Management of Commercially Generated Radioactive Waste - U.S. ...

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wells. The 90 Sr leach rate decreased with time and after about 5 years stabilized at the
very low rate of 5 x 10-11 g glass/cm 2 -day (Merritt 1977).
Recently, development of a stuffed glass process has begun at Catholic University in
Washington, D.C. (Simmons et al. 1979). The process utilizes a high-temperature, high-
silica glass that can be prepared in a porous form outside the radioactive processing cell.
The pre-prepared porous glass is then soaked in HLW solution. After a suitable soaking
period the solution-laden porous glass is removed from solution and the HLW constituents are
precipitated. The porous glass is then soaked in a solvent that removes the waste from a
surface layer of the porous glass. The solvent is subsequently evaporated and the porous
glass is dried at 6250 to 700°C to convert the HLW constituents in the pores to oxides.
Then the temperature is raised to 900°C for sintering. During sintering, the pores col-
lapse. The final product is solid glass that contains the radioactive waste materials
interstitially, and has a high-silica envelope on the outer surface. Alternatively, the
same final form can be obtained by putting waste-laden porous glass granules in an envelope
of waste-free porous glass and sintering to close the pores.
The stuffed glass process potentially yields a product with the durability of a high-
melting glass but utilizes lower processing temperatures. In addition, the product has a
built-in barrier of inert high silica glass on the surface.
Glass-Ceramics
Glass-ceramics are a class of specially formulated materials that can be melted, processed
and formed as glasses and then devitrified, or crystallized, under controlled conditions.
Glass-ceramics have become important commercially in the last 20 years. They are
valued for their thermal stability and physical ruggedness.
Most of the investigations of glass-ceramics as materials for HLW disposal have been
carried out in Germany at the Hahn-Meitner Institute in Berlin and at Karlsruhe (De et al.
1976, Guber et al. 1979). The waste-containing glass-ceramics formulated to date are
usually only about 50% crystalline (commercial glass-ceramics are over 95% crystalline).
Some improvements in thermal stability (higher softening points) and physical ruggedness
have been observed; the leach rates obtained to date are in the same range as those of lowmelting
waste glasses.
4.3.2.3 Crystalline Waste Forms
For the purposes of this discussion all nonvitreous high-level solid waste forms will
be termed crystalline. In general, crystalline waste forms, particularly those that have
undergone extensive thermal treatment and are not approaching solid solution limits, are
thermodynamically more stable than glass waste forms. In some crystalline waste forms the
crystals are "tailored" to resemble minerals that have a demonstrated stability in nature.
Cement
Cements are used routinely to encapsulate low- and intermediate-level radioactive
wastes. Liquid or slurry wastes are mixed with a predetermined weight of dry solids. The