Steam generator

Heat transfer tubes are disposed upwardly in an annular region surrounded by an outer shroud and an inner cylinder, and bent spirally to form a helical bundle. Water in the heat transfer tube is converted into steams by heat exchange with external sodium. The heat transfer pipes rise vertically at the upper portion of the helical tube bundle to form a straight tube portion. The top end of them is connected to a steam tube plate passing through a main body penetration portion. The pitch is set smaller at a portion where the heat transfer tubes pass through the main body penetration portion than the pitch at a portion where they are connected to the steam tube plate. With such a constitution, the diameter of the main body penetration portion can be reduced, and the diameter of the upper portion and the lower portion of the steam generator can be made equal to each other. Accordingly, the axial length of the structure of the water feeding pipe bed at the upper portion of the main body can be shortened, so that the steam generator can be made compact. (I.N.)