Multi-Phase Glass-Ceramics as a Waste Form for Combined Fission Products: Alkalis, Alkaline Earths, Lanthanides, and Transition Metals

In this study, multi-phase borosilicate-based glass-ceramics were investigated as an alternative waste form for immobilizing non-fissionable products from used nuclear fuel. Currently, borosilicate glass is the waste form selected for immobilization of this waste stream, however, the low thermal stability and solubility of MoO3 in borosilicate glass translates into a maximum waste loading in the range 15-20 mass%. Glass-ceramics provide the opportunity to target chemically durable crystalline phases, e.g., powellite, oxyapatite, celsian, and pollucite that will incorporate MoO3 as well as other waste components such as lanthanides, alkalis, and alkaline earths at levels twice the solubility limits of a single-phase glass. In addition a glass-ceramic could provide higher thermal stability, depending upon the properties of the crystalline and amorphous phases. Here, glass-ceramics were synthesized at waste loadings of 42, 45, and 50 mass% with the following glass additives: B2O3, Al2O3, CaO, and SiO2 by slow-cooling from a glass melt. Glass-ceramics were characterized in terms of phase assemblage, morphology, and thermal stability. Only two of the targeted phases, powellite and oxyapatite, were observed, along with lanthanide-borosilicate and cerianite. Results of this initial investigation show promise of glass-ceramics as a potential waste form to replace single-phase borosilicate glass.