The disposal canister plays a central role in ensuring long-term safety in many concepts for the geological disposal of high-level radioactive waste. This role is particularly important in disposal systems that rely on the robustness of engineered barriers as the primary containment, especially during the early thermal phase following waste emplacement. After repository closure, the canister will be exposed to evolving geochemical and physical conditions that gradually lead to its degradation. As a result, the ability of the canister to fulfil its confinement safety function will diminish over time, eventually allowing the release of radionuclides into the surrounding environment. Understanding and quantifying this evolution is therefore essential to demonstrating compliance with regulatory requirements and maintaining confidence in the performance of the disposal system.

To support the development of safety assessments and the broader safety case, a variety of modelling approaches have been developed to evaluate canister behaviour over various timescales and with different levels of detail. These approaches can broadly be categorised as high-level performance assessment models and low-level mechanistic process models. The high-level models are often designed to represent the system in a simplified and conservative manner, providing the quantitative basis for the assessment of canister performance over long timescales. In contrast, mechanistic models aim to improve detailed process understanding, often serving as tools to interpret experimental results and support the justification of model assumptions used in safety assessment.

In this contribution, we present representative examples of both types of modelling approaches. For performance assessment, we discuss modelling strategies developed to estimate the long-term corrosion of copper canisters due to sulphide under repository conditions. These models range from simple back-of-the-envelope mass and flux balance approaches to more complex representations involving 3D reactive transport, allowing different levels of abstraction and caution depending on the safety assessment context and the underlying knowledge base. We discuss how these models are used to underpin safety arguments and to identify and prioritise areas for further research.

To illustrate the application of mechanistic process models, we describe a state-of-the-art model developed to simulate the corrosion of carbon steel canisters under conditions representative of the post-closure phase. This model couples key processes such as gamma-radiation-induced radiolysis of water and chloride, electrochemical corrosion reactions under oxic and anoxic conditions, and geochemical interactions. It comprises tens of reactions and more than 60 chemical species. The model is applied to interpret experiments, providing insights into the chemical evolution at the carbon steel–bentonite interface and within more distal regions, including the precipitation of secondary phases.

Together, these modelling approaches contribute to a robust understanding of canister performance and its evolution, reinforcing the scientific basis of long-term safety assessments.

How to cite: Pekala, M., Pont, A., and Idiart, A.: Approaches to modelling canister corrosion: from mass balance to coupled electrochemical models, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-168, https://doi.org/10.5194/safend2025-168, 2025.

Within the multi-barrier system for deep geological repositories for high-level nuclear waste, the stability of potential canister and geotechnical barrier materials is crucial for long-term safety. Over time, it is expected that the near field of the canister will become saturated with groundwater and the geochemical environment consequently changes from oxic – warm – unsaturated to anoxic – cool – saturated conditions. This change in the geochemical environment impacts the material behavior of canister and buffer materials over long time. Corrosion of the canister materials might occur inducing a change in the properties of the canister and/or buffer material.

The aim of the study presented is to enlighten fundamental corrosion processes of potential canister materials in bentonite suspension under anaerobic conditions at elevated temperature. Four metals were studied: wrought copper cold rolled, oxygen-free phosphorous-doped wrought copper, cast iron with spheroidal graphite (EN-GJS-400-15), and a stainless steel (grade: 1.4021). All metal coupons were ground with 600/800/1200-grit SiC paper and subsequently cleaned with acetone to guarantee a defined surface state before exposure experiments. Grinding removes any oxide layers, ensuring a homogeneous and even surface. Metallographic pre-characterization of the coupons was performed by light optical (LOM) and scanning electron microscopy (SEM). Two different bentonites (1.) commercially available bentonite (Sigma Aldrich) and 2.) granulated Wyoming-type (Bara Kada)) were used for the exposure experiments. Bentonite slurries were prepared by mixing 5 g of bentonite with 100 ml of synthetic Opalinus clay porewater. Two distinct compositions of synthetic Opalinus clay porewater were used. The prescribed bentonite suspension and one coupon each were placed into steel autoclaves equipped with a Teflon liner. To ensure anaerobic conditions, the autoclaves were assembled within a glovebox. After assembling and airtight sealing, the autoclaves were kept at 60 °C for 60 and 90 days. Subsequent to the end of the experiments, the autoclaves were transferred into a glovebox, where they were dismantled and the coupons prepared for further analysis. The following characterization techniques are used: (µ)-X-ray diffraction (XRD), µ-X-ray-fluorescence analysis (µXRF), SEM, LOM and Raman spectroscopy.

The results indicate that localized corrosion is the predominant corrosion mechanism on the iron based materials. The attached digital microscopy image shows the localized corrosion of steel 1_4021 after 60 days at 60 °C. Complementary SEM-EDX and µXRF analysis revealed the formation of a Cr-oxide as the main corrosion product. The impact of the observed corrosion characteristics for the long term safety will be discussed.

How to cite: Panjiyar, L., Haupt, E. M. I. W., and Stephan-Scherb, C.: Comprehensive insights into corrosion processes of potential canister materials in bentonite suspension at elevated temperature, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-28, https://doi.org/10.5194/safend2025-28, 2025.

The stability of canister and geotechnical barrier materials are crucial aspects for the of long-term safety in the established concepts of a multi-barrier system for deep geological repositories of high-level nuclear waste. However, to build confidence in numerical models used to estimate the lifetime of a canister in the safety case, the right experimental data is often a key aspect.

In the context of considering the long-term stability of copper as a container material for the Swedish waste management program, a study from 1986 questioned the thermodynamic stability of copper under anaerobic conditions on the basis of experimental data [1]. This gave rise to a scientific debate that continues to this day and includes various corrosion processes [2]. This prompted the Swedish waste management organization SKB to carry out another comprehensive experimental and theoretical research program to support the numerical model-based prediction of wall thickness loss in copper canister materials [3]. Even today, the investigation of potential corrosion processes that have an influence on material stability in the detection period has not yet been completed internationally. For a timely development of canister materials for the outer wall (in contact with the geotechnical barrier) a sound strategy for analyzing corrosion in an application-relevant environment should be pursued from the outset. At the current status of the German site selection act, the geochemical environment as well as the electrolyte in contact with the canister and surrounding buffer material are not defined yet. However, a change in geochemical conditions over longer time is expected and it will affect the long-term behavior of the canister and buffer materials. The controversy regarding the corrosion of copper in the Swedish disposal program makes it clear how crucial it is to carry out meaningful experiments to support safety analyses. But what experimental data is needed to support each model? How must corrosion experiments be designed and results analyzed and interpreted in order to support a model with confidence?

As an impulse for further discussion, the contribution will review the controversy regarding the corrosion of copper and place it in a current context of existing knowledge on corrosion processes and their modeling in the repository context. Furthermore, own lab-based experiments in two different approaches A. in compacted Bentonite and B. in Bentonite slurries with synthetic pore water compositions are discussed in the context of corrosion under application relevant environments.

[1] G. Hultquist, Hydrogen evolution in corrosion of copper in pure water, 1986, Corros. Sci., 26, 173–177

[2] Entsorgungskommission, Diskussionspapier, „Diskussionspapier zur Kontroverse um die Verwendung kupferbeschichteter Behälter für die Endlagerung hochradioaktiver Abfälle“, 2021

[3] Supplementary information on canister integrity issues; SKB Technical Report TR-19- 15, March 2019

How to cite: Stephan-Scherb, C. and Panjiyar, L.: Trust in corrosion analysis for canister materials: status and challenges, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-134, https://doi.org/10.5194/safend2025-134, 2025.