05000499/LER-2024-001-01, Supplement to Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators

Supplement to Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators

ML24242A163
Person / Time
Site:	South Texas
Issue date:	08/29/2024
From:	Tomlinson J South Texas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-24004057, STI: 35628492 LER 2024-001-01
Download: ML24242A163 (1)

LER-2024-001, Supplement to Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators

Event date:
Report date:
4992024001R01 - NRC Website

text

August 29, 2024 NOC-AE-24004057 10 CFR 50.73 STI: 35628492 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 2 Docket No. STN 50-499 Licensee Event Report 2024-001-01 Supplement to Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators Reference: Letter; K. Harshaw (STP) to Document Control Desk (NRC) - Licensee Event Report 2024-001-00, Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators, July 2, 2024, (NOC-AE-24004049)

(ML24184C083)

On July 2, 2024, 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 evaluations. 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 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 2 LER 2024-001-01, Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators 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: 2024.08.29 09:32:10

- 05'00'

NOC-AE-24004057 Attachment Attachment Unit 2 LER 2024-001-01 Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators

Abstract

South Texas Project Unit 2

00499 9

Automatic Reactor Trip and Actuation of Two of Three Emergency Diesel Generators 05 12 2024 2024 001 01 08 29 2024 N/A N/A 1

15%

Chris Warren, Licensing Engineer 361-972-7293 E

EL RLY-21 W120 Y

X SB SLV W120 Y

On May 12, 2024, STP Unit 2 experienced a partial Loss of Offsite Power (LOOP), resulting in an automatic reactor trip and actuation of Unit 2 Standby Diesel Generators (SBDG) 21 and 23. All three Engineered Safety Feature (ESF) busses were energized. These systems actuated due to the "21/G1" relay inadvertently actuating from the sensitivity of the moving contact being affected by the vibrations caused from the closure of the Main Control Room Panel Section CP010 side door. Following the reactor trip, Steam Generator (S/G) Power Operated Relief Valve (PORV) 2C was declared inoperable due to erratic controls in Manual. Load Center E2A1 supply breaker failed to automatically close, which resulted in loss of power to S/G PORV 2A and SBDG 21 Train 2A Diesel Generator Emergency Supply Fan. With the fan inoperable, SBDG 21 may not have been able to meet its mission time, also challenging the ability of S/G PORV 2D to perform its design functions.

Completed corrective actions included installation of a replacement "21/G1" relay, replacement of the Load Center E2A1 output breaker handswitch, and replacement of the PORV push button, push button sleeve, and potentiometer. The following root causes led to the Unit 2 reactor trip: vendor instructions for calibrating the "21G1" relay contacts were not included in site procedures and the Main Generator protection schedule was not fully updated to prevent the trip function of the "21/G1" relay when the Main Generator is offline and the Main Generator Circuit Breaker is in the open position.

Summary of the Event Timeline (Note: All times are in Central Daylight Time) 05/12/2024 (1641) - Switchyard breakers Y590 and Y600 tripped. Unit 2 reactor tripped and EDGs 21 and 23 automatically actuated and sequenced on a Loss of Offsite Power. The following busses were deenergized: 13.8kV Auxiliary busses 2F, 2G, 2H, and 2J, 13.8kV Standby busses 2F and 2H, and Load Center 2W.

05/12/2024 (1641) - All four Reactor Coolant Pumps (RCP) lost power. Entered Technical Specification (TS) 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."

05/12/2024 (1641) - Reactor Coolant Loop 'B' declared inoperable due to RCP 2B #2 Seal exceeding its leak-off rate limit.

05/12/2024 (1641) - Declared loss of two offsite sources due to two ESF busses not connected to a load tap changer in Auto. Entered TS 3.8.1.1.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 />."

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05/12/2024 (1641) - Load Center E2A1 supply breaker failed to close automatically following LOOP and automatic ESF sequencing. Class 1E 480V Load Center E2A1 is de-energized. Entered TS 3.8.3.1.a, Action 'A': "With one of the required trains of A.C. ESF busses not fully energized, within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> reenergize the train 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 in Cold Shutdown within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.

05/12/2024 (1645) - Isolated Main Steam using safety grade switches from the Control Room.

05/12/2024 (1705) - Steam Generator PORV 2C placed in manual for venting, PORV opened to full travel without required input from operator. Steam Generator PORV 2C declared inoperable.

05/12/2024 (1707) - Load Center E2A1 supply breaker automatically closed after placing it in "pull to lock" and returning the hand switch to Automatic. Class 1E 480V Load Center E2A1 is energized. Exited Technical Specification 3.8.3.1.a, Action 'A'.

