Patent Number: 
Section: claims

1. A method of controlling reactivity in a nuclear fission reactor having a fast neutron spectrum, the method comprising:determining a desired reactivity parameter within a selected portion of a nuclear fission reactor having a fast neutron spectrum; andadjusting at least one reactivity control rod having fast spectrum neutron absorbing material, at least a portion of the fast spectrum neutron absorbing material including fertile nuclear fission fuel material, responsive to the desired reactivity parameter. 2. The method of claim 1, wherein the fast spectrum neutrons are part of a nuclear fission traveling wave. 3. The method of claim 1, wherein determining a desired reactivity parameter within a selected portion of a nuclear fission reactor having a fast neutron spectrum includes determining at least one desired reactivity parameter of the fertile nuclear fission fuel material. 4. The method of claim 1, wherein determining a desired reactivity parameter within a selected portion of a nuclear fission reactor having a fast neutron spectrum includes determining at least one desired reactivity parameter of the at least one reactivity control rod. 5. The method of claim 1, wherein determining a desired reactivity parameter within a selected portion of a nuclear fission reactor having a fast neutron spectrum includes determining at least one desired reactivity parameter of the nuclear fission reactor. 6. The method of claim 1, further comprising determining at least one determined reactivity parameter. 7. The method of claim 6, further comprising determining a difference between the desired reactivity parameter and the at least one determined reactivity parameter. 8. The method of claim 7, wherein adjusting at least one reactivity control rod having fast spectrum neutron absorbing material, at least a portion of the neutron absorbing material including fertile nuclear fission fuel material, responsive to the desired reactivity parameter includes adjusting at least one reactivity control rod having fast spectrum neutron absorbing material, at least a portion of the neutron absorbing material including fertile nuclear fission fuel material, responsive to the difference between the desired reactivity parameter and the at least one determined reactivity parameter. 9. The method of claim 6, wherein determining at least one determined reactivity parameter includes predicting at least one reactivity parameter. 10. The method of claim 6, wherein determining at least one determined reactivity parameter includes modeling at least one reactivity parameter. 11. The method of claim 6, wherein determining at least one determined reactivity parameter includes selecting at least one predetermined reactivity parameter. 12. The method of claim 6, wherein determining at least one determined reactivity parameter includes sensing at least one reactivity parameter. 13. The method of claim 12, wherein sensing at least one reactivity parameter includes sensing a time history of at least one reactivity parameter. 14. The method of claim 12, wherein sensing at least one reactivity parameter includes sensing at least one parameter chosen from a radioactive decay event, fission, neutron flux, neutron fluence, fission products, temperature, pressure, and power level. 15. The method of claim 1, wherein adjusting at least one reactivity control rod having fast spectrum neutron absorbing material, at least a portion of the neutron absorbing material including fertile nuclear fission fuel material, responsive to the desired reactivity parameter includes moving, in at least one of two directions, at least one reactivity control rod having fast spectrum neutron absorbing material, at least a portion of the neutron absorbing material including fertile nuclear fission fuel material, responsive to the desired reactivity parameter. 16. The method of claim 15, wherein the directions include directions chosen from axial directions in the nuclear fission reactor, radial directions in the nuclear fission reactor, and lateral directions in the nuclear fission reactor. 17. The method of claim 12, wherein sensing at least one reactivity parameter includes sensing a difference in reactivity in association with a change in position of the reactivity control rod. 18. The method of claim 12, further comprising calibrating a sensor configured to sense at least one reactivity parameter.