Abstract:
A grid ( 13 ) for supporting nuclear fuel pencils ( 3 ) for a nuclear fuel assembly ( 1 ) comprising a peripheral belt ( 17 ), the peripheral belt ( 17 ) comprising on at least one of its edges ( 35, 37 ) guide fins ( 33 ) is disclosed. The edge ( 35, 37 ) of the peripheral belt ( 17 ) has between the adjacent guide fins ( 33 ) recesses ( 39 ) towards the inside of the grid ( 13 ). The invention is applicable, for example, to pressurized water reactors.

Description:
[0001]    The present invention relates to a grid for spacing nuclear fuel rods in a nuclear fuel assembly, the grid being of the type defining a substantially regular array of cells and including a peripheral belt, the peripheral belt having guide fins on at least one of its edges. The invention applies in particular to fuel assemblies for pressurized water nuclear reactors. 
       BACKGROUND 
       [0002]    Document FR—2 736 190 describes a spacer grid of the above-specified type. The peripheral belt is made up of plane outer plates that are assembled to one another. The guide fins are folded relative to the planes of their respective plates towards the inside of the grid. Such a grid makes it possible to reduce the lateral deformation of the assembly that includes it. Inclining the fins inwards serves in particular to limit any risk of the spacer grids of two adjacent assemblies catching while the assemblies are being handled. 
         [0003]    Nevertheless, a risk still exists of grids catching in certain configurations that are particularly penalizing. This applies in particular when two adjacent assemblies are offset by half the pitch of the array of fuel rods. During a handling operation, the free ends of the guide fins of one grid in a first one of the adjacent assemblies can then become engaged between two adjacent fins of a grid in the second of the two adjacent assemblies, thus catching the edge of the grid of the second assembly. If in addition, one of the two assemblies bears laterally against the other of the two adjacent assemblies to such an extent as to cause the peripheral rods of at least one of the two assemblies to bend and/or be displaced laterally, the risk of the grids of the two adjacent assemblies catching is increased. Such catching can lead to adjacent assemblies being deformed and ought therefore to be avoided. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to solve this problem by limiting the risk of the grids of two adjacent assemblies catching. 
         [0005]    To this end, the invention provides a grid of the above-specified type, characterized in that said edge of the peripheral belt presents setbacks towards the inside of the grid between adjacent guide fins. 
         [0006]    In particular embodiments, the grid may include one or more of the following characteristics, taken in isolation or in any technically feasible combination:
       the setbacks are formed by bosses in the peripheral belt;   through orifices are formed through the peripheral belt to allow cooling fluid to pass laterally;   the through orifices are positioned substantially in the centers of the outside faces of the peripheral cells defined by the peripheral belt;   the peripheral belt does not have springs for laterally holding nuclear fuel rods received in the peripheral cells;   the grid includes crossed inner plates, which inner plates are surrounded by the peripheral belt;   the inner plates present notches for mutually engaging the plates at their cross-points, and the notches have respective regions of reduced width;   the guide fins bear against regions of the inner plates that constitute stiffeners;   the inner plates adjacent to the peripheral belt include abutments for laterally retaining the nuclear fuel rods, which abutments project into the peripheral cells from their inside faces facing the peripheral belt;   the lateral retaining abutments are formed in the bottom edges and the top edges of the inner plates adjacent to the peripheral belt;   the peripheral belt presents cups set back towards the inside of the grid and in which the inner plates are welded to the peripheral belt; and   the bottom edges of the inner plates are substantially rectilinear in register with the faces of cells that are to receive guide tubes of the nuclear fuel assembly.       
