Patent Number: 
Section: claims

1. A fuel rod for a nuclear reactor having a bottom end, a top end, and an intermediate region located between the bottom end and the top end, comprising:a first axial zone positioned proximate to the bottom end;a second axial zone positioned adjacent to the first axial zone in the intermediate region; anda third axial zone positioned proximate to the top end,wherein the first axial zone has an enrichment greater than the second axial zone and the second axial zone has an enrichment greater than or equal to the third axial zone, the first axial zone, the second axial zone, and the third axial zone having different levels of at least one of average enrichment and average gadolinium doping,wherein enrichments for each of the first, second, and third axial zones is uniformly distributed axially within each axial zone. 2. The fuel rod of claim 1 wherein the enrichment of the first axial zone is configured to optimize a local peak power. 3. The fuel rod of claim 2 wherein a zone length of the second axial zone and a zone length of the third axial zone are each dimensioned to optimize an R-factor. 4. The fuel rod of claim 2 wherein the enrichment of the second axial zone and the enrichment of the third axial zones are each selected to optimize for an R-factor. 5. The fuel rod of claim 1, further comprising a bottom end axial zone positioned between the bottom end and the first axial zone, wherein the bottom end axial zone includes at least one of natural uranium and an enrichment about equal to the enrichment of the first axial zone. 6. The fuel rod of claim 5, further comprising a top end axial zone positioned between the third axial zone and the top end, wherein the top end zone includes at least one of natural uranium and an enrichment about equal to the enrichment of the first axial zone. 7. The fuel rod of claim 6 wherein the fuel rod has no more than 5 axial zones. 8. The fuel rod of claim 1 wherein a zone length of the second axial zone and a zone length of the third axial zone are dimensioned for optimizing an R-factor. 9. The fuel rod of claim 1 wherein the fuel rod has no more than 3 axial zones having enrichments so as to be distinct from natural uranium. 10. A fuel assembly for a nuclear reactor comprising a plurality of fuel rods wherein one or more fuel rods include a first axial zone positioned generally at a bottom end, a second axial zone positioned adjacent to the first axial zone in an intermediate region, and a third axial zone positioned generally at a top end, wherein the first axial zone has an enrichment greater than the second axial zone and the second axial zone has an enrichment greater than or equal to the third axial zone, the first axial zone, the second axial zone, and the third axial zone having different levels of at least one of average enrichment and average gadolinium doping,wherein the one or more fuel rods is a substantial portion of the fuel rods in the fuel assembly. 11. The fuel assembly of claim 10 wherein the enrichment of the first axial zone for a substantial portion of the one or more fuel rods is configured to optimize a local peak power. 12. The fuel assembly of claim 11 wherein a zone length of the second axial zone and a zone length of the third axial zone for a substantial portion of the one or more fuel rods are each dimensioned for optimizing an R-factor. 13. The fuel assembly of claim 10 wherein only the first axial zones of fuel rods about the edge of the fuel assembly are optimized for local peak power. 14. The fuel assembly of claim 10 wherein a zone length of the first axial zone for a substantial portion of the one or more fuel rods is dimensioned for optimizing the first axial zone for a local peak power, and a zone length of the second axial zone and a zone length of the third axial zone for the substantial portion of the one or more fuel rods are each dimensioned for optimizing an R-factor. 15. The fuel assembly of claim 10, further comprising a bottom end axial zone positioned between the bottom end and the first axial zone in a portion of the one or more fuel rods and a top end axial zone positioned between the third axial zone and the top end in the portion of the one or more fuel rods, wherein both the bottom end and top end axial zones include at least one of natural uranium and an enrichment about equal to the enrichment of the first axial zone. 16. The fuel assembly of claim 15 wherein a substantial portion of the fuel rods includes the bottom end and top end axial zones and does not include more than 5 axial zones. 17. The fuel assembly of claim 10 wherein the one or more fuel rods includes two or more groups of fuel rods each having a different combination of enrichments and zone lengths for the first axial zone, the second axial zone, and the third axial zone. 18. The fuel assembly of claim 17 wherein each of the different combinations of enrichments and zone lengths for the two or more groups is configured to optimize a local peak power and an R-factor for the fuel assembly. 19. The fuel assembly of claim 17 wherein only the first axial zones of fuel rods about an edge of the fuel assembly are optimized for local peak power and only the second axial zones and third axial zones of the edge fuel rods are optimized for R-factor. 20. The fuel assembly of claim 10 wherein the substantial portion of the fuel rods in the fuel assembly have no more than three enrichment zones each. 21. A fuel assembly for a nuclear reactor comprising a plurality of fuel rods wherein one or more fuel rods includes a first axial zone positioned generally at a bottom end, a second axial zone positioned adjacent to the first axial zone in an intermediate region, and a third axial zone positioned generally at a top end, wherein the first axial zone is configured to optimize a local peak power of the first axial zone and the second and third axial zones are configured to optimize an R-factor for the fuel assembly, the first axial zone, the second axial zone, and the third axial zone having different levels of at least one of average enrichment and average gadolinium doping. 22. The fuel assembly of claim 21 wherein the enrichment of the first axial zone within the fuel assembly is greater than the enrichment of the second axial zone and is greater than the enrichment of the third axial zone. 23. The fuel assembly of claim 22 wherein the enrichment of the second axial zone within the fuel assembly is greater than or equal to the enrichment of the third axial zone.