Patent Number: 
Section: claims

1. A nuclear light water moderated boiling water reactor comprising: a plurality of fuel assemblies arranged vertically in the reactor, the reactor being arranged such that water flows upwards through said fuel assemblies whereby part of the water is transformed into steam;  at least one of the fuel assemblies comprising at least five fuel units stacked on top of each other, each fuel unit comprising a top tie plate and a bottom tie plate and a plurality of fuel rods extending between the top tie plate and the bottom tie plate;  at least one of said fuel units having fuel rods of different diameters from fuel rods of another of said fuel units; and  said at least one fuel assembly having a fuel channel with a substantially square cross section surrounding the stack of said at least five fuel units. 2. A nuclear light water moderated boiling water reactor according to  claim 1 , wherein at least two of the fuel units in said at least one fuel assembly differ from each other in at least one of the following respects: claim 1 the fuel units have different numbers of fuel rods;  the fuel units have different lattice configurations;  the fuel units have different heights;  the fuel units have bottom tie plates with different embodiments with respect to the mixing of coolant; and  the fuel rods have different degrees of enrichment. 3. A nuclear light water moderated boiling water reactor according to  claim 1 , wherein said fuel units are retained by a spring arrangement arranged in an upper part of the fuel assembly. claim 1 4. A nuclear light water moderated boiling water reactor according to  claim 1 , wherein at least one fuel rod in said at least one fuel assembly comprises a top plug for attachment to the top tie plate of the fuel unit in which said at least one fuel rod is positioned and a bottom plug for attachment to the bottom tie plate of said fuel unit in which said at least one fuel rod is positioned, wherein, for receiving fissile gases, at least one of said top and bottom plugs is hollow and has an opening facing the interior of said fuel rod. claim 1 5. A nuclear light water moderated boiling water reactor according to  claim 1 , wherein at least one fuel rod in said at least one fuel assembly has a coating protecting against abrasion damage to that part of the cladding tube of the fuel rod which is nearest the top tie plate of the fuel unit in which the fuel rod is positioned. claim 1 6. A nuclear light water moderated boiling water reactor according to  claim 1 , wherein each of said fuel units comprises at least 80 fuel rod positions. claim 1 7. A nuclear light water moderated boiling water reactor according to  claim 1  wherein the number of fuel units in said at least one fuel assembly is larger than six. claim 1 8. A method comprising: providing a fuel assembly in a nuclear light water moderated boiling water reactor arranged such that water flows upwards through said fuel assembly whereby part of the water is transformed into steam, said fuel assembly being arranged vertically in said reactor;  providing at least five fuel units stacked on top of each other in said fuel assembly, each fuel unit comprising a top tie plate and a bottom tie plate and a plurality of fuel rods extending between the top tie plate and the bottom tie plate;  providing, in at least one of said fuel units, fuel rods of different diameters from fuel rods of another of said fuel units;  wherein said fuel assembly has a fuel channel with a substantially square cross section surrounding the stack of said at least five fuel units. 9. The method according to  claim 8 , wherein at least two of the fuel units in said at least one fuel assembly differ from each other in at least one of the following respects: claim 8 the fuel units have different numbers of fuel rods;  the fuel units have different lattice configurations;  the fuel units have different heights;  the fuel units have bottom tie plates with different embodiments with respect to the mixing of coolant; and  the fuel rods have different degrees of enrichment. 10. The method according to  claim 8 , wherein said fuel units are retained by a spring arrangement arranged in an upper part of the fuel assembly. claim 8 11. The method according to  claim 8 , wherein at least one fuel rod in said fuel assembly comprises a top plug for attachment to the top tie plate of the fuel unit in which said at least one fuel rod is positioned and a bottom plug for attachment to the bottom tie plate of said fuel unit in which said at least one fuel rod is positioned, wherein, for receiving fissile gases, at least one of said top and bottom plugs is hollow and has an opening facing the interior of said fuel rod. claim 8 12. The method according to  claim 8 , wherein at least one fuel rod in said fuel assembly has a coating protecting against abrasion damage to that part of the cladding tube of the fuel rod which is nearest the top tie plate of the fuel unit in which the fuel rod is positioned. claim 8 13. The method according to  claim 8 , wherein each of said fuel units comprises at least 80 fuel rod positions. claim 8 14. The method according to  claim 13 , wherein the number of fuel units in said fuel assembly is larger than six. claim 13