05000446/LER-2025-002, (Cpnpp), Safety Chiller Inoperable Due to Failure of Copper Refrigerant Fittings

(Cpnpp), Safety Chiller Inoperable Due to Failure of Copper Refrigerant Fittings

ML25287A021
Person / Time
Site:	Comanche Peak
Issue date:	10/14/2025
From:	Christopher Jackson Vistra Operations Company
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
CP-202500315, TXX-25055 LER 2025-002-00
Download: ML25287A021 (1)

LER-2025-002, (Cpnpp), Safety Chiller Inoperable Due to Failure of Copper Refrigerant Fittings

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4462025002R00 - NRC Website

text

COMANCHE PEAK NUCLEAR POWER PLANT CP-202500315 TXX-25055 October 14, 2025 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Comanche Peak Nuclear Power Plant (CPNPP)

Docket No. 50-446 Comanche Peak Nuclear Power Plant Christopher J. Jackson Nuclear Site Vice President Vistra Operations Company LLC P.O. Box 1002 6322 North FM 56 Glen Rose, TX 76043 Office: 254.897.6411 Ref 10 CFR 50.73 Safety Chiller Inoperable due to Failure of Copper Refrigerant Fittings Licensee Event Report 2-25-002-00

Dear Sir or Madam:

Attached is a Licensee Event Report (LER) 2-25-002-00, "Safety Chiller Inoperable due to Failure of Copper Refrigerant Fittings" for Comanche Peak Nuclear Power Plant (CPNPP) Unit 2.

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

Should you have any questions, please contact Marianne Burnett at (254) 897 - 5424 or marianne.burnett@vistracorp.com.

Sincerely, Christopher J. Jackson

Abstract

Phone Number (Include area code) 254-897 -5424 Manufacturer Reportable to IRIS Month Day Year On August 14, 2025 at 2125, Safety Chiller 2-06 on Unit 2 Train B tripped on low refrigerant pressure. Initial inspection identified a less than fully torqued valve packing and hand tight pipe cap to be the suspected leak source. Once re-secured, refrigerant was added, system pressure stabilized, and the chiller was declared operable. Over the next five days, refrigerant pressure trended down again. By August 23, 2025, the pressure was approaching operating limits and refrigerant was added.

Subsequently, Safety Chiller 2-06 was declared inoperable, shutdown, and a full leak test identified a leak at the foul gas check valve inlet fitting. The fitting was replaced on August 27, 2025, and the system was returned to service and declared operable.

Due to the same leakage symptoms, it is concluded that the fitting leak was present and uncorrected during the initial chiller lnoperability. A Past Operability evaluation confirmed the chiller was unable to perform its safety function for approximately 293 hours0.00339 days <br />0.0814 hours <br />4.844577e-4 weeks <br />1.114865e-4 months <br />.

This report is required as the lnoperability of Safety Chiller 2-06 existed longer than permitted per Technical Specifications (TS)

Limiting Condition of Operation (LCO) 3. 7.19.

I. DESCRIPTION OF REPORTABLE EVENT.

050 052

2. DOCKET NUMBER

3. LER NUMBER I

00446 D

NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-0 Energy Industry Identification System (EIIS) Codes are identified in the text as [XX). All times listed in this report are in Central Time.

A Reportable Event Classification.

This Licensee Event Report is required per 10 CFR 50.73(a)(2)(i)(B) as the reported condition (one safety chilled water train inoperable) existed longer than permitted by the Technical Specifications (TS) LCO 3. 7.19. Specifically, Unit 2 Train B Safety Chiller [KM] 2-06 was inoperable for approximately 293 hours0.00339 days <br />0.0814 hours <br />4.844577e-4 weeks <br />1.114865e-4 months <br />, exceeding the required action completion time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to return the train to Operability.

B. Plant Condition Prior to Reportable Event.

Comanche Peak Nuclear Power Plant (CPNPP) Unit 2 was operating in Mode 1 at 100% power at the time of discovery.

C. Status of Structures, systems, or components that were inoperable at the start of the event and that contributed to the event.

No structures, systems, or components were inoperable at the start of the event.

D. Narrative Summary of the Event, including dates and approximate times.

On August 14, 2025 at 2125, the Safety Chiller 2-06 [KM] on Unit 2 Train B automatically tripped due to low refrigerant pressure at which point the LCO was entered. Upon investigation, a less than fully torqued valve packing and hand tight pipe cap were identified. The valve packing was re-secured, initially suggesting a corrected main refrigerant leak. Once refrigerant (R-11) was added to the system, the refrigerant low pressure stabilized at 10 inches of mercury within the approved monitoring parameters (8 inches of mercury to 12 inches of mercury). The LCO was exited on August 15, 2025 at 1151.

On August 23, 2025 at 1630, the refrigerant low pressure was recorded to be trending downward at 12 inches of mercury, while the outlet temperature was trending up at 56.5 degrees Fahrenheit. Additional troubleshooting determined that Safety Chiller 2-06 had low refrigerant. The LCO was entered on August 24, 2025 at 1600. A leak test was initiated. The system was shutdown, refrigerant evacuated, and nitrogen leak test performed. The test confirmed a leak at the foul gas check valve [CKV] inlet fitting. The inlet fitting is a copper fitting, which degraded to failure over time. On August 27, 2025 at 0138, the foul gas inlet fitting replacement was complete at which time the LCO was exited. The system returned to service operating under normal parameters.

