The transport of a radioactive solute during the transient thermo-hydraulic regime with gas generation in and around a disposal cell depends on complex multi-phase processes. Numerical simulations can improve the understanding of the system by providing detailed information on the temporal and spatial distribution of the radionuclides. In particular, their fluxes can be computed under the given transient conditions considering radionuclide, heat and gas release from the waste. However, such detailed multi-phase simulations are very demanding with respect to computational resources and time. Based on the knowledge gained from such complex simulations, we have developed a robust simplified single-phase approach for performance and safety assessment, the improved efficiency of which enables extensive parameter studies. The simplified approach comprises, on the one hand, homogenization of features of high detail and, on the other hand, the employment of two-phase simulation results that are used to deduce equivalent single-phase parameterizations. The results have been validated with various benchmark criteria at well-defined interfaces in the modelled disposal cells based on the simulated radionuclides fluxes.