In the year 2002, German nuclear research centres under the leadership of Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) initiated a concerted action for the “Development and Application of Computational Fluid Dynamics (CFD) Programmes for Phenomena in the Primary System of Light Water Reactor”. The goal of this CFD network is the development of a CFD-software package for the efficient and accurate simulation of reactor safety relevant fluid flow and heat transfer processes. The investigations are primarily concerned with three-dimensional flows in the primary system and containment of pressurised water reactors. Later, flows in other reactor component, e.g. in the reactor core will also be investigated.