Proposals of geological sites for L/ILW and HLW repositories. Geological background. Text volume

On April 2008, the Swiss Federal Council approved the conceptual part of the Sectoral Plan for Deep Geological Repositories. The Plan sets out the details of the site selection procedure for geological repositories for low- and intermediate-level waste (L/ILW) and high-level waste (HLW). It specifies that selection of geological siting regions and sites for repositories in Switzerland will be conducted in three stages, the first one (the subject of this report) being the definition of geological siting regions within which the repository projects will be elaborated in more detail in the later stages of the Sectoral Plan. The geoscientific background is based on the one hand on an evaluation of the geological investigations previously carried out by Nagra on deep geological disposal of HLW and L/ILW in Switzerland (investigation programmes in the crystalline basement and Opalinus Clay in Northern Switzerland, investigations of L/ILW sites in the Alps, research in rock laboratories in crystalline rock and clay); on the other hand, new geoscientific studies have also been carried out in connection with the site selection process. Formulation of the siting proposals is conducted in five steps: A) In a first step, the waste inventory is allocated to the L/ILW and HLW repositories; B) The second step involves defining the barrier and safety concepts for the two repositories. With a view to evaluating the geological siting possibilities, quantitative and qualitative guidelines and requirements on the geology are derived on the basis of these concepts. These relate to the time period to be considered, the space requirements for the repository, the properties of the host rock (depth, thickness, lateral extent, hydraulic conductivity), long-term stability, reliability of geological findings and engineering suitability; C) In the third step, the large-scale geological-tectonic situation is assessed and large-scale areas that remain under consideration are defined. For the L/ILW repository, for which the time period considered for long-term safety is 100,000 years, there are no large-scale geotectonic units that, as a whole, would be unsuitable from the viewpoint of long-term geological stability and would have to be excluded. Regionally and locally, however, critical zones will have to be avoided when locating the disposal chambers in order to ensure long-term stability. The space required for the L/ILW repository is comparatively small and there is considerable flexibility in arranging the individual disposal chambers. This means that none of the large-scale units need to be deferred because of spatial conditions, although there are significant differences among the units with respect to tectonic dissection and the resulting spatial situation. For the HLW repository, with a time period of 1 million years being considered for long-term safety, the Alps have to be excluded if long-term stability (including uplift and erosion during the period being considered) is to be assured. There are also reservations regarding the long-term geological stability of the Folded Jura, the western Tabular Jura and the western sub-Jurassic zone. Because of strong tectonic dissection and the resulting insufficient spatial conditions, these three units are excluded for a HLW repository; D) The fourth step involves selecting the preferred host rock formations within the large geotectonic units still under consideration. Proposed for the L/ILW repository are the Opalinus Clay with its confining units, the clay stone sequence 'Brauner Dogger' with its confining units, the Effingen Beds and the marl formations of the Helveticum. For the HLW repository, the Opalinus Clay with its confining units is the preferred host formation. Although the crystalline bedrock and the clay-rich formations of the Lower and Upper Freshwater Molasse fulfil the minimum requirements for a host rock, test options have been deferred because of the large variability of rock properties and the resulting difficulties with exploration. In the case of the Molasse formations, the relatively high horizontal hydraulic conductivity caused by sandstone layers and channels is a further reason for putting these possibilities on hold; E) The configurations of the preferred host rocks within the large-scale geotectonic units under consideration are evaluated in the fifth step. Taking into account the presence of regional geological features (regional fault zones, over-deepened valleys resulting from glacial erosion, zones with indications of small-scale tectonic dissection, other zones to be avoided for reasons of neotectonics), preferred areas are identified within which the preferred host rocks can be found at a suitable depth and with sufficient thickness and lateral extent. The preferred areas are used as the basis for delimiting the geological siting regions. Some siting regions contain several preferred areas and sometimes more than one host rock type. With the 5-step procedure adopted for narrowing down the geological siting possibilities, 7 sites were identified as possible geological siting regions for the L/ILW repository, and 3 for the HLW repository. In three of the geological siting regions, the possibility exists in principle of siting the L/ILW and HLW repositories together as a so-called 'combined repository'