BfS considers stabilisation of an emplacement chamber plug in Asse by way of precaution

The Federal Office for Radiation Protection (BfS), the new operator of the Asse mine near Remlingen since the beginning of the year, intends to stabilise the plug to an emplacement chamber in the Asse II mine with concrete. This will be done shortly and by way of precaution. At the end of 2008, rotational monitoring measurements carried out by the former operator Helmholtz Zentrum München (HMGU) showed that apparently there were damages in the so-called emplacement chamber 4 on the 750-m level which could lead to pieces of rock falling from the roof and onto low-level radioactive waste stored in the chamber. In the opinion of the new operator, BfS, and the regulatory authorities, it is currently not necessary to implement danger prevention measures. Nevertheless, such measures are considered by way of precaution in order to prevent a dispersion of radioactively contaminated dusts and mine air from the emplacement chamber into the mine and into the exhaust air in case of a piece of rock falling from the roof. Therefore, it is intended to enforce the plug to the damaged chamber in a first step. After the ordered rock-mechanical and radiological evaluations have been submitted, additional safety measures will be examined. In doing so, BfS pursues the objective of not impairing decommissioning options for Asse through these measures.

Already since the end of 2007, micro-seismic monitoring measurements have been carried out in chamber 4 and routinely evaluated at the end of each year, when they are submitted to the regulatory authorities. When the data for 2008 were evaluated, it showed that micro-seismic events at the pillar above emplacement chamber 4 on the 750-m level had occurred more frequently. A first evaluation of these results has shown that it cannot be excluded that a piece of rock will fall from the roof. In that case, low-level radioactive waste stored in the chamber could be damaged. Neither can it be excluded at present that on account of the pressure built up in such a case the plugs to the chambers will become permeable and thus radioactivity could be released. Therefore, BfS decided to enforce the plug to the chamber as a precautionary measure. For this purpose, one intends to construct a second barrier of Sorel concrete in front of the old plug which was already constructed some decades ago. This is to prevent the release of radioactivity in case of a piece of rock falling from the roof. In the next days, BfS will file a corresponding application at the competent mining authority, Lower Saxony's "Landesamt für Bergbau, Energie und Geologie" (LBEG). Access of the staff to the affected areas has meanwhile been restricted.

In the medium and the long term, it is planned to stabilise the chamber in such a way that a collapse can be excluded for the time until Asse will be finally closed. This could be achieved, for example, through backfilling the damaged chamber. However, a fundamental rock-mechanical and radiological evaluation of the situation in situ is essential for the further proceedings. BfS has already charged the “Institute for Rock Mechanics” ("Institut für Gebirgsmechanik", IFG) with an evaluation of the rock-mechanical situation. First assessments can be expected at the end of January. The "Institute for Safety Technology" ("Institut für Sicherheitstechnologie", ISTEC), on the other hand, was charged with a fundamental radiological evaluation. In this evaluation, the long-term decommissioning options for Asse including an examination of the option of retrieving the entire waste or of parts of the waste are to be taken into account.