FBR type reactors

Purpose: To surely maintain the soundness of a shielding plug and reactor core upper mechanisms by preventing the spontaneous convection of cover gases in an annular space. Constitution: Spontaneous convection preventive rings are disposed in the annular space formed between the reactor core upper mechanisms and the shielding plug. The rings partition the annular space axially to narrow the free flowing region of the argon gases. Further, the temperature difference between the rings is substantially made equal to the value of the temperature difference in the entire axial direction of the conventional annular space divided by the number of the rings. Accordingly, this can effectively prevent the spontaneous convection resulted from the great temperature difference and the viscosity due to the flow of the argon gases. (Yoshino, Y.)