Spacer for retaining elongated elements such as fuel rods in a nuclear reactor fuel assembly

Abstract: A spacer comprising a cell-formed lattice-work for retaining parallel elongated elements such as fuel rods (5) into a bundle in a nuclear reactor fuel assembly, said spacers being arranged in a suitably spaced relationship to each other along the bundle, and said cells being formed from tubular sleeves (1), each of which offers a passage for one of said elements in order to fix the elements in relation to each other. According to the invention substantially half the number of sleeves (1) in a spacer, distributed substantially evenly over the cross section of the bundle, are located in a first plane (11) across the bundle and joined to each other, thereby forming a lattice of sleeves (1) and gaps (13). The remaining sleeves (1) of the spacer are joined together and located in a second plane (12), separate from said first plane, across the bundle, also forming a lattice of sleeves (1) and gaps (13). The sleeves (1) in the different planes are arranged in such a way that a fuel rod (5) fixed by a sleeve (1) in said first plane traverses a gap (13) arranged in said second plane and vice versa.

Description: The invention relates to a spacer for retaining elongated elements such as fuel rods in a nuclear reactor fuel assembly according to the precharacterising part of claim 1. It is important that the above-mentioned spacers, which are usually arranged at a distance of 5 to 6 dm from each other, do not form too high a resistance to the coolant flow which flows inside the fuel assembly along the bundle and through the spacers. Previous attempts to provide improved cooling in a boiling-type nuclear reactor (BWR) are known, wherein a number of sub-spacers are arranged and each sub-spacer takes up only a small part of the whole cross section of the bundle and thus braces the fuel rods only in this part of the cross section. This may lead to problems in bracing the sub-bundles against each other. Another known solution comprises providing the bundle with spacers which alternately fix only the fuel rods positioned peripherally in the bundle (peripheral spacers) and the centrally located fuel rods positioned inside the former fuel rods (central spacers), respectively. Also in this case problems may arise in fixing the sub-bundles thus obtained in relation to one another. In addition, the lattice-work in the above-mentioned known spacers consist of crossed plate bands standing on edge, which substantially form square cells. In a BWR this type of lattice provides less favourable cooling conditions than a lattice composed of sleeves in that the sleeves are given a circular shape which is able to conform better to the normally round fuel rods. The invention aims at developing a spacer of the above-mentioned kind which does not suffer from the afore-mentioned deficiencies of the known sub-spacer designs while still offering only a reduced flow resistance to the cooling flow through the fuel assembly. To achieve this aim the invention suggests a spacer 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. By means of the invention a spacer is provided in which the material area in the planes separated from each other is reduced to almost half of the material area of a spacer in which all the sleeves are positioned in the same plane across the bundle. Since the material area of the lattice-work in a plane across the bundle is in direct relation to the resistance provided by the lattice-work against the coolant flow, the cooling of the fuel assembly is improved most considerably. Further, the spacers are provided with first and second side plates. In a boiling-water reactor (BWR) these side plates rest against the wall of the fuel box, which surrounds the fuel assembly, and the bundle is therefore fixed in a satisfactory manner in the lateral direction. In a pressurized-water reactor (PWR) the side plates of the spacers in one fuel assembly will rest against the side plates in an adjacent fuel assembly and this contributes in a corresponding manner to the fixing of the bundle. According to a suitable embodiment, the first and second planes of a spacer are joined together by sleeves by joining together the first and second side plates. The lower ends of the sleeves in the first plane may then be positioned at a small distance (about 2-3 mm) from or on the level on which the upper ends of the sleeves in the second plane are positioned. Alternatively, the sleeves may be joined together such that a certain small overlap arises between the adjoining ends of the sleeves in the two planes. According to another embodiment, especially adapted to a boiling reactor with an elongated fuel box against the walls of which the spacers rest, the spacer part with the first plane is arranged at approximately half a normal spacer distance from the spacer part with the second plane. The spacer parts are joined together by means of a guide sleeve surrounding the bundle and provided with windows, the guide sleeve having an external shape which provides a close fit against the walls of the fuel box. The guide sleeve provided with windows will then scrape off coolant flow flowing along the walls of the fuel box and divert it towards the centre of the fuel assembly. This, in combination with the lower flow resistance of the sub-spacers, results in a very efficient cooling of the fuel rods of the bundle. