Abstract: A hydraulic control rod drive for a nuclear reactor, with a a reactor plenum enclosing the drive, comprises first and second hollow bodies together forming a cylinder and a hollow piston. Working fluid is supplied through one of the hollow bodies. The first hollow body is stationary and the second hollow body is disposed coaxially around the first hollow body. The two hollow bodies define an annular gap therebetween so as to allow axial reciprocating movement of the second hollow body, which forms a carrier body for control elements of the control rod. The second hollow body can be lifted, lowered or suspended by feeding working fluid. A portion of the fluid is removed from the inner space via a throttle passage. A positional measurement system determines the relative displacement of the second hollow body by measuring an ultrasonic measurement path. The system includes an ultrasound reflector and an ultrasonic transducer rigidly mounted remote from the ultrasound reflector.