05000414/LER-2025-004, Through Wall Leak in Letdown Piping
| ML25307A221 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 11/03/2025 |
| From: | Huecker J Duke Energy Carolinas |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| RA-25-00254 LER 2025-004-00 | |
| Download: ML25307A221 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 4142025004R00 - NRC Website | |
text
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RA-25-00254 November 03, 2025 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555
Subject:
Duke Energy Carolinas, LLC Catawba Nuclear Station, Unit 2 Docket No. 50-414 Licensee Event Report (LER) 2025-004-00 Jonathan Huecker Vice President Catawba Nuclear Station Duke Energy CN01VP I 4800 Concord Road York, SC 29745 o: 803.701.3349 10 CFR 50.73 Pursuant to 10 CFR 50.73(a)(1) and (d), attached is LER 414/2025-004-00, entitled "Through Wall Leak in Letdown Piping". This report is being submitted in accordance with 1 OCFR50. 73(a)(2)(ii)(A).
There are no regulatory commitments contained in this letter or its attachment.
The impact of this event in relation to plant risk was low and there was no impact 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, Vice President, Catawba Nuclear Station Attachment
United States Nuclear Regulatory Commission Page 2 November 03, 2025 xc (with attachment):
Julio Lara Acting 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 9/4/2025, Catawba Unit 2 was shutdown for a scheduled refueling outage. During initial containment walkdown activities approximately 1 cubic foot of dried boron was observed near a vent valve connected to the Reactor Coolant System letdown piping. Upon additional inspection when conditions allowed on 9/5/2025 the source of the boron was identified as a small crack in a weld between the letdown piping and a vent valve. Upon confirmation of the leak, the flow path was isolated in accordance with plant Technical Specifications. The affected piping section was modified and repaired during the refueling outage.
The leakage rate was not quantifiable through routine Reactor Coolant System leakage calculations; however, it was determined to be part of the Reactor Coolant Pressure Boundary. An eight-hour, non-emergency notification per 10 CFR 50.72(b)(3)(ii)(A), was submitted (Reference Event Notification 57906).
This event is reportable under 1 0CFR50. 73(a)(2)(ii)(A) as a condition of the nuclear power plant, including its
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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 System [EIIS] and component codes are enclosed within brackets.
Catawba's unique system and component identifiers are contained within parentheses.
The Reactor Coolant System [AB](NC) transports heat from the reactor coolant system to the steam generators, where heat is transferred to the Feedwater system and Main Steam System of the secondary side. The reactor coolant system pressure boundary provides a barrier against the release of radioactivity generated within the reactor and is designed to ensure a high degree of integrity throughout the life of the plant. The Reactor Coolant Pressure Boundary (RCPB) extends to the outermost containment isolation valve in system piping which penetrates the containment and is connected to the NC system. Thus, the RCPB contains portions of systems beyond the NC system. The RCPB provides a barrier against the release of radioactivity generated within the reactor.
The Chemical and Volume Control System [CB](NV) functions during both normal unit operation and in support of the Emergency Core Cooling System (ECCS) during mitigation of Small-Break and Large-Break Loss of Coolant Accidents (LOCAs). The specific functions during normal unit operation can be categorized as:
- 1. Letdown, charging and Reactor Coolant Pump seal injection, and
- 2. Chemical control, purification and makeup.
The letdown and charging functions of the NV system are employed to maintain a programmed water level in the Reactor Coolant System Pressurizer, thus maintaining proper reactor coolant inventory during all phases of plant operation. This is achieved by a continuous feed and bleed process from the Reactor Coolant System. The chemical control, purification and makeup functions are used to provide pH control chemicals, control oxygen concentrations, purification of the NC system by removing corrosion products, fission products, lithium, and chemical impurities and addition/removal of boric acid for reactivity control.
TECHNICAL SPECIFICATIONS Catawba TS 3.4.13 governs Reactor Coolant System Operational Leakage in Modes 1, 2, 3, and 4.
Limiting Condition for Operation (LCO) 3.4.13 A requires isolation of affected component, pipe or vessel from the Reactor Coolant System by use of a closed manual valve, closed and de-activated automatic valve, blind flange, or check valve within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> if pressure boundary leakage exists. If the leak is not isolated within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> the unit must 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 in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> per LCO 3.4.13 Required Actions C.1 and C.2.
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 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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On 9/4/2025, Catawba Unit 2 was shutdown for a scheduled refueling outage. During initial containment walkdown activities approximately 1 cubic foot of dried boron was observed near a vent valve connected to the Reactor Coolant System letdown piping. The section of piping is insulated which made determination of the source of the leak difficult. Initially the leakage was thought to be from either the valve packing or a pipe cap downstream of the vent valve. The most recent Reactor Coolant Leakage calculation was reviewed and did not indicate a quantifiable leak in the reactor coolant system.
On 9/5/2025 with the Unit in Mode 4 a more thorough investigation into the source of the leakage was performed. With the insulation removed and the residual boron deposits cleaned, a small crack in a welded connection between the letdown piping and vent valve was identified. Upon confirmation of the leak source, the letdown flow path was isolated in accordance with plant Technical Specifications. It was additionally observed the vent valve was rubbing adjacent piping which left visual markings on the pipe and valve but no indication of degradation. The affected piping section was modified and repaired during the refueling outage.
The affected section of piping is on a vent valve connection on the letdown flow path. The pipe in this location is downstream of the letdown orifices and is ASME Class 2 pipe rated for 615 psia and protected from overpressure by a relief valve designed to lift at 600 psig. Although this is ASME Class 2 pipe in the NV system it does meet the definition of reactor coolant pressure boundary as the location is between the Reactor Coolant System and the outermost containment isolation valve with no normally closed valves in the flow path. Due to the crack in the reactor coolant pressure boundary, this condition is reportable under 10CFR50.73(a)(2)(ii)(A) as a condition of the nuclear power plant, including its principal safety barriers, being seriously degraded.
CAUSAL FACTORS The affected portion of piping was removed from the system and submitted for laboratory analysis. The observations from the evaluation include:
Cracking in the weld was attributed to high-cycle (vibrational) fatigue.
An older portion of the crack was found; this crack may have been present for some time before reaching a critical size and accelerating to the OD surface.
The contact between the valve and pipe may have provided an additional source of vibration.
The cantilever geometry of the valve being supported by the pipe and elbow likely contributed to the vibration.
The weld appeared to have been correctly applied, although the weld profile was slightly underfilled (concave) where the crack occurred.
No issues were observed with the materials of construction.
CORRECTIVE ACTIONS
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 H.
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1-0 A plant modification was performed to replace the existing vent valve with a smaller/lighter valve as well as replace all the 1-inch schedule 40 piping in the affected branch with 3/4 inch schedule 80 piping utilizing 2: 1 taper welds. The pipe configuration was revised to eliminate the vent valve contacting adjacent piping.
SAFETY ANALYSIS
The crack location was in a lower pressure portion of the letdown line downstream of the letdown orifices. The flaw in the pipe was minimal and did not result in quantifiable leakage from the system and did not appear to be active at the time of discovery. In the event the leak was to progress and worsen, the letdown line is easily isolable via numerous remotely operated valves to restore the reactor coolant pressure boundary in a timely manner. Additionally, valves that provide isolation capability automatically close on either a containment isolation signal, or low pressurizer level.
With these factors taken together there was no impact on 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.