Patent Number: 051436914
Section: claims

1. Fuel assembly for boiling water reactor, comprising an elongated case having an interior and inner surfaces, a base part having inlet openings discharging into the interior of said case for coolant flowing in a given flow direction, a head part having outlet openings leading out of the interior of said case for the coolant flowing in said given flow direction and at a given coolant flow cross section, mutually parallel fuel rods containing nuclear fuel and being disposed in said case between said base and head parts, said inner surfaces of said case having a profile with a rectilinear alignment as seen in said given flow direction and flow trippers extending transversely to said alignment in the form of bulges protruding inwardly, each of said bulges having an impact surface facing toward the coolant flow substantially perpendicular to said given flow direction for reducing the coolant flow cross section to produce a pressure difference between the interior and exterior of the wall, and a sloped surface facing away from said impact surface for gradually enlarging the coolant flow cross section in said given flow direction. 2. Fuel assembly according to claim 1, wherein said case has wall parts with a substantially constant wall thickness, said bulges are formed by bending said case wall parts, and said impact surfaces are formed by removing material from said sloped surfaces. 3. Fuel assembly according to claim 1, wherein said case has wall parts with a substantially constant wall thickness, and said bulges are formed of additional wall material disposed on said case wall parts. 4. Fuel assembly according to claim 3, wherein said additional wall material is welded on. 5. Fuel assembly according to claim 4, wherein said additional wall material is welding material, and said impact surfaces are formed by mechanical removal of part of said welding material. 6. Fuel assembly according to claim 3, wherein said additional wall material is in the form of reinforcing metal sheets welded to said wall parts, and said reinforcing sheets have a profile forming said bulges and said impact surfaces. 7. Fuel assembly according to claim 3, wherein said additional wall material is in the form of ribs being attached to said inner surface of said case and perpendicular to said alignment. 8. Fuel assembly according to claim 1, wherein said case has upper and lower portions, and said bulges are disposed only in said upper portion. 9. Fuel assembly according to claim 1, including spacers disposed in said case, said spacers having ribs between said fuel rods extending perpendicular to the longitudinal axis of said case, and each of said bulges being disposed upstream of a respective one of said spacers as seen in said given flow direction. 10. Fuel assembly according to claim 1, including inner walls being attached to said case in the interior of said case, said inner walls having said inner surfaces with said impact surfaces disposed substantially perpendicular to said given flow direction. 11. Fuel assembly for a boiling water reactor, comprising an elongated case having inner surfaces and an interior through which coolant flows in a given flow direction, mutually parallel fuel rods disposed in said case, said inner surfaces of said case being rectilinear as seen in said given flow direction except for flow trippers in the form of inwardly protruding bulges, each of said bulges having an impact surface facing said coolant flow substantially perpendicular to said given flow to produce a pressure difference between the interior and exterior of the wall direction.