FR2499750A1 - ANNULAR OR POLYGONAL GRID FOR SUPPORTING NUCLEAR FUEL BARS ACCORDING TO A TRIANGULAR LAYOUT - Google Patents

Abstract

THE GRID FOR THE SUPPORT OF NUCLEAR FUEL BARS 1 IS ARRANGED IN A TRIANGULAR CONFIGURATION OR NETWORK. THE GRID IS SHAPED BY CONCENTRIC RINGS 4 OF BANDS 3, 18 CONNECTED BY SECTIONS OF BARS 22 ARRANGED RADIALLY.

Description

The present invention relates to a fuel rod support grid.

  a new design for a support grid for triangular arrangements or networks of nuclear fuel rods, especially for hexagonal fuel assemblies

  intended for fast reactors cooled with sodium.

  The nucleus of a nuclear reactor usually consists-

  Lement in a grouping of fuel assemblies. Each fuel assembly is a group of cylindrical fuel rods which are arranged in grids and which are supported by these grates spaced axially along the fuel assembly. The support grids define the geometric design of the cells intended to receive the fuel rods and maintain the correct spacing

between these bars.

  Some nuclear reactor designs specify

  the production of hexagonal fuel assemblies, the

  of fuel being placed in a triangular arrangement

  laire. A triangular arrangement of the bars is defined here as a geometric design formed by the bars and in which three imaginary lines connecting the centers of any group of mutually adjacent bars in the assembly form an equilateral triangle. The triangular arrangement or arrangement is repeated so that the family of all imaginary triangles formed by the centers of all possible groups of three mutually adjacent bars of the assembly is made up of congruent triangles. The prior art gives grids for triangular arrangements, but these grids, as will be described below, have manufacturing drawbacks and technical drawbacks.

  Hexagonal grids for a triangular layout

  bars are made up of stacks of a large number of strongly corrugated strips, this stack being surrounded by a strip which then forms the surrounding element

Grid.

  The stacking of the wavy bands leads to the creation

  double walls at periodic points on the grid.

  These double walls bring limitations to the flow

  in the final fuel assembly.

  The depth of the corrugations in a strip can vary during manufacture due, for example, to the wear of the stamping die. Experience has shown that these depth variations manifest themselves as a trend rather than being statistical, so that

  all bands in a manufactured batch may have a depth

  ripple which is slightly larger (or smaller) than that of design. In the stacking operation currently used to make the grids, the differences are cumulative so that the height of the final grid is distorted. This problem of the alteration of the height arises in the phase of stamping of the strip, but does not appear only thereafter during the phase of stacking of

  manufacturing and, at this stage, the only possibility to perform-

  killing an adjustment according to variations in dimensions is the flexibility of the projections or support bosses, The

  practical result is that the sum of the cumulative differences

  sometimes passes the flexibility of these projections so that some

  some bars cannot be inserted and the grid

whole must be discarded.

  A wide variety of manufacturing issues

  introduce variations in the exact location of the sail-

  support lees or bosses. Such variations result in the bars being held in place more or less firmly relative to what is desired, since even a slight deviation in the position of the projections changes the distance between the contact points of the projections to the interior

of a cell.

  We are currently making an annular grid (a grid with an empty center) by making a complete grid by stacking, cutting to a hexagonal outer shape and strapping the outer surface, then cutting and

  removing the central section and encircling the hexagonal surface

  inner gonal. The need to manufacture a complete grid arises from the nature of the stacking manufacturing process in which the positioning, alignment and support of a strip during welding is obtained by reference to the strip previously welded. The central section of the grid cannot be simply omitted since it plays a role in the manufacturing process. All the work and all the material associated with this central region is currently lost. In addition, the annular grid obtained must have at least two rows of fuel rod housing holes so that a mechanically sound structure is maintained for

the cutting operation.

  This is why the main object of the present invention is to provide a grid supporting fuel rods placed in a triangular arrangement, this grid being able to be manufactured without stacking the strips and without the additional manufacturing operations which have been carried out.

  recourse to form annular or polygonal grids.

  In view of this object, the present invention

  resides in a support grid intended to support bax-

  nuclear fuel res placed in a triangular arrangement, this grid consisting of mutually connected metal bands and being characterized in that said bands being arranged in the form of a plurality of rings or rings arranged concentrically, a plurality of bars extending radially between the adjacent rings and being fixed to the latter, thereby forming cells intended to receive nuclear fuel rods of

  in such a way that the layout of the cells has a configuration

triangular ration.

  This bar and band structure can be applied to fuel assemblies of polygonal cross section,

  like for example a hexagonal assembly. The modes of realization

  preferred sations described below relate to assemblies

hexagonal fuels.

  The present invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is an embodiment with a straight strip according to the present invention; Figure 2 is an embodiment of a corrugated strip according to the present invention; Figures 3 and 4 are views from Figure 1 and showing details of a double support projection; Figures 5 and 6 are views from Figure 2 and showing details of a single projection; and figures 7 and 8 show details of seals

  connecting strips to strips.

  The grid according to the present invention is represented in the form of a first embodiment in FIG. 1, which shows a grid supporting thirty-seven cylindrical bars 1 of fuel in a triangular arrangement 2 * by forming a hexagonal assembly The flat bands 3 form

  hexagonal rings 4 and are welded to their vertices 5.

  Radial bars 22 extend only between adjacent rings 4 and are welded at each of their ends to the flat strips 3 by forming cells 9 intended to receive the fuel bars 1. Double projections or bosses are an integral part of the bars 22. The central cell 8 shown in FIG. 2 has projections or bosses

on the annular band 4.

