Abstract: A nuclear reactor, a control rod drive for finely adjustable normal insertion and withdrawal of control rods from the reactor core, for scramming the control rods, as well as for locking the control rods in their respective rated position, the insertion being effected in a direction opposing the force of gravity, and the withdrawal being effected in direction of the force of gravity, includes a piston rod mounted so as to be displaceable in axial direction, the piston rod being couplable to respective control rods for driving the same, a threaded piston rotatably mounted on the axially displaceable piston rod, screw thread means supporting the threaded piston so that the piston is movable back and forth hydraulically, the screw thread means having controllable locking means for clearing the screw thread means stepwise in the back and forth direction so that the threaded piston is displaceable into and out of the reactor core stepwise to effect fine adjustment, the screw thread means, in condition of the contr