Nepheline Formation Potential in Sludge Batch 4 (SB4)

PEELER, D

Savannah River Site (S.C.)

关键词: Discriminators;    Probability;    12 Management Of Radioactive Wastes, And Non-Radioactive Wastes From Nuclear Facilities;    Hydroxides;    Sludges;   

DOI  :  10.2172/890172
RP-ID  :  WSRC-TR-2005-00153
RP-ID  :  DE-AC09-96SR18500
RP-ID  :  890172

美国|英语

来源: UNT Digital Library

【 摘 要 】

The effect of crystallization on glass durability is complex and depends on several interrelated factors including the change in residual glass composition, the formation of internal stress or microcracks, and the preferential attack at the glass-crystal interface. Perhaps one of the most significant effects is the type and extent (or fraction) of crystallization and the resulting change to the residual glass composition. A strong increase in glass dissolution (or decrease in durability) has been observed in previous studies in glasses that formed aluminum-containing crystals, such as NaAlSiO{sub 4} (nepheline) and LiAlSi{sub 2}O{sub 6}, and crystalline SiO{sub 2}. Although 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{sup +}-[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}). Given the projected high concentration of Al{sub 2}O{sub 3} in SB4 and the potential use of a high Na{sub 2}O based glass (as a result of the use of a high Na{sub 2}O frit and/or a less washed sludge) to improve melt rate, the potential formation of nepheline in various SB4 systems is being assessed. Li et al. (2003) indicate that sodium alumino-borosilicate glasses are prone to nepheline crystallization if their compositions projected on the Na{sub 2}O-Al{sub 2}O{sub 3}-SiO{sub 2} ternary fall within the nepheline primary phase field. In particular, durable glasses with SiO{sub 2}/(SiO{sub 2}+Na{sub 2}O+ Al{sub 2}O{sub 3}) > 0.62, where the oxides are expressed as mass fractions in the glass, do not precipitate nepheline as their primary phase. Twelve SB4-based glasses have been identified or classified as ''prone to nepheline formation'' using a ''less conservative'' discriminator value of 0.65. Ten of the 12 glasses are Frit 320 based, and 8 of these 10 target a 40% WL-independent of the SB4 blending scenario used. This is not unexpected due to the higher alkali content of Frit 320 (12% Na{sub 2}O) relative to Frit 418 (8% Na{sub 2}O) and the fact that as WLs increase, the Na{sub 2}O and Al{sub 2}O{sub 3} concentrations increase and the SiO{sub 2} concentrations decrease in this series of glasses. Using the ''less conservative'' value as a guide will not only increase the probability of forming nepheline but will also allow the assessment of several different blending scenarios, both frits, and different WLs which will provide valuable insight into the frit selection process for SB4. More specifically, blending strategies, frit compositions, and WLs that avoid nepheline formation could be used to guide the frit selection process or to make compositional adjustments to the frit. The durability of these 12 glasses (of both quenched and centerline canister cooled versions) will be measured with the results being documented in a subsequent report.