Patent Number: 043691617
Section: summary

BACKGROUND OF THE INVENTION The invention concerns a rack and pinion mechanism for moving a unit absorbing neutrons, movable vertically in a nuclear reactor for controlling the power and for emergency shutdown of the reactor. PRIOR ART In nuclear reactors, the regulation of power and emergency shutdown of the reactor are generally obtained by inserting control rods, constituted by tubes containing a material strongly absorbent of neutrons, inside the core. Power regulation is obtained by progressive movements of the control rods to increase their degree of insertion in the reactor core if power is required to be decreased or, conversely, by movements to reduce the degree of insertion of the control rods if power is required to be increased. These movements of inserted control rods in the vertical direction from the upper part of the core are made downwards in the first case and upwards in the second case. When the emergency shutdown is tripped, the control rods are caused to fall under the effect of gravity to their position of maximum engagement in the core. Movements of the control rods are generally effected by mechanisms disposed in the upper part of the reactor in engagement with a control shaft extending the control rods upwards into a region distant from the reactor core. The association with the control shafts of racks disposed in the longitudinal direction of these shafts, either machined directly in the shaft or added to this shaft, to move the control rods by the setting in rotation of a pinion engaged with the rack solid with the control rod has already been envisaged. These rack and pinion apparatuses can allow precise movements of the control rods during control of the reactor to be effected, if the pinion is driven in a precisely controlled way. Similarly, it is possible to halt the control rods in a precise position, from the moment the pinion can be kept rotationally stationary. This assumes a suitable motorization of the pinion driving the rack, the means for driving the pinion being associated with a control allowing movements of the control rods to be effected in both directions according to movements with well defined amplitude. To trip the emergency shutdown, however, it is necessary to disengage the rack from its driving means so as not to obstruct the fall of the rods under the effect of their own weight. To allow these emergency shutdowns, means for driving the pinion step by step have been proposed, allowing locking in position and releasing of the pinion at the moment of emergency shutdown. Such apparatuses, of the pawl type, are complicated, however, and their operation is entirely discontinuous. Systems for engaging and disengaging the pinion have also been proposed, allowing release of the rack at the moment of emergency shutdown, but such apparatuses assume movable mounting of the pinion which complicates the apparatus and makes its working less reliable. SUMMARY OF THE INVENTION The object of the invention is therefore a rack and pinion mechanism for moving a unit absorbing neutrons, movable vertically in a nuclear reactor for controlling the power and for emergency shutdown of the reactor, comprising a vertical control shaft whose lower part is connected to the absorbent unit and whose upper part has a rack disposed in the longitudinal direction of the shaft and a pinion driven in rotation by a motor apparatus, in engagement with the rack so as to move the control shaft and the absorbent unit in both directions, this mechanism having to allow precise movements of the control rods and the halting of these rods in well defined positions and the falling of the rods for emergency shutdown of the reactor controlled by a simple apparatus preventing disengagement of the rack and pinion. To this end, the pinion permanently in engagement with the rack is mounted to rotate on a shaft connected to the motor apparatus for setting the pinion in rotation and disposed perpendicularly to the control shaft, and a clutch device solid with the shaft in rotation is mounted to move in translation on this shaft between a disengagement position in which the pinion and the clutch device are brought into contact and connected by a mechanical means allowing the rotary movement of the shaft to be transmitted to the pinion, the translatory movement of the clutch device in the axial direction of the shaft being controlled by a push-rod connected to an actuating member with vertical movement which is maneuverable from the upper part of the apparatus. For a full understanding of the invention, an embodiment of the rack and pinion mechanism according to the invention will now be described by way of example, with reference to the attached drawings.