Research

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 saturated bedrock. Our objective is to support the implementation of geological disposal of spent nuclear fuel and low- and intermediate level waste disposal by improving knowledge base for the safety case.

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. The diffusion of radionuclides (H-3, Na-22, I-131, Cs-134, Ba-133 and Cl-136) will be determined. The in situ through diffusion experiments (TDE) started in 2016 and were finalized in 2019. Results are currently being analyzed.

Our involvement in the Grimsel in situ projects has been linked to the investigations in the KYT cluster, which is funded by the Ministry of Economic Affairs and Employment. The first in situ diffusion experiment in Grimsel (Monopole 1) started in 2007 and was stopped in 2013, followed by the second in situ through diffusion in 2014-2018. The experiment was overcored and the diffusion profiles of the radionuclides (Na-22, Cs-134 and Ba-133) were obtained. Tritiated water and chloride diffused about half a meter in the rock within four years´ time. We have started a research project on subject "In Situ investigations of radionuclide migration in the interface between cement and bedrock (RASK)". The purpose of this research project is to investigate radionuclide retention and migration in cementitious materials, crystalline rock, and especially their interface via in situ- and laboratory experiments. This work is linked to the project C-14 and I-129 in situ migration in cement which is ongoing in Grimsel underground rock laboratory. The research project is also expected to produce information to be used as the basis of long term safety analyses.

Among other activities, our group is taking part in EURAD project, which started in 2019 and is set to be finalized in 2024. Our involvement in RD&D activities of EURAD project consist of HITEC (“Influence of temperature on clay-based material behavior”) and FUTURE (“Fundamental understanding of radionuclide retention”) parts of the project.

In HITEC part of the EURAD project bentonite powder will be treated in high temperatures for extended durations, after which it will be characterized and the effect of temperature on the properties of bentonite will be determined. Properties under study are the buffer swelling pressure, hydraulic conductivity as well as erosion and transport properties. Rheological properties of bentonite sol-gel phases as a function of time will be also determined.

FUTURE project aims to understand how radionuclides retain in clay and crystalline rocks of the repository barrier system. This will be done by quantifying the influence of key parameters of the heterogeneous rock/water system, such as the influence of rock structure on transport of non-sorbing and sorbing radionuclides. Studies on redox sensitive radionuclides´ behavior on mineral surfaces in atoms and molecular levels is emphasized. Through understanding of key parameters, multicomponent mechanistic sorption models and fracture and/or pore scale simulations of radionuclide transport can be developed and migration properties of radionuclides can be predicted.

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