Assessment of the Mechanical Stress Improvement Process for Mitigating Primary Water Stress Corrosion Cracking in Nickel Alloy Butt Welds in Piping Systems Approved for Leak-Before-Break

Abstract

Executive SummaryIn recent years, operating experience has shown that Alloy 82/182/600 materials used in reactor coolant system (RCS) pressure boundaries of pressurized water reactors (PWRs) are susceptible to primary water stress corrosion cracking (PWSCC). Cracking can initiate at the inside surface of these materials, in part, because of tensile residual stresses introduced by welding. These materials are present in piping systems that were approved by the U.S. Nuclear Regulatory Commission (NRC) for leakbefore-break (LBB) before PWSCC was found in RCS dissimilar metal butt welds. The identification of PWSCC led to concerns regarding the potential effect of this degradation on existing LBB analyses.In response to these concerns, the NRC Office of Nuclear Regulatory Research initiated a program entitled, "PWSCC in Leak-Before-Break Systems," and a follow-on program entitled, "Degradation in Extremely Low Probability of Rupture (xLPR) Systems." Under these programs, Pacific Northwest National Laboratory (PNNL) is assessing the various strategies being used by industry to manage potential or existing PWSCC in susceptible welds in piping systems approved for LBB.The commercial nuclear power industry has implemented strategies to manage potential or existing PWSCC at Alloy 82/182 dissimilar metal welds (DMW) in PWRs. One general strategy consists of management by a combination of mitigation plus inspection, where several mitigation techniques have been used by industry. The other general strategy is to manage potential PWSCC by inspection alone.At the request of the NRC, the American Society of Mechanical Engineers (ASME) took actions to develop improvements to the existing Code requirements to address the potential for PWSCC in Class 1 PWR piping butt welds fabricated with Alloy 82/182 weld materials. The ASME developed Code Case N-770, "Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated with UNS N06082 or UNS W86182 Weld Filler Material With or Without Application of Listed Mitigation Activities, Section XI, Division 1." It was realized that the Code Case needed to be revised to address specific examinations such as welds with a design life of less than 10 years and post-weld preservice surface examinations. In general, the revised Code Case (N-770-1) has requirements for inspection of unmitigated as well as mitigated Alloy 82/182 RCS butt welds. As such, specific inspection requirements for welds mitigated by the Mechanical Stress Improvement Process (MSIP®), (a) the subject of this technical letter report (TLR), are contained in the Code Case. The NRC incorporated ASME Code Case N-770-1 by reference into §50.55a (76 FR 36232, p. 36278) in June 2011.This TLR provides an assessment of MSIP as a mitigation strategy and includes an assessment of the MSIP-related inspection requirements of Code Case N-770-1, as conditioned in §50.55a.Depending upon the weld geometry, fabrication practices, and the presence of a nearby safe end-topipe weld, Al...

Behavior, Failure Analysis, and Effectiveness of Mechanical Stress Improvement Process in Residual Stress Relaxations in Butt-Welded Austenitic Piping Using a Numerical Simulation Approach

“…The following year, Ford et al (2012) [18] delved into the intricate realm of intergranular stress corrosion cracking (IGSCC) in BWRs, unraveling the complexities of this speci c form of corrosion within the nuclear domain. Transitioning into 2012, Aoike and Tsuruki's research [19]presents the development of the Residual Stress Improvement Method for Small-Diameter Butt-Welding Pipe, demonstrating the practical application of stress improvement techniques in small-scale, intricate welding scenarios.Sullivan and Anderson's assessment in 2013 of the MSIP for mitigating primary water stress corrosion cracking in nickel alloy butt welds within piping systems approved for leak-before-break signi es an ongoing commitment to nuclear safety [20]. The storyline progresses into 2014 with Bolognesi and their associates shedding light on the modeling and characterization of residual stresses in material processing, presenting a holistic perspective on residual stresses across various manufacturing processes [21].…”