Patent Number: 050323482
Section: description

DESCRIPTION OF INVENTION The invention is a stowage rack for nuclear fuel elements, for storage or preferably dry transport, comprising a plurality of adjacent prismatic cells which are parallel and very long, said cells being designed to receive said fuel elements and forming a unit which must fulfil mechanical strength, thermal conductivity and neutron absorption functions, characterised in that, for the purpose of construction the rack from known, reliable materials which will fulfil the mechanical strength and thermal conductivity functions, and providing at least the neutron absorbing function separately, the walls of the cells are made up of a neat intersecting stack of elongated structural members of the profile type, of constant cross-section and made of common material, disposed in successive layers perpendicular to the axis of the cells, and that said walls control criticality, either due to their thickness and due to neutrophage rods arranged inside said walls parallel or perpendicular to the axis of the cells, or through having the structural members of common material with mechanical strength and/or thermal conductivity functions alternating in the stack with structural members of neutrophage material fulfilling the neutron absorbing function; the stack being such that the rack fulfils all the required functions. Thus the rack according to the invention is used for packing nuclear fuel elements under conditions requiring the presence of a neutron absorber, allowing for their geometry and their composition. A stowage rack of this type may be used either for storing fuel elements in a pool or preferably dry, or more preferably for transporting them under dry conditions. In this rack it is placed in a sheathed container before or after the fuel elements have been loaded. If the elements transported have been irradiated, the container both provides biological and mechanical protection and dissipates heat. The stowage rack acts as a neutron absorber and is the sole means of transferring the heat given off by the irradiated fuel elements to the sheathed container. The rack also has an essential mechanical strength function, particularly in avoiding possible crushing of the elements in the event of an accident, and in supporting the load of packed elements during horizontal transportation. Such a rack may equally be self supporting. All that is required is to reinforce it by any appropriate means and to fit it with handling devices, a base and possibly a cover. The rack according to the invention is made up of a plurality of adjacent prismatic cells, They may have any cross-section and are generally all identical. They have common walls, which are thick enough (in the transverse direction to the cell axis) to carry out the mechanical strength and thermal conductivity functions and, where appropriate, to contain the neutrophage rods, as will be explained later. The structural members chosen to form the neat stack are elongated with a cross-section of substantially constant profile; they are preferably sections of the normal type, with a preferably angular cross-section of simple shapes such as L, U, T, I, H, crosses, tubes or solid bars of square or rectangular section, flat strips and the like. Pairs of elongated thin flat strips may equally be used as structural members. Their long surfaces are arranged parallel with each other and parallel with the axis of the cells, and the space thus defined between the strips must be sufficient to insert the neutrophage rods, parallel or perpendicular to the cell axis. The structural members may equally be elongated extruded flat bars of rectangular section, with aligned tubular receptacles formed inside them to receive the neutrophage rods. All these different types of structural elements may be used either alone or in combination. They are generally straight but may also be evenly bent, with the edges of the bent portions arranged parallel with each other and perpendicular to the plane containing the resultant broken line. The stack is formed parallel with the axis of the cells to be obtained, with the length of the structural members directly perpendicular to the cell axis. The structural members are stacked in successive layers or rows, so that they are in a well-defined general direction in each layer or row, the directions cutting across each other from one layer to another. The number of directions is usually the same as the number of walls required, but this is not essential: two or three directions may, for example, be used for cells or hexagonal section. Structural members extending in the same direction and located in different layers are stacked, aligned, parallel with the axis of the cells, to form the cell walls. They must be long enough for one and the same member to serve a plurality of cells. To obtain a vertical, square mesh cellular rack, for example, a first layer is formed by arranging the structural members in a horizontal plane, parallel with a direction A and spaced by a distance corresponding to the cell mesh; then by forming a second horizontal layer on the first through arranging the structural members in the same way but in a direction B perpendicular to the direction A. Thus a square mesh cellular structure can be obtained by alternating the layers in directions A and B and ensuring that stacked members extending in the same direction are vertically aligned. Similarly cells of hexagonal mesh can be obtained by crossing over the layers of structural members in two or preferably three directions, forming angles of 60.degree. between them. The members must first have been bent evenly at 120.degree., to form a broken line in which each segment has the dimension of one side of the cell. This kind of stack, illustrated in FIG. 5, will be described in greater detail later. The structural members in the different layers or rows are rigidly connected by any known joining means such as pins, bolts, welds, flat bar, staples, angle irons, rivets, tie rods, punched plates etc., so as to maintain the cohesion and rigidity of the stack forming the cellular structure. As a complementary measure the compactness of the stack may be increased, while still maintaining its cohesion and rigidity, if the structural members are provided with positioning notches or incisions cooperating with one another, as in certain bottle cases or in the construction of mountain huts where the walls are assembled by this nesting process. If this is done the walls need not contain any cut outs. Neutrophage rods are preferably placed within the actual walls of the cells, extending in the desired direction, i.e. parallel or perpendicular to the cell axis. If necessary appropriate holes first have to be made in the structural members. Thus if a stack of profiled members other than flat strips is used, e.g. `H` shaped or `U` shaped members, holes are drilled in the sole plates or cores of the sections which are perpendicular to the cells' axis, and the neutrophage rods will then be parallel with that axis. Similarly, if the structural members used are extruded flat bars, which are crossed over and nested by means of incisions, and which are