Fuel assembly with debris catcher for a light-water nuclear reactor

Abstract: Fuel assembly for a light-water nuclear reactor comprising a plurality of vertical fuel rods (10) which are arranged, in spaced relationship in the lateral direction between a bottom tie plate (11) and a top tie plate (12). The bottom tie plate and the top tie plate are provided with through-holes (35) for inlet and outlet of coolant for the fuel rods. Below the bottom tie plate, in the flow path of the water, a debris catcher (36) is arranged. The debris catcher comprises helical springs (40) which are arranged in a frame comprising at least two ends (37,38) in the form of substantially parallel plates, the flat sides of which are interconnected via pins (39). The pins are arranged in spaced relationship in at least two rows between the ends in such a way that the springs, in spaced relationship or adjacent to each other, can be fixed between the rows of pins in one or more layers.

Description: The invention relates to a fuel assembly for a light-water nuclear reactor according to the precharacterising part of claim 1. Experience shows that, for example in connection with repairs and service of a nuclear reactor, debris may enter and then move with the water which circulates through the reactor core. The debris may, inter alia, consist of metal shavings (borings or turnings) formed in connection with the repair of, for example, a steam separator, pieces of metal wire, or other foreign particles, which have entered the system from the outside. The debris may give rise to abrasion damage, which may have serious consequences if it occurs on parts which are particularly easily damaged, such as fuel rods. This may be the case if the debris adheres to a spacer. To avoid damage of the above-mentioned kind in pressurized water reactors, it is known to form the bottom tie plates, already existing under fuel rod bundles, with a large number of bored holes in order for each bottom tie plate, besides 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. A type of bottom tie plate which is also intended to serve as a debris catcher is described in SE-A-91 00 402-8. This bottom tie plate is provided with horizontal channels which cross vertical flow channels, and in at least some of the horizontal channels spiral springs with horizontal symmetry axis are arranged. The disadvantage of this solution is that it is difficult to avoid that the flow resistance across the bottom tie plate becomes too high. A type of separate debris catcher arranged below the bottom tie plate is described in SE-B-465 644. This debris catcher comprises several turns of a wound tape or several concentric rings with spacing elements arranged between the turns of the tape or the rings, which spacing elements maintain a mutual distance between the turns of the tape or the rings in the radial direction. The problem with hitherto known separate debris catchers is that they provide an impermissibly high flow resistance and/or are complicated and expensive to manufacture. The invention aims at developing a fuel assembly for a light-water nuclear reactor with debris catcher means which is simple in design and does not massively increase the flow resistance through the fuel assembly. To achieve this aim the invention suggests a fuel assembly for a light-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. Preferably, the debris catcher is arranged as a separate unit below the bottom tie plate and spaced from this plate in the flow path of the water through the bundle of rods. The debris-catching elements of the debris catcher consist of springs, preferably helical springs. The use of helical springs as debris-catching elements allows an area reduction which is small in cross section and hence a moderate speed variation of the coolant flow when passing through the debris filter. A frame comprising two ends and a number of pins keep the springs in position. These ends consist of substantially parallel plates, the flat sides of which are interconnected via the pins. The pins are arranged spaced from each other in at least two rows between the ends in such a way that a plurality of parallel springs in spaced relationship or adjacent to each other can be fixed between the rows of pins in one or more layers. The debris catcher is normally arranged at least substantially such that the symmetry axis of the springs is horizontal. In a boiling water reactor, the debris catcher can be arranged spaced from the bottom tie plate. In at least certain types of pressurized water reactors, it may be necessary to allow the debris catcher to make contact with the underside of the bottom tie plate or to arrange it at a very small distance from the underside. By designing the debris catcher in the manner described above, it may be given a very low flow resistance and hence constitute a minimal obstacle to the desired flow while at the same time the pins with intermediate springs are active during the debris catching. To compensate for the pressure drop which the debris catcher still causes, the flow holes in the bottom tie plate may be made somewhat larger. One advantage with the invention is that the debris catcher can be inserted into fuel assemblies without necessitating any major modifications and without significantly influencing the flow resistance. Another advantage in the mentioned case is that it is possible, in a simple manner, to supplement already existing fuel designs with debris catcher. A debris catcher according to the invention is very simple to manufacture and very robust since it can be designed with few parts. This also means that the risk of small parts becoming detached in connection with the water flowing through is very small. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1, in a vertical section along line I-I in Figure 2, an embodiment of a fuel assembly composed of four fuel rod bundles for a boiling-water reactor according to the invention with a debris catcher arranged below the bottom tie plate on each fuel rod bundle (sub-assembly), Figure 2 the fuel assembly in Figure 1 contemplated from above (from Line II-II), Figure 3 a horizontal section along line III-III in Figure 1, Figure 4 a view from above of four debris catchers according to Figure 1 arranged in the bottom part of a fuel assembly, Figure 5 a debris catcher included in the fuel assembly according to Figure 1 in a view from above, Figure 6 the debris catcher of Figure 5 in side elevation. Figures 1-4 show a fuel channel 1 of substantially square cross section. The fuel channel 1 surrounds, with no significant free space, 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 1, the bottom part 2 supports a supporting plate 4. At its lowermost 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 supporting member 7 with cruciform cross section, the fuel channel 1 is divided into four vertical tubular parts 6 with at least substantially square cross section. The supporting 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 32 formed by the supporting member 7 is connected at the bottom to an inlet tube 9 for moderator water. Each tubular part 6 comprises a bundle 25 of twenty-five fuel rods 10. The rods 10 are arranged in a symmetrical lattice in five rows each containing five rods 10. Each rod 10 is included in two rows perpendicular to each other. 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 tie plate 11, top tie plate 12, spacer 13 and fuel channel 1 forms a unit which, in this application, is referred to as a sub-assembly, whereas the device illustrated in Figures 1-4 and comprising four such sub-assemblies is referred to as a fuel assembly. The four bottom tie plates 11 are supported in the fuel assembly by the supporting plate 4 and are each partially inserted into a corresponding square hole 14 therein. In each sub-assembly at least one of the fuel rods 10 is made 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) in each sub-assembly is formed in this way. This rod also serves as a spacer holder rod. The holes for the passage of the water through the bottom tie plate 11 are designated 35. 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 emanate from a common central portion. At its outer end each arm 18-21 has an arrow-head-like portion 22, each of which makes contact with the inner wall surface of the fuel channel 1 at a respective corner 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 supporting member 7 by inserting each of four vertical bars 28 into a respective wing 8 of the supporting member 7 and welding them thereto. At the top each bar 28 has a vertical, bolt-like portion 29 which is passed, with a free space, through a corresponding hole in the central 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, arranged intermittently in the longitudinal direction, to which the supporting member 7 is welded. According to the invention the debris catcher 36 is arranged below the bottom tie plates 11 in the bottom part 2, that is, in the flow path for the water which flows to each one of the bottom tie plates 11. The debris catcher 36 is best illustrated in Figures 4-6. Four debris catchers of the type shown in Figure 5 are arranged in a fuel assembly according to Figure 4. In Figures 1, 4, 5 and 6 each debris catcher comprises two ends 37, 38 in the form of substantially parallel plates. The flat sides of the end plates 37, 38 are interconnected via pins 39. The pins 39 are arranged in spaced relationship in two rows between the end plates 37, 38 in such a way that a plurality of parallel helical springs 40, in spaced relationship to each other or placed adjacent to each other, can be fixed between the two rows of pins 39. One of the end plates 37 is provided with a peg 41 and the other end plate 38 with two pegs 42, these pegs allowing the debris catcher to be fixed to the bottom part 2. The two pegs 42 can suitably consist of pins 39 extending through end plate 38 . One of the end plates 37 can also be provided with a flange 43 which can be welded to the bottom part 2 for additional fixing of the debris catcher 36. The shape of the bottom part 2 necessitates bending of one (37) of the end plates 37,38 to accommodate the debris catcher. Alternatively, the whole debris catcher 36 can be made shorter, that is, the length of all the pins 39 and the springs 40 be shortened, but in that case the debris-catching area is reduced. The material in the end plates 37, 38 and the pins 39 consists, for example, of stainless steel or of any other material which is corrosion-resistant to the reactor water. The material in the springs 40 preferably consists of inconel. The pitch of the helical springs 40 is determined by the demands on the debris-catching capacity. The helical springs 40 are manufactured with tolerances which prevent the occurrence of a free space between the springs 40 and the pins 39, which eliminates the risk of abrasion because of vibrations. To compensate for the pressure drop which arises across the debris catcher 36, the diameter of the flow holes 35 in the bottom tie plate 11 may be increased somewhat. According to a particularly preferred embodiment of the invention, the debris catcher 36 consists of a device parallel to the bottom tie plate 11 in such a way that the symmetry axis of the helical springs 40 is horizontal. If the debris catcher 36 is arranged below the bottom tie plate 11, it has, in addition to the advantages mentioned above, the advantage of being able to be inspected and cleaned. It is obvious that a debris catcher 36 of the kind described can be used in a fuel assembly which is not, as in the exemplified case, divided into sub-assemblies with their own bottom tie plates 11 but which comprises only one single fuel rod bundle and one single bottom tie plate and is thus in analogous manner placed in the flow path of the water to the single bottom tie plate. It is also obvious that a debris catcher 36 of the kind described can be arranged below the bottom tie plate of a pressurized-water reactor. In a pressurized-water reactor, because of the constructive design of the flow path of the water to the bottom tie plate, it is normally most suitable to allow the debris catcher 36 to make contact with the underside of the bottom tie plate.

