Three cationic tracers, Sr²⁺, Co²⁺ and Cs⁺ were tested with a modified electromigration device by applying 2V, 3V and 4V voltage gradients over an intact Grimsel granodiorite rock sample. An ideal plug-flow model and an advection-dispersion model were applied to analyze the breakthrough curves. Matrix characterization by C-14-PMMA autoradiography and scanning electron microscopy showed that in the centimeter scale of Grimsel granodiorite rock, the interconnected matrix porosity forms a well-connected network for diffusion. Micrometer-scale fissures are transecting biotite and chlorite minerals, indicating sorption of the studied cations. The ideal plug-flow model indicated that the effective diffusion coefficients (D_e values) for Sr²⁺, Co²⁺ and Cs⁺ tracer ions within the Grimsel granodiorite rock were 3.20 × 10⁻¹³ m²/s, 1.23 × 10⁻¹³ m²/s and 2.25 × 10⁻¹² m²/s, respectively. D_e values were also derived from the advection-dispersion model, from which 2.86 × 10⁻¹³ m²/s, 1.35 × 10⁻¹³ m²/s and 2.26 × 10⁻¹² m²/s were calculated. The diffusion speed for the tracers was in the sequence of Cs⁺ > Sr²⁺ > Co²⁺ that is in the same sequence as their diffusion in diluted water. The distribution coefficients (K_d values) calculated from the models covered the range of two magnitudes (from 10⁻⁷ m³/kg to 10⁻⁵ m³/kg). The result indicated that the sorption process of the studied elements did not reach equilibrium during the electromigration process, mainly due to the too much acceleration of the migration speed by the voltage gradients applied over the rock sample.

Nagra and its international partners have been conducting underground research projects at the Grimsel Test Site (GTS, https://www.grimsel.com, last access: 8 November 2021) for more than 35 years. The results have been incorporated directly into modelling, safety and engineering feasibility studies necessary for the siting and construction of deep geological repositories. Various types of experiments are carried out at the GTS, each involving field testing, laboratory studies, design and modelling tasks, thus integrating all scientific aspects. Projects are typically planned over a 5 year period with the option to extend depending on the latest findings from the experiment. In the current 5 year programme (2019–2023) new phases of running in situ experiments using radionuclides were started and include the Long-Term Diffusion experiment (LTD) and the Colloid Formation and Migration project (CFM). A completely new experiment studying the migration of C-14 and I-129 in aged cement (CIM) was also initiated. Other experiments focusing mostly on engineered barrier materials were continued such as the Material Corrosion Test (MaCoTe), which is studying anaerobic corrosion of candidate canister materials in bentonite (Fig. 1). Also, a 1:1 scale experiment studying the high-temperature (>175∘C) effects on bentonite materials (HotBENT project) was started last year. In this paper we provide an overview of the CIM, LTD and MaCoTe projects, including key findings so far. In addition to research, the GTS, as part of the Grimsel Training Centre (GTC), is also used as an education platform for knowledge transfer to the next generation of scientists and engineers in the area of radioactive waste disposal and geosciences.