Potentially dangerous events involving heavy gas dispersion and their severe consequences have been largely publicized by the media. Simplified models have been widely applied to describe the effects of these accidents. However, most simplified models deal with flat terrain scenarios and are based on quite crude simplifications of the complex phenomenology involved. In this paper the possibility of simulating the dispersion of heavy gas clouds over a large topographically complex area (tens of km) by a general purpose computational fluid dynamics (CFD) code is investigated. The aim is that of setting up a tool able to produce a realistic description of such dispersion processes, whose results can be useful in considering the question of when and by how much one can be misled by a simplified model. The paper is divided in two parts: the first part describes the use of CFD simulations to generate the micrometeorological parameters which characterize the site before the release of the toxic material, while the second part deals with the dispersion process itself. The geographical region examined is the industrial site of Priolo-Gargallo/Augusta in the eastern part of Sicily, covering an extension of 30 x 30 km(2).