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The chemical durability of borosilicate glass used to confine nuclear wastes is known to vary nonlinearly with their composition, making glass dissolution rate predictions difficult. Here, we focus on the effect of Al, an important oxide of these materials. The initial and residual glass dissolution rates were investigated through experiments conducted at 90 °C and pH 9. Our results show that low Al content glasses dissolve initially faster than glasses with higher Al content, but quickly achieve a lower residual rate due to the rapid formation of a more passivating gel. Nevertheless, the high Al content gel will eventually achieve passivation with a slower reorganization. The strengthening effect of Al on the Si-O bond hydrolysis will result in more stable gels, suggesting that slightly higher Al content in the gel will improve nuclear high-level waste glass durability in geological disposal conditions. [ABSTRACT FROM AUTHOR]

Chemical durability is not a single material property that can be uniquely measured. Instead, it is the response to a host of coupled material and environmental processes whose rates are estimated by a combination of theory, experiment and modeling. High-level nuclear waste ( HLW) glass is perhaps the most studied of any material yet there remain significant technical gaps regarding their chemical durability. The phenomena affecting the long-term performance of HLW glasses in their disposal environment include surface reactions, transport properties to and from the reacting glass surface, and ion exchange between the solid glass and the surrounding solution and alteration products. The rates of these processes are strongly influenced and are coupled through the solution chemistry, which is in turn influenced by the reacting glass and also by reaction with the near-field materials and precipitation of alteration products. Therefore, those processes must be understood sufficiently well to estimate or bound the performance of HLW glass in its disposal environment over geologic time scales. This article summarizes the current state of understanding of surface reactions, transport properties and ion exchange along with the near-field materials and alteration products influences on solution chemistry and glass reaction rates. Also summarized are the remaining technical gaps along with recommended approaches to fill those technical gaps. [ABSTRACT FROM AUTHOR]

The methodology developed for predicting nuclear waste behavior under disposal conditions combines experimental approaches and modeling. A waste glass canister placed in contact with water undergoes irreversible chemical processes leading to its degradation into more stable phases. This transformation occurs in three kinetic stages: the initial alteration rate (stage I), the residual rate (stage II), and, in some cases, a resumption of alteration (stage III) related to zeolites precipitation. Affinity effects based on the transition state theory are used to account for the rate drop from stage I to stage II. However, modeling of stage III has not been extensively studied. This study investigates the ability of the "glass reactivity with allowance for the alteration layer" (GRAAL) model to describe the effect of zeolite precipitation on the dissolution kinetics of the international simple glass (ISG). The GRAAL model-based description of the alteration layer was adapted to account for alkaline pH alteration mechanisms. A model describing the nucleation and growth kinetics of zeolites was proposed based on simple formalisms whose parameters can be inferred from previous studies' results. These improvements give a description of the moment where a resumption of alteration occurs. As the predictive capacity of the GRAAL model is strongly dependent on the appropriateness of the alteration layers' description, this work shows the need to develop new functions to describe the evolution of their compositions and solubilities with pH changes. Calculations also show the importance of Al and Ca activities and the effect of Al on the silica solubility. Nuclear glass: Next top model The ability of a model to describe the effect of zeolite precipitation on the dissolution kinetics of a nuclear glass has been studied. Nuclear glasses, when in contact with water, degrade and form more stable phases. This alteration process occurs in three stages with different kinetics: an initial rate (I) is followed by a rate drop (II), which is then followed by a 'resumption of alteration' ('RA' - stage III) linked to the precipitation of zeolites. A model known as the 'glass reactivity with allowance for the alteration layer' (GRAAL) model has previously been used to better understand various aspects of long-term glass behavior. Now, a team led by Maxime Fournier at the CEA, Marcoule, France has investigated its potential for describing the processes involved in the RA stage undergone by international simple glass. [ABSTRACT FROM AUTHOR]

