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After the end of rock salt and potash salt extraction in the Asse II mine located in Northern Germany low- and medium-level radioactive waste was emplaced. The waste volume was around 47,000 m³ and the work was carried out from 1967 to 1978. The occurrence of solution inflows and the difficulty of predicting their progression led to the decision of...
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... reason, a proof must be carried out, which in principle includes a comparison of the stresses caused by rock convergence and the build-up of a solution column. Particularly sensitive is the EDZ that is characterized by more or less cross-linked cracks and fissures. Likewise in this case, the pilot structures provided important information (cf. Fig. 10). They showed that after just a few years the radial stresses did not correspond to the original rock pressure, but are significantly higher than the fluid pressure expected at the respective depth. For example, stresses between 13 and 14 MPa are currently being determined at so-called pilot flow barrier A1 on the 950 m level. This ...
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... such as technical equipment an extensive site-specific survey starts. It includes, for example, mine surveying of the sites, geological mapping, and the performance of geotechnical measurements. Now the barrier construction can be planned in detail. In particular, the extent of the re-cutting work and the dimension of the barrier are determined. Fig. 11 shows a visual inspection of a barrier site during this working phase. Then, drilling of the backfill and vent boreholes and the shaping of the profile can be ...
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... from re-cutting the rock surface to concreting the Sorel concrete A1. Experience shows that a period of three months must not be exceeded in order to limit the extent of the EDZ to the required extent. The construction of the formwork walls for the concreting of the abutments is always based on a proof of stability considering static calculations (Fig. 12). The work ends with a control of final acceptance. The magnesium oxide and the aggregate are pneumatically conveyed to underground mix-pump-units. To guarantee undisturbed production and backfilling processes, tests of the technical equipment and the Sorel concrete are necessary. Quality specifications have been agreed with the ...
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... mixtures and hardened test specimens ensure the conformity with the requirements [13]. For this purpose, two laboratories were installed at the Asse II mine. An above-ground laboratory is primarily intended for acceptance tests, and an underground laboratory for rheological tests. In the underground laboratory, measurements of the temperature (Fig. 13), the volume increase during hardening, changes in density, and the swelling pressure development of the Sorel concrete A1 are investigated (Fig. 14). All test methods meet the requirements of maximum accuracy and sensitivity, in the sense that measured values change significantly when the properties of the concrete change. In addition, ...
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... II mine. An above-ground laboratory is primarily intended for acceptance tests, and an underground laboratory for rheological tests. In the underground laboratory, measurements of the temperature (Fig. 13), the volume increase during hardening, changes in density, and the swelling pressure development of the Sorel concrete A1 are investigated (Fig. 14). All test methods meet the requirements of maximum accuracy and sensitivity, in the sense that measured values change significantly when the properties of the concrete change. In addition, the test procedures were optimized according to the characteristics of the concrete. In this regard, BGE TECHNOLOGY GmbH used the comprehensive ...
Citations
... As mentioned before, seals are not only required in deep boreholes, furthermore in mine shafts, underground drifts and exploration boreholes, seals need to be planned and constructed. The procedure and experience in the field of drift seals in the Asse II mine is presented by Engelhardt et al. [1] for example. In principle, the approach from the sealing of underground drifts can be adapted for the planning of seals in deep boreholes. ...
Deep borehole disposal (DBD) enables the disposal of radioactive waste in deep rock formations that have favourable properties for long-term isolation. The borehole is the preferred pathway for fluids that could mobilize radionuclides and for contaminated fluids. For this reason, seals have to separate the disposal zone from the areas near the earth's surface. The required seal performance depends on the waste characteristics and the isolation capacity of the host rock. In addition, the safety concept can take into account the isolation effect of the waste canisters or sealing measures that are carried out in the disposal zone. Stagnant groundwater conditions in the deep rock areas can also be relevant. Regardless of individual concepts, it can be assumed that seals as engineered barriers are intended to restore the integrity of the host rock. Based on this fact, a methodical approach was developed for planning sealing systems. This approach takes account of the experience that BGE TECHNOLOGY GmbH has gained in the course of the planning and construction of seals in mine shafts, underground drifts and exploration boreholes (e.g. [1]). The final seal system is derived stepwise, taking into account the geological framework conditions. Experiences show that contact zones, damaged rock zones and materials that are not corrosion-resistant impair the function of a seal. As a result, these areas, such as casing and its cementation, have to be removed; furthermore damaged rock areas are recut shortly before the emplacement of the sealing material. If necessary, special measures such as rock injections need to be performed to improve the quality of the host rock and to seal flow paths. The use of steel reinforcement within the seal is not permitted due to potential corrosion and backfill pipelines are generally pulled so that they do not remain in the seal. Seals need to fulfil their task for a very long time, due to that there is a high probability that a range of mechanical, thermal and chemical processes could affect their function. In order to take into account uncertainties in the prognosis, a system of sealing elements should be set up. The elements have to work in different ways, so that the requirement of redundancy and diversity is met. Sealing materials that have a positive influence on the tightness of the contact zone and can contribute to the sealing of damaged rock zones are to be preferred. These include, for example, bentonites, which apply a permanent pressure to the rock surface, or bitumen (asphalt), which can penetrate cracks. Materials based on silicate solutions (water glass) such as geopolymers are also advantageous because they react with saline solutions and have a sealing effect. In general sealing materials can be divided into materials that are self-supporting or that require additional elements to ensure their function. These can be abutments or filter elements. Abutments or support elements are load-bearing elements that ensure the positional stability of the seal and can prevent its erosion. Filter elements (filter frits) prevent erosion or suffusion of clay particles and homogenize the pressure or stresses on the face of the seals or abutments. Moreover, backfill (ballast) can be used to avoid or delay a pressure-build-up due to high porosity and the guarantee the position of seal elements. This results in a basic design of a sealing system with different functional elements, which can be simplified after the selection of the materials and the respective requirements. The choice of materials is primarily based on the structural properties, such as long-term stability and the effect of the materials on the solubility of radionuclides. Chemical reactions that have a positive influence on the properties of the elements are allowed or desired. Thus, elements can control the chemical environment, i.e. to reduce radionuclide solubility or to fix mobilized radionuclides. They can serve as chemical barriers.
