In excess of 100 metric tons of technetium-99 (t1/2 = 2.13x105 years) has been produced in thermal nuclear reactors as a result of the fissioning of U-235. The isolation and immobilization of technetium from the raffinate of used fuel reprocessing by durable solid waste-forms are a challenge. In the technical process to vitrify high-level radioactive waste effluents, a significant part of the technetium inventory will be oxidized or subjected to disproportionation to consequently volatilize as heptoxide. Oxidation and volatilization of Tc-99 are process-related and a result of slightly oxidizing redox conditions in combination of temperatures targeting 1200°C in the glass melter.
In this research we show that Tc-99 can be successfully immobilized as tetravalent cation in solid state refractory oxides such as pyrochlores and perovskites. Pyrochlores have shown 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 the conditions of a generic repository.
We have fabricated lanthanide technetium oxides using either dry-chemical ceramic processing, or wet-chemical coprecipitation methods. The Tc pyrochlores have shown better Tc retention and corrosion resistance in ASTM C1220-10 based testing 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 metal Ru were partially reduced to the metals. The presence of metallic inclusions has strong impact on Tc retention and release rates are increased 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 75% combined with sufficient mechanical strength, while maintaining ruthenium in the tetravalent state. To subsequently introduce technetium into the coprecipitation-based process route and to fabricate waste-forms with the stoichiometry Sm2(Tc0.25Ru0.25Ti0.5)2O7 we have successfully tested the performance of reducing agents on pertechnetate. The precipitation of technetium from a mock-up washer solution allows now the incorporation of Tc-99 into the wet-chemical fabrication route. Our aim hereby was to provide a potential solution for continuously depleting the technetium content from a waste treatment facility when vitrifying high and lower-level waste effluents.