Experimental study on sealing properties of commercial bentonites related to bentonite mineralogy and water solution composition

Several concepts for repositories of spent nuclear fuel include bentonite material as an important part of the fuel isolating system. The bentonite is proposed as buffer material between the spent fuel canisters and the surrounding rock, and in some concepts also as a sealing component of the tunnel backfill material. Here we report the results from analyses and tests of sixteen potential buffer and backfill materials, which have been examined with respect to mineralogy and sealing properties. The overarching aims were to: correlate mineralogy of the different bentonites to their sealing properties, i.e. swelling pressure and hydraulic conductivity; quantify the effects of saline solutions on the sealing properties Here, mineralogy refers to the mineral composition of bulk material, to the type of cations in exchange position, and finally to the mineralogical structure of the swelling mineral in the bentonite. The materials represent actual and potential commercial bentonites from Wyoming (US), Milos island (Greece), Kutch area (India), Mecklenburg-Vorpommern (Germany), Western Czech Republic, and Fyn island (Denmark). All bulk materials were examined by use of powder X-ray diffraction (XRD) in order to determine the mineralogical composition. Mineral quantification was made by use of the Rietveld technique (Siroquant software and database). Microscopic swelling characteristics were determined by use of XRD of oriented samples from the clay fractions, before and after treatment with ethylene glycol. Element content of the total materials and of the clay fractions were determined by use of ICP/AES analyses. The cation exchange capacity (CEC) of the bulk materials and of the clay fractions was determined by Cu(II) exchange, and the distribution of charge compensating cations in the bulk material was determined by ammonium exchange. Special analyses were made on several materials when this was assumed essential for the buffer functions, e.g. regarding organic carbon and sulfide content. (authors)