PURPOSE: To make uniform the flow rate of a primary coolant supplied from an outer cylinder, by providing a channeling-preventing weir along the outside periphery of a shroud, in a heat exchanger constituted of an outer cylinder and a shroud concentrically placed on the outside of the cylinder.

CONSTITUTION: The channeling-preventing weir 12 is provided along the circumferential direction of the outer shroud 3 on the forward side of inflow ports 4 provided between the outer cylinder 2 and the shroud 3. The weir 12 is constituted of a substantially cylindrical ring 12a and a flange 12b formed at the lower end of the ring 12a, has an inside peripheral surface formed to be concentric with the shroud 3, and has a wall thickness increased as a primary-side inlet nozzle 1 becomes farther. Therefore, the gap between the cylinder 2 and the weir 12 becomes narrower as the nozzle 1 becomes farther. Accordingly, the primary coolant introduced through the inlet nozzle 1 flows uniformly into the interior of the shroud 3 through each of the inflow ports.