Cement-based materials in the multi-barrier system of nuclear waste repositories – impact on radionuclide retention

The safe disposal of long-lived nuclear waste is a grand societal challenge and part of the current energy transition in Germany. Safety concepts regarding nuclear waste disposal in underground repositories generally rely on a combination of engineered and geological barriers, which minimize the potential release of radionuclides out of the containment providing rock zone or even the transport into the biosphere. Cementitious materials are very important in this context. They are used for conditioning of certain nuclear waste types, as components of waste containers and overpacks as well as constituents of structural materials at the interface between backfill and host-rocks in some repository concepts. In order to assess the potential impact of cement-based materials on the (geo)chemical behaviour of radionuclides in a repository system, targeted studies on the interaction of radionuclides with cementitious materials are required.
In the event of water interacting with cementitious materials, pore water solutions characterized by (highly) alkaline pH conditions will form. These boundary conditions define the chemical response of the radionuclides, but also influence the behaviour of neighbouring components of the multi-barrier system, e.g. bentonitic or argillaceous backfill and host-rocks, respectively. Hardened cement pastes are considered to be the main sorbing materials present in the near field of repositories for low and intermediate level waste. Hence, interactions of radionuclides with cementitious materials represent a very important mechanism retarding their mobility and potential migration from the near field. In addition to a robust quantitative description of the sorption processes (usually in terms of sorption coefficients, i.e. Kd values), the detailed mechanistic analysis and understanding of sorption phenomena provide additional scientific arguments and important process understanding and thus enhance both the quality of safety arguments and the overall confidence in the safety assessment process.
The NUSAFE partners KIT, HZDR and FZJ have contributed to a significantly improved understanding on various processes related to the retention of radionuclides on cement-based materials. Within the framework of the EU project EURAD (https://www.ejp-eurad.eu/), NUSAFE partners are contributing to and coordinating the workpackage (WP) Cement Organic Radionuclide Interactions - CORI, providing new insights into the role of organic molecules with respect to radionuclide mobility. With a focus on the elevated ionic strength conditions expected for the north German clay formations, NUSAFE partners likewise collaborate within the GRAZ II project, funded by the Federal Ministry for the Environment, Nature Conservation, Nuclear Safety, and Consumer Protection (BMUV). Further work is performed within bilateral international contracts or cooperations. Apart from the frequently studied actinide elements, long-lived fission and activation products receive specific attention. The combination of classical experimental (wet-chemistry) methods, advanced spectroscopic techniques, and theoretical calculations provides both an accurate quantitative evaluation and a fundamental understanding of the sorption processes. The present state-of-knowledge as well as main remaining uncertainties affecting the retention processes of radionuclides in cementitious environments under different conditions will be critically discussed.