Pressurized water reactor

A nuclear reactor comprising a pressure enclosure, a reactor core contained in said enclosure, at least one primary circuit in which a primary fluid circulates, a secondary circuit in which a secondary fluid circulates, a steam generator through which said primary fluid transfers heat given off in said reactor core to said secondary fluid, said steam generator having a vertically axed cylindrical envelope and a group of tubes in which the primary fluid circulates, said envelope having a lower intake for entry of the secondary fluid in a liquid state and an upper outlet whereby said secondary fluid leaves the envelope in a gaseous state, a reserve tank positioned in said secondary circuit parallel and at the same time level as the steam generator, said reserve tank being permanently connected with the steam generator and containing a volume of said secondary fluid in the liquid state surmounted by a volume of said secondary fluid in the gaseous state, and a cooling circuit incorporating a heat exchanger located within the reserve tank and below a minimum level of said secondary fluid in the liquid state, said cooling circuit being adapted for removing heat transmitted to the secondary fluid contained in the reserve tank via the steam generator in case of failure of the secondary circuit.