Asse II mine

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Asse II headframe

Coordinates: 52°08′38″N 10°38′32″E / 52.14389°N 10.64222°E / 52.14389; 10.64222

The Asse II pit (Schacht Asse II) is a former salt mine used as a deep geological repository for radioactive waste in the mountain range of Asse in district Wolfenbüttel in Lower Saxony, Germany.

History[edit]

Asse II was allegedly used as a research mine since 1965. Between 1967 and 1978 radioactive waste was placed in storage. The mine is operated by the German government, and was executed by the Helmholtz Zentrum München. Research was stopped in 1995; between 1995 and 2004 cavinates were filled with salt. After media reports in 2008[1][2] about brine contaminated with radioactive caesium-137, plutonium and strontium, politicians accused the operator of not having informed the inspecting authorities. On September 8, 2008, the responsible ministers of Lower Saxony and the German government concluded to change the operator. The new one, the Bundesamt für Strahlenschutz (federal office for radiation protection), will close the mine according to atomic law instead of mining law.[3]

Instability of the pit[edit]

Schacht Asse II at ca. 490m depth

No columns and struts are used in a salt mine. The extra stress in the remaining salt structure (the pit building) by constructing the chamber is handled in the capping mass. Plasticity effects are taken into account as they naturally occur in salt domes. Significant mechanical stress is built up between the surrounding diapirs and the artificial construction. The capping mass in Asse II moves 15 cm a year which undermines the strength of the construction.

Because of the high number of tunnel constructions and the decades of use, the deformation in Asse II has reached the state that the pressurised salt is losing its stability: "The supporting construction is softening by creep deformation, plasticity effects and local fractures from the ground pressure."[4] The Institut für Gebirgsmechanik (IfG) in Leipzig which has been monitoring Asse II since 1996 predicted in 2007 that, from the beginning of 2014, an increase in the loss of the load carrying capacity will result in an increased displacement of the capping mass.[5] The shifts may lead to an uncontrollable increase in water inflow and make the continued operation as a dry pit impossible.

In 1979 a report on the stability of the pit building was released by a working group under the leadership of HH Juergens[6] which describes the now imminent scenario of uncontrolled inflow from the capping mass in 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.

Water inflow[edit]

Tank trucks with salt brine from the mine (08.2009)

Water will always flow into a salt mine where the salt barrier in the surrounding structure is breached. This happens when the salt barrier is damaged in the drilling operation or by plasticity deformation of the salt resulting in cracks. Asse II is particularly threatened by water because the salt barrier is in some places only a few meters thick.

For the period 1906 to 1988 there were 29 documented water breaches. They were sometimes successfully sealed off, partly dry, sometimes negligible (less than 0.5 m³ per day). For the current operational security, they are meaningless.[7]

Between 1988 and 2008 32 new entry points were recorded. Part of the solution is explained as coming from the diapir in the south area. The solution is collected at 658, 725, 750 m and since 2005 at 950 m,[8] the inflow is currently (2008) 11.8 m³/Day.[9] The liquid is tested for radioactivity and if levels are passed pumped into a tank truck and transported to the abandoned K+S AG mines (Bad Salzdetfurth, Adolfsglück and Mariaglück)[10][11][12] The brine in Mariaglück was tested again at the end of 2008 with results for caesium-137 and tritium.[13]

The German mineral- and table water decree (Mineral- und Tafelwasserverordnung, 24.5.2004) is set at 120 milli-Bq/l U-233 or U-235 for drinking water.[14]

Inventory[edit]

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 pit all material was once again reviewed in August 2008:[15]

1) 125,787 drums of low level radioactive waste stored from 1967 to 1978 in various chambers at 750 meters depth. The containers are mostly drums with volumes from 100 to 400 liters or concrete vessels. The declared total activity at the time of the respective storage amounted to 1.8·1015  Bq. Around 50% of the containers came from the nuclear reprocessing plant of the former Forschungszentrum Karlsruhe, 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.[16]

