Patent Number: 043691617
Section: claims

1. 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 said reactor, comprising (a) a vertical control shaft whose lower part is connected to said absorbent unit and whose upper part constitutes a rack disposed in the longitudinal direction of said shaft;  (b) a first pinion rotatably driven by drive means, in engagement with said rack for moving said control shaft and said absorbent unit in both directions;  (c) said first pinion being in permanent engagement with said rack and being mounted to rotate on a shaft connected to said drive means for setting said first pinion in rotation perpendicularly to said control shaft, said first pinion having teeth on one of its lateral faces;  (d) a clutch pinion rotatably engaged with said shaft and mounted to move in translation on said shaft, between a disengaged position in which said first pinion and said clutch pinion are at a distance and an engaged position in which said pinion and said clutch pinion are in contact, said clutch pinion having teeth on its face directed toward said lateral face of said first pinion provided with teeth;  (e) whereby said first pinion and said clutch pinion are connected for transmitting the rotary movement of said shaft to said first pinion, the translatory movement of said clutch pinion in the axial direction of said shaft being controlled by a push-rod connected to a vertically moving actuating member which is maneuverable from the upper part of the apparatus. 2. Rack and pinion mechanism according to claim 1, wherein said actuating member of said push-rod comprises a sleeve coaxial with said control shaft connected at its upper part to a magnetic means and retained in a raised position, in which said push-rod keeps said clutch pinion in an engaged position, by a magnetic coil, stopping of the supply of this magnetic coil causing said sleeve to descend and said push-rod and said clutch pinion to move, so as to achieve emergency shutdown and descent of said control rod under the effect of its own weight. 3. Rack and pinion mechanism according to claim 2, comprising a return spring associated with said sleeve so as to return it to a lower position, said push-rod returning said clutch pinion back into disengaged position. 4. Rack and pinion mechanism according to claim 1, wherein said drive means comprises a three-phase motor locked in position by braking by lack of current disposed outside a sealed casing in communication with the reactor vessel and containing said rack and pinion mechanism, said motor being associated, for transmitting rotary movement to said rack and pinion mechanism, with a permanent magnetic coupling.