Nepheline Precipitation in High-Level Waste Glasses : Compositional Effects and Impact on the Waste Form Acceptability

Abstract

ABSTRACTThe impact of crystalline phase precipitation in glass during canister cooling on chemical durability of the waste form limits waste loading in glass, especially for vitrification of certain high-level waste (HLW) streams rich in Na2O and Al2O3. This study investigates compositional effects on nepheline precipitation in simulated Hanford HLW glasses during canister centerline cooling (CCC) heat treatment. It has been demonstrated that the nepheline primary phase field defined by the Na2O-Al2O3-SiO2 ternary system can be used as an indicator for screening HLW glass compositions that are prone to nepheline formation. Based on the CCC results, the component effects on increasing nepheline precipitation can be approximately ranked as Al2O3 > Na2O > Li2O ≈ K2O ≈ Fe2O3 > CaO > SiC2. The presence of nepheline in glass is usually detrimental to chemical durability. Using x-ray diffraction data in conjunction with a mass balance and a second-order mixture model for 7-day product consistency test (PCT) normalized B release, the effect of glass crystallization on glass durability can be predicted with an uncertainty less than 50% if the residual glass composition is within the range of the PCT model.