Ion/Water Migration Phenomea in Dense Bentionites
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ION/WATER MIGRATION PHENOMEA IN DENSE BENTIONITES
Roland Pusch+, Trygve Eriksen++ and Arvid Jacobsson+..,
+ +++
Div. of Soil
Mechanics, University of LuleA, 951 87 Lule& (Sweden), ++ Dept. of Nuclear Chemistry, Royal Institute of Technology, 100 44 Stockholm (Sweden).
INTRODUCTION The development of a suitable technique for isolating unreprocessed nuclear
reactor wastes from the biosphere has led to the Swedish multibarrier concept KBS 21 with two engineered ccmponents, a thick-walled copper canister and a clay body which confines the canister, Fig. 1. The clay consists of well fitting blocks of highly compacted Na bentonite made by "isostatic" compression of bentonite powder. They are not water saturated when placed in the deposition holes but take up water from the surrounding rock, swell and ultimately form a tight contact with the rock and the canisters. When the bentonite is in physical equilibrium with the surroundings it forms a medium with a number of valuable properties, such as self-healing and ion exchange capacities. The healing means that initial joints between blocks and voids, or local passages in the clay cased by slight rock or canister displacements, will be sealed by the swelling potential of the clay. The high ion exchange capacity retards the migration of radionuclides through the clay barrier after corrosion of the canisters. The most important feature is, however, the very low hydraulic conductivity and the low ion diffusivity. These properties will be discussed in the present article. CLAY MATERIAL AND PORE WATER Granulcmetry and mineralogy
One purpose of the KBS study of clay buffers is to
develop a technique for preparation and application of rather large quantities of clay masses, which implies that only easily available and cheap soils are considered. Thus, once the decision was made that smectites are most suitable, the interest was focused on commercial bentonites. Two, fairly well defined materials were selected for the study: The American Colloid Co. type MX-80 (Wyoming Na bentonite) being the main KBS reference material, and the Erbsl6h Ca-bentonite, representing natural calcium-saturated clay materials. Both bentonites are characterized by a clay content ( 97%),
they were subjected to an external water pressure. In most tests, the pressure was varied systematically in the order 500 kPa, 250 kPa, 100 kPa, and 50 kPa to find out whether the hydraulic gradient affects the permeability. Each resulting gradient (104, 5 10 3 , 2 - 10 3 , and 10 respectively) was held constant until stationary flow conditions were approached. The total testing time required for each sample was therefore rather long; 3-4 weeks as an average.
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The percolated water quantities were extremely small and the flow rate was therefore determined by observing the rate of displacement of a water meniscus in a calibrated capillary connected to the filter at the water exit. The accuracy of the evaluated permeability was estimated at approximately t 5 -10-15 m/s. The evaluated perme
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