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Amorphous silica-rich surface layers, also called gels, can passivate silicate glass and minerals depending on environmental conditions. However, several uncertainties remain on the mechanisms controlling the formation of these layers. In this paper, the influence of exogenous ions supplied by solutions is studied, both on the formation and on the properties of the gel formed on international simple glass (ISG). ISG was altered at 90 °C, pH90 °C 7, in silica-saturated solutions containing various alkaline cations separately (Li+, Na+, K+, and Cs+). The alteration kinetics observed with Li and Na in the solution is similar to that observed with no ions, while K and Cs in the solution tend to decrease glass alteration. Furthermore, for K or Cs ions, the kinetics decreases as the ionic strength of the solution increases. The passivation layer formed in these solutions shows a selectivity toward cations following the series K > Cs > Na >> Li. These alkalis replace Ca from pristine glass in the altered structures, leading to differences in [AlO4]− units charge compensation. Importantly, exchange between Ca and alkali also affects the total quantity of water inside each gel and this effect is well correlated with the observed drop in glass alteration. The presence of ions has been observed to affect the speed at which aqueous solutions alter the surface of a glass of nuclear interest. Borosilicate glasses are used to store highly radioactive waste in deep geological repositories, however, they are expected eventually to come into contact with ground water. Therefore, how they change when exposed to aqueous environments must be understood. Now, a team, led by Stephane Gin at the CEA, France, has studied the effects that various ionic solutions have on the formation and properties of the passivating silica-rich layers that can form on the surface of a reference glass known as International Simple Glass. They see that K+ and Cs+ ions decrease glass alteration kinetics, while the kinetics observed with Li+ and Na+ present are similar to solutions with no ions.

For the handling of high level nuclear waste (HLW), new glass formulations with a high waste capacity and an enhanced thermal stability, chemical durability, and processability are under consideration. This study focuses on the durability of peraluminous glasses in the SiO2–Al2O3–B2O3–Na2O–CaO–La2O3 system, defined by an excess of Al3+ ions compared with the network-modifying cations Na+ and Ca2+. To qualify the behavior of such a peraluminous glass in a geological storage situation, its chemical durability was studied in various environments (pure water, groundwater, and alkaline solutions related to a cement environment) and glass alteration regimes (initial rate, residual rate, and resumption of alteration). The alteration solution was characterized by inductively coupled plasma, and the altered glass by scanning electron microscopy, X-ray diffraction and secondary ion mass spectrometry. A comparative study of the chemical durability of these and reference glasses (ISG and SON68) over all timescales highlights the remarkable properties of the former. While their initial dissolution rate is of the same order as the reference glasses, the gel formed under silica saturation conditions is more passivating, making its dissolution rate at least one order of magnitude lower, while its low alkalinity makes it less susceptible to clayey groundwater and highly alkaline solutions. How a ‘peraluminous’ glass—of interest in nuclear waste storage—changes in solution has been studied. The long term behaviour of borosilicate nuclear glasses, used to store high-level nuclear waste, has important safety ramifications and it is known that dissolution and precipitation processes in water can affect their durability. A team, led by Maxime Fournier at the CEA, Marcoule, France, has now studied the durability of a ‘peraluminous’ glass, which is defined by its excess of Al3+ ions as compared to more standard ‘peralkaline’ glasses. Such materials are known to form passivating layers on their surface in aqueous solution and the researchers observe that the layer that forms on the surface of the ‘peraluminous’ glass is more passivating than that on the peralkaline reference glasses used for comparison, resulting in a significantly lower dissolution rate.

Understanding the effect of near-field materials, such as iron corrosion products, on the alteration of vitreous nuclear waste is essential for modeling long-term stability of these waste forms in a geological repository. This work presents experimental results for which monoliths of International Simple Glass—a six oxide borosilicate glass–, with polished and unpolished cut sides, were aged for 70 days under oxic conditions at 90 °C in a solution initially saturated in 29SiO2 at pH 7; then magnetite was added to the leaching environment. Solution and solid analyses were performed to correlate the changes in the surface features and dissolution kinetics. It was found that magnetite primarily influences the mechanically constrained surface of the non-polished sides of the monoliths, with little to no effect on the polished surfaces. This work highlights the importance of the unique chemistry within surface cracks that invokes a drastic change in alteration of glass in environments containing iron corrosion products. Immobilization in glass based hosts is the current geological method for disposal of long-lived radioactive waste from used nuclear fuels. A key factor that has to be understood is the fundamental mechanism that controls the glass dissolution in a geological repository involving complex reactions between glass and iron, and iron corrosion products. The team of Stephane Gin from DTCD SECM in France and Nathalie Wall from Washington State University in the USA, together with their co-workers, are seeking to decode the alteration of glass waste in the presence of iron corrosion products, specifically magnetite. It is determined that products such as magnetite primarily impact a mechanically constrained surface, particularly in the case of non-polished sides. Conversely, almost no influence can be observed on the polished surface. Such findings have implication for the long term durability of nuclear glasses.