Spacer for a nuclear reactor fuel assembly

Abstract: Spacer for a nuclear reactor fuel assembly the spacer comprising a cell-formed lattice work for retaining parallel, elongated elements such as fuel rods (4) into a bundle with substantially rectangular cross section. The cells are formed from tubular, preferably round sleeves (1), each of which offering a passage for one of said elements in order to mutually fix said elements. According to the invention each spacer consists of two sub-spacers (5, 6) arranged in a certain spaced relationship to each other in the longitudinal direction of the bundle. One of said sub-spacers (5) in a pair fixes elements lying substantially on one side of a diagonal in the cross-section through the bundle, and the other sub-spacers (6) in the pair fixes the other elements such that elements passing through the sleeves of said first sub-spacer (5) pass, possibly with some exception, at the side of the sleeves in said second subspacer (6) and vice versa. Some elements may pass through sleeves in both sub-spacers in order to fix them in relation to each other.

Description: The present invention relates to a spacer for a nuclear reactor fuel assembly according to the precharacterising part of claim 1. Known embodiments of such spacers are normally arranged at a distance of 5-6 dm from each other along the bundle. In the arrangement referred to here, the cell-formed lattice is formed from tubular sleeves, each of which offers a passage for one of the elongated elements such as fuel rods in order to fix the elements mutually. It is important that the spacers do not provide too high resistance to the coolant flow which flows inside the fuel assembly along the bundle and through the spacers. It is previously known to try to achieve lower spacer resistance and hence improved cooling in a boiling reactor with a fuel assembly in the form of an elongated polygonal channel in which the bundle of fuel rods is arranged. This has been done by the use of sub-spacers, each of which extends only over part of the total cross section of the bundle and thus braces the fuel rods in this cross section part only. Another known solution disclosed in EP-A-0 347 674 is to provide the bundle with spacers which alternately fix only the fuel rods positioned peripherally in the bundle (peripheral spacers), or the fuel rods located centrally inside the peripheral fuel rods (central spacers). The lattice-work in the above-mentioned known spacers consists of plate bands, standing on edge, which are arranged crosswise to form square cells. In certain types of reactors, this type of lattice-work provides less favourable cooling conditions than a lattice-work composed of tubular, preferably round sleeves because the sleeves can be fitted in a better way around the normally round fuel rods. The invention aims at developing a spacer for a nuclear reactor fuel assembly of the above-mentioned kind whose cells are formed from tubular, preferably round sleeves, each of which offers a passage for one of the above-mentioned elements in order to mutually fix these elements into a bundle of substantially rectangular cross section, whereby the resistance to the coolant flow caused by the spacer shall be as small as possible. To achieve this aim the invention suggests a spacer for a nuclear reactor fuel assembly 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 obtained in which the material area in one of the sub-spacers separated from each other is reduced to near half of the material area of a spacer in which all the sleeves lie in one and 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 offered by the lattice work to the coolant flow, the cooling of the fuel assembly is improved most considerably. The diagonal division of each spacer into two sub-spacers also provides an improved mixing of the coolant flow along the bundle. According to a suitable embodiment of the invention, two associated sub-spacers are connected together by providing both sub-spacers with sleeves in the corners positioned on the above-mentioned diagonal such that elements in these corners of the bundle pass through sleeves arranged in both sub-spacers. In addition, the sub-spacers can be formed such that also an element running centrally in the bundle passes through sleeves in both sub-spacers. Each sub-spacer can be provided with a four-sided frame embracing the entire bundle. Inside this frame the sleeves are arranged in a substantially triangular formation resting against each other and against two of the sides of the frame. In fuel assemblies for nuclear reactors of BWR type (boiling water reactor), the bundle of elements is arranged in a channel for the coolant. In this type of fuel assemblies the frame can suitably be formed as a skirt hanging under the sleeves, the upper part of the skirt being fixed to the lower edge of the sleeves adjoining the frame. The upper part of the skirt is thereby perforated by a plurality of openings, into which fins are inserted. These fins are formed or bent such as to guide the coolant flowing through the fuel assembly towards the central parts of the bundle. In addition, the skirt rests against the walls of the channel and functions in such a way that coolant flowing along these walls is scraped off the walls and directed towards the centre 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 one of the sleeves forming the lattice work in the spacer, Figure 2 one of the sub-spacers belonging to a pair of sub-spacers seen from above, Figure 3 the same sub-spacer seen from the righthand side, Figure 4 the same sub-spacer seen from the front, Figures 5-7 in the same way the other sub-spacer belonging to a pair of sub-spacers, Figure 8 how the two sub-spacers in a pair can be joined to a square sleeve which is partially provided with a large number of openings. Figure 1 shows a tubular, preferably round sleeve 1 included in the spacer, the sleeve having fixed supports 2 and a spring 3 opposite thereto. A fuel rod 4 is also indicated in Figure 1 by dot-dash lines. Figure 2 shows one of the sub-spacers 5 in a pair. In Figure 2, the same designations are used as in Figure 1. As will be clear from Figure 2, the bundle comprises altogether 25 fuel rods 4. The sub-spacer 5 contains only 14 sleeves 1 arranged on substantially one side of a diagonal through the bundle. Certain of the sleeves 1 lie on the actual diagonal, and of these the corner sleeves 1a and 1b as well as the central sleeve 1c have corresponding sleeves 1a′, 1b′ and 1c′ (Figure 5) located in the same horizontal positions in the other sub-spacer 6 shown in Figure 5. Fuel rods 4 which are located in these positions in the bundle thus pass through sleeves 1 in both sub-spacers 5, 6 whereby the sub-spacers 5, 6 in one pair are fixed in relation to each other. This, in turn, means that all the fuel rods 4 in the bundle of a pair of sub-spacers 5, 6 are fixed in relation to each other. The sub-spacer 5 can be provided with a frame 7. In the current embodiment the frame 7 is formed as a square and the triangular lattice work of sleeves 1 is supported on two sides by the frame 7. The frame 7 is formed as a skirt 8, the upper part of which, as will be clear from Figures 3 and 4, is connected by means of poles 9 to the lower edge of the outer sleeves 1. The upper part of the skirt 8 is provided with a number of openings 10. In these openings 10, fins 11 are arranged. The fins are intended to guide coolant flowing through the fuel assembly towards the central part of the bundle in order thus to improve the cooling of the bundle. In Figures 5-7, which show the other sub-spacer 6, designations corresponding to those used in Figures 2-4 have been introduced. Figure 8 shows how the two sub-spacers 5, 6 in a pair can be connected together with a long square sleeve 12 as an alternative to the frame 7 described above. The square sleeve is provided with a large number of openings 13. The sub-spacers 5, 6 are then placed, for example, at a distance of half a normal spacer (ca 3 dm) from each other. To increase the turbulence further in the coolant, an associated pair of sub-spacers may be rotated, on the bundle, through 90 in relation to the following pair placed along the bundle, and so on, thus guiding the coolant flow along the bundle in a spiral. The steam which is generated in the fuel assembly is diverted more easily than the cooling water, and a thought behind the design is that the steam should take the path of least resistance through the sub-spacers, that is, through that part of the spacers which has no cells. In this way the steam, which prevents the cooling of the fuel rods, as opposed to the cooling water, should be brought as fast as possible upwards through the fuel assembly. The intention is for the cooling of the fuel rods to be considerably improved in this way.

