PURPOSE: To enable manufacturing a reactor vessel having both a vapor/liquid separation effect and a prompting effect for an increased natural convection amount, and to contrive decreasing a cost of the reactor vessel by making a riser and a separator to be of an integrated structure and by providing a piping with rotating vanes in the riser.

CONSTITUTION: A reactor core 51 is arranged at a lower part of a reactor vessel 1 and a cylindrical riser 2 which surrounds the reactor core 51 and extends far above the core, is provided. A downcomer 16 is formed between the vessel 1 and the riser 2. Also, a plurality of cylinders 3 stand to form a group of the cylinders at a place above the core 51 and below an upper end of the riser 2. Both ends of the cylinder 3 are opened and one rotating vane 4 is arranged at each lower end of the cylinder 3. In such a way that a plurality of rotating vanes 4 are housed in an upper part of the riser 2 being above the core 51, and a vapor/liquid is made to be separated by clinging a liquid film to an inner side of the riser 2 with a centrifugal force given by the vanes 4, when a two-phase flow passes through the wings 4, an axial height for a natural circulation can be restricted and therefore the vessel 1 can be compactly manufactured.