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

摘要

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 MoO_3 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 MoO_3 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: B_2O_3, Al_2O_3, CaO, and SiO_2 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.