Claims: Fuel assembly for a light-water nuclear reactor comprising a plurality of vertical fuel rods (10) which are arranged, in spaced relationship, between at least one bottom tie plate (11) and at least one top tie plate (12) each being provided with through-holes (35) for inlet and outlet of coolant, and comprising a substantially horizontally arranged debris catcher (36) below the bottom tie plate in the flow path of the water, characterized in that said debris catcher comprises at least two end plates (37,38) arranged with their large sides substantially vertically and substantially parallel to each other, a plurality of pins (39) extending parallel to one another between said end plates in at least two vertically spaced rows, and a plurality of longitudinal springs extending also in parallel with one another and between said pins and between said end plates, the spacing between any two adjacent pins in both the vertical direction and the horizontal direction being less than the diameter of any spring so as to maintain each spring in its desired position. Fuel assembly according to claim 1, characterized in that the springs (40) are arranged in one or more layers. Fuel assembly according to any claim 1 or 2, characterized in that the springs (40) are of helical type. Fuel assembly according to any of the preceding claims, characterized in that at least one of the end plates (37,38) in the debris catcher (36) is provided with at least one peg (41,42) by means of which the debris catcher is fixed to the bottom part (2) of the fuel assembly. Fuel assembly according to any of the preceding claims, characterized in that the debris catcher (36) is arranged in spaced relationship to the bottom tie plate (11).

Inventors: OLSSON TOERSTEN, SOEDERLUND ANDERS, OLSSON, TOERSTEN, SOEDERLUND, ANDERS, Olsson, Törsten, Söderlund, Anders
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1993
Main classification: G21C3
IPC codes: G21C3