Vitrification of High-Level Waste at the Savannah River Site

ABSTRACT The objective of this study was to experimentally measure the properties and performance of a series of glasses with compositions that could represent high level waste Sludge Batch 5 (SB5) as vitrified at the Savannah River Site Defense Waste Processing Facility. These data were used to guide frit optimization efforts as the SB5 composition was finalized. Glass compositions for this study were developed by combining a series of SB5 composition projections with a group of candidate frits. The study glasses were fabricated using depleted uranium and their chemical compositions, crystalline contents and chemical durabilities were characterized. Trevorite was the only crystalline phase that was identified in a few of the study glasses after slow cooling, and is not of concern as spinels have been shown to have little impact on the durability of high level waste glasses. Chemical durability was quantified using the Product Consistency Test (PCT). All of the glasses had very acceptable durability performance. The results of this study indicate that a frit composition can be identified that will provide a processable and durable glass when combined with SB5

GLASS SELECTION STRATEGY: DEVELOPMENT OF US AND KRI TEST MATRICIES

High-level radioactive wastes are stored as liquids in underground storage tanks at the Department of Energy's (DOE) Savannah River Site (SRS) and Hanford Reservation. These wastes are to be prepared for permanent disposition in a geologic repository by vitrification with glass forming additives (e.g., frit), creating a waste form with long-term durability. Wastes at SRS are being vitrified in the Defense Waste Processing Facility (DWPF). Vitrification of the wastes stored at Hanford is planned for the Waste Treatment and Immobilization Plant (WTP) when completed. Some of the wastes at SRS, and particularly those at Hanford, contain high concentrations of aluminum, chromium and sulfate. These elements make it more difficult to produce a waste glass with a high waste loading (WL) without crystallization occurring in the glass (either within the melter or upon cooling of the glass), potentially exceeding the solubility limit of critical components, having negative impacts on durability, and/or resulting in the formation of a sulfate salt layer on the molten glass surface. Although the overall scope of the task is focused on all three critical, chemical components, the current work will primarily address the potential for crystallization (e.g., nepheline and/or spinel) in high level waste (HLW) glasses. Recent work at the Savannah River National Laboratory (SRNL) and by other groups has shown that nepheline (NaAlSiO{sub 4}), which is likely to crystallize in high-alumina glasses, has a detrimental effect on the durability of the glass. The objective of this task is to develop glass formulations for specific SRS and Hanford waste streams to avoid nepheline formation while meeting waste loading and waste throughput expectations, as well as satisfying critical process and product performance related constraints. Secondary objectives of this task are to assess the sulfate solubility limit for the DWPF composition and spinel settling for the WTP composition. SRNL has partnered with Pacific Northwest National Laboratory (PNNL) and the V.G. Khlopin Radium Institute (KRI) to complete this task

HIGH LEVEL WASTE (HLW) SLUDGE BATCH 4 (SB4): SELECTING GLASSES FOR A VARIABILITY STUDY

A critical step in the Sludge Batch 4 (SB4) qualification process is to demonstrate the applicability of the durability models, which are used as part of the Defense Waste Processing Facility's (DWPF's) process control strategy, to the frit / SB4 glass system via a variability study. A variability study is an experimentally-driven assessment of the predictability and acceptability of the vitrified waste product quality that is anticipated from the processing of a sludge batch. The quality of the waste form is a measure of its durability as determined by the Product Consistency Test (PCT). At the DWPF, the durability of the vitrified waste product is not directly measured by this test during normal operation. Instead, the durability is predicted using a set of models that relate the PCT response of a glass to the chemical composition of that glass. The main objective of a variability study is to demonstrate that these models are applicable to the glass composition region anticipated during the processing of the sludge batch. The success of this demonstration allows the DWPF to confidently rely on the predictions of the durability/composition models as they are used in the control of the DWPF process. The glass region for the SB4 variability study was determined using the most recent projections of the compositions of this sludge batch. Variation was introduced into the composition of the sludge to account for the uncertainty present in these projections as well as for process variation that may be experienced at the DWPF during its normal operations. The primary focus will be on the use of Frit 503, as this frit was recommended for SB4 processing. However, the frit recommendation memorandum also stated that Frit 418 is a viable option, especially for DWPF processing during the transition from SB3 to SB4 (i.e., an acceptable product can be produced with both SB3 and SB4 when Frit 418 is used).a As a result, there is interest in selecting some glasses from the SB4 / Frit 418 system. In this report, glasses are selected for the variability study using a nominal SB4 composition combined with Frits 418 or 503, covering a range of waste loadings (WLs) that are likely to be processed at DWPF. In addition, three sets of corner points or extreme vertices (EVs) for regions representing different levels of variation in the SB4 composition are combined with Frit 503 to identify glasses that will allow for an evaluation of the effect of sludge variation on the durability of the vitrified waste product. These glasses also cover a range of WLs that are likely to be processed at DWPF. A thorough statistical analysis is used to allow for a wide range of sludge compositions to be examined while minimizing the number of glasses that must be made in the laboratory. A total of 35 glasses are selected for the SB4 variability study. These glasses will be batched and melted following standard SRNL procedures, and testing will be completed to measure the chemical durability of each glass composition. A subsequent report will document the results of the experimental portion of the SB4 variability study

