Abstract:
A fuel element for a nuclear reactor has a fuel rod bundle, at least one spacer with cells defined by at least one web section made from a first material and several guide tubes each running through a cell and axially fixed thereto made from a second material. The first and second materials have differing thermal expansion coefficients. The connection between the guide tube and the spacer is embodied as follows: first and second projections are directly or indirectly fixed to the guide tube. The first projections are disposed in a first axial position and the second projections are arranged at a second axial position and the projections each engage in an opening through the web section to give an axially-acting undercut.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is application is a continuation, under 35 U.S.C. §120, of copending international application PCT/EP2006/003729, filed Apr. 22, 2006, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2005 020 292.6, filed Apr. 30, 2005; the prior applications are herewith incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a fuel element for a pressurized water reactor. Such a fuel element comprises a fuel rod bundle, at least one spacer with cells, for example of square shape, which are delimited by at least one web portion consisting of a first material, and a plurality of guide tubes passing in each case through a cell and consisting of a second material. The spacers are fixed axially to the guide tubes. If the spacers and guide tubes consist of zirconium alloys, connection by welding is possible. If, however, one of the components in question is manufactured from Inconel or steel and the other component from a zirconium alloy, for example Zircaloy, a welded connection is disqualified. The problem is, furthermore, that said materials have different coefficients of thermal expansion and, correspondingly, different thermal expansions. When a fuel element is heated to operating temperature, stresses at welded and soldered connections may consequently arise. Positive connections tend to be no longer exactly as firm when the fuel element is heated as in the cold state. In fuel elements known from German published patent applications DE 2 259 495 A and DE 26 05 594 (cf. U.S. Pat. No. 4,120,751), the connection between guide tube and spacer is configured such that a thermally induced relative expansion does not weaken the strength of said connection. This is achieved in that first and second projections are fixed indirectly or directly to the guide tube, a first projection being arranged in a first axial position and a second projection being arranged in a second axial position, and the projections engaging in each case into an aperture piercing a web portion. 
     BRIEF SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a fuel assembly for a pressurized water reactor (PWR) which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for spacers and guide tubes consisting of materials having different thermal expansions to be connected to one another in an alternative way, specifically, in particular, such that material deformation in the region of the apertures is at least reduced. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel element for a nuclear reactor, comprising:
         a fuel rod bundle, at least one spacer formed with cells that are bounded by at least one web portion consisting of a first material, and a plurality of guide tubes each passing through a respective cell and being axially fixed to the cell and consisting of a second material having a different thermal expansion from the first material;   an assembly for connecting a respective the guide tube and the spacer, the assembly including:   first projections directly or indirectly fixed to the guide tube in a first axial position thereof and second projections directly or indirectly fixed to the guide tube in a second axial position thereof; and   the first and second projections each engaging in an aperture formed in a respective the web portion, so as to form an axially effective undercut;   wherein, on occasion of a higher degree of thermal expansion of the guide tube than the web portion, those sides of the projections facing away from one another cooperate in each case with an abutment region of the apertures and, on occasion of a higher degree of thermal expansion of the web portion than the guide tube, those sides of the projections facing one another cooperate in each case with an abutment region of the apertures; and   wherein the abutment region of the apertures is formed with at least one oblique edge running obliquely with respect to a longitudinal direction of the spacer or of the guide tube and, together with a projection, forming a push-and-wedge connection.       

