A repository in crystalline rock? A new study by GRS deals with conceptual issues of a possible crystalline repository for Germany

14.11.2016

Picture © Posiva Oy

When it comes to the disposal  of high-level radioactive waste, there are three rock types that are mainly discussed worldwide: salt, clay and crystalline rock. In Germany, the focus had been on the host rocks  salt and clay for a long time. In 2002, in its recommendations on a site selection procedure, the Committee on a Selection Procedure for Repository Sites (AkEnd)  , established by the German Federal Environment Ministry, had given priority to a safety concept according to which the radioactive waste is isolated in a so-called containment-providing rock zone (CPRZ or CRZ). This should consist of practically impermeable formations with adequate thickness, thus suggesting salt and clay as host rocks. The key elements of the concept were laid down in the Safety Requirements Governing the Final Disposal of Heat-Generating Radioactive Waste in 2010.

With the Repository Site Selection Act (StandAG) of 2013, crystalline formations also came into consideration for the selection of a repository site in addition to clay and salt. In 2016, the Commission on the Storage of High-Level Radioactive Waste of the German Bundestag confirmed the consideration of crystalline rock in its recommendations.

Research projects on crystalline rock formations

Together with the Federal Institute for Geosciences and Natural Resources (BGR) and DBE Technology, GRS investigates in a current project whether the concept of disposal in a deep geological formation with a high containment capacity, as required in today's German safety requirements, can also be applied to crystalline.

For geological reasons, Sweden and Finland are already pursuing the disposal in crystalline rock. The researchers of GRS were interested in whether or not the repository concepts developed there for specific sites up to the readiness for licensing could be applied to German conditions.

Can Scandinavian concepts be applied to Germany?

According to the Scandinavian repository concepts, the high-level radioactive waste is stored in fuel assembly containers enclosed by a copper shell. The copper shell ensures that the containers will have a long service life. Moreover, the containers are embedded in bentonite with a thickness of several decimetres as an additional barrier.

However, the Scandinavian concepts cannot be applied to German conditions without adaptation. In Finland and Sweden, the safety of the repository must be demonstrated for 100,000 years. In Germany, the assessment period is 1 million years. In addition, no MOX fuel assemblies  have to be disposed of in Sweden and Finland. MOX fuel assemblies require special precautions to prevent self-sustaining nuclear reactions and are characterised by a higher thermal output.

How would a repository in crystalline have to look like?

From the joint R&D project it was concluded that the following crystalline formations could meet the requirements:
1.    a formation with a high containment capacity (Fig. A)
2.    a crystalline formation covered by layers with a high containment capacity (Fig. B)
3.    several areas with a high containment capacity within a crystalline formation (Fig. C)

Overburden (Picture: GRS)

The prerequisites for the first option are rather difficult to find due to the geological conditions in Germany. In case of the second option, the crystalline formation is covered by a containment-providing rock zone. This rock zone ensures that radioactive substances are enclosed and retained. According to the third and final option, the waste is positioned in low-fractured rock blocks in the host rock. Here, the containment capacity is based on the combination of the low hydraulic permeability of the rock blocks and additional engineered barriers. However, based on the current state of knowledge, it is difficult to assess which of the three options will be appropriate for Germany.

Further research required

The researchers of GRS conclude that there is a greater need for research and development to develop concepts and a robust safety case for a repository in crystalline rock. This work is the prerequisite for a “host-rock-open” site selection.
The study sponsored by the German Federal Ministry for Economic Affairs and Energy is expected to be published in mid-November.