Optical inspection of coated particle nuclear fuel

In this paper, we describe the inspection of coated particle nuclear fuel using optical microscopy. Each ideally spherical particle possesses four coating layers surrounding a fuel kernel. Kernels are designed with diameters of either 350 or 500 microns and the other four layers, from the kernel outward, are 100, 45, 35, and 45 microns, respectively. The inspection of the particles is undertaken in two phases. In the first phase, multiple particles are imaged via back-lighting in a single 3900 χ 3090 image at a resolution of about 1.12 pixels/micron. The distance transform, watershed segmentation, edge detection, and the Kasa circle fitting algorithm are employed to compute total outer diameters only. In the second inspection phase, the particles are embedded in an epoxy and cleaved (via polishing) to reveal the cross-section structure of all layers simultaneously. These cleaved particles are imaged individually at a resolution of about 2.27 pixels/micron. We first find points on the kernel boundary and then employ the Kasa algorithm to estimate the overall particle center. We then find boundary points between the remaining layers along rays emanating from the particle center. Kernel and layer boundaries are detected using a novel segmentation approach. From these boundary points, we compute and store layer thickness data.