Abstract:
A spacer for a fuel assembly for a light water nuclear reactor contains a plurality of intercrossed segments, which form a grid. The segments are formed from first and second metal strips which are assembled and provided with protruding parts or corrugations in such a way that the adjacent protruding parts are embodied in such a way that a flow component perpendicular to a vertical central plane which extends between the metal strips is applied to cold water running out from the flow sub-channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP2004/001515, filed Feb. 18, 2004, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 103 09 742.2, filed Mar. 6, 2003; the prior applications are herewith incorporated by reference in their entirety. 
   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a spacer for a fuel assembly of a nuclear reactor cooled by light water, as is disclosed, for example, in published European patent application EP 0 237 064 A2 (corresponding to U.S. Pat. No. 4,726,926). 
   The known spacer is constructed from a multiplicity of crisscrossing webs that form a grid with a multiplicity of meshes. Each web is formed by two thin sheet-metal strips welded to one another. The sheet-metal strips are provided in each case with raised corrugations that extend into the interior of the grid mesh bounded in each case by the sheet-metal strip. Neighboring corrugations, respectively opposite one another, of the sheet-metal strips assembled to form a web form an approximately tubular flow sub-channel extending in a vertical direction. The flow sub-channels are inclined relative to the vertical and produce a flow component of the cooling liquid that is oriented parallel to the web and directed to a crossing point of the webs. The component produces a swirl flow around the fuel rods respectively penetrating the meshes. 
   In the case of the known spacer, these corrugations serve at the same time, moreover, as a bearing for the fuel rods penetrating the meshes. The fuel rod bearing has proved to be particularly advantageous in practice, since only slight fretting defects are observed on the fuel cans when use is made of such spacers. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a spacer that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which simultaneously exhibits improved thermohydraulic properties in conjunction with a high level of resistance to fretting. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a spacer for a fuel assembly of a nuclear reactor cooled by light water. The spacer contains a multiplicity of crisscrossing webs forming a grid. Each of the webs are formed of interconnected first and second sheet-metal strips having corrugations such that in each case neighboring corrugations form a flow sub-channel running oblique to a vertical. The neighboring corrugations impart to cooling water emerging from the flow sub-channel a flow component perpendicular to a vertical middle plane running between the sheet-metal strips. 
   Such a spacer for a fuel assembly of a nuclear reactor cooled by light water is constructed from a multiplicity of crisscrossing webs that form a grid and respectively are formed of interconnected first and second sheet-metal strips. The sheet-metal strips have corrugations in such a way that in each case neighboring corrugations form a flow sub-channel and are fashioned in such a way that they impart to the cooling water emerging from the flow sub-channel a flow component perpendicular to a vertical middle plane running between the sheet-metal strips. An improved lateral mixing of the cooling water is rendered possible thereby. 
   In particular, the cross section of the partial channel formed by a first corrugation decreases in the flow direction of the cooling water, and the cross section of the partial channel formed by a second, neighboring corrugation increases in this direction. In other words, the cross section of the partial channel formed by the first corrugation is greater at the inlet opening than at the outlet opening, and the cross section of the partial channel formed by the second corrugation is smaller at the inlet opening than at the outlet opening. This increase or decrease in the cross section can be affected continuously over the entire web height. However, forms of corrugation in which a middle region of constant cross section is present are also known. Owing to the mutually differing fashioning of the respectively neighboring corrugations, it is possible to produce a flow component directed perpendicular to the web plane by shaping the sheet-metal strips in a particularly simple way as regards production engineering. 
   In a further advantageous refinement of the invention, the first and the second sheet-metal strips in each case have first and second corrugations that are alternately disposed in the longitudinal direction of the first and second sheet-metal strips. The first and second sheet-metal strips are assembled to form the web in such a way that each flow sub-channel is formed by a first and second corrugation. Such a sheet-metal strip and the spacer formed by it are easy to fabricate. 
   In a preferred refinement of the invention, the flow sub-channel runs oblique, or at an inclination, to the vertical at least at its downstream end. 
