Fracture system flow test. Experimental and numerical investigations of mass transport in fractured rock

Investigation of the effectiveness of the host rock as a geological barrier and hydraulic studies are important for the safety analysis of a potential final repository for radioactive or toxic wastes. The flow patterns of water in the rock and the capacity of the water for transporting dissolved substances are studied using conservative (salt) and non-conservative (heat) tracers to assess the potential migration of contaminants and toxic substances in the rock. Within the scope of the German-Swiss cooperation in the field of final disposal, the Rock Mechanics and Civil Engineering section of the Federal Institute for Geosciences and Natural Resources (BGR) was engaged in the third phase of the 'Fracture System Flow Test' project from 1991 to 1993. As in the preceding phases, interest was focused on the development of methods and techniques. In the present report, the future development of the equipment and numerical methods used for evaluation of the data are described in detail. The findings of the in-situ tests at the NAGRA Grimsel Test Site are summarised and the results of the simulation of flow and transport processes using both numerical (DURST) and analytical methods are presented. The basics of the DURST (DURchstroemung and STofftransport) finite element program system were developed during the first two phases of the project, together with the Institute for Flow Mechanics and Computing of Hannover University. Newly developed multipacker combined probes were tested during the several-month tracer tests. The computer program developed for this type of equipment proved to be reliable. The speed of the DURST/ROCKFLOW finite element program was increased by implementing faster algorithms. Introducing the possibility of selecting the initial boundary conditions improves the adaptation of the program to complex model structures. (author) figs., tabs., refs