The atmospheric dispersion of CBRN (Chemical, Biological, Radiological and Nuclear) toxic substances, of accidental or intentional origin, on an urban or industrial complex built area, is a subject of major concern for the safety of people and the protection of infrastructures. In order to face these different types of risks and threats, industrial companies and public authorities need operational simulation tools for assessing, in advance or in emergency, the health consequences of harmful discharges on the population and on first responders. In this context, a new software called BUILD (Building, Urban and Industrial Lagrangian Dispersion model) has been developed to simulate the atmospheric dispersion of gases and particles in a complex built environment. In order to validate this new model, simulations have been performed on the wind tunnel experimental test case of continuous and instantaneous releases behind a 2D square obstacle (Gamel, 2015) and in a regular network of streets (Cierco et al., 2010, Ben Salem et al., 2015). Results of the operational BUILD model are compared with experimental data and with detailed CFD calculations.