Journal of the American Ceramic Society, Vol.102, No.10, 6284-6297, 2019
Thermodynamic assessment of the hollandite high-level radioactive waste form
Hollandite has been studied as a candidate ceramic waste form for the disposal of high-level radioactive waste due to its inherent leach resistance and ability to immobilize alkaline-earth metals such as Cs and Ba at defined lattice sites in the crystallographic structure. The chemical and structural complexity of hollandite-type phases developed for high-level waste immobilization limits the systematic experimental research that is required to understand phase development due to the large number of potential additives and compositional ranges that must be evaluated. Modeling the equilibrium behavior of the complex hollandite-forming oxide waste system would aid in the design and processing of hollandite waste forms by predicting their thermodynamic stability. Thus, a BaO-Cs2O-TiO2-Cr2O3-Al2O3-Fe2O3-FeO-Ga2O3 thermodynamic database was developed in this work according to the CALPHAD methodology. The compound energy formalism was used to model solid solution phases such as hollandite while the two-sublattice partially ionic liquid model characterized the oxide melt. Results of model optimizations are presented and discussed including a 1473 K isothermal BaO-Cs2O-TiO2 pseudo-ternary diagram that extrapolates phase equilibrium behavior to regions not experimentally explored.