3, Clemson University, Clemson, South Carolina, United States
2, Savannah River National Laboratory, Aiken, South Carolina, United States
Hollandite-type crystalline ceramics are viable waste forms for immobilizing Cs and Ba found in waste generated from nuclear material processing and treatment. Previous research has demonstrated replacing a portion of the Ti in the framework with a tri-valent cation (e.g. Cr, Al, Fe, and Ga) to improve the stability of the hollandite phase while simultaneously incorporating additional waste constituents. Other elements such as Sb, In, Mo, and Tc are also present in typical waste streams and may potentially co-substitute for Ti. The resulting doped hollandite phase is considered a hierarchical waste form material due to the ability to immobilize multiple species in both channel and framework lattice sites of the hollandite phase resulting in enhanced waste storage capacity.
To provide an understanding of the stability of hollandite waste form with different dopants, the Cs2O-BaO-TiO2-Sb2O3-In2O3-TcO2-MoO3 system has been thermodynamically assessed according to the CALPHAD methodology encompassing all relevant crystalline phases/solid solutions including hollandite and the liquid phase(s) characterized using the compound energy formalism and two-sublattice partially ionic liquid models, respectively. The resultant thermochemical models and values can allow determination of hollandite phase stability, including limits to substitution/dopant concentrations, temperature behavior, and thermodynamic representations usable for evaluating aqueous solubility and leaching.
This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.