Abstract
The current reference recycle fuel particle for steam-cycle high temperature gas-cooled nuclear reactors is a UO 2-UC 2 kernel with a multilayered coating (Triso) deposited in a fluidized bed. The coating comprises a low density pyrolytic carbon buffer, a high density inner pyrolytic carbon coating, SiC and a high density outer pyrolytic carbon coating. The kernel is fabricated from a weak acid ion exchange resin. Microradiographic examination of coated weak acid, ion exchange resin particles demonstrated that a considerable quantity of uranium can be transferred from the kernel into the buffer coating during fabrication. Since one function of the buffer coating is to absord fission product recoil, this behavior is a potential source of failure of the high density inner pyrolytic carbon coating. Fuel redistribution in Triso particles can result from permeation of the inner pyrolytic carbon by HCl during SiC deposition. Since the SiC deposition employs decomposition of methyltrichlorosilane (CH 3SiCl 3), HCl is readily available during this process. The permeability of the inner pyrolytic carbon coating to HCl gas has a marked effect on the extent of this fuel dispersion for certain fuel kernel conditions. Permeability of pyrolytic carbon to chlorine was determined by 1500°C chlorine-leaching studies to be a strong function of the coating density, the hydrocarbon gas dilution in the coating furnace and the coating temperature but to be relatively unaffected by application of a “seal” coat, by the coating thickness or by subsequent annealing of the pyrolytic carbon coatings at 1800°C. With this information it is possible to modify the inner pyrolytic carbon coating to prevent uranium redistribution due to coating permeability.
Published Version
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