Claims: Spacer for a nuclear reactor fuel assembly the spacer comprising a cell-formed lattice work for retaining parallel, elongated elements such as fuel rods (4) into a bundle with substantially rectangular cross section, said cells being formed from tubular, preferably round sleeves (1), each of which offering a passage for one of said elements in order to fix said elements mutually, characterized in that each spacer comprises two sub-spacers (5, 6) arranged in a certain spaced relationship to each other in the longitudinal direction of the bundle, a first one of said sub-spacers (5) in a pair fixing elements lying substantially on one side of a diagonal in the cross-section through the bundle, and a second one of said sub-spacers (6) fixing the other elements such that elements passing through the sleeves of said first sub-spacer (5) pass, possibly with some exception, at the side of the sleeves in said second subspacer (6) and vice versa. Spacer according to claim 1, characterized in that one or more elements extending through a pair of associated sub-spacers (5, 6) pass through sleeves (1a, 1b, 1a′, 1b′) in both sub-spacers (5, 6), thus fixing said sub-spacers (5, 6) in relation to each other. Spacer according to claim 2, characterized in that said elements which pass through respective sleeves in both associated sub-spacers are positioned at the corners (1a, 1b, 1a′, 1b′) of said diagonal and/or at the centre (1c, 1c') of the bundle. Spacer according to any of the preceding claims, characterized in that each sub-spacer is provided with a four-sided frame (7) embracing the entire bundle. Spacer according to claim 4, characterized in that inside said frame (7), sleeves (1) are arranged in a substantially triangular formation resting against each other and two of the sides of the frame (7). Spacer according to claim 4, intended for a fuel assembly (4) with a rectangular channel containing said bundle, characterized in that the frame (7) consists of a skirt (8) hanging under the sleeves (1), the upper part of said skirt being fixed to the lower edge of the sleeves (1) adjoining the frame (7). Spacer according to claim 6, characterized in that the upper part of the skirt (8), which upper part adjoins the sleeves (1), is perforated by a plurality of openings (10). Spacer according to claim 7, characterized in that fins (11) are arranged in the openings (10), said fins being intended to guide coolant flowing through the fuel assembly towards the central parts of the bundle. Spacer according to any of the preceding claims, characterized in that pairs of sub-spacers (5, 6) which belong together are arranged at a distance of about half a normal spacer from each other. Spacer according to any of the preceding claims, characterized in that the sub-spacers (5, 6) in a pair are joined together with a square-formed sleeve (12), the walls of which are provided with a plurality of openings (13).

Inventors: MANSSON RAGNAR, NYLUND OLOV, WIKTOR CLAS-GOERAN, MANSSON, RAGNAR, NYLUND, OLOV, WIKTOR, CLAS-GOERAN, Wiktor, Clas-Göran
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1992
Main classification: G21C3
IPC codes: G21C3