Article pubs.acs.org/est
Silicate Glass Alteration Enhanced by Iron: Origin and Long-Term Implications A. Michelin,†,‡ E. Burger,† D. Rebiscoul,† D. Neff,‡ F. Bruguier,† E. Drouet,‡ P. Dillmann,‡ and S. Gin*,† †
CEA, DEN, (DTCD/SECM/LCLT) −Marcoule, F-30207 Bagnols-sur-Cèze Cedex, France LAPA SIS2M UMR 3299 CNRS/CEA and LMC IRAMAT UMR CNRS 5060, CEA Saclay, F-91191 Gif-sur-Yvette, France
‡
S Supporting Information *
ABSTRACT: Silicate glasses are used as containment matrices for deep geological disposal of nuclear waste arising from spent fuel reprocessing. Understanding the dissolution mechanisms of glasses in contact with iron, an element present in large amounts in the immediate environment (overpack, claystone, etc.) would be a major breakthrough toward predicting radionuclide release in the geosphere after disposal. Two different reacted glass−iron interfacesa short-term nuclear system and a long-term archeological systemwere examined using a multiscale and multianalytical approach including, for the first time on samples of this type, STXM under synchrotron radiation. Comparisons revealed remarkable similarities between the two systems and shed light on Fe−Si interactions, including migration of iron within a porous gel layer and precipitation of Fe-silicates that locally increase short-term glass alteration and are sustainable over the long-term.
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INTRODUCTION Reliable prediction of material durability over the very longterm is one of the most challenging research issues in material, earth, and environmental sciences. Borosilicate glasses are used as containment matrices for long-lived radionuclides arising from spent nuclear fuel reprocessing and destined for deep geological disposal. If a glass package lifetime of several hundred thousand years can be demonstrated, it will help to guarantee the safety of the geological repository and increase confidence in this solution. The only way to estimate the behavior of such materials over geological time scales, and then derive the radionuclide source term,1 is to combine the examination of natural or archeological analogs with simulation on laboratory systems under well controlled conditions. This approach helps focus on relevant mechanisms and increases the reliability of predictive models.2,3 The corrosion of silicate glass arises from several coupled mechanisms: hydration, ion exchange between alkali or alkalineearth ions and protons in solution,4 hydrolysis of the ioniccovalent network,5 condensation of freshly dissolved species,6 and precipitation of more stable crystalline phases.7 These processes depend on several parameters such as temperature, pH, water composition and flow rate, and thus indirectly on other nearby solid phases. For example, some geological disposal designs include a glass package with an overpack or liner made of stainless steel or carbon steel, providing a large amount of iron very near the glass.8,9 According to previous studies, glass durability could be adversely affected by the presence of iron but the mechanisms involved remain insufficiently understood for reliable modeling of the long-term behavior of glass packages.10−13 Through a novel approach based on thorough characterization of a short-term nuclear system and a long-term archeological one, © 2012 American Chemical Society
we show how iron affects the fate of silicon and thus the glass durability in both systems. Both systems consist of glass altered in contact with metallic iron in clay media. They have been investigated to bridge the gap between short-term laboratory experiments and long-term field studies, as well as to rank the phenomena for modeling purposes.
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METHODOLOGY The systems are shown in Figure 1. The short-term system (STS) was designed to reproduce as closely as possible nuclear waste conditions in the French concept,8 and consisted of a vertical core of Callovo-Oxfordian claystone on which 2 iron disks were placed (5 mm thick, 39 mm diam., Armco, 99.8% purity). Holes 2 mm in diameter were drilled and partially filled with iron powder (1−6 μm size fraction, 98% purity, Goodfelow). This steel−iron composite was covered with SON68 nuclear glass powder from the 63−125 μm size fraction. The injected synthetic solution, estimated to be at thermodynamic equilibrium with the claystone,14 flowed vertically from the claystone to the glass with an overpressure of 15 bar. This pressure imposed a low flow rate (around 1 mL/month) and a Peclet number below 1, so that the transport of species was predominantly diffusion-controlled. Anoxic conditions were preserved throughout the experiment by the use of an airtight vessel (initially purged with Ar). The experiment was performed at 50 °C and the leachate was collected continuously for analysis. Received: Revised: Accepted: Published: 750
May 9, 2012 December 3, 2012 December 12, 2012 December 13, 2012 dx.doi.org/10.1021/es304057y | Environ. Sci. Technol. 2013, 47, 750−756
Environmental Science & Technology
Article
Figure 1. Schematic view of short-term system (STS) and long-term system (LTS).
Table 1. Comparison of the Main Characteristics of the Materials, Alteration Conditions, and Duration of the Two Experiments short-term system (STS)
long-term system (LTS)
glass
homogeneous SON68 borosilicate glass 2 wt % Fe (valence +III)a SiO2: 45.48 wt %; B2O3: 14.02 wt %; Na2O: 9.86 wt %; Al2O3: wt %; CaO: 4.04 wt %; ZrO2: 2.65 wt %; Li2O: 1.98 wt %; other elements
