The groundwater flow and radionuclide (3H, 14C and 137Cs) transport model was performed for the model domain of the experiment site near Ignalina nuclear power plant (INPP) in Lithuania assuming that the radionuclides will reach groundwater straight after the new NPP operation begins. The task of the study is to predict the transportation of radionuclide from the hypothetical reactor of the new NPP and perform a sensitivity analysis in order to predict the effects of radiation on the environment. The main focus of attention was the evaluation of ranges of the parameters, such as effective porosity, hydraulic conductivity and dispersivity, mostly influencing the assessment results. The obtained results show that the activity concentration of the modelled radionuclides strongly depends on the hydrogeological properties of the aquifer. The largest variability of activity concentration is established for the radiocarbon determined by the dispersivity and hydraulic conductivity.