Thermal radiation and conduction are the main modes of heat transfer in a packed bed with internal heat generation during a loss of forced cooling incident. Due to the large number of spheres, the only feasible way of treating the bed in Computational Fluid Dynamics is through a porous medium approach. Traditionally, CFD treats a porous medium as a fluid zone, whilst fluid properties are adjusted to account for the presence of the solids. In this work, the bed is treated as a gray gass, emitting and absorbing thermal radiation at the solid temperature. A unit cell approach is used to determine the absorption and scattering coefficients. This approach prevents direct radiation between surfaces on opposite sides of the bed, but allows the free surface of the bed to exchange radiation with surrounding structures. Results generated with the proposed model were compared to modelling results using an effective conductivity (Zehner-Schlu¨nder model [1]) to combine the contributions of conduction and radiation.

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