Hydraulic and Diffusive Properties of Clay-Based Backfill Material for a Low-and Intermediate-Level Waste Repository
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HYDRAULIC AND DIFFUSIVE PROPERTIES OF CLAY-BASED BACKFILL MATERIAL FOR A LOWAND INTERMEDIATE-LEVEL WASTE REPOSITORY W.J. Cho, J.O. Lee, P.S. Hahn and H.H. Park Nuclear Environment Management Center Korea Atomic Energy Research Institute, P.O. Box 105, Yoo-Sung, Taejon 305-606, Korea ABSTRACT The results of experimental studies performed to determine the radionuclide diffusion coefficients in a compacted clay and the hydraulic conductivities of clay/crushed granite mixtures with various clay contents are presented. Clay used in the experiments is a natural clay from the southeastern part of Korea, and it contains mainly calcium bentonite. The hydraulic conductivities of clay/crushed granite mixtures decreased with increasing clay content. In case of clay content of 50 wt.%, they maintain the considerably lower values even at the dry density of 1.5 Mg/m 3. The diffusion coefficients for 9°Sr, 137Cs, 6°Co and 125I in water saturated clay at a dry density of 1.4 Mg/m3 were measured at room temperature. The average apparent diffusion coefficients obtained are 4.5 x 10-12 m 2 /s, 9.0 xI0-13 m 2/s, 3.4 x 10-13 m 2 /s, and 6.7 x 1011 m 2/s for 9°Sr, 1S•Cs, 60Co, and 12I, respectively.
INTRODUCTION
A repository for lowand intermediate-level radioactive wastes arising from nuclear power plant operation and radioisotope application in Korea will be constructed in the bedrock below ground surface, and the site selection is under way. Present design concept suggests that the intermediate level wastes will be emplaced into the compartments made of reinforced concrete, and the space between the concrete wall and the cavern surface will be backfilled with a mixture of clay and crushed granite [1]. As compacted clay-based materials have a low hydraulic conductivity, and the hydraulic gradient in a disposal cavern is expected to be relatively low, molecular diffusion will be the principal mechanism by which radionuclides will migrate through the backfill. Bentonite has been considered as a potential backfill material because of its low permeability, high sorption capacity, self sealing characteristics, and durability in nature [2]. In Korea, as high quality sodium bentonite might not be available in sufficient quantities, the mixture of natural clay and crushed rock is being suggested as a candidate backfill material [3]. It is however possible that the addition of crushed rock to clay would increase the hydraulic conductivity. This study intended to examine the hydraulic conductivities of the Mat. Res. Soc. Symp. Proc. Vol. 353 ©1995 Materials Research Society
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mixtures with various clay contents and the radionuclide coefficients in compacted clay.
diffusion
MATERIALS AND METHODS Materials Clay and rock aggregates used in this study were selected as follows. The clay was a calcium bentonite from Kampo, Kyungsangbuk-do, Korea, and contains approximately 53.2 % Si0 2, 22.1 % A12 0 3 , 8.4 % F20 3 . 2.6% CaO, 2.0% MgO, 1.4% Na2 O, and some
minor elements. Its detailed mineralogical composition and some basic
properties wer
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