Colloid and Radionuclide Transport in Granite Under Low Water Flow Rates Expected in a Geological Repository
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Colloid and Radionuclide Transport in Granite Under Low Water Flow Rates Expected in a Geological Repository Nairoby Albarran, Tiziana Missana, Ursula Alonso, Miguel Garcia-Gutierrez, Manuel Mingarro, Trinidad Lopez CIEMAT, Environmental Department, Avda. Complutense 22, 28040 Madrid, Spain
ABSTRACT Colloids generated from the engineered barriers of a high level radioactive waste repository (HLWR) emplaced in crystalline rock could play a significant role in radionuclide transport and they are of concern for the safety assessment of these repositories. The main objectives of this study are: a) to analyze the transport properties of colloids in a crystalline fractured rock under hydrodynamic conditions as similar as possible to those expected in a repository (i.e. low flow rates) and b) to discuss the effects of their presence on the transport of radionuclides. Transport experiments with bentonite and latex colloids in a fractured granite column from the Grimsel Test Site (Switzerland) were carried out, under geochemical conditions ensuring colloid stability (alkaline and low ionic strength water). Transport experiments were also carried out with 85Sr and 233U and the results with and without the presence of bentonite colloids were compared. Colloid filtration in the fracture was always observed, even when colloids presented high stability and the conditions were unfavorable to colloid attachment to rock surfaces, being both the colloids and the rock negatively charged and the fracture surface smooth. The retention in the fracture depended on the water flow rate, increasing the retention as the water flow decreased. This work illustrates as both the mobile and retained fraction of colloids, which strongly depend on the hydrodynamic conditions, are of importance in the overall radionuclide mobility. INTRODUCTION Colloids are potential carrier of contaminant in groundwater because they are particles suspended in a fluid with a size smaller than 1 m presenting high surface area and electrostatic charge. Plutonium migration along kilometers distances close to nuclear test areas in Nevada (USA) was attributed to mobile colloidal phases [1-2]. In a high level radioactive waste repository (HLWR), different types of colloids can be present. All those radionuclides presenting small solubility (i.e. tetravalent actinides), may be present in a colloidal form. All the engineered barriers forming the repository, for example metal canisters and compacted bentonite clays, could be a source of colloidal particles [3]. In particular, bentonite has very high sorption capability for most radionuclides (RN) and it is expected to hinder their migration towards the geosphere. However, mobile bentonite particles could facilitate the migration of adsorbed radionuclides. Negatively charged colloids as latex particle or bentonite colloids are very stable in alkaline and low ionic strength water as Grimsel water (GW). At the Grimsel Test Site (GTS, Switzerland) bentonite colloids were showed to be extremely mobile in highly conductive fractu
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