Comparison of Ceramic Waste forms Produced by hot Uniaxial Pressing and by Cold Pressing and Sintering
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COMPARISON OF CERAMIC WASTE FORMS PRODUCED BY HOT UNIAXIAL PRESSING AND BY COLD PRESSING AND SINTERING
V.M. Oversby* and E. R. Vance# *Lawrence Livermore National Laboratory, Livermore, CA 94550 USA and #Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia ABSTRACT
Synroc C waste form specimens prepared using the Australian-developed technology are uniaxially pressed in stainless steel bellows at 12000C and 20MPa. This produces a material with high chemical and physical durability and with the radioactivity enclosed inside both the waste form and the bellows. An alternative method of producing the ceramic product is to use cold pressing of pellets followed by reactive sintering to provide densification and mineralization. Depending on the scale of waste form preparation required and on the activity level and nature of the waste streams, the cold press and sinter method may have advantages. To evaluate the effects of production method on waste form characteristics, especially resistance to dissolution or leaching of waste elements, we have prepared two simulated waste samples for evaluation. Both samples were prepared from liquid precursor materials (alkoxides, nitrates, and colloidal silica) and then doped with waste elements. The precursor material in each case corresponded to a basic phase assemblage of 60% zirconolite, 15% nepheline, 10% spinel, 10% perovskite, and 5% rutile. One sample was doped with 25% by weight of U; the other with 10% by weight each of U and Gd. Each sample was calcined at 7500C for 1 hr. in a 3.5% H2 in N2 atmosphere. Then one portion of each sample was hot pressed at temperatures ranging from 1120 to 12500C and 20MPa pressure in steel bellows. A separate portion of each sample was formed into pellets, cold pressed, and sintered in various atmospheres at 12000C to produce final products about 2/3 cm in diameter. Samples were then examined to determine density of the product, grain sizes of the phases, phase
assemblage, and the location of the U and Gd in the final phases. Density data
indicate that sintering gives good results provided that the samples are held at 2000C for long enough to allow trapped gases to escape. INTRODUCTION
Many radioactive waste management options currently under consideration would generate waste streams that are high in actinide content. Examples of these waste streams are actinides that are separated from fission products during the reprocessing of commercial power reactor spent fuels, the fuel debris from reactor accidents, such as Chernobyl and Three Mile Island fuel residues, and plutonium from dismantled weapons. Two of these potential waste streams would consist of well characterized input streams to the immobilization process. The fuel residues from reactor accidents, however, would be rather poorly characterized and might present substantial challenges if the nature of the waste form required detailed chemical characterization of the waste prior to processing. We performed a set of experiments to evaluate two factors rele
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