Abstract
From the view point of the safety characteristic, the passive cooling system should be designed for the Very High Temperature Reactor (VHTR) as the best way of the reactor and vessel cooling systems (VCS). So, the gas cooling system by natural convection is the one of candidate system for the VCS of the VHTR. This study is to develop the passive cooling system for the VHTR using the vertical rectangular channel inserting porous materials. From the results obtained in this analysis, it was found that the amount of transferred heat from the heated wall will be increased even if the heat removed by natural convection. It will be important to consider the thermal radiation from the vessel wall to the concrete vessel.