In a nuclear reactor, a core barrel (46) is arranged in a reactor vessel (41) having an inlet nozzle (44) and an outlet nozzle (45), the core (53) is arranged in the core barrel (46), a lower plenum (58) is sectioned by the reactor vessel (41) and the bottom of the core barrel (46), a downcomer section (59) is sectioned by the reactor vessel (41) and the sidewall of the core barrel (46), and heat exchange efficiency is enhanced by providing the lower plenum (58) with a ring-shaped upper ring (65) and lower ring (69) and a straightener member (61) consisting of a plurality of spokes (64, 68) arranged radially on the inside of each ring (65, 69) thereby supplying coolant introduced into a pressure vessel uniformly from the lower plenum in the radial direction and circumferential direction for the core, thus improving the heat-exchage efficiency.