Session T4b

T4b | Safety across time – methods for evaluating the post-closure safety of a deep geological repository

Safety across time – methods for evaluating the post-closure safety of a deep geological repository

Main Session Organizers: Johan Liakka, Sari Peura, Fabien Magri

Orals

| Thu, 18 Sep, 13:40–15:00 (CEST)|Room Plenary

Posters

| Attendance Thu, 18 Sep, 17:20–18:20 (CEST)|Poster area

Storing the radioactive waste in a deep geological repository (DGR) is considered a robust solution to minimize the risks to human health and the environment, even in the far future. However, how do we know it is truly safe? How can we manage uncertainties in an analysis extending up to 1 million years into the future? This session will focus on the methods used to assess post-closure safety of a DGR. We invite contributions related to all aspects of post-closure safety, from evaluating how the technical barriers contribute to the long-term safety of a DGR to calculating radiation doses to humans as well as non-human biota. Suggested topics may include (but are not limited to):
• Quantifying uncertainties related to long-term evolution of a DGR and its environment
• Defining scenarios for post-closure safety analyses, including different climate evolutions
• Evaluating how natural and technical barriers contribute to post-closure safety
• Determining representative persons and life habits over long timescales
• Evaluating radionuclide transport and dose to humans and non-human biota
• Assessing landscape development and identifying potential release areas
• Strategies for using generic versus site-specific data in post-closure safety analyses
• Methods for choosing and evaluating Features, Events and Processes (FEPs) of importance for post-closure safety
• Engineering and modelling perspectives on post closure safety
• Are there alternative waste-management solutions to DGRs?

Orals: Thu, 18 Sep, 13:40–15:00 | Room Plenary

13:40–14:00

safeND2025-119

Sensitivity analysis of the natural-barrier-based Swiss repository system: The safety is in the rock

Tim Vietor, Michael Schnellmann, Xiaoshuo Li, and Olivier Leupin

To protect humans and the environment from radiological risks, deep geological repositories for high-level waste rely on a system of staggered passive natural and technical barriers that collectively ensure post-closure safety. In Switzerland, the protection criteria were defined by the regulator as a dose rate. Given that the next 1 Million years have to be considered for post-closure safety, the significance of the natural barrier i.e. the clay rock sequence plays a particularly relevant role in limiting the release of radionuclides.

At the designated location close to Zurich a 250 m thick claystone sequence forms the geological barrier of the repository. This natural barrier exhibits very low hydraulic conductivities and the hydraulic gradient between the overlying and underlying aquifers is small. Thus - under present conditions - advective flow through the repository zone is practically absent. Future fault-related flow is unlikely due to the tectonic stability of location and the self-sealing processes in clay rich rocks, for which evidence is available over ranges of spatial and temporal scales.

In the absence of relevant water flux, waste particles released from the technical barrier system can only leave the repository by diffusion through the pore space of the rock. The corresponding transport parameters can be derived from lab studies and -at the meter-scale- from rock lab experiments. Transport parameters are systematically related to the rock’s clay content, as it controls the pore space geometry. These insights lead to confidence in the parametrization of nuclide species retention properties of the geological barrier.

The dose calculations for the current concept indicate a large robustness of the system due to the extremely slow transport through the natural barrier. Even with canister lifetimes at the regulatory minimum of 1000 years the clay formation limits the maximum individual dose contribution from the geological repository to a small fraction of the regulatory threshold of 0.1 mS/yr. Most of the nuclides are immobilized in the first few meters of the clay formation and decay in the barrier system. The robustness of the system is supported by the results of further alternative cases with reduced sorption in the backfill material. The deviations from the reference case in any of these cases are very small.

In comparison to technical-barrier-based systems (e.g. in crystalline rocks) canister lifetime and nearfield properties are less relevant for the radiological risk of a clay-based repository for high-level radioactive waste or a combined repository. In the Swiss clay-based repository concept the geological barrier alone is sufficient to ensure a dose far below the regulatory limit. In a stable, high clay content geological situation the technical barriers can therefore be designed with a specific focus on operational and early post closure aspects.

How to cite: Vietor, T., Schnellmann, M., Li, X., and Leupin, O.: Sensitivity analysis of the natural-barrier-based Swiss repository system: The safety is in the rock, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-119, https://doi.org/10.5194/safend2025-119, 2025.