SLUDGE BATCH 4 (SB4) AFTER A TANK 40 DECANT: CANDIDATE FRITS, MAR ASSESSMENTS, AND GLASSES FOR A VARIABILITY STUDY

In early October 2006, the Liquid Waste Organization (LWO) began to consider decanting Tank 40 at the end of Sludge Batch 3 (SB3) processing and transferring the aqueous phase from the decant to Tank 51. This transfer would be done to decrease Tank 51 yield stress and facilitate the transfer of the contents of Tank 51 to Tank 40. The projected composition of Sludge Batch 4 (SB4) was adjusted by LWO to reflect the impact of the Tank 40 decant leading to new projected compositions for SB4, designated as the 10-04-06 and the 10-10-06 compositions. A comparison between these SB4 compositions and those provided in June 2006 indicates that the new compositions are slightly higher in Al2O3, Fe2O3, and U3O8 and slightly lower in SiO2. The most dramatic change, however, is the new projection's Na2O concentration, which is more than 4.5 wt% lower than the June 2006 projection. This is a significant change due to the frit development team's approach of aligning the Na2O concentration in a candidate frit to the Na2O content of the sludge. This approach enhances the projected operating window and the waste throughput potential for the resulting glass system while eliminating the potential for nepheline crystallization. Nepheline can have a detrimental impact on durability. Questions surfaced regarding the applicability of Frit 503 to these revised compositions since the Savannah River National Laboratory (SRNL) recommended Frit 503 for use with SB4 based on the June 2006 compositional projection without the Tank 40 decant. Based on the paper study assessments, the change in SB4's expected Na2O content had a significant, negative impact on the projected operating window for the Frit 503/SB4 glass system. While Frit 418 had slightly smaller waste loading (WL) intervals for the June 2006 SB4 projections as compared to Frit 503 and the Frit 418 glass systems were nepheline limited, Frit 418 had a slightly larger operating window for the 10-04-06 projection (as compared to Frit 503) and the Frit 418/10-04-06 glass system was no longer nepheline limited. Thus, strictly from the perspective of this paper study, Frit 418 was more attractive than Frit 503 for the new SB4 projected compositions. This comparison, however, does not reflect other aspects of interest for the glass systems such as their respective melt rates or the development of alternative frits to balance the projected operating windows, melt rate, waste throughput, and robustness to compositional variation. In discussions with Waste Solidification Engineering (WS-E) regarding the results being presented in this report, their decision was to utilize Frit 418 for initial processing of SB4. This decision was not only based on the paper study assessments presented in this report, but also on the fact that Frit 418 is currently being used to process SB3 and, perhaps more importantly, frit optimization efforts for SB4 may be premature given the uncertainties in tank transfer and heel volumes associated with the SB4 flowsheet. More specifically, WS-E indicated their plan to initiate processing with Frit 418 with subsequent authorization for the frit development team to optimize a frit based on the measured composition of SB4 after determination of the actual SB4 blend composition (i.e., both the SB3 and SB4 compositions and masses are known). Given this decision and recognizing that a SB4/Frit 503 variability study had been initiated as part of the qualification process, questions regarding the need for a supplemental variability study to demonstrate applicability of the process control models for a Frit 418 based system surfaced. This report addresses the need for a supplemental study and defines additional glasses to fill the compositional gaps. A total of 13 glasses (based on the 10-10-06 projection) were selected for the supplemental SB4/Frit 418 variability study. These glasses will be batched and melted following standard SRNL procedures, and a suite of characterization testing will be completed to measure the chemical durability of each glass composition

