An Evaluation of Ultrasonic Phased Array Testing for Cast Austenitic Stainless Steel Pressurizer Surge Line Piping Welds (NUREG/CR-7122, PNNL-19497)
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Manuscript Completed: December 2011
Date Published: March 2012
A. A. Diaz, A. D. Cinson, S. L. Crawford, R. A. Mathews, T. L. Moran,
M. S. Prowant and M. T. Anderson
Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352
W. E., NRC Project Manager
NRC Job Code N6398
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001
Confirmatory 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 methods for the inspection of primary system pressure boundary components and materials in light-water reactors. The work reported here provides a technical evaluation to assess the capabilities of phased-array (PA) ultrasonic testing (UT) methods as applied to the inspection of welds in cast austenitic stainless steel (CASS) pressurizer (PZR) surge line piping. A set of thermal fatigue cracks (TFCs) were implanted into CASS PZR surgeline specimens (pipe-to-elbow welds) salvaged from cancelled nuclear power plants that were fabricated of vintage materials formed in the 1970s. Responses from these cracks were used to evaluate detection and sizing performance of the PA-UT methods applied. Custom arrays, operating nominally at 800 kHz, 1.0 MHz, 1.5 MHz, and 2.0 MHz were employed. Raster and line-scan data were acquired as a function of probe frequency and angle from both the centrifugally and statically cast base materials adjacent to the welds.
All implanted TFCs were easily detected in these specimens. The results reported here show that longitudinal mode, transmit-receive matrix phased-array probes, over the applied frequency range, provides effective sound fields for detection and characterization of TFCs in CASS PZR surge line components. PA-UT results were compared against true-state data for all cracks, and root mean square error was computed as a metric for both length- and depth-sizing of these cracks. Signal-to-noise ratio measurements were documented and analyses made to quantify the potential impact of material-induced attenuation and redirection of the sound fields on crack detection and localization.
In addition to the implanted TFCs in the weld region, five in-situ grown TFCs were placed in parent pipe and elbow regions. Unlike implanted flaws, this cyclic induction heating process grows TFCs directly in the specimen material. The resultant crack morphology is a product of the specimen composition and microstructure. It is believed that certain characteristics of in-situ grown cracks, such as branching, surface facets, and local discontinuities, are more representative of service-induced cracks. All of these cracks, implanted and in-situ grown, were detected and accurately characterized at multiple frequencies.
Based upon the results of this work, state-of-the-art phased-array inspection approaches are shown to be rapidly evolving and the capability to detect cracks in CASS components where the wall thickness is generally less than 50 mm (2.0 in.) has been demonstrated. While additional questions remain to be answered, long-wavelength ultrasonic approaches coupled with advanced signal processing technologies are beginning to show signs of success toward addressing this challenging inspection issue.
In addition, to provide an external confirmatory data set, an inservice inspection (ISI) supplier was contracted to examine two of the CASS PZR specimens using Performance Demonstration Initiative (PDI) qualified procedures and equipment. Their crack detection and characterization results are also reported here.
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