## 1 Introduction  
In any industry, including nuclear energy, huge benefits inevitably bring some less beneficial residual effects. These might be in the form of emissions (like the coal industry) or in the form of solid materials (microplastics, toxic waste, radioactive waste, etc.). As a society thinking about the future, we need to take care of those materials in a responsible way. In the case of highly radioactive waste, it is internationally demonstrated that reprocessing and transmutation of specific radionuclides do not eliminate the need for permanent disposal underground but would help reduce its footprint. Therefore, a number of disposal concepts have been developed, and much research has been initiated over the last decades. Geological disposal (repository) is realized by emplacing high-level radioactive waste in a geological environment surrounded by carefully selected and designed engineered barriers. Such a system of barriers compatible with each other (multibarrier system) ensures the containment and isolation of radionuclide-containing material over a hundred thousand years. In a carefully selected stable geological environment, the characteristics of the host rock, aquifers, aquitards up to biosphere, and prevailing conditions (stresses, groundwater pressures, temperature, etc.) are determined by processes and conditions billions of yearsearlier, and the engineered barrier properties are those that could be adjusted to be compatible with the surrounding geological environment. Thus, the processes taking place in engineered barriers and their state at different times in the geological repository lifetime is crucial. Bentonite material is among other materials considered for the construction of engineered barriers in geological repositories. Due to its favourable characteristics, such as low hydraulic conductivity, diffusion dominated transport, high sorption capacity, etc., it is often foreseen as having a use in spent nuclear fuel or high-level waste disposal in granite in such countries as Sweden, Finland, Canada, and Japan [1]. Under geological disposal conditions, a number of processes will take place in the bentonite barrier such as heat transfer, groundwater flow, vapour transport, transport of diluted species, chemical interaction with accessory minerals in the bentonite and bentonite structure itself, etc. From the perspective of long-term safety of the repository, it is important to assess the possibilities and extent of radionuclide release and transport from the waste matrix. Released radionuclides could dissolve in porewater and be transported further with it or could be transported in gaseous phase. In low permeability porous media, radionuclides are transported by diffusion mainly. Therefore, an effective diffusivity is a key parameter describing this transport mechanism. It is well known that diffusivity in a porous medium is lower than in free water due to pore structure. The latter could be affected by a number of geochemical processes, which, in turn, depend on thermal, hydraulic, mechanical, etc., conditions. Apart from the porosity change, geochemical processes are important from the perspective of sorption of radionuclides (pH [2; 3], redox conditions, availability of complexing agents, etc.). In order to evaluate the long-term safety of a disposal facility, a reliable prediction should be made about the state of the repository barriers (engineered and natural). The complexity of the repository and disposal conditions requires a multidisciplinary approach. In addition to a number of in situ heating experiments in the underground research laboratories [4] with the focus on the thermo-hydraulic (TH), thermo-hydro-mechanical (THM) state of the host rock (such as ALC experiment in the Callovo-Oxfordian claystone [5], FE experiment in the Opalinus clay [6] or the PRACLAY test in the Boom clay [7]), there are also experiments focused on the bentonite barrier. The FEBEX (full-scale engineered barrier experiment in crystalline host rock) in situ test is among these.