Head-End Separation of Biso-Coated Fissile and Fertile Particles for High-Temperature Gas-Cooled Reactors

An attractive recycle fuel for advanced HTGRs uses bonded fuel beds containing BISO-coated fissile and fertile particles (i.e., those having buffer and isotropic pyrolytic carbon coatings surrounding the fuel kernel). Two types of fissile material are used, ²³³U and ²³⁵U. The economics of the fuel cycle makes separation of these materials prior to reprocessing desirable. Laboratory-scale studies have shown that a conceptual separation process, based on the stability of (Th, U)O₂ kernels in contrast to (Th, U)C₂, UC₂, or UO₂ kernels under oxidizing conditions, is feasible on unirradiated fuel. However, damage to the oxide microspheres during irradiation is sufficient to result in fragmentation of the kernels upon removal of the pyrolytic carbon coating. Other head-end separation processes will be required to utilize bonded BISO recycle fuel in advanced HTGRs.