Pnl-sa-26071 Waste Forms Based on Cs-loaded Silicotitanates

Silicotitanate ion exchange materials are being considered for removal of radioactive Cs and Sr from tank wastes at the Hanford site. The phase evolution as a function of heat treatment temperature for several sol gel derived compositions within the Cs20-Si02-Ti02 system was investigated, in order to determine if an adequate waste form can be achieved by direct thermal conversion. The Cs leach rates and Cs loss during heat treatment of select materials were measured. Some compositions which contain large amounts of Ti melt to form a glass with reasonably low aqueous leach rates. A new Cs-silicotitanate material with a stsucture isomorphous to pollucite was discovered. This material forms at low temperatures (700-800°C) where Cs volatility is negligible. The silicotitanatepollucite exhibits extremely low leach rates (1.42 g/m2day ) at 9OoC, and has been identified as a promising waste form for Cs containment.