The porosity and pore-size distribution in the glass play an important role in glass corrosion; however, such information is difficult to be obtained non-destructively. Here we report the use of spectroscopic ellipsometry (SE) under controlled humidity environments to determine those structural parameters in the alteration layers formed on international simple glass (ISG), a model nuclear waste glass, in aqueous corrosion conditions. The SE results show that the ISG sample corroded for nearly 4.5 years develops pores larger than 2 nm in diameter, while the alteration layers formed initially in less than 1 month do not have such large pores. The development of larger pores over a long period of corrosion time, while the overall thickness remains relatively constant, implies structural rearrangements of the silicate network occurring within the alteration layer, which could eventually affect the transport of reactants and products as well as the structural stability. Gel layers: Scrutinized by spectroscopy Spectroscopy has been used to study the interfacial silica gel layer created during the corrosion of a model nuclear-waste glass. Understanding the way in which such materials degrade is of great importance because they are used to store radioactive waste. When international simple glass (ISG) corrodes an 'alteration' layer forms on its surface and now Seong H. Kim, from The Pennsylvania State University, and co-workers have used spectroscopic ellipsometry to characterize the thickness, porosity, and pore-size distribution of this layer after exposing it to different corrosive experimental conditions. Their data show that, although their alteration layers are of similar depths, the pore diameters in ISG samples corroded for 7 or 28 days are smaller than 2 nm whereas those corroded for 1625 days are larger, implying some restructuring of the layer over this time. [ABSTRACT FROM AUTHOR]

• Adsorption-induced stress might be generated upon ingress of water to the porous surface of corroded glass, which could be detected by IR spectroscopy. • Filling pores with water alters the effective refractive index of the porous glass, which would also cause changes in the IR spectrum. • Thus, it is difficult to unambiguously determine adsorption-induced stress in the porous surface of corroded glass by means of IR spectroscopy only. Water ingress into porous glass can induce internal stress, resulting in strains in chemical bonds of the network. For silica and silicate glasses, the position of Si–O–Si stretching mode in infrared spectroscopy is known to vary with the degree of strain in the Si–O network. Then, one could hypothesize that the stress due to water ingress could be probed with infrared spectroscopy. We tested this hypothesis using porous layers formed through aqueous corrosion on a model nuclear waste glass. The porosity and thickness of the porous layer were determined using ellipsometry. The humidity-dependent infrared spectra of the samples showed red-shifts of the Si–O–Si stretching band; however, it was difficult to deconvolute the spectral change due to variation of effective refractive index of the sample upon water ingress. Thus, it was infeasible to unambiguously determine the internal stress of porous glass network upon water ingress using the infrared spectroscopy alone. [ABSTRACT FROM AUTHOR]

Ballistic effects due to recoil nuclei will be the predominant mode of long-term radiation damage of nuclear waste glass in disposal facilities. While these ballistic effects are known to modify the glass structure in ways that have been quantified using molecular dynamics (MD) simulations, the effect of irradiation-induced structural changes on glass corrosion is not understood. Monte Carlo (MC) simulations were therefore performed to quantify the effects of structural changes determined from MD simulations on the corrosion of simplified nuclear waste glasses, namely sodium borosilicate and sodium alumino-borosilicate glasses. MC simulations were first performed to determine separately the effects of irradiation-induced changes in boron coordination, the number and nature of Q n species, and the angular distribution of Si O Si linkages, that is, changes to the short-range order of the glass. Generally, the MC simulations showed the resulting effects on the kinetics (B release rate) and extent (B leached thickness) of corrosion were correlated to the extent of depolymerization of the glass network introduced by these local structural changes. The combined effects of these structural changes, as parameterized using the results of MD simulations, were then determined using a glass composition for which relevant experimental data were available for comparison. Here, the combined effect of local changes was lower than the increase in alteration depth observed upon irradiation. This insight from the MC simulations highlighted the need to consider changes in the medium-range order to completely account for experimental observations. • Effects of irradiation-induced structural changes on glass corrosion were investigated with Monte Carlo simulations. • Changes in Boron coordination, number and nature of Q n species, and angular distribution of Si O Si linkages were considered. • The kinetics and extent of corrosion were correlated to the extent of depolymerization of the glass network. • Direct comparison with experimental data pointed to the importance of medium-range structural changes. [ABSTRACT FROM AUTHOR]

