SPACER FOR A FUEL ASSEMBLY AND A FUEL ASSEMBLY

Abstract: The present invention relates to a spacer in a fuel assembly and a fuel assembly. The spacer (7) is intended to retain, at a plurality of levels, longitudinal elements in a nuclear reactor and comprises an orthogonal latticework, surrounded by a sleeve-formed structure (10) arranged to extend both upstream and downstream of the orthogonal latticework. The invention is characterized in that the part of the structure (10) which extends upstream of the orthogonal latticework is arranged substantially at the corner portions of the orthogonal latticework, and in that the part of the structure (10) which extends downstream of the orthogonal latticework is arranged substantially along the sides of the orthogonal latticework.

Description: Spacer for a fuel assembly and a fuel assembly- TECHNICAL FIELD The present invention relates to a spacer for a nuclear fuel assembly for a light-water nuclear reactor, more particularly a boiling water reactor, and a nuclear fuel assembly compri¬ sing spacers according to the present invention. The fuel assembly comprises a bundle of elongated fuel rods which are retained and fixed by a number of spacers arranged in spaced relationship to each other along the bundle. The spacers comprise a number of cells for mutually fixing the fuel rods. The bundle is surrounded by a fuel channel. Further, the fuel assembly normally comprises a top tie plate and a bottom tie plate, to which the ends of the fuel rods are fixed. A coolant, for example water, is adapted to flow from below and up through the normally vertically arranged fuel assembly and, during a nuclear reaction, to cool the fuel rods arranged in the assembly. BACKGROUND ART, PROBLEMS The design of the spacers comprised by the fuel assembly is very important for the performance of the fuel assembly. Above all, it is important to minimize the resistance to the flow which flows from below and up through the fuel assembly and to achieve an even cooling of all the fuel rods arranged in the fuel assembly. In those cases where the cooling is not sufficient, so-called dryout may occur in a boiling water reactor. In serious cases, dryout gives rise to penetration of the fuel rods. Thus, the spacers influence the flow of the coolant and hence the cooling of the fuel. It is known that, in a region imme- diately below the spacer, where the coolant has not yet passed through the spacer, a deterioration of the coolant film occurs on the fuel rods, whereas in a region above the spacer, where the coolant has just passed through the spacer, a reinforce¬ ment of the water film instead occurs. The reinforcement of the coolant film is due to the turbulence which arises in the coolant when it passes through a spacer. The greatest risk of dryout exists in the upper part of the fuel just below the spacers. The object of the present invention is to achieve a spacer with additionally improved properties, primarily as regards cooling of the fuel rods. A further object of the invention is to achieve a spacer which has a low pressure drop and which is robust so as to withstand forces to which it may be subjected during handling, and to minimize the risk of obliquity of the spacer. SUMMARY OF THE INVENTION, ADVANTAGES The present invention relates to a spacer and a fuel assembly comprising such spacers, with properties which provide a low pressure drop, an improved margin with respect to dryout and a greater resistance to damage due to handling. These properties are achieved by means of a spacer according to the characterizing part of claim 1. The spacer comprises an orthogonal latticework which is surrounded by a sleeve-shaped outer structure which extends outside the orthogonal structure both upstream and downstream and has one structural part which creates turbulence and one structural part which facilitates the guiding, which structu¬ ral parts are separated in the axial direction. In a particu¬ larly advantageous embodiment of the invention, the corners, comprising the guiding structure, are formed stiff to absorb the forces acting on the bundle during operation and during handling of the bundle. The turbulence-creating structural part extends downstream of the spacer and is arranged substantially at the sides of the spacer. The turbulence is created by means of vanes which direct the coolant towards the centre of the spacer. The guiding function consists of a part of the structure which extends upstream of the spacer and is arranged substantially at the corner portions of the spacer. The guiding is important when inserting a bundle of elongated elements retained by spacers into a fuel channel. The guiding is achieved by means of a structural part provided with so-called guiding studs which consist of a plate flap or the like, the plane of which is arranged parallel to the direction of flow of the coolant and one side edge of which extends slopingly from the lower edge of the structure in a direction downstream of the spacer towards the corners of the structure. Two such guiding studs are arranged perpendicular to each other in respective corners forming a claw-shaped guiding means. The advantage of the invention is that improved properties regarding cooling, and hence dryout, of a fuel assembly are achieved by means of a structure provided with deflection members according to the invention and by deflecting coolant towards the centre of the bundle while the coolant flows upwards along the wall of the fuel channel. By means of the invention, improved cooling of the elongated elements is achieved, primarily immediately after the passage of the coolant through a spacer but also at some distance up in the bundle to the next spacer level. By the axially displaced location of the sub-structures in relation to the orthogonal latticework, efficient guiding of the coolant to desired regions in the fuel rod bundle is made possible. By separating the sub-structures in the axial direc¬ tion, a spacer with essentially lower pressure drop than known spacer is also achieved because the flow-preventing transverse area in a cross section through the respective structural parts may be reduced. Another