Waste Forms Based on Separations Media (DOE 360-94). The Strategic Environmental Research and Development Program

Abstract

Abstract : Millions of gallons of radioactive wastes contained in tanks at U.S. Department of Energy's (DOE's) Hanford site require immobilization in stable waste forms. Unfortunately, many waste feeds produced during tank waste processing are either insoluble or incompatible with the baseline borosilicate glass host that is currently proposed for containment of Hanford waste. Two feeds that represent significant challenges to borosilicate glass are phosphate-rich tank sludges and silicotitanate ion exchangers (< 1 wt% of each insoluble component is allowed in the baseline glass). If high levels of waste dilution are required to stabilize these wastes, the resulting volume of high level borosilicate waste glass can be up to 100 times greater than that of the waste feed, thus increasing disposal costs by tens of billions of dollars. Researchers at PNNL examined the feasibility of alternative glass or ceramic waste forms for containment of TiO2-rich and phosphate-rich wastes. Tailoring of waste form compositions to specific waste feeds rather than attempting to tailor waste feeds for accommodation by a single waste form will minimize the volume of expensive HLW. For TiO2-rich ion exchanger waste, the high temperature phase selection and properties for three key constituents of silicotitanate ion exchangers: Cs2O, SiO2, and TiO2 was determined. This work has mapped out large portions of the phase diagram and has shown that several glass and ceramic compositions have durabilities comparable to borosilicate glass. While TiO2 may have limited solubility in borosilicate glass, silicate glasses that do not contain boron can incorporate up to 30 wt% TiO2. Two new Cs-silicotitanate zeolites with unique crystal structures that encapsulate the Cs in covalently bonded molecular cages have been discovered.