As a part of the optimization study for achieving the highest possible Hanford and Savannah River Site waste loading into acceptable borosilicate glasses, thirty glass compositions were selected for testing at KRI. These thirty test matrix glasses were designed to augment existing data within the compositional regions of interest with relatively high concentration of Al2O3 between 10 and 20 wt%.

This paper reports experimental data on liquidus temperature (TL) and crystallization behavior of all synthesized glasses as well as durability of quenched and heat-treated glasses. The results of this study will be used to develop glass formulations for specific DOE waste streams to maintain or meet waste loading and/or waste throughput expectations while satisfying critical process and product performance related constraints.

The U.S. Department of Energy (DOE) is currently processing high-level waste (HLW) through a Joule-heated melter (JHM) at the Savannah River Site (SRS) and plans to vitrify HLW and Low activity waste (LAW) at the Hanford Site. Over the past few years at the Defense Waste Processing Facility (DWPF), work has concentrated on increasing waste throughput. These efforts are continuing with an emphasis on high alumina concentration feeds. High alumina feeds have presented specific challenges for the JHM technology regarding the ability to increase waste loading yet still maintain product quality and adequate throughput. Alternatively, vitrification technology innovations are also being investigated as a means to increase waste throughput. The Cold Crucible Induction Melter (CCIM) technology affords the opportunity for higher vitrification process temperatures as compared to the current reference JHM technology. Higher process temperatures may allow for higher waste loading and higher melt rate.

Glass formulation testing to support melter demonstration testing was recently completed. This testing was specifically aimed at high alumina concentration wastes. Glass composition/property models developed for DWPF were utilized as a guide for formulation development. Both CCIM and JHM testing will be conducted so glass formulation testing was targeted at both technologies with a goal to significantly increase waste loading and maintain melt rate without compromising product quality.

Liquidus temperatures (TL) were measured, and primary phases were determined for 50 (from an initial test matrix of 76) compositions within the Al2O3–B2O3–CaO–Na2O–SiO2 glass-forming system and its constituent ternary subsystems. Strong linear correlations have been found between composition and TL for melts within the same primary phase fields. The TL and primary phase data are being used to develop and refine a modified associate species model (ASM). The impacts of Fe2O3, Li2O, NiO, ZrO2, Cr2O3, ZnO, and MnO additions on the TL of two baseline glass compositions are reported. These data are intended as benchmarks for further expansion of the ASM or other silicate melt solution models of nuclear waste glasses.

The successful thermochemical model based on the modified associate species approach for the Na2O-Al2O3-B2O3-SiO2 base glass system has been extended to include a critical constituent, Cr2O3. This includes the Cr2O3-Al2O3 solid solution. For the liquid, and potentially glass phase when undercooling is allowed to occur, the model uses the relative simple, modified associate species method to allow accurate determination of phase relations, including liquidus surfaces. It also allows prediction of chemical activities and vapor pressures, which can be important in both processing and in modeling long-term waste form stability.

Sulfate retention in glass during the vitrification process can be as low as 1/3 of the solubility limit, or can exceed the solubility limit if suspended in the glass in the form of droplets. This study is focused on the mechanism of incorporating and segregating sodium sulfate during the melting of an alkali-alumino-borosilicate glass batch. Batches were ramp heated at 4°C/min to temperatures ranging from 600°C to 1050°C and fractured for examination. Observation of the melts showed that as the batch temperature increases and the primary oxo-anionic, predominantly nitrate melt decomposes, the sulfate residue accumulates inside gas bubbles and is transported in them to the melt surface, where it remains segregated. The degree of sulfate incorporation into the final glass depends on the relative rates of sulfate dissolution in the borosilicate melt and sulfate lifting inside bubbles.

The liquidus temperatures, TL, of rare earth-alumino-borosilicate glasses were measured as a function of glass composition. The TL values ranged from 1153°C to 1405°C. Three primary crystalline phases were identified in the study. The most frequently encountered was a rare earth silicate phase in which the TL values ranged from 1153°C to 1405°C. A12O3 was encountered in glasses with an A12O3:SiO2 mass ratio greater than 1, with TL values ranging from 1242°C to 1305°C. Alumino-silicate crystals were encountered as the primary phase in glasses with less than 43 mass% of mixed rare earth oxides (Ln2O3) with TL values between 1164°C and 1255°C. A linear relationship between total mixed rare earth oxide concentration and TL was found within all three primary phase fields. In the rare earth silicate primary phase field, normalizing the Ln2O3 concentration with its mean ionic radius enhanced this linear relationship.

Crystallization kinetics of a simulated high-level waste (HLW) glass were measured and modelled. Kinetics of acmite growth in the standard HW39–4 glass were measured using the isothermal method. A time-temperature-transformation (TTT) diagram was generated from these data. Classical glass-crystal transformation kinetic models were empirically applied to the crystallization data. These models adequately describe the kinetics of crystallization in complex HLW glasses (i.e., RSquared = 0.908). An approach to measurement, fitting, and use of TTT diagrams for prediction of crystallinity in a HLW glass canister is proposed.

At the U.S. Department of Energy's Hanford Site, processes are being developed to vitrify waste generated during nuclear materials processing. One of the wastes slated for vitrification is known as neutralized current acid waste (NCAW). The batch chemistry of simulated NCAW was varied with oxidants and reductants. Untreated, formated, nitrated, or sugar-added samples were combined with frit to produce melter feed. Offgas measurements of the formated melter feed showed that formates decomposed at temperatures too low for participation in melt redox reactions. Sugar pyrolyzed and produced CO and H2 at temperatures exceeding 665 °C. For the sugar-added samples, the glass quenched from 1200 °C produced an Fe2+ /ΣFe of 0.79. The measured iron redox ratios from the glasses made from untreated, formated, and nitrated wastes were essentially indistinguishable (0.0024 at 1000 °C and 0.032 at 1200 °C). However, the batch chemistry affected volume expansion and the reaction paths.