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.
The Asse II mine was developed between 1906 and 1908 to a depth of 765 metres (2,510 ft). 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 725 m to 750 m below surface. Between 1972 and 1977, exclusively medium-level radioactive waste was emplaced in a chamber on the 511 m 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 - the Federal Office for Radiation Protection. In 2009, Greenpeace published a letter of the Bundesamt für Strahlenschutz to the German Bundesumweltministerium from 1996 in which the former had warned the latter that there is the risk of severe radioactive contamination if the mine runs full of water and that further investigation is urgently needed.
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 15 cm 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.
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 there were 29 documented water breaches. They were sometimes successfully sealed off, partly dry or sometimes with negligible inflows (less than 0.5 m³ per day).
Between 1988 and 2008 32 new entry points were recorded. 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 658, 725 and 750 metre levels and, since 2005, at the 950 metre level. The 2008 influx was 11.8 m3 (3,100 US gal) per day. The liquid is tested for the radionuclide caesium-137. All measured values have been below the detection limit. The liquor 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. 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.
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:
- 125,787 drums of low level radioactive waste stored from 1967 to 1978 in various chambers at the 750 metre level. The containers are mostly drums with volumes from 100 to 400 litres or concrete vessels. The declared total activity at the time of storage was 1.8·1015 Bq. Around 50% of the containers came from the former Forschungszentrum Karlsruhe nuclear reprocessing plant, 20% from nuclear power plants and 10% from the former Jülich Research Centre. The containers typically included mixed and laboratory waste, rubble, scrap, filter residues and combustion residues. Liquids such as evaporator concentrates, sludges, oils, resins and solvents had to be bound as solids. According to some former employees barrels of liquid waste were accepted in the early days of storage.
- 1,293 containers with medium-level radioactive waste stored from 1972 to 1977 in Chamber 8a at the 511 metre level. Only 200-litre roll drums were permitted with waste fixed in concrete or bitumen. The declared total activity at the time of storage was 2.8·1015 Bq. About 97% of the packages (over 90% of the total activity inventory of Asse II) originated from the Karlsruhe reprocessing plant. Some of the Karlsruhe drums contained waste from the reprocessing plant itself, and thus fissile material. Storage limits per drum were 200 grams U-235, 15 g U-233 and 15 g Pu-239. These limits were not reached. Maximum values per drum were 24 g U-235, 5.7 g Pu-239 and less than 1 g U-233 on the 511 metre level.