Modeling the release of radionuclides to the host rock considering retardation by failed (geo‑)engineered barriers

The final disposal of high-level radioactive waste in crystalline host rock poses particularly high requirements on the integrity of engineered and geo-engineered barriers, especially on the waste canister. Therefore, it is essential to understand and be able to predict the corrosion behavior of waste canisters and the implications for the release of radionuclides to the host rock.

This work shortly summarizes various approaches to model the corrosion of canisters and highlights a modeling approach on the release of radionuclides. Details are given in (Bracke & Philipp 2025).

Numerous approaches with different complexity exist for the modelling of corrosion mechanisms for potential canister materials. Widely-used models are based on empirical approaches.  Some models include mass balance or mass transport. Beyond that, very complex mechanistic models (such as reactive transport models) exist, which are crucial for the demonstration of process understanding within the safety case, justifying the simplifications made in larger numeric models.

When a potential failure of (geo-)engineered barriers occurs, radionuclides could be released into the host rock. This work considers that parts of the barriers are still present after a failure. These parts may retard or delay the release of radionuclides or contain a fraction. This would imply that an instant and complete release of all radionuclides to the geosphere does not take place after a failure of all barriers. The modeling approach uses hypothetical functions and hypothetical parameter values to quantify this retardation potential. As a first approximation e-functions were used to simulate some underlying processes for retardation of radionuclide in different technical barriers. These functions describe the onset time of failure and the time-dependent extent/size of barrier defects of zircaloy hulls and outer canisters. Sorption and diffusion processes in corrosion products of the inner canister and in bentonite were simulated using the Fick’s law with a realistic but hypothetical apparent diffusion coefficient.

The modeling results show as a proof of principle that the consideration of a retention potential of failed barriers, especially sorption on canister corrosion products and the buffer, can have a significant effect on overall radionuclide release to the host rock. Since the modeling used hypothetical functions and parameter values the real retardation potential of failed technical barriers and the subsequent radionuclide release still needs to be developed and evaluated in detail.

Bracke, G. & Philipp, T. (2025): Konzepte und Testrechnungen zur Vorhersage der Behälterintegrität und des Einschlussvermögens im Kristallingestein. Forschungsbericht 3 von 3 zum Projekt „Langzeitintegrität von Behäl-tern in Kristallingestein“ (LaKris). Forschungskennzahl 4722B10401, Bundesamt für die Sicherheit der nuklearen Entsorgung (BASE). BASE-Forschungsbericht, BASE-008/25: Berlin, August 2025.