PURPOSE: To reduce thermal stress generated in a shell of a heat exchanger in proximity to a liquid surface, by a method wherein a heat-accumulating body disposed along a suspended shell is driven in the vicinity of a sodium liquid surface when changing the output.

CONSTITUTION: A primary sodium inflow shut-off valve 6 is used also as a heat- accumulating body disposed along the suspended shell 8 and capable of being driven to move to the upper side of the sodium liquid surface. At the time of starting, a steep axial temperature gradient is generated in the sheet 8 in the vicinity of the liquid surface, due to the rise in the sodium temperature and the rise in the sodium liquid surface. To cope with this, the valve 6 immersed in the sodium and heated up to the sodium temperature is raised into a cover gas space to effect heat exchange between the valve 6 and the suspended shell 8, whereby the shell temperature in the gas space in the vicinity of the liquid surface is made to approach the sodium temperature, resulting in a substantially linear temperature distribution. Accordingly, the thermal stress generated in the shell 8 is reduced, and higher reliability of the faste breeder reactor can be contrived.