Patent Number: 047145854
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Turning first to FIGS. 1A-1C, there is depicted an array of fuel pins 10. The pins 10 are spaced and supported by a hexagonal egg-crate interlocking grid formed from the strips 20, 30 and 40. The pins 10 are positioned in a closely packed triangular array having a pitch p between the axis of adjacent pins. In the array depicted, the triangle formed by three adjacent pins 10 forms an equilateral triangle so that the pins may be packed into a tight array. The egg-crate grid of the present invention is formed from interlocking strips 20, 30 and 40 oriented as shown in FIGS. 1A-1C. Each strip is preferably formed "zig-zag" pattern having faces of approximately equal length and bends of approximately 120.degree.. As will be understood with respect to the description of FIG. 5 below, the strips which form the first and third layers (FIGS. 1A and 1C) are similar in configuration but with their orientations reversed. Turning first to FIG. 1A, there are depicted several strips which together form a part of the first layer of the grid. Each strip 20 has a thickness t which is as thin as possible to reduce the amount of parasitic material in the core but which has sufficient strength and rigidity to properly space and support the fuel pins. Each face of the strip 20 has a face 27, the length of which is determined by the pitch p of the fuel pin lattice. Adjacent faces of the strip are formed with an interior angle of approximately 120.degree. therebetween. On each face 27 of the strip 20 is formed a dimple 22 which is a relatively rigid protuberance (with respect to the springs 32) which forms a part of the support for the fuel pins. The third layer of strips, FIG. 1C, are formed in a manner similar to the strips of FIG. 1A but as assembled will be of reverse orientation as described below with respect to FIG. 5. The second or middle layer, depicted in FIG. 1B, is also formed of strips comprised of segments bent at an angle of approximately 120.degree. to each other. Each alternate face 34 of the strips 30 has a spring 32 formed in it. Intervening alternate faces of strip 30 have no spring and are used as the site to interlock the strips as described below. FIG. 2 depicts a detail of a strip of FIGS. 1A or 1C. For convenience, the strip will be referred to as strip 20, but it should be understood such reference and the following description is also applicable to the strips 40 of FIG. 1C. Each strip is comprised of alternating segments having faces 23 and 27. Faces 27 have dimples 22 stamped or otherwise imparted onto them and are generally rectangular in shape. A pair of tabs 26 are provided to engage with the slots 36 as described hereinbelow. Faces 23 also have dimples 22 stamped or otherwise formed on them. The strips preferably are provided with short vanes 29 having beveled sides in order to guide the interlocking strips 30 into correct position upon assembly. Slots 28 are formed in the strip 20 along with vertices 24 of the bends. These slots are dimensioned to accommodate complementary tabs in the strips 30 as detailed below with respect to FIG. 3. Turning now to FIG. 3 there is depicted an exemplary strip from the grid layer of FIG. 1B. Each strip 30 preferably is comprised of two types of faces, 34 and 35, formed by stamping or otherwise at an angle of approximately 120.degree. to each other about the vertices 42. The faces 34 and 35 are preferably of approximately the same length, a, as the faces 23 and 27 of strip 20 and are a function of the pitch, p, of the fuel pin lattice. The height of the face 35, c, is not crucial as long as it is properly dimensioned to accommodate the interleaving of the various strips and provides the necessary mechanical strength and rigidity. The amount of material in the grid should, of course, be minimized to reduce parasitic neutron absorption. The portion 35 of the strip is formed with its upper and lower edges 43 and 44 in the shape of a bent "S", 36, (see FIG. 3A) to form slots 37 and 38 which, during assembly, accommodate tabs 26 on strips 20 and tabs 66 on strips 40 as detailed below. The faces 34 of the strip 30 have portions 45 and 46 extending respectively above and below the face 35. The portions 45 and 46 have a height, b, approximately equal to the height of a section 27 of the strip 20. Pressed, stamped or otherwise formed in the face 34 is a spring 32 which may be formed between two slits 31 cut in the face 34 of the strip 30. The spring may have a nipple or detent 33 formed for making contact with the fuel pin. As will be apparent to the artisan, other spring geometries may be used, such as cantilevered or snap-on leaf springs, the above-described integral spring being illustrative only. Each corner of the face 34 has a tab 39 which cooperates (as described hereinbelow) with the slots 28 in the strips 20 during assembly. Turning now to FIGS. 4 and 4A, the assembly of the strips of FIGS. 1A-1C and the peripheral boundary strip will be described. During assembly the tabs 39 from the strips 30 are inserted into the slots 28 of the strips 20 (and 40). At the same time, the tabs 26 are guided onto opposite sides of the bent "S" slot 36. As can be seen from FIGS. 6A and 6B, at this point of the assembly, face 27 of strip 20 becomes coplanar with the face 35 of strip 30. Each cell 57 formed during the assembly supports the fuel pins 10 within the cell by means of springs 32 holding the pins 10 against the dimples 22 with sufficient force to allow the rods to expand and contract during power excursions of the reactor while restraining the rod against mechanical and hydraulic forces acting upon it. The peripheral row of cells 58 may preferably be formed in the shape of pentagons as opposed to the hexagonal shape of cells 57. The peripheral boundary strip 52 is formed with spring members 32' along it to support the pin against the relatively rigid dimples 22'. For connecting the strips at their ends, there are provided tabs means 54 or the like that fit into slots 55 in the peripheral boundary strip 52. During assembly, the strips may be spot welded or otherwise metallurgically or mechanically bonded at