Patent Number: 
Section: description

FIG. 1 shows an example of a previously known fuel assembly 10 which has already been described above. FIG. 2 shows examples of spacers 14 according to the prior art. FIG. 2a shows here an example of sleeve-formed cells 16 which are welded together. FIG. 2b shows another kind of spacer 14 where the cells 16 are shaped as relatively open elements with support points and opposite resilient members which hold the fuel rods and other parallelly extending elongated elements in position. Although the present invention here below is described primarily in connection with the second kind of cells 16, i.e. similar to those which may be seen in FIG. 2b, it should be noted that the invention in no way is limited to such a kind of cells 16. The invention may thus also be applied to the kind of cells 16 which is shown in FIG. 2a. Also other kinds of cells 16 for spacers 14 may be formed according to the invention. For example, there are spacer cells 14 which consist of completely round tubes which are welded together. FIG. 3 shows a side view of an example of a part which may be formed to a deflecting member 22. The shown part may be produced from a thin metal sheet in some now commonly used material, such as a nickel-based alloy (Inconel), stainless steel or a zirconium alloy. A deflecting member 22 may be formed in that the shown part is folded along the line 30 in such a manner that the right part in the figure is folded away from the plane of the drawing and forms an angle of about 75xc2x0-120xc2x0 with the left part in the figure. The right part thereby forms a vane 24 with a first edge 34 and a second edge 36, which edges meet in a corner portion 38. The left part forms a base portion 28. The base portion 28 may now be arranged vertically next to a flow channel 18 in a spacer 14 or in another part of the fuel assembly 10. The base portion 28 may for example be point-welded to a spacer cell 16, such that the vane 24 extends from the cell 16 into the neighbouring flow channel 18. With a suitable inclination of the vane 24 relative to a vertical plane 26 (see FIG. 5) a controlled vortex formation is formed in a cooling medium flowing in the flow channel 18, which vortex formation is relieved from the vane 24 at the upper part of the vane, primarily at the corner portion 38. FIG. 4 shows an example of one kind of spacer cell 16 for an elongated element. A number of such cells 16 are combined in a manner known by the person skilled in the art to a spacer 14 of similar kind to that shown in FIG. 2b. As has been mentioned above, a fuel assembly 10 is usually positioned such that the fuel rods 12 extend in a vertical direction. Thereby also the cells 16 in the spacer 14 are directed in a vertical direction. In connection with such a vertically positioned fuel assembly 10 it is thus clear what is ment by upwards, downwards, vertical and horizontal. These concepts will therefore be used in this description and in the following claims. It should however be pointed out that the fuel assembly 10 or the fuel rods 12 not necessarily must be positioned completely vertically. This description and the following claims are therefore not limited to such a vertically arranged fuel assembly 10. FIGS. 4 and 5 thus show a cell 16 for a spacer 14. In a usual vertically arranged fuel assembly 10, the cells 16 thus extend in a vertical direction. The upper parts in FIGS. 4 and 5 thus correspond to the upper parts when the cells 16 are positioned in a spacer 14 for a fuel assembly 10 which has en extension in the vertical direction. FIG. 6 shows such a cell 16 seen from above and FIG. 7 shows a flow channel 18 which is formed by four neighbouring cells 16. With reference to the figures, the invention will now be more closely described. A cell 16 of this kind usually has a number of support points 20. Some of these support points have a resilient function for holding the elongated elements, for example the fuel rods 12, in predetermined positions in the fuel assembly 10. The deflecting member 22 which forms part of a spacer 14 according to the invention may form a part which separate from the cell 16 or may form one integrated unit with the cell 16. In the now described embodiment, the deflecting member 22 forms an integrated unit with the cell 16. The deflecting member 22 comprises a vane 24. The vane is inclined relative to a vertical plane 26 (see FIG. 5). A suitable angle of inclination depends, inter alia, on which kind of cooling medium is used in the reactor and on the speed of flow of the cooling medium. A suitable angle of inclination is normally between 5xc2x0 and 30xc2x0. It has been found that a particularly suitable angel of inclination is between 10xc2x0 and 25xc2x0. In the here described embodiment, the deflecting member 22 also includes a base portion 28. The base portion 28 may, but need not, form a unit with the cell 16. The vane 24 may hereby, as is shown in the figures, be formed as a folded-out continuation of the base portion 28. According to such an embodiment, the vane 24 thus meets the base portion 28 along a line 30. As can be seen in the figures, the vane 24 is wider in its upper part than in its lower part. The vane 24 extends in a direction from a cell 16 into the neighbouring flow channel 18 (this can clearly be seen in FIG. 7). Suitably, the vane 24 is folded-out from the base portion 28 such that an angle 32 of about 75xc2x0-120xc2x0, for example 90xc2x0-100xc2x0, is formed therebetween. According to the shown embodiment, the vane 24 has a first edge 34 and a second edge 36. The first edge 34 and the second edge 36 meet in a corner portion 38. The second edge 36 is preferably formed to extend horizontally, but also an inclination relative to a horizontal plane is possible. In the shown embodiment, the first 34 and the second 36 edges are straight. Since the vane 24 is inclined relative to a vertical plane 26, an overpressure is formed on the lower side of the vane 24 by the flowing cooling medium (which normally flows upwards in the figures). In FIG. 5, the vane is directed out towards the viewer. The first edge 34 is thus shown here. In this figure, the lower side of the vane 24 is indicated by 40 and the upper side of the vane 24 is indicated by 42. In the flowing cooling medium, a higher pressure is thus formed on the lower side 40 of the vane 24 than on the upper side 42 of the vane 24. This has as a consequence that the cooling medium, in order to equalize the pressure difference, flows from the lower side 40 of the vane 24 around the first edge 34 to the upper side 42 of the vane 24. A vortex is thus formed at the first edge 34 of the vane 24. Since the cooling medium flows