05/12/2024 (1729) - Energized Standby bus 2F from Standby 2 transformer (restoration of off-site power to ESF Bus A).

05/12/2024 (1733) - Energized Standby bus 2H from Standby 1 transformer (restoration of off-site power to ESF Bus C).

05/12/2024 (1745) - Energized Auxiliary bus 2F from Standby 2 transformer.

05/12/2024 (1747) - Energized Auxiliary bus 2H from Standby 1 transformer.

05/12/2024 (1750) - Auxiliary bus 2J re-energized from off site power.

05/12/2024 (1753) - Auxiliary bus 2G re-energized from off site power.

05/12/2024 (1758) - Started RCP 2D. Reactor Coolant Loop D and its associated steam generator and RCP are in operation with the reactor trip breakers open. Exited TS 3.4.1.2, Action 'C'.

05/12/2024 (1917) - Started Reactor Coolant Pump 2A for plant stabilization.

05/12/2024 (1918) - Load Center 2D1 energized from off site power.

05/12/2024 (1921) - Load Center 2D2 energized from off site power.

05/12/2024 (1950) - Completed NRC 4-hour notification under 10 CFR 50.72(b)(2)(iv)(B) for RPS actuation and 10 CFR 50.72(b)(3)(iv)(A) for EDG 21 and 23 actuations. Event Notification 57124.

F. Method of Discovery The event was self-revealing when the generator backup distance relay "21/G1," time delay relay "62G1," generator lockout relay "86G1" and the main transformer lockout relay "86SY" were flagged in the Unit 2 Control Room. The primary and backup pilot wire relays had the "DTT Key" light illuminated, indicating a transfer trip to the switchyard relays.

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3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.

052 050Planned Corrective Actions

3. Perform a Performance Analysis to determine the process and training needs related to KD-10 and KD-11 relays' adjustments.

4. Include in Electrical Maintenance Advance Protective Relay certification the adjustment of contacts for relays type KD-10 and KD-11 per VTD-B455-0076, Section 14.6.

5. Determine a permanent solution to block the "21/G1" relay trip signal when the Main Generator is offline and perform an Engineering Change.

6. Implement the permanent solution to block the "21/G1" relay trip signal when the Main Generator is offline.

7. Implement PORV potentiometer replacement plan with planned work packages once vendor stock is replenished.

VI. Previous Similar Events The following previous similar events were identified for S/G PORVs:

1. Condition Report (CR) 24-4183: S/G 2C PORV did not respond to the Close (down) pushbutton being depressed for approximately 11 seconds during 0PSP05-MS-7411L. S/G 2C PORV did travel full closed after the delay.

2. CRs 23-10139 and 23-10508: STP received a 10 CFR Part 21 notification (Certrec Event # 56821) from Enertech:

Notification of potential defect for Steam Generator PORV potentiometer part number D2060S. A number of returned potentiometers to Enertech exhibited inconsistent resistance values at certain stroke positions. The potentiometers are used in modulating actuators. Per Enertech STP is impacted. A plant impact form was completed under CR 23-10139.

This CR is tracking additional actions needed to be taken to correct the vulnerabiliites with the impacted PORV potentiometers for Steam Generators 1C, 2A, 2B, and 2C.

3. CR 22-12288: Received the following silent Plant Computer alarms: MSZE7421 S/G 2B PORV POSIT at 2.1% and slowly increasing (~1% / 30 mins) and MSZC7421 S/G 2B PORV OPEN. No observable increases in Steam Flow from S/

G 2B or effects on the Reactor Coolant System were observed.

4. CR 21-10382: While venting S/G 1D during performance procedure 0POP03-ZG-0007, "Plant Cooldown," while opening S/G 1D PORV and with the Valve Position Indicator (VPI) indicating approximately 90% open, we observed VPI peg low to 0% and then return to 90% two times. No other indications occurred to indicate the valve moved.

5. CR 21-8976: During performance of WAN# 605160, Operations used the Operator Interface Module (OIM) up pushbutton to open 2C PORV. After holding the up pushbutton for an adequate time for a full open stroke, the valve never moved.

6. CR 18-5534: S/G 'C' PORV indication on CP-006, plant computer point MSZE7431 showing valve position is oscillating from as high as 99% to as low at 9% open. Investigate cause of valve position indication oscillation. Operator reports no stem movement locally and indicating full open.

No previous, recent similar events were identified for Load Center E2A1 supply breaker 2E or relay "21/G1."

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