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Published: 2005

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The high-density, isotropic pyrolytic carbon layer beneath the silicon carbide (IPyC) plays a key role in the irradiation performance of coated particle fuel. The IPyC layer protects the kernel from reactions with chlorine during deposition of the SiC layer, provides structural support for the SiC layer, and protects the SiC from fission products and carbon monoxide. The process conditions used by the Germans to deposit the IPyC coating produced a highly isotropic, but somewhat permeable IPyC coating. The permeability of the IPyC coating was acceptable for use with the dense German UO2 kernels, but may not be suitable when coating UCO kernels. The UCO kernels are typically more porous and thus have a larger surface area than UO2 kernels. The lower density and the higher surface area of UCO kernels could make them more susceptible to attack by HCl gas during the silicon carbide (SiC) coating process, which could result in heavy metal dispersion into the buffer and IPyC coatings and a higher level of as-manufactured SiC defects. The relationship between IPyC deposition conditions, permeability, and anisotropy must be understood and the appropriate combination of anisotropy and permeability for particle fuel containing UCO kernels selected. A reference set of processing conditions have been determined from review of historical information and results of earlier coating experiments employing 350 and 500 {micro}m UO2 kernels. It was decided that a limited study would be conducted, in which only coating gas fraction (CGF) and temperature would be varied. Coatings would be deposited at different rates and with a range of microstructures. Thickness, density, porosity and anisotropy would be measured and permeability evaluated using a chlorine leach test. The results would be used to select the best IPyC coating conditions for use with the available natural enrichment uranium carbide/uranium oxide (NUCO) kernels. The response plots from the investigation of the deposition of pyrolytic carbon in a fluidized bed graphically depict the relationships between processing parameters and coating properties. The additional figures present optical, scanning electron microscopy, and other images to highlight microstructural details. For the study, only two parameters (factors), coating gas fraction and deposition temperature, were varied. The plots reveal obvious trends and links between factors and responses. The dominant relationships determined by this study for this range of coating conditions are: (1) rate is dependent upon coating gas fraction or in other terms, reactant concentration; (2) density is controlled by deposition temperature; (3) efficiency is influenced by both CGF and temperature; (4) anisotropy is affect by CGF and temperature, however, the relationship is more complex than for other properties; (5) permeability is dependent upon deposition temperature (thus density); and (6) open porosity is affect by CGF thus is influenced by coating rate. The response plots can be used as 'maps' for the deposition process and are thus valuable for selecting coating conditions necessary to produce desired combinations of properties. The information is useful in predicting the effects of changes to processing on properties and is beneficial in optimizing the process and product properties. Although the study was limited to only two parameters, the information provides a foundation from which other aspects of the coating process can be more easily investigated.