ML24080A393

February 21, 2024, Summary of U.S. NRC Public Meeting with the PWROG on Topics Related to Pipe Stress Corrosion Cracking Operating Experience

ML24080A393
Person / Time
Issue date:	04/01/2024
From:	Leslie Fields Licensing Processes Branch
To:	Gerond George Licensing Processes Branch
Shared Package
ML24080A332	List: ML24080A393
References
Download: ML24080A393 (6)

Text

April 1, 2024

MEMORANDUM TO: Gerond A. George, Chief Licensing Projects Branch Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

FROM: Leslie C. Fields, Senior Project Manager /RA/

Licensing Projects Branch Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

SUMMARY

OF FEBRUARY 21, 2024, U.S. NUCLEAR REGULATORY COMMISSION PUBLIC MEETING WITH THE PRESSURIZED WATER REACTOR OWNERS GROUP ON TOPICS RELATED TO PIPING STRESS CORROSION CRACKING OPERATING EXPERIENCE

On February 21, 2024, the U.S. Nuclear Regulatory Commission (NRC) staff held a public observation meeting with the Pressurized Water Reactor Owners Group (PWROG) and industry representatives on items related to the French stress cracking issue and its applicability to the US pressurized water reactors. The meeting was held in a hybrid format (i.e., meeting attendees participated in person, via computer, and by t eleconference) via Microsoft Teams. The meeting provided the opportunity for NRC staff to have an open dialogue with industry representatives in an open and collaborative environment. Presentations were given by David Rudland of NRC, Flavien Simon of Nuclear Safety Authority/Frenc h Electricity (ASN/EDF), a French Regulatory Authority, Anees Udyawar from Westinghouse Electric Company (Westinghouse), Heather Malikowski of the Electric Power Research Institute, and Craig Wicker of Framatome. The meeting notice and presentation slides are publicly available in t he NRCs Agencywide Documents Access and Management System (ADAMS) under Accession Package No. ML24026A180. A list of the meeting attendees is enclosed.

The main purpose of the meeting was to exchange information and discuss the results on the analyses to determine the applicability of the atypical stress corrosion cracking found in safety injection piping in France. Three different topics on piping stress corrosion were presented from three different speakers which were all interrelated. A summary is provided below.

CONTACT: Leslie Fields, NRR/DORL 301-415-1186

G. George

The first presentation entitled Stress Corrosion Cracking in French Reactors was presented by Flavien Simon the Director of Pressure Vessels Division of the ASN/EDF. The presentation provided an overview of the issues concerning piping stress corrosion, the ASN current status/perspective, and the Western European Nuclear Regulator Association (WENRA) recommendations. WENRA was created when the lead nuclear safety regulators for the European Union and Switzerland decided to start a co-operation in 1999.

The speaker went over the past findings of stress corrosion cracking (SCC) and the plans to expand the inspection program in France. The main discussion points here focused on the ever-evolving root cause for this cracking. The EDF root cause assessment is still ongoing, but it was conveyed that the thermal stratification loading on these pipe systems is the main cause of the cracking. Many of the cracks were described as very long and shallow, some reaching 360 degrees (some with multiple flaws that coalesced) in length but only 6mm deep. Thermal stratification loads act like a global bending load on the pipe, i.e., tension on one side, and compression on the other. This type of loading does not lend itself to long shallow cracks and unusual weld residual stress does promote this kind of cracking. ASNs argument (and Framatome agrees) is that the bulk of the cracking has occurred in only the lines with these higher stratification loads, which means it has to be a main contributor.

The EDF inspection program will continue through 2026, and they expect to have inspected over 1000 welds over the course of this inspection program. Currently they have found over 100 indications across the emergency core cooling system (ECCS) piping inspected. Future inspections are going to focus on locations of weld repairs and location where they think SCC might be most likely. They will also analyze the water chemistry and temperature profiles at some of the critical locations and inspect the main coolant loop. It was stated that they have already looked at the pressurizer surge line and have not found any indications.

WENRA representatives made nine recommendations listed in slides 19 and 20 of the presentation enclosed in this meeting summary. These include the design of piping shall limit loads resulting from thermal gradients. Water chemistry, materials, welding procedures, and repairs are also identified as factors.

The next presentation given by PWROG representatives from Framatome, Westinghouse, and EPRI entitled Auxiliary Piping SCC listed above discussed the results of the safety assessment which includes deterministic analyses, a survey of the fleet wide operating experience OpE and past inspection data. They have concluded that the location of the flaws in the EDF fleet have been inspected in the US, but they recommend an enhancement to their inspection procedures and have implemented those through NEI 03-08 guidance (which were used in our LIC-504 analyses). Also, past OpE and inspection do not indicate any SCC similar to that found in France within the applicable piping systems in the US fleet. The representatives also stated that a break in these lines would be bounded by design basis loss-of-coolant accident (LOCA) analyses.

Westinghouse presented information that two US plants have already implemented the updated inspections per the NEI 03-08 guidance, and no reportable indications were found.

The PWROG representatives presented details about the ongoing applicability assessment, which is not yet finalized but should be published in April of 2024. This effort is analyzing root cause with topics including thermal stratification, weld residual stress, material chemistry, and water chemistry. They are still considering how to address the question about 360 degree cracking in those lines with high thermal stratification loads, when those loads would not cause G. George

this type of cracking. The discussion led to possibility that the stratification loads aided in initiating the cracking and the weld residual stress drove the crack propagation. There was also some indication that a combination of fatigue and SCC may be the cause. They will provide updates at the June 2024 Materials technical exchange meeting. The industry representatives also presented how their actions are meeting the WENRA recommendations.

