PURPOSE: To secure the decay heat removing capability of a decay heat removing system by forcefully opening a damper controlling the wind fed to a heat transfer pipe with a damper operating system utilizing thermal expansion.

CONSTITUTION: In a damper operating system 28A, a thermal expansion rod 29 is thermally expanded via heat collecting fins by the heat radiation due to the temperature rise of a sodium inlet distribution pipe 24, and a moving arm 30 in contact with the thermal expansion rod 29 is moved together with the rotation of a roller 36 and dropped by gravity. A damper 26A fitted at the tip of the moving arm 30 is moved downward from the air passage of a cooler main body 21, and the damper 26A is opened. In a damper operating system 28B, the temperature rise of a sodium inlet distribution pipe 24 is transferred to a liquid 39 to cause the thermal expansion of the liquid 39. The liquid surface area of the liquid 39 in a container 38 is formed relatively narrow, the rise quantity of the liquid level is large. A piston ring 40 is lifted by the level rise of the liquid 39, and the coupling between the damper 26B and the piston ring 40 is cut off. The damper 26B is rotated and opened.