Patent Number: 060350113
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2 and 3 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 bottom part 2 which otherwise comprises a conical portion 2b and a cylindrical portion 2c. The bottom part 2 has a downwardly facing inlet opening 3 for cooling water. Besides supporting the fuel channel, the bottom part 2 also supports a supporting plate 4. At the bottom the fuel channel 1 has a relatively thick wall portion which is fixed to the bottom part 2 and the supporting plate 4 by means of a plurality of horizontal bolts indicated by means of dash-dotted lines 5. From FIG. 3 it is clear that the fuel channel 1 according to the invention is provided with a reduced corner portion 41. By means of a hollow support member 7 with a cruciform cross section, the fuel channel 1 is divided into four vertical channel-formed parts 6 with at least substantially square cross section. The support member 7 is welded to the four walls 1a, 1b, 1c, 1d of the fuel channel 1 and has four hollow wings 8. The central channel formed by the support member 7 is designated 32 and is connected at the bottom to an inlet tube 9 for moderator water. Each tubular part 6 comprises a bundle 25 containing twenty-five fuel rods 10. The rods 10 are arranged in a lattice in which each rod 10 is included in two rows perpendicular to each other, apart from the reduced corner portion 41 where one rod 10 has been removed. From FIG. 1 it is clear that each bundle 25 is arranged with a bottom tie plate 11, a top tie plate 12 and a plurality of spacers 13. A fuel rod bundle 25 with bottom Lie plate 11, top tie plate 12, spacer 13 and fuel channel part 1 forms a unit which in this application is referred to as a sub-bundle, whereas a unit comprising four such sub-assemblies is referred to as a fuel assembly. In the drawings, fuel assemblies are denoted by the reference numeral 40 and sub-assemblies by 40a-d. A unit comprising four fuel assemblies 40 and a control rod 38 arranged centrally therebetween constitutes a supercell. The four bottom tie plates 11 are supported in the fuel assembly 40 by the supporting plate 4 and are each partially inserted into a respective square hole 14 therein in each sub-assembly 40a-d, at least one of the fuel rods 10 is formed with relatively long, threaded end plugs 33 and 34 of solid cladding material, the lower end plug 33 being passed through the bottom Lie plate 11 and provided with a nut 15 and the upper end plug 34 being passed through the Lop tie plate 12 and provided with a nut 16. In the embodiment shown, the centre rod 26 in each sub-assembly is formed in this way. This rod 26 also serves as a spacer holder rod. The holes for the passage of the water through the bottom tie plate 11 are designated 35. From FIGS. 1 and 2 it is clear how an upper end portion of the fuel channel 1 surrounds a cruciform lifting plate 17 with four horizontal arms 18, 19, 20 and 21 which extend from a common central portion. At its outer end each arm 18-21 has an arrowhead-like portion 22, each of which, at respective corners of the fuel channel 1, makes contact with the inner wall surface of the fuel channel 1. A lifting handle 23 is fixed to the arms 18-21. The lifting plate 17 and the handle 23 together form a lifting member of steel cast in one piece. The lifting plate 17 is fixed to the support member 7 by inserting four vertical bars 28 into respective wings 8 of the support member 7 and welding them thereto. At its top each bar 28 has a vertical, bolt-like portion 29 which is passed, with a play, through a corresponding hole in the central portion of the lifting plate 17 and provided with a nut 30. As is clear from the figures, the fuel channel 1 is provided with indentations 31, intermittently arranged in the longitudinal direction, to which the support member 7 is welded. FIG. 4 shows part of an asymmetrical core lattice according to the prior art. The section comprises sixteen fuel assemblies 40. The spaces between the fuel rods 10 within each sub-assembly 40a-d are traversed by water, as is the cruciform channel 32 in the fuel assembly 40. The gaps between the fuel assemblies 40 are also traversed by water. In an asymmetrical core lattice, the control rod gaps 37a, into which the control rods 38 can be inserted, are wider than the narrow gaps 37b, into which no control rods 38 can be inserted. In a symmetrical core lattice, the control rod gaps 37a and the narrow gaps 37b have the same width. The control rods 38 have blades 38a-d which form a rectilinear cross. As is clear from FIG. 5, each fuel rod 10 includes a cladding tube 42 and a large number of circular-cylindrical pellets 43 stacked on top of each other in the axial direction of the tube 42. The pellets 43 which are located nearest each end of the fuel rods 10 may possibly consist of natural uranium whereas the rest of the pellets 43 in a conventional manner consist of uranium dioxide enriched with respect to uranium-235. The lowermost pellet rests rigidly on an end plug 44 welded to the lower