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 an example of a suitable sleeve for a spacer according to the invention viewed from I-I in Figure 2, Figure 2 a section through the sleeve in Figure 1 along line II-II in Figure 1, Figure 3 a conventional spacer composed of sleeves as shown in Figures 1 and 2 lying in the same horizontal plane, Figure 4 a section of a spacer taken along line IV-IV in Figure 5, the sleeves being located in two different planes, Figure 5 a view from V-V on the spacer of Figure 4, Figure 6 a spacer embodiment in which the spacer parts are arranged at half a normal spacer distance from each other and the spacer frames are joined together by a sleeve provided with windows, Figure 7 schematically a fuel assembly for a boiling reactor, Figure 8 a spacer with slimmed side plates, seen from above, Figures 9 and 10 the spacer of Figure 8 in two side views. In Figure 1 and 2, which show a tubular sleeve 1 which can be used in a spacer according to the invention, the sleeve 1 is internally provided with fixed supports 2 and a spring member 3 with a contact surface 4 which makes contact with a fuel rod 5 indicated by dot-dashed lines. The sleeve 1 is preferably given a straight circular cylindrical shape to be able to conform to the normally circular fuel rods 5. In Figure 2, 6 designates the actual leaf spring which is cut out from the sleeve by means of grooves 7. Figure 3 schematically shows how the sleeves 1 are composed into a spacer 8 of conventional type with all the sleeves in one and the same plane inside an outer frame 9. Figure 4 shows a spacer 10 according to the invention, in which substantially half of the sleeves 1 are located in a first plane where they are retained by first side plates 11. In a second plane, separate from the first plane, the remaining sleeves 1 are arranged inside second side plates 12. In this particular embodiment every other sleeve is located in the upper first plane and every other in the lower second plane. Since the exemplified spacer is designed for 25 fuel rods 5, this means that 12 sleeves 1 are arranged in the upper plane and 13 in the lower plane. Between the sleeves, gaps 13 are formed. The sleeves 1 in one plane may be joined together by means of plates 14 which, however, must not encroach upon the gaps 13 and prevent the passage of the fuel rods 5 through these gaps. The plates 14 may be given a length corresponding to the length of a sleeve 1 and may be twisted such that they will function as guide vanes for the coolant flow through the fuel assembly and increase the turbulence in the flow. As will be clear from Figures 4 and 5, a fuel rod 5, which passes through a sleeve 1 in the upper first plane and is braced therein by the fixed support 2 of the sleeve 1 and the spring member 3, will freely pass through a gap 13 between the sleeves 1 in the lower second plane. Conversely, a fuel rod 5 which passes through a sleeve 1 in the lower plane will pass through a gap 13 in the upper first plane. In the embodiment shown in Figure 4, the sleeves 1 in the upper plane rest with their edges against the sleeves 1 in the lower plane, and the side plates 11 and 12 have been joined together so that they actually function as a unit. Figure 6 shows a special embodiment in which the spacer parts are located at a distance corresponding to half a normal spacer distance equal to 250-300 mm from each other. In this case, the two spacer parts (11,12) have been joined together by a guide sleeve 16 provided with windows 15. Figure 7 shows a fuel assembly 17 for a boiling reactor which has been divided into four chambers by means of partitions, each chamber intended for a bundle containing 5x5 fuel rods 18. The walls of the fuel assembly 17 are designated 19. The figure shows how a spacer according to the alternative of Figure 6 is intended to be mounted. The same designations as in Figure 6 are used in Figure 7. As mentioned previously, the task of the guide sleeve 16 is to direct coolant flow flowing along the walls 19 of the fuel assembly 17 towards the centre of the fuel assembly 17. In the embodiments shown every other sleeve 1 in a so-called