Full-matrix microscopic ultrasound for 3D reconstruction of leakage microholes in thin-walled stainless steel

The spent fuel pool is constructed by welding stainless steel and is susceptible to corrosion, damage, degradation, and micro-leakage due to prolonged exposure to boric acid and high-radiation environments. Accurate three-dimensional reconstruction of defects is challenging with traditional methods because of the coarse grains in stainless steel, severe scattering attenuation, high background noise, and the complex morphology and small scale of micron-sized pores. In this study, we propose a novel method for detecting and 3D reconstructing internal micropores in stainless steel using an ultrasound microscope. The proposed method involves three key steps: (1) signal denoising and filtering using the adaptive sparrow search algorithm combined with variational mode decomposition (ASFSSA-VMD), (2) slice-by-slice threshold segmentation for image mask extraction, and (3) 3D reconstruction of the segmented masks using the Visualization Toolkit (VTK). The results show that this workflow effectively addresses the challenges of noise interference and segmentation accuracy, enabling high-quality 3D visualization of minor defect. The 3D microleak hole in the thin stainless plate can be shown visually. The results indicate that the maximum reconstruction error in the stainless steel base material is 0.2 mm, while the maximum reconstruction error in the stainless steel cladding layer is 0.5 mm. It provides robust technical support for the safety maintenance and defect characterization of spent fuel pools.