  Figures 3 and 4 are views from Figure 1 and showing the details of a double projection 20. Figures and 6 are views from Figure 2 and showing the details of a single projection 21 used for bear the bar

  central in FIG. 1. These projections 20, 21 are structures

  tures forming an integral part of the material of the strip or

the bar.

  The grid according to the present invention is not manufactured using the stacking process and does not raise any problem of cumulative deviation. The grid can be assembled in a welding device which receives and momentarily supports the

  flat strips in the place and in a suitable form.

  Any variation in cell size is due to the characteristics

  of the welding device, which can be detected and corrected before manufacturing the grid. The grid cells are completed by the choice and placement of bars 22 so that a tight assembly is obtained. If we use butt welded joints as shown in Figure 7, we obtain the tight assembly

  thanks to the use of slightly different length bars

  ferente chosen according to needs. If we use a welded joint with tongue and groove as shown in figure 8,

  the sliding of the joint can be used to obtain the assembly

  tight clamping so that only standardized bars of a

  only dimension are required. The final gap in the dimensions

  sions of cells is not cumulative over the entire grid. Of course, any existing deviation is, in this new design, within the adjustable range of the projections in

  all but the most unusual circumstances

  tuelles. The single and double projections 20, 21 in the form of flat strips have, in the present invention, linear contact zones 16 (FIGS. 1 and 2) so that small differences in position of the fuel rods do not occur.

not translate into play.

  The final grid has no double walls so that one can expect a lesser reduction in flow. The grid shown in FIG. 1 is advantageous for certain applications but cannot lend itself to a layout of the fuel rods in which the rods are close to each other to the point that there is not between

  these bars an adequate interval to admit hex rings

flat band gonals 4.

  FIG. 2 is an embodiment of the invention in which corrugated strips 10 are used to manufacture the

  hexagonal rings 4. This grid has bars of com-

  bustible with little spacing and no free space between the rings

adjacent bars.

  In this design, the protrusions are present both at positions on the bars and at positions on the strips. Figures 3, 4, 5 and 6 show details of the double projections 20 (Figure 4) and single projections 21 (Figures 5, 6). The central bar 8 can be supported by additional projections in addition to the three shown on the

figure 2.

  The construction technique for this second embodiment also involves the use of tongue-and-groove joints or bars 22 of variable length each extending between adjacent rings 4 only. The same advantages are obtained as those listed in connection with the first embodiment. Deviations in the depth of the undulations of the corrugated strip 10 only affects cells adjacent to the particular strip, are not cumulative from one end of the grid to the other, and are therefore within the limits of the range. for adjusting the support projections. The corrugated strip can still be supported in a welding device since the use of tongue and groove joints or bars of different lengths provide the necessary flexibility.

  for an adjustment with regard to depth differences.

  Figure 2 shows a location 23 of the single projections 21 which are unused. These protrusions appear in the drawing because the use of a standard corrugated strip 10 for all the hexagonal rings 4 results in the presence of these superfluous protrusions. These projections can be omitted

in a variant.

  Both embodiments use a structure

  strip and strip ture which involves the connection of the strips and strips while these are supported and arranged in the form of a desired grid in a welding apparatus comprising grooved plates and a welding machine. The strip sections are cut to the appropriate lengths so as to form the desired rings from a standardized strip previously prepared and comprising the projections already formed at the appropriate intervals. The standard tape can be prepared in standard lengths or even in large rolls. The strip sections are introduced into the grooves of the welding machine plates

  which give the strip sections the form of hexagonal rings

  nales of predetermined size. The sections of bars are introduced into grooves of the plates of the welding apparatus which bring these bars into a state of proximity suitable for their welding to the rings. It is at this stage that the bars of different length are chosen to ensure positive contact with the rings and compensate for the slight

  distance variations from ring to ring.

  Welding can be carried out by a beam

  electron, resistance, laser or any other process.

  Figure 7 shows a butt joint between the bar and the strip, joint in which the strip 3 is welded by resistance at points 26 and welded using a beam

  of electrons or by any other process at contact lines 27.

  FIG. 8 shows a joint with tongue and groove between the bar and the strip, with welds with double contact lines 28. The method of assembly by tongue and groove allows a certain flexibility of adjustment of a standardized length of bar to compensate for small variations in ring-to-ring spacing by sliding the tongue 30 in the groove 31 while still allowing welding to the double contact lines 28. This characteristic can reduce, even eliminate, the need for 'a

bar of variable length.

  The grids of Figures 1 and 2 are shown for convenience as being relatively small assemblies. In practice, the grid can be adapted to support a much larger number of bars

  in larger assemblies.

Claims (4)

  1. Support grid for supporting nuclear fuel rods in a triangular network, this grid consisting of metal strips connected to each other and being characterized in that said strips (4, 10) are arranged in the form of a plurality of concentrically arranged rings, a plurality of bars (22) extending radially between adjacent rings (4, 10) and being fixed to these adjacent rings (4, 10) thereby forming cells intended to receive bars (1) nuclear fuel   so that the arrangement of the cells forms a triangle.   2. Support grid according to claim 1, characterized in that said concentric rings (4) are formed by corrugated bands (10) so as to obtain a triangular arrangement, closely spaced, of the bars (1) fuel.   3. Support grid according to claims 1 or   2, characterized in that said bars (22) have different lengths.   4. Support grid according to any one of   Claims 1, 2 or 3, characterized in that   elastic projections are provided to ensure an area of   linear contact (16) intended for said fuel rods (1) tible.