Asse II mine

The Asse II mine (Schacht Asse II) is a former salt mine used as a deep geological repository for radioactive waste in the Asse Mountains of Wolfenbüttel, Lower Saxony, Germany.
History.
The Asse II mine was developed between 1906 and 1908 to a depth of . Initially extracting potash, the mine also produced rock salt from 1916 to 1964. Potash production ceased in 1925.
Between 1965 and 1995, the state-owned Helmholtz Zentrum München used the mine on behalf of the Federal Ministry of Research to test the handling and storage of radioactive waste in a repository. Between 1967 and 1978 low-level and intermediate-level radioactive waste were emplaced in 13 chambers in the Asse II mine. Two chambers are located in the middle part and ten in the southern flank of the mine at depths from below surface. Between 1972 and 1977, exclusively medium-level radioactive waste was emplaced in a chamber on the level. Research was stopped in 1995; between 1995 and 2004 cavities were filled with salt.
After media reports in 2008 about brine contaminated with radioactive caesium-137, plutonium and strontium, politicians accused the operator, Helmholtz Zentrum München – German Research Center for Environmental Health, of not having informed the inspecting authorities. On 8 September 2008, the responsible ministers of Lower Saxony and the German government replaced the operator with Bundesamt für Strahlenschutz (BFS) - the Federal Office for Radiation Protection.
In April 2017, operator responsibility for Asse II was transferred from BFS to the Supervisory Board of the Bundes-Gesellschaft für Endlagerung mbH (BGE), under the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety.
Instability of the mine.
Typically salt mining is structurally unsupported. Stresses produced in the remaining salt structure during construction of the mine voids are accommodated in the overlying rock. Plasticity effects are taken into account as they occur naturally in salt domes. Significant mechanical stress is built up between the surrounding diapir and the mine construction. The overlying rock mass in Asse II moves per year, undermining the strength of the mine construction.
Because of the large number of tunnels and chambers, and the decades of use, deformation in Asse II has reached a state where the pressurised surrounding salt is losing its stability: "The supporting construction is softening by creep deformation, plasticity effects and local fractures from ground pressure."
In 1979 a report on the stability of the mine was released by a working group under HH Juergens, which describes an imminent scenario of uncontrolled plastic flow from the surrounding rock on the southern flank resulting in the subsequent loss of the load carrying capacity. The manager of Asse II in 1979 and his advisers categorised this report as "unscientific" and declared that there were no stability problems.
In 2007 the Institut für Gebirgsmechanik (IfG) in Leipzig, which had been monitoring Asse II since 1996, predicted that an increase in the rate of loss of load carrying capacity would result in an increased displacement of the surrounding rock. The shifts would lead to an uncontrollable increase in water inflow and make continued dry operation impossible.
Water inflow.
A significant inflow of water and a subtle loss of mechanical stability may jeopardise the underground mine integrity – the site is in danger of collapsing and becoming flooded.
For the period 1906 to 1988, when Asse II was an operational salt mine, there were 29 documented water breaches. They were sometimes successfully sealed off, partly dry or sometimes with negligible inflows (less than per day).
Between 1988 and 2008 32 new entry points were recorded. In 1996, the BFS notified the Bundesumweltministerium that there was a risk of severe radioactive contamination if the mine ran full of water and that further investigation was urgently required.
Most of the brine influx is concluded as coming from the diapir in the southern part of the mine. The brine is captured before it comes in contact with the storage drums, at the levels and, since 2005, at the level. The 2008 influx was per day. The liquid is tested for the radionuclide caesium-137. All measured values have been below the detection limit. The liquid is also tested for tritium. The weighted mean concentration is about 100 Bq/litre, which is the value that must be present in accordance with the European drinking water standard (and slightly more than radon levels in Bad Gastein radon spa in Austria). The brine is pumped into a tanker and transported to the abandoned K+S AG mines (Bad Salzdetfurth, Adolfsglück and Mariaglück) The brine in Mariaglück is also tested for caesium-137 and tritium.
Inventory.
Asse II is licensed for the storage of intermediate radioactive waste (LILW-LL, Long lived) and low level waste (LILW-SL, Short lived), defined as waste without significant heat generation. After public speculation about the presence of radioactive high level waste in the mine all material was once again reviewed in August 2008:
However the level of the waste was determined by the measurable amount of radiation outside of the container, not by the actual contents of the vessel. Because of this it must be assumed that the containers with concrete shielding also contained medium level waste, raising their number from 1293 to 16.100. Some of these concrete vessels emitted radiation above the permitted level and had to be stored in special metal shielding.