Role of alkalis on the incorporation of iodine in simple borosilicate glasses

https://doi.org/10.1016/j.jnoncrysol.2021.121278Get rights and content

Highlights

  • The role of alkalis on iodine solubility was studied for a mimic of the R7T7 glass.

  • The more the alkali content, the more iodine solubility in glass.

  • For a given alkali content, the nature of alkalis plays a role on iodine solubility.

  • Sodium is the most favorable alkali to incorporate iodine.

  • Nonlinear trends are observed on I-solubility when substituting an alkali by another.

Abstract

To understand the influence of alkali elements on the incorporation mechanisms and incorporation limit of iodine in borosilicate glasses used for nuclear waste immobilization, series of model glasses with different alkali contents (22 or 35 mol% Na2O, or 22 mol% of a mixture of alkalis: Na substituted by Li, K and Cs) were loaded with iodine (from 1 000 to 10 000 ppm at. under the form of iodide) at 1100 °C. When the incorporation limit of iodine was reached, alkali iodide crystals were observed (e.g., NaI). For oversaturated samples containing two alkalis, iodide crystals were analyzed and it was found that crystals are enriched in the heavier one of the two relative to bulk composition. Crystal-free glasses were studied by Electron Probe Micro-Analysis to measure the incorporation limit of iodine. A complex variation of incorporation limit was found. Initial substitution of Na by other alkalis (<50%) leads to a decrease in I incorporation limit, this decrease being smallest for Li and largest for Cs. For substitution >50%, only the potassium-bearing system could be studied. In this case, the incorporation limit was non-linear and passed through a minimum for a substitution of ∼75%. Glass structure was determined by X-Ray absorption spectroscopy and Nuclear Magnetic Resonance, providing evidence for local I environments rich in alkalis. It is concluded that high alkali content and high concentrations of Na2O are particularly favorable for iodine incorporation.

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.

References (66)

  • P. Maass

    Towards a theory for the mixed alkali effect in glasses

    J. Non Cryst. Solids

    (1999)
  • V. Jolivet et al.

    High pressure experimental study on iodine solution mechanisms in nuclear waste glasses

    J. Nucl. Mater.

    (2020)
  • C. Leroy et al.

    Xenon and iodine behaviour in magmas

    Earth Planet. Sci. Lett.

    (2019)
  • H. Bureau et al.

    Modern and past volcanic degassing of iodine

    Geochim. Cosmochim. Acta

    (2016)
  • V. McGahay et al.

    The origin of phase separation in silicate melts and glasses

    J. Non Cryst. Solids

    (1989)
  • J. Hopf et al.

    Glass–water interaction: effect of high-valence cations on glass structure and chemical durability

    Geochim. Cosmochim. Acta

    (2016)
  • F. Angeli et al.

    Contribution of 43Ca MAS NMR for probing the structural configuration of calcium in glass

    Chem. Phys. Lett.

    (2007)
  • H. Yamashita et al.

    Nuclear magnetic resonance studies of 0.139MO (or M′2O) • 0.673SiO2 • (0.188−x)Al2O3 • xB2O3 (M=Mg, Ca, Sr and Ba, M′=Na and K) glasses

    J. Non Cryst. Solids

    (2003)
  • J. Wu et al.

    Temperature and modifier cation field strength effects on aluminoborosilicate glass network structure

    J. Non Cryst. Solids

    (2013)
  • J. Wu et al.

    Effects of cation field strength on the structure of aluminoborosilicate glasses: high-resolution 11B, 27Al and 23Na MAS NMR

    J. Non Cryst. Solids

    (2009)
  • A. Quintas et al.

    Effect of compositional variations on charge compensation of AlO4 and BO4 entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass

    Mater. Res. Bull.

    (2009)
  • F. Angeli et al.

    Insight into sodium silicate glass structural organization by multinuclear NMR combined with first-principles calculations

    Geochim. Cosmochim. Acta

    (2011)
  • S.K. Lee et al.

    The distribution of sodium ions in aluminosilicate glasses: a high-field Na-23 MAS and 3Q MAS NMR study

    Geochim. Cosmochim. Acta

    (2003)
  • E. Gambuzzi et al.

    Computational interpretation of 23 Na MQMAS NMR spectra: a comprehensive investigation of the Na environment in silicate glasses

    Chem. Phys. Lett.

    (2014)
  • L.-.S. Du et al.

    Network connectivity in aluminoborosilicate glasses: a high-resolution 11B, 27Al and 17O NMR study

    J. Non Cryst. Solids

    (2005)
  • G.H. Frischat et al.

    Nanostructure and atomic structure of glass seen by atomic force microscopy

    J. Non-Crystall. Solids, Phys. Non-Crystall. Solids

    (2004)
  • S. Siwadamrongpong et al.

    Prediction of chloride solubility in CaO–Al2O3–SiO2 glass systems

    J. Non Cryst. Solids

    (2004)
  • S. Tan et al.

    Incorporation and phase separation of Cl in alkaline earth luminosilicate glasses

    J. Nucl. Mater.

    (2018)
  • W. Zhao et al.

    Dissolution of Cl in alkaline earth (Ca, Sr, Ba) aluminosilicate glasses

    J. Non Cryst. Solids

    (2019)
  • L.-.S. Du et al.

    Solid-state NMR study of metastable immiscibility in alkali borosilicate glasses

    J. Non Cryst. Solids

    (2003)
  • M. Neyret et al.

    Ionic transport of alkali in borosilicate glass. Role of alkali nature on glass structure and on ionic conductivity at the glassy state

    J. Non Cryst. Solids

    (2015)
  • J.O. Isard

    The mixed alkali effect in glass

    J. Non Cryst. Solids

    (1969)
  • P. Richet

    Viscosity and configurational entropy of silicate melts

    Geochim. Cosmochim. Acta

    (1984)
  • Cited by (6)

    View full text