05000498/LER-2024-004, Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps

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Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps
ML24263A145
Person / Time
Site: South Texas  STP Nuclear Operating Company icon.png
Issue date: 09/19/2024
From: Tomlinson J
South Texas
To:
Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-24004064 LER 2024-004-00
Download: ML24263A145 (1)
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LER-2024-004, Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4982024004R00 - NRC Website
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text

September 19, 2024 NOC-AE-24004064 10 CFR 50.73 STI: 35638176 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498 and 50-499 Licensee Event Report 2024-004-00 Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps Pursuant to reporting requirements in 10 CFR 50.73(a)(2)(iv)(A), STP Nuclear Operating Company (STPNOC) hereby submits the attached Licensee Event Report 2024-004-00 for South Texas Project Units 1 and 2.

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: Units 1 and 2 LER 2024-004-00, Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps cc:

Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511 Jason R Tomlinson Digitally signed by Jason R Tomlinson Date: 2024.09.19 09:32:57 -05'00'

NOC-AE-24004064 Attachment Attachment Units 1 and 2 LER 2024-004-00 Units 1 and 2 LER 2024-004-00, Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps

Abstract

South Texas Project Unit 1

00498 6

Loss of Offsite Power Resulting in Unit 1 Automatic Reactor Trip and Actuation of Emergency Diesel Generators and Auxiliary Feedwater Pumps 07 24 2024 2024 004 00 09 19 2024 South Texas Project Unit 2 N/A 00499 1

100%

Chris Warren, Licensing Engineer 361-972-7293 N/A COMP G080 Y

11 14 2024 On July 24, 2024, STP Unit 1 experienced a Loss of Offsite Power (LOOP), resulting in an automatic reactor trip and actuation of all three Unit 1 Standby Diesel Generators (SBDG) and all four Auxiliary Feedwater (AFW) pumps. All three trains of Engineered Safety Feature (ESF) buses were energized and all equipment responded as expected without any complications with the exception of Steam Generator (S/G) Power Operated Relief Valve (PORV) 1C. Unit 2 experienced a partial LOOP and automatic actuation of SBDG 22 and one of four AFW pumps.

The systems for each unit actuated due to the switchyard experiencing electrical faults during the Shunt Reactor RT-2 fire event and momentarily de-energizing the 345kV North Bus twice, locking out the 345kV South Bus, as well as initiating protective relaying to isolate Unit 1's main transformers from the switchyard.

Immediate corrective actions included restoring offsite power to all affected ESF trains. A third-party failure analysis of Shunt Reactor RT-2 is ongoing to determine causes for the event and any additional corrective actions.

Summary of the Event Timeline (Note: All times are in Central Daylight Time)

Unit 1 - 07/24/2024 (0702) - Unit 1 reactor trip due to fire in the switchyard. The 345kV North Bus and the Unit 1 Generator tripped. All SBDGs automatically actuated and sequenced on a Loss of Offsite Power. The following busses were deenergized: 13.8kV Auxiliary busses 1F, 1G, 1H, and 1J, 13.8kV Standby busses 1F, 1H, and 1G, and 480V Load Center 1W.

Units 1 and 2 - 07/24/2024 (0702) - Entered Technical Specification 3.8.1.1, Action 'e': With two of the required offsite A.C.

circuits inoperable, within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> restore at least one of the inoperable offsite sources 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 />.

Unit 1 - 07/24/2024 (0702) - All four Reactor Coolant Pumps (RCP) lost power. Entered Technical Specification 3.4.1.2:

"At least two of the reactor coolant loops listed below shall be OPERABLE and with two reactor coolant loops in operation when the reactor trip system breakers are closed and one reactor coolant loop in operation when the reactor trip system breakers are open", Action 'C': "With no reactor coolant pump in operation, suspend operations that would cause introduction into the Reactor Coolant System (RCS) of coolant with boron concentration less than required to meet Shutdown Margin of Limiting Condition of Operation (LCO) 3.1.1 and immediately initiate corrective action to return the required reactor coolant loop to operation."

Unit 2 - 07/24/2024 (0703) - Report of explosion and fire in the switchyard. Entered off-normal procedure 0POP04-ZO-0008, Fire/Explosion.

Unit 1 - 07/24/2024 (0735) - Verified natural circulation with Steam Generator (S/G) Power Operated Relief Valves (PORV) open in Manual.

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South Texas Project Unit 1 00498 2024 004 00

Timeline (Note: All times are in Central Daylight Time)

Unit 2 - 07/24/2024 (0740) - Commenced load reduction to 1200 MWe net due to Electric Reliability Council of Texas (ERCOT) request.

07/24/2024 (0800) - Bay City Fire Department arrive on site at the switchyard.

07/24/2024 (0810) - CenterPoint Energy (Electric Transmission Utility Company) reports fire is shifting and CenterPoint Energy will de-energize lines to Standby Transformer 1.

Unit 2 - 07/24/2024 (0838) - ERCOT rescinded grid restrictions. Unit 2 is able to return to 100% power.

Unit 1 - 07/24/2024 (0848) - Spent Fuel Pool pump 1B is running, restoring cooling to the Spent Fuel Pool.

07/24/2024 (0925) - Fire in Shunt Reactor RT-2 is extinguished.

Unit 1 - 07/24/2024 (0953) - S/G PORV 1C declared inoperable and nonfunctional due to not operating in AUTO or MANUAL from the Control Room.

Unit 1 - 07/24/2024 (1054) - Completed NRC 4-hour Non-Emergency Notification under 10 CFR 50.72(b)(2)(iv)(B) for a Reactor Trip from 100% power (RPS actuation). NRC Form 361 was emailed, read and discussed with the NRC. Event Notification Time was 1154 EDT, Event Notification 57237.

