Patent Number: 053717694
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel rod support grid in which straps and springs are made of different materials. 2. Technological Background A fuel assembly of the following construction is generally known. In a typical fuel assembly, a plurality of support grids are disposed at a predetermined spacing between the upper nozzle and the lower nozzle. A grid comprises a plurality of straps intersecting at right angles to each other thereby defining a plurality of grid cells. In the grid cells are disposed a control rod guide tube and instrument tubes which are retained by the grid and whose ends are attached to the upper and lower nozzles. A plurality of fuel rods pass through the grid cells, in which each rod is elastically retained in the grid cell by the springs joined to the straps of the grids. In the conventional technology presented above, the material of construction of the grid is Inconel (nickel alloys for use in nuclear reactors) which provide excellent elastic properties. However, it is desirable to make the grid with zircalloy (zirconium alloys), in view of improving the neutron economy and lowering the exposure to neutron irradiation. However, the use of zircalloy alloys for the springs presented problems in that this material, which is inherently not highly elastic, is prone to degradation after neutron irradiation. Therefore, zircalloy springs are inferior in performance to Inconel springs. A solution to the above problem which has been under consideration is to use zircalloy for the straps, and Inconel for the springs. A problem in this combination of different materials for making a grid is that when the straps and springs are made of different materials, welding cannot be used for joining the straps to springs. Therefore, there has been an outstanding need to develop a grid configuration in which the springs are firmly joined to the straps, the joint structure is relatively simple and grid manufacturing is facilitated. SUMMARY OF THE INVENTION The present invention relates to a fuel rod support grid (hereinafter referred to as grid) and an objective is to present a grid configuration of a relatively simple structure which can be manufactured readily, so that the straps are engaged firmly with the springs, the neutron economy is improved, the exposure to neutron irradiation is reduced, and the elastic characteristics of the springs are maintained during the service life. Another objective of the present invention is to present a grid configuration which enables welding to be applied to join the same metals even though the straps and the springs are made from different metals. Yet another objective of the present invention is to present a grid having a structure which prevents thermal effects of welding of the spring frame members to adversely affect the performance of the straps. According to an aspect of the present invention, there is provided a fuel rod support grid for nuclear fuel assembly, the grid defining grid cells for holding fuel rods, said grid comprising: a plurality of straps intersecting each other thereby constituting a grid, the straps having a plurality of bosses; a plurality of spring frame members engaged with the straps and intersecting each other thereby constituting a grid, said spring frame members having a plurality of spring sections disposed in a direction perpendicular to the longitudinal direction of the fuel rods; wherein each of the fuel rods is resiliently held in each of the grid cell by the bosses and said spring sections. In the grid of such a configuration, fuel rods are retained in the grid cells with mutually crossing spring frame members engaged with the straps in the horizontal direction. The spring frame members themselves support each other firmly and the mutually orthogonal spring frame members are attached firmly to the straps. The grid cell configuration of the present invention thus allows engaging of the spring frame members smoothly but firmly with the straps. Furthermore, since the straps and the spring frame members are made of different materials, it provides advantages in neutron economy and lowering of neutron exposure, thereby enabling the maintaining of the spring properties for an extended period of time. The construction is relatively simple, and thus facilitates the manufacturing of the grids.