Patent Number: 054835658
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1-4 show a fuel channel 1 with a substantially square cross section. The fuel channel 1 surrounds with no significant play an upper square portion 2a of a transition section 2 which otherwise comprises a conical portion 2b and a cylindrical portion 2c (see also FIGS. 5a-c). The transition section 2 has a downwardly-facing inlet opening 3 for cooling water. The transition section 2 is formed with a guide member which is intended to guide the fuel assembly 16 into an assembly supporting plate (not shown) and comprises a plurality of guiding spokes 2d. Besides supporting the fuel channel 1, the transition section 2 also supports a bottom support 4. At its bottom the fuel channel 1 has a relatively thick wall portion which is fixed to the transition section 2 and the bottom support 4 by means of a plurality of horizontal bolts 5. According to the embodiment shown, with a hollow support member 6 of cruciform cross section, the fuel channel 1 is divided into four vertical tubular parts 7a-d with at least substantially square cross section. The support member 6 is welded to the four walls 1a-d of the fuel channel 1 and has four hollow wings 8. The central channel formed by the support member 6 is designated 9 and at its bottom extended down through the bottom support 4 with an inlet 10 for moderator water. Each tubular part 7a-d comprises a bundle of twenty-four fuel rods 12. The rods 12 are arranged in a symmetrical lattice in rows in which each rod 12 is included in two rows perpendicular to each other. Each bundle is arranged with a bottom tie plate 13, a top tie plate 14 and a plurality of spacers 15. A fuel rod bundle with a bottom tie plate 3, a top tie plate 14, spacers 15 and fuel channel part 1 forms a unit which is referred to as a sub-assembly, whereas the device illustrated in FIGS. 1-4 and comprising four such sub-assemblies is referred to as a fuel assembly 16. A unit comprising four fuel assemblies 16 and a control rod 17 arranged centrally therebetween constitutes a supercell. The spaces between the fuel rods 12 within each sub-assembly 7 are traversed by water, as is the hollow support member 6 of cruciform cross section in the fuel assembly 16. The gaps 18a-b between the fuel assemblies 16 are also traversed by water. The four bottom tie plates 13 are supported in the fuel assembly 16 by the bottom support 4 and are each partially inserted into a respective square hole therein. The holes for the passage of the water through the bottom tie plate 13 are designated 19. FIG. 3 shows part of a symmetrical core lattice according to the prior art. The section comprises a supercell. In a symmetrical core lattice, the control rod gaps 18a, into which the control rods 17 can be inserted, have the same width as the narrow gaps 18b, into which no control rods 17 can be inserted. The control rods 17 have blades which form a rectilinear cross and are arranged centrally in the supercell. Two continuous holes 20a-b for by-pass flow are arranged in the wall portion of the transition section 2, facing away from the control rod 17. To illustrate the by-pass holes more clearly in the FIGS. 3 and 4a, one fuel assembly 16 in each figure is shown without the cruciform water channel 19. FIGS. 4a-c show part of a symmetrical core lattice, a supercell, according to the invention. FIG. 4a shows a supercell with fuel assemblies with by-pass holes 20a-d arranged in the transition section in a position corresponding to the center of the side surfaces of the fuel channel 1. FIGS. 5a-b show how the transition section is provided with a turnable valve, a turning ring 21, with holes 22a-b arranged such that a turning of the ring allows opening of two holes 21a-d at a time, which are located such that the by-pass flow is directed away from the control rod. The by-pass holes 20a-d may be arranged at any location between the assembly supporting plate and the lowest part of a fuel pellets column 24. FIG. 4b shows a supercell with fuel assemblies where the by-pass holes 20a-d are arranged in a position corresponding to the corner of the fuel channel 1 where the transition section 2, in a manner corresponding to the embodiment of FIG. 4a, is provided with a turning ring 21 according to FIGS. 5a-b. According to FIG. 4a, in the original operating position of the fuel assembly 16, the by-pass flow passes through the two holes 20a and 20b which are facing the narrow gaps 18b, whereas in the operating position when the fuel assembly 16 has been turned, for example, 180.degree., the by-pass flow passes through the holes 20c and 20d which in the original operating position were facing the control rod gaps 18a, but which after the turning are facing the narrow gaps 18b. This change of flow is made possible by causing the turning ring 21 to open/close two of the four holes 20a-d with which the transition section 2 is provided. Thus, by turning the turning ring 21, the two holes 20c and 20d, which in the original operating position are closed and facing the control rod gap 18a, open for the by-pass flow whereas the other two holes 20a and 20b are closed. In FIG. 4b, in the original position of the fuel assembly 16, the by-pass flow passes through the holes 20b and 20d whereas the holes 20a and 20c are closed by means of the turning ring 21. When turning the upper lefthand fuel assembly 16, shown in the figure, for example 90.degree. in the clockwise direction, the by-pass flow, after turning the turning ring 21, instead passes through the holes 20a and 20c when are then facing away from the control rod 17 in the supercell. FIG. 4c shows an embodiment intended for fuel assemblies 16 which can only be turned 180.degree. around their longitudinal axis. The embodiment shows two holes 20a-b for by-pass flow. The holes 20a-b are diametrically opposed and arranged symmetrically in the wall portion of the transition section 2 such that both holes 20a-b substantially have the same distance to the control rod 17. No turning ring 21 is needed since a 180.degree. turn of the fuel assembly 16 results in the location of the holes 20a-b in relation to the control rod 17 being the same as before the turning. A transition section 2 according to FIG. 4c is also clear from FIG. 5c. FIG. 5b shows the design of the turning ring 21 and its arrangement around the transition section 2. The front ring 23 shown in FIG. 5b is intended to fix the turning ring 21 in the desired position. In this case, the transition section 2 is provided with four holes 20a-d for the by-pass flow arranged around the conical wall portion 2b of the transition section 2 with an approximately 90.degree. pitch angle. The turning ring 21 is provided with two holes 22a-b.