Site selection procedure in Germany: Considerations on a depth limit for a high-level radioactive waste repository in claystone

The depth of a repository for high-level radioactive waste is an important aspect with respect to both long-term and operational safety. In contrast to the minimum depth, no relevant constraints are given in the German legal regulations for a maximum value. To define such a limit, various safety-related aspects of claystone host rock are considered. This evaluation takes place within an early stage of the German site selection process. The available data are evaluated, and the respective assessments are discussed, ranging from aspects of construction and operation of the repository to its post-closure period. Furthermore, tunneling experience in relevant claystones is evaluated. It is concluded that a favorable depth is neither the maximum depth of existing underground excavations nor the maximum achievable with available techniques. Instead, it is the depth for which experience indicates that under the current assumptions, the extended operation of a nuclear waste repository can be well conducted without jeopardizing operational and long-term safety. The major argument for limiting the repository depth is robustness, which means that the repository as a whole can be operated safely, requires no extraordinary measures like resources, techniques, or time, and can be built with well-established techniques. It can be summarized that those requirements are unlikely to be met for a repository in claystone at depths greater than 1000 m.