Understanding the Brittle Deformation of the Host Rock – An Example of Investigations Needed for the Safety Case Development Supporting Radioactive Waste Management

The most important requirements for safe nuclear waste management are related to the long-term safety of the repository. In case of deep geological disposal in crystalline bedrock environment, the main role of the bedrock is to provide a stable environment for the engineered barrier system. It is recognized that the requirements for the host rock should cover the issues of mechanical stability and favorable groundwater conditions. The stability, local seismic risks, groundwater flux and evolution, and transport properties are all strongly linked to the brittle deformation features: faults and fractures. This is considered in developing the geological understanding: in mapping, interpreting and modeling the structural elements of the bedrock in a selected site. It needs to cover the fabric of intact rock, general characteristics of brittle deformation, fracture data collection and processing, characterization of brittle deformation zones and identification of fracture and fault systems. The geological data are used for conceptual understanding and building deterministic and stochastic models. In a nuclear waste disposal program, typical downstream users of these data and models are modelers of other disciplines, safety assessment team and those responsible of design, construction, licensing, and operation of the repository. Therefore, geologists need to be able to answer questions about the host rock during the operation but also about the future evolution. One example of extensive characterization programs of brittle deformation is Posiva's project in the Olkiluoto site, Finland. Geological investigations started in the early 1980s, and after a multi-stage process, Olkiluoto was selected for the final disposal site of spent fuel from the Olkiluoto and Loviisa nuclear reactors. During this long process, geological investigations have been in a major role in the disposal project and its licensing and acceptance, but mutually the disposal project has been beneficial for developing investigation methods and better knowledge of bedrock in general. (author)