Compensation for attenuation on ultrasonic testing at TF coil structure of ITER

Abstract

The ITER Toroidal Field Coil (TFC) is a massive 16 m × 9 m “D”-shape-superconducting magnet composed of winding pack (WP) and TF coil structure (TFCS). The TFCS is made of thick austenitic stainless steel with over 100 mm. The ultrasonic testing (UT) is used to the welding examination. The longitudinal wave is selected since it is generally used for UT of austenitic stainless-steel welds. There is a concern about the attenuation of the signal level of UT, because the ultrasonic wave is scattered through the weld metal of austenitic stainless steel. This paper reports the following compensation method against the attenuation of the UT signal with actual data applied for ITER TFCS. In the ITER TFCS fabrication, the attenuation of the UT beam in the weld is compensated by the transfer correction factor obtained from a test piece made of the actual TFCS material and weld metal during calibration. UT is performed using distance amplitude characteristic (DAC) curve obtained with a calibration block made from the base material of TFCS with side drill holes (SDHs). In order to capture the sensitivity difference between base material and weld metal, signal obtained with a reference block made from welding joint of TFCS with the same SDHs in the weld are plotted on the DAC. The sensitivity difference is obtained by difference of signal level on DAC and the plot. Then, relationship between the sensitivity difference and distance propagating in the weld metal is obtained. The sensitivity difference is used for assessment of UT signal level to correct decreasing by weld metal selecting proper sensitivity difference with respect to distance propagating in the weld metal in actual UT. This paper includes test results obtained by manufacturing companies using the actual UT probes for TFCS fabrication. The paper also provides obtained correlation between propagation distance in the weld metal and sensitivity difference.