 
         [0018]    The invention also provides a nuclear fuel assembly comprising nuclear fuel rods, a skeleton for supporting the rods, the support skeleton comprising guide tubes, nozzles disposed on the longitudinal ends of the guide tubes, and grids for spacing the rods, the assembly being characterized in that at least one grid is a grid as defined above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention can be better understood on reading the following description given purely by way of example, and made with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a diagrammatic side view of a nuclear fuel assembly for a pressurized water reactor; 
           [0021]      FIGS. 2 and 3  are fragmentary diagrammatic views in perspective showing a spacer grid suitable for forming part of the  FIG. 1  assembly, seen respectively from above and from below; 
           [0022]      FIG. 4  is a fragmentary diagrammatic view in lateral section of the grid of  FIGS. 2 and 3 ; 
           [0023]      FIG. 5  is a fragmentary diagrammatic view of the grid of  FIGS. 2 and 3  seen from above; 
           [0024]      FIGS. 6 and 7  are diagrammatic side views showing two internal plates used for making the grid of  FIGS. 2 and 3 ; 
           [0025]      FIG. 8  is a diagrammatic view on a larger scale the portion in circle VIII of  FIG. 6 ; 
           [0026]      FIG. 9  is a diagrammatic face view showing the relative position of two adjacent assembly grids during a handling operation; and 
           [0027]      FIG. 10  is a fragmentary diagrammatic side view of the two assemblies of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In order to illustrate the context of the invention,  FIG. 1  is a diagram showing a nuclear fuel assembly  1  for a pressurized water reactor. Water thus performs therein both a cooling function and a moderation function, i.e. it slows down neutrons produced by the nuclear fuel. 
         [0029]    The assembly  1  extends vertically in rectilinear manner along a longitudinal direction A. This direction is the flow direction of the cooling fluid, i.e. water, when the assembly  1  is placed in a core. More precisely, the water flows upwards therethrough. 
         [0030]    In conventional manner, the assembly  1  mainly comprises nuclear fuel rods  3  and a structure or skeleton  5  for supporting the rods  3 . 
         [0031]    The support skeleton conventionally comprises:
       a bottom nozzle  7  and a top nozzle  9  disposed at the longitudinal ends of the assembly  1 ;   guide tubes  11  for receiving the rods of a control and stop cluster (not shown) for the nuclear reactor; and   grids  13  for spacing the rods  3  apart.       
 
         [0035]    The nozzles  7  and  9  are secured to the longitudinal ends of the guide tubes  11 . 
         [0036]    The rods  3  extend vertically between the nozzles  7  and  9 . The rods  3  are disposed at the nodes of a substantially regular square-based array where they are spaced apart laterally by the grids  13 . Some of the nodes of the array are occupied by the guide tubes  11 , and possibly also by a central instrumentation tube. 
         [0037]    Generally, the top grid  13  and possibly the bottom grid  13  serve not only to hold the rods spaced apart laterally as mentioned above, but also to support the rods  3  longitudinally. The other grids  13  serve solely to space the rods apart laterally. 
         [0038]    Below, the invention is described with reference to an intermediate grid  13 , i.e. a grid that serves only to provide lateral spacing. Nevertheless, it can also be applied to grids  13  that also perform a function of supporting rods  3 . 
         [0039]    As shown in  FIGS. 2 to 5 , the grid  13  comprises:
       crossed inner plates  15 ; and   a peripheral belt  17  surrounding the inner plates  15  and made up of outer plates  19 .       
 
         [0042]      FIGS. 6 and 7  show two of the sixteen different inner plates  15  used for making the grid  13 . 
         [0043]    Between them, the inner and outer plates  15  and  19  define a square array of cells  21  and  23 . In the example shown, this array comprises 17×17 cells, however in other variants the grid  13  could comprise some other number of cells, e.g. 15×15, 16×16, . . . . 
         [0044]    Numerical reference  21  designates the inner cells, i.e. those that are defined solely by inner plates  15 . Numerical reference  23  designates the peripheral cells that are bordered by the peripheral belt  17  and that are therefore defined both by the belt and by inner plates  15 . 
         [0045]    The cells  21  and  23  are centered on the nodes of the array of fuel rods  3 , and for the most part they receive the fuel rods  3 . Some of the cells  21  receive guide tubes  11 , and the central cell  21  can receive a central instrumentation tube. Each cell  21 ,  23  in plan view is substantially square in shape and thus presents four side walls that are opposite in pairs. 
         [0046]    The peripheral belt  17  in plan view is substantially square in shape with four corners  25 . 
         [0047]    Each outer plate  19  forms one of the corners  25 ; and the plates  19  are assembled together in regions  27  that are distinct from the corners  25 , e.g. by being welded together. 
         [0048]    The outer plates  19  present cups  29  for assembly with the inner plates  15 . 
         [0049]    These cups  29 , e.g. made by stamping, are set back towards the inside of the grid  13  and they extend longitudinally across the plates  19 . They are pierced to receive the lateral ends of the inner plates  15  which are welded in the bottoms of the cups  29 . 