RECOMMENDED FRIT COMPOSITION FOR INITIAL SLUDGE BATCH 5 PROCESSING AT THE DEFENSE WASTE PROCESSING FACILITY

The Savannah River National Laboratory (SRNL) Frit Development Team recommends that the Defense Waste Processing Facility (DWPF) utilize Frit 418 for initial processing of high level waste (HLW) Sludge Batch 5 (SB5). The extended SB5 preparation time and need for DWPF feed have necessitated the use of a frit that is already included on the DWPF procurement specification. Frit 418 has been used previously in vitrification of Sludge Batches 3 and 4. Paper study assessments predict that Frit 418 will form an acceptable glass when combined with SB5 over a range of waste loadings (WLs), typically 30-41% based on nominal projected SB5 compositions. Frit 418 has a relatively high degree of robustness with regard to variation in the projected SB5 composition, particularly when the Na{sub 2}O concentration is varied. The acceptability (chemical durability) and model applicability of the Frit 418-SB5 system will be verified experimentally through a variability study, to be documented separately. Frit 418 has not been designed to provide an optimal melt rate with SB5, but is recommended for initial processing of SB5 until experimental testing to optimize a frit composition for melt rate can be completed. Melt rate performance can not be predicted at this time and must be determined experimentally. Note that melt rate testing may either identify an improved frit for SB5 processing (one which produces an acceptable glass at a faster rate than Frit 418) or confirm that Frit 418 is the best option

INTERNATIONAL STUDIES OF ENHANCED WASTE LOADING AND IMPROVED MELT RATE FOR HIGH ALUMINA CONCENTRATION NUCLEAR WASTE GLASSES

The goal of this study was to determine the impacts of glass compositions with high aluminum concentrations on melter performance, crystallization and chemical durability for Savannah River Site (SRS) and Hanford waste streams. Glass compositions for Hanford targeted both high aluminum concentrations in waste sludge and a high waste loading in the glass. Compositions for SRS targeted Sludge Batch 5, the next sludge batch to be processed in the Defense Waste Processing Facility (DWPF), which also has a relatively high aluminum concentration. Three frits were selected for combination with the SRS waste to evaluate their impact on melt rate. The glasses were melted in two small-scale test melters at the V. G. Khlopin Radium Institute. The results showed varying degrees of spinel formation in each of the glasses. Some improvements in melt rate were made by tailoring the frit composition for the SRS feeds. All of the Hanford and SRS compositions had acceptable chemical durability

TECHNOLOGY DEMONSTRATION OF SLUDGE MASS REDUCTION VIA ALUMINUM DISSOLUTION: GLASS FORMULATION PROCESSING WINDOW PREDICTIONS FOR SB5

Composition projections for Sludge Batch 5 (SB5) were developed, based on a modeling approach at the Savannah River National Laboratory (SRNL), to evaluate possible impacts of the Al-dissolution process on the availability of viable frit compositions for vitrification at the Defense Waste Processing Facility (DWPF). The study included two projected SB5 compositions that bound potential outcomes (or degrees of effectiveness) of the Al-dissolution process, as well as a nominal SB5 composition projection based on the results of the recent Al-dissolution demonstration at SRNL. The three SB5 projections were the focus of a two-stage paper study assessment. A Nominal Stage assessment combined each of the SB5 composition projections with an array of 19,305 frit compositions over a wide range of waste loading (WL) values and evaluated them against the DWPF process control models. The Nominal Stage results allowed for the down-selection of a small number of frits that provided reasonable projected operating windows (typically 27 to 42 wt% WL). The frit/sludge systems were mostly limited by process related constraints, with only one system being limited by predictions of nepheline crystallization, a waste form affecting constraint. The criteria applied in selecting the frit compositions somewhat restricted the compositional flexibility of the candidate frits for each individual SB5 composition projection, which may limit the ability to further tailor the frit for improved melt rate. Variation Stage assessments were then performed using the down-selected frits and the three SB5 composition projections with variation applied to each sludge component. The Variation Stage results showed that the operating windows were reduced in width, as expected when variation in the sludge composition is applied. However, several of the down-selected frits exhibited a relatively high degree of robustness to the applied sludge variation, providing WL windows of approximately 30 to 39 wt%. The maximum WLs were limited by processing constraints, liquidus temperature and low viscosity, rather than a waste form affecting constraint (e.g., nepheline crystallization) in the Variation Stage assessments. These paper study assessments have identified candidate frits which, when combined with the SRNL projected SB5 compositions after Al-dissolution, have projected operating windows that should be reasonable for DWPF processing. As more information is obtained on the SB5 composition to be processed in DWPF, including the actual Al removed and Tank 7 mass transferred, additional paper study assessments will be performed as well as experimental frit development studies. The frits identified in this study provide insight into potential processing windows but are not the recommended frits for SB5. No information regarding melt rate can be inferred from the paper study results. Experimental studies to evaluate this critical factor in DWPF processing must be performed on the best SB5 projection before a frit recommendation could be made for any projected sludge composition

EM-21 Higher Waste Loading Glasses for Enhanced DOE High-Level Waste Melter Throughput Studies -10194

