A heat exchanger according to the present invention has a plurality of heat transfer tubes (10) fastened to upper and lower tube plates (5, 23) provided in a heat exchanger body (3). An upper plenum (24) is formed in the portion of the interior of the heat exchanger body (3) which is above the upper tube plate (23), and a lower plenum (6) in the portion thereof which is below the lower tube plate (5). The upper and lower plenums (24, 6) are communicated with each other via the heat transfer tubes (10). A downwardly-extending tube (16) extending through the upper plenum (24) upper tube plate (23) and used to introduce secondary sodium into the lower plenum (6) extends among the heat transfer tubes (10) to be fastened to the lower tube plate (5) and communicated with the lower plenum (6). The downwardly-extending tube (16) is stretched through the lower plenum in the axial direction thereof to contact at its lower end a bottom surface (34) of the heat exchanger body (3). A plurality of openings (17) are provided in a wall of the portion of the downwardly-extending tube which is in the lower plenum (6).
The primary sodium of a high temperature flows into the heat exchanger body from the portion thereof which is below the upper tube plate (23), and flows out of the heat exchanger body from the portion thereof which is above the lower tube plate. The method for supplying the secondary sodium of a low temperature is characterized in that the secondary sodium flows downward through the downwardly-extending tube (16) to be ejected in the horizontal direction from the above-mentioned openings (17) into the lower plenum (6). The secondary sodium ejected into the lower plenum flows in parallel currents therein to enter each of the heat transfer tubes and advance upward. The secondary sodium heated in the heat transfer tubes (10) is ejected into the upper plenum (24).