Patent Number: 
Section: claims

1. A nuclear fuel assembly, comprising:a plurality of nuclear fuel pins;a fuel duct configured for insertion into a nuclear reactor vessel, including:a first hollow structure having a first cross-sectional geometry configured to expand in at least one dimension under stress, the first hollow structure configured to receive the plurality of nuclear fuel pins, anda second hollow structure disposed around the first hollow structure having a second cross-sectional geometry being different from the first cross-sectional geometry, the second hollow structure configured to distribute therethrough at least a portion of the stress of the first hollow structure,wherein under the stress, the first hollow structure and the second hollow structure define an interior space between the first hollow structure and the second hollow structure with at least a portion of the first hollow structure in physical contact with the second hollow structure, andwherein the first hollow structure permits fluid communication between the interior space and a region inside the first hollow structure. 2. The fuel assembly of claim 1, wherein the first cross-sectional geometry includes a polygon having more sides than the second cross-sectional geometry. 3. The fuel assembly of claim 1, wherein the first cross-sectional geometry includes a polygon having fewer sides than the second cross-sectional geometry. 4. The fuel assembly of claim 1, wherein the first cross-sectional geometry includes a dodecagon. 5. The fuel assembly of claim 1, wherein the second cross-sectional geometry includes a hexagon. 6. The fuel assembly of claim 1, wherein the first cross-sectional geometry includes a dodecagon and the second cross-sectional geometry includes a hexagon. 7. The fuel assembly of claim 1, wherein at least one of the first hollow structure and the second hollow structure has a wall thickness of between about 0.2 mm and about 5 mm. 8. The fuel assembly of claim 1, wherein at least one of the first hollow structure and the second hollow structure has a wall thickness varying along at least a portion of a respective circumference of the first cross-sectional geometry and the second cross-sectional geometry. 9. The fuel assembly of claim 1, wherein at least one of the first hollow structure and the second hollow structure includes at least one steel chosen from ferritic steel, martensitic steel, and non-ferritic steel. 10. The fuel assembly of claim 1, wherein at least one of the first hollow structure and the second hollow structure includes at least one material chosen from a Zr-based alloy, a Fe-based alloy, a ceramic, a refractory metal, a refractory alloy, and a composite material. 11. The fuel assembly of claim 1, wherein the first hollow structure is spaced apart from the second hollow structure. 12. The fuel assembly of claim 1, wherein at least a portion of the first hollow structure is coupled to at least a portion of the second hollow structure by at least one structural member. 13. The fuel assembly of claim 1, further including a coolant disposed in an interior of the first hollow structure. 14. The fuel assembly of claim 1, further including at least one instrument disposed interior the first hollow structure, the at least one instrument being configured to perform at least one function chosen from test, observe, and provide feedback regarding operational conditions. 15. The fuel assembly of claim 1, wherein at least a portion of the first hollow structure is in direct physical contact with a portion of the second hollow structure. 16. The fuel assembly of claim 15, wherein the first hollow structure has a periphery that is in direct physical contact with the second hollow structure. 17. The fuel assembly of claim 15, wherein the first hollow structure has at least one side that is in direct physical contact with the second hollow structure. 18. A nuclear fuel assembly, comprisinga nuclear fuel,a plurality of nuclear fuel elements, anda plurality of fuel ducts having the plurality of nuclear fuel elements disposed therein, at least one of the plurality of the fuel ducts including:a first hollow structure having a first cross-sectional geometry, the first hollow structure containing more than one of the plurality of nuclear fuel elements, the first hollow structure configured to expand radially outward under stress, anda second hollow structure disposed around the first hollow structure, and having a second cross-sectional geometry different from the first cross-sectional geometry, the second hollow structure configured to distribute therethrough at least a portion of the stress from the first hollow structure, with the first hollow structure in contact with the second hollow structure,wherein the first hollow structure and the second hollow structure define an interior space between the first hollow structure and the second hollow structure, andwherein the first hollow structure permits fluid communication between the interior space and a region inside the first hollow structure. 19. The fuel assembly of claim 18, wherein the at least one of the plurality of fuel ducts further includes at least one structure member that connects a point on a side of the first cross-sectional geometry of the first hollow structure to a corner of the second cross-sectional geometry of the second hollow structure. 20. The fuel assembly of claim 18, wherein the plurality of nuclear fuel elements are disposed in an interior of the first hollow structure. 21. The fuel assembly of claim 18, wherein the plurality of the fuel ducts define interstitial spaces therebetween, at least one of a coolant, inert gas, fuel material, and a monitoring device, being disposed in the interstitial spaces.