Radiocarbon (C-14) is potentially important in radioactive waste. Because the carbon transfer through the biosphere is principally different from the transfer of other radionuclides, using an empiric soil-to-plant transfer coefficient (also called soil/plant concentration ratio) in the same way as with other nuclides is inappropriate and can lead to wrong results in biosphere assessment models. C-14 concentrations and fluxes in the biosphere are strongly influenced by the concentrations and fluxes of stable carbon. We developed a new process-based model of the soil to plant transfer of C-14 for the use in the safety assessments of radioactive waste repositories. The model describes the steady-state nuclide abundance in the plants due to the fluxes of water and solid matter within the biosphere system. To study the model sensitivity to parameter variations, parameter variations were studied. We found that the carbon cycle plant => soil => canopy determines the uptake of C-14 by crops. The major parameters are the plant productivity (yield) and the exchange rate of carbon between the air within the canopy and the general atmosphere. The quantification of this exchange rate of carbon is, however, a major open issue.