An Assessment of Ultrasonic Techniques for Far-Side Examinations of Austenitic Stainless Steel Piping Welds (NUREG/CR-7113, PNNL-19353)
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Publication Information
Manuscript Completed: October 2011
Date Published: November 2011
Prepared by:
M. T. Anderson, A. A. Diaz, A. D. Cinson, S. L. Crawford,
S. E. Cumblidge, S. R. Doctor, K. M. Denslow, and S. Ahmed
Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352
W. E. Norris, NRC Project Manager
NRC Job Code N6398
Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
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Abstract
Austenitic stainless steels are used in operating nuclear power plants primary loop piping because of their resistance to corrosion, or in locations where high strength and creep resistance are required because of elevated temperatures. Austenitic stainless steel piping welds are susceptible to stress corrosion cracking so it is essential to perform effective and reliable inspections. However, their anisotropic and large-grained structure strongly affects the propagation of ultrasound making it difficult to inspect. Another impediment to performing satisfactory inspections is that a significant number of weld locations exist where component geometry or permanent obstructions only provide suitable access to one side of the weld. These "far-side inspections" of piping welds currently are performed on a "best-effort" basis and with the exception of single-side dissimilar metal welds, do not conform to ASME Code Section XI, Appendix VIII, performance demonstration requirements. The industry, through the Electric Power Research Institute (EPRI), is actively involved in improving far-side inspections through the use of phased-array probes.
As a result, confirmatory research is being conducted for the U.S. Nuclear Regulatory Commission at the Pacific Northwest National Laboratory to assess the capabilities of ultrasonic testing (UT) for the examination of austenitic stainless steel piping welds from the far side. Specifically, studies were conducted to assess the ability of advanced UT techniques to detect and determine the size of flaws from the far-side of wrought austenitic piping welds. Far-side inspections of nuclear system piping welds are currently performed on a "best-effort" basis and do not conform to ASME Code Section XI Appendix VIII performance demonstration requirements. To ensure safety in the case of less than 100% examination coverage of a particular weld, the staff requires that the licensee demonstrate in its relief request that other welds of the same examination category have had 100% coverage with no flaws, and that the subject weld has no active degradation mechanism based on operating experience. The staff may also require the licensee to improve its examination coverage by improving its examination technique (e.g., using certain transducers). The staff approves these less-than-100% examination coverage relief requests based on rigorous evaluation to ensure that structural integrity of the piping is maintained in accordance with the ASME Code, Section XI. For the laboratory work in this study, four circumferential welds in 610-mm-diameter, 36-mm-thick ASTM A-358, Grade 304 vintage austenitic stainless steel pipe were examined. The welds were fabricated with varied welding parameters; both horizontal and vertical pipe orientations were used, with air and water backing, to simulate field welding conditions. A series of saws, electro-discharge machined notches, and implanted fatigue cracks were placed into the heataffected zones of the welds. The saws and notches ranged in depth from 7.5% to 28.4% through-wall. The implanted cracks ranged in depth from 5% through-wall to 64% through-wall. The welds were examined with phased-array technology operating at 1.5 MHz, and with lowfrequency/Synthetic Aperture Focusing Technique (SAFT) methods in the 250–400 kHz regime. These results were compared to encoded conventional (monolithic transducer element) ultrasonic techniques as a baseline at 2.0 MHz, a frequency typical for examinations of piping welds in the field. The examinations showed that while phased-array and low-frequency/SAFT were each able to detect and accurately length-size, but not depth-size, the notches and flaws through the welds, phased-array ultrasonic inspection provided the best results, detecting nearly all of the flaws from the far side. The ultrasonic results were insensitive to the different welding techniques used in each weld.
Based on the laboratory work conducted in this study, phased-array ultrasonic inspection provided the best results, detecting nearly all of the flaws from the far side. These results were presented at the Fifth International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components in 2006. To better understand acoustic propagation through austenitic welds to determine whether the phased-array inspections currently being conducted are effective and reliable, sound-field mapping was conducted. At the invitation of EPRI, PNNL examined field-removed specimens containing service-induced intergranular stress corrosion cracks at the EPRI Nondestructive Evaluation Center, in Charlotte, North Carolina, to compare results. Collective results from the activities described above are presented here.
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