Patent Number: 043550029
Section: claims

1. In a nuclear fuel assembly including first nuclear fuel elements containing fissionable material and a non-fissionable burnable poison, and second adjacent nuclear fuel elements containing the same fissionable material as contained in said first nuclear fuel elements but being free of said non-fissionable burnable poison, the improvement comprising the enrichment of said fissionable material in said first nuclear fuel elements at zero megawatt.day/ton burn-up being less than the enrichment of said same fissionable material in said second adjacent nuclear fuel elements at zero megawatt.day/ton burn-up, whereby the danger of damage to said first nuclear fuel elements is reduced. 2. A nuclear fuel assembly according to claim 1, wherein the enrichment of said fissionable material in said first nuclear fuel elements is less than about 72% of the enrichment of said fissionable material in said second adjacent nuclear fuel elements. 3. A nuclear fuel assembly according to claim 2, wherein said nuclear fuel elements are in the form of axially elongated rods, and wherein the enrichment of fissionable material in the axially middle portions of said first nuclear fuel elements is less than the enrichment of fissionable material in the same portion of said second adjacent nuclear fuel elements. 4. A nuclear fuel assembly according to claim 3, wherein the enrichment of fissionable material in said axially middle portions of said first nuclear fuel elements is less than the enrichment of the same fissionable material in the axially end portions of said first nuclear fuel elements. 5. A nuclear fuel assembly according to claim 3, wherein the enrichment of fissionable material in said axially middle portions of said first nuclear fuel elements is less than about 72% of the enrichment of fissionable material in said second adjacent nuclear fuel elements. 6. A nuclear fuel assembly according to claim 5, wherein the enrichment of fissionable material in said axially middle portions of said first nuclear fuel elements is less than the enrichment of the same fissionable material in the axially end portions of said first nuclear fuel elements. 7. A nuclear fuel assembly according to claim 3, wherein said fissionable material contained in said first and second nuclear fuel elements is present in pellets of UO.sub.2, and wherein said non-fissionable burnable poison contained in said first nuclear fuel elements is gadolinium oxide. 8. A nuclear fuel assembly according to claim 1, wherein said first nuclear fuel elements contain a single fissionable material. 9. A nuclear fuel assembly according to claim 1, wherein said nuclear fuel elements are in the form of axially elongated rods, and wherein the enrichment of said fissionable material in the axially middle portions of said first nuclear fuel elements is less than the enrichment of said fissionable material in identical portions of said second adjacent nuclear fuel elements. 10. A nuclear fuel assembly according to claim 9, wherein the enrichment of said fissionable material in the axially middle portions of said first nuclear fuel elements is less than about 72% of the enrichment of said second adjacent nuclear fuel elements. 11. A nuclear fuel assembly according to claim 1, wherein said second adjacent nuclear fuel elements are arranged adjacently surrounding each of said first nuclear fuel elements. 12. A nuclear fuel assembly according to claim 1, wherein said fissionable material contained in said first and second nuclear fuel elements is present in pellets of UO.sub.2, and wherein said non-fissionable burnable poison contained in said first nuclear fuel elements is gadolinium oxide. 13. A nuclear fuel assembly according to claim 12, wherein the enrichment of said fissionable material in said first nuclear fuel elements is less than about 72% of the enrichment of said fissionable material in said second adjacent nuclear fuel elements. 14. A nuclear fuel assembly according to claim 1, wherein said nuclear fuel elements are in the form of axially elongated rods. 15. A nuclear fuel assembly according to claim 1, wherein said non-fissionable burnable poison is a material having a high thermal neutron absorption cross-section. 16. A nuclear fuel assembly according to claim 15, wherein said non-fissionable burnable poison is a material containing at least one of boron, cadmium, erbium, europium, hafnium, samarium, and chemical compounds thereof. 17. In a nuclear fuel assembly including first nuclear fuel elements containing fissionable material and a non-fissionable burnable poison, and second adjacent nuclear fuel elements containing the same fissionable material as contained in said first nuclear elements but being free of said non-fissionable burnable poison, a method for preventing failure of said first nuclear fuel elements comprising the step of loading fissionable material in said first nuclear fuel elements so that the enrichment of the fissionable material in said first nuclear fuel elements at zero megawatt.day/ton burn-up is less than the enrichment of the same fissionable material in said second adjacent nuclear fuel elements at zero megawatt.day/ton burn-up, whereby the danger of damage to said first nuclear fuel elements is reduced. 18. A method according to claim 17, wherein the enrichment of fissionable material loaded in said first nuclear fuel elements is less than about 72% of the enrichment of fissionable material in said second adjacent nuclear fuel elements.