Considerations of Future Human Actions for a Deep Geological Repository in Switzerland

Passive safety is a fundamental principle of any deep geological disposal for radioactive waste, aimed at protecting humans and the environment until the activity of the radioactive waste decays to levels comparable to those of natural rocks. In Switzerland, Nagra submitted a general licence application in November 2024 for a deep geological repository to be constructed in the Opalinus Clay host rock in the Nördlich Lägern siting region.  

The general licence application is the conclusion of Switzerland’s multi-stage site selection process to determine the most effective and ideal site for the long-term isolation of the radioactive waste. The Nördlich Lägern region lacks significant natural resources (hydrocarbons, salt, water, minerals, etc.) at or below the level of the repository. This limits motivation for future human intrusion related to the exploration of resources, thereby reducing future human actions that could compromise repository safety. The repository design incorporates a multi-barrier system that includes high-level waste disposal canisters that safely enclose the waste for thousands of years, well beyond the thermal phase. In combination with the significant repository depth, foreseeable inadvertent human intrusion is unlikely and difficult.  

Due to the depth of the repository (for both high-level waste and low- and intermediate-level waste), drilling is the only foreseeable future human action via which recovered waste material could reach the biosphere, potentially impacting health. Additionally, the disposal canisters for high-level waste present very small targets, reducing the likelihood of being hit by wildcat drilling activities.  

This paper explores  

1) design, strategies and measures implemented to ensure post-closure safety of the repository, and  

2) potential radiological consequences of an unlikely penetration of waste by drilling.   

To assess an unlikely event of future human action leading to the intrusion into the radioactive waste in Nördlich Lägern, stylised scenarios were developed using features, event, and processes. The intrusion scenarios focus on borehole scenarios. Consequences for the drilling team are excluded due to the regulatory framework. The borehole scenarios consider pessimistic parameter assumptions to assess the potential radiological consequences for the biosphere in the region between 100 and 50,000 years post closure. In the short term, key dose-relevant radionuclides include Sr-90, Am-241, Cs-137, Ni-63, Pu-239, and Ag-108. Over longer time scales, Cl-36, I-129, Pu-240, Nb-94, Th-230, Ra-226, and U-234 become more relevant. All scenarios result in dose consequences below the regulatory limit prescribed and include an unrealistic exfiltration pathway for radionuclides which is implausible in the context of the clay-rich sedimentary environment.  

Overall, the Swiss siting region represents an ideal site for the long-term disposal of radioactive waste due to its high level of passive safety and lack of potential resources. Only worst-case conservatisms and assumptions allow us to identify consequences for the regional biosphere for drilling events and even under such conditions, dose consequences remain well below the regulatory limits for future human actions, supporting the safety case and the long-term isolation of waste from the biosphere.