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Ensuring the long-term chemical durability of glasses is critical for nuclear waste immobilization operations. Durable glasses usually undergo qualification for disposal based on their response to standardized tests such as the product consistency test or the vapor hydration test (VHT). The VHT uses elevated temperature and water vapor to accelerate glass alteration and the formation of secondary phases. Understanding the relationship between glass composition and VHT response is of fundamental and practical interest. However, this relationship is complex, non-linear, and sometimes fairly variable, posing challenges in identifying the distinct effect of individual oxides on VHT response. Here, we leverage a dataset comprising 654 Hanford low-activity waste (LAW) glasses across a wide compositional envelope and employ various machine learning techniques to explore this relationship. We find that Gaussian process regression (GPR), a nonparametric regression method, yields the highest predictive accuracy. By utilizing the trained model, we discern the influence of each oxide on the glasses' VHT response. Moreover, we discuss the trade-off between underfitting and overfitting for extrapolating the material performance in the context of sparse and heterogeneous datasets. [ABSTRACT FROM AUTHOR]

Re is often used as a nonradioactive surrogate for Tc in borosilicate melts because both elements have similar valences, ionic radii, and coordination environments. Re solubility in high sodium borosilicate waste glasses is sensitive to melter redox conditions; at 1150°C the solubility is ∼4130 ppm under oxidizing atmospheres. Raman, X‐ray absorption spectroscopy, and X‐ray diffraction show that Re dissolved in waste glasses under oxidizing conditions is in perrhenate tetrahedra; under more reducing conditions, Re0 separates from the melt. From air to PO2$ {\rm P}_{{\rm O}_2} $∼ 10−4 atm, both Re7+ and Tc7+ are dominant species in borosilicate melts. However, Tc and Re behaviors deviate between PO2$ {\rm P}_{{\rm O}_2} $< 10−4 atm and PO2$ {\rm P}_{{\rm O}_2} $> 10−8 atm, where Tc4+ becomes dominant, whereas Re7+ remains stable. Unlike Tc4+, Re4+ is not observed in borosilicate melts at 1150°C and associated glasses. Near PO2$ {\rm P}_{{\rm O}_2} $= 10−8 atm, Tc4+ reduces to Tc0, whereas below PO2$ {\rm P}_{{\rm O}_2} $= 10−8 atm, Re7+ reduces to Re0. Re is found to be a good surrogate for Tc under oxidizing conditions (PO2$ {\rm P}_{{\rm O}_2} $> 10−4 atm) and under highly reducing conditions (PO2$ {\rm P}_{{\rm O}_2} $< 10−9 atm). Redox and solubility behaviors of Tc and Re are also sensitive to glass chemistry, such that the fair assessment of Re modeling Tc behavior should be glass specific. [ABSTRACT FROM AUTHOR]

For nuclear waste vitrification in the Tank Waste Treatment and Immobilization Plant (WTP) at the Hanford site, waste feed is charged into a continuously fed electric melter and the molten glass is poured into stainless steel containers to solidify. The heat from the molten glass must be removed from the facility within plant operating constraints, making knowledge of thermal properties of the waste glasses critical. The heat capacity of Hanford low‐activity waste glasses with systematic variations in composition were studied using complementary thermal analyses – power‐compensated differential scanning calorimetry (pcDSC) at 50–630°C, heat‐flux differential scanning calorimetry (hfDSC) at 200–800°C, and drop calorimetry at 600–1200°C. Variations of the heat capacity with composition have been observed around the transition temperature (Tg) and above, particularly when B2O3 and Al2O3 is varied. Above Tg, all glasses exhibit decreasing heat capacity with increasing temperature. A correlation was found between this curve and the fraction B[4]2O3 in glasses, suggesting boron coordination plays a role in this anomalous behavior. These measurements will be used as critical input to validate the heat balance calculations used for facility design and thus the overall operation of the WTP. [ABSTRACT FROM AUTHOR]

