The KBS-3H disposal alternative is composed by horizontally placed supercontainers comprising the canisters with the spent nuclear fuel surrounded in both drift axis and radial directions by compacted bentonite blocks (buffer) enclosed in a perforated shell. The different internal gaps between the supercontainer components and the one between the buffer blocks and the host rock have direct effects on the buffer behaviour. This paper presents a Thermo-Hydraulic (TH) Three-Dimensional (3D) numerical model developed to analyse a particular geometry assuming three different gap state conditions and providing results of the temperature, liquid pressure, and evolution of the degree of saturation. The material parameters, constitutive models, and assumptions made were carefully selected with regards to laboratory measurements reported in directly-related bibliography. The modelling settles the importance of understanding the groundwater flow through the rock mass and from fractures in the rock in order to achieve reliable predictions regarding buffer saturation, since it is known that the saturation times could range from few years to one thousand years depending on the hydrogeological conditions in the rock. The obtained results lead to full saturation times of 50 to 100 years. In addition to the rock hydraulic conductivity and fracture transmissivity, the saturation process was directly affected by the material properties of the buffer and gap presence between the buffer blocks and the host rock. Finally, in connection with thermal evolution, the thermal conductivity of repository components and the behaviour of air gaps in the buffer were key variables.

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