United States Nuclear Regulatory Commission - Protecting People and the Environment

Field Evaluation of Low-Frequency SAFT-UT on Cast Stainless Steel and Dissimilar Metal Weld Components (NUREG/CR-6984)

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Publication Information

Manuscript Completed: August 2008
Date Published: November 2008

Prepared by:
A.A. Diaz, R.V. Harris, S.R. Doctor

Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352

D.A. Jackson and W.E. Norris, NRC Project Managers

Prepared for:
Office of Nuclear Regulatory Research
US Nuclear Regulatory Commission
Washington, DC 20555-0001

RC Job Code N6398

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Abstract

This report documents work performed at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, and at the Electric Power Research Institute's (EPRI) Nondestructive Examination (NDE) Center in Charlotte, North Carolina, on evaluating a low-frequency ultrasonic inspection technique used for examination of cast austenitic stainless steel (CASS) and dissimilar metal (DMW) reactor piping components. It should be noted that this work predates efforts recently published in two reports: NUREG/CR-6933, “Assessment of Crack Detection in Heavy-Walled Cast Stainless Steel Piping Welds Using Advanced Low-Frequency Ultrasonic Methods,” published in March 2007 (Anderson et al. 2007), and NUREG/CR-6929, “Assessment of Eddy Current Testing for the Detection of Cracks in Cast Stainless Steel Reactor Piping Components,” published in February 2007 (Diaz et al. 2007). The results of the earlier work are being published because it was conducted semi-blind and will be a valuable source of information relative to performance demonstration assessments. In addition, there were some examinations of DMWs, and the number of studies published to date providing DMW examination results are limited.

The early work demonstrated the potential for using low-frequency ultrasound coupled with synthetic aperture focusing technique (SAFT) signal processing to detect cracking in coarse-grained stainless steels. The follow-on efforts are detailed in NUREG/CR-6929 and NUREG/CR-6933. It should be noted that the inspection techniques have greatly improved since the initial work; in particular, the use of low-frequency phased arrays which permits faster inspections, more flexible and precise scans, and better detectability.

The technique discussed in this report uses a zone-focused, multi-incident angle, low-frequency (250–450 kHz) inspection protocol coupled with the synthetic aperture focusing technique (SAFT). The primary focus of this work is to provide information to the United States Nuclear Regulatory Commission on the utility, effectiveness and reliability of ultrasonic testing (UT) inspection techniques as related to the inservice ultrasonic inspection of coarse grained primary piping components in pressurized water reactors (PWRs).

Experiments were conducted in order to assess the low-frequency (350 kHz) ultrasonic inspection technique for coarse-grained stainless steel components. Software was modified and experiments were performed for applying a noise reduction algorithm to the pre- and post-SAFT processed data sets. PNNL staff traveled to the EPRI NDE Center to examine samples from the inventory of Westinghouse Owner's Group (WOG) CASS and DMW sections. The results reported here do not represent data from a statistically large number of field-representative CASS samples. Approximately 20 CASS specimens (PNNL and EPRI specimens) were examined using this examination protocol. Results from this field test clearly show that the low-frequency/SAFT inspection technique is capable of providing quality detection and localization data, and accurate length sizing information.

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