Integrating the management of radioactive isotopes into a reactive transport code

Abstract The modeling of radioactive decay chains within reactive transport frameworks remains a complex challenge due to the coupling of chemical reactions, physical transport, and decay processes. Traditional approaches often simplify decay by neglecting chemical interactions or approximating decay chains with first-order reactions. This study introduces a modular methodology employing an analytical solution to decay equations, allowing seamless integration into reactive transport codes. This approach overcomes the classical timestep limitations imposed by element half-lives, while maintaining accuracy through controlled management of numerical errors introduced by the operator splitting technique during decay integration. Implemented in the HYTEC code using a Sequential Iterative Approach (SIA), the method was validated on four test cases involving multiple isotopes, including parent-daughter pairs, coupled with sorption and mineral reactions under equilibrium and kinetic conditions. Results demonstrate robust and accurate simulation of isotope transport and decay dynamics. While validation was primarily one-dimensional, the framework provides a strong foundation for extension to multidimensional, multiphase systems. Future work will focus on applying this model to more realistic and complex scenarios, thereby further validating its robustness and expanding its practical relevance for nuclear waste disposal and environmental geochemistry.