Study of Ultrasonic Techniques on the Inspection of NPP Components

2009

Research is being conducted for the U.S. Nuclear Regulatory Commission at the Pacific Northwest National Laboratory to assess the effectiveness and reliability of advanced nondestructive examination (NDE) methods for the inspection of light-water reactor components. The scope of this research encompasses primary system pressure boundary materials including dissimilar metal welds (DMWs), cast austenitic stainless steels (CASS), piping with corrosion-resistant cladding, weld overlays, inlays and onlays, and far-side examinations of austenitic piping welds. A primary objective of this work is to evaluate various NDE methods to assess their ability to detect, localize, and size cracks in steel components that challenge standard and/or conventional inspection methodologies. This interim technical letter report (TLR) provides a summary of a technical evaluation aimed at assessing the capabilities of phased-array (PA) ultrasonic testing (UT) methods as applied to the inspection of small-bore DMW components that exist in the reactor coolant systems (RCS) of pressurized water reactors (PWRs). Operating experience and events such as the circumferential cracking in the reactor vessel nozzle-to-RCS hot leg pipe at V.C. Summer nuclear power station, identified in 2000, show that in PWRs where primary coolant water (or steam) are present under normal operation, Alloy 82/182 materials are susceptible to pressurized water stress corrosion cracking (PWSCC) (NRC 2008b, 2009). The extent and number of occurrences of DMW cracking in nuclear power plants (domestically and internationally) indicate the necessity for reliable and effective inspection techniques. The work described herein was performed to provide insights for evaluating the utility of advanced NDE approaches for the inspection of DMW components such as a pressurizer (PZR) surge nozzle DMW, a shutdown cooling pipe DMW, and a ferritic (low-alloy carbon steel)-to-CASS pipe DMW configuration. In this study, a set of flaws, including both thermal fatigue cracks and electrical discharge machining (EDM) notches that were hot isostatically pressed (HIP'ed) were inserted into five DMW mock-up specimens. The DMW specimens used in this study were either salvaged from a cancelled plant that had never been in an operational state, or were fabricated from new and/or vintage materials. PA-UT flaw responses from these cracks were used to evaluate detection and characterization performance. The PA-UT examination approach used on the DMW component mock-ups focused on acquiring data as a function of inspection frequency (2.0 MHz, 1.5 MHz, and 1.0 MHz), inspection angle (30° to 70° in 1° increments), and by employing both raster and line-scan data acquisition. Data were acquired from both sides of the welds; however, the focus in this document is to evaluate inspection performance by assessing the data taken from the austenitic side of the DMW configuration, as this is the side where an inspector typically has access in the field. The data analyses conducted here provide quantitative information regarding detection and sizing performance against the true state. Additional metrics used for these analyses included signal-to-noise ratio (SNR) and root-mean-square error (RMSE) for length and depth sizing. Also, a comparison of detection and sizing performance for line-scan versus raster-scan data was also conducted to determine if inspection performance is significantly enhanced by performing more rigorous and time-consuming raster scans over simple line scans. Finally, a qualitative assessment of the effect of angular flaw tilt (from the normal to the inner-diameter [ID] surface) on detection and flaw characterization performance was conducted.

2009

Research is being conducted for the U.S. Nuclear Regulatory Commission (NRC) at the Pacific Northwest National Laboratory (PNNL) to assess the effectiveness and reliability of advanced nondestructive examination (NDE) methods for the inspection of light water reactor components. The scope of this research encompasses primary system pressure boundary materials including cast austenitic stainless steels (CASS); dissimilar metal welds; piping with corrosion-resistant cladding; weld overlays, inlays and onlays; and far-side examinations of austenitic piping welds. A primary objective of this work is to evaluate various NDE methods to assess their ability to detect, localize, and size cracks in coarsegrained steel components. In this effort, PNNL supports cooperation with Commissariat à l'Energie Atomique (CEA) to assess reliable inspection of CASS materials. The NRC Project Manager has established a cooperative effort with the Institut de Radioprotection et de Surete Nucleaire (IRSN). CEA, under funding from IRSN, are supporting collaborative efforts with the NRC and PNNL. Regarding its work on the NDE of materials, CEA is providing its modeling software (CIVA) in exchange for PNNL offering expertise and data related to phased-array detection and sizing, acoustic attenuation, and back scattering on CASS materials. This collaboration benefits the NRC because CEA performs research and development on CASS for Électricité de France (EdF). This technical letter report provides a summary of a technical evaluation aimed at assessing the capabilities of phased-array (PA) ultrasonic testing (UT) methods as applied to the inspection of welds in CASS pressurizer (PZR) surge line nuclear reactor piping. A set of thermal fatigue cracks (TFCs) was implanted into three CASS PZR surge-line specimens (pipe-to-elbow welds) that were fabricated using vintage CASS materials formed in the 1970s, and flaw responses from these cracks were used to evaluate detection and sizing performance of the PA-UT methods applied. This effort was comprised of multiple elements that included use of microstructural knowledge (dimensional analysis, grain orientation, and grain type) as well as sound field modeling to more effectively modify inspection parameters and enhance the inspection outcomes. Advanced probe design and sound field simulations were employed to enhance detection and characterization of circumferentially oriented flaws, and an assessment of lateral (circumferential) flaw localization capability and performance was also conducted. An evaluation of flaw detection, length sizing, depth sizing, and signal-to-noise ratio was performed for all flaws in the subject specimens, as a function of various inspection parameters, and finally, measurements were made to quantify and assess the baseline CASS material noise and its potential impact on flaw detection.

