This work aims at modelling thermal radiation in a nuclear reactor, in the course of a severe accident leading to its degradation. Because the reactor coolant is water, radiative heat transfer occurs in presence of steam and water droplets. The 3D geometry of a fuel bundle with 21 damaged rods has been characterized from tomography images. The degradation of the rods has been simulated in the experimental small-scale facility PHEBUS.

The homogenized radiative properties of the considered configurations with a transparent fluid phase have been completely characterized by both the extinction cumulated distribution function Gext and the scattering phase functions p. Gext strongly differs from the exponential function associated with the Beer law and p strongly depends on both the incidence and the scattering directions. By using the radiative transfer equation generalized for non Beerian porous media by Taine et al. the radiative conductivity tensor has been first determined with a transparent fluid phase, by a numerical perturbation method. Only the diagonal radial and axial components of this tensor are not equal to zero. They have been fitted by a simple law only depending on the porosity, the specific area and the wall absorptivity. In a second step, a radiative transfer equation based on three temperatures is established. This model takes into account a semi transparent fluid phase by coupling the radiative properties of fluid and solid phases. The radiative properties of water steam and droplets are calculated respectively with the CK approach and Mie theory, in typical thermal hydraulics conditions of reactor accidents. The radiative fluxes verify the Fourier law and are characterized by radiative coupled conductivity tensors associated with the temperatures of each phase. The radiative powers exchanged between phases per unit volume are also calculated from this model.