Browsing by Subject "final disposal of spent nuclear fuel"

In Finland, the final disposal of spent nuclear fuel will start in the 2020s where spent nuclear fuel will be disposed 400-450 meters deep into the crystalline bedrock. Disposal will follow Swedish KBS-3 principle where spent nuclear fuel canisters will be protected by multiple barriers, which have been planned to prevent radionuclides´ migration to the surrounding biosphere. With multiple barriers, failure of one barrier will not endanger the isolation of spent nuclear fuel. Insoluble spent nuclear fuel will be stored in ironcopper canisters and placed in vertical tunnels within bedrock. Iron-copper canisters are surrounded with bentonite buffer to protect them from groundwater and from movements of the bedrock. MX-80 bentonite has been proposed to be used as a bentonite buffer in Finnish spent nuclear fuel repository. In a case of canister failure, bentonite buffer is expected to absorb and retain radionuclides originating from the spent nuclear fuel. If salinity of Olkiluoto island´s groundwater would decrease, chemical erosion of bentonite buffer could result in a generation of small particles called colloids. Under suitable conditions, these colloids could act as potential carriers for immobile radionuclides and transport them outside of facility area to the surrounding biosphere. Object of this thesis work was to study the effect of MX-80 bentonite colloids on radionuclide migration within two granitic drill core columns (VGN and KGG) by using two different radionuclides 134Cs and 85Sr. Batch type sorption and desorption experiments were conducted to gain information of sorption mechanisms of two radionuclides as well as of sorption competition between MX-80 bentonite colloids and crushed VGN rock. Colloids were characterized with scanning electron microscopy (SEM) and particle concentrations were determined with dynamic light scattering (DLS). Allard water mixed with MX-80 bentonite powder was used to imitate groundwater conditions of low salinity and colloids. Strontium´s breakthrough from VGN drill core column was found to be successful, whereas caesium did not breakthrough from VGN nor KGG columns. Caesium´s sorption showed more irreversible nature than strontium and was thus retained strongly within both columns. With both radionuclides, presence of colloids did not seem to enhance radionuclide´s migration notably. Breakthrough from columns was affected by both radionuclide properties and colloid filtration within tubes, stagnant pools and fractures. Experiments could be further complemented by conducting batch type sorption experiments with crushed KGG and by introducing new factors to column experiments. The experimental work was carried out at the Department of Chemistry, Radiochemistry in the University of Helsinki.