fig1 shows the cylindrical housing 1 containing the apparatus for moving the control rods disposed with its vertical axis at the upper part of the vessel of the nuclear reactor , this very thick housing being pressure - resistant and communicating at its lower part with the interior of the vessel . the housing 1 is closed sealingly at its upper part not represented in fig1 . the control shaft 2 disposed along the axis of the housing 1 , i . e ., disposed with its axis vertical , is connected at its lower part ( not shown in fig1 ) to the upper part of the control rod which is solid with the control shaft in its movements in the vertical direction . a rack 3 is machined on one part of the control shaft in the longitudinal direction of this control rod . a pinion 5 whose axis is perpendicular to the axis of the control shaft is in engagement with the rack . the rack occupies a length of the control shaft which is such that the pinion 5 is able to move the control shaft upwards with the aid of the rack with which it is in engagement to the position of the control shaft corresponding to the position of maximum extraction of the control rod . the rack is also machined towards the top of the control shaft over a length sufficient for the pinion to remain engaged with this rack when the control rod is in its position of maximum insertion , for example at the time of an emergency shutdown . the pinion 5 remains constantly engaged with the rack 3 , whatever the vertical position of the control shaft 2 . as fig1 and 2 show , the pinion 5 is freely mounted on a shaft 7 itself mounted to rotate inside the support 8 of the apparatus for moving the control rod by two bearings 9 and 10 . the pinion 5 is freely mounted on the shaft 7 by bearings 11 and 12 whose inner rings are solid with the shaft 7 . in this way , the pinion 5 remains in a fixed position with respect to the rack 3 , the shaft 7 being fixed in axial position by a stop 14 and by the back part of a conical pinion 15 , both solid with the shaft 7 . the conical pinion 15 itself engages with a conical pinion 16 solid with a shaft 18 mounted to rotate in the support 8 and itself solid at its end opposite the conical pinion 16 with a pinion 19 . the pinion 19 is itself solid in rotation with a ring 20 connected to the output of a reducer 21 whose input is connected to the driven part of a magnetic coupling ( not represented in fig1 ). the motor apparatus for driving the shaft 7 , through the magnetic coupling of the reducer and the set of pinions described , is constituted by a three - phase motor disposed outside the magnetic casing 1 which causes a locking in position when it is not supplied . the three - phase driving motor transmits its rotary movement to the driven part of the magnetic coupling through the casing 1 which therefore remains totally sealed and isolated from the external environment . when the three - phase motor is set in rotation , this rotary movement is then transmitted to the shaft 7 which continues to move as long as the three - phase motor is supplied and which locks in position when the three - phase motor is no longer supplied . as fig2 shows , the shaft 7 has grooves 24 over one part of its length while a clutch device made in the form of a pinion 25 has corresponding grooves over its inner bore . the pinion 25 is engaged on the shaft 7 so as to be solid in rotation with the shaft 7 but movable in translation in the axial direction of this shaft . the pinion 25 is engaged on the shaft 7 so that its front face 26 comes into contact , at the forward translatory movement of the pinion 25 , with the back face 27 of the pinion 5 whose axial position on the shaft 7 is fixed . the front face 26 of the pinion 25 and the back face 27 of the pinion 5 have corresponding teeth of the &# 34 ; jaw &# 34 ; type which come into engagement at the end of movement . in this way , the shaft 7 , the clutch ring 25 and the driving pinion 5 are solid in rotation when the clutch pinion 25 is in its front position as represented in fig2 . a push - rod 30 is also mounted articulatedly via a shaft 31 on the support 8 of the apparatus for moving the control rods , one of the ends of this push - rod 30 being connected articulatedly to the lower part of an actuating sleeve 32 coaxial with the control shaft 2 and extending to the upper part of the control apparatus . the sleeve 32 is kept in its high position as represented in fig1 by a magnetic coil , disposed at the upper part of the apparatus and not represented , acting on a magnetic part solid with the upper part of the sleeve 32 . at its base , the sleeve 32 is connected to an actuating ring 34 on which is articulated , at 35 , one of the arms of the bent push - rod 30 whose other end is constituted by an actuating fork 36 in engagement with the clutch pinion 25 via two wheels 37 and 38 . in its high position represented in fig1 the actuating sleeve holds , via the bent push - rod 30 , the clutch pinion 25 in an advanced position as represented in fig1 and 2 . in this position the teeth of the front face of the clutch 25 are engaged in the