Spacer for retaining fuel rods in a nuclear reactor fuel assembly

Abstract: Spacer with a plurality of cells for retaining a plurality of parallel elongated fuel rods (5), extending through the cells, into a fuel rods bundle in a nuclear reactor fuel assembly. The cells are formed from tubular sleeves (1) internally provided with fixed supports (2) and at least one spring means (3) arranged opposite to said fixed supports (2). The spring means (3) are adapted to be pushed into tensioned state through contact with a fuel rod (5) inserted into a sleeve (1) and are formed as a leaf spring (6) which is cut out of the wall of the sleeve (1) and which, when the sleeve (1) is empty, extends into the inner space of the sleeve (1). According to the invention said leaf spring (6) in tensioned position, when the fuel rod (5) is inserted into the sleeve (1), rests in a position substantially coinciding with the wall of the sleeve (1). The leaf spring (6) extends transversally in relation to the longitudinal axis of the sleeve (1) thus allowing the use of cells of relatively short longitudinal extension.

Description: The invention relates to a spacer for retaining fuel rods in a nuclear reactor fuel assembly according to the precharacterising part of claim 1. It is important that these spacers, which are placed spaced from each other along the bundle, do not form too high a resistance to the coolant flow which flows inside the fuel assembly along the bundle and through the spacers. There are known spacers in which the cells of the spacer are formed from tubular sleeves which are composed into a socalled spacer grid for positioning the fuel rods extending through the sleeves. Each sleeve has been internally provided with fixed supports and spring means for pressing the fuel rod extending through the sleeve against said supports and thus fixing the position of the fuel rod. The spring means is formed as a leaf spring cut out of the wall of the sleeve. Also, when a fuel rod is in position in the tubular sleeve, the leaf spring belonging to this sleeve still extends for the most part into the sleeve. A considerable part of the free space between the fuel rod and the sleeve is then occupied by the leaf spring. This increases the unwanted flow resistance through the spacer. The FR-A-2 457 544 describes a spacer with square shaped cells formed by crosswise arranged straight strips of sheet. Cut out from the cell walls are leaf springs which emerge with their base from the cell wall and are slightly bent towards the centre of the cell. When a fuel rod is inserted the planes of the springs are mainly pressed back into the plane of the cell wall except for the portions at the free ends of the leaf springs which contact the fuel rod. The leaf springs are oriented in the axial direction of the spacer. Since the planes of these contact portions are inclined relative to the flow direction of the coolant they form a relatively large blocking area in the flow direction of the coolant the size of which increases with the size of the contact surface. A similarly designed spacer is known from the EP-A-0 025 395. The DE-A-1 589 463 describes a spacer the cells of which consist of tubular sleeves. To the inner side wall of each sleeve there is connected in axially oriented leaf spring for positioning of the fuel rods. This leaf spring provides a considerably resistance to the coolant flow between the cell wall and fuel rod partly because it is entirely arranged in this small circular gap and partly because its plane and its contact portions are more or less inclined to the direction of the cooling flow. The invention aims at developing a spacer for retaining fuel rods in a nuclear reactor fuel assembly of the abovementioned kind which exhibits a reduced flow resistance. To achieve this aim the invention suggests a spacer for retaining fuel rods in 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. According to the invention, the increased flow resistance caused by the leaf spring is substantially avoided in that the leaf spring, in tensioned position when the fuel rod is inserted into the sleeve, is returned by the fuel rod to a position in which the leaf spring substantially coincides with the wall of the sleeve. In this way, the space between the fuel rod and the sleeve is almost entirely freed from the flow-preventing effect of the spring means. According to a particularly suitable embodiment, the spring means is formed from a leaf spring which is cut out of the wall of the sleeve and extends transversally in relation to the longitudinal axis of the sleeve. The respective sleeve is then suitably internally provided with at least one first pair of laterally separated fixed supports at one level in the sleeve and a second, similarly arranged, pair of laterally separated fixed supports at another level in the sleeve. The spring means is arranged opposite to the fixed supports such that it makes contact with a fuel rod, inserted into the sleeve, at a point between the levels for the fixed supports. Because the leaf spring is arranged to extend transversally in relation to the longitudinal axis of the sleeve, the leaf spring can be made sufficiently long to provide a suitably soft resilience without the need to increase the length of the sleeve, and hence the flow resistance of the spacer. To facilitate returning the leaf spring to a position coinciding with the sleeve wall, the leaf spring is provided at its end with a contact surface which is bent out towards the centre of the sleeve. An additional way to improve the resilience of the leaf spring is to design it to be wider at its base than at its free end. Further, the leaf spring should be cut out with as narrow slots in the sleeve as possible to prevent lateral flow of the cooling water in the spacer. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a top view on a sleeve in a spacer according to the invention seen in the direction of the arrows A in Figure 2, Figure 2 a section of the sleeve taken along B-B in Figure 1, Figure 3, schematically, sleeves composed into a spacer inside an outer frame. In Figure 1, numeral 1 designates a tubular sleeve which is internally provided with fixed supports 2 and a spring means 3 with a bent-out contact surface 4 which makes contact with an outlined fuel rod 5. Figure 2 shows how the spring means 3, which consists of a leaf spring 6 with a bent-out contact surface 4, has been cut out of the wall of the sleeve 1 by means of narrow slots 7. The leaf spring 6 with the contact surface 4 is bent such that it extends into the sleeve 1 when the sleeve 1 is empty. In Figures 1 and 2 it has been assumed that a fuel rod is in position in the sleeve 1. This causes the leaf spring 6 to be bent back to a position which, according to the invention, coincides with the sleeve wall. The figures also show that the total number of the fixed supports 2 is four, two of which being arranged at an upper level and two at a lower level. The fuel rod 5 is pressed against these four fixed supports 2 by the oppositely positioned spring means 3 which abuts in tensioned state against the fuel rod 5 at a point midway between the fixed supports 2. From Figure 2 it is further clear that the leaf spring 6 cut out of the sleeve is considerably wider at its base, that is, where the leaf spring 6 is joined to the sleeve 1, than at its free end, thus obtaining improved resilience. Figure 3, finally, schematically shows how the sleeves 1 have been composed to form a spacer 8 with several cells arranged inside an outer frame 9.

