Patent Number: 
Section: claims

1. A control rod drive for a nuclear reactor, comprising:a drive housing;a throttle bush disposed at least partially in said drive housing;a drive unit; anda control rod carrying element disposed in said drive housing and moveable between a basic position and a moved-in end position, a part of said control rod carrying element being guided in said throttle bush and said control rod carrying element having a lower end cooperating with said drive unit, said control rod carrying element and said throttle bush defining there-between a flow path for a pressure fluid, said flow path leading beyond said throttle bush and having a free flow cross section varying in dependence on a position of said control rod carrying element. 2. The control rod drive according to claim 1, wherein said free flow cross section increases when the control rod carrying element moves from the basic position to the moved-in end position. 3. The control rod drive according to claim 1, wherein said control rod carrying element has a length and an outer diameter varying over said length of said control rod carrying element. 4. The control rod drive according to claim 3, wherein said control rod carrying element has a lower region facing said drive unit and an upper region, said outer diameter of said control rod carrying element is smaller in said lower region than said outer diameter in said upper region resulting in a reduced outer diameter in said lower region. 5. The control rod drive according to claim 4, wherein said outer diameter of said control rod carrying element has a narrowing region that narrows continuously to said reduced outer diameter. 6. The control rod drive according to claim 4, wherein said reduced outer diameter extends constantly over a part of a length of said control rod carrying element. 7. The control rod drive according to claim 5, wherein said control rod carrying element is a hollow piston having a wall with at least one bypass orifice formed in said wall. 8. The control rod drive according to claim 7, wherein said bypass orifice is disposed one of in a region upstream of said narrowing region and in a region of said narrowing region of said control rod carrying element. 9. The control rod drive according to claim 4, wherein said control rod carrying element has, in said lower region having said reduced outer diameter, an outer web, said outer web disposed within said throttle bush when said control rod carrying element is positioned in the moved-in end position. 10. The control rod drive according to claim 9, wherein said throttle bush having an inside diameter, and said outer web is a peripheral annular web having an outer diameter corresponding approximately to said inside diameter of said throttle bush. 11. The control rod drive according to claim 4, further comprising longitudinal webs disposed in a region of said reduced outer diameter on said control rod carrying element.