advantage of the invention is that a large damping surface between the structure and the wall of the fuel channel is obtained by the plane surfaces of the structure facing the fuel channel wall. By damping vibrations induced in the fuel rods by the coolant flow, also the risk of abrasion damage is reduced. Additional advantages will be clear from the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a boiling water fuel assembly according to the prior art. Figure 2 shows a perspective view of a spacer with a surroun¬ ding structure according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a boiling water reactor fuel assembly 1 which comprises a long tubular container, of rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends so as to form a continuous flow passage through which the coolant of the reactor flows. The fuel assembly 1 comprises a large number of equally long tubular fuel rods 3, arranged in parallel in a bundle, in which pellets 4 of a nuclear fuel are arranged. The fuel rods 3 are retained at the top by a top tie plate 5 and at the bottom by a bottom tie plate 6. The fuel rods 3 are kept spaced from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. Figure 2 shows a spacer 7 with an orthogonal latticework com¬ posed of sleeves 9 where each sleeve 9 is intended to position a fuel rod 3 (not shown) extending through the sleeve. The orthogonal latticework is surrounded by a sleeve-shaped struc- ture 10. A spacer latticework composed of sleeves and being of the same type as that shown in Figures 1 and 2 is disclosed in Swedish patent application 9303583-0. The sleeve-shaped structure 10 is arranged extending both up- stream and downstream of the axial extent of the orthogonal latticework, that is, of the sleeves 9. The structural part extending upstream of the orthogonal latticework is arranged substantially at the corner portions of the orthogonal latticework and is designed for guiding the spacer into the fuel channel 2. Guiding is achieved by means of plate flaps 11 or the like, the plane of which is arranged parallel to the direction of flow of the coolant and one edge of which extends in an inclined manner from the lower part of the spacer in a direction downstream of the orthogonal latticework towards the corners of the orthogonal latticework. Since the side surface of the guiding stud is arranged parallel to the direction of flow, only the cross-section area of the guiding stud 11 perpendicular to the direction of flow will constitute an obstacle to the passage of the coolant through the bundle and thus will not give rise to any signi¬ ficant flow resistance. Two such guiding studs 11 are arranged perpendicular to each other in the respective corners of the structure 10 forming a claw-shaped guiding means. Upstream of the orthogonal latticework, the structure extends at least over a distance corresponding to half the axial extent of the orthogonal latticework. That part of the structure 10 which extends downstream of the orthogonal latticework is arranged substantially at the sides of the latticework and is designed for creating turbulence in the coolant flowing upwards along the fuel channel wall 22 and to direct the coolant towards the elongated elements 3 which are arranged at the centre of the orthogonal latticework. Turbulence is achieved by means of flaps 12 extending from the structure 10 and bent towards the central part of the structure 10 and centred between the elongated element 3 (not shown) extending through the orthogonal latticework. The flaps 12 are shaped such that turbulence is achieved which reinfor¬ ces the water film adhering to the fuel rods 3 so that its cooling effect reaches to a downstream spacer 7, located adjacent thereto, thus preventing dryout. Downstream of the orthogonal latticework, the structure extends over at least a distance corresponding to half the axial extent of the orthogonal latticework. Figure 2 shows an advantageous embodiment where only the corner portions of the orthogonal latticework are surrounded by the structure 10 to minimize the pressure drop. Further, the structure 10 at the respective corner portions of the orthogonal latticework is provided with at least one first slit 13 for scraping off coolant flowing upwards through the fuel channel 2 and guiding this coolant towards the elongated elements. Further, a second slit 14 is arranged at the res¬ pective corners of the structure between the orthogonal latticework and the structure extending upwards for guiding coolant at the elongated elements which are arranged in the corner portions of the latticework. The structure 10 is provided with means in the form of flaps 15 extending from the upstream edge thereof and in an upstream direction. The flaps 15 are arranged for attachment to the orthogonal latticework by means of, for example, spot welding. In an advantageous embodiment, the flaps 15 are made long and narrow in the axial direction so as to have a minimum influence on the upstream and guiding edge of the structure 10. It is also advantageous to arrange the flaps 15 so as to lie in the same vertical plane as the structure 10 from which they extend. By making the flaps 15 long and narrow and/or in the same vertical plane as the structure 10, a minimum influence on the pressure drop across the spacer 7 is obtained. In an advantageous embodiment of the invention, the upstream edge surfaces 16 of the structure, both along the side surfa¬ ces of the structure 10 and along the oblique edges of the flaps 11, are bevelled. The bevel is preferably about 45° and adapted to guide the water flowing upwards along the fuel channel wall into the central part of the fuel channel wall. The bevel permits a reduced pressure drop compared with non- bevelled transverse edges. It is especially suitable to construct the orthogonal lattice¬ work and the surrounding structure 10 in zircaloy. It is also possible to construct the orthogonal latticework in inconel whereas the surrounding structure 10 is made in zircaloy. The orthogonal latticework is shown in the figure as composed of sleeves. However, the latticework can also consist of cells or of crossed strip elements standing on end.