Unit 2 - 07/24/2024 (1054) - Energized 13.8kV Standby Bus 2G from 13.8kV Auxiliary Bus 2G.

Unit 2 - 07/24/2024 (1212) - Energized 345kV South Bus and Standby Transformer 2.

Unit 2 - 07/24/2024 (1223) - Closed 4.16kV ESF Bus E2B supply breaker.

Unit 1 - 07/24/2024 (1223) - Energized 13.8kV Standby Bus 1F from Standby Transformer 2.

Unit 1 - 07/24/2024 (1237) - Energized 13.8kV Auxiliary Bus 1F from 13.8 kV Standby Bus 1F Unit 1 - 07/24/2024 (1249) - Energized 13.8 kV Auxiliary Bus 1J from Standby Transformer 2.

Unit 1 - 07/24/2024 (1346) - Started Reactor Coolant Pump 1D. Reactor Coolant Loop D and its associated steam generator and reactor coolant pump are in operation with reactor trip breakers OPEN. Exited LCO 3.4.1.2, Action 'c'.

Unit 2 - 07/24/2024 (1350) - Both 345kV North and South buses are OPERABLE and all three ESF buses are powered by the Unit Auxiliary Transformer. Exited Technical Specification 3.8.1.1, Action 'e' and entered Action 'a': With one offsite circuits inoperable, demonstrate the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirement 4.8.1.1.1.a within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter. Within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> restore the offsite circuit to OPERABLE status or apply the requirements of Configuration Risk Management Program (CRMP), or be in at least HOT SHUTDOWN within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and 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 />.

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South Texas Project Unit 1 00498 2024 004 00

Unit 1 - 07/24/2024 (1421) - Energized 13.8kV Standby Bus 1G from Standby Transformer 2.

Unit 1 - 07/24/2024 (1429) - Energized 13.8 kV Auxiliary Bus 1G from 13.8 kV Standby Bus 1G.

Unit 1 - 07/24/2024 (1442) - Energized 4.16kV Bus 1D1.

07/24/2024 (1455) - Completed NRC 8-hour non-emergency notification under 10 CFR 50.72(b)(3)(iv)(A) as an event that resulted in the valid actuation of a PWR auxiliary system and emergency AC electrical power system. Completed NRC 4-hour non-emergency notification under 10 CFR 50.72(b)(2)(xi) for the issuance of a news release. Any event or situation, related to the health and safety of the public or onsite personnel, or protection of the environment, for which a news release is planned or notification to other government agencies has been or will be made. NRC Form 361 was emailed, read and discussed with the NRC. Event Notification Time was 1555 EDT. Event Notification EN 57237.

Unit 1 - 07/24/2024 (1502) - Energized 13.8kV Standby Bus 1H from Standby Transformer 2.

Unit 1 - 07/24/2024 (1508) - Energized 13.8kV Auxiliary Bus 1H from 13.8kV Standby Bus 1H.

Unit 2 - 07/24/2024 (1537) - Rolled 13.8kV bus 2G from Unit 2 Auxiliary Transformer to Standby Transformer 2. ESF Transformer E2B Load Tap Changer returned to AUTO. Exited Technical Specification 3.8.1.1, Action 'a': With one offsite circuit of the required AC electrical power sources INOPERABLE, demonstrate operability of the remaining AC sources by performing 0PSP03-EA-0002 within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter. Within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, restore the offsite circuit to OPERABLE status or apply the requirements of the Configuration Risk Management Program (CRMP), or be in at least HOT SHUTDOWN within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and 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 />.

Unit 1 - 07/25/2024 (0057) - Switchyard Breakers Y0520 and Y0510 closed. Main and Auxiliary transformers are energized.

Unit 1 - 07/25/2024 (0305) - Restored 4.16kV E1B bus back to offsite power.

Unit 1 - 07/25/2024 (0514) - Restored 4.16kV E1C bus back to offsite power.

Unit 1 - 07/25/2024 (0514) - E1B and E1C ESF buses are connected to the operable 345kV North and South buses through an automatic load tap changer. E1A ESF bus remains power solely from SBDG 11. Exited Technical Specification 3.8.1.1, Action 'e'. Entered Technical Specification 3.8.1.1, Action 'a'.

Unit 1 - 07/25/2024 (0933) - 4.16kV E1A bus aligned to 13.8kV Standby Bus. Exited Technical Specification 3.8.1.1, Action 'a'.

F. Method of Discovery The event was self-revealing when Unit 1 experienced an automatic reactor trip coincident with a Loss of Offsite Power and actuation of the SBDGs for all three ESF buses.

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3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.

052 050 C. Safety Consequences and Implications Analysis for the LOOP event involves three calculations:

(1) Conditional Core Damage Probability (CCDP) estimation (initiating event analysis) associated with just the initiating

event, (2) Delta Core Damage Probability (CDP) estimation (condition analysis) associated with any SSC unavailability over the exposure time, and (3) the sum of the CCDP and Delta CDP results.

Based on the above analysis, the switchyard-centered LOOP CCDP exceeds the risk thresholds of 1.0E-05 for Delta CDF and 1.0E-06 for Delta Large Early Release Frequency (LERF). The Total Incremental CCDP and Conditional Large Early Release Probability are of substantial safety significance. The additional risk incurred due to S/G PORV 1C unavailability during the event was of low risk safety significance.

IV. Cause of Event Failure analysis is ongoing. A supplement to this report will be provided detailing causes upon completion of the failure analysis.

V. Corrective Actions Failure analysis is ongoing. A supplement to this report will be provided detailing additional corrective actions upon completion of the failure analysis.

VI. Previous Similar Events No previous similar events were identified.

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