a rack and pinion mechanism for vertical up - and - down movement of neutron - absorbing control elements to control the power and to shut down a nuclear reactor in case of emergency . the upper portions of the elements constitute the rack , engaging a rotatably driven pinion perpendicular thereto and having teeth on a lateral face engaging corresponding teeth of a clutch pinion . the rack and pinion remain on continuous enagement , while the clutch pinion selectively engages and disenages the pinion , the translatory axial movement of the clutch pinion being controlled by a vertically moving actuating member controllable from the upper part of the apparatus .