Chemistry of Glass Corrosion in High Saline Brines
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CHEMISTRY
OF GLASS CORROSION IN HIGH SALINE BRINES.
B. Grambow, R. MUller, Hahn-Meitner-Institut Glienicker Str. 100, D-1000 Berlin 39, FRG
Berlin,
ABSTRACT Experimental data were obtained for conventional pH values corrected for liquid junction, amorphous silica solubility and glass corrosion in concentrated salt brines. The data were interpreted with a geochemical model. The brine chemistry was described with the Pitzer formalism [1] using a data base which allows calculation of brine compositions in equilibrium with salt minerals at temperatures up to 2000C. In MgCl2 dominated brines Mg silicates form and due to the consumption of Mg the pH decreases with proceeding reaction. A constant pH (about 4) and composition of alteration products is achieved, when the alkali release from the glass balances the Mg consumption. The low pH results in high release of rare earth elements REE and U from the glass. In the NaCl dominated brine MgCl2 becomes exhausted by Mg silicate formation. As long as there is still Mg left in solution the pH decreases. After exhaustion of Mg the pH rises with the alkali release from the glass and analcime is formed. INTRODUCTION Corrosion data obtained in laboratory tests can be used for the performance assessment of nuclear waste glasses in a repository if the data are quantitatively described in the frame of a geochemical model. For granitic environment a comparison of model and experimental data is described recently [2,31. Geochemical modeling of glass corrosion in salt brines is more difficult because of the ionic strength dependence of activity coefficients. In order to evaluate the long-term performance of nuclear waste glass in a repository in salt formations, a large experimental data base was generated which covers the effect of temperature, surface area, solution volume, brine composition and time [4,5]. As in diluted groundwaters it was confirmed that the reaction rate decreases with accumulation of dissolved silica in solution and that the reaction continues with a low rate after reaching saturation of silica [5]. This paper describes the chemical aspects of the reaction of glass in salt brines; the time dependence is subject of a future communication. The Pitzer formalism [1] which recently was applied with increasing success for modeling solubility constraints in highly concentrated salt brines [6,7,8] was used to model salt/brine interaction, pH and the glass/brine reaction. Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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EXPERIMENTAL Static corrosion experiments with the simulated borosilicate nuclear waste glass SON68 (for composition see [3]) for up to 3 years duration at temperatures of 110, 150 and 190 C were performed in three different brines typical of a repository in the Zechstein salt formation at Gorleben. The composition of the brines is given in Table 1. The glass surface area to solution volume ratio, S/V, was varied from 10 to 10000 m- (powdered samples for S/V > 10). Details of experimental procedures are given in ref. [4]. Tabl
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