     In other words, the objects of the invention are achieved by way of a connection between guide tube and spacer in which, in the event of a higher thermal expansion of the guide tubes, those sides of the projections facing away from one another and, in the event of a higher thermal expansion of a spacer or of a web portion forming its cells, those sides of the projections facing one another cooperate in each case with an abutment region of an aperture. This first ensures that, in the event of heating-induced relative movement between spacer and guide tubes, the mechanical connection between the parts does not come loose, but, on the contrary, becomes firmer, to be precise in that the projections are pressed more firmly onto the respective abutment regions of the apertures. There is therefore no need, even in the mounting state, that is to say at room temperature, to achieve the subsequent final strength required in the operating state. To be precise, a slight play present between the projection and an abutment region of the aperture after mounting disappears at the latest when the operating temperature is reached. Owing to the relative movement between spacer and guide tubes during the heating of the fuel element, a deformation of the abutment region of an aperture may occur. However, the deformation can be kept low by means of a play present at room temperature between the projections and the abutment regions. A greater material deformation in the region of an aperture is reduced or completely prevented in that an abutment region of an aperture has at least one lateral oblique edge which runs obliquely with respect to the longitudinal direction of the spacer or of a guide tube and which forms with a projection a push-and-wedge connection. With an increasing relative movement between spacer and guide tubes, this type of connection becomes firmer under increasing heating on account of the wedge action. By the amount of obliquity of the oblique edges, adaption to more or less pronounced differences in the thermal expansion behavior of the materials combined with one another can be carried out. The deformation of a web portion in the aperture margin region can also be counteracted in that a projection is configured with an oblique surface cooperating with the oblique region. As a result, the mutual bearing surface is enlarged, and, correspondingly, the forces acting on an oblique edge are distributed. The lower surface pressure achieved thereby has the effect of a correspondingly lower deformation of a web portion in the region of the oblique edge. 
     In a further preferred refinement, there is provision for the oblique surface of a projection to have additionally an obliquity in the radial direction and to engage at least partially behind the oblique edge of an aperture in the circumferential direction, the web portion being acted upon by a radially inward-acting force component during a relative movement between web portion and guide tube. Thus, during a relative movement between spacer and guide tube, a web portion is pressed against the circumferential surface of the guide tube by virtue of the force action. Moreover, a web portion is prevented from curving radially outward by overcoming the axially effective undercut between projection and aperture. 
     In principle, it is conceivable to provide the projections directly on the guide tube, and to press these radially out of the circumferential surface with the aid of a mandrel tool. In a preferred refinement, however, there is provision for the projections not to be connected directly to the guide tube, but, instead, to be carried by an upper and a lower sleeve which are pushed over a guide tube and surround the latter, fixed axially, in the region of a spacer. The fixing of the sleeve on the guide tube preferably takes place by means of welding, which presupposes that the sleeves and guide tubes consist of materials weldable to one another, for example both of stainless steel. Mounting is made easier by sleeves carrying the projections. The sleeves are first fixed on the top side and on the underside to a spacer by being plugged into a cell until the projections latch into the apertures of the web portions. A guide tube is subsequently plugged through the sleeves fixed to the spacer. Since the projections are to pass with an axially effective undercut through an aperture, the wrench size of two diametrically opposite projections is necessarily greater than the clear width of a, for example, square spacer cell. In order to make it possible to introduce a sleeve into a cell, therefore, the web portions would first have to be widened radially outward. In a preferred refinement, then, there is provision for the projections not to be arranged directly on the outer circumference of the sleeve, but, instead, on the outside of arms which extend axially away from that end face of a sleeve which faces the spacer. During mounting, the arms can be pressed radially inward, so that the sleeve can be introduced, with its arms in front, into a cell. When the projections have reached the apertures, the arms spring radially outward, the projections engaging behind the apertures. 
     In a further preferred refinement, there is provision for a spacer to have rectangular cells formed by four web portions, and for four arms oriented in each case centrally with respect to a web portion to be present on a sleeve, the web portions having a radially widened reception region extending axially away from their upper margin and a radially widened reception region extending axially away from their lower margin, into which reception region the arm assigned to it extends axially and radially and engages with its projection into an aperture. 
     In another preferred refinement, a spacer with rectangular cells formed by four web portions is likewise present. However, the sleeve is arranged such that its four arms are oriented in each case toward the cell corner regions. An upper and a lower aperture is provided in the cell corner region of each web portion, the projection of an arm engaging simultaneously into two upper and two lower apertures. The advantage of this refinement is, for example, that a web portion is more stable in the cell corner region than in its middle portion and accordingly better withstands forces exerted by a projection in the operating state, in particular without pronounced deformations or distortions. It is advantageous, furthermore, that the middle portion of a web portion remains free and this space may serve, for example, for arranging a spring element serving for the radial support of a fuel rod. 
