Potential fires in the COLUMBUS Attached Pressurized Module (APM) shall be extinguished by reducing the O2 concentration in the atmosphere below 15 %. For this purpose a CO2-distribution system is foreseen. It injects CO2 stored in a tank into the volume where fire is to be extinguished.Due to its dimensions the most critical of these volumes is the subfloor with the stand-off areas. To investigate the fire suppression process a detailed three dimensional computational fluid dynamic analysis (CFD-analysis) was performed. The transient CO2-distribution mechanisms, forced convection and diffusion, were analyzed to examine the feasibility of the foreseen system and to optimize it.In this paper the governing physical processes and their implementation in the mathematical model of the problem are described. The very complex inlet conditions - speed of sound, tiny nozzles - are examined in detail to investigate a proper method for implementation in the mathematical model. Several parameter runs with different nozzle positions and mass flow rates were carried out. They will be described and evaluated w.r.t. their potential for fire suppression purposes.The computation showed that a fire in the subfloor can be extinguished by blowing CO2 into the volume. The system was optimized in order to reduce leakages into other volumes and to reduce the time required for fire suppression. After optimizing the system this time could be reduced significantly.