Clearing out Asse 2

The Asse 2 mine near Braunschweig, lower Saxony, produced potash from 1908 until 1925 and rock salt from 1916 to 1964 from a salt dome. It is the last remaining dry mine in the area; both the Asse 1 potassium salt mine and the Asse 3 rock salt mine nearby were flooded by unmanageable inflow of water in the first decades of the last century. After the Asse 2 mine closed, the German government bought it with the purpose of installing a pilot plant for nuclear waste disposal. The mine had a 765 m deep shaft, which later was extended to 950 m depth in the 1980s for scientific research on topics such as on the effects of cobalt-60 radiation on rock salt. In the Zechstein salt dome, there were 113 open rock salt mining chambers without backfill, the total volume of which was 3.4 million m³. In contrast, a large proportion of the potash mining chambers with a total volume a total 1.5 million m3 in the dome’s carnallite seam had been backfilled with humid residue from a potassium mineral processing plant.

The disposal of nuclear material took place from 1967 to 1978 under the authority of the Federal Ministry for Scientific Research and Technology, now the Helmholtz Centre, Munich. About 1,300 drums with 260 m3 of medium-level waste with an activity of 1.2 x 10¹⁵ Bq (1 January 2002) were deposited in a 8,400 m³ subsection of mining chamber 8 on the -511 mining level (Fig. 1). The subsection is separated from the main chamber by a concrete wall. The southwest end of the main chamber may only be 5 m away from the Mesozoic country rock. The thin salt barrier left in between salt and rock has possibly already disintegrated in the course of the deformation of the overall mine structure [1]. Furthermore, there are 125,000 drums with low-level waste distributed over 11 mining chambers on the -750 m level and one chamber on the -725 m level. The total radioactivity of the low-level material is 1.7 x 10¹⁵ Bq. These figures probably need revision, since the disposal activities are badly documented. A Lower Saxony state parliamentary enquiry committee was set up in May 2009 in order to shed light upon the activities in the mine. But this body has not yet discovered what quantities of radioactive material lie underground. It remains unclear whether high-level waste has been deposited as well.

Fig 1. Vertical section through the Asse anticline. Dashed line refers to area of Fig. 2

In 1988, water infiltrated the -532 m mining level at the southwest flank of the salt dome, that is, relatively close to the medium-level waste deposits. The inflow increased over the years to a nearly constant value fluctuating around 12 m³ per day. Fortunately, the leakage, which probably originates in the Mesozoic country rock nearby, is not radioactive. But there are minor amounts of radioactive fluids at deeper levels. They come from either the contact between waste and water flowing out of mining chambers with humid backfill, or accidents during the transport of waste drums. A further possibility is that even liquid radioactive waste was disposed in the mine.

About 80% of the old salt mining cavities were backfilled between 1995 and 2004 with salt residue from the mine dumps at Ronnenberg. The material has an original porosity of 40% (percentage of voids) and will consolidate under the pressure of the mine structure as it deforms elastically–a process called convergence. Depending on the type of assumptions made, it will take between several thousand and ten thousand years until convergence will have completed its job. At the end of the process, the porosity of the backfill will be reduced to an insignificant level and nearly all fluid will be expelled from the mine. Sorel (magnesium oxychloride) concrete is used for secondary backfilling and for closing the drifts connecting the mining chambers. Magnesium oxide mixed with rock salt and magnesium chloride dissolved in water is pumped into the open space. Magnesium oxide is the binding agent and rock salt is the filling material. Concrete has the advantage of near-zero porosity but the disadvantage of non-elastic behaviour. Cracks in concrete will be preferential paths for fluids expelled from the mine workings in the process of convergence.

Decommissioning

In 2009, the Federal Office for Radiation Protection, the BfS, took over the site.

The change followed a government decision that the site should not be seen as an experimental repository (which follows mining rules), but an operational nuclear waste repository (which follows nuclear rules). Nuclear law makes greater demands on the operation, decommissioning, and radiation protection of the facility, according to the BfS. In particular, it requires a proof of long-term safety.

Until 15 January 2010, the mine operator could not decide how to finally close Asse 2. A variety of schemes have been considered which could not have been more diverse [1-4]. One former alternative was an in-situ option, that is, leaving the waste where it is. Another former alternative was restoring the waste within the mine. A new cavern at 1,200 m depth would be the new destination.