     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 fuel element for a pressured 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 DESCRIPTIONS OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a fuel element of a pressurized water reactor in a perspective view; 
         FIG. 2  shows a detail of a first exemplary embodiment in a perspective illustration; 
         FIG. 2A  shows a modification of the exemplary embodiment of  FIG. 2 ; 
         FIG. 3  shows a partially cut away top view in the direction of the arrow III in  FIG. 2 ; 
         FIG. 4  shows a detail of a second exemplary embodiment in a perspective view; 
         FIG. 5  shows a top view in the direction of the arrow V in  FIG. 4 ; 
         FIG. 6  shows a longitudinal section along the line VI-VI in  FIG. 5 ; 
         FIG. 7  shows a cross section along the line VII-VII in  FIG. 5 ; 
         FIG. 8  shows a detail of a third exemplary embodiment in a perspective illustration; 
         FIG. 9  shows a sleeve in a perspective illustration; 
         FIG. 10  shows the sleeve of  FIG. 9  in a second perspective illustration; 
         FIG. 11  shows a side view of the sleeve of  FIG. 9 ; 
         FIG. 12  shows a cross section along the line XII-XII in  FIG. 11 ; 
         FIG. 13  shows a cross section along the line XIII-XIII in  FIG. 11 ; 
         FIG. 14  shows a cross section along the line XIV-XIV in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, a fuel element of a pressurized water reactor comprises a bundle of a multiplicity of fuel rods  1  which are held laterally by a plurality of spacers  2 . 
     The fuel element  1  comprises, furthermore, a plurality of guide tubes  3 . The guide tubes  3  and the spacers  2  consist of different materials which cannot be welded to one another and which have different coefficients of thermal expansion and thermal expansion behavior. In the exemplary embodiments according to  FIGS. 2 ,  4  and  8 , the guide tubes consist of a material with a higher thermal expansion than the material of the spacers. The guide tubes  3  consist of stainless steel or of a nickel alloy, for example Inconel®, and the spacers  2  consist of a zirconium alloy, for example Zircaloy. In the exemplary embodiment of  FIG. 2A , the circumstances are reversed. Here, the spacers have the higher thermal expansion, that is to say consist, for example, of Inconel or stainless steel. By contrast, the guide tubes  3  are manufactured from Zircaloy or the like. In all the exemplary embodiments, a spacer is composed essentially of webs  4  arranged crosswise, a cell  5  of a spacer being formed by four web portions  6 . The spacers  2  are fixed in the axial direction to the guide tubes  3 . For this purpose, in each case four first projections  7  and second projections  8  project radially in an upper axial position I and in a lower axial position II. The projections  7 ,  8  are spaced apart uniformly in the circumferential direction and are formed by a radially outward-directed embossing of the tube wall of the guide tube  3 . The first and second projections  7 ,  8  have the same rotary position with respect to one another and are, for example, of boss-shaped design. The projections  7 ,  8  are arranged within a cell and pass, so as to form an axially effective undercut, through apertures  9  piercing the web portions  6 . 
     During heating to operating temperature, the material of the guide tubes  3  expands to a greater extent than the zirconium alloy of the spacers  2 . This results, in terms of the upper axial position I, in a relative movement of the first projection  7  with respect to the web portions  6  upward or in the direction of the arrow  10  and, in terms of the lower axial position, in a relative movement downward or in the direction of the arrow  12 . The projections  7 ,  8  are in this case pressed against an abutment region, which, in the present case, is formed by oblique edges  13  of the apertures  9 . The oblique edges  13  are oriented obliquely with respect to the longitudinal mid-axis  14  of the guide tube  3 . The oblique edges  13  of axial position I in this case form an acute angle open in the direction of the arrow  12  and those of axial position II form an acute angle open in the direction of the arrow  10 . With an increasing relative movement of the guide tube  3  in the direction of the arrows  10  and  12 , the projections  7 ,  8  are pressed with their sides pointing in the direction of the arrows  10  and  12  against the oblique edges  13  of the apertures  9 . The projections  7 ,  8  cooperate with the oblique edges  13  with the effect of a push-and-wedge connection. With an increasing relative thermal expansion of the guide tubes  3 , a projection  7 ,  8  is noticeably pressed into the narrowed region of the apertures  9 . The connection between spacer and guide tube  3  thus becomes firmer under heating. 
