[en] This report describes the development of a three-dimensional finite difference model (SWIFT) to simulate the transport by ground water of trace concentrations of radioisotopes. The model was developed for use by NRC in the analysis of deep geological nuclear waste disposal facilities. It is based upon an existing model which treats the injection of liquid chemical wastes into deep saline aquifers. Complete documentation of SWIFT is provided. Comparison of SWIFT results with the analytical solution for constant velocity and diffusivity condition and with the ORIGEN code results for static conditions is presented. A discussion of possible future developments is given. 9 figures, 4 tables