Evaluation of thermally converted silicotitanate waste forms

Crystalline silicotitanate ion exchangers are highly selective for separating Cs from Na-rich waste streams. However, use of these ion exchangers for removal of Cs from radioactive tank waste will result in large volumes of secondary wastes. Thermal conversion of silicotitanates produces a durable waste form with reduced volumes up to 40%. Leach tests (MCC-1 and PCT) have shown that Cs leach rates of IE-911-Na (heat treated at 900 °C for an hour) are extremely low, ranging from 0.1 to 1wt% Cs loss in Cs fraction release, or 10^-1 to 10^-8 g/m² day in normalized Cs mass loss. These are several orders of magnitude lower than that of borosilicate glass. In order to understand the interplay between the structure and high Cs durability, X-ray diffraction, ¹³³Cs NMR, and thermogravimetric analysis have been used to identify phase(s) responsible for trapping Cs in these silicotitanates. Results indicate that Cs is likely to be contained in a crystalline silicate phase.