     The exemplary embodiment of  FIG. 2A  differs from that according to  FIG. 2  in that the material of the guide tube has a lower thermal expansion than the material of the spacer. The guide tube  3  consists, for example, of Zircaloy, whereas the spacer consists of Inconel or stainless steel. During heating to operating temperature, the expansion conditions are reversed. The projections  7  of upper axial position I execute a relative movement with respect to the spacer  3  downward or in the direction of the arrow  15  and the projections  8  of lower axial position II execute a relative movement upward or in the direction of the arrow  21 . With the longitudinal mid-axis  14  of the guide tubes  3 , the oblique edges  13   a  of the upper apertures  9  form an acute angle open upward or in the direction of the arrow  21  and the side edges  13   a  of the lower apertures  9  form an acute angle open downward or in the direction of the arrow  15 . A different thermal expansion between guide tube and spacer  2  thus has the effect that the apertures  9  are pressed with their two oblique edges  13   a  against the projections  7 ,  8 . 
     In the exemplary embodiments of  FIGS. 4 and 8 , the first and second projections  7 ,  8  are connected indirectly to the guide tube  3 . In the region adjacent to a spacer  2  on the top side and on the underside, the guide tube is in each case fixed axially and surrounded, essentially free of play, by a sleeve  16 . The sleeves  16  are fixed to the guide tube  3  by welding and likewise consist, for example, of Inconel or stainless steel or of a material weldable to these materials. Arms  17  are integrally formed onto the end face  32  of the sleeves  16  which faces the spacer  2  and extend away in the axial direction. The arms  17  are cut free from the sleeve wall and at their free end carry the projections  7 ,  8  which project radially from their outsides. The insides of the arms  17  are curved correspondingly to the inner wall of the sleeve  16  and accordingly, in the mounting state, bear against the circumferential surface of the guide tube  3 . 
     In the exemplary embodiment of  FIG. 4 , the sleeves  16  are in a rotary position with respect to a cell  5  of the spacer  2  such that their arms are arranged approximately centrally with respect to a web portion  6 . The upper and the lower region of the web portions  6  which in each case faces a sleeve  16  is widened radially outward and forms a reception region  18  into which the arms  17  extend axially and radially. The width of the reception region  18  is greater than the width of the arms  17 . The web portions  6  are pierced below and above the reception region  18  by an aperture  9 . The latter has a rectangular cross-sectional shape and extends with its longitudinal direction transversely with respect to the longitudinal direction of the fuel element  1  or transversely with respect to the longitudinal mid-axis  14  of the guide tube  3 . The projections  7 ,  8  integrally formed onto the free ends of the arms  17  are wedge-shaped in longitudinal section, and they have an oblique surface  19  pointing away from the circumferential surface of the guide tubes  3  and extending tangentially with respect to this. The projections  7 ,  8  pass through the apertures  9  and bear with their oblique surface  19  against the inner edge  20  of the reception regions  18 . The reception region  18  or the inner edge  20  and the oblique surface  19  likewise cooperate with the effect of a push-and-wedge connection. In the event of thermally induced expansion of the guide tube  3 , the projections  7 ,  8  or their oblique surfaces  19  move in the direction of the arrows  10  and  12 . In this case, the oblique surfaces  19  are pushed onto the inner edges  20 , thus leading, with increasing heating, to a consolidation of the connection between guide tube  3  and spacer  2 . 
     For mounting, first, the two sleeves  16  are plugged on the top side and on the underside onto a spacer. Since the wrench size  22  ( FIG. 6 ) of two diametrically opposite projections is greater than the wrench size  23  of two diametrically opposite reception regions  18 , the arms  17  must be bent radially inward before a sleeve  16  is plugged into the spacer  2 . Another possibility for allowing the sleeves  16  to be plugged in is to select the width of the arms  17  such that these are accommodated in cell corner regions  24  of the spacer  2 . The sleeves  16 , when plugged in, are thus held in a position rotated at 45° with respect to their desired rotary position. After the sleeves  16  have approximately reached their axial desired position, the sleeve is rotated through 45°, the projections  7 ,  8  engaging into the apertures  9 . In order to make this easier, it is expedient for the apertures  9  to project into the cell corner regions  24 . In order to make it easier to plug the sleeves  16  into a spacer  2 , it is conceivable to configure the end faces  25  of the projections  7 ,  8  as run-on slopes, these cooperating with the outer edge of a web portion  6  or of a reception region  18 , with the result that an arm  17  is bent radially inward and it is possible for the sleeve  16  to be plugged into a cell  5 . 
