Jake Amoroso

A-site compositional effects in Ga-doped hollandite materials of the form BaxCsyGa2x+yTi8-2x-yO16: implications for Cs immobilization in crystalline ceramic waste forms.

The hollandite structure is a promising crystalline host for Cs immobilization. A series of Ga-doped hollandite BaxCsyGa2x+yTi8−2x−yO16 (x = 0, 0.667, 1.04, 1.33; y = 1.33, 0.667, 0.24, 0) was synthesized through a solid oxide reaction method resulting in a tetragonal hollandite structure (space group I4/m). The lattice parameter associated with the tunnel dimension was found to increases as Cs substitution in the tunnel increased. A direct investigation of cation mobility in tunnels using electrochemical impedance spectroscopy was conducted to evaluate the ability of the hollandite structure to immobilize cations over a wide compositional range. Hollandite with the largest tunnel size and highest aspect ratio grain morphology resulting in rod-like microstructural features exhibited the highest ionic conductivity. The results indicate that grain size and optimized Cs stoichiometry control cation motion and by extension, the propensity for Cs release from hollandite.

Microstructure-Property Relations in Melt Processed and Spark Plasma Sintered Ceramic Waste Forms

The use of nuclear power to generate electricity necessitates methods to immobilize waste generated from those processes. There are many proposed waste form technologies for the storage of nuclear waste such as vitrification, hot isostatic pressing, melt formation, and spark plasma sintering (SPS). Titanatebased ceramic waste forms specifically SYNROC [1] and derivative materials have been widely studied for their potential to accommodate a large variety of radionuclides. The major phases targeted in SYNROC compositions are zirconolite (CaZrTi2O7), hollandite (BaxCsyAl2x+yTi8-2x-yO16), and perovskite/pyrochlore (CaTiO3/A2B2O7, where A and B are different 3+ or 4+ cations). Zirconolite and perovskite/pyrochlore are targeted to immobilize the majority of actinides, while fission products (Cs and Sr) are immobilized in hollandite phases.