Patent Number: 
Section: claims

1. A passive reactivity control nuclear fuel device for a nuclear reactor core, the passive reactivity control nuclear fuel device comprising:a multiple-walled fuel chamber includingan inner wall chamber configured to position nuclear fuel in a molten fuel state within a high neutron importance region of the nuclear reactor core, the inner wall chamber being further configured to allow at least a portion of the nuclear fuel to move in a molten fuel state into a lower neutron importance region of the nuclear reactor core while remaining within the inner wall chamber as temperature of the nuclear fuel satisfies a negative reactivity feedback expansion temperature condition, the movement of the nuclear fuel in a molten fuel state to the lower neutron importance region increasing negative reactivity feedback in the nuclear reactor core, andan outer wall chamber containing the inner wall chamber, a gap between the outer wall chamber and the inner wall chamber thermally isolating the inner wall chamber. 2. The passive reactivity control nuclear fuel device of claim 1 further wherein thermal isolation of the inner wall chamber contributes to maintaining the nuclear fuel in a molten state. 3. The passive reactivity control nuclear fuel device of claim 1 further comprising:a duct containing the outer wall chamber and configured to flow a heat conducting fluid through the duct and in thermal communication with the outer wall chamber. 4. The passive reactivity control nuclear fuel device of claim 1, wherein movement of the nuclear fuel in a molten fuel state to the lower neutron importance region of the nuclear reactor core decreases reactivity within the nuclear reactor core. 5. The passive reactivity control nuclear fuel device of claim 1, wherein the nuclear fuel is stored in a solid fuel state within the inner wall chamber when temperature of the nuclear fuel does not satisfy a nuclear fuel melting temperature condition. 6. The passive reactivity control nuclear fuel device of claim 1, wherein the melting temperature of the nuclear fuel exceeds a flow temperature of a heat conducting fluid within a duct containing the outer wall chamber. 7. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber is expandable under increased internal temperature of the inner wall chamber. 8. The passive reactivity control nuclear fuel device of claim 1, wherein the nuclear fuel includes a solid porous fertile nuclear fuel and a bonding material. 9. The passive reactivity control nuclear fuel device of claim 1, wherein the nuclear fuel includes solid porous fertile nuclear fuel, a bonding material, and fissile nuclear fuel. 10. The passive reactivity control nuclear fuel device of claim 1, wherein the nuclear fuel in a molten fuel state includes a solution of fissile nuclear fuel and a nuclear translucent carrier medium, the nuclear translucent carrier medium being formed from a melted bonding material. 11. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber is not in physical contact with the outer wall chamber when the inner wall chamber has not expanded under increased internal temperature of the inner wall chamber. 12. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber radiates heat from within the inner wall chamber to a heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded under increased internal temperature of the inner wall chamber. 13. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber conducts heat from within the inner wall chamber to a heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded to physically contact the outer wall chamber under increased internal temperature of the inner wall chamber. 14. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber conducts heat from within the inner wall chamber to a heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded to allow physical contact between the outer wall chamber and contacts affixed to the inner wall chamber under increased internal temperature of the inner wall chamber. 15. The passive reactivity control nuclear fuel device of claim 1, wherein transfer of heat from the inner wall chamber to a heat conducting fluid flowing outside the outer wall chamber reduces the temperature of the nuclear fuel and transitions the nuclear fuel to a solid fuel state. 16. The passive reactivity control nuclear fuel device of claim 1, further comprising:one or more thermally conductive contacts affixed to the inner wall chamber and in thermal communication with the nuclear fuel, the thermally conductive contacts configured to physically contact the outer wall chamber when the inner wall chamber expands. 17. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber conducts heat from within the inner wall chamber to a heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded to physically contact the outer wall chamber under increased internal temperature of the inner wall chamber. 