Selection of regions for surface exploration in the search for a repository for high-level radioactive waste in Germany: Decision-making in a safety-oriented site selection procedure with sparsely available data

The site selection procedure for the deep geological storage of high-level radioactive waste in Germany started with a white map, i.e., with the entire federal territory, and considers all of three potential host rocks: rock salt, clay rock, and crystalline rock. In a first phase of the site selection procedure, Bundesgesellschaft fr Endlagerung (BGE, i.e., Federal Company for Radioactive Waste Disposal) is determining regions for surface exploration based on the previously identified sub-areas that meet the minimum requirements defined by law. As the sub-areas cover approximately 54% of Germany's area, the site selection methodology must not only include safety assessments but must also lead to a significant safety-oriented reduction of the considered area towards the best-suited regions for surface exploration. Several challenges arise for the determination of the regions for surface exploration. As legal regulations provide only a framework for the procedure but largely lack explicit assessment criteria, a methodology must be developed that is in accordance with the law and that also supplies the tools to evaluate and narrow down the remaining areas. In addition, the safety-oriented evaluation and area reduction must be done legally on the basis of available data only, which is sparsely available and mostly not specific to the needs for site evaluation. Only in the next phase of the site selection procedure can BGE collect data during surface exploration for a more detailed site characterization. Finally, the developed methodology, as well as decisions based on its application to the sub-areas, must be transparent and comprehensible to permit discussions with stakeholders and the public. Data availability and associated uncertainties provide a specific challenge in ensuring this transparency during the decision-making. To address the challenges, BGE is developing a methodology that concretizes the legal requirements and translates them into a plausible evaluation system with well-defined criteria. These host-rock-specific evaluation criteria derived from the legal requirements allow a reproducible, systematic and thus transparent evaluation of safety-relevant attributes. The criteria are applied successively in a series of work steps, such that the level of detail and the strictness of requirements increase throughout the selection process. Thus, less-suitable areas are identified early on in the procedure with limited effort, while areas that are more promising are analyzed in greater detail providing also the basis for a comparison between areas and must pass successively higher requirements. The level of knowledge, which increases throughout the selection process, is taken into account when deciding on the evaluation criteria for a specific work step, such that heterogeneous data availability influences the decision-making as little as possible. However, poor data availability and verifiability of geologic realities are fundamental problems for the decision-making process that need to be addressed and discussed openly.