A Study on Conditions for Microbial Transport through Compacted Buffer Material
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A Study on Conditions for Microbial Transport through Compacted Buffer Material Fukunaga1, M. Honya1, E. Yokoyama1, K. Arai1, T. Mine2, M. Mihara2 and T. Senju3 1 IHI (Ishikawajima-Harima Heavy Industries Co., Ltd.), Shin-nakahara-cho 1, Isogo-ku, Yokohama 235-8501, Japan 2 JNC (Japan Nuclear Cycle Development Institute), Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan 3 IIC (Ishikawajima Inspection and Instrumentation Co., Ltd.), Shin-nakahara-cho 1, Isogo-ku, Yokohama 235-8501, Japan ABSTRACT In order to assess microbial transport through compacted buffer material consisting of bentonite and silica sand with a density of 1.2, 1.6 or 1.8 g/cm3, sterilized test pieces with the diameter of 50 mm and the depth of 25 mm were fed with bacteria, Escherichia coli. The bacteria freely diffused when the proportion of Na-bentonite was 20 wt% or that of Ca-bentonite was 50 wt%. Bacterial movement was less than 5 mm in three weeks when the proportion of Nabentonite was 70-100 wt% or that of Ca-bentonite was 100 wt%. A high proportion of bentonite is essential to slow down microbial entrance to the surface of a nuclear-waste container. INTRODUCTION It has long been suggested that microbes could have a range of both direct and indirect effects on waste containment breakdown and subsequent radionuclide migration [1] as well as on gas generation [2] in underground repositories for radioactive waste. As it seems desirable that there are no microbes adjacent to the waste, buffer materials surrounding the waste are considered important as a barrier to protect the waste against microbial entrance from outside. The lethal effect of low water activities [3] and motility of microbes [4] in compacted buffer materials have been investigated. However, there is insufficient data on conditions of the buffer material that affect microbial entrance. The authors, on the basis of their previous studies on bentonite [5], investigated the effects of exchangeable-cation type, proportion of bentonite, and density of the buffer material on microbial transport. MATERIALS AND METHODS Buffer material Test pieces of buffer material were made by compacting a mixture of Na-type bentonite and silica sand. The Na-type bentonite (Na-bentonite) is composed of montmorillonite which has exchangeable sodium ions. A product supplied by Kunimine Industries Co., Ltd., called Kunigel V1, was used in this study. The proportion of montmorillonite in Kunigel V1 was 46 to 49%, which was lower than that (75 wt%) of MX-80. Other components of Kunigel V1
included quartz/chalcedony (29-38 wt%), feldspar, calcite, dolomite, analcite, and pyrite. Its true specific gravity was 2.7 [6]. In some test runs, Na-bentonite was previously converted to Ca-type bentonite (Ca-bentonite) according to a method proposed by JNC [7]. The procedure comprised suspension of Na-bentonite in a calcium-chloride solution, reaction for 1 h, and elution of chloride by distilled water. When Ca-bentonite made by the above method was washed again, the pH of the effluent was 9.4, which was lower
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