Two models with different conceptual and numerical approaches have been used to simulate the dispersion of radionuclides released from Sellafield nuclear fuel reprocessing plant. The first is a long-term box-model for simulating dispersion from Sellafield to the Arctic environment in which water fluxes between boxes are obtained from basic oceanographic information. Water-sediment radionuclide exchanges are described through sedimentation and remobilisation processes on the basis of equilibrium distribution coefficients. The second model is a 3D high-resolution model that explicitly solves tidal mixing and suspended sediment transport in the Irish Sea. Exchanges of radionuclides between the liquid and solid phases are now described in terms of kinetic rates. Thus, this model has a general applicability and can be used in situations out from equilibrium. Both models have been used to simulate the dispersion of Cs and Pu in the Irish Sea. Results from the two models are comparable, being differences in the results smaller than should be expected from the very different modelling approaches.