Imaging and measuring methods for coating layer thickness of TRISO-coated fuel particles with high accuracy

TRISO-coated fuel particles are the main innovation of high-temperature gas-cooled reactors (HTGRs), which effectively improve the safety of reactors for higher stability at high temperature. The structure of the typical ball-type TRISO-coated fuel particle comprises a nuclear fuel particle as a kernel and four outer coating layers. These layers play an important role as a barrier against fission product diffusion, and ensuring the integrity of the particles. As one of the key quality parameters of TRISO particles, the coating layer thickness must be measured accurately by non-destructive means. Conventional X-ray radiography, which is based on attenuation contrast, is a non-destructive alternative to measure the coating layer thickness. However, the attenuation contrast is determined by the density difference of the material. For the weakly absorbing material of coating layers composed of carbon and silicon, the variations of the thickness and density of different coating layers are not large enough to realize accurate thickness measurement. In order to obtain ideal images with enhanced boundaries, we applied in-line phase contrast imaging (PCI) based on a micro-focus X-ray source for the inspection of fuel particles. The method is more powerful for acquiring radiographs with sharper edges than conventional X-ray radiography. An improved NL-means method was adopted to decrease the image noise while preserving the detailed information effectively. Finally, based on the denoised DR image with sharper boundaries obtained by PCI, edge detection with sub-pixel accuracy and thickness measurement results are shown.