Patent Number: 
Section: claims

1. A nuclear fuel assembly for a boiling water reactor comprising:a bundle of fuel rods comprising a set of fuel rods, the fuel rods of the set of fuel rods being arranged at the nodes of a first lattice having a non-uniform distribution of nodes at a lowermost end of the fuel assembly, the fuel rods of the set of fuel rods being arranged at the nodes of a second lattice having a uniform distribution of nodes at an uppermost end of the fuel assembly, the second lattice having the uniform distribution of nodes including the fuel rods of the set arranged at nodes of the second lattice with a same spacing between each pair of adjacent nodes, the first lattice having the non-uniform distribution of nodes including the fuel rods of the set arranged at nodes of the first lattice with at least two different spacings between pairs of adjacent nodes. 2. The nuclear fuel assembly as in claim 1, wherein the spacings between the fuel rods of the set vary monotonously along the length of the fuel assembly. 3. The nuclear fuel assembly as in claim 1, wherein, in the lowermost section, the fuel rods of the set of fuel rods are arranged in groups separated by coolant/moderator gaps. 4. The nuclear fuel assembly as in claim 3, wherein the fuel rods of each group are with a uniform pitch between the fuel rods of said group. 5. The nuclear fuel assembly as in claim 3, wherein the coolant/moderator gaps are wider than passages between the fuel rods or rows of fuel rods of each group. 6. The nuclear fuel assembly as in claim 3, wherein the coolant/moderator gaps include at least one coolant/moderator gap extending substantially radially from a central region of the bundle of fuel rods towards the periphery thereof. 7. The nuclear fuel assembly as in claim 3, wherein the groups including a central group and a peripheral group surrounding the central group, the coolant/moderator gaps including a coolant/moderator gap between the central group and the peripheral group. 8. The nuclear fuel assembly as in claim 3, wherein the coolant/moderator gaps include at least two coolant/moderator gaps extending parallel to each other. 9. The nuclear fuel assembly as in claim 1 further comprising at least one tubular water channel. 10. The nuclear fuel assembly as in claim 9 further comprising at least one coolant/moderator gap, the at least one tubular water channel being surrounded by the at least one coolant/moderator gap. 11. The nuclear fuel assembly as in claim 9, wherein the at least one tubular water channel is of constant cross section along the length of the fuel assembly. 12. The nuclear fuel assembly as in claim 1, wherein the bundle of fuel rods includes at least one individual fuel rod which is offset at least one of from the first lattice in the lowermost section and from the second lattice in the uppermost section of the fuel assembly. 13. The nuclear fuel assembly as in claim 1, wherein a transition zone between the lowermost section with a non-uniform pitch towards the uppermost section with a uniform pitch is positioned at a height comprised between 30% and 70% of the height of an active zone of the fuel assembly. 14. The nuclear fuel assembly as in claim 1, wherein all the fuel rods are full-length fuel rods. 15. The nuclear fuel assembly as in claim 3, wherein the fuel rods of each group are in a square or rectangular lattice arrangement. 16. A nuclear fuel assembly for a boiling water reactor comprising:a bundle of fuel rods comprising a set of fuel rods arranged in a first lattice having a non-uniform pitch in a lowermost section of the fuel assembly and arranged in a second lattice with a uniform pitch in an uppermost section of the fuel assembly,a transition zone between the lowermost section with a non-uniform pitch towards the uppermost section with a uniform pitch is positioned at a height comprised between 30% and 70% of the height of an active zone of the fuel assembly.