Generation and Characterisation of Colloids of the Near Field / Far Field Interface
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CIEMAT, Dpto. Impacto Ambiental de la Energia Avenida Complutense 22, 28040 MADRID (SPAIN) ABSTRACT The generation of bentonite colloids has been investigated by means of column experiments with bentonite compacted at the engineering barrier density and using water flows as low as 0.04 ml/h (aý 350 ml/year). Results on generation of bentonite colloids in these "all-
bentonite" columns suggested that most of the colloidal material observed in the eluted water is due to erosion of the plug. Therefore new column experiments in which granite and compacted bentonite are in contact have been designed and the effects of the water flow analysed. The physico - chemical characteristics of the colloidal material obtained in the "all-hentonite" and "bentonite p/v prami'e " column experiments have been compared as well as its stability. Results show that bentonite colloids that have been in contact with granite always present a higher negative charge, this indicating that they could be stable in the natural environment. INTRODUCTION The conceptual model of a high level waste repository consists of a multi-barrier system in which every component (e.(. vitrified waste, canisters, backfill), could be a potential source of
colloids, if local environment favours colloid generation, either by dissolution / precipitation processes or by dispersion. Bentonite is a potential back-filling material for a high level
radioactive waste repository for its swelling properties and very low hydraulic conductivity, in addition to its good sorption capability. If radionuclides strongly associate with bentonite, the possible generation of bentonite colloids at the near field / far field interface, could have a significant influence on migration processes. The aim of this work is to determine whether bentonite colloids can be released from compacted bentonite at the bentonite/granite interface and to obtain data concerning concentration and stability of these colloids. Even if these data are obtained at a laboratory scale and need to be integrated in a larger scale test, they could be very useful to address some still open questions about which factors are worth to consider in an improved model for colloids in Performance Assessment calculations. Different types of experiments have been designed for this purpose. The first type of experiment consists of a "classical" column experiment in which an "all-hentonite" plug
compacted at the density of the engineering barrier is used. The second type of experiment, that presents novel features, consist of "henlonile plu.s giranile '" column experiments simulating the near / far field interface, which have been carried out both in dynamic or quasi-static conditions. EXPERIMENTAL
All the experiments have been carried out using FEBEX [1] bentonite. Full details of the characteristics of this Ca-bentonite are given elsewhere [2]. Granite used in some column experiments come from the El - Berrocal Spanish site [3] and the composition of the water, typical of a granitic medium which has been used in all the
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