Patent Number: 
Section: claims

1. A fuel bundle for a pressurized heavy water nuclear reactor, the fuel bundle comprising:fuel elements containing fissile content of 235U, wherein each of the fuel elements of the fuel bundle has a fissile content of 235U between about 0.9 wt % 235U and 5.0 wt % 235U, and wherein at least one of the fuel elements is a poisoned low-enriched uranium fuel element further including a neutron poison in a concentration greater than about 5.0 vol %;wherein the at least one poisoned low-enriched uranium fuel element includes a mix of low-enriched uranium and at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired fissile content of 235U. 2. The fuel bundle of claim 1, wherein the fissile content of the at least one poisoned low-enriched uranium fuel element is at least about 3.0 wt % of 235U. 3. The fuel bundle of claim 2, wherein the fissile content of the at least one poisoned low-enriched uranium fuel element is between about 3.0 wt % of 235U and about 3.5 wt % of 235U. 4. The fuel bundle of claim 1, wherein the neutron poison concentration in the at least one poisoned low-enriched uranium fuel element is between about 5.0 vol % and about 8.0 vol %. 5. The fuel bundle of claim 4, wherein the fissile content of the at least one poisoned low-enriched uranium fuel element is about 3.21 wt % of 235U and the neutron poison concentration in the at least one low-enriched uranium fuel element is about 6.82 vol %, and wherein the neutron poison is dysprosium. 6. The fuel bundle of claim 1, wherein the at least one poisoned low-enriched uranium fuel element includes a central fuel element that extends along a fuel bundle axis and a first plurality of fuel elements immediately surrounding the central fuel element, and wherein the remaining fuel elements of the fuel bundle are non-poisoned fuel elements disposed radially outside the poisoned low-enriched uranium fuel elements. 7. The fuel bundle of claim 6, wherein each of the non-poisoned fuel elements has a fissile content of 235U not exceeding the fissile content of the poisoned low-enriched uranium fuel elements, and at least some of the non-poisoned fuel elements have a fissile content of 235U that is less than the fissile content of the poisoned low-enriched uranium. 8. The fuel bundle of claim 7, wherein the non-poisoned fuel elements are arranged to include a second plurality of fuel elements immediately surrounding the first plurality of fuel elements, and a third plurality of fuel elements immediately surrounding the second plurality of fuel elements, and wherein the fuel elements of the second plurality have a higher fissile content that the fuel elements of the third plurality. 9. The fuel bundle of claim 8, wherein the fuel elements of the second plurality have a fissile content between about 3.0 wt % and about 3.5 wt % of 235U, and the fuel elements of the third plurality have a fissile content less than about 2.0 wt % of 235U. 10. The fuel bundle of claim 9, wherein the fuel elements of the second plurality have a fissile content of about 3.18 wt % of 235U, and the fuel elements of the third plurality have a fissile content of about 1.73 wt % of 235U. 11. The fuel bundle of claim 1, wherein the at least one poisoned low-enriched uranium fuel element includes a fissile content of 235U between about 0.9 wt % 235U and 5.0 wt % 235U. 12. The fuel bundle of claim 1, wherein each one of the fuel elements includes at least one of slightly-enriched uranium and low-enriched uranium mixed with at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired predetermined fissile content of 235U. 13. The fuel bundle of claim 1, wherein the fuel bundle includes 37 total fuel elements having substantially uniform size. 14. The fuel bundle of claim 1, wherein the fuel bundle includes 43 total fuel elements, and wherein the at least one poisoned low-enriched fuel element includes 8 enlarged fuel elements positioned at the center. 15. The fuel bundle of claim 1, wherein the neutron poison includes at least one burnable neutron poison. 16. The fuel bundle of claim 15, wherein the neutron poison includes dysprosium. 17. The fuel bundle of claim 15, wherein the neutron poison includes gadolinium. 18. The fuel bundle of claim 1, wherein the neutron poison is a non-burnable neutron poison. 19. A method of operating a pressurized heavy water nuclear reactor comprising:providing a first fuel bundle made up of a plurality of fuel elements each having a fissile content of 235U between about 0.9 wt % 235U and 5.0 wt % 235U, at least one of the fuel elements being a poisoned low-enriched uranium fuel element further including a neutron poison in a concentration greater than about 5.0 vol %, wherein the at least one poisoned low-enriched uranium fuel element includes a mix of low-enriched uranium and at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired fissile content of 235U;inserting the first fuel bundle into a pressure tube of the pressurized heavy water nuclear reactor; andoperating the pressurized heavy water nuclear reactor to burn the fuel elements, producing a power output at least as great as a fuel bundle of natural uranium while providing a negative fuel temperature coefficient (FTC), a negative power coefficient (PC), and a coolant void reactivity (CVR) that is lower than that provided by operating the pressurized heavy water nuclear reactor with natural uranium fuel. 20. The method of claim 19, wherein the first fuel bundle is inserted to replace a fuel bundle of natural uranium. 21. The method of claim 19, further comprising filling the pressure tube with 12 fuel bundles similar to the first fuel bundle. 22. The method of claim 21, further comprising refueling the pressure tube during operation of the nuclear reactor with a 4-bundle-shift. 23. The method of claim 21, further comprising refueling the pressure tube during operation of the nuclear reactor with a 2-bundle-shift. 24. The method of claim 21, further comprising refueling the pressure tube during operation of the nuclear reactor with a mixed 2-and-4-bundle-shift. 25. The method of claim 21, further comprising refueling the pressure tube during operation of the nuclear reactor with a mixed 4-and-8-bundle-shift. 26. A fuel bundle for a nuclear reactor, the fuel bundle comprising:a fuel element including a mix of low-enriched uranium and at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired fissile content, the fuel element containing at least one fissile material selected from the group consisting of 233U, 235U, 239Pu, and 241Pu, the at least one fissile material being mixed with gadolinium and dysprosium. 27. A fuel element for a nuclear reactor, the fuel element comprising:a mix of low-enriched uranium and at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired fissile content; andat least one fissile material selected from the group consisting of 233U, 235U, 239Pu, and 241Pu, the at least one fissile material being mixed with gadolinium and dysprosium. 28. A fuel bundle for a nuclear reactor, the fuel bundle comprising:a plurality of fuel elements including inner elements and outer elements;wherein at least one of the inner elements includes a mixture of low enriched uranium and at least one of: natural uranium, recycled uranium, and depleted uranium to achieve a desired fissile content, a first neutron poison, and a second neutron poison different from the first neutron poison.