Submixture model to predict nepheline precipitation in waste glasses

High-alumina high-level waste (HLW) glasses are prone to nepheline precipitation during canister-centerline cooling (CCC). If sufficient nepheline forms, the chemical durability of the glass will be significantly impacted. Overly conservative constraints have been developed and used to avoid the deleterious effects of nepheline formation in U.S. HLW glasses. The constraints used have been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A 90-glass study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. The CCC crystallinity data from these glasses were combined with 657 glasses found in the literature. The trends showed significant effects of Na2O, Al2O3, SiO2, B2O3, CaO, Li2O, and potentially K2O on the propensity for nepheline formation. A pseudo-ternary submixture model was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo-ternary with axes of SiO2 + 1.98B2O3, Na2O + 0.653Li2O + 0.158CaO, and Al2O3 was found to divide glasses that precipitate nepheline during CCC from those that do not. Application of this constraint is anticipated to increase the loading of Hanford high-alumina HLWs in glass by roughly one-third.