the intersection between the shoulder 53 of the strip (20, 30, 40) and the peripheral strip 52. The tabs 54 may also be fusion welded or otherwise bonded in the slot 55 during final assembly to insure a strong finished assembly. Structural posts 56 which support the fuel bundle are preferably provided at each of the six corners of the hexagonal fuel bundle. It should be readily appreciated, however, that other skeletal support arrangements can be used to axially space the grids from each other along the height of the fuel bundle and to provide support for the fuel bundle. Turning now to FIG. 5 there is a front elevation view of interlocking egg-crate grids supporting fuel pins 10. The upper and lower grid straps 20 and 40 respectively, having dimples 22 for contacting the fuel rods are shown on the faces 23 and 63 of the upper and lower grid straps respectively. On the right side of FIG. 5, the grid strap portions 23 and 63 with the scalloped edges 29 and 69 can be viewed. On the left side of FIG. 5, the faces 27 and 47 of the upper and lower grid straps can be seen with the tabs 26 and 66 clipped onto the "s" slot 36 of face 35 of grid strap 30. Faces 27 and 47 of grid straps also carry dimples 22 for contacting fuel rods 10. The tabs 39 of grid strip 30 fit in interlocking engagement with the slots 28 and 48 of strips 20 and 30, respectively. Note that as assembled the faces 27 and 47 of grid strip 20 and 40 respectively are coplanar with each other and with the face 35 of grid strip 30. Faces 23 and 63 of grid strip 20 and 40 respectively are coplanar with each other forming a wall of a cell 57 having an open space 59 between the grid strips. Each cell 57 has one wall formed from face 34 of the grid strip 30 carrying a spring 32 for contacting the fuel pin. As viewed in FIG. 5, the springs 32 face into the plane of the paper and urge the rods against opposed pairs of dimples 22 on the faces 23, 63 and 27, 47 of the grid strips 20 and 40. This forms a stable five point contact system although it should be appreciated that other contact geometries can also be advantageously employed. The dimples opposed to the spring 32 in FIG. 5 cannot be viewed in that figure inasmuch as they are obscured by the other elements of the grid assembly. Turning to FIGS. 6A and 6B, however, there is depicted isometric drawings of a single cell of the interlocking grid without any fuel pins. FIG. 6A is a view from the direction of the arrow 6A in FIG. 5. FIG. 6B is a view from the direction of the arrow 6B in FIG. 5. In each view, it can be seen that preferably only a single spring 32 is used to support the fuel pin in each cell and the spring is preferably opposed by a pair of dimples 22 located on alternate adjacent sides of the cell. Thus formed the cell is preferably in the shape of a hexagon with support contact on three of the six sides. It should be understood that the assembled grid shown in FIG. 4 has only 37 grid cells. A nuclear reactor fuel bundle will typically have several hundred fuel pins in a bundle and this smaller assembly is depicted for illustrative purposes only and shows all the significant geometric features of a full size assembly. In FIG. 4, the peripheral cells are formed by merely extending the grid strips forming the lattice to the peripheral boundary strip 52. The peripheral grid cells thus formed have a somewhat irregular pentagon shape instead of the regular hexagon shape of the remaining cells. As will be understood by the artisan, as long as the basic nuclear, hydraulic and mechanical design considerations for spacing and supporting the fuel pins are satisfied, the shape of the peripheral cells can be varied to accommodate geometric discontinuities. The grid strips are preferably attached to the peripheral strip 52 with a spot weld to fix the pieces during assembly but would preferably use a fusion weld as alluded to hereinabove for strength in welding the tabs 54. Processes such as electron beam welding or laser welding may be used for this purpose. As depicted in FIG. 4, each of the outside cells has a spring formed in the cell wall formed by the peripheral wall 52. Because of this, the peripheral wall should preferably be the same appropriate height, thickness and material as a grid strip 30. Of course, other geometries where the peripheral wall does not support a spring are within the scope of the present invention. As a further consequence of the peripheral spring placement discussed above, some of the grid strips 20 and 40 should incorporate dimples 22 projecting in both directions from the strip face instead of only one. As will be understood by the artisan, this can be easily accomplished and accommodated in the basic stamped out grid strip design. The use of interlocking strips to form a tight hexagonal fuel pin array as described above significantly enhances manufacturing ease and reduces the costs of production over prior art grid assemblies, particularly LWBR grid assemblies which used many small face pieces. Moreover, the use of pins at the vertices of the grid strip is avoided with the above described interlocking grid design thus simplifying the grid manufacture and enhancing its reliability. It will be apparent to the artisan that the grid assembly described avoids the use of double thickness of metal along any faces of the grid cell (except for the small locations on strip 30 where tabs 26 overlap S-shaped slot 36) thereby introducing a minimum of parasitic neutron absorbing materials and permitting the nuclear fuel pins to be more closely spaced. It is also important to appreciate that the interlocking grid design described above will retain its structural integrity without spot welds at the grid vertices, or if such weld are included, for additional safety margin, their failure will not adversely affect the integrity of the grid assembly. In fact, it is only along the peripheral grid strip that welds may be necessary. Moreover, because of the controlled flexure nature of the assembled grid strips and the fuel pin contact geometry, the grid assembly described herein will support the fuel pin lattice fairly rigidly against distortion. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.