Patent Number: 040615367
Section: summary

The present invention relates to a fuel assembly for a nuclear reactor and, more particularly, to a fuel assembly which minimizes the amount of parasitic structural material therein, yet provides a rigid support means for the fuel elements, together with the ability to accommodate other necessary functions of a fuel assembly. The reactivity and, in turn, the power output of a nuclear reactor is limited by the amount of structural material in the reactor core, as the structural material parasitically absorbs neutrons which could otherwise be used in the fission process. One general structural form commonly used for providing a nuclear fuel inventory in nuclear reactors is that in which a plurality of elongated nuclear fuel elements or rods containing the nuclear fuel material are arranged, within a prescribed volume, in a parallel array in an upstanding direction between the upper and lower reactor core plates. To provide integrity in the support relations, the fuel rods are divided into groups; and the rods in each group are formed as a fuel assembly prior to placement between the reactor core plates. A fluid, having coolant and, if desired, neutron moderating properties, flows longitudinally along and among the fuel rods as a vehicle for energy transfer. Since the fuel assembly became longer with the development of larger nuclear reactors, a can enclosing the fuel rods was used for structural support of the latter. The can, however, has the effect of poisons within the critical region of the reactor core, because the can absorbed neutrons. Furthermore, structural material, such as the above mentioned can, increases the pressure drop through the reactor core, allows by pass coolant flow between cans thereby reducing the effective heat transfer within the reactor core, provides an unheated surface which also may influence the heat transfer within the reactor core, requires a greater spacing between fuel rods of adjacent fuel assemblies than the spacing between fuel rods within the same fuel assembly, and restricts lateral coolant flow mixing between the fuel assemblies of the reactor core. In addition, the can to some extent increases the weight of the fuel assembly, necessitating a heavier core support structure as a considerable number of fuel assemblies are utilized in a reactor core. Accordingly, it is the general object of this invention to provide a new and improved nuclear fuel assembly. Another general object of this invention is to provide a fuel assembly which serves to increase the power output of a reactor core. Another object of this invention is to provide a fuel assembly which contains a minimum of parasitic structural material. Still another object of this invention is to provide a simplified structure for supporting the fuel rods in the fuel assembly. A further object of this invention is to provide a fuel element assembly which accomplishes other necessary functions of a fuel assembly in addition to its function of supporting a plurality of fuel elements. A still further object of this invention is to provide a fuel assembly which serves to reduce the pressure drop through the reactor core. Yet another object of this invention is to provide a fuel assembly which permits the same spacing between fuel elements of adjacent fuel assemblies as the spacing between fuel elements within the fuel assembly so that the reactor core approaches complete homogeneity. A still further object of this invention is to provide a fuel assembly wherein the fuel elements add to the overall stiffness of the fuel assembly. Briefly, the present invention accomplishes the above cited objects by providing an elongated support means in the fuel assembly to vertically support the fuel elements or rods and also to position a plurality of longitudinally spaced grids which extend across and are secured to the elongated support means. The support means comprises a plurality of parallel elongated members or thimbles which also serve as guide tubes for the control rods and are interspersed among and parallel to the fuel rods to extend through the grids. In addition, perforated end plates are secured to the outer end portions of the thimbles. The fuel rods, in turn, extend substantially from one end plate to the other end plate in a parallel array through the remaining openings in the grids and are vertically supported by the bottom end portion of the support means. Each grid has means for laterally positioning the fuel rods and also may have mixing vanes for laterally mixing the longitudinally flowing coolant. More specifically, the nuclear fuel assembly comprises an open elongated support means, a plurality of grids and a plurality of parallel fuel rods. The elongated support means, in turn, comprises a plurality of laterally spaced control rod guide tubes or thimbles having their end portions secured to the transverse perforated end plates. The longitudinally spaced grids extend transversely of and are secured to the thimbles thereby adding rigidity to the entire support means. The fuel rods extend through the grid openings in a parallel array, with the thimbles being parallel to and interspersed among the fuel rods. The fuel rods are disposed between the end plates and are vertically supported by the bottom end plates. The openings in the grid member are formed by a plurality of straps, which are interfitted to provide a structural network, similar to an "egg crate." The grid straps, in turn, are provided with at least resilient means for laterally engaging and positioning the fuel rods. Furthermore, the overall stiffness of the fuel assembly is increased when the fuel rods are in place because of the friction generated between the fuel rods and the lateral supporting means on the grid. The grids can also be further provided with mixing vanes which protrude into adjacent flow channels to laterally deflect the longitudinal coolant flow. In this embodiment the thimbles can also be used for instrumentation purposes, irradiation of materials or for receiving and guiding neutron absorbing control rods. Also, coolant cross flow between fuel assemblies is now freely possible, because the elongated transversely spaced thimbles are utilized to give ridigity to the fuel assembly in lieu of elongated enclosures or cans which completely surround the fuel rods contained within a single fuel assembly. The absence of the outer peripheral can also permits closer spacing between peripheral fuel rods of adjacent assemblies.