Zirconia containing actinides in solid solution, (Zr, An)Ox, where An = U, Pu, Am, Np, Cm is considered as a starting material for the synthesis of two durable crystalline host-phases for actinide immobilization: zircon (Zr, An)SiO4 and cubic zirconia (Zr,An)O2. The use of this starting material is necessary in order to provide for total actinide incorporation into the structures of the final phases, zircon and cubic zirconia, while avoiding the presence of unincorporated actinides. Difficulties arise when the chemically inert pure oxides, ZrO2 and AnOx are used as precursor materials. Melting techniques, which are commonly used to provide solid solution of inert oxides are not acceptable for ceramic preparation, particularly the synthesis of ceramic Pufuels. This paper presents a new approach utilizing plasma calcination. Solids containing both zirconium and actinides are calcined in a plasma. This allows the use of intermediate, stabilized actinide forms, for example, co-precipitated actinide-zirconium oxalates which have a low solubility level or a solid sol-gel precursor, such as (Zr, An)SiOx.
Calcination was completed using an induction plasma in an argon gas mredium. Two types of material were used: co-precipitated oxalates of Zr and Ce with 10 wt.% Ce and a solidified sol-gel (Zr, Ce)SiOx with 10 wt.% Ce. Weakly milled material with a particle size of 0.34 mm was dropped through the plasma at a speed of 40-60 grams per hour. The final product was examined by high resolution transmission electron microscopy (HRTEM). The calcined oxalate material consisted of aggregates of globular particles (tens to hundreds of nm) of monoclinic and tetragonal (Zr,Ce)O2. The calcined sol-gel consisted of globular particles (50 to 100s nm) of monoclinic and tetragonal (or pseudo-cubic) (Zr, Ce)O2 in an amorphous silica matrix. This paper discusses the application of plasma calcination to the immobilization of excess weapons Pu and other actinides. The plasma calcination of solid materials would be used for ceramic Pu-fuel fabrication, as well as for the final disposal in geological formations.