The State of Development of Waste Forms for Mixed Wastes U.S. Department of Energy's Office of Environmental Management (1999) / Chapter Skim
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5. Characterization of Mixed Waste Forms
5. Characterization of Mixed Waste Forms
Pages 70-82
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From page 70... ...
and worldwide to develop methods to determine the physical and chemical integrity of waste forms, contaminant release mechanisms, and waste form degradation rates (Cunnane, 19941. Much of this research has been sponsored by the U.S.
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Determination of performance of the waste form by tests lasting generally a few months and then extrapolated to estimate performance over centuries or millennia introduces considerable uncertainty into predicting long-term performance. The possibility of early failure of the waste form with subsequent generation of contaminated leachates in the disposal facility places an extra burden on design of the engineered and natural barriers and emphasizes the selection of disposal sites offering favorable geology, hydrology, and climatic conditions.
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For cement-based waste forms, the most common means of accelerating testing is through use of higher water flow rates and more aggressive leaching solutions (Quillin, et al., 19941. As will be seen in the next section, this approach has led to the design and implementation of leaching tests that bear little resemblance to the environmental conditions experienced by the disposed waste.
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From page 73... ...
The second tier of testing is designed to determine if conditions in a disposal facility will affect the integrity of the waste form and, therefore, its ability to retain hazardous and radioactive contaminants. These two testing tiers, tests for leachability and the physical durability of waste forms, are discussed in this chapter.
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A summary of some of the leaching tests that are most relevant to mixed waste forms is presented In Table 7.
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I ~ h~m Deionized Up to 90 days water Deionized 1 1 days water' Leaching of crushed sample used to define characteristic hazardous wastes Measures leach rate from monolithic waste form Elevated temperature and leach solution changes used to measure leach rates SOURCE: 40CFR 261, Appendix II, Method 131 1; ANS, 1986; Fuhnnar~n, et al., 1990. The Toxicity Characteristic Leaching Procedure (TCLP)
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From page 76... ...
This test therefore measures leaching as a function of time, in contrast to the single measurement that is made with the TCLP. The ANS test includes calculation of a "leachability index" that can be related to an effective diffusion coefficient for waste forms that leach mainly by diffusion (rather than by dissolution of the waste form matrix itself)
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From page 77... ...
in a Column Apparatus;" · ASTM Method D 5233-92, "Standard Test Method for Single Batch Extraction Method for Wastes;" · ASTM Method D 5284-93, "Standard Test Method for Sequential Batch Extraction of Waste with Acidic Extraction Fluid," for wastes with at least 5% solids; and · ASTM Method D 5369-D, "Standard Practice for the Extraction of Solid Waste Samples for Chemical Analysis Using Soxhiet Extraction" at elevated temperature with cycling organic solvent for the extraction of non-volatile and semi-volatile organic compounds. A more recent ASTM method, Cl308-95, is an "Accelerated Leach Test for Diffusive Releases from Solidified Waste and a Computer Program to Mode!
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Four activities for predicting the long-term durability of waste forms are laboratory testing, field testing, analogue studies,4 and modeling (Means, et al., 1995~. Laboratory testing involves developing procedures for accelerating the effects of degradation processes.
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(1994) , who investigated the performance of cementitious waste forms by considering the chemistry of the calcium-aluminum-silica phases present in ordinary Portland cement.
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Because some mixed wastes contain long-lived radionuclides and chemically hazardous constituents, knowIedge of the {ong-term behavior of waste forms is necessary if credit is to be taken for the waste form in assessing the long-term performance of disposal facilities.
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It is difficult to elucidate the reaction mechanisms pertinent to waste form degradation that can be used to estimate long-term performance. Laboratory methods to accelerate natural processes that govern waste form degradation usually subject waste forms to unrealistic conditions.
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The objective of this rapid testing protocol would be to reduce the need for performing TCLP analyses on every batch of waste prior to its disposal. · OST should work to promote consensus among EPA, USNRC, DOE, and the scientific community on waste gown testing protocols that are generally acceptable for providing at least a qualitative evaluation of long-term waste form performance in disposal environments.
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