Fundamental Properties of Monolithic Bentonite Buffer Material Formed by Cold Isostatic Pressing for High-Level Radioact
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ABSTRACT The methods of fabrication, handling, and emplacement of engineered barriers used in a deep geological repository for high level radioactive waste should be planned as simply as possible from the engineering and economic viewpoints. Therefore, a new concept of a monolithic buffer material around a waste package have been proposed instead of the conventional concept with the use of small blocks, which would decrease the cost for buffer material. The monolithic buffer material is composed of two parts of highly compacted bentonite, a cup type body and a cover. As the forming method of the monolithic buffer material, compaction by the cold isostatic pressing process (CIP) has been employed. In this study, monolithic bentonite bodies with the diameter of about 333 mm and the height of about 455 mm (corresponding to the approx. 1/5 scale for the Japanese reference concept) were made by the CIP of bentonite powder. The dry densities: pd of the bodies as a whole were measured and the small samples were cut from several locations to investigate the density distribution. The swelling pressure and hydraulic conductivity as function of the monolithic body density for CIP-formed specimens were also measured. High density ( pd: 1.4-2.0 Mg/m 3) and homogeneous monolithic bodies were formed by the CIP. The measured results of the swelling pressure (3-15 MPa) and hydraulic conductivity (0.51.4 x10-3 m/s) of the specimens were almost the same as those for the uniaxial compacted bentonite in the literature. It is shown that the vacuum hoist system is an applicable the handling method for emplacement of the monolithic bentonite.
1. INTRODUCTION In our disposal concept, high-level radioactive waste must be emplaced in an isolating buffer. In addition, any material used to form the buffer must have all the properties necessary required for this use. Easy production, transportation, and emplacement at the disposal site are desirable. A monolithic buffer material, as proposed here, is able to satisfy these requirements from both technical and economic points of view for a high level radioactive waste repository. Such integrated construction is possible through cold isostatic pressing (CIP), a straightforward method of forming homogeneous compacts [1] for the buffer material to be emplaced around the waste package. This construction method also facilitates the transportation and burial of formed products, reducing total costs. Fig. 1 shows a disposal pit at a disposal site and monolithic buffers.
671 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
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Monolithic buffer material
Fig.1 Schematic representation of disposal pit for borehole disposal and monolithic buffer material This paper will show that the CIP method is suitable for forming large buffers with thick walls, and that such buffers have the necessary characteristics for a use with high-level radioactive waste, including formability of bentonite powder at a sufficient density, u
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