Nepheline Formation Study for Sludge Batch 4 (SB4): Phase 1 Experimental Results

Abstract

Although it is well known that the addition of Al{sub 2}O{sub 3} to borosilicate glasses enhances the durability of the waste form (through creation of network-forming tetrahedral Na+-[AlO{sub 4/2}]{sup -} pairs), the combination of high Al{sub 2}O{sub 3} and Na{sub 2}O can lead to the formation of nepheline (NaAlSiO{sub 4})--which can negatively impact durability. Given the projected high concentration of Al{sub 2}O{sub 3} in SB4 (Lilliston 2005) and the potential use of a high Na{sub 2}O based frit to improve melt rate and a high Na{sub 2}O sludge due to settling problems, the potential formation of nepheline in various SB4 systems continues to be assessed. Twelve SB4-based glasses were fabricated and their durabilities (via the Product Consistency Test [PCT]) measured to assess the potential for nepheline formation and its potential negative impact on durability. In terms of ''acceptability'', the results indicate that all of the study glasses produced are acceptable with respect to durability as defined by the PCT (normalized boron release values for all nepheline (NEPH) glasses were much lower than that of the Environmental Assessment (EA) glass (16.695 g/L)). The most durable glass is NEPH-04 (quenched) with a normalized boron release (NL [B]) of 0.61 g/L, while themore » least durable glass is NEPH-01 centerline canister cooled (ccc) with an NL [B] of 2.47 g/L (based on the measured composition). In terms of predictability, most of the study glasses are predictable by the {Delta}G{sub p} model. Those that are not predictable (i.e., they fall outside of the prediction limits) actually fall below the prediction interval (i.e., they are over predicted by the model) suggesting the model is conservative. The Phase 1 PCT results suggest that for those glasses prone to nepheline formation (using the 0.62 value developed by Li et al. (2003) as a guide), a statistically significant difference in PCT response was observed for the two heat treatments but the impact on durability was of little or no practical concern. When one couples the PCT responses with the X-Ray Diffraction (XRD) results and/or visual observations, one could conclude that the formation of nepheline in these glasses does have a negative impact on durability. However, that impact may be of statistical significance, but the practical impact may not be sufficient to avoid a specific candidate frit for the SB4 glass system. The results of this study not only suggest that the 0.62 value appears to be a reasonable guide to monitor sludge--frit systems with respect to potential nepheline formation, but also that the impact of nepheline, although statistically significant, has little or no practical impact in the SB4 system to durability as measured by the PCT. This latter statement must be qualified to some extent given only two glasses were selected which were actually ''prone to nepheline formation'' based on this general guide and the relatively volume % of nepheline formed based on XRD results ({approx} 0.5 vol%). If the presence of nepheline has no appreciable, adverse impact on durability for the recently revised SB4 systems, then as decisions regarding the viability of the SB4 options and the down select of candidate frits are pursued, little weight will be given to minimizing the likelihood of nepheline and the decisions will be dominated by waste throughput criteria. That is, the frit selection process will not have to consider the impact of nepheline on the ultimate durability of the product and can focus on recommending a frit that when coupled with the sludge can be processed over a waste loading (WL) interval of interest to the Defense Waste Processing Facility (DWPF) with melt rates meeting production expectations.« less