The safety case for a deep geological repository in Switzerland: radiological consequence analysis and study of the barrier efficiency

Geological repositories for radioactive waste are sited and designed to provide a set of natural and engineered barriers that isolate the waste from the human environment and contain its associated radionuclides. Nonetheless, some release of radionuclides from a geological repository cannot be excluded over the Time frame of hundreds of thousands of years during which the waste presents a potential hazard. Thus, radiological consequence analysis is a key element of the post-closure safety case for deep geological repositories and provides essential insight into the efficiency of the multi-barrier system. This paper presents the current methodology and selected results of the radiological consequence analysis for a combined repository for high-level waste and low- and intermediate-level waste in an Opalinus Clay rock formation hundreds of metres below ground at a proposed site in Northern Switzerland. Deterministic and probabilistic simulations are performed to evaluate radionuclide transport, barrier efficiency, and associated uncertainties. Radionuclide migration in the liquid phase is modelled for a wide range of potentially safety-relevant radionuclides, while gas-phase transport focuses on C-14 released as methane under two-phase flow conditions. Results show that calculated dose rates are several orders of magnitude below Swiss regulatory protection criterion, demonstrating a large safety margin. Long-lived, weakly sorbing radionuclides, particularly I-129, dominate the calculated dose in a reference safety scenario that corresponds to the expected evolution of the repository and in a range of other scenarios, while most other radionuclides are effectively retained within the engineered barriers and the Opalinus Clay host rock. Probabilistic analyses and sensitivity studies identify key parameters controlling radionuclide release. Overall, the analysis confirms the robustness of the multi-barrier system and provides improved understanding of the Swiss deep geological repository system.