Engineering geological methods for proving the barrier efficiency and stability of the host rock of a radioactive waste repository

The suitability of a geological medium for final waste disposal can only be demonstrated through a comprehensive safety analysis, showing that the protection aims are reached within the system consisting of the waste product, the disposal facility and the overall geological situation. For a waste disposal mine, the load bearing capacity of the rock, the protective properties of the surrounding rock formations and the geological stability of the area are important factors in a safety analysis, of which geotechnical stability analysis is an important part. Such an analysis comprises an engineering geological study of the site, laboratory and in situ experiments and model calculations, long term monitoring, and special geological and geochemical investigations. Some general criteria to apply in performing a geotechnical stability analysis have been developed on the basis of theoretical considerations and practical experience. They have been used by the Bundesanstalt fuer Geowissenschaften und Rohstoffe (BGR) in preliminary repository design calculations for the Gorleben site and for proving the safety of the Konrad site. Some results of these calculations are given. The assessment of the integrity of the geological barrier can only be performed by calculation with validated geomechanical models. Natural barriers should ensure that the inflow of water capable of leaching and/or the outflow of contaminated water from the final repository into the biosphere either is fundamentally impossible or at least can be kept within acceptable limits as a result of the natural geohydraulic conditions. Therefore, the determination of water conductivity and permeability along discontinuities is necessary in order to evaluate the barrier efficiency of crystalline host rock. The BGR developed a special water injection test method to measure water percolation through fissured rock, interpreting the test results by use of model tests and numerical analysis. (author)