The chosen option is extraction. During the next fifteen years, the waste and the contaminated salt around it will be put in transport containers in a remote-control operation at a cost of EUR 3.7 billion [6]. This is a highly ambitious plan without precedent.

As it is unknown whether the mine structure will collapse before decommissioning is completed, or whether the mine will flooded while waste remains underground, provisions for an emergency are being accelerated. The pumping system is being upgraded to deal with 500 m3 of inflowing water per day. A drainage system for diverting excess inflow to special reservoir sections within the mine is under construction. As long as the inflow of water proves manageable, medium-level waste will be extracted from chamber 8 and the remaining mining cavities will be backfilled. If the medium-level waste cannot be extracted, it will be covered by Sorel concrete. Artificial flooding with a magnesium chloride solution will also be attempted even when the rate of natural flooding is high. The short-term concern is to stabilize the mine structure, although the results may be counterproductive in the long run.

The plan certainly can be considered feasible, at least under optimistic assumptions. It is technically feasible to put the Asse 2 waste and the contaminated rock salt into transport containers, provided that there is excellent ventilation that keeps the concentration of radioactive aerosols in the mining chamber below the dose limits during the operation. The licensing of the low-level and medium-level Konrad repository in Salzgitter, Germany has passed all national legal hurdles, and Konrad appears to be ready to receive the Asse 2 waste. However, Konrad is not designed to host substantial amounts of rock salt, which speeds up corrosion of metal containers. Peak activity values of individual waste drums will be another problem because they do not comply with the regulatory premises for Konrad, although the average would.

However, there are a lot of unsolved questions, especially with regard to the quality of the waste packages.

When the drums were deposited more than 40 years ago, the major concern was to protect the mining personnel from radioactivity. In order to keep the radioactive dose at a minimum, the drums were unloaded in a hurry. The medium-level drums were thrown through an opening on the -490 m mine level into chamber 8 on the -511 m level. Furthermore, many of the low-level drums experienced a hard landing, too; they rolled along the inclined floor of the mining chamber after being dropped from the shovel of the front loader. In addition to mechanical damage, there has been the destructive work of the corrosive brines. In short, disintegrated drums will have to be sealed and stabilized in order to become fit for remote-controlled transport.

The waste left behind in a converging salt mine is bound to cause gaseous pollution. Hydrogen gas is formed when metal corrodes at low oxygen fugacities (pressures); the oxygen component of water combines with metal to form metal oxides and the hydrogen component of water forms hydrogen gas. Gaseous radioactive components of the waste, such as newly-formed methane and carbon dioxide, and the hydrogen gas, may find their way to the surface.

Groundwater pollution due to radioactive water expelled by convergence takes longer than the release of radioactive gas, but is equally difficult to predict. Figure 2 shows a computer simulation how a plume of contaminated fluid expelled from chamber 8 spreads in the Mesozoic aquifer between 1,000 and 6,000 years from now [5].

Fig 2. Vertical section through the Asse anticline as Fig 1, orientated along the major direction of contaminant transport. It shows the simulated cumulative liquid-phase activity (Bq/L) at the end of simulation periods lasting 1,000, 3,000 and 6,000 years respectively

This simulation is based on the pessimistic assumption that a fault with a high hydraulic conductivity connects the Mesozoic rock next to waste with the surface. But the simulation is far from being pessimistic enough for a risk assessment. The simulation follows common numerical-modelling practice by treating the fractured Mesozoic rock as if it were unconsolidated porous sediment. Fractured hard rock adsorbs pollution to a much smaller degree than porous sediment. For example, the concentration of plutonium radionuclides in a simple porous medium are reduced to an insignificant level at a depth of 400 m whereas, in a fractured-medium, serious groundwater contamination cannot be ruled out as close as 100 m to the earth’s surface, according to scoping calculations. Exact simulations, which show what may happen when the closure scheme fails, are bound to be computationally very expensive but will be worth the effort.

Author Info:

Michael O. Schwartz, environmental geology consultant, MathGeol, P.O. Box 101204, 30833 Langenhagen, Germany

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