For the disposal of high-level radioactive waste in deep geological formations, different host rocks (rock salt, crystalline and clay rock) are investigated internationally. According to the Site Selection Act (StandAG), the three host rocks that are potentially suitable for the disposal of high-level radioactive waste in Germany–rock salt, clay rock and crystalline rock (such as granite)–must be taken into account. Since there is no repository mine for high-level radioactive waste and no underground laboratory in these host rocks for repository research in Germany, German institutions are involved in research work in international rock laboratories in order to gain or expand knowledge of these host rocks. The German institutions have already been involved in geoscientific experiments in the Grimsel (Switzerland) and Äspö (Sweden) rock laboratories, where the properties of the granite formations are being explored.

Due to the rapid economic development in China, the energy requirement there has increased tenfold in recent years. To reduce fossil fuel consumption, 44 nuclear reactors with a capacity of 58 GW are currently (as of the end of 2019) in operation and 13 reactors with a capacity of 30 GW are under construction. The search for a site for a repository for high-level radioactive waste began in 1985. Two host rocks (granite and clay) are being intensively studied in China. In the Gobi desert (granite formation, the climatic (low precipitation, high evaporation), hydrogeological (low water table), and geological (stable crust, low seismicity) conditions as well as the social (low population density) and economic (infrastructure development due to the potential repository site) situations seem to be well suited for a repository. In 2020, the permit to set up an underground research laboratory (URL) has now been granted (Sect. 2.1).

In June 2021, the construction of the underground research laboratory (URL) started in the crystalline rock in the Gobi desert (Gansu province). The ground infrastructure with offices, conference rooms and workshops as well as access road and water supply have been completed now. The excavation phase with three shafts and a ramp of about 7 km length and a maximum depth of 560 m offers the opportunity to observe and evaluate the hydraulic, mechanical and geochemical changes in the fractured rock as a result of this intervention.

The Chinese repository concept for high-level radioactive waste in granitic rock is based on the multi-barrier system with bentonite as the backfilling and sealing material for boreholes, tunnels and and shafts after emplacement. For this purpose, the Chinese bentonite GMZ (GaoMiaoZi) is intended as shaped stones and pellets, which is extracted in an open-pit mine with a deposit reserve of 160 megatons (Chen et al., 2014). Extensive laboratory tests are carried out to determine the thermal, hydraulic, mechanical and chemical properties of the GMZ bentonite. A long-term mock-up test was started in 2011 to study the behavior of GMZ bentonite under the simulated repository conditions (Sect. 2.2).

In 2018, a Joint Declaration of Intent (JDoI) was signed between the Federal Ministry for Economic Affairs and Energy (BMWi), now BMWK and the China National Nuclear Corporation (CNNC). The aim of the JDoI is to promote possible cooperation in the field of repository research and nuclear safety and to create a general framework for cooperation. Under these conditions, the potential German partners have developed the following two project outlines:

  • Study of thermo-hydro-mechanical-chemical processes on GMZ bentonite,

  • ‘Fact-finding study’ on crystalline rock characterization and public acceptance.

On this basis, the project management organization Karlsruhe (PTKA) initiated a pilot project to investigate the GMZ on behalf of the BMWi in 2020. Helmholtz Center for Environmental Research (UFZ), TU Bergakademie Freiberg (TUBAF), Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH (GRS), Technical University of Clausthal (TUC), Friedrich Schiller University Jena (FSU Jena), the Karlsruhe Institute of Technology (KIT), and Federal Institute for Geosciences and Natural Resources (BGR) are involved in these investigations. The pilot project focuses on thermal-hydrogeological-mechanical and chemical processes in bentonites (GMZ and MX-80, a bentonite intensively studied in many countries) and includes both experimental work and numerical modelling. The aim of this project are model and method verification as well as results comparison from different institutions.

The safe deposition of high-level radioactive waste from the energy industry is one of the major societal challenges of this century worldwide. At the same time, final disposal in deep geological formations is considered the safest solution to date–with many scientific and technical challenges still to overcome. The Pilot Project ”Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Bentonite Barrier Systems” is a collaborative project between German and Chinese scientists to organize a bilateral comparison of methods and technologies in (1) experimental laboratory and field research as well as in (2) systems analysis. Both German and Chinese partners obtain excellent complementary experimental and modeling research facilities. China’s first underground research laboratory for geological disposal of high-level radioactive waste (Beishan URL) is currently under construction and will serve as an international collaboration platform for URL research in future. Due to the high cost of scientific programs and the technical operation of URLs, international cooperation is not only essential but also a prerequisite for excellent research and development to provide safe solutions–not only for radioactive waste deposition into deep geological repositories but also for other geoenergy applications such as energy storage and geothermal energy utilization.