Expertise concerning NAGRA's request for NSG 20 drilling in Benken

On November 8, 1994, the National Corporation for the Disposal of Nuclear Wastes (NAGRA) sent a request to the Swiss Federal Transport and Energy Department (EVED) for the granting of a license for deep drilling in Benken. The bore-hole is to clarify the properties of Opalinus clay and the surrounding rock layers for the final storage of highly radioactive wastes. The Federal Agency for the Safety of Nuclear Installations (HSK) has to examine the request from the point of view of the nuclear safety and protection from radioactivity because it is related to the construction of an underground repository for radioactive wastes. In its 1988 report to the Swiss Federal Government, NAGRA provided proof of the safety of an underground repository for radioactive wastes, but the demonstration that there really exists a host rock layer homogeneous enough and with sufficient dimensions for a repository was still needed. Since then, NAGRA has especially concentrated its investigations on Opalinus clay because of the good characteristics of this rock for waste storage. A convenient Opalinus clay layer was found in the Zurich Weinland region, which has been explored using seismic methods. The bore-hole is dedicated to the calibration of the results of the seismic investigations. In comparison with other sites, the layer studied exhibits many geological advantages: its depth is the desired range of 400 to 1000 m; with 110 m the layer is thick enough; and there are no geological faults in the layer. At present, knowledge concerning the hydro-geology of the domain is scarce; its improvement is an important objective of the drilling. The present project foresees coupling the drilling work with a 3D-seismic investigation that should bring detailed information on the geological conditions. HSK requires that the drilling goes deeply enough until crystalline bedrock is reached in order to more accurately calibrate the results of the deep seismic reflections. It will also give information on the water-bearing layers below the Opalinus clay layer. Gas tests have to be performed along the bore-hole because large gas bubbles could jeopardise the waste repository. At the end of the bore-hole study, the bore-hole must be filled in in such a way that the good properties of the host rock remain intact. In this area the technical expertise gained at drilling operations at the Wellenberg site will be useful. On the basis of its examination, HSK concludes that no objection can be raised against the granting of the requested license, under the condition, however, that HSK will be continuously informed over the progress of the work