PNNL

Abstract

High-alumina high-level waste (HLW) glasses are prone to nepheline (nominally NaAlSiO4) 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 vitrification plants. The constraint used has been shown to significantly limit the loading of waste in glass at Hanford and therefore the cost and schedule of cleanup. A study was performed to develop an improved understanding of the impacts of glass composition on the formation of nepheline during CCC. Four experimental phases were conducted in which 90 independent glass compositions were fabricated, subjected to simulated CCC heat-treatments, and characterized for crystallinity – 38 of the 90 test glasses formed nepheline. These data were examined separately and combined with 657 glasses previously tested glasses found in literature. The trends showed that in addition to Na2O, Al2O3, and SiO2 components included in previous constraints B2O3, CaO, K2O, and Li2O also significant impacted the propensity for nepheline formation. A pseudo-ternary submixture approach was proposed to identify the glass composition region prone to nepheline precipitation. This pseudo-ternary with axis of SiO2 + 1.70B2O3, Na2O + 0.813Li2O + 0.439K2O + 0.223CaO, and Al2O3 was found to effectively divide typical U.S. HLW glasses that precipitate nepheline during CCC from those that do not. This approach results in a total misclassification rate of 13%, 7% of which are false negatives (those glasses predicted not to form nepheline that actually do for nepheline). When applied to the 90 glasses developed specifically for Hanford high-alumina HLWs, the misclassification rate is 19% (17/90) with 1/38 false negatives. Application of such a constraint is anticipated to increase the loading of Hanford high-alumina HLWs in glass by roughly one third.