Gorleben salt dome

The salt dome Gorleben is a possible deep geological repository in a salt dome in Gorleben in the Lüchow-Dannenberg district in the far north-east of Lower Saxony for low, medium and highly radioactive waste.

Site selection

At the end of 1973 the search began for a final salt dome storage. The plan was a repository for all types of radioactive waste in a salt dome. 24 salt domes were considered. The federal government asked the company KEWA (Kernbrennstoff-Wiederaufarbeitungs-Gesellschaft) to search for a location.

On July 1 1975 the KEWA proposed three sites in Lower Saxony for further investigation: the salt domes Lutterloh, Lichtenhorst and Wahn. The location Gorleben is not advantageous in this category. The investigation of the sites began with drilling holes. In November 1976 the Lower Saxony cabinet called for the Federal Government to examine the three sites, so they could designate a location.

In February 1977 the Lower Saxony cabinet designated finally the Gorleben salt dome as a single location for the repository and waste disposal center. The choice of Gorleben was the work of a projectgroup which investigated 140 salt domes within a few months. Of these 140 salt domes only four were proposed: Lichtenhorst, Wahn, Maria Glück (Höfer) and Gorleben. Of these Gorleben was chosen. The selection criteria were among others, land use, population density, radiation and repository geology. Today it should be noted that geoscientific arguments played only a small role. For example, the salt dome Höfer (Maria Glück) is too small for a repository but reached the last round of selection. The ultimate decision for Gorleben according to the former Lower Saxony Prime Minister Ernst Albrecht was mainly for structural reasons for the economic development of the zone.

On hinsight it became clear that the original and methodologically correct approach, to investigate three potentially suitable salt domes in parallel and a comparative assessment of the results were abandoned by the choice for Gorleben. Security-oriented geoscientific arguments were ingnored in the evaluation of Gorleben. The lack of transparency and accountability for the decision for Gorleben is still one of the reasons why the opposition to the location is so vehemently.

Surface exploration

The surface exploration of the Gorleben site began in April 1979 and lasted until 1983. The investigation covered drilling 44 boreholes in the salt table, geophysical investigations, including reflection seismic measurements, hydro-geological studies, approximately 500 outcrop- and groundwaterlevel drillings, four deep boreholes to approximately 2,000 m at the edges of the salt stock, two shaft boreholes to approximately 1,000 m depth to confirm the starting points of the selected slot, a seismic networkstation to monitor earthquake activity as well as numerous other studies.

The exploration results and their evaluation were carried out in two reports by the Physikalisch-Technische Bundesanstalt (1983) and the Federal Office for Radiation Protection (1990) and are summarized below.

It says (PTB 1983): "An initial assessment of the capping mass in terms of its barrier function for potential contaminated ground water shows that the occurring clay sediments in the central areas of the salt stock Gorleben lack the indication that it can function as a permanent barrier to prevent contamination of the biosphere."

This assessment still applies today and is supplemented by other negative site characteristics, such as anticipatory selective subrosion and the short time that is needed for the groundwater from the salt dome to reach the biosphere.

The expectations that one of the capping masses could be used as a barrier were not fulfilled. After that there was a change in safety philosophy: the importance of the capping mass as a barrier against the spread of radionuclides has been withdrawn and in return the salt dome alone is used as the crucial barrier. Based on this modification of the safety philosophy the underground exploration started.

Underground Exploration

The drilling for shaft 1 started in 1986 and in October 1996 the punch line was created between shafts 1 and 2 on the 840 m sole. The main objective of the underground exploration is the detection of rocksalts like topanhydrite which are required for the final storage. The exploration area 1 is largely open and under investigation. Comprehensive geoscientific and geotechnical investigations as well as mountaineering technical measurements and experiments were done. The results so far can be classified according to the Federal Institute for Geosciences and Natural Resources (BGR 1998) as followed:

• In the border zone between the core and the southern salt dome flank are heavily deformed layers and their thickness is reduced. Areas of missing topanhydrite and accompanying layers. The topanhydrite is decomposed into individual plates. Larger isolated solutions and gas reserves are possible in the topanhydrite layers.

• Between the core zone and the northern salt dome flank the borderzones are laid in folds, and the layers involved are still largely in its original sedimentary form. The topanhydrite is broken, but not decomposed into individual layers.

• Core zone of the salt stocks with topsalt: Here is a simple solution without solutions and gas reserves.

• In the boreholes located in areas near the border area of Zechstein 2/Zechstein 3 intensive folding of the layers with high reduction of thickness occurs. In the border region Staßfurt to Zechstein 3 are partially disorders healed by secondary rock salts. Limited solution and gas can occur in the areas of disturbance but without connections to the salt levels.