14:00–14:20

safeND2025-100

Code development and verification for the review of long-term safety analyses

Jens Eckel

Long-term safety analyses need to be performed by the implementer to identify adequate siting regions in the course of the site selection process in Germany, regulated by the Site Selection Act (Standortauswahlgesetz - StandAG). The Federal Office for the Safety of Nuclear Waste Management (Bundesamt für die Sicherheit der nuklearen Entsorgung - BASE) as responsible federal authority has to review the implementer’s long-term safety analyses. To perform this duty at the required detailedness, and to identify potentially missing processes, it will be necessary to recalculate important aspects of the analyses by means of numerical computer programs. In addition, this will allow to assess the underlying uncertainties of the implementer’s long-term safety analyses from a regulatory point of view.

Since 2022 BASE operates the research project ERLa (Entwicklung und Verfizierung von Rechenprogrammen zur Bewertung von Langzeitsicherheitsanalysen) which will terminate by the end of 2025.

Within ERLa starting points were set to further develop and use the open source programs PFLOTRAN and FEHM for the review of long-term safety analyses. PFLOTRAN is an open source, multi-phase flow and reactive transport simulator designed to leverage massively-parallel high-performance computing to simulate subsurface earth system processes. FEHM is used to simulate reactive groundwater and contaminant flow and transport in deep and shallow, fractured and unfractured porous media and allows for a coupling of the transport processes with geomechanical processes. Moreover, BASE develops its own transport program MARNIE2 which allows flow and transport calculations including processes which are relevant in long-term safety analyses. Since numerical modelling and the development of the computer programs in use requires a high degree of quality assurance, verification has also been a key issue in the research project. To timely react on the requirements during the site selection process a workflow for the application of the afore mentioned programs has been set up including standard pre- and postprocessing techniques as well as tools for sensitivity analyses. Beyond these technical topics a strategy has been developed considering the main aspects of research and development needed to perform modeling duties at BASE.

This contribution presents selected results of the research project ERLa which have a direct link to the review of long-term safety analyses within the site selection process. Research and development as executed in ERLa need to be carried out before their results like e.g. verified computer programs are actually needed. To this effect a well planned research which anticipates the needs of a regulatory assessment contributes to a time-efficient decision of the regulator which is one part of the topic ‘Time as a safety factor’.

How to cite: Eckel, J.: Code development and verification for the review of long-term safety analyses, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-100, https://doi.org/10.5194/safend2025-100, 2025.

14:20–14:40

safeND2025-65

Long-Term Impact on Humans from a Repository for Radioactive Waste: Experimentally Verified Radioecological Biosphere Model

Veronika Ustohalova and Anna Kopp

Transport processes of long-lived radionuclides potentially released from a repository for radioactive waste in the far field must be known for the long-term prognosis of a repository system. They represent an integral part of radioecological biosphere modelling. For this, a proper description how the radionuclides migrate from the ground water level via the soil into plants and thus into the human food chain is needed. Data from well-defined experiments represent a valuable basis for the model parametrization. The joint project TRAVARIS, funded by the German Federal Ministry of Education and Research, focuses on the experimental investigation of water and radionuclide transport in the soil and the uptake mechanisms of radionuclides in plants that are interlinked with the model development. The overall objective of the joint project two-fold. Firstly, we aim to deepen the mechanistic understanding of micro- to mesoscale mobility processes of selected radionuclides in the pedosphere and rhizosphere including uptake mechanisms in plants. Secondly, we integrate these findings into the PHREEQC calculations of sorption processes and into the macroscale probabilistic biosphere modeling with dose estimation. In coordination with the project partners, the experiments are set up and the measured values are recorded and included in the model approaches at different scales of the process considerations. The measurement results (parameter values) are used to calibrate the model approaches.

In our contribution, we will present the radioecological biosphere model developed with the software AFRY Intelligent Scenario Modelling. It is a compartment model that consists of three main model parts. The model part “soil transport” describes water level fluctuations in the pore space under the influence of evapotranspiration along several soil horizons and the associated interaction of the radionuclides in the pedosphere. The implementation of water movement and hysteresis effects due to water level fluctuations as well as radionuclide migration is based on the results from lysimeter, and column experiments carried out by the project partners. To describe the sorption processes, the K_d-value variability due do different mineral phases, the organic matter content and the pH value is implemented in the transport model based on the “Smart-Kd concept”. The description of the K_d-value variability is also linked with the results of the PHREEQC calculations of sorption processes. The model part “plant” takes into account the microscale accumulation processes of radionuclides in the root area and the distribution in plant compartments. Here, parameter values resulting from the lysimeter experiments with planting and the rhizoboxes are considered as well as the results of the laboratory investigations on the role of the plant transporters and exudates in the rhizosphere. This is followed by the dose estimation over long periods of time and supplemented with a subsequent statistical analysis of the uncertainties in the determination of the exposure using Monte Carlo simulations (model part “dose estimation”). A sensitivity analysis investigates the influence of individual parameters on the overall result of the biosphere model - the dose to humans.

