Abstract: Boiling reactor comprising a reactor vessel and a moderator tank arranged therein which surrounds a reactor core with a plurality of vertically arranged fuel boxes. A moderator tank space located below the reactor core is provided with inlet openings for cooling water streaming through the reactor core and is in direct hydraulic connection with the lower ends of the fuel boxes. The wall of the rector vessel is provided with an opening for the supply of a first feed water flow to the cooling water flowing into the lower end of the reactor core, with the upper ends of the fuel boxes being arranged in hydraulic connection with a plurality of riser tubes by the aid of which a steam and water mixture flowing out from the fuel boxes is conducted to a plurality of water separators.