Patent Number: 
Section: claims

1. A nuclear fuel assembly for boiling water reactors, comprising: a tubular water channel extending along a longitudinal axis and having an upper section of a larger cross-section area than a lower section and at least one fuel rod receiving groove extending longitudinally on an outer surface of the lower section, fuel rods extending parallel to the longitudinal axis of the channel and disposed in an array around the water channel and fixing members fixing at least one of the fuel rods to the water channel in the at least one fuel rod receiving groove below the upper section,wherein the at least one fuel rod receiving groove extends along the upper section such that the at least one fuel rod received in the fixing members is longitudinally extractable or insertable from an upper end side of the fuel assembly without extracting the water channel. 2. The nuclear fuel assembly as recited in claim 1 wherein the at least one groove is of a constant width along the lower section and the upper section. 3. The nuclear fuel assembly as recited in claim 1 wherein the fuel rod receiving groove has a depth larger in the lower section than in the upper section. 4. The nuclear fuel assembly as recited in claim 1 wherein the fuel rod receiving groove is adapted to receive at least one fuel rod. 5. The nuclear fuel assembly as recited in claim 4 wherein the fuel rod receiving groove is adapted to receive at least two fuel rods side-by-side. 6. The nuclear fuel assembly as recited in claim 1 wherein the water channel has a main duct and at least two lateral ducts in fluid communication with the main duct along the whole length of the water channel, the at least two lateral ducts defining between them the at least one fuel rod receiving groove on an outer surface of the water channel. 7. The nuclear fuel assembly as recited in claim 6 wherein each lateral duct has a cross section area being constant along the length of the water channel, and the main duct having a larger cross section area in the upper section than in the lower section. 8. The nuclear fuel assembly as recited in claim 6 wherein each lateral duct replaces at least one fuel rod in an array. 9. The nuclear fuel assembly as recited in claim 8 wherein each lateral duct replaces one fuel rod in the array. 10. The nuclear fuel assembly as recited in claim 6 wherein the main duct replaces at least one fuel rod in an array. 11. The nuclear fuel assembly as recited in claim 10 wherein the main duct replaces a square unit of several fuel rods in the array. 12. The nuclear fuel assembly as recited in claim 11 wherein the main duct replaces a four fuel rods square unit in the array. 13. The nuclear fuel assembly as recited in claim 6 wherein the water channel has four lateral ducts defining between them four fuel rod reception grooves around the water channel. 14. The nuclear fuel assembly as recited in claim 1 wherein the water channel has a cruciform cross section replacing the fuel rods disposed in diagonals of a fuel rods square unit of the array. 15. The nuclear fuel assembly as in claim 14 wherein the water channel replaces the fuel rods disposed in the diagonals of a sixteen fuel rods square unit of the array.