Of fractures and concepts – current research on crystalline rock as a host rock for a repository
Since the search for a repository site was restarted, crystalline rock, such as that found in Bavaria or Saxony, has also been considered as a host rock for a repository for high-level radioactive waste. Currently, experts from the GRS Repository Research Centre are working on several research projects to deepen the understanding of fundamental properties of the host rock and to develop concepts for assessing the safety of a repository in crystalline rock – in their own laboratory tests, in cooperation with foreign research partners, and as part of experiments carried out in the Swiss underground laboratory at Grimsel.
A couple of months ago, construction of the world's first repository for high-level radioactive waste from the civilian use of nuclear energy began on the west coast of Finland. Sweden has also already decided on a repository site. Both repositories will be built in crystalline rock – commonly referred to as "granite." In Germany, too, the Site Selection Act stipulates that crystalline rock, along with salt rock and argillaceous rock, can be considered as host rock for a repository. Corresponding deposits, which are being considered in the current selection procedure for a repository site, are found primarily in Bavaria and Saxony.
For more than 25 years, experts at GRS's Repository Research Centre have been dealing with a wide variety of scientific issues relating to the safety of repositories – ranging from studies into the basic properties of possible host rock types to the development of methods for site selection and the demonstration of the safety of a repository. After the focus of repository research in Germany was for a long time primarily not only on salt rock but also on argillaceous rock, research on crystalline rock has gained in importance with the restart of the site selection process.
Concept for the presentation and assessment of safety
For example, in a project funded by the Federal Ministry of Economics and Technology, GRS, together with the Federal Institute for Geosciences and Natural Resources (BGR) and BGE Technology GmbH (BGE TEC), is developing a concept for presenting and assessing the safety of a repository in crystalline rock. Such a concept describes how the radioactive substances from the waste emplaced can be isolated in a defined area underground – the so-called "containment-providing rock zone" (CRZ for short) – in such a way that they will not reach the biosphere for a period of one million years, or at most only in harmless quantities. Various properties of the respective host rock play a decisive role in this.
Corresponding concepts have already been developed for salt rock and argillaceous rock. However, these cannot be applied to crystalline rock because the host rocks differ greatly in key properties, as Dr Judith Flügge, Head of the Site Selection Department at GRS, explains: "While the salt rock and argillaceous rock formations that come into question as host rocks are found in dense, homogeneous units, crystalline rock can also be strongly fissured in the deep underground. Through such fissures – figuratively speaking: cracks in the rock – groundwater could reach the waste containers through the subsoil." This, in turn, could cause the containers to leak due to corrosion and release radioactive and other substances into the underground; in the planned repositories in Finland and Sweden, this is to be prevented by a corrosion-inhibiting copper layer around the waste containers.
Therefore, two approaches which have already been worked out in previous projects are being pursued for crystalline rock: In the case of a "multiple CRZ", the waste containers are to be emplaced in several smaller areas, each of which is fractured to such a small extent that the safe containment of the radioactive waste in these areas can be ensured. In the case of an "overlying CRZ", the emplacement area in the crystalline rock is to be covered by a salt rock or argillaceous rock formation, which in turn then represents an insurmountable barrier for the radionuclides.
In special simulation software, the scientists are now creating a site-independent geological 3D model for each approach. They will then feed these 3D models into calculation codes (d3f++ and RepoTREND) that were developed for calculating the dispersion of substances in a repository system. In doing so, they assume the case that water penetrates the emplacement area, which leads to a release of radionuclides from the waste containers. In this way, they want to find out under what conditions and over what periods of time radionuclides could reach the biosphere, from which, for example, requirements for a CRZ can then be derived. These requirements, in turn, are necessary in order to be able to develop or assess safety concepts.
