Nuclear Glass Alteration in Clay: Assessment of the Effect of Direct Contact between the Materials through Experimental
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1XFOHDU*ODVV$OWHUDWLRQLQ&OD\ $VVHVVPHQWRIWKH(IIHFWRI'LUHFW&RQWDFWEHWZHHQWKH 0DWHULDOVWKURXJK([SHULPHQWDODQG0RGHOLQJ$SSURDFK S. Gin, F. Thierry and Y. Minet Commissariat à l’Énergie Atomique – CEA Valrhô DIEC/SESC BP 17171, 30207 Bagnols-sur-Cèze Cedex, France $%675$&7 A new approach is proposed for discussing the reliability and predictability of the models intended to evaluate the performance of glass packages under geological disposal conditions. The U(W) model developed by the CEA is used in this study to simulate original laboratory test results, and the validity of the predictions is then verified experimentally. This approach allows us to check that the key mechanisms are correctly simulated, even if they are simplified. A review of the experimental results (glass alteration kinetics and morphology of the alteration film) suggests that physically separating the glass and clay under geological disposal conditions could considerably diminish the silica pump effect attributed to reactive clays such as Boom clay or FoCa7 clay. ,1752'8&7,21 The U(W) operational model 0 was developed to predict the alteration of an R7T7 glass package according to the environmental conditions—notably silica regulation in the materials surrounding the glass, which is considered to be the key mechanism governing the alteration kinetics of this type of glass under repository conditions. The quantity of glass altered under these conditions is calculated from the glass alteration kinetics and the surface area exposed to alteration. Due to fracturing, the last point constitutes a separate problem area 0 and is not discussed here. In order to test the capability of the model to account for the R7T7 glass alteration kinetics in a complex environment representative of disposal conditions, we designed an experiment in which an inactive sample of SON68 glass (inactive surrogate of the R7T7 reference glass) is altered near a sample of FoCa7 clay known to consume silica from the glass. In this experiment the glass and clay are physically separated by an inert diffusion barrier 2 mm thick. The U(W) model was first applied to predict the glass behavior before performing the experiment. The alteration film formed during the experiment was examined by SEM and the results were compared with those obtained in the case of glass alteration directly in contact with the clay. This approach introduces the notion of the “sphere of influence” of the environment on the glass. 7+(U W 02'(/ The three main equations of the U(W) model used to describe the glass dissolution kinetics are the following 0: The glass alteration kinetics follow a first-order law based on silica: & ([, W ) U = U0 1 − &*
where U0 is the initial dissolution rate, & is the silica concentration, and [ is the space variable. &* is the first model parameter. Unlike a simple affinity law in which &* accounts for the thermodynamic
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stability of the glass, in this model &* is a parameter that depends to a large extent on the conditions under which a
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