COCOSYS is being developed and validated for the comprehensive simulation of severe accident propagation in containments of light water reactors. This system is to allow the simulation of all relevant phenomena, containment systems and conditions during the course of design basis accidents and severe accidents. In COCOSYS, mechanistic models are used as far as possible for analyzing the physical and chemical processes in containments. Essential interactions between the individual processes, like e.g. between thermal hydraulics, hydrogen combustion as well as fission product and aerosol behavior, are treated in an comprehensive way. With such a detailed approach, COCOSYS is not restricted to relevant severe accident phenomena, but will also be able to demonstrate interactions between these phenomena as well as the overall behavior of the containment.