05000445/LER-2024-002-01, Condition Prohibited by Technical Specifications Due to Inoperable Remote Shutdown System Functions

Condition Prohibited by Technical Specifications Due to Inoperable Remote Shutdown System Functions

ML24277A774
Person / Time
Site:	Comanche Peak
Issue date:	10/03/2024
From:	Sewell S Vistra Operations Company
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
CP-202400368, TXX-24071 LER 2024-002-01
Download: ML24277A774 (1)

LER-2024-002, Condition Prohibited by Technical Specifications Due to Inoperable Remote Shutdown System Functions

Event date:
Report date:
4452024002R01 - NRC Website

text

COMANCHE PEAK NUCLEAR POWER PLANT CP-202400368 TXX-24071 October 03, 2024 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Subject:

Comanche Peak Nuclear Power Plant (CPNPP)

Docket No. 50-445 Steven K. Sewell Nuclear Site Vice President Vistra Operations Company LLC P.O. Box 1002 6322 North FM 56 Glen Rose, TX 76043 Office: 254.897.6113 Ref 10CFR50.73 Condition Prohibited by Technical Specifications due to Inoperable Remote Shutdown System Functions Licensee Event Report 1-24-002-01 References:

1. Condition Prohibited by Technical Specifications due to Inoperable Remote Shutdown System Functions Licensee Event Report 1-24-002-00. [ML24155A172]

Dear Sir or Madam:

Attached is Licensee Event Report (LER) 1-24-002-01, "Condition Prohibited by Technical Specifications due to Inoperable Remote Shutdown System Functions" for Comanche Peak Nuclear Power Plant (CPNPP) Unit 1.

This communication contains no new commitments regarding CPNPP Units 1 or 2.

Should you have any questions, please contact Kassie Mandrell at (254) 897-6987 or Kassie.Mandrell@vistracorp.com.

Sincerely,

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Steven KSewell Attachment:

Abstract

On October 20, 2023, following the performance of Integrated Test Sequencing (ITS), Unit 1, Train A, Neutron Flux Monitoring System (Gamma Metrics) failed to automatically restart. ITS is performed to verify proper response of SafegXDrds equipment following a Safety Injection (SI) in conjunction with a Loss of Offsite Power (LOOP). Operators questioned the reliability of the Gamma Metrics system during accident conditions and a follow-up evaluation of Gamma Metrics performance on April 3, 2024, found that both trains of the Unit 1 Gamma Metrics would be unable to automatically restart during a LOOP and had been inoperable in the past for longer than allowed by Technical Specifications (TS).

The direct cause of the Gamma Metrics failure was an overYROWDJH trip on the high voltage power supply (HVPS). Corrective actions include modifications to improve power restoration to the system and a procedure revision for compensatory operator actions.

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SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES During the refueling outage in October 2017, the high voltage power supply (HVPS) [EIIS: (IG)(JX)] was replaced on Unit 1, Train B. It was subsequently replaced on Unit 1, Train A on July 7, 2020. The following Unit 1 refueling outage (1RF21 in October 2020 resulted in a trip of the Neutron Flux Monitoring System (Gamma Metrics) [EIIS: (IG)(MON)] during the performance of Integrated Test Sequencing (ITS). ITS is performed to verify proper response of SafeguDrds equipment following a Safety Injection (SI) in conjunction with a Loss of Offsite Power (LOOP). The HVPS was reset to restore the signal prior to plant start-up. The same failure occurred during the next two Unit 1 refueling outages in 2022 and 2023.

Following the failure on October 20, 2023, Operators questioned the reliability of the Gamma Metrics system during accident conditions and potential actuation of SI. On April 3, 2024, Engineering evaluated the condition and found that the Unit 1 Gamma Metrics ZDs not reliable and wRXOG trip during power restoration from any loss of the 6.9 kilovolt (kV) Safety Bus. A past operability evaluation was then completed on April 11, 2024 and determined that this condition had existed for longer than allowed by Technical Specifications (TS). On April 18, 2024, a temporary modification was installed to add a 30 second delay to the power restoration of both trains of the HVPS. As a result of the temporary modification, the system was declared operable.

II. COMPONENT OR SYSTEM FAILURES A. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE The evaluation determined that voltage spikes occur when the 6.9KV bus is lost and the Emergency Diesel Generator (EDG) breaker closes. This cycle occurs in approximately 7 seconds on Unit 2 and approximately 6 seconds on Unit 1.

7he Unit 1 HVPS [EIIS: (IG)(JX)] cannot sustain the shorter cycle, resulting in an overvoltage protection trip.

This failure was caused by inadequate design of the HVPS. The original HVPS for Gamma
Metrics had an overvoltage trip setpoint of 1140 VDC, whereas, the replacement HVPS has an overvoltage trip setpoint of 1053 VDC with WKUHH additional protection circuits. This was not identified in the equivalency evaluation for the replacement.
This condition does not affect Unit 2 Gamma Metrics.

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Comanche Peak Nuclear Power Plant, Unit 1 445 24 002 01

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3. LER NUMBER YEAR SEQUENTIAL NUMBER REV NO.

052 050

• 7 CAUSE OF THE EVENT The direct cause of the Gamma
  Metrics failure was an overYROWDJH trip of the HVPS. Subsequent evaluation determined that this was caused by inadequate design of the HVPS during the initial replacement. Unidentified differences in design resulted in the HVPS being unable to respond to fast power cycling events and actuating protective circuitry following Integrated Test Sequencing. Analysis concluded that Unit 2 was not affected.

7 CORRECTIVE ACTIONS Corrective actions include the following:

Completed

1. Revise applicable procedures to have operators manually reset associated breakers in the event of a station blackout to restore power to the Unit 1 Gamma Metrics. Action completed on April 18, 2024.

2. Install a temporary modification to slow power restoration to the system and prevent the overvoltage trip of the HVPS on Unit 1. Action completed on April 18, 2024.

Planned

1. Create and implement a permanent modification to correct the GammaMetrics HVPS issue.

VI. PREVIOUS SIMILAR EVENTS

There have been no similar events or conditions related to Gamma Metrics being inoperable for a period longer than the TS action statement allowed outage time of 30 days at Comanche Peak Nuclear Power Plant.

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Comanche Peak Nuclear Power Plant, Unit 1 445 2024 002 01

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