05000413/LER-2025-001, Catawba Nuclear Station, Unit 1, Plant Shutdown Required by Technical Specifications Due to Unidentified Reactor Coolant System Leakage

Catawba Nuclear Station, Unit 1, Plant Shutdown Required by Technical Specifications Due to Unidentified Reactor Coolant System Leakage

ML25090A171
Person / Time
Site:	Catawba
Issue date:	03/31/2025
From:	Flippin N Duke Energy Carolinas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
RA-25-0082 LER 2025-001-00
Download: ML25090A171 (1)

LER-2025-001, Catawba Nuclear Station, Unit 1, Plant Shutdown Required by Technical Specifications Due to Unidentified Reactor Coolant System Leakage

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
4132025001R00 - NRC Website

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RA-25-0082 March 31, 2025 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555

Subject:

Duke Energy Carolinas, LLC Catawba Nuclear Station, Unit 1 Docket No. 50-413 Licensee Event Report (LER) 413/2025-001-00 Nicole Flippin Vice President Catawba Nuclear Station Duke Energy CN01VP 14800 Concord Road York, SC 297 45 o: 803.701.3340 10 CFR 50.73 Pursuant to 10 CFR 50.73(a)(1) and (d), attached is LER 413/2025-001-00, entitled "Unit 1 Plant Shutdown Required by Technical Specifications due to Unidentified Reactor Coolant System Leakage". This report is being submitted in accordance with 10 CFR 50. 73(a)(2)(i)(A).

There are no regulatory commitments contained in this letter or its attachment.

This event is considered to be of no significance with respect to the health and safety of the public.

If questions arise regarding this LER, please contact Ari Tuckman of Regulatory Affairs at (803) 701-3771.

Sincerely, Nicole Flippin Vice President, Catawba Nuclear Station Attachment

United States Nuclear Regulatory Commission Page2 March 31, 2025 xc (with attachment):

Laura Dudes Regional Administrator U.S. Nuclear Regulatory Commission - Region II Jack Minzer-Bryant NRC Project Manager (CNS)

U.S. Nuclear Regulatory Commission David Rivard Senior Resident Inspector, Catawba Nuclear Station U.S. Nuclear Regulatory Commission

Abstract

On 1/28/2025, at 0430 EST with Unit 1 in Mode 1 operating at approximately 100 percent power, Operators detected a 1.4 gpm unidentified reactor coolant system leak. This required entry into Technical Specification action 3.4.13, Condition B which has a Required Action to reduce leakage to within limits within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. This Required Action was not completed within the completion time and Operators initiated a Technical Specification required shutdown. The required safety systems and shutdown equipment performed as expected resulting in an uncomplicated shutdown.

Upon entry into the Unit 1 containment, a separated instrument tube associated with Pressurizer Channels 3 and 4 was identified as the source of the leakage. The most probable cause of the fitting/tubing separating is nitride coating influenced corrosion.

This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(A) as completion of a plant shutdown required by the

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BACKGROUND Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

Nuclear Regulatory Commission, Washington, DC 20555-0001, or by email to lnfocollects.Resource@nrc.gov, and the 0MB reviewer at: 0MB Office of Information and Regulatory Affairs, (3150-0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of

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The following information is provided to assist readers in understanding the event described in this LER.

Applicable Energy Industry Identification [EIIS] system and component codes are enclosed within brackets.

Catawba Nuclear Station (CNS) unique system and component identifiers are contained within parentheses.

This event is being reported under the following criterion:

This event was reported to the NRC on January 28, 2025, in Event Notification 57517, as a 4-hour notification under 10 CFR 50. 72(b)(2)(i) for the initiation of any nuclear plant shutdown required by the plant's Technical Specification. It is also reportable as a 60-day written report under 10 CFR 50.73(a)(2)(i)(A).

Reactor Coolant System [AB] (NC)

The Catawba Nuclear Station reactor coolant system (NC) is designed to provide forced coolant flow through the reactor core to remove heat from the nuclear fuel. The pressure containing components of the reactor coolant system and the portions of connecting systems out to and including the isolation valves define the reactor coolant pressure boundary. During plant life, the joint and valve interfaces can produce varying amounts of reactor coolant leakage through either normal operational wear or mechanical deterioration. The safety significance of NC system leakage varies widely depending on its source, rate, and duration. Therefore, detecting and monitoring NC system leakage into the containment area is necessary. Quickly separating the identified leakage from the unidentified leakage is necessary to provide quantitative information to operators, allowing them to take corrective action should a leak occur that is detrimental to the safety of the facility and the public. A limited amount of leakage inside containment is expected from auxiliary systems that cannot be made 100% leak tight. Leakage from these systems should be detected, located, and isolated from the containment atmosphere, if possible, to not interfere with NC system leakage detection.

NC system pressure is controlled by use of the pressurizer where water and steam are maintained in equilibrium by electrical heaters and water sprays. Steam can be formed (by the heaters) or condensed (by the pressurizer spray) to minimize pressure variations due to contraction and expansion of the reactor coolant. Multiple channels of pressurizer level and pressure instrumentation are available to generate appropriate reactor trip and engineered safeguards actuation signals. These instruments are also used for NC pressure and level control.

Technical Specifications:

CNS Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.13, "Reactor Coolant System Operational Leakage" applies in modes 1, 2, 3, and 4 and requires no pressure boundary leakage. Additionally, unidentified leakage shall be limited to 1 gpm and identified leakage shall be limited to 1 O gpm. When unidentified leakage is not within the limits, TS 3.4.13, Action 8.1 requires the leakage to be within limits within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. If leakage cannot be reduced to within limits within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, TS 3.4.13, Action C.1 requires the unit be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.

