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Disposal > Survey – Text Version > Asse Research Mine > Safety
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Safety
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Rock Mechanics
In the more than 100-year-old mine, problems with the stability of the mine have been occurring. These problems are due to the many cavities in the southern flank which have only been loosely backfilled and which reach closely up to the border between salt and adjoining rock, the so-called Röt-anhydrite.
Through yielding of the perforated southern flank, the rock strata move and tear open. Thus, clefts form in the brittle Röt-anhydrite and in the salt itself. Rock is ripped from the roofs – also referred to as hanging walls or raise stopes – and walls, the so-called pillars.
There are many places in the southern flank where the raise stopes between the chambers are already entirely broken.
Saline Water Intrusion
Groundwater can flow in porous rock and alongside existing faults. With increasing depth the groundwater becomes more saline. In salt rock it is generally saturated with salt. Therefore, it cannot dissolve more rock salt.
With the movement of the southern flank of the Asse and the salt barrier that has become cracked as a result, the saline groundwater can now also intrude into the mine. The saline water intrudes into the mine in a depth of 500 m to 600 m. Through loosened pillars and raise stopes it runs from level to level into deeper parts of the mine. The major part of it is collected in this chamber located in a depth of 658 m.
The loose salt material was covered with a canvas collecting the saline water. This again was covered with gravel so that it won’t be damaged by dropping salt chunks. The collected saline water runs over the canvas and is conveyed into a pool. From there it is pumped off.
Below the collecting chamber, the salt barrier is thicker and still intact according to the current state of knowledge. In the loosening areas between the chambers, however, cracks have formed. Via this path, water that has not been collected could reach the emplacement areas further down.
Backfilling
Already during potash winning until 1925, the potash excavations at the northern flank were backfilled with the moist residues originating from processing in the potash factory. From 1967 until 1978, radioactive waste was emplaced mainly in a depth of 750 m.
Until the beginning of the nineties, lower excavations in the southern flank were backfilled with salt from the mine itself. Starting in 1995, the major part of the southern flank was backfilled with material from the former Ronnenberg potash mine near Hanover. As the material is not compact after it has been blown into the cavities, it can only develop an increasing stabilising effect after the excavations have grown together further.
Swamp/Drum Accident
Already when emplacement operations started it was known that there is humidity underneath the ground of the emplacement level. In 1988, a so-called swamp was established in front of the emplacement chamber where saline solution gathers. It was stated that this solution is radioactively contaminated with caesium-137.
The solution comes from the neighbouring former potash excavations. The backfill material placed until 1925 contained humidity. It gets into the swamp through the loosened areas under the galleries. The fact that the solution is contaminated is explained through an accident on December 18, 1973.
Radioactive liquid has entered the soil and has radioactively contaminated it over an area of 250 square metres. It is also possible that single drums were not tight. After the accident, the ground had to be milled off. However, it is assumed that residual contamination has remained.
From 2005 until 2008, solution was removed from the swamp to ensure occupational safety for operational works in this area. The solution was pumped into transport casks and transported to a borehole. Through this borehole it was conveyed to a depth of 975 m. This was interdicted by the regulatory authority.
Swamp in front of Chamber Twelve
The level of radioactivity having occurred in the swamp over the years cannot be explained by the drum accident alone.
It must be assumed that the solution from the factory residues in the potash excavations also flows below emplacement chamber twelve. Thus, radioactive material can be taken up from the soil from solid waste from drums that have meanwhile corroded.
Possibly, radioactive waste that had not been completely solidified was emplaced, too. The radioactive material can, thus, get from the drums into the saline solution and into the swamp located in front of chamber twelve.
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