Patent Number: 043483550
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT In the Figures, the numeral 1 designates cruciform control rods. Between a group of four of these control rods there is arranged a group of four fuel assemblies 2. Each fuel assembly comprises four boxes 2a, 2b, 2c and 2d of square cross-section, each of these boxes containing a respective bundle of fuel rods 3. These fuel rods comprise fuel pellets in sheathing tubes. The boxes 2a-2d of the fuel assembly are connected at the bottom to a common bottom portion 4, which is connected to an assembly supporting plate of the core and distributes the cooling water to the four boxes of the assembly. At the top, the boxes are connected to a common top portion or unit 5. Between the bottom portion 4 and the top unit 5, the boxes 2a-2d may be connected to each other in one or more places by means of connecting elements 6. As shown in FIG. 3, which shows one of the boxes (box 2a), it can be seen that each of the boxes 2a-2d may be formed with corners of large radius and the four fuel rods 3a adjacent the corners of a box may be of smaller diameter than the other fuel rods 3b. By employing a fuel assembly with four boxes 2a-2d having a common bottom portion 4 and a common top unit 5, it is possible to improve the fuel economy of both new reactors and reactors already in operation. Previously known and tested devices may be employed for supporting the fuel assemblies. Because of the cruciform gap 8 between the four boxes 2a-2d of a fuel assembly, a steam-free quantity of water is obtained which results in an increased moderating ability, and thus in a more even neutron flux over the cross-section of the fuel assembly, as well as a more even power developement and burn-up. The need to vary the degree of fuel enrichment within the fuel rod bundle is reduced or eliminated. Since the gaps 9 and 10 between two adjacent fuel assemblies 2 are larger than the gap 8 between the boxes 2a-2d, the neutron flux is somewhat greater nearest the gaps 9 and 10, and thus also the power development. By re-arrangement of the boxes containing the fuel rods or transfer of fuel rod bundles between boxes, as shown by the arrows 11 in FIG. 1, an uneven burn-up may be prevented. As shown, the boxes 2a and 2c change positions and the boxes 2b and 2d change positions. One further possibility of achieving an equalization of the neutron flux, the power development and the burn-up is to replace a fuel rod with an empty tube 7 (see FIG. 3) through which the cooling water and the moderator water pass freely. A rod may be placed in the centre of the fuel assembly, which rod connects and retains the bottom portion 4 and the top unit 5 of the fuel assembly. In a design of a fuel assembly in which the four boxes of the assembly are connected to each other, the fuel assembly may comprise a cruciform temporary absorber rod 12 or individual absorber plates with a burnable absorber which may be removed after a certain time in connection with fuel transfer or refuelling. The bottom portion of the fuel assembly is preferably formed with a cruciform divider 13 which extends down into or through a throttle opening in the bottom portion 4 and the core bottom orifice. The bottom portion 4 is thus divided into four throttled channels, which ensures that the water volume through the four boxes becomes equal and stable even if the flow resistance in the boxes should be different. By the division of a fuel assembly box into four smaller ones, a larger box surface is obtained. However, because the dimensions are smaller, the bending stresses will be reduced to such an extent that the thickness of the metallic sheet material from which the boxes are made may be reduced by more than 50 percent. The amount of box material is considerably reduced, and thus also the undesired neutron absorption. At the same time, a larger space is obtained for a steam-free water volume and a more even distribution thereof.