson J South Texas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-25004111 LER 2024-007-01
Download: ML25163A305 (1)

LER-2024-007, Supplement to Condition Prohibited by Technical Specifications

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4982024007R01 - NRC Website

text

June 12, 2025 NOC-AE-25004111 10 CFR 50.73 STI: 35743909 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 2024-007-01 Supplement to Condition Prohibited by Technical Specifications

Reference:

Letter; J. Tomlinson (STP) to Document Control Desk (NRC); Licensee Event Report 2024-007-00 Condition Prohibited by Technical Specification; January 23, 2025; (NOC-AE-25004085) (ML25023A280).

On January 23, 2025, STP Nuclear Operating Company (STPNOC) submitted the referenced Licensee Event Report. This letter is a supplement to the report to provide updates as a result of the causal evaluation. The updated information is denoted by revision bars located in the right-hand margin. The report is submitted in accordance with 10 CFR 50.73.

The event did not have an adverse effect on the health and safety of the public.

There are no commitments in this letter.

If there are any questions regarding this submittal, please contact Zachary Dibbern at 361-972-4336 or me at 361-972-8945.

Jason R. Tomlinson Site Vice President Enclosure: Unit 1 LER 2024-007-01, Supplement to Condition Prohibited by Technical Specifications 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: 2025.06.12 16:40:51

- 05'00'

NOC-AE-25004111 Enclosure Enclosure Unit 1 LER 2024-007-01 Unit 1 LER 2024-007-01, Supplement to Condition Prohibited by Technical Specifications

Abstract

South Texas Unit 1 n

00498 5

Condition Prohibited by Technical Specifications 11 27 2024 2024 007 01 06 12 2025 1

100 4

Zachary Dibbern, Licensing Engineer 361-972-4336 D

EB FUB G080 Yes 4

It was discovered on November 27, 2024, that all degraded voltage relays were inoperable for one safety-related bus, E1B, and would not have had the required protection afforded to it. The degraded voltage protection for E1B was inoperable for greater than the Technical Specification Allowed Outage Time. The cause was a pull-out fuse block that was not returned to the proper position following maintenance during the 1RE25 refueling outage in Fall 2024 due to (1) inadequate controls in the preventative maintenance program to ensure the work package had sufficient post-maintenance test requirements and that the implementing procedure had the necessary detail, and (2) the standard for recognition of conditions prohibited by Technical Specifications was not enforced by Operations Management. Corrective actions include: (1) restoration of the fuse block and confirming proper alignment of equipment, (2) revision of work process and maintenance procedures to ensure adequate detail is included in post-maintenance procedures to match the ability of new performers, and (3) implementation of a change management plan to reinforce consistent application of operator fundamentals.

A. Reportable Event Classification

This event is reportable per 10 CFR 50.73(a)(2)(i)(B) as a condition that was prohibited by the Plants Technical Specifications (TS). The following Technical Specification Limiting Conditions for Operation (LCOs) were not met, and the required TS Action was not met:

TS 3.3.2, Engineered Safety Features Actuation System Instrumentation, LCO states:

The Engineered Safety Features Actuation System (ESFAS) instruments channels and interlocks shown in Table 3.3-3 shall be OPERABLE with their Trip Setpoints set consistent with the values shown in the Trip Setpoint column of table 3.3-4 and with RESPONSE TIMES as shown in Chapter 16 in the UFSAR.

TS 3.3.2 Action c. states:

With an ESFAS instrumentation channel or interlock inoperable, take the ACTION shown in Table 3.3-3.

TS 3.3.2, Table 3.3-3, Functional Items 8.b and 8.c, Action 20A.b. states:

With the number of OPERABLE channels more than one less than the Total Number of Channels, 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 Operable status for functions with three channels and restore at least 3 channels to OPERABLE status for functions that have four channels, or apply the requirements, of the 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 be in at least 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 />, and be in COLD SHUTDOWN within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. This action is not required for the surveillance testing provision in the note to Action 20A.a.

The 4160 Volt AC Class 1E Power System pull-out fuse block for degraded voltage relays was left in the off position when Class 1E SWGR E1B was returned to service. This resulted in all four channels for the degraded voltage relays being inoperable longer than the allowed outage time without taking the required actions. Consequently, Unit 1 was in a condition prohibited by Technical Specifications.

B. Plant Operating Conditions Prior to Event

Prior to the event on November 27, 2024, Unit 1 was in MODE 1 at 100% power.

C. Status of Structures, Systems, and Components That Were Inoperable at the Start of the Event and That Contributed to the Event There were no inoperable Structures, Systems, or Components at the start of the event that contributed to the event.

