Patent Number: 040627250
Section: claims

1. A moveable control rod having first and second ends for the control of power oscillations in a nuclear reactor having a core with an active length L, said control rod comprising: a. a first portion of said control rod's first end comprising a first neutron poison with a length no longer than 20 percent of the length L;  b. a second portion at said control rod's second end comprising a second neutron poison, said second neutron poison having a smaller macroscopic absorption cross-section than said first neutron poison and having a high number density but having a low microscopic neutron absorption cross-section; and  c. a third portion between said first and second portions, said third portion comprising a material which is substantially non-neutron absorbing relative to said first and second portions, and the sum of the lengths of said first, second, and third portions being substantially equal to the length of the core's active length L.  a. a first portion at said control rod's first end comprising a first neutron poison, said first poison having a length no longer than 20 percent of the length L;  b. a second portion at said control rod's second end comprising a second neutron poison different from said first neutron poison, said second neutron poison having a length between the range of 25 percent of the length of L and 55 percent of the length of L; and  c. a third portion between said first and second portions, said third portion comprising a material which is essentially non-neutron absorbing relative to said first and second portions, and the sum of the lengths of said first, second, and third portions being substantially equal to the length of the core's active length L. 2. The control rod as recited in claim 1 wherein said second portion has a length between the range of 25 percent of the length L and 55 percent of the length L. 3. The control rod as recited in claim 2 wherein said second portion has a length equal to 50 percent of the length L. 4. The control rod as recited in claim 3 wherein said first portion includes boron carbide (B.sub.4 C). 5. The control rod as recited in claim 4 wherein said non-absorbing material is a hydrogeneous material. 6. The control rod as recited in claim 5 wherein said hydrogeneous material is water. 7. The control rod as recited in claim 6 wherein said second portion includes Inconel 625. 8. The control rod as recited in claim 6 wherein said second portion includes Inconel 600. 9. The control rod as recited in claim 6 wherein said third portion includes Inconel 625. 10. A moveable control rod having first and second ends for the control of power distribution in a nuclear reactor having a core with an active length L, said control rod comprising: 11. The control rod as recited in claim 10 wherein said non-absorbing material includes an hydrogeneous material. 12. The control rod as recited in claim 11 wherein said hydrogeneous material includes water. 13. The control rod as recited in claim 12 wherein said second neutron poison has a smaller macroscopic absorption cross-section than said first neutron poison with a high number density but with a low microscopic neutron absorption cross-section. 14. The control rod as recited in claim 13 wherein said second portion has a length equal to 50 percent of the length L. 15. The control rod as recited in claim 14 wherein said first portion includes boron carbide (B.sub.4 C). 16. The control rod as recited in claim 15 wherein said second portion includes Inconel 625. 17. The control rod as recited in claim 15 wherein said second portion includes Inconel 600.