Characteristics of wasteform composing of phosphate and silicate to immobilize radioactive waste salts.

In the radioactive waste management, metal chloride wastes from a pyrochemical process is one of problematic wastes not directly applicable to a conventional solidification process. Different from a use of minerals or a specific phosphate glass for immobilizing radioactive waste salts, our research group applied an inorganic composite, SAP (SiO(2)-Al(2)O(3)-P(2)O(5)), to stabilize them by dechlorination. From this method, a unique wasteform composing of phosphate and silicate could be fabricated. This study described the characteristic of the wasteform on the morphology, chemical durability, and some physical properties. The wasteform has a unique "domain-matrix" structure which would be attributed to the incompatibility between silicate and phosphate glass. At higher amounts of chemical binder, "P-rich phase encapsulated by Si-rich phase" was a dominant morphology, but it was changed to be Si-rich phase encapsulated by P-rich phase at a lower amount of binder. The domain and subdomain size in the wasteform was about 0.5-2 μm and hundreds of nm, respectively. The chemical durability of wasteform was confirmed by various leaching test methods (PCT-A, ISO dynamic leaching test, and MCC-1). From the leaching tests, it was found that the P-rich phase had ten times lower leach-resistance than the Si-rich phase. The leach rates of Cs and Sr in the wasteform were about 10(-3)g/m(2)· day, and the leached fractions of them were about 0.04% and 0.06% at 357 days, respectively. Using this method, we could stabilize and solidify the waste salt to form a monolithic wasteform with good leach-resistance. Also, the decrease of waste volume by the dechlorination approach would be beneficial in the final disposal cost, compared with the present immobilization methods for waste salt.