rod lane is located in a first plane and every other sleeve 1 in a second plane. This distribution of the sleeves 1 is, of course, not necessary. The main thing is that the sleeves 1 are distributed relatively evenly over the cross section of the bundle with substantially half the number of sleeves in one plane and the remaining sleeves 1 in the other plane. The sleeves should also be distributed such that one sleeve 1 in one plane, viewed axially, covers a gap 13 in the other plane. It is possible, however, if it is not desired to join the side plates 11, 12 to each other, to arrange instead within the sub-spacers one or more pairs of sleeves 1 with one sleeve in each sub-spacer such that the two sleeves of each pair are vertically aligned with each other in a manner that the sub-spacers are fixed in relation to each other by means of, for example, two fuel rods which pass through sleeves 1 both in the upper and the lower plane. Figure 8 shows an embodiment of the spacer according to the invention in which the side plates are slimmed down to the smallest possible size to reduce the coolant flow resistance of the spacer. In the figure, 1 designates sleeves, 13 gaps and 5 fuel rods according to the above. The slimmed side plates are designated 20 and 21. The same designations are used in Figures 9 and 10 where the welds 22 are located between the edges of the side plates 20, 21 and the sleeves 1 positioned inside these plates. The side plates 20, 21 have been reduced in size by the provision of recesses 23 and shortened portions 24. In this way, the side plates 20, 21 are given a minimal size and therefore provide reduced resistance to the coolant flow.

Claims: A spacer comprising a cell-formed lattice-work for retaining parallel elongated elements such as fuel rods (5) into a bundle in a nuclear reactor fuel assembly, said spacers (10) being arranged in a suitably spaced relationship to each other along the bundle, and said cells being formed from tubular sleeves (1), each of which offers a passage for one of said elements in order to fix the elements in relation to each other, characterized in that substantially half the number of sleeves (1) in the spacer (10), distributed substantially evenly over the cross section of the bundle, are located in a first plane across the bundle and joined to each other, thereby forming a lattice of sleeves (1) and gaps (13), that the remaining sleeves (1) of the spacer are joined together and located in a second plane, separate from said first plane, across the bundle, also forming a lattice of sleeves (1) and gaps (13), and that the sleeves (1) in the different planes are arranged in such a way that an element fixed by a sleeve (1) in said first plane traverses a gap (13) arranged in said second plane and vice versa. A spacer according to claim 1, characterized in that the sleeves in the first plane are provided with first side plates (11), that the sleeves in the second plane are provided with second side plates (12), and that said first and second side plates are joined to each other. A spacer according to claim 1 in which the bundle is arranged in an elongated fuel assembly (17), characterized in that the spacer part with the sleeves (1) of the first plane is arranged at approximately half a normal spacer distance from the spacer part with the sleeves (1) of the second plane and that the sleeves (1) in the first plane are joined to the sleeves (1) in the second plane by means of a guide sleeve (16) which is provided with windows (15), surrounds the bundle and has an external shape providing a close fit against the walls (19) of the fuel assembly (17). A spacer according to any of the preceding claims, characterized in that every other sleeve (1) is placed in the first plane. A spacer according to any of the preceding claims, characterized in that sleeves (1) adjoining each other are connected together by means of plates (14). A spacer according to claim 5, characterized in that the plates (14) have substantially the same length as the sleeves (1) and are twisted so as to function as guide vanes for the coolant flowing through the fuel assembly (17). A spacer according to any of the preceding claims, characterized in that sleeves (1) adjoining the frame of the spacer (10) are welded to the frame. A spacer according to any of the preceding claims, characterized in that the spacer (10) is provided with side plates (20, 21) which are reduced in size by means of recesses (23) and shortened parts (24).

Inventors: FARKAS TIBOR, MANSSON RAGNAR, NYLUND OLOV, OLSSON TORSTEN, SAHLIN THORBJOERN, FARKAS, TIBOR, MANSSON, RAGNAR, NYLUND, OLOV, OLSSON, TORSTEN, SAHLIN, THORBJOERN, Sahlin, Thorbjörn
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1992
Main classification: G21C3
IPC codes: G21C3