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Summary of the Event Timeline (Note: All times are in Daylight Savings Time) 10/12/24 (0311) - Control Power to Monitoring Relay 27-1, which is in parallel with the four degraded voltage relays, is de-energized for maintenance during the 1RE25 refueling outage. At this point the four degraded voltage relays would have been inoperable.

11/05/24 (1133) - Unit 1 enters Mode 4.

11/27/24 (1526) - E1B 4160 VAC bus cubicle 2 degraded voltage relays flags found dropped and would not reset.

Investigation of the cause began.

11/27/24 (1634) - The 30A pull-out fuse block that supplies power to the E1B 4160 VAC bus cubicle 2 degraded relays 27E, 27F, 27G, and 27H, was found in the OFF position. Without the 30A pull-out fuse block ON, the E1B bus would not have degraded voltage protection. LCO 3.3.2, Action c., Items 8.b and 8.c, Action 20A.b of Table 3.3-3 entered for all four channels being INOPERABLE.

11/27/24 (1734) - Unplanned entry into Risk Managed TS (RMTS) per the configuration risk management program (CRMP) due to reaching the 1-hour front stop completion time for TS required actions.

11/28/24 (0435) - E1B 4160 VAC ESF Bus degraded voltage relays 27E, 27F, 27G, and 27H declared OPERABLE following satisfactory restoration of control power, channel check, verification of relay flags cleared, and satisfactory package review. Exited TS 3.3.2 and the requirements of the CRMP.

E. Method of Discovery

The discovery of the inoperable degraded voltage relays 27E, 27F, 27G, and 27H was self-revealing when it was found that the relays were flagged and could not be cleared during operator rounds on November 27, 2024.

A. Failure Mode, Mechanism, and Effects of Failed Components

Prior to October 12, 2024, at 0311, the degraded voltage relays were OPERABLE, as the control power to the monitoring relay was energized and the 30A pull-out fuse block was in the ON position. The four degraded voltage relays were inoperable when the control power was de-energized to support maintenance and during the course of maintenance the 30A pull-out fuse block was set to the OFF position. When the relays were placed back in service, the 30A pull-out fuse block was not reset to the ON position, and as a result the relays remained inoperable. The mechanism of failure was due to the equipment not being returned to appropriate alignment before returning it to service. Therefore, the degraded voltage protection relays were inoperable on the E1B 4160 VAC bus cubicle 2 from October 12, 2024, until November 28, 2024. The effect of four inoperable degraded voltage relays was that the number of OPERABLE channels was more than one less than the total number of four channels, and would have required Actions per TS Table 3.3-3.

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3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.

052 050

C. Safety Consequences and Implications

A risk evaluation was performed to estimate the Incremental Core Damage Probability (ICDP) associated with this event.

The calculated ICDP from the inoperability of the four degraded voltage relays for the E1B 4160 VAC bus was determined to result in a very small change in risk and is of very low safety significance. 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 Event

The cause of the event was the failure to return the 30A pull-out fuse block for the degraded voltage relays to the ON position post-maintenance. The pull-out fuse block was not returned to proper alignment due to: (1) inadequate controls to ensure that the work packages contained adequate post-maintenance test requirements and that the implementing procedure had the structure to support procedure use control requirements that were written to match the skills and abilities of the performer, and (2) the standards for recognition of conditions prohibited by Technical Specifications and procedure compliance in accordance with station expectations for Conduct of Operations was not enforced by Operations Management.

V. Corrective Actions

1. On November 28, 2024, the degraded voltage relays were returned to OPERABLE status with the restoration of control power and satisfactory close-out.

2. Revise the STP Work Process Program governing the issuance of Preventative Maintenance Work Orders to address requirements for quality, usability and Post Maintenance Tests to require that Technical Reviewers review any associated Plant Maintenance Instructions (PMI)s for any associated PM models for conflicts with other procedures when implementing procedure revisions for maintenance procedures or work instructions.

3. Revise the STP Work Process Program governing the issuance of Preventative Maintenance Work Orders to address requirements for quality, usability and Post Maintenance Tests by updating the electrical post -maintenance test matrices.

4. Revise procedure 0PMP05-PK-1001, 4160V Class 1E Switchgear Maintenance to include improved Post Maintenance Tests, to ensure the procedure is written to match the skills and abilities of a newly qualified performer, and to require a data package is filled out for each cubicle worked.

5. Operations Leadership to reinforce consistent application of operator fundamentals for procedure compliance, addressing Integrated Computer System (ICS) alarms, and Technical Specifications recognition. This will be accomplished through changes to procedures and implementation of a change management plan.

VI. Previous Similar Events

There were no previous recent similar events identified.

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