How to cite: Ustohalova, V. and Kopp, A.: Long-Term Impact on Humans from a Repository for Radioactive Waste: Experimentally Verified Radioecological Biosphere Model , Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-65, https://doi.org/10.5194/safend2025-65, 2025.

14:40–15:00

safeND2025-135

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

Raphael Wuest, Gerhard Mayer, Samuel Diem, Xiaoshuo Li, and Tim Vietor

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.

How to cite: Wuest, R., Mayer, G., Diem, S., Li, X., and Vietor, T.: Considerations of Future Human Actions for a Deep Geological Repository in Switzerland , Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-135, https://doi.org/10.5194/safend2025-135, 2025.

Posters: Thu, 18 Sep, 17:20–18:20 | Poster area

P18

safeND2025-139

Get the current state right first – lessons learnt from a groundwater modelling perspective

Katrin Brömme, Torsten Seidel, and Christoph König

Groundwater models considering density-dependent flow are one of the instruments used to assess post-closure safety of deep geological repositories (DGR). Their task is to assess potential flow paths of radionuclides or other pollutants migrating from the DGR to the near surface environment through long-term prognoses. Often a one million years period is assumed for these long-term prognoses.

Scientific discussions on this topic focus very often on questions whether to include further processes (thermal, mechanical) or how the climate will develop in the next one million years. Somehow, a bit out of focus are the past and the current state where we in the best case have measured values and know much more than about the future in a million years. An important lesson learnt is to get the current state of the hydrological and hydrogeological system right as the main precondition for qualified long-term prognoses. Using examples from the modelling practice, the following aspects will be highlighted.

The analysis should start with a thorough understanding of the near surface groundwater flow system and its main driving forces. On the one hand, a transient groundwater recharge boundary condition is required. It is indispensable for a transient calibration which should be state-of-the-art nowadays. On the other hand, the partially saturated zone and the surface water groundwater interaction need to be described by the model. As the leakage iterations require extra computing time and the focus is shifted too much to the deep groundwater system this part is often omitted. The near surface groundwater flow system influences the deep groundwater flow system and vice versa. Usually, the near surface processes are described through field observation much better than the deep groundwater. Building up a transient 2D model as a start enables a good system understanding from the beginning. After that, the model is upgraded to a fully 3D model including all relevant hydrogeologic units and eventually discrete fractures. But also the 3D model has to include a transient groundwater recharge, the unsaturated zone and the leakage interaction with surface waters. The steady-state calibration using a mean groundwater recharge is the starting point but the model can only be used for prognoses after a good transient calibration result.

Last but not least, the software code used for this kind of analyses should be regularly verified using standard verification examples for flow and density dependent flow and transport.

How to cite: Brömme, K., Seidel, T., and König, C.: Get the current state right first – lessons learnt from a groundwater modelling perspective, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-139, https://doi.org/10.5194/safend2025-139, 2025.

P19

safeND2025-82

Host rock-specific preliminary safety concepts for the representative preliminary safety assessments

Eva-Maria Gottron, Marc Wengler, Bernadette Mensching, Fatemeh Gholami, Lars Wundram, Anne Bartetzko, and Wolfram Rühaak

The Federal Company for Radioactive Waste Disposal (BGE) is responsible for identifying the site with the best-possible safety for the disposal of high-level radioactive waste for at least one million years in Germany. The Site Selection Procedure consists of three phases with an increasing level of detail. The first step of the first phase was completed in September 2020. Ninety sub-areas were identified that are expected to have favorable geological conditions for safe disposal. The potentially suitable sub-areas are located in three different host rocks: rock salt (halite), claystone, and crystalline rock.

