Technical Report NTB 94-02

Grimsel Test SiteFracture system flow test:Experimental and Numercial Investigations of Mass Transport in Fractured Rock

The investigation of the effectiveness of the host rock as a geological barrier and hydraulic studies are important for the safety analysis of a potential permanent waste 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 (temperature) tracers to assess the potential spreading of contaminants and toxic substances in the rock.

Within the scope of the German-Swiss cooperation in the field of permanent repositories, 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" research 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 further development of the equipment and the numerical methods used for evaluation of the data are described in detail. The findings of the in-situ tests in the Nagra Grimsel rock laboratory are summarized and the results of the simulation of flow and transport processes using both numerical methods (DURST) and analytical methods are presented. The basics of the DURST (DUrchströmung 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. The introduction of the possibility of selecting the initial boundary conditions improves the adaption of the program to complex model structures. The simulation of multi-phase flow was theoretically prepared.

The tests during Phase III of the fracture system flow test were also focused on the in­situ hydraulic tests in the BK-section of the rock laboratory. Experiments to determine the permeability of jointed rock as a function of stress were carried out in the peripheral area of the laboratory gallery. The objective of the tracer tests was to determine the transport velocity of toxic substances in jointed rock.