[en] A means for generating vortex flow in an upper plenum is disposed to a suction port of an exit pipeline immersed in primary coolants in the upper plenum. Circumferential distribution at an inner wall of a reactor container is made uniform by the generated vortex flow, to prevent occurrence of thermal stresses. Further, a radiation level of cover gases or FP gases discharged to environment is lowered by extending staying time of the FP gases till they are released to the cover gases from a liquid surface, to transfer radioactive materials in the FP gases into coolants. This can reduce occupational exposure of operators to attain an FBR reactor of high safety. (N.H.)