Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuing application, under 35 U.S.C. § 120, of copending international application No. PCT/EP03/01110, filed Feb. 5, 2003, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. 102 11 179.0, filed Mar. 14, 2002; 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 bundle of a boiling water reactor, having cells formed by inner webs disposed in crosswise fashion and by outer webs surrounding the inner webs in the form of a frame. In the case of a fuel assembly for a boiling water reactor, a number of fuel rods are combined to form a fuel bundle in a fuel assembly channel. A number of spacers serving to guide the fuel rods are disposed over the fuel rod length. During reactor operation, water flows as primary coolant in the boiling water reactor from below into the fuel assembly and flows upward through the latter. The heat dissipated by the fuel rods heats the water as a result of which the latter partially evaporates. There is thus a two phase mixture of water and steam in the upper region of the fuel assembly. At the exit from the fuel element, the steam fraction is usually at approximately 60%. 
     There are special requirements placed on the configuration of a fuel assembly because of the two-phase flow typical of the boiling water reactor. Next to the observance of a suitable neutron moderation, this relates, in particular, to ensuring that the cooling of the fuel rods is sufficient, that is to say, adequate heat dissipation from the surface thereof. It must be ensured that the surface of the fuel rods is wetted with an adequate quantity of water. Specifically, this is a precondition for the so-called “nucleate boiling” in which steam bubbles are produced locally which propagate and finally burst. Heat is transferred in this case substantially through an annular zone below the bubble. However, there is the risk of so-called “foam boiling” because of the high steam quality in the upper region of the fuel element. This is understood as the effect that a water film located on the fuel rod surface boils off over the area without forming bubbles, something which finally leads to the fact that the fuel rod is no longer wetted with water in this region. The lesser degree of cooling thereby engendered raises the temperature sharply in the fuel rod and can lead to damage in the fuel rod. 
     It is known that there flows along the inner wall of the fuel assembly channel a coolant film whose steam quality increases in the upper region of the fuel bundle. It is known to use the cooling potential of the coolant film by leading coolant radially inward into the fuel bundle in the region of a spacer. A measure with the aid of which this can be achieved is described in U.S. Pat. No. 4,999,153. Grooves, so-called flow trippers running transverse to the flow direction of the coolant are worked into the inner wall of the fuel assembly channel. These serve the purpose of turbulently mixing the coolant flow at the inner channel wall. Positioned downstream of the grooves in the flow direction are discharge elements that project from the outer side of the outer webs and, as it were, scrape the water film turbulently mixed by the grooves from the inner channel wall and redirect it via a flow opening into the interior of the fuel bundle. In the case of a spacer known from Japanese Patent JP 05323073 A, a similar effect is achieved with the aid of outer webs penetrated by inflow openings. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a fuel assembly for a boiling water reactor which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which coolant flowing at the inner channel wall can be rendered useful for fuel rod cooling in an alternative way. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water reactor. The fuel assembly contains inner webs disposed in a crosswise fashion and outer webs each having an inner side and surround the inner webs in a form of a frame. The inner webs and the outer webs together define and form a number of spacers having cells for receiving fuel rods. Each of the spacers contains at least one guide device disposed in a respective outer web. The guide device has a flow opening and a guide element. The guide element as viewed in a flow direction of a coolant, is disposed upstream of the flow opening, and projects from the inner side of the respective outer web. The guide element constricts a flow channel formed by a surface of a respective fuel rod and the inner and outer webs surrounding the respective fuel rod. Inwardly projecting clamping springs are provided and each cooperates with a respective one of the fuel rods. The guide device is disposed at a region of the respective outer web extending between a respective inwardly projecting clamping spring cooperating with the respective fuel rod and a respective inner web. 
     The object is achieved by a spacer having a guide device that has a flow opening, disposed at an outer web and a guide element which—seen in the flow direction of the coolant—is disposed upstream of the flow opening, and projects from the inner side of the outer web, and cooperates with the flow opening in the manner of a Venturi tube. In the case of a Venturi tube, a suction port connected to the surroundings opens out at a constricted site of a flow channel. The increased rate of flow at the constricted site of the flow channel results in a subatmospheric pressure that propagates outward via the suction port. A flow channel is formed by the region between the surface of a fuel rod and the inner and outer webs surrounding it, the channel is restricted by the inwardly projecting guide element. By contrast with the coolant flowing past the outer side, the coolant flowing at the inner side of the outer web encounters a cross-sectional constriction in the form of the inwardly projecting guide element, and this leads to the abovementioned increase in the speed or the kinetic energy of the coolant, and through a corresponding lowering of the pressure. The coolant flowing along the inner channel wall is therefore sucked through the flow opening, as it were. If practically every cell is assigned at least one such guide device, the cooling potential of the inner wall flow can be utilized to a marked extent. The distance to the boiling transition line is increased in this way, and the risk of film boiling is thereby reduced. In a preferred embodiment variant, the guide element is a flow vane that is integrally formed on the outer webs and forms an acute angle, opening in the flow direction, with the flat plane of the outer web. Such a flow vane can be produced in a simple way. It is preferably formed by cutouts which extend approximately in the flow direction and open out into the lower edge of the flow opening. When upper and lower are mentioned here and in what follows, they relate to the installed state of the spacer or the fuel assembly. Another refinement, which is preferred in particular from the point of view of production engineering, provides that the flow vane is formed by a deep-drawn wall region of the outer web that adjoins the lower edge of the flow opening. 
