Patent Number: 
Section: claims

1. A fuel bundle comprising:a first fuel rod, the first fuel rod includingfirst enriched uranium in a boost zone of the first fuel rod, the boost zone having a first average composition, the boost zone of the first fuel rod including a first burnable poison and constituting a bottommost portion of the first fuel rod, a percent of enrichment of the first enriched uranium in the boost zone of the first fuel rod being at least one percent,second enriched uranium in a second zone of the first fuel rod, the second zone having a second average composition, the second zone of the first fuel rod being arranged over the boost zone of the first fuel rod, the second average composition of the second zone being different from the first average composition of the boost zone, andnatural uranium in a third zone of the first fuel rod, the third zone having a third average composition, the third zone of the first fuel rod being arranged directly over the second zone of the first fuel rod, a length of the boost zone being equal to a length of the third zone, the third average composition of the third zone being different from the first average composition of the boost zone and the second average composition of the second zone. 2. The fuel bundle according to claim 1, further comprising:a second fuel rod, the second fuel rod includingthird enriched uranium in a boost zone of the second fuel rod, the boost zone of the second fuel rod being arranged directly at a bottom of the second fuel rod,fourth enriched uranium in a second zone of the second fuel rod, the second zone of the second fuel rod being arranged over the boost zone of the second fuel rod, andnatural uranium in a third zone of the second fuel rod, the third zone of the second fuel rod being arranged over the second zone of the second fuel rod, wherein a percent of enrichment of the third enriched uranium in the boost zone of the second fuel rod is equal to or greater than a percent of enrichment of the fourth enriched uranium in the second zone of the second fuel rod, and at least one of the boost and second zones of the second fuel rod includes a second burnable poison. 3. The fuel bundle according to claim 2, wherein the second burnable poison is gadolinia. 4. The fuel bundle according to claim 2, wherein the third zones of the first and second fuel rods have a length of about six inches. 5. The fuel bundle according to claim 2, wherein the second zones of the first and second fuel rods are directly on the boost zones of the first and second fuel rods. 6. The fuel bundle according to claim 2, wherein the boost zone of the second fuel rod does not include the second burnable poison. 7. The fuel bundle according to claim 2, wherein the boost zone and the second zone of the second fuel rod include the second burnable poison. 8. The fuel bundle according to claim 7, wherein a percent by weight of the second burnable poison in the boost zone is less than a percent by weight of the second burnable poison in the second zone. 9. The fuel bundle according to claim 2, wherein the first and second fuel rods are configured to yield a batch fraction of about 48% in a reactor core. 10. The fuel bundle according to claim 1, wherein the percent of enrichment of the first enriched uranium in the boost zone of the first fuel rod is about one percent. 11. The fuel bundle according to claim 1, wherein the percent of enrichment of the first enriched uranium in the boost zone of the first fuel rod is less than a percent of enrichment of the second enriched uranium in the second zone of the first fuel rod. 12. The fuel bundle according to claim 1, wherein the second zone has an enrichment that is greater than that of the boost zone. 13. The fuel bundle according to claim 1, wherein the length of the boost zone is about six inches. 14. The fuel bundle according to claim 1, wherein the first burnable poison is gadolinia.