Patent Number: 054835658
Section: claims

1. A supercell of a boiling-water nuclear reactor which includes four fuel assemblies and a centrally located control rod, each fuel assembly including a plurality of vertical fuel rods containing enriched nuclear fuel material, said rods being arranged between a bottom tie plate and a top tie plate in a surrounding vertical fuel channel, said fuel channel being connected to a transition section, said transition section being provided with holes for a by-pass flow outside the fuel assembly, each fuel assembly being designed with a water inlet for passing water in through the bottom tie plate, through the space between the fuel rods in the vertical fuel channel and through the top tie plate, and each fuel assembly being arranged with intermediate gaps to adjacent fuel assemblies for passing water in the vertical direction through an inlet from the bottom and upwards through the core, and wherein each fuel assembly, including the transition section, is turnable through an angle of 90.degree., 180.degree. or 270.degree. around a longitudinal axis thereof, and  each fuel assembly comprises a flow-preventing means arranged to prevent the passage of the by-pass flow through the hole or holes which, before or after a turning of the fuel assembly, are arranged so as to face the control rod in said supercell, whereby at least one hole faces away from the control rod, such that by-pass flow towards the control rod is prevented.  each fuel assembly, including the transition section, is turnable through an angle of 180.degree. around its longitudinal axis, and  each fuel assembly comprises at least one hole for the by-pass flow, which holes, before and after a turning, are arranged so as to face away from the control rod in said supercell, such that by-pass flow towards the control rod is prevented. 2. A supercell according to claim 1, wherein the holes for the by-pass flow of each fuel assembly are arranged at a lower part of the fuel assembly between an assembly supporting plate and a lowest part of the nuclear fuel material. 3. A supercell according to claim 1, wherein the flow-preventing member of each fuel assembly is a turnable turning ring located over the holes, said turning ring being turnable for opening at least one hole for the by-pass flow, which hole faces away from the control rod in said supercell. 4. A supercell according to claim 1, wherein the transition section of each fuel assembly is provided with four holes for the by-pass flow arranged with a substantially 90.degree. pitch angle. 5. A supercell according to claim 4, wherein the by-pass holes are arranged in the transition section in a position corresponding to a corner of each fuel assembly. 6. A supercell according to claim 4, wherein the by-pass holes are arranged in the transition section in a position substantially corresponding to the center of side surfaces of each fuel assembly. 7. A supercell of a boiling-water nuclear reactor which includes four fuel assemblies and a centrally placed control rod, each fuel assembly including a plurality of vertical fuel rods containing enriched nuclear fuel material, said rods being arranged between a bottom tie plate and a top tie plate in a surrounding vertical fuel channel, said fuel channel being connected to a transition section, said transition section being provided with holes for a by-pass flow outside the fuel assembly, each fuel assembly being designed with a water inlet for passing water in through the bottom tie plate, through the space between the fuel rods in the vertical fuel channel and through the top tie plate, each fuel assembly being arranged with intermediate gaps to adjacent fuel assemblies for passage of water in the vertical direction through an inlet from below and upwards through the core, and wherein 8. A supercell according to claim 7, wherein at a lower part of each fuel assembly at least two-holes for the by-pass flow are arranged diametrically opposite along a symmetry line so as to have substantially the same distance to the control rod in said supercell. 9. A supercell according to claim 7, wherein each fuel assembly includes a turnable turning ring arranged around a lower part of the fuel assembly over the by-pass holes, said turning ring being turnable for opening at least one hole for by-pass flow, said hole being directed away from the control rod in said supercell. 10. A supercell according to claim 1, wherein the by-pass holes are arranged in the transition section in a position corresponding to a corner of each fuel assembly. 11. A supercell according to claim 1, wherein the by-pass holes are arranged in the transition section in a position substantially corresponding to a center of side surfaces of each fuel assembly.