2007

Studies conducted at the Pacific Northwest National Laboratory in Richland, Washington, have focused on assessing the effectiveness and reliability of novel approaches to nondestructive examination (NDE) for inspecting coarse-grained, cast stainless steel reactor components. The primary objective of this work is to provide information to the U.S. Nuclear Regulatory Commission on the effectiveness and reliability of advanced NDE methods as related to the inservice inspection of safety-related components in pressurized water reactors (PWRs). This report provides progress, recent developments, and results from an assessment of low frequency ultrasonic testing (UT) for detection of inside surface-breaking cracks in cast stainless steel reactor piping weldments as applied from the outside surface of the components.

2008

Research is being conducted for the U.S. Nuclear Regulatory Commission (NRC) at the Pacific Northwest National Laboratory (PNNL) to assess the effectiveness and reliability of advanced nondestructive examination (NDE) methods for the inspection of light water reactor (LWR) components. The scope of this research encompasses primary system pressure boundary materials including cast austenitic stainless steels (CASS), dissimilar metal welds (DMWs), piping with corrosion-resistant cladding, weld overlays, and far-side examinations of austenitic piping welds. A primary objective of this work is to evaluate various NDE methods to assess their ability to detect, localize, and size cracks in coarse-grained steel components. This interim technical letter report (TLR) provides a synopsis of recent investigations at PNNL aimed at evaluating the capabilities of phased-array (PA) ultrasonic testing (UT) methods as applied to the inspection of CASS welds in nuclear reactor piping. A description of progress, recent developments and interim results are provided. The ongoing work presented in this report encompasses: • The use of phased-array ultrasonic methods and advanced probe designs to enhance detection and characterization of circumferentially oriented flaws in CASS piping specimens, and • Preliminary investigation of delta-techniques for flaw-depth sizing using tip-diffracted energy from thermal fatigue cracks in CASS components. While previous work primarily focused on thicker primary loop piping components with outer diameters (OD) ranging from 28 to 36 inches (71.12 to 91.44 cm), a variety of thinner cast piping welds exist in pressurized water reactors (PWRs) and an optimum frequency has yet to be established to examine these thin CASS components. While the sample specimens used in this study may not represent worst-case field conditions with regard to microstructures, weld root conditions, counterbore facets, surface roughness/geometry, etc., the sample specimens provide a range of grain structure conditions and are representative of components found in plants. The work reported here is focused toward evaluating UT inspection capabilities in CASS pressurizer surge line piping welds as well as continuing to assess improved PA probe designs on primary loop, CASS piping. PNNL was tasked with acquisition and procurement of materials, fabrication of flawed specimens, modeling and procurement of specialized phased-array probes, and conducting data acquisition and analysis on these specimens. The macrostructure of vintage pressurizer (PZR) surge line piping to determine grain size, shape, and orientation was performed in FY-2007. The material comprising these components include both centrifugally cast (pipe segments) and statically cast (elbows), with dimensions of approximately 12-to 14-inch (30.5 to 35.5 cm) OD and 1.3-inch (33-mm) wall thickness. This thickness is representative of typical PZR surge lines and various safe-ends in Combustion Engineering-designed plants. The ongoing activities reported here included PA-UT examinations on implanted/fabricated thermal fatigue cracks to evaluate detection, localization, and sizing performance as a function of frequency. In conjunction with these activities, PNNL employed customdesigned, low-frequency PA probes for continued evaluation of inspection performance on thicker-section CASS pipe segments, such as the Westinghouse Owner's Group practice set of specimens.

2011

PNNL has been studying and performing confirmatory research on the inspection of piping welds in coarse-grained steels for over 30 years. More recent efforts have been the application of low frequency phased array technology to this difficult to inspect material. The evolution of 500 kHz PA probes and the associated electronics and scanning protocol are documented in this report. The basis for the probe comparisons are responses from one mechanical fatigue crack and two thermal fatigue cracks in largebore cast mockup specimens on loan from the Electric Power Research Institute.