The present study describes the implementation of a radiative heat transfer model in the ICARE/CATHARE software for simulating severe accident sequences in a Pressurized Water Reactor (PWR). During such accidents, the reactor core temperatures can reach values greater than 2500 K and it is of major interest to accurately evaluate the radiative tranfers. In this aim, a model is proposed in which the reactor core is considered as an optically thick porous medium. This assumption is valid since the radiation mean free path is much smaller than the core size and allows us to use the diffusion approximation to treat the radiation propagation. An equivalent reactor core conductivity, depending on the medium geometry, is then determined. In the model presented here, this conductivity is established to account for the drastic variations of the geometry of the core undergoing a degradation process. This method provides a practical means to describe the radiative exchanges and to smoothly calculate the transitions between the successive core configurations. Then, on the basis of a PWR severe accident sequence, the satisfying behavior of this model is shown. A comparison with the more classical Hottel approach allows to demonstrate that the present model leads to a better prediction of the degradation owing to a more accurate estimate of the radial heat transfers.