more than one control rod lead screw without incurring a substantial increase in the size of the mechanism application_number US 44785082 A description the dual action crdm of the present invention performs two independent but sequential functions on two concentric lead screws or actuators to positionally control two sets of reactor control elements utilizing a single activator as best shown in fig1 the mechanism preferably utilizes an inner lead screw 110 which is shown in its fully inserted or 34 down 34 position and a concentric outer lead screw 112 also shown in its fully inserted position as described above one of the lead screws may preferably be used in a thoria finger control system to control the position of poison rods and the other lead screw to control the position of thoria control rods as will be explained more fully below the crdm of the present invention employs a single drive motor for sequentially operating the poison rods and then the thoria control rods to achieve criticality in the reactor for clarity the topmost section of the inner lead screw 110 is drawn twice in fig1 once in the fully inserted position and again as it would appear when captured by the latching device 114 when fully withdrawn the latching device 114 is illustrated as it would appear with the latch coil 116 energized turning briefly to fig2 there is illustrated a latching mechanism according to the present invention showing the inner lead screw 110 in its.