Glass materials, particularly the most common silicate glasses, are sometimes seen as inert relative to their environment. Geologic and historical glasses have, after all, survived hundreds to millions of years exposed to various environments. This is one major reason that glass has been widely used as a host material to immobilize radioactive wastes and to limit the release of radionuclides into the biosphere. The high chemical durability of borosilicate glass compositions designed for this application complicates their study, however. For instance, direct experimental studies of glass behavior at laboratory time scales cannot be simply extrapolated to performance lifetimes of hundreds to thousands of years. Instead, the chemical and physical mechanisms of corrosion must be studied at a fundamental level in order to achieve an actionable understanding of glass corrosion over any time scale.1 With this mechanistic understanding, models can be developed that calculate the expected extent of degradation that accounts for the environmental conditions an individual glass composition will experience over its useful life.2
Another major reason for the use of silicate glasses in the immobilization of nuclear wastes is that the glass can incorporate a wide variety of elemental chemistries and form a stable product. The diversity of glasses in use, however, exhibit substantial differences in glass corrosion behavior, and led to studies that, while independently robust, were difficult to compare to other investigations. Thus, the international waste glass corrosion scientific community determined that a common and relatively simple glass composition—one nevertheless still “close” to the desired composition space—could enable direct comparison of research results and accelerate the pace of accumulated learnings needed for development of reliable models for glass corrosion. This motivated the design and fabrication of the original International Simple Glass (ISG): a six-component borosilicate glass composed of Si, B, Na, Al, Ca, and Zr oxides. The composition struck a balance between (a) being comparative to a variety of waste glass compositions under consideration by the international waste glass community, (b) being simple enough for atomistic computer models, and (c) having aqueous corrosion durability behavior similar to other waste glass compositions of interest. The ISG glass composition has the same molar ratios of the six oxides as is found in the SON68 glass, a non-radioactive simulant glass based on the R7T7 glass composition produced at La Hague in France.3
In 2011, the ISG composition was fabricated by MoSci Corp. into 0.5 kg bars that were subsequently the subject of a multifaceted set of investigations, including physical and mechanical properties, dissolution behavior, and computational modeling (see Reiser et al (2021),4 Kaspar et al. (2019),5 and references therein). The popularity of using this composition in research and its success in corrosion experiments led to a universal desire to continue the general theme as the original 50 kg batch of ISG became depleted. During the annual International Workshop on Long-Term Glass Corrosion in 2019, an international consortium representing five countries voted unanimously that producing more standard glass would be enormously beneficial. Having representatives in the consortium and a long-standing commitment to advancing glass science, Corning Incorporated volunteered to produce this new batch of ISG for the international glass corrosion community at no charge. This production would also make use of a continuous melter at Corning’s research facility, enabling the next production of standard glass to have better intra-sample homogeneity, improved sample-to-sample consistency, and a much larger overall production volume to supply the glass corrosion community for years to come. Raw materials with improved purity were also provided by Corning for this production, in order to yield glass with substantially lower impurity levels compared to the previous ISG production. For example, trace levels of Fe were expected from raw materials, as in the original ISG, albeit at a lower level (50 to 200 ppm, as opposed to roughly 650 ppm in the original ISG).5 All of these suppositions were verified experimentally and are documented in this report.
In terms of candidate compositions, discussions were held that a simple reproduction of ISG would be sufficient, but two major points served to lessen enthusiasm with this simple approach. First, although compositions can be nearly duplicated, often the unique thermal history of a glass can produce measurable differences with a new synthesis of the same material. Similar logic applies in subtleties of glass production for reproducing a composition, such as differences in homogeneity, low-level metallic impurities, and water content. Secondly, it was reasoned that more useful and novel information about the nature of glass corrosion could be obtained by pursuing a different composition. These considerations notwithstanding, it was also agreed that the composition of the new standard glass should be related somewhat closely to the original ISG, such that future research could leverage the relationship between the two compositions.
Thus, the consortium decided that two batches should be produced: (1) a reproduction targeting the original ISG composition, designated as ISG-1 to differentiate this newly-synthesized glass batch from the prior production by MoSci Corp., and (2) a new glass composition referred to as ISG-2. The ISG-1 composition replenishes the diminished original ISG stock and enables comparative testing between the original and new composition, as the newer production of ISG-1 would offer improved compositional uniformity and purity relative to the original ISG glass blocks. During the 2019 discussions on the new ISG-2 composition, the consortium decided that ISG-2 should be strongly related to the original ISG composition, but with notable changes that would both alter its dissolution behavior in meaningful ways and establish ties to other glass compositions of interest in the glass corrosion community. In terms of major changes to the composition, five main options were put forward:
1. Increase in SiO 2 relative to other modifying and intermediate species, targeting a more dramatic change in connectivity of the Si-O-Si network, and potentially helping to discriminate between structural regimes that represent a fully-interconnected (or percolated) Si-O-Si network versus one that is not (e.g., the original ISG). 6,7
2. Increase Al 2 O 3 from 3.8 mol % to 7 mol % Al2O3 and decrease B2O3 and remove ZrO2. An investigation of Al is of interest because of the unique chemistry of Al in the glass (e.g., Al avoidance rule and the need for a charge compensating ion with [AlO4-]) and the interesting observation that increasing Al concentration in the glass results in an increase in durability followed by a decrease in durability if a critical concentration is reached4, 8–10.4, 8–10
3. Replace half or all of the CaO in the original ISG composition by MgO. The Mg role in glass structure and during alteration has long been a focus in the community. It is of particular importance as a major component in certain UK and French nuclear waste glass compositions. Mg is also an important component of key secondary phases (e.g., clays, Mg silicates), that are thought to impact glass dissolution kinetics when present11–14.12,13,15,16
4. Investigate glass compositions out of the nuclear waste glass family. The compositions of the discussed glass families, such as pharmaceutical glasses, would have the same components as ISG (except ZrO2) with a much higher SiO2 content and commensurate lower forward rates. These glasses also have sufficient network formers to meet the intent of option 1 (Network Percolation).
