FLUTAN is a highly vectorized computer code for 3D fluiddynamic and thermal-hydraulic analyses in Cartesian or cylinder coordinates. It is related to the family of COMMIX codes originally developed at Argonne National Laboratory, USA (Shah et al, 1985). FLUTAN was developed in order to simulate single phase flows with small compressibility. The conservation equations for mass, momentum, energy, and turbulence quantities are discretized in a struc­tured grid using a finite volume method. A staggered grid is used for the velocities. The discre­tization of the diffusive terms is performed by a central difference method (CDS). A first order upwind or one of two second order upwind methods (QUICK (Leonard 1979) and LECUSSO (Gunther 1992)) can be chosen for the convective terms. A first order implicit Euler-method is used for time discretization. Several turbulence models are available in FLUTAN. The most important one for buoyant flows is the Turbulence Model for Buoyant Flows (TMBF) which consists of a first order k-s model in a low-Reynolds number formulation and a second order five-equations turbulent heat flux model (Carteciano et al. 1997). In several benchmarks it turned out that the TMBF in its current development status is at least a powerful tool for forced and mixed convection (Baumann et al. 1997). Special thermal boundary conditions are available in order to simulate different thermal situations like a heat exchanger model and a wall model. A 3d heat conduction model for the structures was developed for the investigation of the SUCOS experiments. This is necessary for simulating solid structures with internal non­uniform transport of heat; it was required to achieve realistic boundary conditions for the fluid domain at the heated copper plate in the SUCOS experiments. The structure temperatures are discretized on an own grid on which the heat conduction equation is solved in all dimensions independent of the solution of the corresponding equation in the fluid domain.