Patent Number: 
Section: claims

1. A passive reactivity control apparatus comprising:a first reservoir having a first reservoir portion and a second reservoir portion, the first reservoir portion and the second reservoir portion being spaced apart;a first conduit interposed between and fluidly connecting the first reservoir portion and the second reservoir portion of the first reservoir;a second reservoir spaced apart from the first reservoir, the second reservoir interposed between the first reservoir portion and the second reservoir portion of the first reservoir, the second reservoir being located within a selected portion of a nuclear fission reactor core;a second conduit interposed between and fluidly connecting the second reservoir portion and the second reservoir;a driver material disposed in the first reservoir portion and the second reservoir portion of the first reservoir and the first conduit, a volume of the driver material thermally expanding and contracting in response to an increase and decrease of a nuclear fission reactor thermal operational parameter, respectively; anda neutron absorption parameter modifying material disposed in the second reservoir portion and the second reservoir, the neutron absorption parameter modifying material being different from the driver material, a portion of the neutron absorption parameter modifying material being in physical contact with a portion of the driver material in the second reservoir portion, the neutron absorption parameter modifying material being driveable by the driver material from the second reservoir portion to the second reservoir through the second conduit upon expansion of the volume of the driver material, the neutron absorption parameter modifying material being drivable by the driver material from the second reservoir to the second reservoir portion through the second conduit upon contraction of the volume of the driver material. 2. The apparatus of claim 1, further comprising a plurality of members disposed in the second reservoir, the plurality of members being arranged to mitigate free-surface effect in the second reservoir. 3. The apparatus of claim 1, wherein the driver material includes a gas. 4. The apparatus of claim 3, wherein the gas includes at least one gas chosen from He, Xe, Kr, Ar, Ne, Rn, N2, CO2, and NH3. 5. The apparatus of claim 1, wherein the driver material includes a liquid. 6. The apparatus of claim 5, wherein the neutron absorption parameter modifying material includes a liquid that is immiscible with the driver material. 7. The apparatus of claim 1, wherein the driver material includes a solid. 8. The apparatus of claim 7, wherein the solid includes at least one solid chosen from a ferritic martensitic steel and a zirconium alloy. 9. The apparatus of claim 1, wherein the nuclear fission reactor thermal operational parameter includes at least one temperature chosen from reactor coolant temperature, reactor coolant vapor temperature, and fuel temperature. 10. The apparatus of claim 1, wherein the nuclear fission reactor thermal operational parameter includes at least one flux chosen from neutron flux, beta flux, gamma flux, and neutrino flux. 11. The apparatus of claim 1, wherein the neutron absorption parameter modifying material includes a neutron absorber. 12. The apparatus of claim 11, wherein the neutron absorber includes at least one neutron absorber chosen from In, Li-6, Eu, Ag, Dy, B, Hf, Gd, Pm, Cd, Sm, binary combinations thereof, and eutectic combinations thereof. 13. The apparatus of claim 11, wherein the neutron absorber includes nuclear fission fuel material. 14. The apparatus of claim 13, wherein the nuclear fission fuel material includes at least one nuclear fission fuel material chosen from U dissolved in Pb, U—Fe, U—Mn, Pu—Mn, U—Cr, Pu—Cr, Pu—Fe eutectic, and Pu—Mg eutectic. 15. The apparatus of claim 1, wherein the neutron absorption parameter modifying material includes a moderator. 16. The apparatus of claim 15, wherein the moderator includes at least one moderator chosen from Li-7, C, SiC, a hydrogenous material, water, ammonia, acetone, a metal hydride, a metal deuteride, a suspension of carbon in water, and a suspension of SiC in water. 17. The apparatus of claim 1, further comprising a high-Z material distributed in the driver material. 18. The apparatus of claim 17, wherein the high-Z material includes at least one material chosen from W wool, Ta, Au, Ag, Re, and Os. 19. The apparatus of claim 1, wherein:the driver material has a first density;the neutron absorption parameter modifying material has a second density that is different from the first density; andthe driver material is immiscible with the neutron absorption parameter modifying material. 20. The apparatus of claim 19, wherein the second density is greater than the first density. 21. The apparatus of claim 19, wherein the first density is greater than the second density.