STUDY ON THE PERMEABILITY OF ENGINEERED BARRIER FOR THE ENHANCEMENT OF A RADIOACTIVE-WASTE REPOSITORY SYSTEM

To develop engineered barriers and construction methods for an enhance d radioactive waste repository an advanced application of cement/concr ete and bentonite was studied. On the basis of the tests on fundamenta l properties of the materials, model structures were prepared by actua l construction methods, and then the permeability was evaluated. For c ement/concrete, two model silos were constructed by different methods and then the reduction in permeability was evaluated One was construct ed by an ordinary method and then grouted with cement milk containing fine cement and silica fume. The whole permeability of the silo after grouting decreased to one-sixteenth of the value before grouting. The other was constructed by a crack-controlling method. This method could make the whole permeability of the silo as low as one-tenth of that o f the cracked silo. For bentonite, a compaction method and a spraying method were examined with a mixture of sodium-type bentonite and sand. To demonstrate these methods, model structures were constructed using fill-scale machines. Then the relationship between the dry density an d the permeability was examined. For a 20 to 30% bentonite mixture, th e permeability was almost equally low for both methods, while the dens ity was lower for the spraying method than for the compaction method. In contrast, for a 10% bentonite content, low permeability could only be obtained with the high-density structure. The permeability of both the concrete structures and the bentonite-sand structures was signific antly low as engineered barriers, showing some differences with the st ructures and their construction methods. Referring to the test results , an engineered barrier system proving low permeability was suggested by a combination of the structures and the construction methods.