Representative preliminary safety assessments are one of the tools in the current phase of the Site Selection Procedure, which assess the extent to which the safe containment of radioactive waste can be expected. In this context, host rock-specific preliminary safety concepts describe how the safe containment in the disposal system is achieved. The disposal system comprises a graded system of geological (host rock and overburden) as well as engineered (backfill, sealing, canister, and waste) barriers. The key safety-relevant features of the disposal system can be summarized in the main safety functions, integrity of the barrier system and retardation of radionuclides. These safety functions are differentiated into a number of subordinate safety functions for each barrier. The interaction of the barriers and their various safety functions ensures that safe containment is achieved by the system as a whole over the assessment period.

Within the preliminary safety assessments, the behavior of the disposal system is analyzed in its entirety, under consideration of possible future evolutions of the disposal system with regard to the safe containment of the radioactive waste. The safety functions provide the understanding of the safety features of the disposal system and mark the starting point for the assessment of key issues with regard to safe containment.

An overview of the host rock-specific preliminary safety concepts will be provided and an insight into their barrier-specific safety functions is given. In addition, the role of the safety functions in the post-closure safety assessment will be outlined.

How to cite: Gottron, E.-M., Wengler, M., Mensching, B., Gholami, F., Wundram, L., Bartetzko, A., and Rühaak, W.: Host rock-specific preliminary safety concepts for the representative preliminary safety assessments, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-82, https://doi.org/10.5194/safend2025-82, 2025.

P20

safeND2025-84

Impact of processes and their qualitative evaluation on the safety functions in a repository system

Yvonne Messerschmidt, Christine Fahrenholz, Bernd Förster, Kim-Marisa Mayer, Florian Panitz, André Rübel, Tobias Wengorsch, Phillip Kreye, Anne Bartetzko, Jens Wolf, and Wolfram Rühaak

The Federal Company for Radioactive Waste Disposal (BGE) is responsible for implementing and performing the Site Selection Procedure in Germany. The Site Selection Procedure consists of three phases. During Step 1 of Phase I, ninety sub-areas that have favorable geological conditions for safe disposal for one million years were identified. The sub-areas cover approximately 54 % of Germany and are located in three different host rocks: claystone, rock salt (halite), and crystalline rock.

In Step 2 of Phase I, which is currently in progress, the ninety sub-areas will be reduced to a limited number of smaller areas that are suitable for exploration. Within this step, so-called representative preliminary safety assessments (rvSU) are applied. These include an evaluation that the safe containment of radioactive waste for the expected evolutions can be achieved. A maximum limit of a fraction of 10^-4 in total and a fraction of 10^-9 annually of both the mass and number of atoms over 1 million years is allowed to be released outside the containment-providing rock zone. A quantitative method was developed based on a 1D finite-differences code for modeling the transport of radionuclides in the subsurface. This method is only suitable for claystones. Rock salt is considered impermeable, and the models do not apply with the data at hand. For crystalline rocks, the currently available data is not sufficient for transport modeling as well.

As an alternative approach to transport modeling, a qualitative method was developed. The EVENT method (Evaluation of developments in the rvSU) evaluates the impact of geogenic processes on the safety functions of the geological barriers (host rock and overburden). Safety functions are defined within the preliminary safety concept. They include geometry – for example, thickness – or hydraulic properties. Geogenic processes include glacial processes, erosion, or volcanism. They are described in FEP (features, events, processes) – catalogues, which are comprehensive, structured descriptions of a repository system and the interactions and dependencies of processes and components. To carry out the evaluation, the assessment period of 1 million years is subdivided into four periods, the cooling phase of the containers, the rest of the current interglacial period, the first glacial period, and the rest of the assessment period. Continuation of the glacial cycles as in the Pleistocene is expected.

The impact of each process on the safety functions is assessed for each period. Not all processes will take place during all periods. Processes can have a positive effect on the safety functions; however, this case is documented but not taken into account for the assessment. Negative impacts are classified (and justified) as “negligible,” “significant,” or “very significant.” In case of the occurrence of a very significant negative impact or a considerable number of significant negative impacts, the safe containment is not ensured for the area.

Assessment tables were set up for each host rock first and are adjusted for each area during the rvSU. All assessments are documented and stored in a sophisticated in-house database.

How to cite: Messerschmidt, Y., Fahrenholz, C., Förster, B., Mayer, K.-M., Panitz, F., Rübel, A., Wengorsch, T., Kreye, P., Bartetzko, A., Wolf, J., and Rühaak, W.: Impact of processes and their qualitative evaluation on the safety functions in a repository system, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-84, https://doi.org/10.5194/safend2025-84, 2025.