To prove the suitability and safety of underground structures for the disposal of radioactive wastes, an extensive safety analysis has to be carried out. Basic steps of the analysis are the geological modelling of the entire structure including the host rock, the overburden and the repository geometry, the idealization of the geology as well as the geomechanical modelling taking into account the modelling of the underground structure and the appropriate material models with respect to creep and dilatancy of rock salt. For the geomechanical analysis the ANSALT finite-element code was compared to the JIFE code which allows the consideration of large three-dimensional structures with complex inelastic material behaviour. To establish the finite-element models needed for stability and integrity calculations, the geological models are simplified with respect to homogenous rock layers with uniform creep behaviour. The modelling results are basic values for the evaluation of the stability of the repository and the long-term integrity of the geological barrier. Concerning the barrier integrity, two geomechanical criteria are considered, the dilatancy criterion and the frac criterion. As an example of application, the analysis of the central part of the Morsleben site, used for the disposal of low and medium radioactive waste, is described.
Over the last three decades, the Federal Institute for Geosciences and Natural Resources (BGR), Germany, has been carrying out extensive geoscientific research and practical project work on domal salt structures to prove their suitability for the disposal of high and low level radioactive wastes. The objective of a waste repository, i.e., prevention of hazardous substances from entering the biosphere, is attained through the use of a system of barriers. Public acceptance of a waste repository depends on the assurance that these barriers are sufficient to provide the necessary protection. Therefore, the safety analysis has a prime importance to the planning and authorization of a repository.
The natural geological barrier is an important part of the multiple-barrier system of repositories. Thus, the load-bearing capacity and geomechanical integrity of the rock, its geological and tectonic stability, and its geochemical and hydrogeological development are important aspects of the safety analysis. It is, therefore, not only an engineering problem, but must include geological aspects. The safety analysis must be based on a safety concept that takes into consideration the possibilities for failure that could occur during excavation, operation, and post-operation phases, as well as measures to avoid such failures.
The safety analysis must include several steps as geological investigations to provide the basic geological data, mine observations and mining experience, geotechnical in-situ measurements to provide the necessary parameters of the host rock and the overburden, monitoring of the long-term rock behaviour, geomechanical laboratory investigations to determine the relevant properties of the rock and to develop adequate material models, geomechanical and, if required, thermomechanical or hydromechanical model calculations to analyse the stability and integrity of the structure and the repository, as well as evaluation and assessment of the safety taking all geological, experimental and theoretical investigation results into account (Langer & Heusermann 2001).