Patent Number: 053295714
Section: description

In FIGS. 1-3, 1 designates a fuel channel of substantially square cross section. The fuel channel surrounds with no mentionable play an upper, square portion of a bottom part 2 with a circular, downwardly facing inlet opening 3 for cooling water and moderator water. The bottom part 2 supports, in addition to the fuel channel 1, a supporting plate 4. At its lowest part 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. The fuel channel 1 is divided, by means of a hollow support member 7 of cruciform cross section, into four vertical tubular parts 6, hereinafter referred to as casings, with at least substantially square cross section. The support member 7 is welded to the four walls 1a, 1b, 1c and 1d of the fuel channel 1 and has four hollow wings 8. The cruciform space formed from the support member is designated 32 and is connected at its lowest part to an inlet tube 9 for moderator water. Each casing 6 comprises a bundle 25 containing twenty-five fuel rods 10. The rods are arranged in a symmetrical lattice in five rows each containing five rods. Each rod is included in two rows perpendicular to each other. Each bundle 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 a bottom-tie plate 11, a top-tie plate 12, a spacer 13 and a casing 6 forms a unit which in this application is referred to as fuel assembly and is designated 40a, whereas the device illustrated in FIGS. 1-3 comprising four such fuel assemblies is referred to as a composed fuel assembly and is designated 40. In the composed fuel assembly 40 each fuel assembly 40a is arranged with interspaces 8 with respect to adjacent fuel assemblies 40a in that the cruciform space 32 extends through the composed fuel assembly in a vertical direction. The four bottom-tie plates 11 are supported in the composed fuel assembly by the supporting plate 4 and are each partially inserted into a respective square hole 14 in this plate. In each fuel assembly at least one of the fuel rods is designed with relatively long, threaded end plugs 33 and 34 of solid cladding material, the lower end plug 33 being passed through the bottom-tie plate 11 and provided with a nut 15, the upper end plug 34 being passed through the top-tie plate 12 and provided with a nut 16. In the embodiment shown, the centre rod 26 is designed in this way. This rod also serves as a spacer holder rod. 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 the outer end each arm has an arrow-head-like portion 22 which, in 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 20 and 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 each of four vertical bars 28 into a respective wing 8 of the support member 7 and welding them thereto, At the top each bar 28 has a vertical, bolt-like portion 29 which is passed with a play through a corresponding hole in the mid-portion of the lifting plate 17 and provided with a nut 30. As will be 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 a small part of a reactor core. The section comprises nine whole composed fuel assemblies 40 of the kind illustrated in FIGS. 1-3. Of the fuel assemblies only one is shown in detail, the other ones only as empty squares. The spaces between the fuel rods 10 within each fuel assembly 40a is traversed by water as is the cruciform space 32 in the composed fuel assembly 40. The interspaces in the form of gaps 37a and 37b between the fuel assemblies 40 are also traversed by water. Those gaps 37a into which control rods 38 can be inserted are wider than those gaps 37b into which no control rods can be inserted. The control rods 38 have blades 38a, 38b, 38c and 38d which form a right-angled cross. The fuel assembly 40a schematically shown in FIG. 5 has, in the exemplified case, six spacers 13a-f. Each fuel rod 10, of which only one is shown in the figure, contains between the two end plugs 33 and 34 (FIG. 1) a large number of pellets, stacked one above the other, of uranium dioxide enriched with U 235. At the top there is a plenum 10a filled with helium. This plenum without nuclear fuel material is thus not included in the active length of the fuel rod. On two sides the fuel assembly is provided with outlet holes 41 for releasing water. The outlet holes are located at a level above the centre of the active length of the fuel rods and below the two uppermost spacers 13a and 13b. FIG. 5 does not show the front side of the casing 6 but only the positions of the outlet holes thereon. According to the embodiment of the present invention illustrated in FIG. 6, four fuel assemblies 40a of the kind shown in FIG. 5 are arranged to form a composed fuel assembly 40 of the kind described with reference to FIGS. 1-3. The outlet holes 41 for water for release of water from the four fuel assemblies 40a to the space 32 in the composed fuel assembly 40 are arranged on those sides of the casing 6 facing the space 32. Outlet holes could also be arranged in those sides of the casing 6 facing the water gaps 37a and 37b (FIG. 4). The water gaps 37a and 37b shown in FIG. 4 are in FIG. 6, as well as in FIGS. 7 and 8, marked by dashed lines. In the exemplified case, 40% of the total number of fuel assemblies in the core are arranged with outlet holes for water of the kind described. The fuel assembly 42 illustrated in FIG. 7 corresponds to the composed fuel assembly 40 shown in FIGS. 3 and 6 and each one of the composed fuel assemblies 40 in the reactor core according to FIG. 4 can be replaced by such a fuel assembly 42. The fuel assembly 42 comprises 8.times.8 fuel rods and has no cruciform internal space traversed by water. According to the present invention, outlet holes 43 for water are arranged on all side of the casing 6 which are facing the water gaps 37a and 37b. As in the case illustrated in FIGS. 5 and 6, the outlet holes are arranged at a level above the centre of the active length of the fuel rods and below the two uppermost spacers. The fuel assembly 44 illustrated in FIG. 8, like the fuel assembly illustrated in FIG. 7, corresponds to the composed fuel assembly 40 shown in FIGS. 3 and 6 and can replace each one of the composed fuel assemblies 40 in the reactor core according to FIG. 4. The fuel assembly 44 is provided with an internally arranged vertical channel 45, through which water is conducted in a vertical direction from below and upwards through the assembly. The wall of the channel is designated 46. The fuel assembly 44 comprises 9.times.9 fuel rods, with 3.times.3 removed for the arrangement of the channel 45 that is 72 fuel rods in all. According to the present invention, outlet holes 47 for water are arranged in the walls of the vertical channel for releasing water from the space between the fuel rods to the channel. The outlet holes 47 are located at the same level as the outlet holes 41 and 43 in the examples illustrated in FIGS. 6 and 7. Outlet holes could also be arranged in those sides of the casing facing the water gaps 37a and 37b. In the cases exemplified in FIGS. 5, 6, 7 and 8, the total area of the outlet holes in each one of the fuel assemblies constitutes 30% of the flow area in the respective fuel assembly (40a, 42, 44).