Patent Number: 
Section: claims

1. A nuclear reactor, comprising:a vessel closed at a top thereof by a radially external fixed closing structure and by a radially internal mobile closing structure, wherein:the vessel contains a core that is immersed in a primary cooling fluid of the vessel; andthe core includes a plurality of fuel elements, control rods, shutdown rods, and a hydraulic separation structure delimiting a hot manifold and a cold manifold;the primary cooling fluid of the core circulates in the hot manifold and the cold manifold and, during normal operation of the nuclear reactor, a first free surface level of the primary fluid in the hot manifold differs from a second free surface level of the primary fluid in the cold manifold;circulation pumps for circulating the primary fluid; andheat exchangers;wherein the control rods and the shutdown rods are inserted in respective penetrations of the fixed closing structure and located radially external to the radially internal mobile closing structure and external to an upper portion of the hydraulic separation structure containing respective heads of the plurality of fuel elements;wherein the control rods and the shutdown rods are inserted in a radially wider lower part of the hydraulic separation structure via respective ducts that extend from a wall of the hydraulic separation structure above the second free surface of the primary fluid in the cold manifold. 2. The nuclear reactor of claim 1, wherein the control rods and shutdown rods have no mechanical connections with the core. 3. The nuclear reactor of claim 1, wherein rotation of the control rods around an axis of a control mechanism modifies a distance of respective neutron absorbers from an active part of the core. 4. The nuclear reactor of claim 1, wherein the shutdown rods are operated by a float which, via a down-up movement, positions respective neutron absorbers near an active part of the core when the circulation pumps of the primary fluid slow down. 5. The nuclear reactor of claim 4, further comprising a non-return device that allows upward displacement of the shutdown rods and blocks downward displacements to prevent falling in an event of untimely acceleration of the circulation pumps. 6. The nuclear reactor of claim 5, further comprising a control mechanism that engages first on a head of the shutdown rod and then on the non-return device allows, by a programmed intervention, safe re-actuation of the shutdown rod and re-location of the respective absorbers in a position farthest from the core.