teeth of the back face of the pinion 5 for driving the rack . as long as the coil for holding the sleeve 32 in the high position is supplied , the driving pinion 5 is consequently in mechanical connection with the pinion 25 and the shaft 7 which is itself in mechanical connection with the rotary driving means described . it is therefore possible to move the control rod in this position , by causing a controlled rotation of the driving pinion 5 by means of the three - phase motor and the whole kinematic chain previously described . similarly , it is possible to hold the control rod in position , since the three - phase motor is locked against rotation when it is not supplied . it is therefore possible to obtain all movement of the control rod and any holding of position by control of the supply of the three - phase motor . if the supply to the coil for holding the sleeve 32 in the high position is interrupted , this sleeve falls under the action of its own weight and drives the push - rod 30 in rotation so that the fork 36 drives the clutch pinion in translation backwards , the clutch teeth 25 and 5 being then disengaged . the pinion 5 freely mounted for rotation on the shaft 7 then no longer holds the rack 3 and the control rod 2 in position and the control rod is able to fall under the action of its own weight until this control rod , connected to the control shaft 2 , arrives at its position of maximum insertion in the reactor core . emergency shutdown of the reactor can therefore be obtained by cutting off the supply to the coil for holding the sleeve 32 in position , this being particularly simple and quick . throughout its movements , the control shaft 2 is guided by wheels such as 40 and 41 freely mounted on shafts solid with the support 8 of the control apparatus . clearly , the principal advantages of the invention are that it allows the use of a rack and pinion apparatus to move a control rod in a reactor during control of this reactor , and holding of the control rod in position , while allowing tripping of the emergency shutdown in an extremely reliable and very simple way , by releasing an actuating sleeve which falls under its own weight . in this way , all the advantages of the pinion - rack apparatus which allows movements of the control rod in a continuous and shockless motion with precise stopping at any level of the stroke of the control rod are obtained , this mechanism also taking up less room inside the housing over the reactor vessel . in addition , the mounting of the rack and pinion control apparatus inside the housing over the reactor vessel is particularly simple and does not require a precise positioning in rotation of the apparatus with respect to the control rod . in addition , after an emergency shutdown , the driving pinion 5 remains engaged with the rack 3 , so that all that is required to return the apparatus for moving the control rod to service is to cause the sleeve 32 to return to the high position by supplying the coil for holding this sleeve and to supply the three - phase motor for raising the rod and any other subsequent movement for controlling the reactor . in addition , the emergency shutdown can be easily and reliably achieved and , to further increase the reliability of the apparatus , a return spring can be associated with the sleeve 32 to reinforce the action of the weight so as to bring the sleeve 32 into the low position . furthermore , in the case of the use of a motor for driving the pinion apparatus in rotation , the motor being locked against rotation by cutting off of its supply , holding in position of the control rod is assured without consumption of energy other than the energy supplied to the coil for holding the actuating sleeve in the high position . in addition , when a magnetic coupling is used to traverse the wall of the housing of the mechanism , for example a permanent magnet apparatus which allows the motor torque to be transmitted through the sealed casing 1 synchronously and with a high output , an automatic limitation of the stresses exerted on the control rod is thus assured since , in the case of excessive stress , the driving and driven parts of the magnetic coupling become uncoupled . the invention is not limited to the embodiment just described ; it includes all the variants thereof and points of detail can be changed without exceeding the scope of the invention . thus , instead of a three - phase motor effecting a locking in position by lack of current associated with a magnetic coupling for traversing the sealed housing , any type of braking motor can be used , associated with a rotary mechanical traversing allowing the movement to be transmitted into the sealed housing . use of a stepping motor with a shaded rotor ( or with a polegap sleeve ) is also possible to assure motorization of the apparatus for driving the control rod . the invention applies to all nuclear reactors in which power control and emergency shutdown is effected by movement in the vertical direction of a unit absorbing neutrons which can be inserted to a greater or lesser depth in the reactor core .