     In a further preferred embodiment, each cell adjoining an outer web is assigned at least one guide device, in order to increase the inwardly directed coolant fraction. The flow openings preferably have a greater width than the guide elements. It has emerged that it is possible by this refinement to make better use of the suction effect of the guide elements, that is to say a larger coolant quantity can be guided to the fuel bundle. 
     Clamping springs that cooperate with the fuel rods, are present both at the inner webs and at the outer webs in the case of spacers of the type under discussion. Such clamping springs are disposed in the middle of the outer web region defining the edge length of a cell. It is therefore expedient to dispose the guide devices at the region of an outer web extending from a clamping spring up to the next inner web in each case. 
     The flow openings are preferably configured the form of an elongated hole assigned obliquely to the flow direction. Disposed in this refinement, a flow component that is radially fastened to a fuel rod can be imposed on the coolant flow penetrating the flow opening. Preference is accorded to two adjacently disposed flow openings that are aligned obliquely in opposite senses with the formation of an acute angle opening against the flow direction. In particular, in this case, the two flow openings are assigned to neighboring cells. The efficiency of the guide device is increased by virtue of the fact that the upper edge of the flow opening and the wall region of the outer web, subsequent thereto in the flow direction, are cambered convexly outward. 
     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 fuel assembly for a boiling water reactor, 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, side-elevational view of a detail of a first exemplary embodiment of a spacer according to the invention; 
         FIG. 2  is a plan view taken in a direction of arrow II shown in  FIG. 1 ; 
         FIG. 3A  is a longitudinal section view taken along the line III—III shown in  FIG. 1 , clamping springs and the inner web having been omitted for reasons of clarity; 
         FIG. 3B  is a section view showing an enlarged detail of an area of  FIG. 3A  shown in dotted lines; 
         FIG. 4  is a diagrammatic, side-elevational view of a second exemplary embodiment of a spacer in a representation corresponding to  FIG. 1 ; 
         FIG. 5  is a plan view taken in a direction of arrow V shown in  FIG. 4 ; 
         FIG. 6A  is a longitudinal sectional view taken along the line VI—VI shown in  FIG. 4 , in a representation corresponding to  FIG. 3A ; 
         FIG. 6B  is a sectional view showing an enlarged detail of the area shown by dotted lines in  FIG. 6A ; 
         FIG. 7  is a side-elevational view of detail VII shown in  FIG. 4 , in an enlarged perspective illustration; 
         FIG. 8  is a side-elevational view of a third exemplary embodiment of the spacer, in a representation corresponding to  FIG. 1 ; 
         FIG. 9  is a plan view taken in a direction of arrow  1 ×shown in  FIG. 8 ; 
         FIG. 10A  is a longitudinal sectional view taken along the line X—X shown in  FIG. 8 , in a representation corresponding to  FIG. 3A ; and 
         FIG. 10B  is a sectional view showing an enlarged detail of an area shown by dotted lines in  FIG. 10A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1 and 2  thereof, there is shown a spacer  1  of a fuel assembly of a boiling water reactor that is composed of inner webs  2  which are plugged into one another in crosswise fashion, and outer webs  3  surrounding the inner webs  2  in the form of a frame. The inner and outer webs  2 ,  3  enclose cells  4  of which at least some are penetrated in the mounted state by a fuel rod  5  in each case. Projecting from an inner side  6  of the outer webs  3  are clamping springs  7  that are loaded by the fuel rods  5  approximately in a radial direction. Slots  8  are present in the outer webs  3 . Extending into the slots are the inner webs  2 , which do so with fixing sections  2   a  projecting from their lateral end edges, and are welded to the outer web  3  from the outer side thereof. Swirl vanes  9  are disposed at the outer edge of the outer web  3 , and deflector vanes  10  are disposed at the lower edge. The first serve the purpose chiefly of turbulently mixing the coolant flowing through a fuel element from bottom to top, and of guiding it to a surface of the fuel rods, while the latter serve as a threading aid when introducing a fuel element bundle into a fuel assembly channel  21 . 
