Patent Number: 
Section: claims

1. A reactivity control assembly for a nuclear fission reactor,the reactivity control assembly comprising:a reactivity control rod including neutron absorbing material configured to absorb neutrons, at least a portion of the neutron absorbing material including fertile nuclear fission fuel material and a neutron absorbing poison, the reactivity control rod has a first region and a second region;a first concentration of the neutron absorbing poison and a third concentration of the fertile nuclear fission fuel material are disposed in the first region of the reactivity control rod, anda second concentration of the neutron absorbing poison and a fourth concentration of the fertile nuclear fission fuel material are disposed in the second region of the reactivity control rod; andwherein a reactivity effect of the first concentration of the neutron absorbing poison is substantially equalized with a reactivity effect of the fourth concentration of the fertile nuclear fission fuel material; andat least one sensor associated with the reactivity control rod, the at least one sensor being configured to sense status of at least one reactivity parameter associated with the reactivity control rod. 2. The assembly of claim 1, wherein the neutron absorbing material is configured to absorb fast spectrum neutrons. 3. The assembly of claim 2, wherein the neutron absorbing material is further configured to reduce moderation of neutrons. 4. The assembly of claim 2, wherein the fast spectrum neutrons are part of a nuclear fission traveling wave. 5. The assembly of claim 1, wherein the fertile nuclear fission fuel material includes at least one element chosen from uranium and thorium. 6. The assembly of claim 1, wherein the reactivity control rod further includes neutron moderating material. 7. The assembly of claim 6, wherein the neutron moderating material includes at least one neutron moderator chosen from hydrogen, deuterium, helium, lithium, boron, carbon, graphite, sodium, and lead. 8. The assembly of claim 6, wherein the neutron moderating material is substantially heterogeneously distributed within the reactivity control rod. 9. The assembly of claim 6, wherein the neutron moderating material is substantially homogeneously distributed within the reactivity control rod. 10. The assembly of claim 1, wherein the neutron absorbing poison includes at least one poison chosen from silver, indium, cadmium, gadolinium, hafnium, lithium, 3He, fission products, protactinium, neptunium, and boron. 11. The assembly of claim 1, wherein the neutron absorbing poison is substantially heterogeneously distributed within the reactivity control rod. 12. The assembly of claim 1, wherein the neutron absorbing poison is substantially homogeneously distributed within the reactivity control rod. 13. The assembly of claim 1, wherein an effect on reactivity achievable by the fertile nuclear fission fuel material is equalized toward an effect on reactivity achievable by the neutron absorbing poison. 14. The assembly of claim 1, wherein a reactivity effect of the second concentration of the neutron absorbing poison is substantially equalized with a reactivity effect of the third concentration of the fertile nuclear fission fuel material. 15. The assembly of claim 1, wherein a reactivity effect of the second concentration of the neutron absorbing poison is different from a reactivity effect of the third concentration of the fertile nuclear fission fuel material. 16. The assembly of claim 1, wherein a sum of reactivity effects of the first and third concentrations is substantially equalized toward a sum of reactivity effects of the second and fourth concentrations. 17. The assembly of claim 1, wherein reactivity effect is substantially constant between the first region and the second region. 18. The assembly of claim 1, wherein concentration of at least one of the fertile nuclear fission fuel material and the neutron absorbing poison changes along a continuous gradient. 19. The assembly of claim 1, wherein at least one of the fertile nuclear fission fuel material and the neutron absorbing poison are provided in powdered form. 20. The assembly of claim 1, wherein concentration of at least one of the fertile nuclear fission fuel material and the neutron absorbing poison changes along a non-continuous gradient. 21. The assembly of claim 1, wherein at least one of the fertile nuclear fission fuel material and the neutron absorbing poison are provided in discrete particle form. 22. The assembly of claim 1, wherein the fertile nuclear fission fuel material and the neutron absorbing poison are spatially fixed relative to each other. 23. The assembly of claim 1, wherein physical association includes at least one association chosen from being located within an interior of the reactivity control rod and being attached to an exterior of the reactivity control rod. 24. The assembly of claim 1, wherein the reactivity parameter includes at least one parameter chosen from neutron fluence, neutron flux, neutron fissions, fission products, radioactive decay events, temperature, pressure, power, isotopic concentration, burnup, and neutron spectrum. 25. The assembly of claim 1, wherein the sensor includes at least one sensor chosen from a fission detector, a neutron flux monitor, a neutron fluence sensor, a fission product detector, a temperature sensor, a pressure sensor, and a power sensor. 26. The assembly of claim 1, wherein the at least one sensor is replaceable. 27. The assembly of claim 1, wherein the reactivity control rod defines at least one chamber configured to accumulate fission products. 28. The assembly of claim 27, wherein the chamber includes a plenum. 29. The assembly of claim 28, wherein the plenum is located at least one mean free path for fission-inducing neutrons from the fertile nuclear fission fuel material. 30. The assembly of claim 1, further comprising a calibration device configured to calibrate the at least one sensor. 31. The assembly of claim 1, further comprising at least one communications device operatively coupled to the at least one sensor. 32. The reactor of claim 1, wherein the sensor is physically associated with the reactivity control rod. 33. The reactor of claim 32, wherein the sensor is physically attached to the reactivity control rod. 34. The reactor of claim 33, wherein the sensor is physically attached to an interior of the reactivity control rod. 35. The reactor of claim 33, where in the sensor is physically attached to an exterior surface of the reactivity control rod.