PURPOSE: To form respective heat transfer pipes with optimum materials meeting temperature conditions and lower a cost by using the heat transfer pipes made of different materials in a high-temperature steam outlet side and a low- temperature water feed side in a sodium heating steam generator used under high temperature.

CONSTITUTION: In a sodium heating steam generator, the material of a helical coil type heat transfer pipe 1 is changed in its intermediate part so that the heat transfer tube is provided with the range of a heat resistant alloy in a high-temperature steam outlet side which is limited to a minimum range and a low-temperature water feed side of a liquid phase side made of a material conforming to general specifications. For example, for the heat transfer pipe in a steam outlet side near a sodium inlet side which becomes high temperature during operation of the steam generator, an improved 9Cr-1Mo steel high in its strength under the high temperature is used. For the low-temperature water feed side during operation of the steam generator, an inexpensive 21/4Cr-1Mo steel is used. Thus, the helical coil type heat transfer pipe 1 can maintain required strength during operation of the steam generator and the cost can be lowered.