Patent Number: 
Section: claims

1. A fuel assembly for a nuclear power boiling water reactor, comprising:a plurality of fuel rods, each comprising a cladding tube and nuclear fuel material positioned in the cladding tube; anda fuel channel surrounding said plurality of fuel rods;wherein each one of at least 95% of the plurality of fuel rods comprises nuclear fuel material in the form of Uranium (“U”) enriched in 235U;wherein each one of at least 20% of the plurality of fuel rods belongs to a first set of fuel rods;wherein each fuel rod in the first set of fuel rods comprises both U enriched in 235U and Thorium (“Th”); andwherein the first set of fuel rods comprises at least a first and a second subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subsetwherein each one of between 20% and 90% of the plurality of fuel rods belongs to a second set of fuel rods, wherein each fuel rod in the second set comprises U enriched in 235U, but does not comprise a substantial amount of Th or a burnable absorber, wherein the enrichment of 235U in the U in the second set is within the range 4.00-6.00%. 2. A fuel assembly according to claim 1, wherein said first set of fuel rods further comprises a third subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said third subset, is lower than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset. 3. A fuel assembly according to claim 2, wherein said first set of fuel rods further comprises a fourth subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said fourth subset, is lower than the ratio, with regard to weight, between Th and U, in each fuel rod of said third subset. 4. A fuel assembly according to claim 1, wherein, in all of the fuel rods of each subset, the enrichment of 235U in the U is within the range 4.00-6.00%. 5. A fuel assembly according to claim 1, wherein the fuel rods of said subsets have the same enrichment of 235U in the U. 6. A fuel assembly according to claim 1, wherein said subsets of fuel rods are positioned in the fuel assembly such that fuel rods with a higher ratio, with regard to weight, between Th and U are positioned where there will be more moderator when the fuel assembly is in use in the nuclear power boiling water reactor, than where the fuel rods of a subset with a lower ratio, with regard to weight, between Th and U are positioned. 7. A fuel assembly according to claim 1, wherein between 25% and 80% of the plurality of fuel rods belong to said first set of fuel rods. 8. A fuel assembly according to claim 1, wherein each one of between 3% and 20% of the fuel rods belongs to a third set of fuel rods, wherein each fuel rod in the third set comprises U enriched in 235U and at least one kind of burnable absorber. 9. A fuel assembly according to claim 1, wherein in each fuel rod of said second set, the enrichment of 235U in the U is substantially the same as the enrichment of 235U in the U in the fuel rods of said subsets. 10. A fuel assembly according to claim 8, wherein in each fuel rod of said second set, the enrichment of 235U in the U is substantially the same as the enrichment of 235U in the U in the fuel rods of said subsets. 11. A fuel assembly according to claim 8, wherein each one of between 2% and 10% of the plurality of fuel rods belongs to a fourth set of fuel rods, wherein each fuel rod in the fourth set comprises U enriched in 235U, but does not comprise a substantial amount of Th or a burnable absorber, wherein the enrichment of 235U in the U in the fourth set is less than 4.00%. 12. A fuel assembly according to claim 1, wherein the U in the nuclear fuel material in the plurality of fuel rods is present in the form of Uranium dioxide (“UO2”) and the Th is present in the form of Thorium dioxide (“ThO2”). 13. A fuel assembly according to claim 1, wherein the nuclear fuel material in the plurality of fuel rods is in the form of pellets arranged on top of each other such that they form a stack of nuclear fuel pellets in the cladding tubes, wherein the different fuel pellets with nuclear fuel material within one and the same fuel rod have the same composition.