Nuclear reactor, comprising: - a vessel having a center axis containing a primary liquid; - a plurality of nuclear fuel assemblies forming a core placed in the vessel; - a plurality of members for controlling the reactivity of the core; - a plurality of mechanisms for driving the control members parallel to the center axis; wherein each drive mechanism comprises: - a motor comprising a stator and a rotor capable of being rotated by the stator; - a driving member comprising a driving part forming one of a screw or a nut; - a device for rotating connection of the driving member with the rotor, having a member for applying a rotational torque of the rotor to the driving member; - a driven member translationally connected to one of the control members and comprising the other of a screw or a nut, wherein the screw and the nut interact 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; - a blocking device comprising at least one blocking member 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; characterized in that in each drive mechanism: - the motor is fully immersed in the primary liquid inside the vessel; - the rotor has a central passage, wherein the rotary torque application member is located in or close to the central passage; - the driving member comprises a connecting part engaged in the central passage and interacting with the rotary torque application member, wherein the connecting part is free in translation in the central passage relative to the rotor when the, or each, blocking member is in the releasing position. - a vessel having a center axis containing a primary liquid; - a plurality of nuclear fuel assemblies forming a core placed in the vessel; - a plurality of members for controlling the reactivity of the core; - a plurality of mechanisms for driving the control members parallel to the center axis; - a motor comprising a stator and a rotor capable of being rotated by the stator; - a driving member comprising a driving part forming one of a screw or a nut; - a device for rotating connection of the driving member with the rotor, having a member for applying a rotational torque of the rotor to the driving member; - a driven member translationally connected to one of the control members and comprising the other of a screw or a nut, wherein the screw and the nut interact 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; - a blocking device comprising at least one blocking member 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 is fully immersed in the primary liquid inside the vessel; - the rotor has a central passage, wherein the rotary torque application member is located in or close to the central passage; - the driving member comprises a connecting part engaged in the central passage and interacting with the rotary torque application member, wherein the connecting part is free in translation in the central passage relative to the rotor when the, or each, blocking member is in the releasing position.