ABSTRACT Supplemental validation data have been generated that will be used to determine the applicability of the current Defense Waste Processing Facility (DWPF) liquidus temperature (T L ) model to expanded DWPF glass regions of interest based on higher waste loadings. For those study glasses which had very close compositional overlap with the model development and/or model validation ranges (except TiO 2 and MgO concentrations), there was very little difference in the predicted and measured T L values, even though the TiO 2 contents were above the 2 wt% upper concentration covered by the model. The results indicate that the current T L model is applicable in these compositional regions. As the compositional overlap between the model validation ranges diverged from the target glass compositions, the T L data suggest that the model underpredicted the measured values. These discrepancies imply that the influence of individual oxides or their combinations at concentrations outside of the model development and/or previous validation regions may not be adequately assessed by the current model. These oxides include B 2 O 3 , SiO 2 , MnO, TiO 2 and/or their combinations. More data are required to fill in these anticipated DWPF compositional regions so that the model coefficients could be refit to account for these influences. INTRODUCTION High-level waste (HLW) throughput (i.e., the amount of waste processed per unit time) is a function of several parameters, two of which are extremely critical: waste loading (WL) and melt rate. For the Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS), increasing HLW throughput would significantly reduce the overall mission life cycle costs for the Department of Energy (DOE)

Control of Nepheline Crystallization in Nuclear Waste Glass

Abstract Glass frits with a high B 2 O 3 concentration have been designed which, when combined with high-alumina concentration nuclear waste streams, will form glasses with durabilities that are acceptable for repository disposal and predictable using a free energy of hydration model. Two glasses with nepheline discriminator values closest to 0.62 showed significant differences in normalized boron release between the quenched and heat treated versions of each glass. X-ray diffraction confirmed that nepheline crystallized in the glass with the lowest nepheline discriminator value, and nepheline may also exist in the second glass as small nanocrystals. The high-B 2 O 3 frit was successful in producing simulated waste glasses with no detectable nepheline crystallization at waste loadings of up to 45 wt%. The melt rate of this frit was also considerably better than other frits with increased concentrations of Na 2 O. Introduction High-level wastes, a legacy of Cold War production of nuclear materials for defense, are stored at several Department of Energy (DOE) facilities in the United States. These wastes, typically in the form of a sludge stored in underground tanks, are being vitrified with a glass frit or mined, glass-forming minerals to form a glass wasteform suitable for final disposal in the national repository. The waste glass must meet DOE requirements for durability, or resistance to chemical leaching in water at elevated temperatures, in order to be acceptable to the repository. A significant volume of these wastes has been identified as containing high concentrations of Al 2 O 3 and Na 2 O. Based on current blending and washing strategies, waste streams at the Savannah River Site (SRS) have been identified with Al 2 O 3 concentrations from 25 to 40 wt%. Concentrations in some wastes at Hanford are as high as 80 wt%. The combination of high Al 2 O 3 and Na 2 O concentrations in the waste, coupled with lower SiO 2 concentrations in the glass as waste loadings (WLs) increase, can lead to the crystallization of nepheline (NaAlSiO 4 ). The impact of crystallization, or devitrification, on the durability of the glass wasteform is dependent upon the type and extent of the crystalline phases that develop

FRIT SELECTION TO SUPPORT STEKLO METALLICHESKIE KONSTRUKTSII MELTER TESTING WITH SRNL FEEDS

Four frits were developed for possible use in melter testing with V.G. Khlopin Radium Institute's Steklo Metallicheskie Konstruktsii (SMK) melter. The frits were selected using Measurement Acceptability Region (MAR) assessments of an array of frit formulations and two Sludge Batch 5 (SB5) flowsheets, one with the anticipated effect of the implementation of Al-dissolution and one without. Test glasses were fabricated in the laboratory to verify that the property and performance models used to select the frits were applicable to the frit/sludge systems of interest. Each of the four frits was tested with each of the two sludges at two different waste loadings, for a total of 16 test glasses. Each glass was both quenched and subjected to the canister centerline cooled (CCC) thermal profile. Samples of each glass were examined for crystallization by X-ray diffraction (XRD) and durability using the Product Consistency Test (PCT). The quenched version of each glass appeared amorphous by visual observations, although XRD results indicated a small amount of crystallization in four of the quenched glasses. Visual observations identified surface crystallization on the CCC versions of all 16 glasses. Three of the 35% waste loading (WL), CCC glasses were found to contain trevorite (a spinel) by XRD, and all of the 40% WL CCC glasses were found to contain trevorite. Nepheline was not observed in any of the test glasses, which is consistent with model predictions