"Low-Li2O" Frits: Selecting Glasses that Support the Melt Rate Studies and Challenge the Current Durability Model

Abstract

During the progressive development of the cold cap model (as it applies to a potential melt rate predictive tool), the formation of an Al-Li-silicate phase was identified as an intermediate reaction phase that could possibly hinder melt rate for SB4. To test this theory, six glasses were designed (using Frit 320's composition as the baseline) to maintain a constant 20 wt% sum of alkali content (in frit) by varying Na{sub 2}O to Li{sub 2}O ratios. The Li{sub 2}O concentration ranged from 8 wt% down to 0% in either 2% or 1% increments with the differences being accounted for by an increase in Na{sub 2}O concentration. Although the primary objective of the ''lower Li{sub 2}O'' frits was to evaluate the potential for melt rate improvements, assessments of durability (as measured by the Product Consistency Test (PCT)) were also performed. The results suggest that durable glasses can be produced with these ''lower Li{sub 2}O'' frits should it be necessary to pursue this option for improving melt rate. In addition to the series of glasses to support melt rate assessments, a series of frits were also developed to challenge the current durability model based on the limits proposed by Edwards et al. (2004).more » Although the ''new'' limits allow access into compositional regions of interest (i.e., higher alkali systems) which can improve melt rate and/or waste loading, there may still be ''additional'' conservatism. In this report, two series of glasses were developed to challenge the ''new'' durability limits for the SB4 system. In the first series, the total alkali of the Frit 320-based glasses (designed to support the melt rate program) was increased from 20 wt% to 21 wt% (in the frit), but the series also evaluated the possible impact of various Na{sub 2}O and Li{sub 2}O mass ratio differences. The second series pushed the alkali limit in the frit even further with frits containing either 22 or 24 wt% total alkali as well as various Na{sub 2}O and Li{sub 2}O mass ratios. The results of the PCT evaluation indicated that all of the ''higher alkali'' glasses are acceptable as defined by their NL [B]'s as compared to the Environmental Assessment (EA) glass (with a 16.695 g/L NL [B])--regardless of the compositional view (measured or target) or thermal heat treatment (quenched versus centerline canister cooled). The least durable glass (based on NL [B] and target compositions) was Low-Li-7 (quenched) with a NL [B] of 1.11 g/L. With the measured PCT responses being acceptable (i.e., all < 1.11 g/L), the results suggest additional conservatism exists within the current durability model even with the ''proposed'' limits. More specifically, the ''proposed'' limits still appear to restrict access to compositional regions of interest (higher alkali glasses) even though their measured PCT responses are acceptable.« less