Clay mineralogical investigations related to nuclear waste disposal

Mineralogical and geotechnical investigations on the possible use of compacted bentonite as a buffer material in nuclear waste repositories are reported. The swelling capacity is highly dependent on the density of the compacted bentonite. Swelling pressures >30 MPa were measured for dry densities of similar to 2.0 g/cm(3). Added iron or magnetite powder up to 20 wt% had no influence on the swelling capacity. Compacted mixtures of 20 wt% ground set cement and bentonite showed higher swelling pressures but lower swelling strain capability than compacted bentonite alone. Steam lowered the swelling pressure of compacted bentonite to similar to 60% of the original value. The influence was, however, reversible by ultrasonic treatment. The thermal conductivity of saturated compacted bentonite at a density of 2.0-2.1 g/cm(3) is similar to 1.35-1.45 W/m degrees K. The volumetric heat capacity ranges from 3.1 x 106 to 3.4 x 10(6) J/m(3) degrees K. The saturated hydraulic conductivity of the compacted bentonite is <10(-12) m/s. The apparent diffusion coefficients for various ions in compacted bentonite for water contents in the range of 20 to 25 wt% are: K+:5 x 10(-11), Cs+:6 x 10(-12), Sr2+:3 x 10(-11), UO22+:<10(-13), Th4+:<10(-13), Fe2+:4 x 10(-11), Fe3+:4 x 10(-11), Cl-: 1 x 10(-10), and I-:1 x 10(-10) m(2)/s. The 'breakthrough time' for an apparent diffusion coefficient of 10(-11) m(2)/s in compacted bentonite 1 m thick was estimated to be similar to 3000 years. The mineralogical longevity was investigated on natural K-bentonites from Kinnekulle, Sweden, and Montana, USA. Although these materials have undergone considerable changes during diagenesis and contain various amounts of mixed-layer illite-smectite, they still have a substantial swelling and adsorption capacity. The investigations demonstrate that although the properties of bentonite are negatively influenced to a certain extent by heat, hot steam, iron and cement, compacted bentonite is still the best choice to act as a buffer material in a nuclear waste repository.