Geowissenschaftliche Probleme bei der Endlagerung radioaktiver Substanzen in Salzdiapiren Norddeutschlands

In the Federal Republic of Germany disposal of low-level radioactive waste is practiced in rock salt of the salt dome Asse since years. Projects also exist for the disposal of high-level waste from nuclear reactors in salt domes. The feasibility of the “evaporite concept? has to be founded on some basic principles of the origin and metamorphism of marine salt deposits. The German Zechstein salt deposits (Permian) were formed about 230 million years ago. Since that time the salt formations have had a chequered history: 1. Solution metamorphism and thermal metamorphism have altered the primary chemical and mineralogical composition of salt rocks in the presence of unsaturated solutions and/or temperatures up to 100? C and more. 2. Mechanical deformation and dynamic metamorphism processes have preserved the chemical and mineralogical composition of the evaporites. Solution metamorphism: The minerals halite, sylvite, and carnallite are very sensitive to subsequent penetrating unsaturated solutions. Very important in this context is the origin of specific equilibrium solutions and mineral associations. Primary salt rocks (e. g. carnallite rock) have been altered to secondary mineral associations (e. g. Hartsalze and sylvite rocks of the potash seams Thüringen, Hessen, Stassfurt). Most of the solutions which infiltrated the salt deposits from outside are later squeezed out into the adjacent rocks. Geochemical and physicochemical processes of the solution metamorphism are not limited to the geologic past. They are also efficient at the present time as a result of deformation processes in partially extracted salt deposits, for example. Solution metamorphism processes are also possible in the future during the period of about one million years which is necessary for the disposal of high-level radioactive waste in salt domes. A concept which is based upon observed properties of salt formations and with secure scientific foundation must guarantee no radioactive contamination of salt solutions in the case of long time contacts between radionuclides and brines. An excellent method for the immobilisation of radioactive waste seems to be production of a synthetic igneous rock system SYNROC (Ringwood, 1978). In such a rock the radionuclides are fixed in the lattices of minerals. Additional safety barriers which prevent the entry of radionuclides from the geologic barrier into the biosphere are the metallic alloy Ni₃Fe, for waste containers and layers of basalt and corundum. Borosilicate glasses are not suitable for the safe immobilisation of radionuclides because the glasses readly devitrify in contact with solutions and steam at elevated pressures and temperatures. Thermal metamorphism: The heat-generation of high-level waste produces zones of different temperatures in the evaporites of the salt domes. The salt hydrates kainite and carnallite react at relative low temperatures (> 072? C and > 80? C) and form solid phases with less hydrated compounds and equilibrium solutions. For example, effects of thermal metamorphism are to be expected in the case of influencing higher temperatures upon carnallite rocks which were squeezed into parts of rock salt during the geologic past. Increasing temperatures generated by radionuclides must be limited in carnallite rocks to 30 %–50 % of the recent rock temperatures in salt domes. A difference of 10? C should be guaranteed between the temperature zones around the high-level waste and the beginning of carnallite dehydratation at 80? C–85? C under normal pressure. Dynamic metamorphism: Dynamic metamorphism is concerned mainly with isochemical and isophase recrystallizations. The original thickness and composition of evaporites can be altered by the property of plasticity and by creep deformations of salt minerals and rocks. Solutions (e. g. pore solution) and increasing temperatures lead to the effects of dynamic metamorphism. A concept which is based upon observed properties of salt formations and with secure scientific foundation must guarantee temperatures ≦ 100? C in the vicinity around the high-level waste containers. Only for temperatures up to 100? C are relevant geologic data about the plastic deformation of rock salt available in the German salt deposits. The effects of salt rock deformation with increasing temperatures > 100? C could be examined only by laboratory experiments. It seems to be a considerable uncertainty and a risk to develop experimental deformation models for salt rocks valid for a time of 10²–10³ years. The present program in the Federal Republic of Germany for a geologic disposal of radionuclides in evaporites is sometimes characterized as the best concept of the world (e. g. v.Weizsäcker, 1978). For such a general statement no geoscientific foundation is available today. Alternative geologic disposal programs seem to be more acceptable in view of their geoscientific and technical base (e. g.Ringwood, 1978).