Influence of a Backfilling Material on Borosilicate Glass Leaching
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FLUENCE OF A BACKFILLING MATERIAL ON BOROSILICATE GLASS LEACHING F. LANZA and C. RONSECCO Commission of the European Communities, Establishment, 1-21020 Ispra (Va), Italy
Joint Research Centre - Ispra
INTRODUCTION When a canister containing a borosilicate glass incorporating highly radioactive waste is put into an underground repository, care must be taken in favouring the dispersion of the generated decay heat by conduction in the surrounding rock. For this purpose the space existing between the glass container and the repository walls is filled with a material having an acceptable thermal conductivity. If the backfilling material is highly compacted, it can form a zone 1 of very low permeability reducing in this way the flow of water around the canister. In general, a mixture of clay and sand is considered as backfilling material, sand being added in order to improve the heat conductivity of the system. Clays present the advantage of having a good ion exchange capacity 2 (around 100 meq/100 g) , of being plastic and of increasing in volume when in contact with water. It has to be noted, however, that when a dry pressed clay is put into contact with water, the affinity of water with clay produced potential gradients which facilitate water diffusion and, in a relatively short time, 3 create homogeneous distribution of water in the clay . Consequently, clay or clay mixtures, though they prevent an elevated flow of water due to their very low permeability, do not prevent water from coming into contact with the canister and, eventually, after the canister's disintegration due to corrosion, to come into contact with the glass conditioning the high level waste. It seemed worthwhile to analyze the influence of the backfilling on the glass leaching in order to better evaluate backfilling properties and to be able to calculate the release and diffusion of radioactive elements in repository conditions. In particular it seems interesting to compare the values obtained in clay with those obtained in closed systems to verify whether even in a porous system it is possible to reach near-saturation conditions.
126 DESCRIPTION OF THE EXPERIMENT Since the scope of this work is the characterization of the phenomena of leaching in a porous, absorbing medium,
it was decided to choose a simple
testing procedure. Mixing a clay with a limited amount of water is possible only under an elevated pressure due to the swelling properties of the mixture. On the contrary, using a dilute paste it is possible to obtain a fluid paste which can be contained in an ampoule at atmospheric pressure. For this reason pastes containing around 55% of water were used. To emphasize the influence
of clay, a dry mixture composed of 80% of clay and 20% of sand was chosen. The choice of the type of clay presented another difficulty. Bentonite was first proposed by KBS
However, a dilute suspension of bentonite in distilled 4 water tends to give a paste with a high pH value, which, as is well known , .
gives rise to elevated leaching rates. Montmorillonite,
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