Fabrication and Chemical Durability of Ceramic Technetium-Based Pyrochlores and Perovskites as Potential Waste Forms

Abstract

Technetium-99 is a key radioisotope from a nuclear waste perspective because of its long half-life (t1/2= 2.13x105 years) and its abundance in used nuclear fuel. As such, it is targeted in separation strategies such as UREX+, for isolation and encapsulation in solid waste forms for final disposal in a nuclear repository. We report here results regarding the incorporation of Tc-99 into ternary oxides of different structure types: pyrochlore (Nd2Tc2O7), perovskite (SrTcO3), and layered perovskite (Sr2TcO4). The objective was to determine synthesis conditions of these ceramic waste forms to immobilize Tc-99 as Tc (IV) and to harvest crystallographic, thermophysical and hydrodynamic data. The fabricated ceramic technetates exhibit good crystallinity and lattice parameters and atomic coordinates could be refined with high accuracies. Thermophysical properties of the oxides, such as the critical temperature (Tc) for superconductivity, were analyzed using AC magnetic susceptibility measurements. In our efforts to compare hydrodynamic properties of the ceramic waste forms with those to Tc-bearing borosilicate glass, we applied the standard test method ASTM C1220-10. Hereby matrix corrosion and Tc-leaching of Tc-99 containing LAWE4 type borosilicate glass and ceramic specimens were compared by static leaching experiments.