Ultrasonic testing of ITER toroidal field coil cases closure welds

The ITER Toroidal Field Coil Cases (TFCC) are massive components made of austenitic stainless steel with a ''D''-shape and composed of a winding pack enclosed in a welded coil case. Closure welds range from 35 mm to over 120 mm in thickness, making more than 70 m of fused metal per coil. To ensure the mechanical integrity of these components, all the ITER TFCC closure welds have to be 100% examined. As a result of its inherent nature, closure welding can only be inspected from one side, ultrasonic testing (UT) being the method chosen. Postulated defects include volumetric flaws and open, sub-surface and embedded planar flaws (from 10 mm²), with orientations parallel and transversal to the weld centre line. It is recognized that ultrasonic testing of austenitic stainless steel welds are severely hampered by the anisotropy and sound scattering at the grain boundaries. In the present case, weld inspection is also subject to restricted accessibility, large and variable thicknesses, and highly demanding qualification requirements. Since the inspection conditions are exceptionally challenging, devoted qualified ultrasonic techniques are needed. To overcome these challenges, Tecnatom has proposed ultrasonic techniques based on Dual Matrix Arrays (DMA) which combine the advantages of TRL (Transmit-Receive-Longitudinal) probes with those of phased-array (PA) technology. Dual probes create a natural focus in which sensitivity is maximized, while unwanted wedge echoes are minimized. On the other hand, two-dimensional PA probes allow electronic steering in the primary axis along with skewing the beam in the secondary axis with the main purpose of focalizing at different depths. The present paper presents the performance of DMA techniques for the inspection of thick austenitic welds aimed at the qualification of ultrasonic procedures for the non-destructive testing of the ITER TFCC closure welds.