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Title: NEPHELINE FORMATION POTENTIAL IN SLUDGE BATCH 4 AND ITS IMPACT ON DURABILITY: SELECTING GLASSES FOR A PHASE 3 STUDY

Abstract

Savannah River National Laboratory's frit development effort for SB4 is being driven by the most current CBU option for this sludge, referred to as Case 15C Blend 1. Candidate frits have been identified for this option via a paper study approach developed by Peeler and Edwards with the intent of down-selecting to a set of key frits whose operating windows (i.e., WL intervals that meet PCCS MAR criteria) are robust to and/or selectively optimal for this sludge option. The primary frits that appear attractive on paper (i.e., down-selected via the paper study) are now being incorporated into this experimental study. The potential for the formation of a nepheline primary crystalline phase is an important factor in frit development for SB4, due to the high Al{sub 2}O{sub 3} content of this sludge. Based upon earlier work by Li et al., glasses that do not satisfy the constraint: (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, will precipitate nepheline as their primary crystalline phase, hindering the durability of the glass. Based on the most recent compositional projection from the CBU for SB4 (Case 15C Blend 1), 16more » glasses have been selected to complement the earlier work by continuing the investigation into the ability of the above constraint to predict the occurrence of a nepheline primary crystalline phase for SB4 glasses and into the impact of such phases on the durability of the SB4 glasses. Glasses were selected to cover WLs which tightly bound the nepheline discriminator value of 0.62, with the intent of refining this value to a level of confidence where it can be incorporated into offline administrative controls and/or the PCCS to support SME acceptability decisions. In addition, glass specimens at WLs of 35 and 40% will be prepared and analyzed to contribute needed data to the ComPro{trademark} database in anticipation of a variability study for SB4. The glasses in Table 4-3 are to batched and fabricated using standard procedures. Visual observations and other analytical techniques are to be used, as needed, to assess the presence of crystals with specific interest in the nepheline primary phase. The durability of these glasses (for both quenched and centerline canister cooled versions) is to be measured using the ASTM PCT Method A. The results from these efforts are to be documented in a subsequent report. The results of this study will provide valuable input for the frit development efforts and subsequent feedback to the CBU regarding the relative viability of the current SB4 option under consideration. The refined nepheline discriminator value will provide a guideline for the avoidance of nepheline crystallization in SB4 glasses and aid in down-selection of frit compositions. These data will be combined with the results of melt rate studies and a paper study of the frits robustness with regard to variability in the sludge composition to provide an optimized frit recommendation to DWPF for immobilization of SB4.« less

Authors:
Publication Date:
Research Org.:
SRS
Sponsoring Org.:
USDOE
OSTI Identifier:
901319
Report Number(s):
WSRC-TR-2006-00053
TRN: US0702605
DOE Contract Number:
DE-AC09-96SR18500
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; SLUDGES; RADIOACTIVE WASTE PROCESSING; VITRIFICATION; GLASS; CORROSION RESISTANCE; WASTE FORMS; SAVANNAH RIVER PLANT; CRYSTAL STRUCTURE