EVENT DESCRIPTION

Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Library, and lnfonnation Collections Branch {T-6 A10M}, U. S.

Nuclear Regulatory Commission, Washington, DC 20555-0001, or by email to lnfocollects.Resource@nrc.gov, and the 0MB reviewer at: 0MB Office of lnfonnation and Regulatory Affairs, {3150-0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of J.1..-

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On January 28, 2025, Catawba Unit 1 was operating in Mode 1 at approximately 100% power. At 0115, indications were received in the main control room indicative of a reactor coolant system leak. Abnormal operating procedure AP-10, Reactor Coolant Leak, was entered at 0225. Control room operators initiated an NC leakage calculation per procedure and at 0402 an unidentified leakage rate of 1.414 gpm was determined. At 0430 Unit 1 entered LCO 3.4.13 Condition B. A containment entry was made to inspect accessible areas for sources of leakage. No leakage was identified and the Unit entered Mode 3 on 1/28/25 at 1255. Additional containment entries were made in Mode 3 and the source of leakage was identified in the pressurizer cavity; however, in Mode 3 conditions did not permit additional inspection and operators placed the plant in Mode 5.

Once conditions allowed, plant personnel identified tubing separation at a compression fitting inside the pressurizer cavity. The separated section of tubing originates on a pressurizer high-pressure tap and supports multiple instruments, including pressurizer level and pressure. This tubing was previously modified after the first operating cycle in 1986 to re-route pressurizer instrument lines and replace compression fittings with welded connections. As a result of this plant modification the section of tubing that separated remained in place and was not isolated, creating a "dead-leg" extension of the reactor coolant pressure boundary.

Prior to Unit 1 returning to power operations, the affected tubing was disconnected and plugged and isolation valves were verified to be closed. During the shutdown the affected tubing section and fitting were not retrieved from containment; however, work orders have been generated to retrieve the tubing and fitting for forensic analysis at the first available opportunity.

CAUSAL FACTORS A Root Cause Evaluation was completed for the reactor coolant system leak. Since the separated fitting was not available for analysis, a most probable cause was identified based on operating experience and photographs collected of the separated compression fitting. The compression fitting is suspected to be a Parker Hannifin CPI connector. Previous industry operating experience identified some Parker Hannifin supplied compression fittings contained nitride-coated ferrules until a new hardening process was implemented in 1986. The nitriding process sensitized the stainless steel of the ferrules leaving them susceptible to corrosion. Previous operating experience confirmed that nitride coated ferrule compression fittings were supplied to Catawba that resulted in separated tubing in other plant systems. Given the vintage of the separated fitting and available photographic evidence, the most probable cause is corrosion induced failure of Parker Hannifin supplied CPI compression fittings with nitride coated ferrules.

Additionally, the failure to remove existing tubing and isolation valves when the tubing was modified in 1986 left system vulnerabilities and did not fully address the intent of the modification. Therefore, a second cause was identified as a failure to eliminate risk from an unused branch line connected to the reactor coolant system due to less than adequate Engineering Change procedure instructions for equipment abandonment.

NRCFORM

CORRECTIVE ACTIONS

Immediate:

050 052 Estimated burden per response to comply with this mandatory collection request: 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Library, and Information Collections Branch (T-6 A10M), U. S.

Nuclear Regulatory Commission, Washington, DC 20555-0001, or by email to lnfocollects.Resource@nrc.gov, and the 0MB reviewer at: 0MB Office of Information and Regulatory Affairs, (3150-0104), Attn: Desk Officer for the Nuclear Regulatory Commission, 725 17th Street NW, Washington, DC 20503. The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of

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1. Closed the root valves and plugged tubing as close as practical in the instrument tube associated with the leaking fitting in the Unit 1 pressurizer cavity.

Planned:

1. Revise AD-EG-ALL-1132 (Preparation and Control of Engineering Changes) to clarify the requirement to physically separate SSCs no longer intended to be used (i.e., abandoned) from in service plant systems, and provide additional details related to roles and requirements associated with the abandonment process. In cases where these SSCs cannot be fully disconnected, a detailed operational risk assessment is to be performed based on the consequences of the equipment failing.

2. Obtain Met Lab analysis of failed connector. Update RCE and LER based on conclusion. Enter Part 21 and License Renewal Aging Management processes based on conclusions reached.

3. Ensure all Parker Hannifin fittings in stock were purchased after the change to an alternate hardening method for the ferrule.

SAFETY ANALYSIS

The reactor coolant system leak from a fitting on a legacy pressurizer instrument line presents a reduction in the reliability and availability of the Catawba Unit 1 NC system. The leak resulted in a controlled shutdown to Mode 5 due to NC unidentified leakage exceeded Technical Specifications requirements.

There was no plant scram, other plant transient, radiological release, or other challenge to normal plant operation to cause any impacts to the public health and safety. A full evaluation of the impacts of this event is in progress and not complete at the time of this report. However, based on preliminary analysis and qualitative considerations, the overall risk impact is expected to be low.

Catawba Unit 1 has a considerable level of defense-in-depth for the inventory of the reactor coolant system, and for the ability to trip on pressurizer low level. These defense in depth systems were not needed in this event and the operators were able to perform an orderly shutdown.

With these factors taken together the impact of this event on plant risk was very low and there was no impact to the health and safety of the public.

ADDITIONAL INFORMATION

There have been no previous Licensee Event Reports at Catawba Nuclear Station in the last three years with the same causal factor as this issue.

NRCFORM