of rectangular cross-section and adequate thickness, tubular receptacles can be formed in the extruding direction to receive the neutrophage rods, e.g. by means of a bridge type die. The structural members may also be reinforced locally, e.g. by extra thick portions judiciously arranged, particularly near the zones which have been drilled to receive the neutrophage rods. If on the other hand the cell walls are formed by a stack of structural members comprising pairs of thin flat strips, with their surfaces parallel to each other and to the cell axis, then the neutrophage rods will be inserted directly into the space defined by the parallel surfaces of the bars facing one another, as illustrated in FIG. 4. The neutrophage rods may be arranged perpendicular to the axis of the cells; in this case they will improve the other functions (mechanical strength and heat transfer). The structural members forming the cell walls must a least fulfil the mechanical strength and heat transfer functions. For this purpose they can be made from a choice of materials fulfilling both functions, for example steels (normal, stainless or special), aluminium, copper or magnesium or alloys thereof, and generally any metals or materials which will form sections with the required mechanical and thermal properties. Materials which only fulfil one function may equally be used; for example, layers of structural members made of materials with high mechanical strength such as steel may be alternated with others with good heat conductivity such as copper. In some special cases structural members of neutrophage material may be inserted in the stack forming the cellular rack. Thus the members of neutrophage material may be stacked alternately with those providing the mechanical strength and heat transfer. This alteration may take place either from one layer to another or within the same layer or there may be a combination of both. It is compulsory for the neutrophage material used in the invention to contain at least one neutrophage element such as B, Gd, Hf, Cd, In, Li, etc. The material is made up of that element itself or one of its compounds, used as such or combined with a different material to give a composite. These other materials may be: metals or metal alloys such as Al, Cu or their alloys, charged with boron or another neutrophage element, PA1 the various types of fritted products (including fritting by extrusion through a die) containing boron or alternative material, e.g. B.sub.4 C.Al or B.sub.4 C.Cu PA1 ceramics or glasses containing boron or alternative material. These various composites are generally used in rod form with or without a sheath. The rods may equally be obtained by extruding billets of alloy or cermet obtained by known processes, including those of the spray deposition type. However, it is an advantage to use any material which is readily available on the market, particularly wire made of boron-aluminium master alloy, containing 2.5 or 3.5% of boron. This is easily available as it is used in other applications which consume large quantities of this kind of product (e.g. aluminium refining). The neutrophage rods cover at least part of the length or width of the cell walls, according to the direction in which they are installed, and may cover them all. The installation density of the bars is adapted to requirements. A neutrophage bar may be made up of a succession of component bars of reduced length. To illustrate the invention FIGS. 1 to 7 give different examples of stacks using various types of simple sections. FIG. 1 shows a stack formed to create a stowage rack with cells of rectangular or square cross-section; the stack is made up of structural members 1 and 2, which are `U` shaped sections but could equally well be solid bars or hollow tubes of the same cross-section. The members 1 are all arranged in the same direction; so in one layer they are parallel and separated by a distance equal to the size of the cells 3. The members 2 in the adjacent layers are also parallel with one another but perpendicular to the members 1 and separated by a distance equal to the cell mesh in that direction. Holes 4 have been drilled in the horizontal sole plates, enabling neutrophage rods 5 to be inserted in the wall of the cell 3 parallel with its axis. In this case the cell walls contain cut-outs. The parts are assembled by tie rods 6 located at the intersection of the structural members 1 and 2. They are made of a material with good mechanical strength, for example stainless steel; the assembly can be made rigid by end plates (not shown) containing cut outs as in the drawing of the cells. FIG. 2 shows the detail of an assembly of structural members 1 and 2, which are `H` sections, using notches 7. With this type of assembly it is not necessary to have cut outs in the cell walls. The neutrophage wires 5 are arranged parallel with the cell axis in two rows within the walls of the said cell. FIG. 3 shows a portion of a vertical stack of structural members 1-1a-1b in one direction. The structural members are flat sections of rectangular cross-section with tubular receptacles 4 arranged inside them. The receptacles 4 are designed to receive the neutrophage rods 5, which in this case are perpendicular to the axis of the cells. Notches 7 enable the member 1 to be connected rigidly to the member perpendicular to it (not shown). FIG. 4 is an assembly where the structural members are pairs 1 and 2 made up of two parallel flat bars. The pairs 1 and 2 are arranged in orthogonal directions and rigidly connected by means of notches 7. They form the walls of the cell 3. The neutrophage rods 5 are inserted in the actual walls parallel with the axis of the cell and with minimum diametric clearance, to avoid the risk of misalignment of the rods within the wall. FIG. 5 represents a rack with hexagonal cells 3. It is formed by a stack of structural members 1-2-8. These are evenly bent `U` sections (or any other shaped sections) in the form of a broken line extending in three directions 1-2-8. The directions are staggered at 60.degree., so that each cell wall is open over one-third of its height. The rack structure is completed as before by neutron absorber rods 5 and tie rods 6 inserted in the holes 4. The neutron absorber 5 may either be continuous, as are rods 11, or a series of abutting shorter rods 12. FIG. 6 shows a stowage rack according to the invention, comprising an intersecting stack of sections in the form of flat strips. FIG. 7 shows an enlarged view of a portion of the rack of FIG. 6. Strips 1, comprising intersecting, perpendicular strips 1a and 1b with mechanical strength and heat transfer functions, alternate with strips 5 of boron alloy, which absorb neutrons and contribute to the heat transfer. The strips 1 may of course contain openings and/or be replaced by any sections of the same thickness as the strips 5. The use of standard materials for the invention has the advantage of facilitating construction of the stowage racks, lowering their cost and facilitating their homologation. In racks where the neutrophage rods are metallic and perpendicular to the axis of the cells these rods have the further advantage of improving heat transfer.