HYDRO-MECHANICAL AND CHEMICAL-MINERALOGICAL ANALYSES OF THE BENTONITE BUFFER FROM A FULL-SCALE FIELD EXPERIMENT SIMULATING A HIGH-LEVEL WASTE REPOSITORY

The effect of exposure to repository-like conditions on compacted Wyoming bentonite was determined by comparing the hydraulic, mechanical, and mineralogical properties of samples from the bentonite buffer of the Canister Retrieval Test (CRT) with those of reference material. The CRT, located at the Swedish Aspo Hard Rock Laboratory (HRL), was a full-scale field experiment simulating conditions relevant for the Swedish, so called KBS-3, concept for disposal of high-level radioactive waste in crystalline host rock.. The compacted bentonite, surrounding a copper canister equipped with heaters, had been subjected to heating at temperatures up to 95 degrees C and hydration by natural Na-Ca-Cl type groundwater for almost 5 y at the time of retrieval. Under the thermal and hydration gradients that prevailed during the test, sulfate in the bentonite was redistributed and accumulated as anhydrite close to the canister. The major change in the exchangeable cation pool was a loss in Mg in the outer parts of the blocks, suggesting replacement of Mg mainly by Ca along with the hydration with groundwater. Close to the Cu canister, small amounts of Cu were incorporated into the bentonite. A reduction of strain at failure was observed in the innermost part of the bentonite buffer, but no influence was noted on the shear strength. No change in swelling pressure was observed, while a modest decrease in hydraulic conductivity was found for the samples with the highest densities. No coupling was found between these changes in the hydro-mechanical properties and the montmorillonite - the X-ray diffraction characteristics, the cation exchange properties, and the average crystal chemistry of the Na-converted <1 mu m fractions provided no evidence of any chemical/structural changes in the montmorillonite after the 5 y hydrothermal test.