P. Kamlot’s research while affiliated with Vrana GmbH (Germany) and other places

The Asse II mine is an over 100 years old salt mine in a steep inclining structure in a salt ridge in which radioactive waste was embedded from 1967 to 1978. The mining chambers were excavated leaving pillars and stopes stable only in a short-term sense. Because the upper chambers at the southern flank are located very close to the neighboring overburden rocks, a persistent brine inflow exists since 1988. According to the Atomic Energy Act (Lex Asse) the mine shall be closed after complete retrieval of all radioactive waste. But, this will be done only if the geomechanical and radiation conditions allow a safe removal and no brine inflow escalation takes place. Because of the time period of about two decades until start of the removal, mine stabilization using sorel-concrete is of the utmost importance. IfG Leipzig has been involved in the geomechanical assessment of the Asse mine for about 19 years. Aiming at simulation of the geomechanical behavior of the pillars and stopes, different modeling concepts and constitutive laws have been developed. After explanation of the site conditions, two 3D-models of half of the mine are described and the validity is proven on the basis of the data of mine surveying. The models are used for evaluation of the current geomechanical state and for prediction of the further development.

Since 1 January 2009, the Asse II mine is subjected to the provisions of nuclear law. The new operator, the Federal Office for Radiation Protection (BfS), has to ensure the safe operation and closure of the nuclear facility. At the beginning of 2010, the decision for retrieval of all radioactive waste was made as the best closure option in comparison to two others. Today, the conditions for the retrieval such as state of the barrels, radiation and chemo-toxic exposure, state of the chambers and boundary conditions for the technical handling are investigated. For this purpose, two emplacement chambers will be drilled into exemplarily. The decision for retrieval was not declared as final one and there are several reservations. Only if the geomechanical and radiation conditions let expect a safe removal and no brine inflow escalation takes place, all rooms shall be opened. Furthermore, on account of the permanent risk of brine inflow escalation, an emergency concept was developed with included prevention, emergency preparation and emergency measures. The construction of drift sealing elements is a main element of the emergency plan and aims at reinforcement and permeability reduction in the operation phase and fluid flow regulation in the post operation phase (after the emergency caused brine filling). In the article, the most important elements for performance assessment of the drift seal's functionality are presented: The used Sorel-concrete must own a sufficient strength, stiffness and low permeability. Special experiments for determination of the long-term mechanical strength are depicted. Because the EDZ-generation and its permeability are of utmost importance for the tightness of a drift plug, a visco-elasto-plastic constitutive law for calculation of softening and dilatancy in the contour is introduced, calibrated and using a dilatancy-permeability relation the qualification of a special site for testing a pilot seal is proofed. The in-situ experiment lasted 7.5 years and revealed that even at a brine pressure application more than 2 MPa higher than the former (in dry state) generated radial (minimal) stresses a sufficient low permeability without any damaging and tightness loss in the contour zone could be reached. So far, the pilot test was a unique project and is of vital importance for understanding of hydro-mechanical coupled processes in rock mechanics.

Since 1 January 2009, the Asse II mine is subjected to the provisions of nuclear law. The new operator, the Federal Office for Radiation Protection (BfS), has to ensure the safe operation and closure of the nuclear facility. At the beginning of 2010, the decision for retrieval of all radioactive waste was made as the best closure option in comparison to two others. Today, the conditions for the retrieval such as state of the barrels, radiation and chemo-toxic exposure, state of the chambers and boundary conditions for the technical handling are investigated. For this purpose, two emplacement chambers will be drilled into exemplarily. The decision for retrieval was not declared as final one and there are several reservations. Only if the geomechanical and radiation conditions let expect a safe removal and no brine inflow escalation takes place, all rooms shall be opened. Furthermore, on account of the permanent risk of brine inflow escalation, an emergency concept was developed with included prevention, emergency preparation and emergency measures. The construction of drift sealing elements is a main element of the emergency plan and aims at reinforcement and permeability reduction in the operation phase and fluid flow regulation in the post operation phase (after the emergency caused brine filling). In the article, the most important elements for performance assessment of the drift seal’s functionality are presented: The used Sorel-concrete must own a sufficient strength, stiffness and low permeability. Special experiments for determination of the long-term mechanical strength are depicted. Because the EDZ-generation and its permeability are of utmost importance for the tightness of a drift plug, a visco-elasto-plastic constitutive law for calculation of softening and dilatancy in the contour is introduced, calibrated and using a dilatancy-permeability relation the qualification of a special site for testing a pilot seal is proofed. The in-situ experiment lasted 7.5 years and revealed that even at a brine pressure application more than 2 MPa higher than the former (in dry state) generated radial (minimal) stresses a sufficient low permeability without any damaging and tightness loss in the contour zone could be reached. So far, the pilot test was a unique project and is of vital importance for understanding of hydro-mechanical coupled processes in rock mechanics.