Radiological Consequence Analysis for a Deep Geological Repository in Northern Switzerland
Nagra (2024): Radiological Consequence Analysis for a Deep Geological Repository in Northern Switzerland. Nagra Technical Report NTB 24-18.
The radiological consequence analysis assesses the potential radiation dose to individuals from a deep geological repository in Opalinus Clay. This analysis is conducted as part of the general license application process, following regulatory guidelines, for two key purposes: A) to support the safety-based site comparison, and B) as part of the safety case for the repository at the selected site. The analysis includes various cases defined by safety scenarios: a reference safety scenario, several alternative safety scenarios, and a series of “what-if?” cases. Dose calculations based on both the reference and alternative safety scenarios are carried out for both site comparison and the site-specific safety case. Additionally, “what-if?” cases are evaluated exclusively for the safety case of the repository at the chosen location, in accordance with ENSI requirements (ENSI 2018).
Two modelling approaches are adopted, one addressing the transport of radionuclides that are dissolved in porewater within the repository and the surrounding rock, and the other addressing the transport of C-14 methane, along with other repository-generated gases. Model concepts and assumptions are based on system understanding from the assessment basis, including site characterisation and an understanding of radionuclide release, retention and migration processes acquired within Nagra’s RD&D programme, as well as from performance assessment, where, for example, detailed modelling of the fate of repository-generated gas has been calculated. Model assumptions and simplifications are justified on the grounds that they are either in line with this system understanding or are conservative in that they are expected to overestimate the radiological consequences. The models have undergone validation and are benchmarked. The used numerical codes have been verified and shown to be reliable.
Probabilistic simulations are performed for the reference safety scenario for each siting region. The probabilistic simulations are analysed using graphical methods and by evaluating a range of sensitivity measures. These probabilistic simulations are complemented by deterministic calculation cases, in which selected sensitive parameters (identified by the sensitivity analysis) are set to pessimistic bounding values. These are accompanied by the analysis of “combined pessimistic cases”, in which the selected sensitive parameters are assigned pessimistic bounding values. Deterministic analysis is then performed for the alternative safety scenarios. In addition, for the repository at the selected site, “what-if?” cases have been analysed deterministically to assess safety margins in less likely and in extreme, hypothetical cases, and to test the robustness of the system.
The radiological consequence analysis demonstrates that all siting regions offer a robust safety margin across all repository types, with equivalent radiological impact. Even in the highly conservative or hypothetical “what-if?” cases (such as case assuming no sorption throughout the entire containment-providing rock zone), substantial safety margins are maintained. This underscores the strength and safety of the repository, ensuring long-term protection.