Fire simulations have gained more and more significance, particularly in the context of deterministic as well as probabilistic fire safety analyses for operating nuclear power plants (NPP). An “International Collaborative Project to Evaluate Fire Models for Nuclear Power Plant Applications (ICFMP)” has been started in 1999, to make use of the experience and knowledge of a variety of expert institutions in this field worldwide to assess and improve, if necessary, the state-of-the-art with respect to modelling fires for application to nuclear installations/plants. In the frame of ICFMP two fuel fire experiments have been selected for the Benchmark Exercise No. 4 and four cable tray fire tests for the Benchmark Exercise No. 5, all of these performed in the fire compartment OSKAR of the iBMB (Institut für Baustoffe, Massivbau und Brandschutz) in Braunschweig. In this paper an overview of the calculated results will be given. The simulations of the two large fuel pool fire experiments still indicate difficulties to simulate under-ventilated conditions. The results provide a first indication on the uncertainties of such simulations, although it has to be considered, that the fire load was quite large compared to the compartment volume. The ignition of cables trays as well as the flame propagation on these trays could still not be simulated in an adequate way by the fire codes applied. Different boundary conditions such as pre-heating or the use of more complex cable insulation materials make the situation even more complex.