Patent Number: 043426204
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 are general arrangement views wherein an egg-crate frame or rack 10 is formed of stainless steel grids or wall members 12. These grids 12 extend through the entire height of the rack and must generally be equal to or greater in length than the active length of the fuel assemblies to be stored. Support bars 14 pass through the lower portion of the rack for support of the fuel assemblies to be stored. The frame 10 thus forms a plurality of vertically extending rectangular openings 16. The maximum storage capacity of the frame 10 is obtained with the use of box inserts. FIG. 3 show the preferred embodiment of the invention wherein the box insert 20 is located within the confines of the opening 16 defined by the frame grids 12 shown in FIG. 1. Each insert 20 is formed by joining together a plurality of metal plates 22 arranged as an oen-ended polygon having the same general shape but smaller area than the opening 16 provided by the frame 10. In the illustrated embodiment, each opening 16 is a square and accordingly the plates 22 form a square. FIGS. 3 and 4 show one plate 22' which will be described in detail. Each plate 22' has a flat or face portion 24' which forms one of the sides of the container 26, and has integral tab portions 28' rigidly projecting outwardly at an angle to the face portion 24'. Each plate 22' extends in the longitudinal direction at least the length of the fuel assembly to be stored therein (not shown), and each face portion extends laterally at least the distance corresponding to the lateral dimension of the fuel assembly. It can be readily appreciated that the individual plates 22 are in a very convenient form for shipment to the plant site and for storage at the site. Shortly before the fuel assemblies are to be located in the frame 10 for storage, the box inserts 20 are formed and inserted into the openings 16 in the frame, as shown in FIGS. 1 and 3. In the preferred embodiment, the plate 22' is oriented at right angle to another plate 22" whereby the tabs 28', 28" mate with each other over substantially the entire length of the plates 22', 22". The tabs are then joined together, for example, by spot welding or bolts 30 at spaced intervals over the length of the plates. Another plate 22'" is then brought at right angles to plate 22" and their respective tabs are also connected, and the fourth plate is then connected in turn to the third and first plates. Thus the box insert 20 is formed having a rigid container portion 26 and having a plurality of outwardly projecting ribs 32. In the preferred embodiment each plate 22 is made from type 304 stainless steel and has a thickness in the range of about 0.050-0.065 inch (0.125-0.165 cm). The present invention permits the use of relatively thinner stainless steel that is generally permissible with other fuel storage rack inserts because of the rigidity provided by the ribs 32, as described below. The box insert 20 is placed within the opening 16. The ribs 32 fit into the corners 34 of the openings formed by the wall members 12 of the frame. The ribs thus provide two important functions. First they provide rigidity to the container portion 26 of the box 20. Any lateral loads transmitted from the frame to the container plates 22 are primarily transmitted as a compressive force along the lateral dimension of the plates. This permits the use of thinner plate material than is generally used. Secondly, the ribs 32 keep the container 26 in a well-maintained, spaced relationship from the grids 12. This insures that, within close uncertainties and tolerances, the fuel assembly to be located within the container portion 26 will have a minimum separation from the fuel assembly to be located in the closest box insert in the adjacent opening 16a in the frame (see FIG. 1). The assurance of a minimum separation permits the user of the invention to take full advantage of the flux-trap affect which is known to those skilled in the art as a way of avoiding excessive neutronic coupling of adjacent fuel assemblies in a moderating medium such a fuel storage pool. In the preferred embodiment of the invention, the neutronic coupling of adjacent assemblies can be further reduced by providing a layer of neutron absorbing poison material 36 on the outer side of the face portion 24' of the plate 22". The layer could be a coating of B.sub.4 C that is bonded directly to the plate before shipment, as by a spraying technique available from the Carborandum Company, Niagara Falls, N.Y. In another form, the layer could be a sheet of B.sub.4 C in a matrix of ethylene propylene diene rubber, as is available, for example, from the Raybestos Manhattan Industrial Products Company, or a polymeric silicone encapsulant fixing fine particles of B.sub.4 C as is available from Brand Industrial Services, Inc., Park Ridge, Ill. With the sheet form, a thin foil of stainless steel (not shown) may be rivited over the sheet to hold it in place against the plate surface 24'. The maximum thickness of the poison layer 36, including foil cover where needed, will generally be in the range of 0.040-0.090 inch (0.100-0.230 cm). As a further aid in locating the fuel assemblies within the container portion 26 of the box insert 20, the plates may have formed on their upper ends a flared member 38 which acts as a guide for the lower portion of the fuel assembly as it is lowered into the box insert. The lower ends 40 of the plates 22 are not required to have any special structure since the ends merely rest on the support bars 14 as shown in FIG. 1. It may be appreciated that the present invention provides a simple way of constructing a box insert for placement in a frame of a nuclear fuel storage facility. Although the preferred embodiment shows a square box insert formed by connecting four plates each having tabs extending at a 45 degree angle from the faces of the plates, the ordinary practitioner in this field could easily adapt the design to provide box inserts for polygonal fuel assemblies and frame openings.