end of the rod 10, and the uppermost pellet is pressed downwards by a spiral spring 45, which is tensioned against an end plug 46 welded to the upper end of the tube 42, thus obtaining a plenum 47 filled with helium. This plenum 47 without nuclear fuel material is thus not included in the active length of the fuel rod 10. Pellets 43 of natural uranium are considered as belonging to the active length of the fuel rod 10. FIG. 6 shows a particularly advantageous embodiment of the invention. Contrary to the fuel assemblies shown in FIG. 4, the fuel assemblies 40 are provided with an enlarged centre by forming the cruciform channel 32 with inner corner reductions in the fuel assembly 40 (prior art according to Swedish patent 454 822), and further each assembly 40 is provided with a reduced corner portion 41, thus displaying a pentagonal configuration. The reduced corner portion 41 consists of that corner portion 41 in the supercell which is arranged at the largest distance from the centre of the control rod 38. The reduction of the corner portion 41 is combined with removal of at least one fuel rod 10 at the reduced corner portion 41 such that the rectangular rod positioning, with rods arranged in rows perpendicular to each other, can be maintained. The corner reduction permits more non-boiling water to be introduced into the core. In hot state with a mixture of steam and water, this gives a better neutron moderation and increased reactivity. In cold state the neutrons have a considerably shorter diffusion length. This means that the corner reduction contributes to an increased neutron absorption, whereby the reactivity in cold state is reduced and the shutdown margin is increased. When applying the invention an additionally improved shutdown margin is achieved by the removal of a fuel rod 10 such that the mean distance of the fissile material to the control rod 38 in a supercell is reduced, whereby the neutron-absorbing effect of the control rod 38 is improved and thus also the shutdown margin in cold state. A still further improvement of the shutdown margin is obtained by reducing the enrichment content in the fuel rods 10 arranged nearest the reduced corner portion (or portions) 41 and increasing the enrichment content in the corresponding fuel rods 10 in that corner in the fuel assembly 40 which is opposite to the reduced corner 41 such that the mean distance of the fissile material to the control rod 38 is further reduced. In asymmetrical core lattices, the reduction of the enrichment content at the reduced corner portion 41 is an advantage also in that an equalization of the enrichment contents in the fuel assemblies 40 is obtained. FIGS. 7 and 8 illustrate in an asymmetrical core lattice other fuel assemblies 40, each having a reduced corner portion 41, suitable for use according to the present invention. The fuel assembly 40 according to FIG. 7 is provided with an internally arranged vertical channel 48, through which water is led in a vertical direction from the bottom and upwards through the assembly 40. The channel 48 has a substantially square cross section corresponding to nine removed fuel rods 10 and is displaced in relation to the centre of the assembly 40. FIG. 8 shows a fuel assembly 40 which is provided with two centrally arranged vertical water rods 49, through which water is led in a vertical direction from the bottom and upwards through the assembly 40. The water rods 49 have a diameter which is somewhat larger than the diameter of the fuel rods 10 and are designed with a substantially circular cross section. The arrangement of the two water rods 49 centrally in the assembly 40 takes place at the expense of seven fuel rods 10. The assembly 40 also includes partial-length fuel rods 10a, which are dashed in FIG. 8. FIGS. 9, 10 and 11 show in an asymmetrical core lattice fuel assemblies 40 with three reduced corner portions 41, thus displaying a heptagonal configuration. The reduced corner portion 41 are arranged facing away from the centre of the control rod 38. The fuel assembly 40 according to FIG. 9 is of the same type as those shown in FIG. 6. The fuel assembly 40 according to FIG. 10 is of the same type as those shown in FIG. 4. The embodiments according to FIGS. 6-11 are also suitable for symmetrical core lattices, particularly the embodiments in FIGS. 9-11 since the removal of three corners out of four means that the symmetrical enrichment distribution can be retained largely symmetrical. The fuel assembly 40 according to FIG. 11 is provided with two centrally arranged vertical water rods 50 through which water is led in a vertical direction from the bottom and upwards through the assembly 40. The water rods 50 have a diameter which approximately corresponds to the diameter of the fuel rods 10 and are designed with substantially circular cross section. The arrangement of the two water rods 50 centrally in the assembly 40 takes place at the expense of two fuel rods 10. FIGS. 12, 13 and 14 illustrate in a symmetrical core lattice fuel assemblies 40 with two reduced