Tomographic monitoring of circumferential welds in canisters using helical guided ultrasonic waves

Abstract The structural integrity of canisters that store spent nuclear fuel can be monitored reliably and remotely, either periodically or on demand, with helically propagating guided ultrasonic waves (HGUWs). Specifically, by generating and capturing HGUWs along a network of paths within the structure, damage areas can be identified from a tomographic map. Researchers have used this method to map corrosive damage, such as wall thinning, on canister shells. Although these studies have shown the effectiveness of this technique for long-term canister monitoring, it has not yet been tested on a critical part of the structure--namely, the weld zones. Welded regions in canisters are critical and prone to defects because factors like residual stresses, material heterogeneity, and geometric discontinuities increase their susceptibility to stress corrosion cracking and fatigue failure. Therefore, this work explores the feasibility of using HGUW-based tomography to detect damage within canister welds. Experiments were performed on a 5.6-inch-tall, 18-inch-diameter canister section to locate three different types of damage in the circumferential weld. The first damage simulated weld erosion, the second mimicked a shallow surface flaw, and the third represented an early-stage defect such as a crack. Using two arrays of circumferentially distributed PZT discs, HGUWs were generated and recorded across a dense network of paths. The wave data from all paths were then used to create tomographic maps with the RAPID algorithm. These maps successfully pinpointed each damage, thus demonstrating the method's effectiveness in monitoring canister weld integrity.