   In a further preferred embodiment, the cooling water, respectively emerging from the flow sub-channel of a web, which are mutually neighboring and inclined to a crossing point of two webs, has mutually opposed flow components perpendicular to the middle plane such that a swirl flow is produced around the crossing point, in which case, in particular, the swirl flows around mutually neighboring crossing points are respectively directed in an opposed fashion along a web. This prevents the occurrence of an overall torque produced by the swirl flows and acting on the entire fuel element. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a spacer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, plan view of a detail from a spacer in accordance with the invention; 
       FIG. 2  is an enlarged detailed plan view of the spacer in the region of a crossing point of two webs; 
       FIG. 3  is a perspective view of a detail of a web in the region of a corrugation; 
       FIG. 4  a side view of a detail of the web; and 
       FIG. 5  is a diagrammatic, perspective view of a further refinement of a flow sub-channel according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1 and 2  thereof, there is shown a spacer that is constructed from a multiplicity of crisscrossing webs  2  that form a grid with polygonal meshes  4 , square ones in the exemplary embodiment, through which fuel rods  5  are guided. Each web  2  is assembled from a first and second sheet-metal strip  6  and  8 , respectively, that are welded to one another at their mutually touching upper and lower longitudinal edges. 
   The first and second sheet-metal strips  6  and  8 , respectively, are provided in each case with corrugations  10 ,  12  and  14 ,  16 , respectively, that extend in each case into an interior of the mesh  4  respectively bounded by the sheet-metal strips  6  and  8 . The corrugations  10 ,  12 ,  14 ,  16  serve simultaneously as bearings for the fuel rods  5  penetrating the meshes  4 . In this way, there is formed between the corrugations  10 ,  14  and  12 ,  16  of the first and second sheet-metal strips  6 ,  8  respectively forming a web  2 , a flow sub-channel  20  in which cooling water flows upward in a vertical direction through the spacer (out of the plane of the drawing). 
   It is to be seen in  FIGS. 1 and 2  that over their entire length in the web plane the flow sub-channels  20  are inclined to the vertical, that is to say inclined to a direction that runs perpendicular to the plane of the drawing. This inclination effects a deflection of the flow inclined to the vertical, but still parallel to the web plane, as before. Respectively neighboring flow sub-channels  20  of a web  2  have an opposing inclination. The four flow sub-channels  20  neighboring a crossing point P are oriented in this case such that two flow sub-channels  20  disposed in a common web  2  are inclined toward one another, while the two flow sub-channels  20  belonging to the other web  2  are inclined away from one another. 
   Each flow sub-channel  20  has a shape that is asymmetric in relation to a middle plane  24  located between the sheet-metal strips  6 ,  8  and oriented perpendicular to the plane of the drawing. The corrugations  10 ,  16  are provided for this purpose in each case with a lower convex arch  101  and  161 , respectively, such that at this location the corrugation  10  or  16  lies closer to the crossing point P. The corrugations  14  and  16  assigned respectively to the corrugations  10  and  12  therefore have upper convex arches  121  and  141 , respectively, in their upper region, and so the cross-sectional area of the flow sub-channel  20  remains approximately the same over the entire height of the web  2 . 
   Because of the convex arches  101  and  161 , respectively, at the inlet of the flow sub-channel  20 , the partial channel  110  or  116  respectively formed by the corrugations  10 ,  16  has a larger cross-sectional area than the partial channel  112  or  114  respectively formed by the corrugations  12 ,  14 . The partial channels  110 ,  116  therefore branch off a larger quantity of cooling water from the main channel formed by the mesh  4  than do the partial channels  112 ,  114 . Since the cross sections of the partial channels  110 ,  116  narrow in the flow direction, and the cross sections of the partial channels  112 ,  114  widen, the cooling water flowing in the flow sub-channel  20  is displaced to the partial channels  112  and  114  and in this way acquires a flow component perpendicular to the web or middle plane  24 . 
