Patent Number: 043483550
Section: summary

TECHNICAL FIELD This invention relates to a fuel assembly for the core of a boiling water nuclear reactor. PRIOR ART The most common fuel assembly used today in boiling water nuclear reactor cores comprises a fuel rod bundle with sixty-four elongated fuel rods enclosed in an elongated box of square cross-section which, in use of the fuel assembly, has its longitudinal axis substantially vertical and controls the flow of cooling water past the fuel rods. The fuel rods consist of fuel pellets in sheathing tubes, and the box of the assembly has a bottom portion, intended to be connected to an opening in a supporting plate in the core, and a top portion, intended to support against a grid in the core. Fuel assemblies of this kind are described in more detail in, for example, U.S. Pat. No. 3,697,375. In use of these known fuel assemblies, because of the flow resistance in the box, the cooling water has to be forced under pressure through the box along the bundle of fuel rods. Consequently, an internal overpressure exists, which gives rise to a bending stress in the plane sides of the box. Because of this bending stress at elevated temperature and simultaneous neutron radiation, an inconvenient permanent deformation may arise. To be able to restrict this deformation to within reasonable limits, thick metallic sheet material must be used for the box. In a box for an assembly of sixty-four fuel rods, the thickness of the metallic sheet material is usually about 3 mm. The large cross-section of the fuel bundle results in a variation of the neutron flux because this varies with the steam content of the cooling water, so that the neutron flux becomes lowest at the centre of the cross-section. One aim of the present invention is to provide a fuel assembly having a more even neutron flux over the cross-section of a fuel rod bundle so that a more even power distribution and burn-up may be obtained, thus making it possible to use fuel with substantially the same degree of enrichment in all the fuel rods within a fuel rod bundle. Another aim is to reduce the amount of constructional material in the boxes which surround the fuel rod bundles, thus reducing the amount of neutron-absorbing material in the core. The fuel assembly according to the invention may be used in existing reactor cores, which is particularly valuable, and it offers the possibility of reducing the fuel cost and facilitates the administration and the handling of fuel rods, since fuel of only one degree of enrichment, or fewer degrees of enrichment than previously, need be used. DISCLOSURE OF INVENTION According to the invention, a fuel assembly for the core of a nuclear reactor comprises a plurality of elongated boxes, each surrounding a respective bundle of elongated fuel rods, and first and second end portions receiving opposite ends respectively of the plurality of boxes and holding the boxes in substantially parallel spaced relationship, which end portions are intended to be supported in the core. In a fuel assembly in accordance with the invention, the boxes may be detachably joined to the first and second end portions. It is then possible to rearrange the boxes, together with their fuel rod bundles, within the fuel assembly, or to rotate the boxes with their fuel rod bundles, but allow the boxes to maintain their positions within the fuel assembly. Alternatively, the fuel assembly may be constructed with the boxes fixedly connected to the first and second end portions, but with the fuel rod bundles withdrawable from the boxes to enable rearrangement within the fuel assembly. From the point of view of strength, it may be suitable to make the boxes with corners of large radius. Fuel rods having a smaller diameter than the other rods in the bundle are then suitably inserted in the corner positions. A further possibility of improving the moderating effect, so as to achieve a more even neutron flux and power development within the fuel rod bundles, is to replace a fuel rod at the inner part of a fuel rod bundle with an empty fuel rod tube with free through-flow for cooling and moderator water. To prevent oscillations or vibrations, the boxes of the fuel assembly may be connected to a connecting element at one or more places between the first and second end portions.