ENSI reviewed the 6 siting areas proposed by NAGRA for low and intermediate level waste (L+ILW) and the 3 siting areas proposed for high level waste (HLW). ENSI considered all siting areas to be suitable and recommended that all should be considered further in Stage 2 of the Sectoral Plan for Deep Geological Repositories.
In some cases, the ENSI rating for safety and feasibility is lower than the NAGRA rating. However, it should be noted that evaluations are based on current knowledge and the opinions expressed by individual experts often vary only slightly. The following paragraphs comment on the criteria, which were rated differently by Nagra and ENSI as well as instances where the ENSI rating is only “relatively suitable”.
Spatial extent: In the North of Lägeren siting area, there are uncertainties regarding the extent of local dissection and so the available space. As a result, ENSI rates spatial extent as only suitable and not very suitable. For Wellenberg, the ENSI demand of a minimum depth of 500m and a minimum distance of 200 m from adjacent limestone with potential water flows reduces the amount of available space and so this rating is only suitable.
Erosion: ENSI interprets the hardened gravel residues on the top of the Südranden and Klettgau and the minimal change in 1 million years to the Klettgau valley form from glacial action as evidence of very low levels of erosion. As a result, ENSI has given the Südranden a rating of very suitable for erosion. In contrast, ENSI rates the Wellenberg siting area with its potential for future glacial erosion and major landslides as only relatively suitable. However, such erosion processes could be partly resolved by locating the repository at a greater depth.
Repository-induced influences: Rocks rich in clay have some advantages but also some disadvantages compared with other host rocks (e.g. granite). Disadvantages are the development of excavation damaged zones around underground structures and the behaviour of clay-rich rocks in terms of temperature, gas and higher pH values. As a result, ENSI has given a lower rating to clay-rich host rocks in all siting areas than Nagra. However, the disadvantages could be resolved by technical measures.
Conflicts of use: Drilling for natural gas is taking place near the siting area of North of Lägeren. There are plans to extract raw materials for cement manufacture close to Bözberg. In the Jura-Südfuss, the conditions for the exploitation of geothermal energy are particularly favourable. There could, therefore be conflicts of use in the long term. ENSI has taken account of this possibility in its evaluation.
Explorability of spatial conditions: The hilly relief on the Bözberg makes seismic investigations more difficult and so ENSI has rated its explorability as only suitable. ENSI rates the explorability of the Jura-Südfuss as only relatively suitable (large areas of water, young gravels and existing development). Similarly, ENSI rates Wellenberg as only relatively suitable because seismic investigations cannot be conducted in marl accumulations.
Predictability of long-term changes: It is less easy to predict long-term changes from glacial erosion or landslides in the Wellenberg siting area – as in the Alps generally. As a result, ENSI has given it a rating of only relatively suitable.
Rock-mechanical properties and conditions: Because marl is highly tectonised, ENSI rates the rock-mechanical properties in the Wellenberg siting area as only relatively suitable. Similarly, the rating is only relatively suitable – because of localised tectonic dissection – for the siting areas in the precursory folding zone (North of Lägeren, Bözberg) and the Jura-Südfuss. In contrast, such dissections are rare in the relatively quiet beds in the Tabular Jura (Südranden, Zürcher Weinland). At a depth of less than 650 m, engineering conditions for a deep HLW repository would be difficult in all siting areas.
Underground access and water drainage: In the Jura-Südfuss the proximity of the water-bearing chalk of the Jura is likely to increase the engineering cost.
To sum up, the ENSI and NAGRA ratings are only slightly different. These differences will have no impact on the process to refine the number of potential siting areas.