Abstract: Apparatus for controlling a nuclear reactor by vertical displacement of a unit absorbing neutrons, comprising, inside a sealed enclosure (1) in communication with the interior of the reactor, a movable magnetic piece (6) connected to a control shaft (3) which is itself connected to the absorbent unit. This magnetic piece (6) has at least two radial projections (7). The magnetic piece (6) is displaced by an inductor (10) with at least two pole shoes (16) corresponding to the projections (7) on the magnetic piece (6) and allowing magnetic coupling between the inductor (10) and the magnetic piece (6). The inductor (10) and its displacement device (30, 32, 33) are disposed outside the sealed enclosure (1). A control means (38) allows the control shaft (3) to be uncoupled from a member (36, 37) assuring its suspension so as to drop the absorbent unit in the event of emergency shutdown. The apparatus is particularly applicable to control rods of pressurized water nuclear reactors.