corner portions 41. The reduced corner portions consist of the two corners which are arranged at the same distance from the control rod 38. This symmetrical corner reduction (with respect to a sub-assembly) is particularly suitable for symmetrical core lattices. The shutdown margin is considerably improved by the introduction of more water into the core. Another advantage of this embodiment is that the signal from a possible detector, arranged between the supercells at a maximum distance from the centre of the control rod 38, is not affected by the corner reduction. The fuel assembly 40 according to FIG. 12 is of the same type as those shown in FIGS. 6 and 9. The fuel assembly 40 according to FIG. 13 is provided with an internally arranged vertical channel 51, through which water is led in a vertical direction from the bottom and upwards through the assembly 40. The channel 51 has a substantially circular cross section corresponding to four removed fuel rods 10 and is centrally located. FIG. 14 shows a fuel assembly 40 of the same type as in FIG. 7 but with a water channel 48 centrally located in the fuel assembly 40. FIG. 15 shows a symmetrical core lattice with fuel assemblies 40 of the same type as those shown in FIGS. 4 and 10. The fuel assemblies 40 are each provided with four reduced corner portions 41. Thus, this embodiment differs from that shown in FIG. 10 in that also the inner tube, arranged at the shortest distance from the control rod 38, is reduced. Admittedly, the reduction of the corner facing the control rod 38 prevents the mean distance of the fissile material to the control rod from being reduced but has the advantage that the fuel rod 10 which is normally arranged in this corner is removed. This rod 10 is subjected to fast and considerable power variations in connection with the insertion and the withdrawal of t he control rod, whereby it is very heavily loaded from the point of view of power. In certain cases, it may therefore be an advantage to remove this fuel rod 10 from the assembly 40. In addition, it is an advantage that the symmetrical enrichment distribution can be maintained when all the corners are reduced. FIG. 16 shows an asymmetrical core with fuel assemblies 40 of the same type as those shown in FIGS. 6, 9 and 12. The fuel assemblies 40 are provided with two reduced corner portions 41, namely, those corner portions which are arranged at, respectively, the shortest distance and the longest distance from the centre of the control rod 38. This embodiment is advantageous from the same point of view as that stated with reference to FIG. 15, that is, that the sensitive rod 10 nearest the centre of the control rod 38 is removed. FIG. 17 shows an asymmetrical core lattice with fuel assemblies 40 of the same type as those shown in FIGS. 4, 10 an d 15. The fuel assemblies 40 are provided with three reduced corner portions 41. The reduced corner portions 41 are arranged facing the centre of the control rod 38. A particular advantage of this embodiment, as well as in the cores shown in FIGS. 12, 13 and 14, is that the detector signal is not affected by the invention. In addition, the embodiment has the same advantage as the cores shown in FIGS. 15 and 16 in that the sensitive rod 10 nearest the centre of the control rod 38 is removed. According to the particularly advantageous embodiment of the invention, that is, the embodiment shown in FIG. 6, the enrichment content B of those rods 10 which are arranged nearest the reduced corner portion 41 in a fuel assembly 40, with one fuel rod removed, is determined according to the following empirical relationship: EQU B=A.multidot.F.sub.k (F.sub.s (a/b-1)+1) where B=enrichment content in a rod 10 arranged near a reduced corner portion 41 with one fuel rod removed PA1 A=enrichment content in the corresponding rod 10 in a non-reduced corner portion 41 in the fuel assembly 40 which is opposite to the reduced corner portion PA1 The factor F.sub.k describes how the ratio B/A is influenced by a reduced corner portion 41 in a lattice with symmetrical water gaps 37a, 37b PA1 The factor F.sub.s indicates a symmetry factor which describes how the ratio B/A is dependent on the ratio between the control rod gaps 37a and the narrow gaps 37b for a lattice with asymmetrical water gaps 37a, 37b PA1 a=gap width, control rod gap 37a PA1 b=gap width, narrow gap 37b. In both symmetrical and asymmetrical core lattices, the factor F.sub.k for fuel assemblies in FIG. 6 is suitably chosen in the interval 0.72.ltoreq.F.sub.k .ltoreq.0.92 and F.sub.s =0.72. In all the fuel assemblies 40 shown and in embodiments associated therewith, one or more corner portions 41 can be reduced such that an improved shutdown margin is obtained by letting more non-boiling water into the reactor core. One or more rods 10 at the reduced corner portions 41 are removed such that, at least in asymmetrical core lattices, fissile material is arranged nearest the control rod 38 such that an additionally improved shutdown margin is obtained by improved control rod effect.