Application of x-ray imaging for HTR fuel characterization

HTR fuel consists of particles containing a fissile kernel and surrounding layers, whose densities and thicknesses are two of the fundamental fuel specifications. Destructive methods, i.e. respectively the sink float method and image analysis of ceramography, were developed and are still the reference techniques to evaluate these parameters. Exhibiting great accuracy, these methods have nevertheless the drawback of generating effluents and wastes, and of being cost/time consuming. A new technique based on X-Ray Phase Contrast Imaging has been developed by AREVA NP for determining layer thickness and density via data and image processing. The first step consists in the acquisition of particle high resolution X-Ray images. Then, an intensity profile is extracted from one of the images and is used as reference signal. After characterizing the detector response and the X-Ray source spectrum, an initial simulated intensity profile is obtained using nominal specifications of thicknesses and densities. Via an iterative process, layer densities and thicknesses are adjusted to yield minimization of the error between the extracted profile and the simulated one. As an application, layer thickness and density were determined on batches of TRISO particles provided by countries participating in the International Atomic Energy Agency (IAEA) Coordinated Research Project on 'Advances in HTGR Fuel Technology Development'. In parallel, particles issued from the same batches were characterized by Commissariat a l'Energie Atomique using optical microscopy on ceramography samples for thickness and sink float method for density measurements. OPyC and SiC results obtained with the two methods were compared via mean values and differences are discussed. Besides, X-Ray Phase Contrast Imaging technique provided IPyC density results on fully coated particles contrarily to sink float. The newly developed X-Ray Phase Contrast Imaging based technique is non destructive, cost effective and non contaminating technique and yielded comparable results to those obtained with the reference destructive methods. (author)