Alexandros Papafotiou, Typhaine Guillemot, Lukas Martin, Nikitas Diomidis, Paul Marschall
The post-closure safety case for a deep geological repository requires a transparent and traceable demonstration that the repository system fulfils its safety functions and remains robust against uncertainties over relevant timescales. This paper presents the performance assessment workflow implemented in the context of the safety case for a general license application of a deep geological repository for both high-level waste (HLW) and low- and intermediate-level waste (L/ILW) at the Haberstal site in Switzerland. The safety concept relies on a staggered system of passive and redundant barriers, the so-called multibarrier system, each performing specified safety functions in the general post-closure safety architecture. The performance assessment addresses the performance of each component of the multibarrier system, as well as the overall performance of the total system, building upon claims, arguments, and evidence in a transparent and traceable manner. Rigorous handling of uncertainty is an indispensable element in this assessment workflow. The uncertainty framework aims to bracket the confidence bounds on the expected system response evaluated through the simplified model representations of the repository system and the internal or external perturbations acting on it. The possible paths of repository performance are formulated into an expected performance and safety-relevant deviations thereof, assessed in a probabilistic framework with an indicator-based approach. Their safety relevance and likelihood of occurrence form the main input for the development of safety scenarios, which is the next step in the safety assessment workflow. The analysis demonstrates a robust performance of the geological repository through a redundant barrier system that fulfils the intended safety functions under both expected repository performance and deviations from it. The presented methodology provides a transparent and systematic framework for evaluating performance in support of the safety case. The assessment integrates quantitative indicator-based analyses, management of uncertainty, and evidencing of argumentation, contributing to confidence building in the long-term safety of the Swiss deep geological repository.
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.
Recently, several authors have proposed that multi-physics calculations involving coupled neutronics and thermal-hydraulics are most efficiently performed using tight or ‘inline’ coupling. This consists of passing neutronic information to the thermal-hydraulics solver before the neutronics solution is fully converged, providing benefits in stability and runtime. In the context of Monte Carlo neutronics, this work considers the effects of correlation on the results of these calculations by varying both the frequency of data exchanges and the neutron population used each cycle. This is done using newly developed multi-physics capabilities in the Monte Carlo code SCONE. The results show that inline coupling throughout the entire Monte Carlo calculation is not only robust to the choice of neutron population, but also reduces the inter-generational correlation between neutrons, producing lower variance results than if the coupled physics fields were not updated. For the 3D pin problem considered, this implies about 2.89× fewer active particles can be simulated in a coupled calculation to achieve the same power errors as an uncoupled calculation using the correct temperature and density fields.
Nuclear security event response within all-hazards systems requires coordination among emergency management, regulatory, law enforcement, technical and forensic actors operating under distinct mandates. This study examines how nuclear security event response is specified within Türkiye’s national emergency management system. Based on structured documentary analysis of publicly available planning, regulatory, strategic and review documents, the study applies the Four C framework to assess communication, coordination, collaboration, and cooperation across institutional interfaces. The findings show that Türkiye’s planning architecture provides a coordinated basis for radiological emergency response. However, nuclear security events are less clearly specified as a distinct response problem. Partially specified interfaces include command transition, forensic evidence management, security-sensitive information sharing, detection-to-response activation, and international notification timing. The study does not assess operational performance or classified capabilities. It provides a case-based diagnostic of how formal plan alignment may not translate into clearly specified operational response arrangements when nuclear security events require emergency management, regulatory, law enforcement, technical, and forensic functions to operate simultaneously.
