Patent Number: 
Section: claims

1. A nuclear reactor comprising:a vessel having a central axis and containing a primary liquid;a core comprising nuclear fuel, the core being arranged in an internal volume of the vessel;at least one primary pump generating a main primary flow of the primary liquid in the vessel, the main primary flow passing through the core, rising in a direction parallel to the central axis in a rising part of the main primary flow and descending through at least one vapor generator located on a periphery in the vessel in a descending part of the main primary flow, the main primary flow having a reversal zone between the rising part and the descending part, a circulation of the primary liquid forming a turn back in the reversal zone;at least one control member, the reactivity of the core being controlled by the at least one control member, the at least one control member extending in a respective direction parallel to the central axis and being aligned with the core in the respective direction parallel to the central axis;at least one movement mechanism configured for moving the at least one control member, the at least one movement mechanism being arranged in the internal volume of the vessel and linked to the at least one control member; anda pressurizer situated in a top portion of the vessel along the central axis, the pressurizer being in communication with the primary liquid,the movement mechanism comprising an electric actuator and a transmission mechanism,the electric actuator being completely immersed in the primary liquid, the electric actuator being situated outside the main primary flow, the electric actuator being located above the reversal zone in the direction of the central axis and below the pressurizer in a direction parallel to the central axis,the electric actuator comprising:a motor comprising a stator and a rotor configured to apply a rotary torque, andthe control member being configured to be released into the core by gravitational drop by a release system;the transmission mechanism comprising:a driving device comprising a movement part, the movement part being linked to the motor such that the rotary torque of the motor is applied to the movement part,a driven member comprising one of a screw or a nut, the driven member being driven in rotation by the movement part, anda transmission member forming the other of the screw or the nut secured to the control member,the screw and the nut cooperating such that a rotation of the movement part of the driving device relative to the stator results in a translation of the control member parallel to the central axis,the driven member has an orifice extending along the respective direction parallel to the central axis at least in an upper part of the driven member over a length greater than a maximum movement of the control member, the movement part of the driving device having a polygonal section extending at least partially in the orifice over a length of the orifice, the orifice having a corresponding polygonal section at least in an upper part of the orifice, the driven member and the movement part cooperating such that when one of the driven member and the movement part rotates, the other of the driven member and the movement part rotates,the orifice being an internal orifice in the driven member. 2. The nuclear reactor according to claim 1, wherein the transmission member bears the nut, the driven member comprising the screw, the nut cooperating with the screw on the outer diameter of the screw. 3. The nuclear reactor according to claim 1, wherein the driven member has, in the upper part of the driven member, at least one through hole, the through hole connecting the orifice to an outside of the driven member. 4. The nuclear reactor according to claim 1, wherein releasing of the control member is obtained by releasing the driven member. 5. The nuclear reactor according to claim 4, wherein the driven member comprises a shoulder, the release system comprising at least one element movable between a holding position and a release position, the movable element extending below the shoulder in the respective direction parallel to the central axis in the holding position and not extending below the shoulder in the respective direction parallel to the central axis in the release position. 6. The nuclear reactor according to claim 1, further comprising a cooling line for cooling the electric actuator, the cooling line withdrawing the primary liquid from the main primary flow leaving the core or at a discharge from the primary pump, the cooling line further passing by the electrical actuator for cooling said electrical actuator, the withdrawn primary liquid travelling from the withdrawing from the main primary flow to the passing by the electric actuator in a travel time, the cooling line being configured such that the travel time is greater than a minimum duration. 7. The nuclear reactor according to claim 6, wherein the minimum duration is greater than 50 seconds. 8. The nuclear reactor according to claim 7, wherein the minimum duration is between 50 seconds and 150 seconds. 9. The nuclear reactor according to claim 1, wherein the movement part extends inside of the driven member longitudinally in the direction of the central axis. 10. The nuclear reactor according to claim 1, wherein the movement part contacts the inner surface of the driven member. 11. The nuclear reactor according to claim 1, wherein a portion of the driving device extends inside of the rotor longitudinally in the direction of the central axis. 12. The nuclear reactor according to claim 1, wherein the driving device comprises a rod extending parallel to the central axis, the movement part making up a lower segment of the rod. 13. The nuclear reactor according to claim 1, wherein a top part of the driven member has a polygonal outer shape. 14. The nuclear reactor according to claim 13, wherein the top part of the driven member forms a polyhedral ring. 15. The nuclear reactor according to claim 13, wherein the top part includes a shoulder configured for being held by the release system. 16. The nuclear reactor according to claim 13, wherein below the top part of the driven member, the drive member has a circular outer shape. 17. The nuclear reactor according to claim 1, wherein the orifice has a circular section below the polygonal section of orifice. 18. The nuclear reactor according to claim 1, wherein a portion of the polygonal section of the movement part of the driving device is received in polygonal section of a top part of the driven member to define the polygonal section of orifice. 19. The nuclear reactor according to claim 18, wherein a portion of the polygonal section of the movement part of the driving device is received in a circular section of the driven member to define the circular section of orifice. 20. The nuclear reactor according to claim 1, wherein the orifice extends an entire height of the driven member.