Sedimentary rocks: defining the options

In its evaluation of the Project Gewaehr study, one of the requirements of the Federal Government was that, in addition to the crystalline basement, sedimentary rocks should also be investigated more intensively as to their suitability as potential host formations for a repository for high-level waste and long-lived intermediate-level waste. Nagra responded to this requirement with a further investigation programme directed specifically at sediments. The results of regional investigations performed to date indicate that, particularly in a tectonically inactive region, the Opalinus Clay fulfills the main requirements for a host rock. The next step is to initiate localised investigations (demonstrating of siting feasibility) which will be concentrated in the region known as the 'Zuercher Weinland'. Supraregional studies of the Lower Freshwater Molasse have been enabled a distinction to be drawn between clay-rich (i.e. potentially suitable) and sand-rich, permeable sequences. On the basis of these results, Nagra is treating the USM as a reserve option with large spatial potential. (author) 6 figs., refs