Patent Number: 051006111
Section: description

In FIGS. 1-3, 1 designates a fuel channel with substantially square cross section. The fuel channel surrounds, with no significant play, an upper square portion of a bottom part 2 with a circular, downwardly-facing inlet opening 3 for cooling water and moderator water. In addition to supporting the fuel channel 1, the bottom part 2 also supports a supporting plate 4. At the lower 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. By means of a hollow support member 7 of cruciform cross section, the fuel channel 1 is divided into four vertical tubular parts 6 having 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 central channel formed by the support member is designated 32 and is connected at its lower part to an inlet tube 9 for moderator water. Each tubular part 6 contains a bundle 25 of 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, spacers 13 and a casing part 6 forms a unit which in this application is referred to as a fuel assembly, whereas the device comprising four such fuel assemblies shown in FIGS. 1-3 is referred to as a composed fuel assembly. In the composed fuel assembly the four bottom tie plates 11 are supported by the supporting plate 4 and are partially each inserted into a corresponding square hole 14 therein. In each fuel assembly at least one of the fuel rods is provided 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 and 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 formed in this way. This rod also serves as 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 arrowhead-like portion 22, which in each respective corner 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 steel lifting member 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 the upper end 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, against which the support member 7 is welded. In FIG. 4, which shows three adjacently positioned fuel assemblies for a pressurized water reactor, the vertical fuel rods are designated 42, the rectangular top tie plates 43, the rectangular bottom tie plates 44 and the spacer members by means of which the fuel rods are positioned are designated 45. Guide tube members 46 for control rod pins are fixed at their upper ends to the top tie plates 43 and at the lower ends to the bottom tie plates 44. In addition, they are fixed to the spacer members 45. The bottom tie plate according to FIGS. 6 and 7 have through-holes 50 for conducting water through the bottom tie plate. The holes have parts 50a and 50b, the centre lines of which are displaced in relation to each other. One part 50a of a through-hole on the inlet side 51 of the plate for the water is common to several parts 50b of through-holes on the outlet side 52 of the plate. One part 50a of a through-hole on the inlet side 51 of the bottom tie plate has a larger cross section than one part 50b of a through-hole on the outlet side 52 of the bottom tie plate. Parts 50a of through-holes are in the exemplified case--but need not be so--arranged in open communication with one or more edge sides 53 by way of transverse channels 54 which at the edge have orifices 55 which are arranged in open communication with the spaces between the fuel rods in the same fuel assembly for a boiling water reactor or in the same and adjacent fuel assemblies in a pressurized water reactor. The open communication is achieved by means of recesses 56 in the edge side 53 of the bottom tie plate. When the bottom tie plate according to the FIGS. 6 and 7 is used in a fuel assembly for a boiling water reactor, for example in the fuel assembly shown in FIG. 1, the surface 57 makes contact with the supporting plate 4 and the edge side 53 makes contact with the casing part 1, which means that the water which has passed through the orifices 55 flows up to the space between the fuel rods 10. The water which passes through the holes 50a and 50b also flows up to the space between the fuel rods 10. In FIG. 1--for reasons of space--the holes 50a and 50b are drawn schematically as holes of a conventional kind. The holes for the end plugs of the fuel rods in the bottom tie plate are designated 59. When the bottom tie plate according to FIGS. 6 and 7 is used in a fuel assembly for a pressurized water reactor, for example the fuel assembly shown in FIG. 4, the bottom tie plate has no holes (59) for fuel rods and the edge side 53 makes contact with an edge side in another fuel assembly of the same kind. Nor do the recesses 56 extend all the way down to the lower edge of the edge side 53 but two adjacent bottom tie plates make contact with each other along their entire horizontal extension along an edge at the bottom of each edge side. The water which has passed through the orifices 55 thereby flows via the recesses 56 up to the spaces between the fuel rods 42 above the bottom tie plate and in spaces between adjacent fuel assemblies since there are no partitions between the fuel assemblies. Parts 50a of through-holes are also arranged in the illustrated case in open communication with each other via transverse channels 58. The embodiment of the bottom tie plate according to FIGS. 8 and 9 differs from the embodiment according to FIGS. 6 and 7 in that it has several transverse channels. In addition to transverse channels 54 which connect parts 50a of through-holes with edge sides 53 and transverse channels 58 which connect parts 50a with each other, there are transverse channels 60 which connect parts 50b of through-holes with each other and with edge sides 53. The bottom tie plate according to FIGS. 8 and 9 may be used in the same way as the bottom tie plate according to FIGS. 6 and 7, i.e. as such in a fuel assembly according to FIG. 1 and in modified form without holes (59) for fuel rods and with recesses (56) which are shut off at the bottom in a pressurized water reactor. In the bottom tie plates according to FIGS. 10-12, the inlet side for the water is designated 61 and the outlet side, facing the fuel rods, for the water is designated 62. The through-holes in the bottom tie plate according to FIG. 10 have parts 70a and 70b, the centre lines of which make an angle with each other. In the embodiment according to FIG. 11 the through-holes are arranged with a pocket 71 for capturing debris. In the bottom tie plate according to FIG. 12, a part which extends from one side, for example a part 70a extending from the side 61, is arranged in open communication with two or three parts which extend from the other side of the bottom tie plate, i.e. two or three parts 70b extending from the side 62. The bottom tie plate is also provided with pockets 71. A bottom tie plate of the kind illustrated in FIG. 12 has an exceedingly large open inner volume. The bottom tie plates according to FIGS. 10-12 are used in the illustrated embodiment in a fuel assembly according to FIG. 4. When using it in a fuel assembly according to FIG. 1, it is provided with holes corresponding to the holes 59 in the bottom tie plates according to FIGS. 6-9, for end plugs for fuel rods 10. Such holes are not visible in the shown cross section. It may be suitable to combine the use of the described bottom tie plates with the use of a separate strainer means with a low flow resistance to water to ensure that objects of debris with different shapes are captured and prevented from entering sensitive parts of the fuel assembly. The use of such a separate strainer means in the form of a strainer plate 80 is shown in dashed lines in FIG. 1. The strainer plate may, for example, be fixed to the end plug 33 which is extended with an extra nut 81.