Role of alkalis on the incorporation of iodine in simple borosilicate glasses
Introduction
In France, the choice has been made to reprocess spent nuclear fuels in order to recycle uranium and plutonium to produce new fuel assemblies. This is performed through a series of chemical extractions called the Plutonium and Uranium Refining by Extraction (PUREX) process [1,2]. However, the PUREX process also produces a “final waste” of Fission Products (FPs) and minor actinides that is a major potential environmental hazard. Reducing the danger of this final waste is made possible by conditioning in an aluminoborosilicate glass, called “R7T7” [3]. Borosilicate glasses are commonly used matrices for several kinds of nuclear waste thanks to a large incorporation capacity of a wide range of elements, good chemical durability and a radiation-tolerant structure [4], [5], [6]. ANDRA, the French organization responsible for identifying, implementing and guaranteeing safe management solutions for all French radioactive waste, is studying the disposal of High Level Waste (HLW) glasses in a deep geological repository [7]. In this context, it has been found that iodine (more specifically 129I) would be the main contributor to potential contamination of the biosphere [8,9]. This is related to the rapid transport of iodine in a clay environment in the presence of water. Indeed, iodine present as iodide I− (the expected oxidation state present in the glass [10], [11], [12]) is readily transferred to a fluid phase, leading to migration of I on the same time-scales as the transport of water [7,8]. In the light of such considerations, several studies have been devoted to optimize iodine capture and immobilization (e.g., [13]).
Before addressing the question of iodine transport in geological environments over long time spans, it is essential to quantify just how much iodine is expected in the initial waste containment glasses. This is not a simple question as the concentration of I in these pristine glasses has never been directly measured, but will depend on various parameters such as the initial iodine concentration in the waste, the losses by volatilization during vitrification and the formation of iodide salts that can migrate to the top of the batch during melting. In the U.S., studies of the incorporation of iodine in borosilicate glasses have focused on the treatment of Low-Activity Waste (LAW) from the Hanford site [10,12,14,15]. These studies showed that the dominant valence state of iodine is I−, and that iodine is surrounded by alkali elements within the glass network [10,12]. Riley et al. [12] measured an incorporation limit of about 1 wt.% for iodine at 1000 °C under conditions that minimized volatilization (sealed quartz tubes). Moreover, McKeown et al. [10] highlighted a link between the proportion of lithium and sodium in the glass and the retention of I during heating, possibly related to the local environment around iodine atoms (i.e. the nature of chemical bonds between iodine and sodium or lithium which could lead to different trends for iodine volatility). As such, glass composition and more particularly alkali content and the nature of these alkalis are expected to play an important role on iodine incorporation. The well-known mixed alkali effect [16], [17], [18], [19], [20], [21] could also influence this property. However, chemistry is not the only factor that controls iodine incorporation, as melting conditions can also be of importance. For instance, the influence of pressure for simplified silicate, borate and borosilicate glasses have been intensively studied for different pressure ranges [22], [23], [24], [25], [26], [27], [28], [29]. For borosilicate glasses with a B2O3/SiO2 ratio close to that of R7T7 glass, Grousset concluded that there was only a weak impact on the incorporation limit of iodine between atmospheric pressure and 0.2 GPa (melting in confined conditions to limit iodine volatility in both cases) [22]. However, for higher pressure ranges (> 1 GPa), an increase of the incorporation limit was observed by Jolivet et al. [12]. At such conditions, iodine may occur in the form of iodate, leading to increased incorporation compared to iodide [29]. Actually, pressure (typically on the order of 1 GPa) is a key parameter to incorporate volatile elements in glasses and to understand their behavior in magmatic context [26,28].
In this study, we have studied the role of composition on the incorporation of iodine for glasses similar to R7T7 at a pressure close to 0,1 MPa to highlight the effect of composition. As the currently available data indicate that alkali species should be present in the vicinity of iodine and that they could influence the incorporation of iodine, this study focuses on: i) the influence of total alkali content, and ii) the mixed alkali effect at a fixed total alkali content.
Section snippets
Glass composition
A simplified glass derived from the R7T7 glass, containing only sodium, aluminum, boron and silicon oxides was studied in order to identify individual compositional effects. In detail, the R7T7 composition [3] was simplified by ignoring minor components and considering only the eight most abundant oxides (ZrO2, ZnO, CaO, Li2O, B2O3, SiO2, Al2O3, Na2O). This list includes the four oxides retained for study (B2O3, SiO2, Al2O3, Na2O). To take account of the four other oxides, the concentrations of
Glass composition analysis
The variation of iodine concentration in glass as a function of its nominal content (related to the weighing of the reactants) can typically be divided into two parts. At low iodine concentrations, the nominal and incorporated concentrations are close (if volatilization is weak). For increasing iodine concentrations, an incorporation limit is reached, as previously described (e.g., [24]). Above this point, samples contain small iodine-bearing crystals (typically 100 to 500 nm in diameter).
Influence of the total alkali content in pure Na-containing glass
As shown in Fig. 1, there is a strong increase in the saturation limit of iodine with increasing Na2O content. This behavior is consistent with the evolution of iodine retention measured by McKeown et al. [10] with increasing Na2O + Li2O (mol%) content in aluminoborosilicate glasses. For the G-NaI-22–22-xx glasses, the incorporation limit (∼3000 ppm at.) is also higher than that measured by Riley et al. (1641 ppm at.) for a glass with a similar total alkali contents made in a confined
Conclusion
The incorporation limit of iodine was experimentally determined for a simplified aluminoborosilicate version of the R7T7 glass (Linc (G-NaI-22–22) = 3046 ppm at.) and for a sodium-enriched analog (Linc (G-NaI-35–35) = 7260 ppm at.). The significant increase in the incorporation of iodine at higher sodium content is interpreted by the direct interaction between “free” sodium (i.e. that is not required to charge balance cations such as [4]B3+ and [4]Al3+) and iodine.
Replacing sodium by other
Autorship contributions
Please indicate the specific contributions made by each author (list the authors’ initials followed by their surnames, e.g., Y.L. Cheung). The name of each author must appear at least once in each of the three categories below. Category 1 Conception and design of study: B. Vénague, L. Campayo, M.J. Toplis, T. Charpentier, M. Moskura, J.-L. Dussossoy; Acquisition of data: B. Vénague, L. Campayo, T. Charpentier, M. Moskura; Analysis and/or interpretation of data: B. Vénague, L. Campayo, M.J.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank ORANO and EDF for their financial support and their involvement in this project. We are also grateful to Séverine Bellayer, who carried out the EPMA analyses, for her efficiency and her implication.
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