engleski

Correlation between mechanical properties of the bentonite and its swelling behavior

Engineered barrier systems (EBS) used in radioactive waste disposal facilities are part of the system which must prevent radionuclide from reaching the biosphere, for up to 1 000 000 years, migrating from deep (more than 300 m), stable geological environment. EBS is the second ‘line of defense’ against the migration of radionuclides, after the corrosion of containers. They also shield containers from possible environmental impact and the host rock from the heat released by the high level radioactive waste (HLW) and/or spent nuclear fuel (SF). Engineered barrier systems are usually made of cement, salt, clay (bentonite) or clay/rock mixtures. Quality control tests of clays are the same for all engineered barrier systems either used in the environmental engineering or standard geotechnical constructions. Differences may show up in the special required criteria related to specific application. Recommended clay properties depend also on the type of host rocks. Standard quality control procedures for bentonite used as a sealing barrier in radioactive waste disposal sites are described as some personal experiences. The research to determine correlation between mechanical properties of the bentonite and its swelling behavior was conducted on the granular bentonite. X-ray powder diffraction showed that the bentonitne contains 80-85% of montmorillonite. Bentonite properties were as follows: liquid limit 437%, plastic limit 53%, specific surface area 700 m2/g and CEC 89 meq/100g. The influence of swelling on the bentonitne shear strength was studied in the direct shear device. During this process, hydration times (7 to 21 days) were changed in a series of tests at different normal stresses (50 to 200 kPa) simulating bentonite clay swelling. The impact of swelling on the bentonitne consolidation and hydraulic properties were studied in oedometer device and triaxial device under isotropic compression conditions. Demineralized water was used as the test fluid. Primary swelling phase was reached after 31 days regardless of the intensity of the normal stress. Shear tests show a significant reduction in cohesion with the extension of the hydration of bentonite, and the friction angle increases with hydration. There is no significant change in the value of the friction angle by extending hydration after 14 days.

Engineered barrier systems; bentonite; mechanical properties

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