PURPOSE: To enhance the safety by a structure wherein a guide part serving as an annulus part for a primary coolant, which is provided on the outside of the shell of the heat exchanger, a vent pipe, one end of which is open to the guide part and the other end of which is open outside the nuclear reactor container, and a valve installed for the vent pipe are provided.

CONSTITUTION: The annulus part 22 is formed between the shell 11 of the heat exchanger 1 and the guide part 21 surrounding the shell 11. The inlet 23 of the annulus part 22 is open below the minimum low liquid level (b) of the primary coolant of the nuclear reactor containment 2, while the upper end of the annulus part 22 is shielded with an upper plate 24 above the primary side inlet 6a and connected with the vent pipe 25, the other side of which is open through a valve 26 to the outside of the container 2 by piercing through a plug 3. Owing to the structure as described above, the primary coolant 5 ascends the annulus part 22 and flows through the inlet 6a into the shell 11 and then descends the shell 11 while heat-exchanging with the secondary coolant in heat pipes and flows back again through an outlet 7 into the container 2. On the other hand, the gas in the pipe 25 is discharged from the valve 26, which is located at the position with the height too high to produce the syphon braking. In such a manner as mentioned above, the sure cooling is accomplished even when the liquid level of the primary coolant changes.