The work planned at KIT-INE within RULET aims at improving our understanding of the physicochemical processes (complex formation, redox reactions, retention by sorption and solid phase formation) that define the solution chemistry retention of technetium-99 and carbon-14 in carbonate, silicate and sulfide systems. The focus is on solid phases, oxidation states and solution species as they are to be expected in the immediate vicinity of the waste containers. A particular focus is on chemical reactions and equilibrium processes with iron(II)-containing secondary corrosion phases. The results of the investigations of all project partners will be summarized and used in a simplified transport model for a generic repository. This will be used to assess whether the new scientific findings could contribute to a reduction in conservatism or whether new mobility-increasing processes that have not yet been taken into account could occur. Based on the results, recommendations will be derived as to how the retention and mobilization mechanisms found should be taken into account in safety assessments. The results obtained in this project also extend the physico-chemical data basis required for both thermodynamic calculations of the radionuclide source term and for reactive mass transport models.