## 4 Conclusions  
A numerical model for THC processes in the bentonite barrier of the geological repository in granite was developed to analyse the behaviour of the THC state of the bentonite under repository conditions and its impact on long-term safety. The model was implemented in COMSOL Multiphysics, coupled with the geochemical solver Phreeqc via the iCP interface. The model formulation was based on the two-phase flow of miscible fluids (water and air) considering important phenomena such as heat transfer, gas dissolution, advective-diffusive transport in the gaseous phase, CO\({}_{2}\) transport, and chemical interactions (speciation, mineral dissolution/precipitation, ion exchange, and surface complexation). Three cases were analysed: a TH model with tracer transport (Case A), a THC model without consideration of CO\({}_{2}\) transport (Case B), and a THC model where CO\({}_{2}\) transport in a gas phase was taken into account (Case C). Two variants of Case C were distinguished depending on boundary conditions (low CO\({}_{2}\) pressure at the bentonite-granite interface (Case C1) and high CO\({}_{2}\) pressure at the bentonite-granite interface (Case C2). After the analysis of the modelling results, the following conclusions can be drawn:  
* Mineral dissolution/precipitation induced slight changes in the bentonite porosity. The most significant porosity increase was observed near the bentonite-granite interface (by \(\sim\)3%) in Case B (THC without consideration of CO\({}_{2}\) gas transport);
* Changes in porosity depend on the partial CO\({}_{2}\) pressure at the bentonite-granite boundary. In the case of a low pCO\({}_{2}\) at the bentonite-granite interface (Case C1), the calcite dissolution led to a slight porosity increase, while a higher pCO\({}_{2}\) (Case C2) led to decreased porosity. In the latter case, the changes were observed at the larger distance from the bentonite-granite interface;
* Consideration of chemical processes taking place in the bentonite had no significant influence on non-reactive Cl transport for the considered period of time, but it would be important for other radionuclides whose sorption in porous media depends on porewater pH;
* CO\({}_{2}\) de-gassing due to temperature-dependent solubility did not have a significant impact on porosity change;
* Different boundary conditions in terms of pCO\({}_{2}\) at the bentonite-granite interface had a larger impact on the sulphate concentration compared to cation distribution in the bentonite barrier;
* Modelling results presented in this study could be useful for further interpretation of the FEBEX test measurement results before model application for large scale predictions;
* In the future, the modelling of the FEBEX in situ test considering more detailed experiment geometry and heating history is foreseen.  
Investigation, A.N. and D.G.; writing--original draft preparation, A.N.; visualisation, A.N.; writing--review and editing, D.G. and G.P. All authors have read and agreed to the published version of the manuscript.  
The EURAD program has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 847593, as well as the Lithuanian State Budget.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.