An accurate model of atmospheric dispersion of radionuclides over the complex terrain of La Hague reprocessing plant (North Cotentin, France) has been developed by COGEMA, in partnership with Paris VI University, and has been yet adapted for an other industrial site (Marcoule). This model, called CEDRAT, takes into account areas typically outside the validity range of Gaussian models: topography and building influence, short-distance (beyond 500 m from the release point) and various atmospheric stability conditions. CEDRAT allows to reach an effective compromise between rapidity (30 min on a 6000 nodes grid, on a standard PC), robustness and accuracy, coupled with a user-friendly interface. The validation process consisted first of a comparison with the 3D reference model MERCURE, developed by EDF, and then, results of both models were compared on release scenarios thanks to tracer data (Krypton-85, an inert gas released from a height of 100 m) collected at five monitoring stations around the site. Then a second level of comparison, presented here, was made in collaboration with the IRSN Cherbourg Radioecological Laboratory, on ground and altitude (collected with a tethered balloon) field data.