Modeling of radiative heat transfer during a PWR severe accident

In the frame of Institut de Radioprotection et de Surete Nucleaire research programs on severe accidents in pressurized water reactors (PWRs), a new radiative heat transfer model to be used in the ICARE/CATHARE software is presented. The reactor core is considered an optically thick porous medium, and the diffusion approximation is adopted. The equivalent conductivity of the medium is determined. Its expression is carefully established to take into account the strong geometrical variations occurring in a reactor core undergoing a severe accident sequence (as observed in Three Mile Island Unit 2). After describing the theoretical basis of our approach, it is shown that the continuity of the equivalent conductivity is ensured when the geometry evolves from an array of intact cylinders to a particle bed. When compared to the more classical radiation method used in most severe accident codes, this approach better predicts the radial temperature gradient obtained by Cox in his experiment in bundle geometry. The same comparison on a PWR vessel undergoing an accidental sequence brings to the fore the impact of the radiation modeling on the degradation process: The sideward heat losses predicted by the method selected in this work are more limited, which slows the radial progression of the degradation.