Citation Formats

Fox, K. NEPHELINE FORMATION POTENTIAL IN SLUDGE BATCH 4 AND ITS IMPACT ON DURABILITY: SELECTING GLASSES FOR A PHASE 3 STUDY. United States: N. p., 2006. Web. doi:10.2172/901319.
Fox, K. NEPHELINE FORMATION POTENTIAL IN SLUDGE BATCH 4 AND ITS IMPACT ON DURABILITY: SELECTING GLASSES FOR A PHASE 3 STUDY. United States. doi:10.2172/901319.
Fox, K. Fri . "NEPHELINE FORMATION POTENTIAL IN SLUDGE BATCH 4 AND ITS IMPACT ON DURABILITY: SELECTING GLASSES FOR A PHASE 3 STUDY". United States. doi:10.2172/901319. https://www.osti.gov/servlets/purl/901319.
@article{osti_901319,
title = {NEPHELINE FORMATION POTENTIAL IN SLUDGE BATCH 4 AND ITS IMPACT ON DURABILITY: SELECTING GLASSES FOR A PHASE 3 STUDY},
author = {Fox, K},
abstractNote = {Savannah River National Laboratory's frit development effort for SB4 is being driven by the most current CBU option for this sludge, referred to as Case 15C Blend 1. Candidate frits have been identified for this option via a paper study approach developed by Peeler and Edwards with the intent of down-selecting to a set of key frits whose operating windows (i.e., WL intervals that meet PCCS MAR criteria) are robust to and/or selectively optimal for this sludge option. The primary frits that appear attractive on paper (i.e., down-selected via the paper study) are now being incorporated into this experimental study. The potential for the formation of a nepheline primary crystalline phase is an important factor in frit development for SB4, due to the high Al{sub 2}O{sub 3} content of this sludge. Based upon earlier work by Li et al., glasses that do not satisfy the constraint: (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, will precipitate nepheline as their primary crystalline phase, hindering the durability of the glass. Based on the most recent compositional projection from the CBU for SB4 (Case 15C Blend 1), 16 glasses have been selected to complement the earlier work by continuing the investigation into the ability of the above constraint to predict the occurrence of a nepheline primary crystalline phase for SB4 glasses and into the impact of such phases on the durability of the SB4 glasses. Glasses were selected to cover WLs which tightly bound the nepheline discriminator value of 0.62, with the intent of refining this value to a level of confidence where it can be incorporated into offline administrative controls and/or the PCCS to support SME acceptability decisions. In addition, glass specimens at WLs of 35 and 40% will be prepared and analyzed to contribute needed data to the ComPro{trademark} database in anticipation of a variability study for SB4. The glasses in Table 4-3 are to batched and fabricated using standard procedures. Visual observations and other analytical techniques are to be used, as needed, to assess the presence of crystals with specific interest in the nepheline primary phase. The durability of these glasses (for both quenched and centerline canister cooled versions) is to be measured using the ASTM PCT Method A. The results from these efforts are to be documented in a subsequent report. The results of this study will provide valuable input for the frit development efforts and subsequent feedback to the CBU regarding the relative viability of the current SB4 option under consideration. The refined nepheline discriminator value will provide a guideline for the avoidance of nepheline crystallization in SB4 glasses and aid in down-selection of frit compositions. These data will be combined with the results of melt rate studies and a paper study of the frits robustness with regard to variability in the sludge composition to provide an optimized frit recommendation to DWPF for immobilization of SB4.},
doi = {10.2172/901319},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Jan 27 00:00:00 EST 2006},
month = {Fri Jan 27 00:00:00 EST 2006}
}

Technical Report:

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  • The likelihood for the formation of nepheline in Sludge Batch 4 (SB4) glass systems and the potential impact of nepheline on the durability of these systems is part of the frit development efforts for SB4. 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 change to the residual glass composition. A strong increase inmore » 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 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{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 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. The most recent compositional projections from the Closure Business Unit (CBU) for SB4 may be framed around three decision areas: the sodium molarity of the sludge (at values of 1M Na and 1.6M Na), the SB3 heel that will be included in the batch (expressed in inches of SB3 sludge with values of 0, 40, and 127''), and the introduction of an ARP stream into the sludge (which is represented by six options: no ARP, ARPa, ARPe, ARPk, ARPm, and ARPv). Candidate frits are being identified for these options via a paper study approach with the intent of downselecting to a set of key frits whose operating windows (i.e., waste loading intervals that meet Product Composition Control System (PCCS) Measurement Acceptability Region (MAR) criteria) are robust to and/or selectively optimal for these sludge options. The primary or key frits that appear attractive on paper (i.e., down selected via the paper study) will be transferred into SRNL's experimental studies supporting SB4; specifically, the melt-rate studies, chemical process cell flowsheet runs and, if needed, a glass variability study.« less
  • The Defense Waste Processing Facility (DWPF) is about to process High Level Waste (HLW) Sludge Batch 4 (SB4). This sludge batch is high in alumina and nepheline can crystallize readily depending on the glass composition. Large concentrations of crystallized nepheline can have an adverse effect on HLW glass durability. Several studies have been performed to study the potential for nepheline formation in SB4. The Phase 3 Nepheline Formation study of SB4 glasses examined sixteen different glasses made with four different frits. Melt rate experiments were performed by the Process Science and Engineering Section (PS&E) of the Savannah River National Laboratorymore » (SRNL) using the four frits from the Phase 3 work, plus additional high B2O3/high Fe2O3 frits. Preliminary results from these tests showed the potential for significant improvements in melt rate for SB4 glasses using a higher B2O3-containing frit, particularly Frit 503. The main objective of this study was to investigate the durability of SB4 glasses produced with a high B2O3 frit likely to be recommended for SB4 processing. In addition, a range of waste loadings (WLs) was selected to continue to assess the effectiveness of a nepheline discriminator in predicting concentrations of nepheline crystallization that would be sufficient to influence the durability response of the glass. Five glasses were selected for this study, covering a WL range of 30 to 50 wt% in 5 wt% increments. The Frit 503 glasses were batched and melted. Specimens of each glass were heat-treated to simulate cooling along the centerline of a DWPF-type canister (ccc) to gauge the effects of thermal history on product performance. Visual observations on both quenched and ccc glasses were documented. A representative sample from each glass was submitted to the SRNL Process Science Analytical Laboratory (PSAL) for chemical analysis to confirm that the as-fabricated glasses corresponded to the defined target compositions. The Product Consistency Test (PCT, ASTM C1285) was performed in triplicate on each Frit 503 quenched and ccc glass to assess chemical durability. The experimental test matrix also included the Environmental Assessment (EA) glass and the Approved Reference Material (ARM-1) glass. Representative samples of all the ccc glasses were examined for homogeneity visually and by X-ray diffraction (XRD) analysis. Chemical composition measurements indicated that the experimental glasses were close to their target compositions. PCT results showed that all of the Fit 503 quenched glasses had an acceptable durability compared to the EA benchmark glass. The durability of one of the ccc glasses, NEPHB-04, was statistically greater than its quenched counterpart. However, this was shown to be of little practical significance, as the durability of the NEPHB-04 ccc glass was acceptable when compared to the durability of the EA benchmark glass. Visual observations and PCT results indicated that all of the Frit 503 quenched glasses were free of any crystallization that impacts durability. For the ccc glasses, XRD results indicated that the lower WL glasses (30 to 40 wt%) were amorphous, which was consistent with visual observations and PCT responses. The higher WL glasses (45 and 50 wt%) were shown by XRD to contain spinel (trevorite, NiFe2O4). It is possible that some of the other high WL glasses also contained some nepheline, but that the amount of nepheline crystallization was below the detection limit (0.5 vol%) associated with XRD. The results indicate that Frit 503 is a good candidate for SB4 processing, based on chemical durability of homogeneous and devitrified glasses over a WL range of 30 - 50%. It should be noted that the higher WL glasses would not be fit for processing in DWPF as they exceed other process related criteria (such as liquidus temperature). However, this is only one of many factors influencing the frit selection. Melt rate and the final SB4 composition are also important factors in frit selection. Additional melt rate studies are currently underway, and the final composition projection for SB4 is expected shortly.« less
  • This Phase 3 study was undertaken to complement the previous phases of the nepheline formation studies1, 2 by continuing the investigation into the ability of the nepheline discriminator to predict the occurrence of nepheline crystallization in Sludge Batch 4 (SB4) glasses and into the impact of such phases on the durability of the SB4 glasses. The Phase 3 study had two primary objectives. The first was to continue to demonstrate the ability of the discriminator value to adequately predict the nepheline formation potential for specific glass systems of interest. The second was to generate additional data that have a highmore » probability of supporting the SB4 variability study. To support these two objectives, sixteen glasses were selected based on the most recent SB4 compositional projection, Case 15C Blend 1.3 Four different frits were included, based on previous assessments of projected operating windows and melt rate,4, 5 with four WLs selected for each frit. Eight of these frit-sludge combinations covered WLs which tightly bound the nepheline discriminator value of 0.62, with the intent of refining this value to a level of confidence where it can be incorporated into offline administrative controls and/or the Process Composition Control System (PCCS) to support Slurry Mix Evaporator (SME) acceptability decisions. The remaining eight frit-sludge combinations targeted lower WLs (35 and 40%) and were prepared and analyzed to contribute needed data to the ComPro database6 to support a potential variability study for SB4.« less
  • 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 SB4more » 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 the 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