Patent Number: 
Section: claims

1. A nuclear reactor comprising:a vessel having a center axis and containing a primary liquid;a plurality of nuclear fuel assemblies forming a core placed in the vessel;a plurality of core reactivity controllers configured for controlling the reactivity of the core;a plurality of drive mechanisms configured for driving the core reactivity controllers parallel to the center axis, each drive mechanism comprising:a motor comprising a stator and a rotor configured for being rotated by the stator;a driving member comprising a driving part forming one of a screw or a nut;a connector for rotating connection of the driving member with the rotor, having a torquer for applying a rotational torque of the rotor to the driving member;a driven member translationally connected to one of the core reactivity controllers and comprising the other of a screw or a nut, the screw and the nut configured for interacting so that a rotation of the driving member relative to the stator results in a translation of the driven member parallel to the center axis relative to the stator; anda blocking device comprising at least one blocker selectively movable between a blocking position in which the drive member is locked in translation parallel to the center axis relative to the rotor, and a releasing position in which the driving member and the driven member are free in translation parallel to the center axis relative to the rotor between an extreme high position and an extreme low position,the motor of each drive mechanism being fully immersed in the primary liquid inside the vessel,the rotor of each drive mechanism having a central passage, the torquer being located in or close to the central passage;the driving member including a connecting part engaged in the central passage and interacting with the torquer, wherein the connecting part is free in translation in the central passage relative to the rotor when the or each blocker is in the releasing position. 2. The nuclear reactor according to claim 1, wherein the connecting part of the driving member comprises a first section interacting with the torquer in the extreme high position, and a second section interacting with the torquer in the extreme low position. 3. The nuclear reactor according to claim 1, wherein the torquer comprises a plurality of rotary elements configured for rolling along the connecting part when the connecting part moves parallel to the center axis. 4. The nuclear reactor according to claim 1, wherein the connecting part has at least one flat side in a section perpendicular to the center axis. 5. The nuclear reactor according to claim 1, wherein the passage is delimited by a peripheral wall separated from the connecting part by a gap with a width greater than 10 mm. 6. The nuclear reactor according to claim 1, wherein the or each blocker is mounted on a support integral with the rotor. 7. The nuclear reactor according to claim 6, wherein the blocking device comprises at least one polar mass connected to the one or more blockers, and at least one electromagnetic coil interacting with the or each polar mass, wherein the or each electromagnetic coil is fixed in translation and in rotation with respect to the stator. 8. The nuclear reactor according to claim 6, wherein the blocking device comprises at least one elastic member interposed between the driving member and the support, the at least one elastic member urging the driving member to the extreme low position. 9. The nuclear reactor according to claim 1, wherein each drive mechanism comprises a guide configured for guiding the driven member in translation and blocking the driven member in rotation relative to the stator. 10. The nuclear reactor according to claim 9, wherein each drive mechanism comprises an upper frame on which are mounted the motor and the blocking device, and a lower frame on which is mounted the device for translational guiding and rotational blocking, wherein the upper and lower frames are removably attached to one another so that the lower frame is closer to the core than the upper frame. 11. The nuclear reactor according to claim 1, wherein the screw and the nut constitute an irreversible connection, arranged such that a vertical bias applied to the driven member is not converted into a rotational movement of the driving member.