Radioactive waste immobilization is a means to limit the release of radionuclides from various waste streams into the environment over a timescale of hundreds to many thousands of years. Incorporation of radionuclide-containing wastes into borosilicate glass during vitrification is one potential route to accomplish such immobilization. To facilitate comparisons and assessments of reproducibility across experiments and laboratories, a six-component borosilicate glass (Si, B, Na, Al, Ca, Zr) known as the International Simple Glass (ISG) was developed by international consensus as a compromise between simplicity and similarity to waste glasses. Focusing on a single glass composition with a multi-pronged approach utilizing state-of-the-art, multi-scale experimental and theoretical tools provides a common database that can be used to assess relative importance of mechanisms and models. Here we present physical property data (both published and previously unpublished) on a single batch of ISG, which was cast into individual ingots that were distributed to the collaborators. Properties from the atomic scale to the macroscale, including composition and elemental impurities, phase purity, density, thermal properties, mechanical properties, optical and vibrational properties, and the results of molecular dynamics simulations are presented. In addition, information on the surface composition and morphology after polishing is included. Although the existing literature on the alteration of ISG is not extensively reviewed here, the results of well-controlled static alteration experiments are presented here as a point of reference for other performance investigations.

Glass corrosion data from the ALTGLASS™ database were used to determine if gel compositions, which evolve as glass systems corrode, are correlated with the generation of zeolites and subsequent increase in the glass dissolution rate at long times. The gel compositions were estimated based on the difference between the elemental glass starting compositions and the measured elemental leachate concentrations from the long‐term product consistency tests (ASTM C1285) at various stages of dissolution, ie, reaction progress. A well‐characterized subset of high level waste glasses from the database was selected: these glasses had been leached for 15‐20 years at reaction progresses up to ~80%. The gel composition data, at various reaction progresses, were subjected to a step‐wise regression, which demonstrated that hydrogel compositions with Si*/Al* ratios of <1.0 did not generate zeolites and maintained low dissolution rates for the duration of the experiments. Glasses that formed hydrogel compositions with Si^*/Al^* ratios ≥1, generated zeolites accompanied by a resumption in the glass dissolution rate. The role of the gel Si/Al ratio, and the interactions with the leachate, provides the fundamental understanding needed to predict if and when the glass dissolution rate will increase due to zeolitization.

The durability of high level nuclear waste glasses must be predicted on geological time scales. Waste glass surfaces form hydrogels when in contact with water for varying test durations. As the glass hydrogels age, some exhibit an undesirable resumption of dissolution at long times while others exhibit near steady‐state dissolution, that is, nonresumption of dissolution. Resumption of dissolution is associated with the formation of zeolitic phases while nonresumption of dissolution is associated with the formation of clay minerals. Hydrogels with a stoichiometry close to that of imogolite, (Al2O3·Si(OH)4), with ferrihydrite (Fe2O3·0.5H2O), have been shown to be associated with waste glasses that resume dissolution. Aluminosilicate hydrogels with a stoichiometry of allophane‐hisingerite ((Al,Fe)2O3·1.3‐2Si(OH)4) have been shown to be associated with waste glasses that exhibit near steady‐state dissolution at long times. These phases are all amorphous and poorly crystalline and are also found on natural weathered basalt glasses. Interaction of these hydrogels with excess alkali and OH− (strong base) in the leachates, causes the Al2O3·nSiO2 (where n=1‐2) hydrogels to mineralize to zeolites. Excess alkali in the leachate is generated by alkali in the glass. Preliminary rate‐determining leach layer forming exchange reactions are hypothesized based on these findings.

Radioactive high level waste (HLW) at the Savannah River Site (SRS) has successfully been vitrified into borosilicate glass in the DWPF since 1996. Vitrification requires stringent product/process (P/P) constraints since the glass cannot be reworked once it has been poured into ten foot tall by two foot diameter canisters. A unique "feed forward" statistical process control (SPC) was developed for this control rather than relying on statistical quality control (SQC). In SPC, the feed composition to the DWPF melter is controlled prior to vitrification. In SQC, the glass product would be sampled after it is vitrified. Individual glass property-composition models form the basis for the "feed forward" SPC. The models transform constraints on the melt and glass properties into constraints on the feed composition going to the melter in order to determine, at the 95% confidence level, that the feed will be processable and that the durability of the resulting waste form will be acceptable to a geologic repository.The DWPF SPC system is known as the Product Composition Control System (PCCS). One of the process models within PCCS is known as the Thermodynamic Hydration Energy Reaction MOdel (THERMO™), which was developed in 1995 and utilizes data from the short term (7-day) durability test given in the ASTM standard C1285A. The DWPF durability model is based on a free energy of hydration function calculated from the molar glass compositions. An individual component free energy, ∆G i , exists for each oxide in a HLW glass and the ∆G i 's are weighted by the molar concentration of each oxide in the glass. This gives an overall preliminary hydration free energy, ∆G p , for a given glass, which is predictive and independent of any leachate solution pH impacts. The less negative the ∆G p the more durable the glass; the more negative the ∆G p the less durable the glass.The DWPF PCCS models are parsimonious in that the oxide terms in each model are only those which are necessary and sufficient to describe the glass property of interest. This approach excludes composition terms that are unnecessary to the implementation of the DWPF flowsheets, and helps to minimize the sources of error in the PCCS models. These parsimonious models have successfully operated the DWPF vitrification process over the last 20 years.

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