Fuel assembly for a lightwater nuclear reactor

Abstract: Fuel assembly for a light-water nuclear reactor, comprising a plurality of vertical fuel rods (10,42), which are arranged, spaced from each other in the lateral direction, between a bottom tie plate (11, 44) and a top tie plate (12, 43), the bottom tie plate being provided with through-holes for conducting water through the bottom tie plate and into the spaces between the fuel rods. According to the invention the through-holes in the bottom tie plate have parts (50a, 50b, 70a, 70b) the centre lines of which are displaced in relation to each other in the lateral direction or make an angle with each other. (Figure 7).

Description: The invention relates to a fuel assembly for a light-water nuclear reactor according to the precharacterising part of claim 1. Such an assembly is known from document SE-B-450 177 Experience shows that, for example in connection with repair and service of a nuclear reactor , debris may enter, which debris then move with the water circulating through the reactor core. The debris may for example, consist of metal chips (borings) formed in connection with repair of, for example, a steam separator, pieces of metal wire, or other foreign particles which have entered the system from outside. The debris may give rise to abrasion damage, which has serious consequences if the damage arises on parts which are particularly easily damaged, such as fuel rods. To avoid damage of the above-mentioned kind in pressurized water reactors, it is known to provide the bottom tie plates which already exist below the fuel rod bundle with a large number of holes to enable each bottom tie plate, in addition to its normal function, to function as a debris catcher in the form of a strainer and prevent debris from reaching the fuel rod bundle with control rod guide tubes and spacers. The invention aims at a fuel assembly for a light-water nuclear reactor of the above-mentioned kind which is designed such that its debris capturing capacity is markedly improved. To achieve this aim the invention suggests a fuel assembly for a boiling water nuclear reactor according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention, through-holes in the bottom tie plate of a fuel assembly in a pressurized water or boiling water nuclear reactor are designed in a special way, which provides efficient capture of debris, especially if the debris consists of thin, elongated objects, such as pieces of wire and borings. The favourable result is obtained according to the invention by forming the through-holes in the bottom tie plate with parts, the centre lines of which are displaced in relation to each other in the lateral direction or make an angle with each other. As will be clear from the text below, it may be suitable to combine the use of a bottom tie plate according to the invention with another debris catcher to ensure that objects of debris of different shapes are taken care of. According to a preferred embodiment of the invention, in which through-holes in the bottom tie plate have parts the centre lines of which are displaced in relation to each other, a part of a through-hole on the inlet side of the bottom tie plate for the water is common to several parts of through-holes on the outlet side of the bottom tie plate for the water. A part of a through-hole on the inlet side of the bottom tie plate thereby normally has a larger cross section than a part of a through-hole on the outlet side of the bottom tie plate. To increase the water flow through the bottom tie plate, part of through-holes in the bottom tie plate may be arranged in open communication with one or more edge sides of the bottom tie plate by way of transverse channels which at each respective edge side have orifices which are arranged in open communication with the spaces between the fuel rods in the same fuel assembly in a boiling water reactor or in the same and adjacent fuel assemblies in a pressurizes water reactor. The open communication may advantageously be achieved via vertical recesses in the respective edge side on the bottom tie plate. A further increase of the water flow through the bottom tie plate may be obtained by arranging parts of the same kind of through-holes in the bottom tie plate in open communication with each other via transverse channels. According to another embodiment of the invention, in which through-holes in the bottom tie plate have parts, the centre lines of which make an angle with each other, a through-hole is arranged with a pocket in which one part of the through-hole changes into another part. To reduce the risk of clogging of the bottom tie plate, a part of the through-hole which extends from one side of the bottom tie plate may be arranged in open communication with at least two parts of through-holes which extend from the other side of the bottom tie plate. This results in redundant flow paths and a large open volume in the interior of the bottom tie plate. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a vertical section through the line I-I in Figure 2 of an embodiment of a composed fuel assembly for a boiling water reactor composed of four fuel assemblies according to the invention with a bottom tie plate on each fuel assembly, Figure 2 the same composed fuel assembly as in Figure 1 contemplated from above, Figure 3 a horizontal section along the line III-III in Figure 1, Figure 4 a side view of an embodiment of a fuel assembly for a pressurized water reactor according to the invention together with two adjacent fuel assemblies of the same find, Figure 5 a horizontal section along I-I through one fuel assembly in Figure 4, Figure 6 a view from above on a bottom tie plate for a fuel assembly according to Figure 1 and in modified form for a fuel assembly according to Figure 4 on an enlarged scale, Figure 7 the bottom tie plate in Figure 6, partly in vertical section along the line I-I in Figure 6, and partly in side view, Figure 8 a view from above on another bottom tie plate for a fuel assembly according to Figure 1 and in modified form for a fuel assembly according to Figure 4 on a enlarged scale, Figure 9 the bottom tie plate in Figure 8, partly in vertical section along the line I-I in Figure 8, and partly in side view, Figures 10, 11 and 12 vertical sections of parts of three different bottom tie plates for a fuel assembly according to Figure 4 and in modified form for a fuel assembly according to Figure 1. In Figures 1 to 3, numeral 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, which has 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 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 th 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 20. 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, 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 Figures 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 arms 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 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 wings 8 of the support member 7 are welded. In Figure 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 their lower ends to the bottom tie plates 44. In addition, they are fixed to the spacer members 45. The bottom tie plate according to Figures 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. The 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. The 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 on the edge side 53 of the bottom tie plate. When the bottom tie plate according to the Figures 6 and 7 is used in a fuel assembly shown in Figure 1, the surface 57 makes contact with the supporting plate 4 and the edge side 53 makes contact with the casing of the fuel channel 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. In Figure 1 - for reasons of limited drawing 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 Figures 6 and 7 is used in a fuel assembly for a pressurized water reactor, for example the fuel assembly shown in Figure 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 Figures 8 and 9 differs from the embodiment according to Figures 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 Figures 8 and 9 may be used in the same way as the bottom tie plate according to Figures 6 and 7, i.e. as such in a fuel assembly according to Figure 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 Figures 10-12, the inlet side for the water is designated 61 and the outlet side, facing the fuel rods, is designated 62. The through-holes in the bottom tie plate according to Figure 10 have parts 70a and 70b, the centre lines of which make an angle with each other. In the embodiment according to Figure 11 the through-holes are arranged with a pocket 71 for capturing debris. In the bottom tie plate according to Figure 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 Figure 12 has an exceedingly large open inner volume. The bottom tie plates according to Figures 10-12 are used in the illustrated embodiment in a fuel assembly according to Figure 4. When using it in a fuel assembly according to Figure 1, it is provided with holes corresponding to the holes 59 in the bottom tie plates according to Figures 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 Figure 1. The strainer plate may, for example, be fixed to the end plug 33 which is extended with an extra nut 81.