   In other words, the asymmetric shaping of the corrugations  10 ,  14  and  12 ,  16 , that is to say the offset configuration of the convex arches  101 ,  121 ,  141 ,  161 , additionally lends the cooling water flowing between the corrugations  10 ,  14  and  12 ,  16  a velocity component perpendicular to the middle plane  24  of the web  2 , since the cooling water experiences a deflection toward the convex arch  121  or  141 . 
   As an alternative to the configuration of the corrugations that is illustrated in  FIGS. 1 to 3  and in the case of which the convex arches form an enlargement of the corrugation only in a direction of the middle plane  24  (web plane), it is also possible to provide convex arches that extend more deeply into the interior of the mesh  4 , as is indicated by dashes, with the aid of a convex arch  200 , in the right-hand lower mesh of  FIG. 1 , and to make better use of the space present in the corners and left free by the fuel rod  5 . 
   The effect of the convex arches  121  and  141  is then that, because of the velocity component perpendicular to the longitudinal direction  24 , the cooling water flowing out of the flow sub-channel  20  is not directed straight onto the crossing point P but is directed past the latter obliquely. This produces a swirl flow around the crossing point P that leads to an improved heat transfer between the fuel rod and the fluid. Moreover, the corrugations  10 ,  12 ,  14 ,  16  are disposed in such a way that the direction of the swirl flow of respectively neighboring crossing points  22  is opposed. This prevents the torques respectively exerted by the swirl flows from adding up to produce an overall torque acting on the fuel assembly. 
   In the exemplary embodiment, the corrugations  10 ,  12 ,  14 ,  16  fundamentally have the same shape. First and second sheet-metal strips  6  and  8 , respectively, are, however, disposed rotated relative to one another about an axis perpendicular to the plane of the sheet-metal strip or middle plane  24 . 
   The shape of the flow sub-channels  20  in particular emerges clearly from the diagram of  FIG. 3 . It is clearly to be seen in  FIG. 3  that the majority of the cooling water flowing in from below and branched out of the main channels of the flow sub-channel  20  is taken up by the partial channel  110  that is formed by the corrugation  10  that has a lower convex arch  101 . Because of the cross-sectional narrowing of the partial channel  110 , the cooling water, which flows upward oblique to the vertical (z direction) in the middle plane because of the flow sub-channel  20  running obliquely over its entire length is directed into the partial channel  114  of the neighboring corrugation  14  and thereby acquires, in addition to a velocity component v x  directed toward the crossing point, a velocity component v y  perpendicular thereto. 
   As is shown in  FIG. 4 , the corrugations  10 ,  14  are respectively provided with longitudinal slots  26  in order to improve the mixing of the cooling liquid between the individual main channels, that is to say in order to increase the lateral mass flow. 
   A further increase in the lateral mass flow can also be achieved by providing windows  28  at the crossing points P. Just as in the case of the known HTP spacer, the corrugations  10 ,  12 ,  14 ,  16  can still have, in the middle of the web  2 , elongated convex arches that are on both sides of the slot  26 , are orientated into the interior of the mesh  4  and, owing to their shaping, form a linear bearing for the fuel rod such that the latter is held resiliently in the mesh overall on eight lines. 
   This is indicated in the exemplary embodiment in accordance with  FIG. 5 . Convex arches  30  are illustrated in this  FIG. 5  on both sides of the slot  26 . Moreover, in a departure from the exemplary embodiment illustrated in  FIG. 3 , the flow sub-channel  20  formed by the corrugations  14  and  10  is not inclined to the vertical z over its entire length l (=web height), but only over a part a of its length l at its downstream end and, if appropriate, over a part b of its length l upstream. In the remaining part l-a or l-a-b, the flow sub-channel  20  runs substantially parallel to the vertical z. It is possible in this way, given small mesh widths and large web heights (length l of the flow sub-channel  20 ), to produce a relatively large velocity component v x  parallel to the web plane  24 , that is to say a velocity component directed away from a crossing point P or toward a crossing point P. 
   This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 103 09 742.2, filed Mar. 6, 2003; the entire disclosure of the prior application is herewith incorporated by reference.