Ceramic titanate waste form

Figure 2. Transmission electron photomicrograph of a ceramic titanate waste form. The sample was prepared by pressure sintering a titanate fully loaded with fission waste oxides and includes zeolite and silicon additions.

The preparation, composition, structure and leaching characteristics of a crystalline, ceramic radioactive waste form have been discussed, and where applicable, compared with vitrified waste forms. The inorganic ion exchange materials used such as sodium titanate were prepared from the corresponding metal alkoxide. The alkoxides were reacted in methanol with a base containing the desired exchangeable cation and the final powder form was produced by hydrolysis in an acetone-water mixture followed by vacuum drying the precipitate at ambient temperature. 

ICENHOWER, J. P., STRACHAN, D. M., LlNDBERG, M. M., Rodriquez, E. A. Steele, J. L. 2003. Dissolution Kinetics of Titanate-Based Ceramic Waste Forms Results from Single-Pass Flow Tests on Radiation Damaged Specimens. Pacific Northwest National Laboratory, Report No. PNNL-14252. 

The use of ion exchange resins and natural or synthetic inorganic exchange materials in the nuclear industry is well documented ( ). In the waste solidification application, the titanates or niobates offer no unique sorption properties. They do, however, provide a relatively high overall sorption capacity for a variety of nuclides in materials which can be converted into a stable ceramic host for the sorbed ions. After the sorption process, the column bed must be consolidated to reduce surface area. The project emphasis was directed toward a stable waste form and a considerable effort was devoted to producing and characterizing a highly dense form with favorable physical, chemical and thermal properties (l ). 

The characterization work has shown that, although the titanate ceramic appears to be homogeneous on a macro scale, it is actually quite heterogeneous on an atomic scale and contains at least l4 different phases and possibly more. Since interactions of the waste form with any environment surrounding it will also occur on an atomic scale, it is important that this type of information be available when speculating on the long term stability of any waste form. 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

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