Claims: Fuel assembly for a light-water nuclear reactor, comprising a plurality of vertical fuel rods (10,42), which are arranged, spaced from each other in the lateral direction, between a bottom tie plate (11, 44) and a top tie plate (12, 43), the bottom tie plate being provided with through-holes for conducting water through the bottom tie plate and into the spaces between the fuel rods, characterized in that the through-holes in the bottom tie plate have parts (50a, 50b, 70a, 70b) the centre lines of which are displaced in relation to each other in the lateral direction or make an angle with each other. A fuel assembly according to claim 1, in which through-holes in the bottom tie plate have parts (50a, 50b) the centre lines of which are displaced in relation to each other in the lateral direction, characterized in that a part (50a) of a through-hole on the inlet side (51) of the bottom tie plate for the water is common to several parts (50b) of through-holes on the water outlet side (52) of the plate. A fuel assembly according to claim 1 or 2, in which through-holes in the bottom tie plate have parts (50a, 50b) the centre lines of which are displaced in relation to each other in the lateral direction, characterized in that a part (50a) of a through-hole on the inlet side (51) of the bottom tie plate has a larger cross section than a part (50b) of the through-hole on the outlet side (52) of the bottom tie plate. A fuel assembly according to claim 2 or 3, characterized in that parts (50a) of through-holes in the bottom tie plate are arranged in open communication with at least one edge side (53) on the bottom tie plate by way of transverse channels (54), which at the edge side have orifices (55) which are arranged in open communication with the spaces between the fuel rods (10, 42) in the same fuel assembly or in the same and adjacent fuel assemblies. A fuel assembly according to any of claims 2-4, characterized in that the orifices (55) of the transverse channels (54) at the edge side (53) of the bottom tie plate are arranged in open communication with the spaces between the fuel rods via vertical recesses (56) in the edge side of the bottom tie plate. A fuel assembly according to any of claims 2-5, characterized in that parts (50a) of the same kind of through-holes in the bottom tie plate are arranged in open communication with each other by way of transverse channels (58). A fuel assembly according to claim 1, in which through-holes in the bottom tie plate have parts (70a, 70b), the centre lines of which make an angle with each other, characterized in that a through-hole is arranged with a pocket (71) in which one part (70a) changes into another part (70b) of the through-hole. A fuel assembly according to any of claims 1 or 7, in which through-holes in the bottom tie plate have parts (70a, 70b), the centre lines of which make an angle with each other, characterized in that one part (70a) which extends from one side (61) of the bottom tie plate is arranged in open communication with at least two parts (70b) of through-holes which extend from the other side (62) of the bottom tie plate.

Inventors: NYLUND OLOV, NYLUND, OLOV
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1990
Main classification: G21C3
IPC codes: G21C3, G21C19