Project Opalinus Clay - Synthesis of the geological research results - Feasibility of disposing spent fuel elements, vitrified high-level radioactive as well as long-lived intermediate-level waste

This geosynthesis report produced for the 'Entsorgungsnachweis' demonstrates the basic feasibility of disposing of spent fuel, vitrified high-level waste and long-lived intermediate-level waste in Switzerland. As host rock Opalinus Clay, an approximately 180 million year old marine clay stone formation, has a sufficient extent in a potential siting area in the Zuercher Weinland. The past geological evolution of the investigation area and the processes occurring in the geosphere are documented. Scenarios for the geological environment are derived. The assessment of long-term safety and of construction feasibility is based on the results of detailed site-related investigations, specific laboratory and field tests, practical experience in underground structures and independent checks of consistency and comparisons with other clay rocks. Scientific understanding is demonstrated using model calculations. A broadly based, stepwise evaluation procedure aimed at narrowing down potential siting options was conducted. Based on safety and engineering criteria, the search was for a geologically simple, tectonically quiet area with Opalinus Clay at a depth between 400 and 900 m. The key elements of this research programme were a 3D seismic campaign in the Zuercher Weinland covering an area of around 50 km², an exploratory borehole at Benken, experiments as part of the international research programme in the Mont Terri Rock Laboratory, comparative regional studies on the Opalinus Clay and comparisons with clay formations under investigation in other countries. The most important properties of the selected area in the Zuercher Weinland are: Long-term geological stability, favourable host rock properties, sufficient extent of host rock body, avoidance of, and insensitivity to, detrimental phenomena and perturbations, explorability with high resolution 3D seismics, and predictability because the geological evolution of the investigation area is well known. For a deep repository with a required area of around two square kilometres at a depth between 400 and 900 m, the potential repository area in the Zuercher Weinland is around 35 km² in extent and the host rock is at least 100 m thick. The key phenomenon for understanding the transport processes and properties of the Opalinus Clay is its microstructure. With a porosity of around 12 volume percent, the rock does contain a significant component of water but, because of the extreme filigree structure of the pore space, the porewater and dissolved substances are practically immobile. The hydraulic characterisation of the host rock in the Benken borehole showed consistent hydraulic conductivities with little scatter, typically between 2x10^-14 and 1x10^-13 m/s. Hydraulic overpressures were measured in the Opalinus Clay. Model calculations have shown that overpressures such as those observed in the Zuercher Weinland only remain intact over geological time periods if the effective hydraulic conductivity is smaller than those derived from hydraulic tests. The understanding of the geological conditions in the investigation area can be considered to be very good, based on the high quality of the 2D and 3D seismic campaigns, the general homogeneity of the host rock without significant lateral variation in rock composition, the cored and intensively tested Benken borehole and neotectonic observation programmes extending over several years. The lateral extent and thickness of the host rock and the absence of large fault zones in the central part of the investigation area are well documented. Understanding of the host rock properties in terms of solute transport and engineering suitability is widely supported by investigations in other boreholes in Northern Switzerland and in the Mont Terri Rock Laboratory. The parameter values measured lie in the range expected from studies of other clay formations. Information on possible long-term evolution of the tectonic, hydrogeological and geochemical conditions forms a sound basis for evaluating the long-term protection and barrier functions of the geosphere. To take account of existing uncertainties, for nearly all aspects considered parameter ranges or pessimistic alternative values and alternative conceptual models were considered besides reference parameters. The effect of uncertainties is evaluated as part of the safety assessment. It can be assumed that uncertainties will be further reduced with future investigations. The geological situation in the investigation area, including the host rock properties, meets the requirements placed on a potential siting area. There is no geoscientific information that could question the implementation and safety of a deep geological repository in the Zuercher Weinland.