The vast majority of High‐Level Waste (HLW) originating from defense nuclear programs is sequestered and immobilized in borosilicate glass. Borosilicate glass is universally accepted for immobilizing HLW, but its efficiency has limitations based on the compositional makeup of the waste stream. The chemical durability of the glass is the most important factor in determining the longevity and usefulness of the final glass waste form. The primary detriment to this durability in glasses containing high levels of aluminum is nepheline (NaAlSiO4) crystallization, as it is generally accompanied by a measurable decrease in the glass's chemical durability. This work seeks to understand nepheline crystallization, within the context of thermal history, and to elucidate the influence of compositional shifts in the residual glass (after crystallization) on the measured durability. The results presented within show a distinct deviation in leaching behavior as a function of structural makeup (calculated Q units). This understanding will provide practical information required for broadening glass compositional regions needed to more efficiently vitrify HLW.

This report provides the results from the chemical analyses of the glass compositions of the Low-Activity Waste High PCT and VHT Response study glasses, a series of simulated nuclear waste glasses designed and fabricated at Pacific Northwest National Laboratory. These data will be used in the development, validation, and implementation of enhanced property/composition models for waste glass vitrification at Hanford. Chemical analyses were performed on a representative sample of each of the quenched glasses to allow for comparisons with targeted compositions. The relative differences between the targeted and measured concentrations of Cl -, K2O, Na2O, and ZrO2 for several of the glasses were greater than 10%. These results can be used in further characterization of this series of glasses, including the normalization of Product Consistency Test results.

In this report, the Savannah River National Laboratory (SRNL) provides chemical analyses and Product Consistency Test (PCT) results for a series of simulated Low-Activity Waste (LAW) glass compositions. A procedure developed at the Pacific Northwest National Laboratory (PNNL) for producing sulfur saturated melts (SSMs) was carried out at both SRNL and PNNL to fabricate the glasses characterized in this report. This method includes triplicate melting steps with excess sodium sulfate, followed by grinding and washing to remove unincorporated sulfur salts. The wash solutions were also analyzed as part of this study. These data will be used in the development of improved property/composition models for LAW glass.Chemical analyses were performed on a representative sample of each of the baseline and sulfur saturated glasses to allow for comparisons with the targeted compositions. An additional dissolution technique, sodium peroxide fusion with the addition of sulfuric acid, was used to obtain improved measurements of the P and Zr concentrations of the study glasses. Minor differences between the targeted and measured concentrations of some of the baseline (quenched) glass components were noted. The measured concentrations of several components of the SSM glasses were low as compared to those of the quenched versions. The measured SO3 concentrations were higher for SSM versions of the study glasses, as expected.The PCT Method-A was performed in triplicate on each of the quenched and canister centerline cooled (CCC) versions of the baseline glasses to assess chemical durability. It was noted that some of the leachates had a yellow color after the PCT. For some of the study glasses, the CCC heat treatment resulted in increased normalized release values as compared to those of the quenched versions. Several of the study glasses have normalized concentration (NCi) values that are higher than those of the Hanford Waste Treatment and Immobilization Plant (WTP) contract limit of 2.0 g/m 2 (~4 g/L) for B, Na, and Si.Chemical analyses were also performed on a representative sample of each of the wash solutions resulting from the preparation of the SSM versions of the study glasses. The measured concentrations of B, Ca, Cr, K, and V in the wash solutions may be related to the lower measured values for these components noted in the SSM versions of the study glasses. The measured concentrations of S were in the range of about 325-1775 mg/L. Glass LP2-OL-13 stood out as generating the highest concentrations of several species in its wash solution, although there didn't appear to be any obvious link to composition or PCT performance. It is recommended that PNNL examine this result further as part of its broader review of these data. Revision vii Revision 0 Table 2-1. Identifier and Lab Responsible for Preparing Each Sulfur Saturated Melt (SSM)

A new model based on glass structure to allow for enhanced waste loading in nuclear waste glass while maintaining chemical durability is proposed. The model is derived by splitting the molar concentrations of the targeted starting glass composition into theoretical crystalline phases anticipated to be observed during devitrification and a residual glass. An empirically derived relationship based on maintaining the residual glass structure, determined from a calculated non-bridging oxygen content, was demonstrated to successfully screen glasses for acceptable durability. The proposed model can successfully identify durable glass compositions containing 20-35 wt% Al 2 O 3 , a concentration that would significantly increase the projected waste loading in glasses processed at the Hanford Tank Waste Treatment and Immobilization Plant.

The Accelerated Basin De-inventory (ABD) Program has been proposed as an alternative for future spent nuclear fuel (SNF) and nuclear material processing at the Savannah River Site (SRS). This approach would change the baseline H-Canyon (HCAN), Concentrate, Storage, and Transfer Facility (CSTF), and Defense Waste Processing Facility (DWPF) operations. The ABD Program would require that all domestic and foreign research reactor SNF currently at SRS be dissolved, stored, and then transferred to CSTF without the recovery of uranium. Preliminary assessments in the ABD Program plan have shown that ~5000 extra SRS high-level waste (HLW) canisters would be produced if the fissile mass loading remains at the current 897 g/m 3 limit; however, increasing the limit to 2500 g/m 3 would result in ~520 extra canisters. Thus, the ABD Program plan requires an increase of the DWPF fissile mass loading limit to 2500 g/m 3 to minimize canister production.The Department of Energy (DOE) Office of Environmental Management (EM) authorized a phased approach to increase the fissile mass loading of vitrified HLW at DWPF to 2500 g/m 3 . Phase 1 begins with testing actual radioactive waste samples to determine if the DWPF glass will meet relevant requirements. Additionally, Savannah River Remediation (SRR) has performed a Nuclear Criticality Safety Evaluation (NCSE) for the increased fissile mass loading limit. The Savannah River Nuclear Solutions (SRNS) Materials Disposition Engineering group requested that the Savannah River National Laboratory (SRNL) demonstrate that increasing the fissile mass loading in vitrified HLW to 2500 g/m 3 will have no adverse effects on glass quality specifications. The objective of this task was to fabricate and characterize two radioactive glass samples prepared with actual radioactive SRS HLW samples. One glass was prepared with a fissile mass loading below the current 897 g/m 3 limit, and a second glass was prepared with a fissile mass loading bounding the 2500 g/m 3 concentration. Both glasses were evaluated for product consistency to confirm the Waste Acceptance Product Specifications (WAPS) acceptance criterion (section 1.3) can be met with the increased fissile mass loading. Glass product consistency was determined using the Product Consistency Test (PCT) per ASTM C1285-14. Complementary X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to compare phase homogeneity among both glass samples.Most of the fissile mass from the ABD Program will be attributed to U-235. DWPF has produced homogeneous HLW glasses with total U3O8 concentrations of 1.1-3.5 weight percent (wt.%) that meet the WAPS acceptance criterion for product consistency; however, these glasses have different fissile uranium isotope distributions than those expected for HLW glasses containing ABD material. Therefore, an additional goal of this study was to confirm that DWPF HLW glass durability remains predictable with the DWPF Product Composition Control System (PCCS) durability models when the percentage of U-235 is inc...