Alkoxide Derived Amorphous Solids as an Alternate Nuclear Waste Form
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by Elsevier Science Publishing Company, BASIS FOR NUCLEAR WASTE MANAGEMENT
Stephen V. Topp,
Inc.
133
editor
ALKOXIDE DERIVED A!ORPHOUS SOLIDS AS AE ALTERNATE NUCLEAR WASTE '"OPI'
J. M. POPE AND D. E. HARRISON Westinghouse Research and Development Center, Pittsburgh, Pennsylvania 15235
1310 Beulah Road
INTRODUCTION Nitric acid waste can be immobilized in an amorphous aluminosilicate matrix that is produced by polymerization of metal alkoxides [1]. Unlike vitreous aluminosilicate host matrices obtained by melting, neither alkalies, alkaline earths, nor borates are required since no melting is involved. Instead, the alkoxide glass formers and radionuclides react directly to form a randomly cross-linked structure of silica and alumina polyhedra. Structurally, the amorphous product formed through chemical polymerization is similar to a true glass except that it has not experienced melting. As a result, neither a glass transition temperature nor a specific volume temperature dependence would be expected in such an amorphous material. As a chemically durable waste form, the "gel-glass" product should be substantially better than ordinary waste glass because it can be made without the addition of either alkalies or boron. A major difference between the gel-glass and true glass is in the physical form or shape of the product. As shown in the process flowchart in Figure 1, gelation is an essential step in the process and marks the successful incorporation of the liquid nitrate into a clear aluminosilicate gel at temperatures 0 between 25 to 75 C. Heating this gel to 15001C drives off the residual alkyls and water and causes a volume reduction in excess of 75%. Because of the magnitude of this shrinkage, the dried gel is comminuted into millimeter sized granules. After a 5001C calcination, the granules are amorphous and quite 2 porous with an apparent surface area of %300 m /g. Due to their high surface area, these granules may be consolidated to %70-90% of theoretical density by warm pressing at 5001C. Alternatively, depending on their composition, they 0 may be dissolved in reference borosilicate glass. Upon heating them to 1850 C, 2 the surface area of the granules is decreased to less than 1 m /g without any agglomeration taking place. These individually dense granules could represent a final waste form of radionuclides bound in an alkali-free, boron-free, aluminosilicate host matrix. If desired, consolidation of these granules can be accomplished using either a low melting metal or glass, or they could be used as a grog in concrete. Another noteworthy characteristic of the gel-granules is the high waste loading that is possible in comparison to conventional vitreous waste forms. For example, Thorex waste loadings approaching 40 wt% thoria in the final product far exceeds the solubility limit of thoria in conventional waste glasses. Thus, the alkoxide chemical polymerization route offers a means for immobilizing complex acid wastes, which in some situations, may be an attractive alternative for either intermediate
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