Patent Number: 052767191
Section: claims

1. A hydraulic control rod drive for a nuclear reactor, with a piston/cylinder drive associated with a control rod and a reactor plenum enclosing the drive, comprising: a) two hollow bodies together forming a cylinder-piston unit and defining an inner space;  b) means for supplying a working fluid through one of said hollow bodies;  c) a first one of said hollow bodies being stationary and a second one of said hollow bodies being disposed coaxially with respect to said first hollow body and defining an annular gap therebetween so as to allow axial movement of said second hollow body; said second hollow body forming a carrier body for control elements of a control rod;  d) said first hollow body having a lower region with a working fluid supply channel formed therein;  e) means for lifting, lowering or suspending said second hollow body by feeding working fluid through said supply channel to said inner space and removing the working fluid from said inner space via a throttle passage;  f) a positional measurement system for said second hollow body with an ultrasonic measurement path, including an ultrasound reflector disposed at an upper end of said second hollow body, and an ultrasonic transducer rigidly mounted above and remote from said ultrasound reflector; and  g) means for venting said inner space, said venting means being in the form of a venting channel configuration disposed at an upper end in said second hollow body, said venting channel configuration having channels opening into the reactor plenum at a given distance from said ultrasonic measurement path, so as not to cause density fluctuations in the working fluid disposed in said ultrasonic measurement path.  a movable cylinder having control rod elements disposed thereon;  a stationary piston rigidly mounted in the fuel assembly, said piston having a bottom, an open top, and an axial opening formed therein for allowing communication between said bottom and said open top;  said cylinder being disposed coaxially around said piston and defining a gap therebetween so as to allow axial movement of said cylinder;  means for supplying a working fluid to said piston for lifting, lowering or suspending said cylinder;  means for measuring a vertical displacement of said cylinder; and  means for venting said gap between said piston and said cylinder. 2. The hydraulic control rod drive according to claim 1, wherein said throttle passage is formed by said annular gap. 3. The control rod drive according to claim 1, wherein said first hollow body is a hollow piston being open at an upper end thereof, said second hollow body is a hollow cylinder coaxially surrounding and covering said first hollow body and defining said annular gap therebetween, and whereby an axial position of said hollow cylinder relative to said hollow piston is determined by an amount of working fluid supplied through said hollow piston into said hollow cylinder. 4. The control rod drive according to claim 3, including a cross-shaped attachment part having a central channel part and cross legs extending away from said central channel part, said cross-shaped attachment part being disposed on the upper end of said hollow cylinder and having said venting channel configuration formed therein, said venting channel configuration including radial channels originating from said central channel part, communicating with an interior of said hollow cylinder, extending through said cross legs, and opening into the reactor plenum. 5. The control rod drive according to claim 4, wherein the control elements of the control rod are absorber plates disposed around said hollow cylinder in a cross-shaped configuration and having distal edges facing away from said hollow cylinder, said radial channels opening into the reactor plenum approximately in the region of said distal edges. 6. The control rod drive according to claim 3, wherein the control elements of the control rod are embodied as cross-shaped absorber plates extending radially from said hollow cylinder, including a head plate disposed on said hollow cylinder and having said venting channel configuration formed therein, said head plate having a diameter approximately corresponding to a diameter of said hollow cylinder, said venting channel configuration including channels being T-shaped as seen in axial section, and including outlet pipes having outlet openings and communicating with said channels, said outlet openings being disposed at a given distance from said ultrasonic measurement path. 7. The control rod drive according to claim 1, wherein said venting means are a venting channel configuration disposed at an upper end in said second hollow body, said venting channel configuration including venting channels extending at an angle of between 5.degree. and 10.degree. relative to the horizontal. 8. The control rod drive according to claim 1, including first annular protrusions and recesses formed about the outer circumference of said first hollow body, second annular protrusions and recesses formed on an inner surface of said second hollow body, said first and second annular protrusions and recesses defining annular gaps therebetween, said annular gaps forming the throttle passage for the working fluid emanating from the inner space of said first and second hollow bodies. 9. In a nulcear reactor having a fuel assembly with a control rod and a reactor plenum enclosing the fuel assembly, a hydraulic control rod drive, comprising: 10. The control rod drive according to claim 9, wherein said measuring means include an ultrasonic reflector disposed on said cylinder, and an ultrasonic transducer rigidly mounted above said cylinder in alignment with a longitudinal axis of said cylinder. 11. The control rod drive according to claim 9, wherein said venting means are in the form of a venting channel formed in an upper end of said cylinder for allowing working fluid with gas bubbles to escape from said cylinder.