Patent Number: 
Section: claims

1. A fuel assembly for a pressure-tube nuclear reactor, comprising:a fuel channel assembly comprising:an outer conduit having an open upper end and a closed lower end that is spaced apart from the upper end along a fuel channel assembly axis;an inner conduit received within the outer conduit and having an inner upper end, an inner lower end axially spaced apart from the inner upper end and disposed toward the lower end of the outer conduit and a central flow passage extending between the inner upper end and inner lower end for receiving a flow of a coolant in one direction;an annular fuel bundle chamber defined between the outer conduit and the inner conduit, wherein the coolant exiting the inner lower end is redirected by the closed lower end of the outer conduit to flow through the annular fuel bundle chamber in an opposite direction; anda fuel bundle positioned within the fuel bundle chamber, the fuel bundle comprising a plurality of fuel elements, and consisting of an inner ring of inner fuel elements surrounding the inner conduit, and an outer ring of outer fuel elements surrounding the inner ring, wherein the fuel elements of the inner ring have a smaller axial cross sectional area than the fuel elements of the outer ring. 2. The fuel assembly of claim 1, wherein a first ratio of a cross sectional area of the coolant in the fuel bundle chamber and the central flow passage to a cross sectional area of the fuel elements is between approximately 2.6 and 7.5. 3. The fuel assembly of claim 1, wherein a second ratio of a cross sectional area of the coolant in the central flow passage to a cross sectional area of the coolant in the fuel bundle chamber is between approximately 0.8 and 1.3. 4. A nuclear reactor comprising a plurality of the fuel assemblies according to claim 1 arranged in a lattice, wherein a moderator region laterally surrounds the outer conduit of each of the fuel assemblies, the moderator region retaining a moderator therein. 5. The fuel assembly of claim 1, wherein the inner and outer conduits have generally circular axial cross sectional shapes. 6. The fuel assembly of claim 5, wherein the central flow passage is laterally surrounded by the fuel bundle. 7. The fuel assembly of claim 6, wherein a central axis of the central flow passage is laterally centered relative to the fuel bundle. 8. The fuel assembly of claim 7, wherein the fuel bundle is rotationally symmetrical about the central axis. 9. The fuel assembly of claim 8, wherein the fuel elements of the inner ring are positioned along a first common circumference about the central axis, and the fuel elements of the outer ring are positioned along a second common circumference about the central axis that is concentric with and laterally outboard of the first common circumference. 10. The fuel assembly of claim 9, wherein a number of the fuel elements in the inner ring is equal to a number of the fuel elements in the outer ring. 11. The fuel assembly of claim 10, wherein a subchannel distance between each of the fuel elements in the inner ring and the corresponding adjacent one of the fuel elements of the outer ring is approximately equal to a subchannel distance between each of the fuel elements in the inner ring. 12. The fuel assembly of claim 9, wherein the fuel elements have generally circular axial cross sections. 13. The fuel assembly of claim 9, wherein axial cross sectional areas of each of the fuel elements in the inner ring are different than axial cross sectional areas of each of the fuel elements in the outer ring. 14. The fuel assembly of claim 1, wherein the fuel channel assembly comprises an insulator that is positioned radially intermediate of the fuel bundle chamber and the outer conduit. 15. The fuel assembly of claim 14, wherein the insulator is encapsulated between inner and outer liner tubes, the outer liner tube being arranged along an interior surface of the outer conduit. 16. The fuel assembly of claim 15, wherein the insulator is formed of a solid material. 17. The fuel assembly of claim 16, wherein the inner and outer liner tubes are formed of different materials. 18. A fuel assembly for a pressure-tube nuclear reactor, the fuel assembly comprising:a fuel channel assembly comprising an outer conduit, an inner conduit received within the outer conduit and defining an annular fuel bundle chamber therebetween for receiving a flow of coolant in one direction, the inner conduit comprising a central flow passage for receiving a flow of the coolant in an opposite direction; anda fuel bundle positioned within the fuel bundle chamber, the fuel bundle comprising a plurality of fuel elements,wherein both of the following conditions are satisfied:(i) a first ratio of a cross sectional area of the coolant in the fuel bundle chamber and the central flow passage to a cross sectional area of the fuel elements is between approximately 2.6 and 7.5; and(ii) a second ratio of a cross sectional area of the coolant in the central flow passage to a cross sectional area of the coolant in the fuel bundle chamber is between approximately 0.8 and 1.3. 19. A pressure-tube nuclear reactor, comprising:a plurality of fuel assemblies arranged in a lattice, each of the fuel assemblies comprisinga fuel channel assembly comprising an outer conduit, an inner conduit received within the outer conduit and defining an annular fuel bundle chamber therebetween receiving a flow of a coolant liquid at a coolant pressure and in one direction, the inner conduit comprising a central flow passage receiving a flow of the coolant liquid in an opposite direction, anda fuel bundle positioned within the fuel bundle chamber, the fuel bundle comprising a plurality of fuel elements; anda moderator region laterally surrounding the outer conduit of each of the fuel assemblies, the moderator region retaining a liquid moderator therein, the liquid moderator being fluidly isolated from the coolant liquid and being at a moderator pressure that is less than the coolant pressure;wherein at least one of the following conditions is satisfied:(i) a first ratio of a cross sectional area of the moderator in the moderator region to a cross sectional area of the fuel elements is between approximately 10 and 20; and(ii) a second ratio of a cross sectional area of the moderator in the moderator region to a cross sectional area of the coolant in the fuel bundle chamber and the central flow passage is between approximately 2.7 and 3.7.