The article reports on the structural dependence of crystallization in Na2O–Al2O3–B2O3–P2O5–SiO2‐based glasses over a broad compositional space. The structure of melt‐quenched glasses has been investigated using 11B, 27Al, 29Si, and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, while the crystallization behavior has been followed using X‐ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy. In general, the integration of phosphate into the sodium aluminoborosilicate network is mainly accomplished via the formation of Al–O–P and B–O–P linkages with the possibility of formation of Si–O–P linkages playing only a minor role. In terms of crystallization, at low concentrations (≤5 mol.%), P2O5 promotes the crystallization of nepheline (NaAlSiO4), while at higher concentrations (≥10 mol.%), it tends to suppress (completely or incompletely depending on the glass chemistry) the crystallization in glasses. When correlating the structure of glasses with their crystallization behavior, the MAS NMR results highlight the importance of the substitution/replacement of Si–O–Al linkages by Al–O–P, Si–O–B, and B–O–P linkages in the suppression of nepheline crystallization in glasses. The results have been discussed in the context of (1) the problem of nepheline crystallization in Hanford high‐level waste glasses and (2) designing vitreous waste forms for the immobilization of phosphate‐rich dehalogenated Echem salt waste. [ABSTRACT FROM AUTHOR]

[Display omitted] • Computational fluid dynamics and heat transfer models of waste glass melters. • Computational modeling is a powerful complement to testing. • Models offer insights into physicochemical phenomena occurring in the melters. • Models provide information on melter behavior and processing throughput. • Rigorous validation using lab-scale experiments and pilot-scale melter tests. Computational fluid dynamics and heat transfer models of waste glass melters are being developed to support the vitrification campaign at the Waste Treatment and Immobilization Plant (WTP) as part of the United States nuclear waste stewardship and environmental protection programs. The WTP mission aims to achieve the stable and durable processing of vitrified waste in the safest, most efficient, and economical manner possible. Hanford's tank waste is far more complex and varied than the waste treated by other vitrification facilities and both startup and long-term operational challenges are likely to be even greater than prior experience has shown. There are thousands of different waste compositions, which makes processing the waste a complex and challenging task. Models of test melters, pilot-scale, and full-scale melters have been developed to provide understanding and insight into the various physical and chemical phenomena occurring during vitrification. The results of these modeling activities will be used to support the WTP operations to process legacy nuclear waste into a stable form for disposal. [ABSTRACT FROM AUTHOR]

Hanford low‐activity waste (LAW) glasses with high sulfur concentrations are subject to salt segregation in the melter, which hinders melter operation by corroding components and shortening the melter life. To better predict the point at which salt accumulates on the melt surface, the development of sulfate solubility models is needed. Using a sulfur saturation method, crucible scale melts for 13 LAW glasses with varying sulfur solubilities were conducted. The resulting salt and glass compositions were reported and the change in component partitioning following the saturation process was examined to better understand potential changes in overall glass composition. It was shown that both Cr2O3 and Cl experience significant losses, with ~28% of Cr2O3 partitioning into the salt phase and Cl primarily volatilizing out of the melt (~23% partitioned to salt and ~40% lost as off gas). These patterns can be accounted for during model development. Measured sulfur solubilities were also compared to previously reported data. It was found that crucible sulfur solubility ranged from 0.95 to 2.14 wt% SO3 with a high correlation ([Rfit2=0.94]) between crucible solubility and melter tolerance. These results suggest that crucible scale sulfate solubility data can be used to predict SO3 tolerance in the melter feed. [ABSTRACT FROM AUTHOR]

