In Finland, the spent nuclear fuel will be deposited at a depth of 400 m in the granitic bedrock. The disposal is based on KBS-3 concept, which relies on the multi-barrier principle, where different successive barriers prevent the migration of radionuclides to biosphere. The spent nuclear fuel is placed in the disposal tunnels in copper-iron canisters, which are surrounded by bentonite clay to insulate them from the groundwater flow and protect from the movements of the bedrock. Bentonite clay consists mainly of montmorillonite, which like the other aluminosilicates are known to retain radionuclides thus, contributing to the retention or immobilization of them. Besides the contribution to the multi-barrier system, the bentonite buffer is assumed to be a potential source of colloids due to the erosion of bentonite in certain conditions. Colloids in the context of radionuclide migration are nanoparticles in the size range from 1 to 1000 nm that remain suspended in water.
The montmorillonite colloids could potentially act as carriers for otherwise immobile radionuclides like transuranium elements in the case of canister failure. Especially, 241Am is an important radionuclide regarding the long-term safety of the final disposal as after a few hundred years 241Am and its mother 241Pu contribute most to the radiotoxicity of the spent nuclear fuel. The relevance of the colloids to the long-term performance is depending on several factors like colloid stability, mobility and their interaction with radionuclides. The colloid stability is depending on the groundwater conditions like ionic strength and pH. In low salinity groundwaters, the montmorillonite colloids have been shown to be stable. On the other hand, the collective processes of the rock matrix, bentonite colloids and radionuclides have to be investigated to assess the long-term performance of the multi-barrier system. It requires the combination of the different scale experiments from the simple laboratory experiments to large, natural scale in-situ experiments to understand the complex processes affecting the colloid-facilitated radionuclide migration. The large-scale laboratory experiments conducted with granite blocks offer an intermediate between the two extremes having a more natural system than the former and a better controllability than the latter.
In this study, the radionuclide migration was studied in different scale laboratory experiments. The colloid-facilitated transport of Eu was studied with a block-scale experiment using a granite block with a natural water conducting fracture. The suitability of the block was assessed by conducting several experiments using different non-sorbing and sorbing tracer and montmorillonite colloids separated from synthetic Ni-labeled montmorillonite and Nanocor PGN Montmorillonite (98 %). Laser-induced breakdown detection (LIBD), photon correlation spectroscopy (PCS) and ICP-/MP-OES were utilized in colloid detection. Supportive batch experiments were conducted to study the colloid stability in different ground waters and the interaction between the granite, different montmorillonite colloids and Eu, an analog to Am.
Good reproducibility was obtained with non-sorbing tracers. The breakthrough of the radioactive 3H, 36Cl and fluoresceine and Amino-G dyes showed similar behavior. On the other hand, no breakthrough of montmorillonite colloids or 152Eu occurred. Based on the literature review, the low flow rates used could be the reason for this. Low flow rate (50 μl/min) could affect the colloid mobility strongly which could explain that Eu retained in the fracture. More experiments with higher flow velocities would be required. Different montmorillonite materials showed similar but not exact the same sorption behavior of Eu. The fraction of Eu attached to colloids decreased during the experiments and correspondingly the fraction attached to the granite increased. At the same time, colloids remained stable during the expertiments. This indicates that desorption of Eu from the colloids is taking place in the presence of granite. Also, the effect of different water composition on the stability of colloids was clearly seen on the preparation of colloid suspensions in different water simulants. Even a small increase in the ionic strength of the solution made the especially Ni-montmorillonite colloids instable.
