Control rod drive for nuclear reactors

In a nuclear reactor, an hydraulic control rod drive comprising a vertically disposed tube, a piston rod mounted in said tube so as to be axially displaceable therein, means for hydraulically driving said piston rod in axial direction in said tube, means located at the upper end of said piston rod for coupling a control rod to said piston, and means for controllably blocking axial displacement of said piston rod in said tube, said blocking means being ineffective during rapid shut-down of the nuclear reactor, said blocking means comprising a threaded piston rotatably mounted on said piston rod and fixed thereon against axial displacement relative thereto, said threaded piston having a thread flank located at the outer periphery thereof, said vertically disposed tube being formed with an inner thread corresponding to said thread flank, said thread flank being received in said thread, said hydraulically driving means comprising at least two hydraulically controlled, axially displaceable switching rods mounted in axial guidance grooves formed at the inner perihery of said vertically disposed tube and interrupting said thread, said switching rods, in a clear position thereof, permitting rotary movement of said threaded piston and, in a locking position thereof, forming a stop for said thread flank and blocking rotary movement of said threaded piston, said switch rods being hydraulically controlled so as to be displaceable stepwise in axial direction during normal operation of the control rod drive.