The mine

Date

18/02/2010

As the Asse mine was formerly a salt mine, it was not intended right from the start for use as an experimental repository for radioactive waste. The original goal was to make use of the salt deposit in the Asse as effectively as possible. In the course of this, chambers were mined that reach up to the outermost edge of the salt layer. A look at the Asse's geology explains why this is a problem for the mine’s stability today.

Geology: Rock Structure

The sectional drawing of the mountain shows how close the mining chambers come up to the overlying rock. Source: Helmholtz Zentrum München

The Asse-Heeseberg mountain ridge consists of different Zechstein (approximately 230 million-year-old salt rock) sequences. One property of salt rocks is that they intrude into weak zones of the overburden (such as fault zones) when overlapping with overlying strata. In the case of the Asse, they were upfolded to a saddle below the younger overlying rock and are therefore referred to as a “saddle structure”. The core of this saddle consists of the older Staßfurt rock salt (dark blue). Above this is the Staßfurt potash seam (carnallitite), which is likewise a salt rock (pink). Further strata above are the younger Leine rock salt (mauve) as well as Anhydritmittelsalze (light blue), a mixture of rock salt and the mineral anhydrite.

Overlying and adjoining rock surrounding the salt structure, mainly consist of different Bunter layers (mudstone, sandstone, and limestone) as well as of Muschelkalk and Keuper which developed later in the earth's history. Directly above the centre of the saddle, the rock layers are interrupted and displaced against each other: The overlying rock is disrupted here. Besides, there are numerous faults in the south-western flank of the saddle (left part of the figure) which can be envisaged as cracks in the rock. In the figure they are displayed as narrow broken lines.

The Mine: Stability Problems

During the period of salt production, numerous chambers were driven in the Asse II mine that lie closely on top of each other in the south-western flank of the mine. In order to waste as little of the resource as possible, part of the salt was mined immediately up to the adjoining rock. At some places, the chambers in the salt layers reach up to five metres to the adjoining rock. Also, the chambers are sometimes only a few metres apart from each other.

The very large total volume of open drifts and chambers and the closeness of the chambers to the adjoining rock cause the major problem in the Asse today. The natural movement of the rock presses the chambers together, which causes the salt rock and the adjoining rock to break up. That way, clefts developed through which the groundwater saturated with salt in the area of the salt structure, can flow. Since 1988, these influent waters have been entering the mine in the upper part of the southern flank, in depths between about 500 and 575 metres. Moreover, the chambers themselves are becoming instable as a result of the rock movement. Parts of the intermediate roofs between the chambers have already collapsed. Without additional stabilisation measures, the whole mine threatens to collapse. Uncontrolled penetration of groundwater must be feared as well. In this case, inflowing freshwater could trigger further dissolution processes.

Tests to Stabilise the Mine

To slow down the shifting of the rock and prevent further breaches, the former operator of the Asse mine, Helmholtz Zentrum München, backfilled the chambers in the southern flank of the mine with salt grit (fine-grained salt) through pneumatic stowing. Approximately 2.2 million tons were blown into the cavities between 1995 and 2003. As the material is relatively loose - the pore volume is approximately 40 percent – , it has slumped in the course of time due to its own weight, resulting in the generation of cavities at the roof, so-called “roof clefts”.

Despite this backfilling with salt grit, the movement of the rock proceeds continuously. Currently, the rock moves approximately 130 millimetres per year. Therefore, new clefts can generate anytime through which more water could get into the mine. In the extreme case, the inflow of groundwater from the overlying rock could increase so dramatically that the mine could drown before the closing measures had been completed and, thus, groundwater could come into contact with radioactive waste.

In order for the salt grit to finally support the mine effectively and prevent such an inflow of groundwater, it has first to be pressed together through the rock movement to the extent that these cavities have closed, too. After that, it can hardly be distinguished from the surrounding salt dome and is of a similar stability. However, years will pass until this will have happened. To intensify and bring forward the stabilising effect, the Federal Office for Radiation Protection (BfS) plans to backfill this roof cleft with a special type of concrete. Thus, the cavity volume will be reduced and bearing capacity will be accelerated. Further options to stabilise the mine, e.g. through injections, are being researched. Successful implementation of these measures is the requirement to conclude a regular decommissioning procedure according to nuclear law.