In the present study, a numerical investigation of natural convection and volumetric radiation interactions has been analysed in an annulus between two isothermal concentric square ducts filled with a semi-transparent medium. Two-dimensional solution was obtained using a discrete ordinates method based on a combined finite volume-immersed boundary method. The fluid is assumed to be gray absorbing-emitting and non-scattering and all walls are gray, diffuse and opaque. The Rayleigh number is fixed to 106, and a study of the effect of three optical thicknesses τ=0.2-1-5 is performed on the thermal and dynamic fields and consequently on the heat transfer rate has been examined. A pure natural convection case has been shown for a comparative study. It is found that the radiation has a large influence on the flow and surface averaged Nusselt number and especially for high Rayleigh numbers.