In the late phase of severe accident phenomena in nuclear reactors a porous debris bed of degraded core materials is formed in the lower head of reactor pressure vessel. Natural convective cooling of this debris bed is of considerable importance for limiting the consequences of the accident. In this study we analyze the related convection processes in porous cavities with internal heat generation. We investigated (i) Natural convection in a square cavity with internal heat generation with adiabatic and isothermal boundary conditions (ii) Natural convection in parallelopiped geometry with convective boundary conditions. Through these problems we study the basic nature of convection in internally heated porous regions and change in flow patterns with change in intensity of internal heating expressed through internal Rayleigh number.

Keywords: Natural convection, Porous media, Internal heat generation, Severe nuclear accident.