5. Produce a simplified version of a different nuclear waste glass analogue. The AFCI glass, a composition previously studied at PNNL 17,18 was considered for this option.17,18
In each case, the impact of such structural changes on dissolution rates and mechanism could be correspondingly inferred. Potential drawbacks for each option were discussed. For options 1 and 4, the calculations to estimate the targeted network percolation transition points remain under debate, such that an appropriate target might be difficult to design. Many options (1, 2, and 4) would likely result in much slower dissolution rates, leading to impractically-long experiment timeframes. Further, options 1, 4, and 5 would represent a significant departure from the composition space seen in most nuclear waste glasses amongst the majority of nations—the last being one of the motivations behind the original ISG target. Following these discussions, a vote was taken and fourteen of the twenty-one voters listed a composition based on the original ISG but with half of the Ca replaced by Mg as their first choice (Option 3), with 4 more voters listing it as their second preference.
After a composition option was selected for ISG-2, discussions focused on dopant additions at a trace level (0.1 wt. %) in the glass, where elements could be added for use as a tracers of network dissolution and retention in the alteration products. Due to process constraints, Mo, a common alternative tracer of network glass dissolution, could not be added. Thus, La was selected to act both as an element fully retained in the amorphous alteration layer and optical dopant for laser-assisted atom probe tomography (APT) analysis19. The international consortium also decided to add the La tracer to the ISG-1 composition so future efforts with the original composition could exploit its addition.
Based on the above considerations, the final compositions and dopant concentrations of ISG-1 and ISG-2 were decided. Target compositions of the original ISG, ISG-1, and ISG-2 are given in Table 1. Emphasis is placed here on the 50% swap of MgO-for-CaO, on a molar basis in ISG-2, in comparison to the Mg-free composition of the original ISG and ISG-1. Meanwhile the tracer species lanthanum oxide (La2O3) was incorporated as a + 0.12 wt. % target addition “on top” of the equivalent major oxide composition in wt.%. Notable differences in the final compositions of ISG-1 versus original ISG are discussed in the results section.
Table 1
Target compositions for ISG-1 and ISG-2, in comparison to original ISG batch (from MoSci). Highlighted are compositions in both wt% and mole%, showing the 50% MgO-for-CaO replacement, on molar basis, in the target ISG-2 composition.
Species
|
|
Batch Target Compositions (this work)
|
Original ISG
|
ISG-1
|
ISG-2
|
ISG-1
|
ISG-2
|
wt%
|
wt%
|
wt%
|
mole%
|
mole%
|
SiO2
|
56.20
|
56.20
|
56.54
|
60.1
|
60.1
|
Al2O3
|
6.10
|
6.10
|
6.14
|
3.84
|
3.84
|
B2O3
|
17.30
|
17.30
|
17.40
|
15.97
|
15.97
|
Na2O
|
12.20
|
12.20
|
12.27
|
12.65
|
12.65
|
MgO
|
< 0.17
|
< 0.17
|
1.80
|
0
|
2.86
|
CaO
|
5.00
|
5.00
|
2.52
|
5.73
|
2.87
|
ZrO2
|
3.30
|
3.30
|
3.32
|
1.72
|
1.72
|
La2O3
|
|
0.12
|
0.12
|
0.02
|
0.02
|
Sum
|
100.1
|
100.22
|
100.12
|
100.0
|
100.0
|
In the present paper, the production of ISG-1 and ISG-2 glasses at Corning’s facilities is described, including the limited quantity of intermediate compositions between ISG-1 and ISG-2 collected during the composition transition. Additionally, the chemical, thermal, and physical properties of ISG-1 and ISG-2 glasses are summarized in comparison to the original ISG composition. The consistency, homogeneity, and purity of these glass samples is also shown herein to be markedly improved relative to the original ISG samples, and benefits the original goal of the international glass corrosion community to provide consistent, comparative standard materials that enable coordinated research into the fundamental mechanisms of glass corrosion. This work is the next step in the ongoing effort to developing accurate aqueous corrosion models applicable over wide ranges of environmental conditions and time scales.