Clay mineralogical investigations related to nuclear waste disposal

Abstract

Abstract 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 ~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 ~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 ~1.35-1.45 W/m°K The volumetric heat capacity ranges from 3.1 x 10 6 to 3.4 x 10 6 j/m 3 °C 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 , Sr 2+ : 3 x 10 -11 , UO 2 2+ : <10 -13 , Th 4+ : <10 -13 , Fe 2+ : 4 x 10 -11 , Fe 3+ : 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 ~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.