Fire is a major concern for the nuclear safety due to potential severe consequences of an uncontrolled fire on the surroundings of a nuclear plant. Since more a twenty years, a research program addressing this topic is in progress at the french "Institut de Radioprotection et de S^uret_e Nucl_eaire" (IRSN). Within this framework, a computational code, named ISIS, dedicated to the simulation of buoyant fire in a compartment mechanically ventilated, is developed. Physical models of this code are based on the Reynolds-Averaged Navier-Stokes equations, supplemented by a two-equation closure for turbulent ows and the eddy viscosity model. The turbulent production term is adapted to cope with buoyancy effects. Combustion modelling relies on classical eddy dissipation approaches and the ux-method is employed to treat radiation exchanges. Both incompressible and low Mach number ows are dealt with. For the numerical solution, a fractional step algorithm has been developed. To ensure stability and positivity of the discrete operators, the spatial discretization combines mixed finite element for the Navier-Stokes equations and finite volumes scheme for transport (advection-diffusion-reaction) equations. For the verification of the code, a wide range of techniques is employed: comparison to analytical solution for model problems, use of manufactured solution and comparison to benchmark result. We detail in this paper particular application of each kind; in all cases, convergence properties of the scheme are assessed. Validation is now underway, and is based on the so-called building-block approach. We shortly describe some of the obtained results, first for an unit problem and then for a large-scale realistic experiment.