Finally, the EPRI representatives presented on an ongoing testing program they are conducting to investigate the high hardness that was discovered near some of these welds removed from the French fleet. One theory is the high hardness may have lowered the resistance to this cracking. The program is using Gleeble testing, which is a small specimen testing arrangement typically used in weld development processes to quickly measure metallurgical properties across a range of processing conditions, to simulate the strain hardening that occurs during welding. Initial testing suggests that they cannot recreate the hardness levels measured in the EDF fleet materials, but their program is still underway. They will also provide updates at the June Materials technical exchange meeting.

David Rudland of the NRC gave a presentation showing the strength of how the NRCs nuclear power plant materials team is using NRC analyse s, results, and risk-informed decision making (RIDM) to determine how safety margin and performance monitoring (with low risk) can affect RIDM. In fact, even the industry work, which concluded low risk, increased the performance monitoring requirements through NEI 03-08. Pa rallel investigations from both the NRC and industry yielded about the same results.

At the conclusion of the meeting, there was an opportunity for the public to ask questions and provide comments related to the meeting topic. A member of the Union of Concerned Scientists asked a question about seismic loading and made an inquiry about how the NRC is convinced that it does not have this type of cracking in the licensing fleet. The NRC staff stated that during past, current, and ongoing inspections, in addition to results of risk-informed analyses, it can be assumed that this cracking exists, but it was not safety significant.

Please direct any inquiries to me at (301) 415-1186 or by e-mail at Leslie.Fields@nrc.gov.

Docket No. 99902037

Enclosure:

List of Meeting Attendees

ML24080A332 (Meeting Package)

ML24051A163 (Meeting Notice)

ML24051A014 (NRC Slides)

ML24051A015 (PWROG Slides)

ML24051A016 (EPRI Slides)

ML24051A082 (ASN Slides)

ML24080A393 (Summary)

OFFICE NRR/DORL/LLPB/PM NRR/DORL/LLPB/LA NRR/DNRL/SLA NRR/DORL/LLPB/BC NAME LFields DHarrison DRudland LFields DATE 3/22/2024 3/25/2024 3/27/2024 4/1/2024 FEBRUARY 21, 2024, U.S. NUCLEAR REGULATORY COMMISSION PUBLIC MEETING WITH THE PRESSURIZED WATER REACTOR OWNERS GROUP ATTENDANCE LIST

Name Organization Name Organization Leslie Fields U.S. Nuclear Regulatory James Lynde Pressurized Water Reactor Owners Commission (NRC) Group (PWROG)

David Rudland NRC Tyler Whaley PWROG Seung Min NRC Jim Molkenthin PWROG Matthew Mitchell NRC Brian Mount Dominion Energy Angie Buford NRC Flavien Simon ASN (French Regulatory Authority)

John Honcharik NRC Heather Malikowski Electric Power Research Institute (EPRI)

Matthew Leech NRC James Cirilli EPRI Matthew Homiack NRC Sarah Davidsaver Framatome Inc. (Framatome)

Chris Nellis NRC Craig Wicker Framatome Samuel Lee NRC Stacy Yoder Framatome Robert Tregoning NRC Leslie White Framatome John Tsao NRC Pascal Brocheny Framatome Jay Collins NRC Ryan Hosler Framatome David Dijamco NRC Anees Udyawar Westinghouse Electric Company (Westinghouse)

Stephen Cumblidge NRC Dewey Olinski Westinghouse Dan Widrevitz NRC James Andrachek Westinghouse Matthew Mitchell NRC Abbas Mohammadi Westinghouse Bart Fu NRC David Barton Westinghouse Dale E Turinetti NRC Dulal Bhowmick Westinghouse Ali Rezai NRC Brian Hall Westinghouse Varoujan Kalikian NRC Melinda J Higby Westinghouse Eric Reichelt NRC Thomas Laubham Westinghouse Steven Levitus NRC Bryan Wilson Westinghouse Robert Davis NRC Christopher Koehler Xcel Energy Scottman Ammons INPO Paul Guill Duke Energy H. Kobayashi EPRI International Mark Honeycutt Duke Energy Robert Magnusson (GN-UQM) Beth Haluska Dominion Energy Kevin Hall IDDEAL Charles Tomes Dominion Energy Michel De Smet TRACTEBEL - Corey Thomas Southern Nuclear BELGIUM Jana Bergman Curtiss Wright Lora Drenth Constellation Edwin Lyman Union of Concerned Osvaldo Cruz Constellation Scientists Gabor Petofi IAEA Jovica Riznic Canadian Nuclear Safety Commission Ubert Coetzee National Nuclear Bogdan Wasiluk Canadian Nuclear Safety Regulator Commission Glenn Chatterton American Electric Power Josh Morton Energy Harbor Theresa Willson American Electric Power Michael Garner South Texas Project Enclosure Name Organization Name Organization Sue Liu Alexandria Scott Suyash Deshpande Jasmyn Bone Maria Garcia Dan Solitz Conrad Wyffels Anthony Bushmire Li Qingquan Yasaman Ghaffari Bret Flesner Marie Darling Daisuke Nio DJ Shim Derrick Moreau Lionel Dejoux Kevin Hacker Suqiang Xu Wijay Heinemann Xuejun Wei Kevin Allen Yong-Zhi Wang Eiji Machida Jinseok Park Shuichi Orita Daniel Laudien Bengt Lydell Robson Alves Francois Foct Uwe Jendrich Maria Rizzilli Denis Gomes Keith Harrison Remington Iddings Joaquín Fole Gurunathan Hernández Ramaraj

• • Attendance list based on Microsoft Teams Participant list and in-room sign-in sheet.

This list does not include individuals that connected via phone.