Evaluation of Thermally Converted Silicotitanate Waste Forms
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A viable waste form must be chemically durable (aqueous environments are of
primary concern) and thermally stable under repository conditions over a geologic time scale. In addition, an adequate waste form should be capable of incorporating specific waste feeds to form a stable glass or ceramic material (no fines) with a minimum of waste dilution (to minimize waste volumes) and be easy to process under remote handling conditions. Relatively low processing temperatures are desirable, as are simple heat treatment cycles. Borosilicate glass has been chosen as the baseline host for immobilization of high activity waste (HAW) present at the Hanford site. However, crystalline silicotitanate (CST), the most promising candidate for removal of Cs and Sr from tank wastes, has been identified as a risk to the borosilicate vitrification process. TiO 2 in the CST promotes crystallization and immiscible phase separation and affects the redox state and solubility of uranium in glass.l4 Therefore, a TiO 2 limit of 1 wt% is set for borosilicate waste glass at the Savannah River Defense Waste Processing Facility (DWPF).4 If high levels of waste dilution are required to stabilize CST waste, the volume of expensive high activity borosilicate waste glass produced for subsequent storage will be substantially increased. Dissolution of the CST in borosilicate glass (as opposed to direct thermal conversion) necessitates removal and transfer of the CST from the column, mixing with glass frit and melting. Each of these steps significantly increases the risk of contamination to workers or the environment. The volume of HAW can be minimized and the process can be simplified by converting the separated, compositionally homogenous, loaded exchanger into an alternate waste form rather than recombining it with the HAW for dissolution 77 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
in glass. Because there is uncertainty in repository availability and in waste acceptance criteria, it is likely that Cs and Sr loaded ion exchangers will require short-term storage at Hanford. Thermal treatment removes water and hydroxyl groups, thus eliminating the possibility of radiolytic hydrogen generation during short-term storage. Furthermore, general waste acceptance treatment consolidates and immobilizes criteria requires the removal of particulates. Thermal 10 CST particulates, thus minimizing handling risks. Cesium-loaded silicotitanate ion exchangers contain the basic ingredients that can form a ceramic or glass at high temperature. Therefore, CST ion exchange waste forms can be tailored to specific waste feeds rather than attempting to tailor waste feeds for accommodation by a single waste form. A limited set of such waste forms will allow for the safe disposal of problematic HAW streams while minimizing the volume of expensive HAW. In a previous paper, we have demonstrated that TAM-5, a CST formulation (synthesized by Universal Oil Products [UOP]) can form a durable waste form by direct thermal conversion.5 In this paper, we exa
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