upwards, the so formed vortex or, vortices are relieved from the vane 24 at the upper part of the vane, i.e. close to the corner portion 38. If the vane extends in towards and reaches about the middle of the flow channel (see FIG. 7), then the vortex is thus relieved from the vane approximately in the middle of the flow channel. It should be noted that when here the middle of the flow channel 18 is mentioned, then this does not necessarily mean also in the middle as seen in a vertical direction, i.e. the vane 24, may also be formed to project up from the spacer 14 and to still reach into the middle of the axial extension of the flow channel 18. Since a controlled vortex formation in this manner is produced by the deflecting member in the axial centre of the flow channel 18, a controlled distribution of the cooling medium out towards the neighbouring parallelly extending elongated elements, for example the fuel rods 12, is achieved. Suitably, the vane 24 or the vanes are positioned asymmetrically in the flow channels 18. In the described embodiment, there is only one vane 24 in the flow channel 18 (see FIG. 7). Such an asymmetrical positioning of the vane 24 in the flow channel 18 has several advantages. The vortex which is formed by the vane in the flowing cooling medium is not disturbed by vortices from any other closely positioned vanes. By only using one vane 24 in the flow channel 18, the vane 24 does not cause any high pressure drop in the flowing cooling medium even if the vane 24 is comparatively large. Similarly, since the vane 24 has a relatively small angle of inclination against the vertical plane 26, the vane 24 does not cause any higher pressure drop in the flow channel 18 even if the vane 24 is relatively long. Since the vane 24 is formed as a folded-out portion directly in the metal sheet of the spacer cell 16, a mechanically very stable construction is achieved. Furthermore, the base portion 28 of the vane 24 may thereby consist of a vertical part of the cell 16 which has the advantage that unwanted vortex or turbulence formation at the base portion 28 is avoided. Thereby a controlled vortex may be formed by the vane 24, which vortex is not essentially disturbed by other turbulence. The base portion 28 of the deflecting member 22 may also be formed to be slightly bent in order to correspond to the bent surface of the elongated element which is held in place by the cell 16. The spacer according to the invention may for example be made in a zirconium alloy or in other presently common materials such as Inconel or stainless steel. With a spacer 14 with a deflecting member 22 according to the invention, several advantages are thus achieved. Since the vane 24 may be made to be relatively long with a relatively small angle between the vane 24 and a vertical plane 26, an orderly vortex formation may thus be achieved, which vortex formation has a clear net rotational movement. The relatively random turbulence which has been created by previously known constructions of deflecting members 22 may thereby be avoided. The orderly vortex formation leads to the effect that the cooling medium is moved out towards the elongated elements, and the steam which is formed in the fuel assembly next to the fuel rods 12 will thus be concentrated close to the middle of the flow channels 18. By the invention, an efficient cooling of the fuel rods 12 is thus achieved. The fuel assembly 10 and thereby the nuclear boiling water reactor thus achieve an improved dryout performance. It should be noted that the deflecting member 22 does not need to be positioned at or formed in a cell 16 which holds the elongated elements in position. The deflecting member 22 may also be positioned close to or formed in the metal sheet of the frame of the spacer 14; i.e. the metal sheet which constitutes the outer limitation of the spacer 14. As has been previously explained it is also possible to arrange the vanes in a separate structure (as so-called intermediate spacer) which does not support the rods but which has the purpose to hold all the vanes in accurate positions between the rods. The supporting function and the vortex formation function are thereby separated, which, hopefully, results in a lower total pressure drop. A deflecting member 22 according to the invention may also be positioned in another position in the fuel assembly 10 than at the spacer 14 or at a so-called intermediate spacer. In a fuel assembly 10, there are other kinds of flow channels. For example, there are types of fuel assemblies 10 which have so-called part length rods, i.e. fuel rods which do not extend along the whole fuel assembly 10 but which end at a lower level. Above such part length rods 12A (FIG. 1) a larger flow channel may thus be formed. Also in such a flow channel, deflecting members 22 according to the invention may be arranged. The principle is thereby the same as that which has been described above. That is, in order to achieve a controlled vortex formation, a long vane 24 with a relatively small inclination is used, which vane is positioned such that the vortex is relieved from the vane 24 close to the corner portion 38 of the vane 24, and, suitably, the vane 24 is arranged such that the vane 24 extends in towards the middle of the flow channel such that a controlled vortex is relieved from the vane 24 approximately in the middle of the flow channel. Such a deflecting member 22 may of course be constructed in accordance with the different embodiments which have been described above in connection with the spacer. As an example of such an application of the invention, the vane may be arranged in a spacer positioned at a level which is above the level where the part length rod ends. Above such a part length rod, a larger flow channel is formed. This flow channel, in this case thus consists of the cell in the spacer which is positioned above the part length rod and of the four neighbouring flow channels corresponding to the flow channels which are formed between the fuel rods when these reach up through the spacer. In such a larger flow channel, a vane may be arranged in the manner which has been described in the previous embodiments above. Since this flow channel is larger than the flow channels which have been described above, the vane which has been arranged in such a channel is suitably larger to a corresponding degree. The flow channel may comprise more than one vane. Suitably, the vanes are thereby asymmetrically arranged in the flow channel. Preferably, also this kind of flow channel comprises only one vane. The present invention is not limited to the described embodiments but may be varied and modified within the scope of the following claims.