Scientific and technical cooperation for over 20 years
In addition to such conceptual issues, there are also unanswered questions at a more fundamental level, for example about certain properties of crystalline rock that are relevant to its function as a host rock. Various research projects are also underway in this area, including projects within the framework of the Scientific and Technical Cooperation with Russia supported by the Federal Ministry of Economics and Technology. For example, GRS researchers are involved in several scientific projects in crystalline rock in the Krasnoyarsk region, where a Russian underground laboratory is currently being built. On the one hand, they are evaluating sample material from the Yeniseysky area in order to collect experimental data. Among other things, they are looking into the sorption behaviour of substances on fractured rocks in GRS's own geoscientific laboratory. Sorption – figuratively speaking, the "adhesion" of substances to the rock – has a significant influence on whether or how these substances could be transported away from the containers through the subsurface with groundwater that has entered.
On the other hand, such transport processes are simulated with the help of 3D models. A particular challenge here is the question of how the fracturing of the crystalline rock can be taken into account in the models. Judith Flügge, who wrote her doctoral thesis on the effects of extreme climatic conditions on radionuclide transport in the overburden of a repository, heads the project. She explains: "Of course, not every single fissure can be depicted faithfully in such a model – the exact location and size of each one is not known in reality either. This makes it all the more important that we develop a method with which we can model the rock in such a way that we can then calculate the transport of radionuclides as realistically as possible." In order to be able to visualise the results of the calculations in three dimensions, the experts also make use of the VIRTUS virtual underground laboratory, which GRS developed together with BGR, BGE TEC and the Fraunhofer Institute for Factory Operation and Automation. For example, VIRTUS enables researchers to use VR goggles to reproduce details of the modelling in a 3D model.
Another project also aims to deepen knowledge about the sorption of radionuclides on crystalline rock. To this end, the experts in the laboratory of GRS's Braunschweig Repository Research Centre are examining rock samples from sites in the Czech Republic, Russia, and South Korea. The data obtained in this way are then used in simulation calculations that can be used to analyse radionuclide dispersion in the geological underground.
Long-term stability and radionuclide mobility
In addition to laboratory research, GRS experts are also involved in experiments in underground laboratories abroad. For example, in a joint project with the Karlsruhe Institute of Technology and the University of Jena, they are investigating how geotechnical barriers made of bentonite in a repository in crystalline rock behave over long periods of time. The work is taking place in the international Swiss Grimsel Rock Laboratory in close exchange with scientists from Finland, Sweden, Switzerland, Spain, and the USA.
Building on the results from a total of four predecessor projects, questions are currently being examined there that deal primarily with processes at the interface of crystalline rock and the geotechnical barrier. The researchers are investigating the extent to which the bentonite barrier is affected by external influences. If groundwater reaches these barriers, for example, small particles can be dissolved from the bentonite. With the help of in-situ experiments and simulation calculations based on them, the researchers are investigating the influence of these processes on radionuclide transport and thus on long-term safety.
Crystalline Club: International exchange and joint studies
However, an international exchange also takes place unrelated to projects. For example, four years ago, the Nuclear Energy Agency of the OECD set up the Crystalline Club, an international expert body in which scientists from all over the world regularly exchange information – the Clay Club and the Salt Club for the other two host rock types had already been established years earlier. Judith Flügge is currently chair of the Crystalline Club. "Being personally in a network with colleagues from other countries is very conducive to professional exchange. Especially with complex issues such as the modelling of fissures, it helps us to be able to compare and discuss different approaches," explains Flügge. In addition to regular video conferences, a plenary meeting is held annually; the next plenary meeting is planned to take place in Dresden in 2022.
About the GRS Repository Research Centre
The Repository Research Centre in Braunschweig has been part of GRS for more than 25 years. Here, about 50 scientists carry out application-oriented basic research on a wide range of questions concerning the safety of repositories. The spectrum of work ranges from laboratory studies on the fundamental properties of host rock types and cooperation in international underground laboratories up to the development of concepts for assessing the long-term safety of repositories and corresponding simulation software. The underground disposal of hazardous chemical waste has also been and continues to be the subject of research projects.
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