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

Abstract

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 similar to 56.8 wt%. Viscosity and the electrical conductivity of the CAB melt at 1300 degrees 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 similar to 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.