A Past Operability concluded that the events on August 14, 2025 and August 23, 2025 were caused by the same issue and that the leakage observed was a result of a failed inlet fitting on the foul gas check valve. Investigation confirmed that the initial leak was masked by the purge system cycling through the fill and drain cycle delaying identification of the failed component. The Safety Chiller was unable to perform its safety function between the period of August 14, 2025 at 2125 through August 27, 2025 at 0138.

050 052

2. DOCKET NUMBER

3. LER NUMBER I

00446 D

NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-0 Consequences. The Safety Chilled Water System (CHS) is a redundant system, where two separate (independent) trains provide essential chilled water to the emergency fan coil units (EFCUs) during normal and accident conditions. Only Unit 2 Train B Safety Chiller 2-06 was declared inoperable. Unit 2 Train A Safety Chiller 2-05 remained operable throughout the duration of this event.

E. The method of discovery of each component or system failure, or procedural personnel error.

The event was self-identified due to the trip, and by plant personnel during routine operations and applicable evaluations.

II. COMPONENT OR SYSTEM FAILURES.

A. Cause of each component or system failure.

An Equipment Failure Investigation was completed per station procedures. The inlet fitting on the foul gas check valve was identified as degraded to failure. The inlet fitting was composed of copper. Factors including vibration or mechanical stress and thermal cycling fatigue degraded the fitting over time resulting in the refrigerant leak.

B. Failure mode, mechanism, and effects of each failed component.

Unit 2 Train B Safety Chiller 2-06 was evaluated as a condition monitoring event (CME), which allowed for identification of the refrigerant leak on August 23, 2025 by plant personnel during routine operations.

CHS for each unit consists of two separate and redundant safety trains. Each train includes a centrifugal chiller, a centrifugal water recirculation pump, interconnecting piping, valves, and controls and instruments. Each train supports the EFCUs that maintain air temperatures as required in selected rooms containing safety-related equipment during normal operation and during and after a design basis accident or blackout.

C. Systems or secondary functions that were affected by failure of components with multiple functions.

No systems or secondary functions were affected by the failure of components.

One train of the CHS provides 100% of the required cooling for the associated train of EFCUs.

Unit 2 Train A Safety Chiller 2-05 remained operable throughout the duration of this event.

D. Failed component information

050 052

2. DOCKET NUMBER

3. LER NUMBER I

00446 D

NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-0 The following information corresponds to the individual component that was replaced in the Safety Chiller 2-06.

Manufacturer: Mueller Industries Part No. 925983 Material: Copper Ill. ANALYSIS OF THE EVENT A. Safety system responses that occurred.

No safety system responses occurred due to the event.

B. Duration of safety system train lnoperability.

Total duration for lnoperability was determined to be approximately 293 hours0.00339 days <br />0.0814 hours <br />4.844577e-4 weeks <br />1.114865e-4 months <br />. LCO was entered August 14, 2025 at 2125 and exited August 27, 2025 at 0138.

C. Safety consequences and implications of the event.

The CHS is a redundant system, where two trains provide essential chilled water to the emergency fan coil units (EFCUs) during normal and accident conditions. The CHS system is required to start automatically and manually to mitigate the consequences of the Design Basis Accidents including remove heat from the Engineered Safety Function (ESF) pump motors, Uninterruptible Power Supply (UPS) equipment, electrical switchgear, and spent fuel pumps, to maintain ambient temperatures below design limits within the applicable rooms for all modes of Operation.

Only Safety Chiller 2-06 was declared inoperable. No opposite train lnoperability existed concurrently. Therefore, no safety system functional failure existed during the period of lnoperability for the 2-06 Safety Chiller. Consequently, this event had no impact on nuclear safety, reactor safety, radiological safety, environmental safety, or the safety of the public.

These events have been evaluated as not meeting the definition of a safety system functional failure per 10 CFR 50.73(a)

(2)(v).

IV. CAUSE OF THE EVENT

Investigation concluded that the inlet fitting on the foul gas check valve of Safety Chiller 2-06 failed resulting in the refrigerant leak. The degradation of the inlet fitting is associated with the copper material not withstanding factors like vibration or mechanical stress and thermal cycling over time. Further investigation confirmed that the initial leak on August 14, 2025 was masked by the purge system cycling through the fill and drain cycle delaying identification of the failed component.

V. CORRECTIVE ACTIONS

I

2. DOCKET NUMBER

3. LER NUMBER IYEAR SEQUENTIAL REV NUMBER NO.

00446 l202sl -I 1-0 002 The following actions were implemented as an immediate corrective actions. The inlet fitting was replaced. The monitoring specifications for low refrigerant pressure for all Safety Chillers was updated to 8 - 12 inches of mercury in operation procedures to improve early detection by plant personnel. Both actions have been completed.

The following action has been identified as a long-term corrective action. A Design Change Notice (DCN) was approved to update all remaining copper valves and fittings to stainless steel for all safety chiller units. Work orders have been initiated to update the fittings accordingly.

VI. SIMILAR EVENTS

No licensee reportable events related to the safety chiller system have been documented in the last three years.

However, similar conditions of safety chiller lnoperability have been reported using the Corrective Action Program (CAP) at CPNPP over the last three years. CPNPP is tracking all adverse trends associated with safety chiller performance and equipment reliability. Page 5

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