Approaches to modelling canister corrosion: from mass balance to coupled electrochemical models

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.