• Performed a systematic evaluation of Hanford low-activity waste loading in glasses. • Constraint impacts on the glass mass were calculated and compared. • ε, VHT, SO 3 solubility, and crystal constraints have the largest effects. • Product Consistency Test and viscosity have relatively small impact. • Model prediction and process uncertainties effects were quantified. A systematic evaluation of Hanford low-activity waste (LAW) loading in alkali-aluminoborosilicate glasses was performed. A set of 22 waste compositions were selected to represent the range of wastes anticipated across the Hanford mission. Glass composition was formulated for each waste to simultaneously satisfy a full cadre of property and composition constraints while maximizing waste loading. Glass properties were estimated using property models recently developed for Hanford LAW Facility operation. The impacts from the property and composition constraints on the glass mass to be produced were calculated and compared. Electrical conductivity, Vapor Hydration Test and SO 3 melter tolerance, and canister-centerline cooling crystal constraints have a large effect on reducing total glass mass, while the impacts from Product Consistency Test, viscosity, and isothermal crystal constraints are relatively small. Additionally, effects of model prediction and process/composition uncertainties were quantified. The results identified areas of future research that could be useful to improve the efficiency of plant operation. [ABSTRACT FROM AUTHOR]

The study aims at understanding the structural role of P 2 O 5 in meta- and per-aluminous sodium aluminosilicate glasses, followed by its correlation with their crystallization behavior. Experimental results suggest an increase in the degree of polymerization of the phosphate network and a change in the aluminum environment in the glasses from Al(OSi) 4 to Al(OP) 4 with an increasing P 2 O 5 /SiO 2 ratio. The substitution of SiO 2 –by–P 2 O 5 in the amount varying between 10 – 20 mol.% suppresses the crystallization of nepheline. This has been attributed to the unavailability of Al3+ (to form Si–O–Al linkages) due to the formation of Al–O–P linkages in the glass structure. An increase in P 2 O 5 content (>20 mol.%) in peraluminous glasses results in the crystallization of AlPO 4 as the primary phase. The results have been discussed in the light of various models developed to predict the compositional dependence of nepheline crystallization in Hanford's high-level waste glasses. [ABSTRACT FROM AUTHOR]

Volatilization of technetium-99 (99Tc) is of concern during vitrification of Hanford low-activity waste (LAW) to produce a borosilicate glass waste form at the Waste Treatment and Immobilization Plant. Addition of reducing agents to LAW glass feed has been known to increase the retention of 99Tc in the final glass product. We investigated rhenium incorporation into glass by performing crucible melting tests on simulated LAW glass feeds, prepared without and with iron oxalate as a reducing agent, to understand the mechanism how the reducing agent influences Re retention in glass. Deionized water leaches were conducted on the dried feed and heat-treated samples to analyze the compositions of soluble and insoluble phases separately and to profile the partitioning of components into different phases during melting. The results in this study do not support the previous hypothesis that the low volatility of reduced Re4+ and Tc4+ is responsible for higher Re and Tc retention in glass when a reducing agent is added. We propose that early reaction of nitrate/nitrite and faster incorporation of sulfate into glass caused by iron oxalate are the two contributing factors that can explain the experimental findings in this study regarding the positive effect of iron oxalate on Re incorporation and retention. [ABSTRACT FROM AUTHOR]

Technetium-99 (99Tc) is a major radionuclide of concern in the Hanford low-activity waste (LAW), which will be vitrified into borosilicate glass at the Waste Treatment and Immobilization Plant (WTP). Sulfate in LAW has been known to be a critical factor affecting the volatile loss of 99Tc. We investigated rhenium (a nonradioactive surrogate for 99Tc) incorporation by crucible melting tests of two representative simulated LAW glass feeds, each prepared with varied sulfur concentrations in glass. The slurry feeds were dried and heated to 400–1100 °C. Soluble and insoluble phases of heated feeds were analyzed to profile the partitioning of various components during the melting process. The mechanism of how sulfate affects rhenium incorporation during feed-to-glass conversion reactions, and so final retention of rhenium in glass, is proposed based on the two hypotheses that segregated sulfate-rich phase forms during feed conversion reactions and Re preferentially partitions to this sulfate-rich phase. [ABSTRACT FROM AUTHOR]