Deep geological repositories are being considered for long-term disposal of spent nuclear fuel in multiple countries. We investigate the transport properties of safety-relevant radionuclides in the geosphere and engineered barriers to provide parameters for the safety analysis of the repository. Our reseach topics include e.g. sorption, diffusion and structure characterization in laboratory and in situ experiments.

Safe disposal of spent nuclear fuel requires information about the radionuclide transport and retention properties within the porous and water-containing rock matrix. Our objective is to support the implementation of geological disposal of spent nuclear fuel by improving knowledge base for the safety case. To this end, we have been involved in several long-term in situ experiments in Olkiluoto, Finland and Grimsel, Switzerland. In addition, we have conducted supporting laboratory studies and interpretation of the results through modelling.

We have been working in ONKALO, the underground rock characterization facility in Olkiluoto, as part of the project "rock matrix REtention PROperties" (REPRO). The research site is located at a depth of 420 meters close to the repository site and the aim is to study the diffusion and sorption properties of radionuclides in the rock matrix in real in situ conditions.

Our involvement in the Grimsel in situ project has been funded by the Ministry of Economic Affairs and Employment under the KYT cluster. The first in situ diffusion experiment in Grimsel (Monopole 1), started in 2007 and was stopped in 2013. The experiment was overcored and the diffusion profiles of the radionuclides were obtained.

The second in situ diffusion experiment (Monopole 2) started in 2013 and was stopped in 2017. The experiment has been overcored and the diffusion profiles of the radionuclides (H-3, Na-22, I-131, Cs-134, Ba-133 and Cl-36) will be determined during 2018.

Repro research niche

When assessing the safety of nuclear waste management, sorption onto mineral surfaces and diffusion into the pore network of rock are the most significant processes that retard the transport of radionuclides from the repository in the water conducting fractures of the rock. In our research group, sorption of safety relevant radionuclides has been investigated from molecular scale up to in situ scale. Distribution coefficients are commonly determined using batch sorption experiments and experimental results are interpreted with hydrogeochemical modelling tools to determine the sorption mechanisms. In addition, sorption is studied in intact rocks with laboratory diffusion methods in drillcore samples and larger rock blocks to combine sorption with diffusion. Finally, we have investigated the transport mechanisms of sorbing and non-sorbing radionuclides in geomaterials with advection experiments.

Schematic advection matrix diffusion experiment

Mechanistic understanding of sorption and diffusion processes under a wide range of geochemical conditions is important for the evaluation of radionuclide transport. To this end, transport modelling plays a key role when extracting numerical results and mechanistic understanding from laboratory and in situ experiments. We have used e.g. Time Domain Random Walk (TDRW) simulations, COMSOL Multiphysics, PhreeqC as well as mathematical solutions to solve various transport problems. These studies have focused on the effects of structural and mineralogical heterogeneities, sorption and diffusion of radionuclides in various hydrogeochemical conditions, and transport of radionuclides in water conducting fractures.

Modelled concentration profile of cesium

Investigations related to the material properties of the different geological formations that serve as host rocks for nuclear waste repositories call for thorough evaluation of the transport and retardation properties of geomaterials. The relevant rock properties to be quantified are the accessible internal pore volume that determines the diffusive transport/retardation of non-sorbing and slightly sorbing radionuclides and the accessible internal surface area that controls sorption/fixation of radionuclides and chemical interactions. These properties are linked to the spatial porosity distribution of the geomaterials, which have been studied over the last decades with the C-14-PMMA autoradiography technique.

The C-14-PMMA technique involves the impregnation of centimeter-scale samples with C-14-labeled methylmethacrylate, a low molecular weight and low-viscosity monomer that mimics water in its behaviour in the pore space. The labeled MMA is then polymerized with gamma radiation or heating, which results in a solid radiolabeled polymer, PMMA, within the pore network. The impregnated rock samples are analyzed with autoradiographic methods and digital image analysis of autoradiographs provides spatial porosity distribution of the geomaterials.

Laboratory experiments

Colloid-facilitated transport of radionuclides may be significant to the long-term performance of a spent nuclear fuel repository. The potential relevance of colloids for radionuclide transport is highly dependent on the bentonite buffer erosion, the stability of formed colloids and their interaction with radionuclides in different chemical environments.

Bentonite erosion kinetics and mechanisms are studied by applying a batch type method and an artificial or rock fracture. The colloid generation and stability are followed by analysing particle size distribution and concentration applying the photon correlation spectroscopy (PCS) and Zeta potential applying the dynamic electrophoretic mobility (Malvern Zetasizer Nano ZS).

The radionuclide sorption on the bentonite colloids, bentonite suspensions and granitic rock are investigated by the batch sorption method. The Zeta potential of the system is determined as a function of pH with and without a studied radionuclide in order to provide information about the adsorption mechanisms. Colloid mobility and the effect of the stable and mobile colloids on the migration of radionuclides are studied in rock-core and crushed granitic rock columns under flowing water conditions. Rock–block migration experiments were introduced to examine fracture flow and radionuclide transport in a metric scale horizontal natural fracture.

Experimental work and modelling are combined to give atomic level information about clay compositions and interpret the sorption mechanisms and radionuclide transport.

Laboratory block experiment

Molecular modelling is used to increase and deepen the molecular level understanding of chemical phenomena. It can be used to study sterical and electrostatic factors of compounds, heat of formation, reaction mechanism and bulk and surface structures of inorganic materials. Modelling can be performed using molecular mechanics, semi-empirical or quantum chemical methods. The focus of research is on the sorption onto mineral surfaces, especially phyllosilicate surfaces. The greatest benefit will be achieved when molecular modelling work is performed in close co-operation with experimental work.

Additional information from Eini Puhakka

Structure of montmorillonite