18. The passive reactivity control nuclear fuel device of claim 1, wherein a gap between the inner wall chamber and the outer wall chamber is filled at least in part by a tag gas. 19. The passive reactivity control nuclear fuel device of claim 1, wherein the inner wall chamber includes a plenum into which the nuclear fuel in a molten fuel state expands as temperature of the nuclear fuel in the molten fuel state increases. 20. A nuclear reactor having a nuclear reactor core comprising:a passive reactivity control nuclear fuel device located in the nuclear reactor core, the passive reactivity control nuclear fuel device includinga multiple-walled fuel chamber includingan inner wall chamber configured to position nuclear fuel in a molten fuel state within a high neutron importance region of the nuclear reactor core, the inner wall chamber being further configured to allow at least a portion of the nuclear fuel to move in a molten fuel state into a lower neutron importance region of the nuclear reactor core while remaining within the inner wall chamber as temperature of the nuclear fuel satisfies a negative reactivity feedback expansion temperature condition, the movement of the nuclear fuel in a molten fuel state to the lower neutron importance region increasing negative reactivity feedback in the nuclear reactor core, andan outer wall chamber containing the inner wall chamber, a gap between the outer wall chamber and the inner wall chamber thermally isolating the inner wall chamber; anda duct containing the multiple-walled fuel chamber and configured to flow a heat conducting fluid through the duct and in thermal communication with the outer wall chamber. 21. The nuclear reactor of claim 20, wherein movement of the nuclear fuel in a molten fuel state into the lower neutron importance region of the nuclear reactor core decreases reactivity within the nuclear reactor core. 22. The nuclear reactor of claim 20, wherein the nuclear fuel is stored in a solid fuel state within the inner wall chamber when temperature of the nuclear fuel does not satisfy a nuclear fuel melting temperature condition. 23. The nuclear reactor of claim 20, wherein the melting temperature of the nuclear fuel exceeds a flow temperature of the heat conducting fluid within the duct. 24. The nuclear reactor of claim 20, wherein the inner wall chamber is expandable under temperature of the inner wall chamber. 25. The nuclear reactor of claim 20, wherein the inner wall chamber is not in physical contact with the outer wall chamber when the inner wall chamber has not expanded under increased internal temperature of the inner wall chamber. 26. The nuclear reactor of claim 20, wherein the inner wall chamber radiates heat from within the inner wall chamber to the heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded under increased internal temperature of the inner wall chamber. 27. The nuclear reactor of claim 20, wherein the inner wall chamber conducts heat from within the inner wall chamber to the heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded to physically contact the outer wall chamber under increased internal temperature of the inner wall chamber. 28. The nuclear reactor of claim 20, wherein the inner wall chamber conducts heat from within the inner wall chamber to the heat conducting fluid flowing outside the outer wall chamber when the inner wall chamber has expanded to allow physical contact between the outer wall chamber and contacts affixed to the inner wall chamber under increased internal temperature of the inner wall chamber. 29. The nuclear reactor of claim 20, wherein transfer of heat from the inner wall chamber to the heat conducting fluid flowing outside the outer wall chamber reduces the temperature of the nuclear fuel and transitions the nuclear fuel to a solid fuel state. 30. A nuclear reactor having a nuclear reactor core comprising:a multiple-walled fuel chamber includingan inner wall chamber configured to contain the nuclear reactor core and to position nuclear fuel in a molten fuel state within a high neutron importance region of the nuclear reactor core, the inner wall chamber being further configured to allow at least a portion of the nuclear fuel to move in a molten fuel state into a lower neutron importance region of the nuclear reactor core while remaining within the inner wall chamber as temperature of the nuclear fuel satisfies a negative reactivity feedback expansion temperature condition, the movement of the nuclear fuel in a molten fuel state to the lower neutron importance region increasing negative reactivity feedback in the nuclear reactor core, andan outer wall chamber containing the inner wall chamber, a gap between the outer wall chamber and the inner wall chamber thermally isolating the inner wall chamber.