Assessments on Eddy Current Detection of Cracking Near Volumetric Indications in Steam Generator Tubes (NUREG/CR-7291, ANL-18/41)
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Manuscript Completed: July 2021
Date Published: March 2022
C. B. Bahn
Argonne National Laboratory
9700 South Cass Avenue
Lemont, IL 60439
P. Purtscher, NRC Project Manager
Office of Nuclear Regulatory Research
United States Nuclear Regulatory Commission
Washington, DC 20555-0001
Stress corrosion cracking (SCC) in steam generator (SG) tubes can occur in conjunction with volumetric degradation. When a flaw-like indication is detected by eddy current (EC) bobbin probe examination, the tube is usually re-inspected with a rotating probe to help better characterize the signal. In subsequent inspections, the location affected by volumetric degradation (e.g., a manufacturing burnishing mark [MBM] or a wear scar) may not be re-inspected with a rotating probe, unless the bobbin probe signal exhibits a measurable change from the previous inspection. If the source of an EC signal is not properly determined (i.e., SCC vs. volumetric flaw), analysts may not be able to accurately characterize and size the indication using the most appropriate EC nondestructive examination (NDE) technique. Missing this determination is of particular concern if SCC were to develop in, or near, volumetric degradation and as a result, not be detected.
Research was conducted at Argonne National Laboratory (Argonne) to assess the ability of conventional EC inspection techniques to detect and characterize cracks located at the same axial elevation as a volumetric flaw in an SG tube. Investigations were also carried out on alternative signal processing methods that could help improve the detection of a crack-like signal affected by interaction with a more dominant volumetric signal. To augment the limited EC data available for this particular mode of degradation, a set of specimens were assembled in-house for this study. Specimens containing a wear scar and SCC located at the same axial position along the tube were manufactured. Cracks were produced at different circumferential positions, relative to the mechanically induced wear mark in each tube. EC inspections were performed, using bobbin and rotating probes, in accordance with generically qualified examination techniques. The specimens were examined at different stages of the flaw manufacturing process. The EC data were subsequently analyzed using both conventional and alternative data analysis methods. For the latter approach, background suppression algorithms for the processing of spatially one- and two-dimensional data were implemented and evaluated, using both actual and simulated data generated through signal superposition. The viability of the conventional and alternative methods were further evaluated using a database of laboratory-produced specimens for a pertinent degradation mechanism, which was provided by Atomic Energy of Canada Limited (AECL), through the International Steam Generator Tube Integrity Program (ISG-TIP). Additional analyses were also performed using a limited set of data from tubes with field-induced damage and degradation. Assessment of EC inspection technique capability with regard to detection of cracking near volumetric degradation was ultimately evaluated through comparison with destructive examination (DE) data for the laboratory-produced specimens at Argonne.
The results of this investigation indicate that detection of an outer-diameter stress corrosion crack (ODSCC) that is axially collocated with a volumetric degradation can pose a challenge to conventional EC examination techniques used for inspection of SG tubing. This finding holds true particularly for cracks with small signal amplitudes relative to the interfering volumetric signal. Utilization of complementary inspection techniques, such as those based on rotating and array probe examinations, can help improve the detection probability of cracks for this rather complex form of degradation. Furthermore, in the presence of volumetric degradation, crack-like indications detected through bobbin probe examination may not be conservatively dismissed based on the absence of a confirmatory signal in the data obtained through rotating probe examination. The results also indicate that background suppression algorithms can improve the detection of crack signals affected by nearby volumetric degradation.
Page Last Reviewed/Updated Friday, March 25, 2022