Abstract: Due to the geometrical and flow complexity in nuclear containment under severe accident, a reliable program is required to analyze and assess the flow. The CFD program Code_Saturne was used in this paper which satisfied the nuclear safety standards to simulate the hydrogen and steam injection and flow procedure. This involved the multi-species low speed gas flow, buoyancy induced stratification, heat conduction inside structures, the convective heat transfer and condensation on the gas-structure interface. This program adopted the SIMPLEC-based numerical scheme and low Mach flow algorithm, the ideal-gas based multi-species model and the one-dimensional heat conduction on thin wall structures. Meanwhile, this paper improved the wall function approach for the calculation of convective heat and mass transfer. Finally two international standard problems were calculated to validate the program and improved model.