       FIGS. 8 to 13  illustrate an exemplary embodiment in which a fixing of a spacer  2  having square cells  5  to a guide tube  3  takes place with the aid of sleeves  16 , on whose end face  32  facing the spacer  2  arms  17  project axially. Here, too, the arms  17  are cut free from the sleeve wall and at their free end carry a projection  7  and  8  on the outside. In the mounting state, the arms  17  extend into the cell corner regions  24  of the spacer  2 . An aperture  9  is arranged in each case in upper axial position I and in lower axial position II in each cell corner region  24  of a web portion  6 . A web portion  6  is thus pierced overall by four apertures  9 . The abutment region of the apertures  9  which cooperates with the projections  7 ,  8  is formed by the inner edges, designed as oblique edges  27 , of the apertures. An oblique edge  27  forms with the longitudinal mid-axis  14  of a guide tube  3  an acute angle opening upward (arrow  10 ) or downward (arrow  12 ). The projection  7 ,  8  of an arm  17  engages simultaneously into two upper and two lower apertures of a cell corner region  24 . 
     The more detailed configuration of a sleeve  16  and, in particular, of the projections  7 ,  8  may be gathered best from  FIGS. 9 to 12 . The side surfaces  28  of the arms  17  do not extend in the radial direction, but in the tangential direction, with respect to the circumferential surface of the sleeve  16 . As may be gathered particularly from  FIG. 12 , the side surfaces  28  form overall a square, the wrench size  29  of which is slightly smaller than the clear width  30  of a cell  4 . A sleeve  16  can be plugged into a spacer  2  at most until its end face  32  carrying the arms  17  sits on the webs  4  or the web portions  6 . The projections  7 ,  8  are thickenings which resemble essentially an annular bead and which project radially from the outside of the arms  17 . The outside  31  of the projections  7 ,  8  is part of a cylinder surface which coaxially surrounds the circumferential surface of the sleeves  16 . The side surfaces of the projections  7 ,  8  are designed as oblique surfaces  33  which with the longitudinal mid-axis  34  of a sleeve  16  or the longitudinal mid-axis  14  of a guide tube  3  form an acute angle opening upward (arrow  10 ) or downward (arrow  12 ). In the mounting state, the projections  7 ,  8  pass with their side regions  35  through two apertures  9  adjacent to a cell corner region  24 . In this case, the oblique surfaces  33  of the projections  7 ,  8  cooperate with the oblique edges  27  of the apertures  9  with the effect of a push-and-wedge connection. So that the arms  17  fit with the projections  7 ,  8  into the cell corner regions  24 , the projections  7 ,  8  should not project too far from the outer circumferential surface of the sleeve  16 . The radial extent of the oblique surfaces  33  is therefore relatively small, so that there is the risk that, in the event of heating-induced expansion of the guide tubes  3 , an oblique surface  33  comes out of engagement with the oblique edge  27 . In to prevent this, the oblique surfaces  33  are oriented, as seen in a radial plane, such that, during a relative movement between guide tube  3  and web portion  6  in the direction of the arrow  10  or  12 , a projection  7 ,  8  exerts a radially inward-directed force component on an oblique edge  27 . The strength of the connection between guide tube  3  and spacer  2  is thus consolidated in the heated state not only in the axial direction, but also in the radial direction. 
     Mounting takes place, for example, as already described above. The sleeves  16  are plugged on the top side and on the underside into a cell, the arms  17  being oriented toward the cell corner regions  24 . In order to facilitate the radial inwardly directed bending of the arms  17 , their end face is designed as a run-on slope  36 .