Control rod driving mechanism

Object: To form an outer peripheral surface of a control rod driving rod with a spiral cut having a notched hole with which a pawl mechanism engages to thereby reduce pull-out speed of the control rod, thus avoiding rapid change in output. Structure: When an index tube is moved up in order to insert control rods into the reactor, a collet finger is outwardly biased by a diagonally chamfered inclined surface under the spiral notch to disengage the finger from the notched hole. Then, the index tube moves up along the collet finger, and if the collet finger is axially moved up, it is disengaged from the notched hole when the finger is moved down, whereby the index tube will move down at a speed adjusted by adjusting the thread pitch of the spiral cut and spacing of the notched hole. (Kamimura, M.)