2) 1293 containers with medium-level radioactive waste stored from 1972 to 1977 in the chamber 8a on 511-meter depth. Only 200-liter roll drums were allowed with waste fixed in concrete or bitumen. The declared total activity at the time of the respective storage amounted to 2.8·1015  Bq. About 97% of the packages (and thus over 90% of the total activity inventory of Asse II) originated from the reprocessing plant in Karlsruhe. A part of the Karlsruhe drums contained waste from the reprocessing plant itself and thus produced fissile material. With storage limits of 200 grams U-235, 15 g U-233 and 15 g Pu-239 per drum. These limits were not reached, the maximum values were 24 g U-235, 5.7 g Pu-239 and less than 1 g U-233 per drum on 511-meter.[17]

The stored radioactivity of 4.6·1015 Bq is not the only measure to evaluate, the largest part of the radiologically most effective and long-lived alpha radiation occurs in the low-level radioactive waste. Therefore, the low level active waste is of particular importance for the long-term security by creating the biggest problems. The medium-active waste contains mostly relatively short-lived radionuclides which is of minor importance to the long-term safety.

See also[edit]

References[edit]

  1. ^ German Leaks Raise More Nuclear Fears
  2. ^ Problems at Germany's Asse II Nuclear Waste Repository
  3. ^ Shaft ASSE II
  4. ^ Dreidimensionale gebirgsmechanische Modellrechnungen zur Standsicherheitsanalyse des Bergwerkes Asse. Institut für Gebirgsmechanik GmbH, Leipzig, 2006.
  5. ^ Gebirgsmechanische Zustandsanalyse des Tragsystems der Schachtanlage Asse II – Kurzbericht. Institut für Gebirgsmechanik GmbH, Leipzig, 2007.
  6. ^ Hans-Helge Jürgens, Katrin Hille: Atommülldeponie Salzbergwerk Asse II - Gefährdung der Biosphäre durch mangelnde Standsicherheit und das Ersaufen des Grubengebäudes, Braunschweiger Arbeitskreis gegen Atomenergie, 2. Auflage, März 1979
  7. ^ Statusbericht des Niedersächsischen Ministeriums für Umwelt und Klimaschutz über die Schachtanlage Asse II, Seite 11. Niedersächsisches Ministerium für Umwelt und Klimaschutz, Hannover, 2008.
  8. ^ Die Asse Chronik
  9. ^ Statusbericht des Niedersächsischen Ministeriums für Umwelt und Klimaschutz über die Schachtanlage Asse II, Seite 12. Niedersächsisches Ministerium für Umwelt und Klimaschutz, Hannover, 2008.
  10. ^ Zutritt und Verbleib der Salzlösung aus dem Deckgebirge (German)
  11. ^ Hildesheimer Allgemeine Zeitung vom 16. August 2008, S. 17.
  12. ^ Halbjahresbericht über den Stand der BMBF-Stilllegungsprojekte und der vom BMBF geförderten FuE-Arbeiten zu „Stilllegung/Rückbau kerntechnischer Anlagen“. Forschungszentrum Karlsruhe, 2007.
  13. ^ Mariaglück Testresults
  14. ^ Radioactivity in drinking-, mineral- and spa waters
  15. ^ Statusbericht des Niedersächsischen Ministeriums für Umwelt und Klimaschutz über die Schachtanlage Asse II, Seiten 93–128. Niedersächsisches Ministerium für Umwelt und Klimaschutz, Hannover, 2008.
  16. ^ Schachtanlage Asse – Befragung früherer Mitarbeiter. Helmholtz Zentrum München, 2008.
  17. ^ Die Asse Chronik – Vom Umgang mit Atommüll in Niedersachsen. Fraktion von Bündnis 90/Die Grünen im Niedersächsischen Landtag, 2008.

External links[edit]