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Challenges in the Fabrication of Ceramic Technetium Waste Forms

Published online by Cambridge University Press:  28 June 2018

Thomas Hartmann  and
Annita Martinelli-Becker
Thomas Hartmann*
Affiliation:
University of Nevada, Las Vegas, Department of Mechanical Engineering, 4505 S. Maryland Parkway, Box 4009, Las Vegas, NV 89154-4009, USA, thomas.hartmann@unlv.edu
Annita Martinelli-Becker
Affiliation:
University of Nevada, Las Vegas, Department of Mechanical Engineering, 4505 S. Maryland Parkway, Box 4009, Las Vegas, NV 89154-4009, USA, thomas.hartmann@unlv.edu
*
*(Email: thomas.hartmann@unlv.edu)
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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.

Keywords

nuclear materialsceramicx-ray diffraction (XRD)corrosion
Type
Articles
Information
MRS Advances , Volume 3 , Issue 51: Energy Materials and Technologies , 2018 , pp. 3041 - 3052
Copyright
Copyright © Materials Research Society 2018 

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