Management of influent saline solutions

Date

06/05/2010

About 12,000 litres of saline solutions are collected in the Asse II mine daily. In order to keep the influent saline solution taken to the Mariaglück mine harmless, the locations where the solution is collected and the reservoirs on the 658-m level and the 490-m level are covered and provided with fresh air to keep the taking up of tritium through the mine air as low as possible. Before they are taken away, the influent saline solutions are cleared. About 12,000 litres of saline solutions are collected in the Asse II mine daily. In order to keep the influent saline solution taken to the Mariaglück mine harmless, the locations where the solution is collected and the Saline solutions collected below the 658-m remain underground.

Points of inflow of saline solutions

Point of Inflow of the intruding waters

Saline solutions enter the salt dome from the adjoining rock in depths between 500 m and 575 m and, from there, enter the mine. The major part of the influent saline solutions is collected on the 658-m level (ca. 10 cubic metres per day). Additional collecting points are on the 725-m level (ca. 1.4 cubic metres per day) and on the 750-m level (ca. 0.2 cubic metres per day).

Contaminated brines on the 750-m level are not to be mixed up with the influent saline solutions. They do not originate from the influent saline solutions but from the time the radioactive waste was emplaced.

Method applied since January 2009

The former operator of Asse, Helmholtz Zentrum München (HMGU) did not immediately take the collected saline solutions to the area above ground but initially collected them in an interim reservoir. Thus, the influent saline solutions collected on the different levels were mixed. Until their removal, the solutions remained underground for quite some time. During this period they took up the radioactive substance tritium from the mine air.

intake of tritium from the mine air

BfS has examined how much tritium from the mine air is taken up by the influent saline solutions. Measurement results have shown that the solutions take up approximately 4 becquerel per litre per day when they remain underground or are not isolated from the mine air. The longer they remain in the mine the higher is the tritium exposure. If they are covered respectively provided with fresh air as soon as possible, the tritium exposure can be reduced.

After the change of operatorship a new method for the management of the saline solutions was introduced. It reduces the taking up of tritium from the mine air clearly and includes the following central measures:

Method beginning January 2009

• The saline solutions flowing in on the 658-m level are collected at covered collecting points and pumped into the covered interim reservoir on the 490-m level. They are strictly separated from all other liquids in the Asse mine.
• The area on the 490-m level and on the 658-m level where the solutions are stored intermediately and covered is supplied with fresh air from above ground. Thus clearly less tritium gets into the influent saline solutions.
• The influent saline solutions are removed from the Asse II mine once a month.
• The influent saline solutions collected on the 725-m and the 750-m level are not mixed with other solutions. They remain in the mine and are internally for the production of Sorel concrete.

Radiation protection

Comparison of the measured tritium activity to the release values

BfS has set itself the goal not to exceed the value of 40 becquerel per litre. This is clearly below the tritium concentration whose maximum (100 becquerel per litre) is permissible according to the German Drinking Water Ordinance. With the brine (or saline solution) management of the former operator the values were generally in the range of circa 200 becquerel per litre.

Site-specific release value: The release value is 140,000 becquerel per litre. That corresponds to about one hundredth of the mean natural radiation exposure.

The figure compares the valid release value with the reduced tritium exposure that can be achieved with the new management of the influent saline solutions.