         [0050]    The lateral ends of the plates  15  situated in register with assembly regions  27  are held captive by the lateral edges of the adjacent outer plates  19 . In the example described, the peripheral belt  17  does not present any cups  29  in the assembly regions  27 . 
         [0051]    The plates  19  of the peripheral belt  17  also present orifices  31  for allowing the cooling fluid to pass laterally. 
         [0052]    In the example shown, these orifices  31  are circular and they are positioned halfway up the belt  17 . Each orifice  31  opens out into a peripheral cell  23  and is substantially centered relative to said peripheral cell  23 . In other words, the orifice  31  is substantially in register with the longitudinal axis of the rod  3  received in the peripheral cell  23  in question. 
         [0053]    In register with the junctions with the plates  15 , the peripheral belt  17  has fins  33  for guiding an assembly  1  adjacent to the assembly in question, and for guiding the cooling fluid. 
         [0054]    These guide fins  33  project upwards and downwards and they are folded towards the inside of the grid  13 . 
         [0055]    Between the guide fins  33 , the bottom and top edges  35  and  37  of the belt  17  present setbacks  39  towards the inside of the grid  13 . The setbacks  39  formed in the bottom edge  35  extend upwards and the setbacks  39  formed in the top edge  37  extend downwards such that in face view the setbacks  39  are oblong in shape. 
         [0056]    The setbacks  39  are made by stamping the outer plates  19  towards the inside of the grid  13  so as to form bosses  40  projecting towards the insides of the peripheral cells  23 . 
         [0057]    As shown more particularly by  FIGS. 4 ,  6 , and  7 , the inner plates  15  have stiffeners  41  at their lateral ends projecting upwards and downwards and against which the fins  33  bear. The fins  33  may be welded to the stiffeners  41 . 
         [0058]    Inside an inner cell  21  receiving a rod  3 , the rod  3  is held laterally by dimples  43  made by cutting and embossing the metal of the inner plates  15 , and also by springs  45 , e.g. fitted on the inner plates  15 . 
         [0059]    More precisely ( FIG. 5 ), for each inner cell  21 , each of the faces of the cell  21  is provided either with a pair of dimples  43  projecting towards the inside of the cell, or with a spring  45 , each spring  45  facing a pair of dimples  43 . 
         [0060]    Concerning those cells  21  that are to receive guide tubes  11 , it can be seen that the inner plates  15  are also provided with welding tabs  46  that project upwards from the lateral faces of each cell  21  in question, and that are for welding to the guide tubes  11 . These tabs  46  are shorter than in the prior art, for example they have a height of 8 mm. 
         [0061]    Concerning the peripheral cells  23 , and as shown in  FIG. 5 , the rods  3  that are received therein are held laterally in each cell  23  by two springs  45 , a pair of dimples  43 , and a pair of dimples  40 . The dimples  40  formed in the peripheral belt  17  face one of the springs  45 , and the pair of dimples  43  likewise face one of the springs  45 . 
         [0062]    For the peripheral cells  23  situated in the corners  25 , the rods are held by two springs  45  and by two pairs of circular dimples  48  formed in the peripheral belt  17  and facing the springs  45 . 
         [0063]    For each of the peripheral cells  23 , including the corner cells  23 , the inner face(s) opposite the peripheral belt  17  is/are provided with a pair of abutments  47  for laterally retaining the rod  3  received in the cell  23  in question. These abutments  47  are formed in the inner plates  15  lying beside the peripheral belt  17 . 
         [0064]    One such inner plate  15  is shown in  FIG. 6  where it can be seen that for each cell  23  one of the abutments  47  is situated at the bottom edge  49  of the plate  15 , and the other at the top edge  51  of the plate  15 . These abutments  47  are formed by cutting and embossing the material of the plate  15  towards the insides of the cells  23  in question. 
         [0065]    In the example described, each pair of abutments  47  lies on either side of a spring  45  facing the peripheral belt  17 . 
         [0066]    Also in this example, the peripheral belt  17  is not provided with a spring for holding the peripheral rods  3  laterally. 