Claims: Spacer (8) with a plurality of cells for retaining a plurality of parallel elongated fuel rods (5), extending through the cells, into a fuel rod bundle in a nuclear reactor fuel assembly, said cells being formed from tubular sleeves (1) internally provided with fixed supports (2) and at least one spring means (3) arranged opposite to said fixed supports (2), said spring means (3) being adapted to be pushed into tensioned state through contact with a fuel rod (5) inserted into a sleeve (1) and being formed as a leaf spring (6) which is cut out of the wall of the sleeve (1) and which, when the sleeve (1) is empty, extends into the inner space of the sleeve (1), characterized in that said leaf spring (6) extends transversally in relation to the longitudinal axis of the sleeve (1) and that the leaf spring (6) in tensioned position, when the fuel rod (5) is inserted into the sleeve (1), is resting in a position substantially coinciding with the wall of the sleeve (1). Spacer according to claim 1 with the respective sleeve (1) internally provided with at least one first pair of laterally separated fixed supports (2) at one level in the sleeve (1) and a second, similarly arranged, pair of laterally separated fixed supports (2) at another level in the sleeve (1), characterized in that said spring means (3) is arranged opposite to said fixed supports (2) to make contact with a fuel rod (5), inserted into the sleeve (1), at a point between the levels for the fixed supports (2). Spacer according to claim 1 or 2, characterized in that the leaf spring (6) is provided at its free end with a contact surface (4) bent out towards the centre of the sleeve (1). Spacer according to any of the preceding claims, characterized in that the leaf spring (6) is made wider at its base than at its free end. Spacer according to any of the preceding claims, characterized in that the leaf spring (6) has been cut out of the sleeve (1) by means of very narrow slots (7).

Inventors: FARKAS TIBOR, MANSSON RAGNAR, NYLUND OLOV, FARKAS, TIBOR, MANSSON,RAGNAR, NYLUND, OLOV
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1992
Main classification: G21C3
IPC codes: G21C3