Interim Waste Forms for High-Level Radioactive Wastes: Processing and Properties
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INTERIM WASTE FORMS FOR HIGH-LEVEL RADIOACTIVE WASTES: PROPERTIES
PROCESSING AND
G. BANDYOPADHYAY Materials Science Division, Argonne National Laboratory, Avenue, Argonne, Illinois 60439, U.S.A.
9700 South Cass
ABSTRACT Several simulated interim waste forms have been investigated in the laboratory to study their suitability for application in handling and transportation of high-level radioactive wastes to terminal processing sites. In the fused-salt/sludge option, the neutralized supernatant liquid and the precipitated sludge are treated simultaneously to form fused-salt cakes. Silicate-based options, in which sodium silicate or sodium silicate and Ca(OH) 2 act as binders for the sludge, require prior separation of the sludge and the soluble radioactive constituents from the supernatant before the waste form can be prepared. The results from tests on simulated fused-salt waste forms indicated that the process simplicity of this option is partially offset by the high water solubility and hygroscopicity of the product, which would necessitate special precautions during transportation and storage. The most promising silicate-based option is the ambienttemperature silicate/sludge process, in which the sludge is mixed with sodium silicate [and sometimes with Ca(OH) 2 ] and subsequently exposed to a contrelled-humidity environment at room temperature to form a chemical bond. Solid material containing 75 wt % synthetic calcined sludge, prepared by
this process, has sufficient physical, chemical, and mechanical stability for use as an interim waste form. INTRODUCTION High-level liquid radioactive wastes are often produced at facilities in which terminal processing of the waste is not practical because the quantities of wastes are small and the capital cost of terminal processing equipment and facilities is high. Current regulations do not permit shipment of high-level liquid waste. Under these circumstances, the waste could be incorporated into an interim solid form for shipment to a terminal processing site. Because the shipment waste form is not expected to remain intact for more than a few years, the criteria for an acceptable
interim form may not be as stringent as required
for geological storage of a final waste form; thus, the effort to prepare interim forms may be only a fraction of that required to prepare the same waste for terminal storage. Several criteria related to the interiT form products, processes, and shipping requirements have been proposed. The interim form should require only simple processing methods, so that extensive additional facilities will not be required at the originating site. Additionally, the interim waste form must satisfy shipping criteria, particularly in relation to transportation accidents; should be mechanically and chemically stable in the ambient environment; and should be readily compatible with the terminal waste form processes.
624 Several candidate materials have been consideredI for the interim solidifiThe interim waste forms that are cation of
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