This geosynthesis provides the geological basis for comparing the three siting regions JO, NL and ZNO, selected for further evaluation as a repository site in SGT Stage 3 based on a defined set of safety-based criteria (Nagra 2025a and references therein; Fig. 1‑2). The report sets out the geological basis for demonstrating post-closure safety for the proposed site (Nagra 2024r) and references therein). Specifically, the report describes the current geological situation in Northern Switzerland and its past evolution with an emphasis on the rock formations that form the geological barrier of the planned repository. Particular emphasis is placed on the geological properties and processes relevant for the long-term stability of the barrier. As well as describing the current geological situation, the report presents scenarios of the natural processes that could affect the barrier or modify its geological environment over the next one million years, that is the time period under consideration for the high-level waste repository (HLW; Nagra 2024r). The scenarios are based on a detailed reconstruction of geological history and related processes. To ensure a robust geological argumentation, key findings relevant to site selection and safety draw on multiple lines of arguments and different, independent methodologies.

This geosynthesis is the central document integrating the geological knowledge base for site comparison and demonstration of long-term safety. It is complemented by two further reports addressing specific user groups (Fig. 1‑1):

  • NAB 24-10 (Nagra 2024m) provides quantitative geological descriptions of the three sites in relation to repository performance assessment (Nagra 2024v) and dose calculations (Nagra 2024u). It documents how the sedimentary column is divided into geological units with similar properties, presents the 3D geometry of these units in the relevant area of the siting regions and suggests unit-specific parameter ranges for key properties.

  • NAB 23-01 / Volumes 2 – 4 (Nagra 2023a, Nagra 2023b, Nagra 2023c) contains the engineering geological characterisation of the three siting regions with regard to construction planning and construction risk analyses.

In contrast to earlier geosynthesis reports (e.g. Nagra 2002), this report only addresses the relevant natural processes and their effect on the geological barrier. The interactions between waste inventory, engineered barriers and the near-repository geological barrier (repository-induced effects) are treated in the performance assessment reports (Nagra 2024v and references therein), including the parametrisation of the excavation damaged zone and gas transport through the geological barrier.

The reports and scientific papers forming the underpinning knowledge base upon which this geosynthesis is predicated are cited in the present document. A selection of Nagra reports detailing the geological findings of the investigations undertaken in SGT Stage 3 is shown in Fig. 1‑1.