According to ENSI (2018), the geological investigations in SGT Stage 3 must provide the level of site-specific knowledge required to evaluate the 13 safety criteria, undertake a comparative safety analysis for the siting regions, and develop a safety case for the proposed site with robust and well evidenced arguments. Based on the considerations summarised in Sections 1.4.1 and 1.4.2, the geological investigations should therefore provide the underpinning evidence and argumentation base to support the demonstration of the following required characteristics:

• The host rock provides the primary geological containment function and is a particularly efficient and robust transport barrier. This requires a sufficiently thick host rock, limited llithological and mineralogical variability, very low hydraulic conductivities, low hydraulic gradients, a lack of distinct transport paths and thus diffusion-dominated transport conditions. Furthermore, the mineralogical composition and the related pore structure should favour the sorption of radionuclides and the geochemical conditions needed to promote the stability of the engineered barriers.

• Low-permeability confining units above and below the host rock contribute to the barrier effect. Compared to the host rock, the properties of the confining units may show a higher variability (e.g. concerning the clay-mineral content), but, as a whole, the CRZ should be characterised by low hydraulic conductivities, and transport should be diffusion-dominated.

• The efficiency of the geological barrier over long timescales is independently evidenced.

• The host rock occurs in a simple, subhorizontal to slightly inclined bedding configuration wherby areas devoid of seismically mappable faults should be as large as posible, providing flexibility for repository placement and design.

• The key properties of the host rock remain stable over the period under consideration with regard to long-term geological evolution. This requires that porewater composition, hydraulic conductivity, self-sealing capacity and diffusion coefficients in the CRZ do not change in a detrimental way in response to natural processes that may be active during the next 100'000 to 1 million years. This includes climate change, glacial loading and unloading, erosion, geodynamic activity or rock dissolution phenomena. The depth of the host rock and the lateral distance to regional tectonic fault systems are pivotal considerations for long-term stability.

The points above address the geological barrier and its contribution to post-closure safety. In addition, geological investigations will provide the basis for the construction risk engineering analyses in the three siting regions as well as for planning the provisional engineering design of the repository at the proposed site⁸.

The investigation campaign described in Chapter 2 focused on the provision of the data and infor­mation to support assessment of the required characteristics in line with the regulatory require­ments (ENSI 2018).