Flat salt formations - an option for disposal?

18.02.2019

For the first time, researchers develop a concept for a nuclear repository in flat salt formations

Rock salt, along with clay and crystalline rock, is one of the three host rock types which, according to the Site Selection Act, are suitable for the disposal of high-level radioactive waste in deep geological formations. Previous research on salt as host rock has focused primarily on salt domes. Flat salt formations differ from salt domes (also known as salt diapirs) in that they are deeper, flat salt structures.

Captions: Schematic representation of a salt dome and a flat salt formation (salt shaded in grey, surrounded by differently coloured rock layers). These formations have vertical extensions ranging from a few hundred metres (flat formation) to more than one kilometre (salt dome). (Source: GRS)

Due to their wide distribution and favourable geological properties, they have moved into the focus of research. The Waste Isolation Pilot Plant (WIPP) in the USA is the first repository in a flat salt formation. This is mainly used to dispose of so-called transuranium waste from military applications. In Germany, such salt formations are already being used for the underground disposal of hazardous chemical waste.

Researchers from GRS, BGE Technology, the Federal Institute for Geosciences and Natural Resources (BGR) and the Leipzig Institute for Rock Mechanics (IfG) have now for the first time developed a site-independent concept for disposal in flat salt formations. The results are impressive: they form the safety-related basis for the comparison of repositories in different geological formations. Flat salt formations may thus be considered when selecting a repository site.

How did the researchers proceed?

Captions: Schematic representation of a salt dome and a flat salt formation (salt shaded in grey, surrounded by differently coloured rock layers). These formations have vertical extensions ranging from a few hundred metres (flat formation) to more than one kilometre (salt dome). (Source: GRS)In a first step, the experts started by developing a prototype model of a flat salt formation as it may be found in Germany. Furthermore, a repository model was designed for this site. Based on this model, they developed a safety concept for a repository in such formations.

In a second step, the project participants carried out model calculations on possible releases of radioactive substances from the repository. To do so, the simulation software RepoTREND, developed by GRS, was used. Three emplacement methods and two types of storage containers were considered:
•    drift emplacement in so-called POLLUX casks
•    storage in horizontal boreholes in canisters for vitrified waste
•    storage in vertical boreholes in canisters for vitrified waste.

As a third and final step, on the basis of the results obtained, the researchers assessed the extent to which a release of radioactive substances from the waste containers may lead to radiologically relevant effects in the biosphere.

What were the results of the study?

The results for the scenarios considered showed that the releases and the resulting radiological consequences for the conditions assumed in the model were far below radiologically significant values. This means that the safety requirements for repositories can be met in the scenarios investigated for flat salt formations. A comparison with similar studies in salt domes shows that the level of safety of flat formations corresponds to that of salt domes.

The detailed results of the project are summarised in the report "Bewertung der Wirksamkeit des Radionuklideinschlusses für ein Endlager in flach lagernden Salzformationen“ (Evaluation of the effectiveness of radionuclide containment for a repository in flat salt formations).

Find out more

Repository research at GRS
Disposal: GRS develops simulation software RepoTREND for long-term safety analysis
A look under the ground: GRS researches measurement methods for surface exploration of a repository site