PURPOSE: To increase the amount of a natural circulation, to make a reactor core internal structure compact and to contrive an improvement of an operational safety by eliminating a vapor/liquid separator and by decreasing a pressure loss thereby.

CONSTITUTION: A cooling water heated in a reactor core 2 boils up and rises up in a riser 3 in a form of a void-containing two-phase flow of vapor and liquid, and then a steam containing liquid drop-lets goes up directly from a two-phase interface of vapor and liquid, and finally flows into a steam separator consisting of conical perforated-plates 23 to be separated into vapor and liquid. A separated dry steam flows into a turbine from a main steam pipe 10, passing through a large number of holes of a steam drier. Condensed droplets pass through holes of the lowermost plate 23, flow down in a drain pipe 25 and then flow into a lower part of the reactor core 2 through a down-comer 4. A liquid which flows into the reactor core 2, is reheated at the reactor core 2 and forms a vapor/liquid flow in the same manner as the steam, to be separat ed by the steam separator. In this way to utilize an inner wall of a vessel 1 as a part of the pipe 25, a decreament of a pressure drop during a steam separation process can be realized.