     There is the risk in the region of the two-phase flow of a boiling water fuel element that parts of the surface of the fuel rods are not adequately supplied with water so that so-called “film boiling” occurs there. In this case, a cooling water film is evaporated from the surface without local formation of steam bubbles. The consequence is that the heat produced by the fuel rods is not dissipated or is dissipated unsatisfactorily. In order to counteract this effect, a number of guide devices  11  are present on the outer webs  3 . The devices  11  respectively contain a flow opening  12  and guide element  13  that is assigned to the latter and projects from the inner side of the outer web  3  and cooperates with the flow opening in the manner of a Venturi tube. The guide element  13  disposed on the inner side  6  of the outer web  3  effects a constriction of the flow cross section of a flow channel  14  formed by the surface of the fuel rod  5 , the inner web  2  and the outer web  3 . A subatmospheric pressure is produced in the region of the constriction by comparison with the coolant flowing outside the outer web  3 , specifically in the flow channel  15  enclosed by and the fuel assembly channel  21  and the outer webs  3 . Consequently, coolant is sucked out of the flow channel  15  into the flow channel  14  via the flow opening  12  (see arrow  29  in  FIG. 3B ). The cooling potential of this coolant fraction can then be used to cool the fuel rods  5 . 
     The guide elements  13  are flow vanes that are integrally formed on the inner side  6  of the outer webs  3  and enclose with the flat plane of the outer web  3  an acute angle α ( FIG. 3B ) opening in a longitudinal direction of a fuel rod or in the flow direction  16 . In the exemplary embodiments illustrated in  FIGS. 1 to 7 , the guide element  13  is a flow vane that is formed by a deep-drawn wall region  18  of the outer web  3  adjoining a lower edge  17  of the flow opening. The flow openings  12  are configured in the form of elongated holes, their lower edge  17  and their upper edge  19  extending upwards parallel to one another. Their side edges  20  extend approximately in the flow direction  16 . Furthermore, the flow openings  12  are wider than the guide elements  13  such that they project laterally over the latter with an overhang  22 . 
     Each cell  4  adjoining an outer web  3  is assigned a guide device  11 , the latter being positioned in each case in a region of the outer web  3  that extends between the clamping spring  7  and the inner web  2 . The guide devices  11 ,  11   a  of two cells  4 ,  4   a  separated from one another by an inner web  2  are positioned at the regions  23 ,  23   a  extending away on both sides from the inner web  2 . The flow openings  12  can extend, for example, in a fashion transverse to the flow direction  16 . However, they are openings aligned obliquely in the case of the exemplary embodiments illustrated in the drawing. The oblique position of two flow openings  12 ,  12   a  assigned to neighboring cells  4 ,  4   a  is in opposite senses, the openings enclosing an acute angle β ( FIG. 1 ) opening against the flow direction  16 . The oblique position imposes a swirl corresponding, for example, to the flow arrows  24  in  FIG. 2  from the coolant flow penetrating the flow openings  12 ,  12   a . The coolant entering via the flow opening  12 ,  12   a  is therefore guided around the fuel rods  5  in a circumferential direction. 
     In the exemplary embodiment illustrated in  FIGS. 4 to 7 , the upper edge  19  of the flow opening  12 ′,  12 ′ a , and the wall region  25 ,  25   a , subsequent thereto, of the outer webs  3  are pre-cambered convexly outward. The pre-cambered wall regions  25  configured in the manner of blades or scoops, project into the flow channel  15  running between the outer web  3  and fuel assembly channel  21 , and guide coolant on to the inner side  6  of the outer webs  3  or into the flow channel  14 . The refinement adds yet a further coolant fraction to the coolant fraction stemming from the Venturi effect described above. The opposite pre-cambering of the upper edge  19  and the lower edge  17  of the flow openings  12 ′,  12 ′ a  increases the passage cross section  26  ( FIGS. 6A ,  6 B) by comparison with openings whose edges run in the flat plane of the outer web  3 , and thereby facilitates the flowing in of coolant. 
     A pair of guide devices  11 ′,  11 ′a assigned to neighboring cells  4 ,  4   a , is shown in  FIG. 7 . Disposed between the wall regions  25 ,  25   a  is a wall section  27  that is situated deeper and is penetrated by a slot  8   a  running in the flow direction  16 . Adjoining the wall section  27  is a further, obliquely running wall section  28  that separates the two flow openings  12 ′,  12 ′ a  from one another. In the case of the embodiment under discussion, the lateral edge of an inner web  2  is shaped in such a way that it extends into the pre-cambering formed by the wall sections  27  and  28 . As already described, in the case of the slots  8 , the slot  8   a  is penetrated by a fixing section  2   a  of the inner web  2 . Owing to the configuration of the wall section  27  which is offset inward or deepened, the fixing section  2   a  can be fixed in the region of the slot  8   a  from the outside with the aid of welding, without the weld seam projecting beyond the wall regions  25 ,  25   a.    
     In addition to the flow openings  12 ,  12 ′ described, it is also possible for flow openings  12 ″ to be present, in the case of which only the upper edge  19  and a wall region  25 ′ subsequent thereto are pre-cambered outward, the lower edge  17  running in the flat plane of the outer web  3  that is to say no inwardly projecting guide element  13  is present (see  FIG. 4 ). In the exemplary embodiment illustrated in  FIGS. 8 to 10B , the guide device  11 ″ has a guide element  13 ′ that is likewise an obliquely inwardly projecting wall region  18 ′. However, the latter is formed in cutouts  30  that run approximately in the flow direction  16  and open into the lower edge  17  of the flow opening  12 ′.

Technology Category: 3