Claims: CLAIMS 1. A spacer (7) intended to retain, at a plurality of levels, elongated elements in a nuclear reactor, comprising an ortho- gonal latticework surrounded by a sleeve-shaped structure (10) arranged to extend both upstream and downstream of the ortho¬ gonal latticework, characterized in that that part of the structure (10) which extends upstream of the orthogonal latticework is arranged substantially at the corner portions of the orthogonal latticework and that that part of the structure (10) which extends downstream of the orthogonal latticework is arranged substantially along the sides of the. orthogonal latticework. 2. A spacer (7) according to claim 1, characterized in that upstream of the orthogonal latticework, the structure (10) extends at least over a distance corresponding to half the axial extent of the orthogonal latticework. 3. A spacer (7) according to any of the preceding claims, characterized in that downstream of the orthogonal lattice¬ work, the structure (10) extends at least over a distance corresponding to half the axial extent of the orthogonal latticework. 4. A spacer (7) according to any of the preceding claims, characterized in that that part of the structure (10) which extends downstream of the orthogonal latticework is formed with flaps (12) which are bent towards the central part of the structure and centred between the longitudinal elements (3) adapted to run through cells in the orthogonal latticework. 5. A spacer (7) according to any of the preceding claims, characterized in that that part of the structure (10) which extends upstream of the orthogonal latticework is formed with guiding studs (11) comprising a plate flap or the like, the plane of which is arranged parallel to the flow direction of the coolant and one side edge of which extends in an inclined manner from the lower edge of the structure in a direction downstream of the orthogonal latticework towards the corners thereof. 6. A spacer (7) according to any of the preceding claims, characterized in that only the corner portions of the orthogonal latticework are surrounded by the structure (10) . 7. A spacer (7) according to any of the preceding claims, characterized in that the structure (10) forms a sleeve. 8. A spacer (7) according to any of the preceding claims, characterized in that in the respective corners of the structure (10) , a slit (14) is arranged between the orthogonal latticework and the structure extending upstream for guiding the coolant at the elongated elements (3) adapted to extend through cells in the corner portions of the orthogonal latticework. 9. A spacer (7) according to any of the preceding claims, characterized in that at least some of the upstream edge surfaces (16) of the structure (10) are bevelled. 10. A spacer (7) according to any of the preceding claims, characterized in that the structure (10) is made of zircaloy. 11. A spacer (7) according to any of the preceding claims, characterized in that the structure (10) is provided with members (15) which are long and narrow in the axial direction, said members extending from the upstream edge thereof and in an upstream direction for attachment to the orthogonal latticework. 12. A spacer (7) according to claim 11, characterized in that the members (15) are arranged in the same vertical plane as the structure (10) from which they extend. 13. A fuel assembly (1) for a nuclear reactor, characterized in that it comprises a spacer (7) according to any of the preceding claims.

Inventors: GUSTAFSSON TOMMY, MAJED MAHDI, WOLFRAM DIETMAR, GUSTAFSSON, TOMMY, MAJED, MAHDI, WOLFRAM, DIETMAR
Applicants: ASEA ATOM AB, GUSTAFSSON TOMMY, MAJED MAHDI, WOLFRAM DIETMAR, ABB ATOM AB, GUSTAFSSON, TOMMY, MAJED, MAHDI, WOLFRAM, DIETMAR
Application year: 1996
Main classification: G21C3
IPC codes: G21C3