Enhancement of the Glass Corrosion in the Presence of Clay Minerals: Testing Experimental Results with an Integrated Gla
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ENHANCEMENT OF THE GLASS CORROSION IN THE PRESENCE OF CLAY MINERALS: TESTING EXPERIMENTAL RESULTS WITH AN INTEGRATED GLASS DISSOLUTION MODEL ENZO CURTI*, N. GODON** AND E.Y. VERNAZ** * Paul Scherrer Institut, Villigen and Wdirenlingen, 5232 Villigen PSI, Switzerland ** CEN-VALRHO,B.P. 171, 30205 Bagnols-sur-C~ze Cedex, France. ABSTRACT Recent glass dissolution experiments, conducted at 900 C in the presence of potential backfill materials, indicate remarkably faster glass corrosion in the presence of clay, compared to tests where the glass is leached either alone or with alternative backfill materials. This effect correlates with the clay content in the backfill, and may be attributed to the removal of silica from solution. Sorption, or dissolution with reprecipitation of a silica-rich clay, have been proposed as possible mechanisms for the silica consumption. The results of some experiments have been tested against a glass dissolution model, in which a widely used kinetic equation for glass corrosion is coupled with diffusive silica transport through a single porosity, linearly sorbing medium, which represents the backfilling. Because the glass corrosion rates imposed by the kinetic equation are inversely proportional to the silicic acid concentration of the leachant contacting the glass, the model predicts enhanced glass dissolution if silica is sorbed by the porous medium. The experimental data proved to be consistent with the predicted enhancement of the glass dissolution. Moreover, the model-estimated distribution coefficients for silica sorption (Kd) fall within the range of values extracted from available literature data, thus supporting the hypothesis that the observed high corrosion rates are due to sorption of silica on the clay mineral surfaces. INTRODUCTION Borosilicate glass is a promising material for high-level radioactive waste (HLW) immobilization [1] and will probably serve as a matrix for spent nuclear fuel in many countries, including Switzerland. Studies of glass corrosion kinetics are essential in determining a sourceterm function for the release of radionuclides from the waste matrix. A vast literature exists on this topic, including experimental as well as model studies [2]. One of the widely accepted results emerging from the experimental data, is that glass corrosion depends on the silicic acid concentration according to a first-order equation of the form [3]: R
R =k+
(I
[H4 S' 041
( 1-
[H 4 10)
is the normalized glass corrosion rate where: R is the corrosion rate in Si-free solutions k+ [H4 SiO4 ] is the activity of orthosilicic acid in the solution contacting the glass surface K is an empirically derived constant
(1)
[kg m- 2 s"1 1 [kg m 2 s ]
[ [
-
]
A similar equation is used by Vernaz et al. in France [4]. Equation (1) imposes a linear decrease of the corrosion rate with increasing silicic acid concentration in the leachant. This concentration, however, cannot increase indefinitely. Instead, it reaches a limiting value Mat. Res. Soc. Symp. Proc. Vol. 294. 1ý1993 Materials
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