Patent Number: 050323482
Section: summary

TECHNICAL FIELD The invention concerns a stowage rack for nuclear fuel elements comprising a plurality of cells, a fuel element to be stowed being inserted in each cell. These stowage racks are used to store the fuel element in a pool or under dry conditions and/or to transport them in a shielded container, which is dried after loading. STATE OF THE ART Stowage racks for nuclear fuel elements are normally made up of adjacent prismatic cells, usually of square cross-section and of elongated shape with a long axis. The cross-sectional shape of the cells is generally identical with that of the fuel elements to be stowed, and the height of the cells is at least equal to that of the elements. The racks according to the invention are suitable for stowing non-irradiated nuclear fuel elements requiring sub-critical conditions, and the fuel may be based on uranium oxide exclusively or on any combustible oxide mixtures. However, they are particularly adapted to stowing and transporting irradiated fuel elements under dry conditions in a sheathed container. In this application any rack--also known as a stowage rack--must simultaneously fulfil several functions: transfer of the heat generated by the irradiated fuel elements stowed in it, to the wall of the sheathed container in order to dissipate it. The better the thermal conductivity of the rack material and the better the contact between the rack and the wall of the container, the better this function is fulfilled. neutron absorption to guarantee that the rack filled with fuel is in a sub-critical state, either under dry conditions or when immersed in water during storage in a pool or during operations in which sheathed enclosures are loaded and unloaded; these may also be carried out in a pool. This function is fulfilled by using materials containing neutron absorbing elements such as B, Gd, Hf, Dc, In, Li and the like, said materials being used directly in the design of the rack, or by using neutron absorbers inserted in the fuel elements and by good neutron degradation obtained through forming spaces near said neutron absorbing and neutrophage materials. high enough mechanical strength to support the load of combustible elements during transportation and to maintain the geometry of the rack even in the event of impact, thereby maintaining sub-critical conditions and avoiding the risks of the fuel elements and rods deteriorating as a result of heating and/or crushing. These functions are normally fulfilled by making the walls of the cells from materials arranged in a plurality of superposed layers. For example, material of the sandwich type may be used, comprising at least two layers: a layer of an alloy fulfilling the mechanical strength and heat transfer functions, with preferably homologated properties, and a layer of an alloy or composite material containing a neutron absorber; here the mechanical and thermal properties are not generally homologated. The layers are combined by an known means, e.g. by rolling them together or by electroplating, mechanical assembly, welding etc. The material containing the neutron absorber may, for example, be stainless steel containing approximately 1% of boron or an aluminium alloy containing approximately 3% or boron. Alternatively it may be a fritted boron carbide/aluminium product which may or may not be coated with aluminium, or cadmium, deposited electrolytically on a metal carrier. When an aluminium alloy is used, it is generally supplied in the form of strips which are then attached to the other layers as indicated above. The strips may be obtained by rolling or extrusion from bars of adequate size. The bars must be very homogeneous and very sound (no blistering, cracks etc.), and the larger the bars are the more precautions have to be taken. In spite of the precautions taken, the guaranteed minimum boron content of the flat product is often one point below that of the starting product. The common, cheap form of these materials is the wire supplied in coil form. These are obtained by continuous casting e.g. of a ring a few centimeters in diameter, which is then rolled and/or drawn. Boron aluminium wire with a diameter of approximately 10 mm is manufactured in this way, and its guaranteed boron content is generally 2.5 or 3.5%. Other cell wall designs have been described, also fulfilling the neutron absorbing function. For example, U.S. Pat. No. 4,034,227 (Soot) describes wall members which may be assembled with special tenons to form a rack. The members are pieces which are specially extruded in the cell length direction. They have a complicated cross-sectional shape with a series of projecting notches zig-zagging from one side of a flat wall to the other, parallel with the extruding direction. The notches are open along a generatrix and designed to receive neutrophage rods. Although a design of this type, using neutron absorbing rods, avoids the tricky processing of the kind of material described above, it nevertheless involves extruding pieces with a large cross-section and a complicated profile. This greatly restricts the number of potential suppliers, given the size of the presses which would have to be used and the resultant extrusion problems. An assembly of this type also has the drawback of having a multiplicity of mortise and tenon type joints (36, 38, 39 . . . ) in the corners of the cells; these make it difficult to obtain good mechanical strength and adequate thermal conductivity. OBJECT OF THE INVENTION The object of the invention is a stowage rack for irradiated or non-irradiated fuel elements, which may be used for dry storage or for transporting the elements dry in a sheathed container. The rack must fulfil mechanical strength, heat transfer and neutron absorbing functions. The invention aims to simplify the processing of the materials used in making the racks and consequently to reduce their cost; at the same time the mechanical, thermal and neutron performance of the racks must be easily homologated. Its purpose is therefore to use materials and semi-finished products which are readily available on the market and have known properties. It must be possible to use them directly as they are, without any intermediate metallurgical transformation and without requiring more than simple assembly means. Since these materials and semi-finished products are preferably standardized, they are generally more reliable and less expensive than extruded, rolled or composite items which have been specially researched and developed. Another object of the invention is to have a rack in which at least the mechanical strength and heat transfer functions are separated from the neutron absorbing function, with a further possibility of separating all three functions, thus making it easier to calculate and homologate the performance of the rack. A further object of the invention is to have a rack in which the neutron barrier may optionally be continuous or, preferably, discontinuous.