Technischer Bericht NTB 86-26

Überlegungen zur Ausdehnung kristalliner Wirtgesteine für die Endlagerung radioaktiver Abfälle in der Nordschweiz

Structure of the bedrock of Northern Switzerland

The crystalline bedrock formation underlying the North of Switzerland is exposed in the southern part of the Black Forest. Hence, conclusions can be drawn concerning the expected surface area distribution of the various types of rock on the basis of geological investigations in the Black Forest. This study covered an area of about 1,100 square kilometers, for which the surface area distribution of the main lithological groups (granite; gneiss, micaschist, anatexite; rhyolite porphyry veins) has been mapped. Three different statistical evaluations were made of these data. An average surface area of 59 to 71 percent was calculated for granite. Gneiss accounts for 25 to 38 percent of the area. A similar percentage distribution may also be assumed for the crystalline bedrock of Northern Switzerland. Predictions of the extent of the crystalline rock types in the bedrock of Northern Switzerland were made on the basis of both the study in the southern part of the Black Forest and the information obtained from exploratory boreholes drilled by Nagra. Faults, which in turn delimit block formations, are defined on the basis of their depth and mapped lengths. In the southern part of the Black Forest many of the smaller geological weaknesses appear to be kakiritic zones, which have mappable lengths of several hundred meters and widths varying from 0.5 to 2 meters. These small disturbances are characterized by offsets which are often less than a meter. Hence, the entire area within the limits of block formations can be described as tectonically «stable».

Hydraulic tests to determine the water flow systems

A characteristic of the crystalline rock formation is that the principal water movements are limited to discrete joints and veins. This was clearly seen in the deep boreholes drilled by Nagra in Northern Switzerland, where in the crystalline bedrock water usually flows into borehole only at discrete sites. In order that water flow can occur over longer distances in the crystalline formation, the joints must form a connected joint network system. The most varied type of evidence concerning the water flow systems was obtained from the deep boreholes. The geological information obtained from the drilled core samples, together with the results of the hydraulic and geophysical investigations in the boreholes, makes it possible to distinguish between different water flow systems. The localization of the discrete sites at which water flows into the borehole was carried out by means of fluid-logging. Packer tests were used for the hydraulic characterization of the flow systems discharging into the borehole. Long-term observations were made using a multi-packer system.

Geological characterization of the water flow systems

The drilling core samples taken in the region of the sites where water flows into the borehole were subjected to detailed petrographic analysis. It was apparent that the water flow channels in the boreholes drilled by Nagra are associated with the following elements and structures:

  • Open, macroporous to drusy, hydrothermal mineral veins, joints and dykes
  • Aplitic and pegmatitic dykes with open joints
  • Open joints in gneiss.

The water-bearing joints in granite have bounding zones of hydrothermally transformed rock. These zones show a more open microporosity, as indicated by the results of mercury pressure porosimetry. Hence, during radionuclide transport, increased retention of the radionuclides can occur as a consequence of matrix diffusion and sorption in these porous borders.

Effects of disturbed zones on water movements

Using the local hydrodynamic model FEM301, the effect of deep-reaching, hydraulic, disturbed zones on water movements in the geosphere was calculated. The results show only negligible changes in the upper crystalline formation. On the other hand, at the base of the 500 meter deep middle crystalline formation, significant effects can be detected. A lowering of the hydraulic potential is revealed running NW-SE along the Eggberg and Vorwald fault zones. Refined two-dimensional modelling using the FREESURF program with 4 scenarios was also carried out. In all of the scenarios a minimal distance of 200 meters between the repository and the disturbed zone was assumed; they differ with respect to the relative location of the disturbed zone to the repository. The case in which the disturbed zone lies directly above the repository proved to be the most unfavourable for safety reasons. The total water flow through the repository in this case differs from the case in which there are no disturbed zones by 7 percent. The average flow rates along the flow channels are up to 60 percent higher, the flow channels are up to 2.5 times shorter.

The effects on the individual doses

Calculations pertaining to the diffusion of radionuclides released from a repository enable the effects of hydraulic, disturbed zones on the water movements to be expressed in terms of the potential exposure to radiation of a population in the future. For the purposes of the calculations, it was assumed that the total transport path from the repository to the disturbed zone or the upper crystalline formation passes through dyke rocks. On account of the lower sorption capacity of rocks along open joints in dykes as compared with those in kakiritic zones, this is a very conservative assumption and leads to higher individual doses for humans. No additional transport delays were supposed for transport in the hydraulically active, disturbed zone or in the upper crystalline formation. Nonetheless, the calculations showed that the effects of hydraulically active disturbed zones are of minor importance compared with other parameters. Even for the most pessimistic estimates, the doses are always well below the guideline value of 10 mrem per year.