Experimental study on swelling characteristics of sand-bentonite mixture for nuclear waste disposal

A mixture of sand and bentonite is seen as an effective component of artificial barriers for disposal facilities of radioactive wastes from nuclear power stations. Therefore, to design and construct these facilities, we must evaluate the swelling characteristics of sand-bentonite mixtures. For this purpose, various laboratory tests on the swelling pressure and swelling deformation of sand-bentonite mixtures were performed. To discuss the process of the swelling characteristics of these mixtures, this study also observed the swelling behavior of bentonite in the mixtures using a scanning electron microscope which can control the temperature and vapor pressure around samples. The following conclusions were obtained through this study. (i) The swelling characteristics of sand-bentonite mixtures is dependent on the dry density and bentonite content of mixtures. (ii) From observation with the scanning electron microscope, we found that the voids of mixtures are filled up by the volume increase when bentonite absorbs water. Furthermore, the swelling process of sand-bentonite mixtures was considered on the basis of the swelling behavior of montmorillonite, a swelling clay mineral. (iii) This study proposes the simplified evaluation of the swelling characteristics of sand-bentonite mixtures using the parameter "Swelling volumetric strain of montmorillonite". This evaluation can obtain the maximum swelling pressure and the maximum swelling strain of sand-bentonite mixtures at various dry densities and bentonite contents. Therefore, this simplified evaluation is available when the disposal facilities of nuclear wastes will be designed and constructed.