Coupled modelling of naturally occurring radionuclides in a cementitious engineered barrier

Constructing a robust numerical model that captures multi-mineral transformations, multiple chemical reactions, and secondary phase pathways in geological repositories is challenging due to uncertainties in parameters and a limited available database describing the kinetics of dissolution/precipitation reactions. In this work, combined with experiments, a comprehensive reactive transport model is used to study the chemical and physical interactions among radionuclides, cement leachate and the host rock in a nuclear waste repository. Hence, the modelling efforts will enhance the understanding of the transport of radionuclides in complex soil/rock systems and highlight the critical factors driving their migration in soils/rocks. To achieve these aims, the modelling of the radionuclide migration process was first investigated, considering all possible reactions that could take place. Then, the PHREEQC software was used for the numerical simulation, and experimental data were used to validate the model. The experiment studied a system for 15 months and 15 years with young cement leachate (pH=13) and intermediate cement leachate (pH = 10.8), respectively. Then, with the dissolution/precipitation kinetics implemented and verified, the transport process was incorporated with the aim of building a geochemical model that will describe the multimineral mass transfer under different conditions. Furthermore, the geochemical model was constructed to ensure the porosity evolution of the porous medium. Finally, radionuclide migration was incorporated into the model to characterise the effect of the sorption process. These studies showed that fluid chemistry controls the dissolution/precipitation of the primary minerals, which will control the long-term chemical equilibria and mineralogical composition of the host rock impacted by the alkaline leachate. Meanwhile, the chemical interaction between hyper-alkaline leachate and the host rock results in a series of mineralogical reactions, including cycles of minerals dissolution and precipitation (calcium silicate hydrate gel, C-S-H phases, C-A-S-H phases, hydrated silicate, and Na-Ca zeolites). Furthermore, by coupling the mineral volume changes and porosity evolution to the dissolution/precipitation reaction model, the results showed a better fit in ion concentration compared to the fixed porosity model, as it led to a more reactive surface area with the cement leachate. Moreover, the model shows that the dissolution of primary minerals in the host rock is the initial driving mechanism for the chemical evolution of the system. At the same time, the subsequent precipitation of several secondary phases controls the host rock's long-term chemical equilibria and mineralogical composition. Lastly, the sorption of uranyl (UO2_2+ (U_VI)) was found to strongly depend on the surface complexation model assumed, with no significant removal of U_VI by precipitation or ion exchange process. Furthermore, uranyl adsorption by the C-S-H phase was found to be minimum, which could be related to the lack of surface complexation parameters for C-S-H minerals