         [0067]    It should be observed that the bottom edges  49  of the bottom plates  15  are rectilinear in the faces  53  of the cells  21  that receive the guide tubes  11  ( FIGS. 3 and 7 ). In particular, these faces  53  do not present any bottom cutout as in the prior art, thereby improving the ability of the grid  13  to withstand buckling. 
         [0068]    In order to assemble the plates  15  at their cross-points, notches  55  are formed in the plates  15 . For half of the plates, the notches  55  open out into their bottom edges  49 , for the other half, the notches  55  open out into their edges  51 , thus enabling all of the inner plates  15  to be mutually engaged via their notches  55 . 
         [0069]    As shown very diagrammatically in  FIG. 8 , each notch  55  presents at its open end a constriction  57  leading to the corresponding edge  49  or  51  via chamfers  59 . These chamfers  59  make it easier to engage the inner plates  15  mutually via the notches  55  at their cross-points. 
         [0070]    The constrictions  57 , which correspond to a reduction in width of the order of 100 micrometers (μm) to 200 μm, for example, serve to provide satisfactory mechanical retention between the inner plates  15  at their cross-points. This makes it possible locally to reduce clearance between the mutually engaged inner plates  15  and thus to improve the quality of the welding performed subsequently at the cross-points. 
         [0071]      FIGS. 9 and 10  show the configuration mentioned in the introduction of the present description, constituting one of the situations in which the risk of catching between two adjacent assemblies  1  used to be the greatest. 
         [0072]    In  FIG. 9 , there can be seen the peripheral belts  17  of two grids  13  of two adjacent assemblies  1 . These two grids  13  are offset laterally by half the pitch of the arrays of nuclear fuel rods. 
         [0073]    The bottom grid  13  belongs to a first assembly  1  and the top grid  13  belongs to a second assembly  1  that is being handled, which explains why one is situated above the other. 
         [0074]    Because of the lateral offset, the top fins  33  of the bottom grid  13  lie between the bottom guide fins  33  of the top grid  13  and are engaged in the bottom setbacks  39  of the top grid  13 . 
         [0075]    Similarly, the bottom fins  33  of the top grid  13  are engaged in the top setbacks  39  of the bottom grid  13 . 
         [0076]    Thus, the risk that the free ends of the guide fins  33  of a grid  13  might catch a longitudinal edge of the adjacent grid  13  is small. 
         [0077]    This risk is made smaller still in that in the event of one of the assemblies  1  pushing the peripheral rods  3  of the other assembly laterally towards the inside, these peripheral rods  3  will come to bear against the abutments  47 , thereby limiting both the bending and the displacement of the rods  3  towards the inside of the assembly  1 . This is shown in  FIG. 10  where the rods  3  directly adjacent to the peripheral rods  3  are omitted. 
         [0078]    The abutments  47  limit any displacement or bending of the rods  3  to a greater extent by being placed at the bottom and top edges  49  and  51  of the plates  15  adjacent to the peripheral belt  17 . 
         [0079]    The dimples  40  formed by the setbacks  39  in the peripheral belt  17  are disposed at the longitudinal ends of the grid  13  and thus provide good lateral holding and increased restriction on lateral displacement and bending of the rods  3 . 
         [0080]    The use of setback cups  29  for the purpose of welding the peripheral belt  17  to the inner plates  15  likewise makes it possible to limit any risk of adjacent assemblies  1  catching. 
         [0081]    Thus, the risks of the grids  13  of adjacent assemblies  1  catching are limited, particularly during handling. 
         [0082]    The presence of the stiffeners  41  for the guide fins  33  serves to increase the stiffness of the guide fins  33  and limits any risk of them deforming, and also any risk of catching that might result from deformation of the outlet plates  19 . 
         [0083]    The orifices  41  serve to increase the transverse hydraulic transparency of the grid  13  and thus to increase the ability of the assembly  1  to withstand buckling, and thus reduce the risk of two adjacent assemblies  1  catching. 
         [0084]    The characteristics described above, and in particular the setbacks  39 , the abutments  47 , the constrictions  57 , the orifices  31 , the stiffeners  39 , the cups  29 , and the rectilinear bottom edges of the faces  53  of the cells that receive guide tubes  11 , can be used independently of one another for achieving the overall object of limiting deformation of nuclear fuel assemblies in a reactor. 
         [0085]    In certain configurations, the setbacks  39  may be present only at one longitudinal edge of the grid  8 .