Abstract: A first assembly for low-speed translational displacement and dropping of a control rod comprises a leak-tight casing tube which communicates with the reactor vessel, a screw and nut system coupled to the control rod by means of the nut and a screw-retaining device which is capable of releasing the screw. In the event of accidental failure of the casing tube, upward displacement of the control rod and of the broken portion of the casing tube is limited by a second assembly comprising means for rotational locking of the screw drive rotor and calibrated elastic means for producing action on the rotor in opposition to the vertical thrust produced by failure of the casing tube.