For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM nuclear waste glass simulants was investigated. Three complex borosilicate glasses (>20 oxides) along with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were altered at 50 °C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose molar ratio of Al 2 O 3 /(CaO or MgO) is ≥ 1, seems to be limited by network-hydrolysis, whereas the overall glass alteration rate of the less durable glasses, whose Al 2 O 3 /MgO ratio is < 1, seems to be driven by the precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity of secondary phases precipitated on their surface. The less durable glasses have a relatively significant quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable glasses was 10–20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated an inflexion in the vapor hydration rate after 120–200 days of alteration, likely due to a passivating effect of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in certain vapor hydrated glasses measured by SAXS is similar to the pore sizes of the gel layer formed in aqueous medium and varies very slightly with glass composition. • Rate-controlling vapor hydration mechanism is dependent on glass composition. • The molar ratio of Al 2 O 3 /MgO affects glass alteration kinetics in vapor phase. • (Al 2 O 3 /MgO) < 1 seems to result in significant precipitation of Mg-smectites. • Difference between relative glass durability in aqueous medium and vapor phase. • Porosity and pore-size of altered layer is similar in the two alteration media. [ABSTRACT FROM AUTHOR]

High-level, long-lived nuclear waste arising from spent fuel reprocessing is vitrified in silicate glasses for final disposal in deep geologic formations. In order to better understand the mechanisms driving glass dissolution, glass alteration studies, based on silicon isotope ratio monitoring of 29 Si-doped aqueous solutions, were carried out in laboratories. This work explores the capabilities of the new type of quadrupole-based ICP-MS, the Agilent 8800 tandem quadrupole ICP-MS/MS, for accurate silicon isotope ratio determination for alteration studies of nuclear waste glasses. In order to avoid silicon polyatomic interferences, a new analytical method was developed using O 2 as the reaction gas in the Octopole Reaction System (ORS), and silicon isotopes were measured in mass-shift mode. A careful analysis of the potential polyatomic interferences on SiO + and SiO 2 + ion species was performed, and we found that SiO + ion species suffer from important polyatomic interferences coming from the matrix of sample and standard solutions (0.5M HNO 3 ). For SiO 2 + , no interferences were detected, and thus, these ion species were chosen for silicon isotope ratio determination. A number of key settings for accurate isotope ratio analysis like, detector dead time, integration time, number of sweeps, wait time offset, memory blank and instrumental mass fractionation, were considered and optimized. Particular attention was paid to the optimization of abundance sensitivity of the quadrupole mass filter before the ORS. We showed that poor abundance sensitivity leads to a significant shift of the data away from the Exponential Mass Fractionation Law (EMFL) due to the spectral overlaps of silicon isotopes combined with different oxygen isotopes (i.e. 28 Si 16 O 18 O + , 30 Si 16 O 16 O + ). The developed method was validated by measuring a series of reference solutions with different 29 Si enrichment. Isotope ratio trueness, uncertainty and repeatability were found to be <0.2%, <0.5% and <0.6%, respectively. These performances meet the requirements of the studies of nuclear glasses alteration and open up possibilities to use this method for precise determination of silicon content in natural samples by Isotope Dilution. [ABSTRACT FROM AUTHOR]

• Mechanisms of boron release during glass vapor hydration are proposed. • Link among mechanical properties, glass structures, and glass chemical durability is made. • Beilby layer formed due to surface polishing is mechanically and chemically characterized. • Preferential release of three-coordinated 3B in the vapor phase is confirmed. Under the context of long-term geological disposal of nuclear waste glass, the present work investigates the effect of initial states on the glass chemical durability in the vapor phase and during subsequent leaching. Glass with different finishing steps of polishing and thermal annealing were prepared, vapor-hydrated, and aqueous-altered. Multiple characterizations of mechanical properties and structural modifications were performed to understand the reduction of vapor hydration rates with annealed glasses. Discussions on the degradation of mechanical properties provide insights to understanding the variations of glass chemical durability, which shows a feasible approach to link mechanical and chemical studies together. By looking into the release of boron in the vapor phase, similarities between glass vapor hydration and glass aqueous alteration at residual rate are noticed, which allow the proposition of mechanisms of boron release. Characterizations of the hydrated-then-altered glass reveal the transport-limiting property of the vapor hydrated layer. [ABSTRACT FROM AUTHOR]