Challenges in the Fabrication of Ceramic Technetium Waste Forms
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.518
Challenges in the Fabrication of Ceramic Technetium Waste Forms Thomas Hartmann and Annita Martinelli-Becker University of Nevada, Las Vegas, Department of Mechanical Engineering, 4505 S. Maryland Parkway, Box 4009, Las Vegas, NV 89154-4009, USA, [email protected]
ABSTRACT
In this research we can demonstrate, that fission technetium-99 can be successfully immobilized as tetravalent cation in solid state refractory oxides such as pyrochlores and perovskites. Pyrochlores show excellent performance in ASTM C1220-10 type corrosion testing and have the ability to structurally bond Tc-99 and therefore avoid the formation of highly-mobile, pertechnetate species under conditions of a generic repository. We have fabricated lanthanide technetium oxides using either dry-chemical ceramic processing, or wet-chemical coprecipitation methods. Tc pyrochlores have shown better Tc retention and corrosion resistance compared with Tc-containing LAWE4-type borosilicate glass, combined with 50-times higher waste loading. However, mechanical properties (fracture toughness, compressive strength) of the pyrochlores are lacking and the microstructure shows high open porosity of about 50 %. To improve these properties we tested a variety of measures such as hot-pressing or the combination of hot pressing and high-temperature synthesis, but the improvement was minor and Tc and the surrogate Ru were partly reduced. The presence of metallic inclusions has strong impact on Tc retention and release rates increased more than tenfold. We have further developed a wet-chemical coprecipitation synthesis route followed by calcination and a 4-days high-temperature sintering cycle for the model composition Sm2(Ru0.5Ti0.5)2O7 where titanium oxide was added as sintering agent. The ceramic surrogate waste forms showed improved theoretical densities of about 73 % combined with sufficient mechanical strength, while maintaining ruthenium in the tetravalent state.
INTRODUCTION
Technetium-99 from used nuclear fuel is a major environmental concern due to its long halflife and the high mobility of heptavalent Tc-species such as pertechnetate ion (TcO4-) in
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groundwater. At the Hanford Site for example, Tc is found in both, the high level waste (HLW) solids and the Cs-137 containing low activity waste (LAW) feed, both of which will be converted to vitreous waste forms. Tc is volatile at temperatures used in vitrification (10001200 °C) requiring off-gas capture and recycling systems. It has been determined that the solubility of Tc in glass (~2,000 ppm) is orders of magnitude higher than the anticipated waste stream composition (10-100s ppm) and is therefore not limiting retention during processing [1]. Other factors to influence retention in
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