Development, characterization and dissolution behavior of calcium-Aluminoborate glass wasteforms to immobilize rare-earth oxides

Calcium-Aluminoborate (CAB) glasses were developed to sequester new waste compositions made of several rare-earth oxides generated from the pyrochemical reprocessing of spent nuclear fuel. Several important wasteform properties such as waste loading, processability and chemical durability were evaluated. The maximum waste loading of the CAB compositions was determined to be ~56.8 wt%. Viscosity and the electrical conductivity of the CAB melt at 1300 °C were 7.817 Pa·s and 0.4603 S/cm, respectively, which satisfies the conditions for commercial cold-crucible induction melting (CCIM) process. Addition of rare-earth oxides to CAB glasses resulted in dramatic decreases in the elemental releases of B and Ca in aqueous dissolution experiments. Normalized elemental releases from product consistency standard chemical durability test were <3.62·10^-5 g·m^-2 for Nd, 0.009 g·m^-2 for Al, 0.067 g·m^-2 for B and 0.073 g·m^-2 for Ca (at 90, after 7 days, for SA/V = 2000m^-1); all meet European and US regulation limits. After 20 d of dissolution, a hydrated alteration layer of ~ 200-nm-Thick, Ca-depleted and Nd-rich, was formed at the surface of CAB glasses with 20 mol% Nd₂O₃ whereas boehmite [AlO(OH)] secondary crystalline phases were formed in pure CAB glass that contained no Nd₂O₃.