Patent Number: 051184672
Section: claims

1. Fuel assembly for a boiling water reactor, comprising an elongated fuel assembly case having an interior, mutually parallel fuel rods having longitudinal axes and being disposed in said case, and inner walls disposed in said fuel assembly case and extending longitudinally through the interior of said case and having lateral surfaces facing toward said fuel rods, at least some of said internal surfaces having grooves formed therein extending perpendicular to the longitudinal axes of said fuel rods. 2. Fuel assembly according to claim 1, wherein said inner walls form at least one water channel for non-boiling water being laterally closed off from said fuel rods. 3. Fuel assembly according to claim 2, wherein said grooves extend continuously around the outside of said water channel. 4. Fuel assembly according to claim 2, wherein said lateral surfaces with said grooves are planar outer surfaces of said water channel. 5. Fuel assembly according to claim 1, wherein said case has mutually opposite walls, a plurality of said inner walls interconnect at least two of said mutually opposite walls, and said inner walls each have two of said lateral surfaces in which said grooves are formed. 6. Fuel assembly according to claim 5, wherein said inner walls have sides being parallel to the longitudinal axes of said fuel rods, and said grooves formed in one of said lateral surfaces each extend as far as said sides of said inner walls. 7. Fuel assembly according to claim 5, wherein said inner walls have sides being parallel to the longitudinal axes of said fuel rods, and said grooves formed in one of said inner walls end in a rounded portion at a distance from said sides of said inner walls. 8. Fuel assembly according to claim 1, wherein said case has a polygonal cross section and oppositely disposed side walls, a plurality of said inner walls are parallel to said oppositely disposed side walls of said case, each of said inner walls interconnect two respective oppositely disposed side walls of said case, and said grooves are formed in all of said lateral surfaces facing toward said fuel rods. 9. Fuel assembly according to claim 8, wherein said polygonal cross section is a square cross section. 10. Fuel assembly according to claim 1, wherein said case has a regular polygonal cross section and side walls, said inner walls include a first group of inner walls forming a water channel for non-boiling water extending at least approximately centrally in said case and being laterally closed off from said fuel rods, and a second group of inner walls connecting each of said side walls of said case to said water channel, and at least said second group of inner walls has said grooves formed in all of said surfaces facing toward said fuel rods. 11. Fuel assembly according to claim 10, wherein said regular polygonal cross section is a square cross section. 12. Fuel assembly according to claim 1, wherein said grooves have narrow sides perpendicular to the longitudinal axes of said fuel rods. 13. Fuel assembly according to claim 1, wherein said case has a lower end with a base having inlet openings for boiling water, an upper portion with a top part having outlet openings for boiling water, and a plurality of spacers being longitudinally spaced apart from one another and having ribs between said fuel rods extending transversely to the longitudinal axes of said fuel rods, said grooves being formed only said upper portion and each of said grooves being upstream of a respective one of said spacers, as seen in flow direction of boiling water. 14. Fuel assembly according to claim 1, wherein said inner walls extend longitudinally between said fuel rods. 15. Boiling water reactor, comprising a boiling water circuit having a pressure vessel and a steam turbine, and a plurality of mutually parallel fuel assemblies disposed in said pressure vessel, each of said fuel assemblies including an elongated fuel assembly case, mutually parallel fuel rods having longitudinal axes and being disposed in said case, and longitudinally extending inner walls having lateral surfaces facing toward said fuel rods, at least some of said lateral surfaces having grooves formed therein extending perpendicular to the longitudinal axes of said fuel rods, and said fuel rods being bathed by boiling water flowing around them in the longitudinal direction of said fuel rods.