1. Field of the Invention
This invention relates to a nuclear fuel assembly and to the core of a reactor and, more particularly, to a fuel assembly and to a reactor core of a boiling water reactor.
2. Description of the Prior Art
The fuel assembly used in a boiling water reactor typically includes fuel rods, water rods and upper and lower tie plates for supporting the upper and lower end portions of the fuel rods and water rods. The water rods are disposed among the fuel rods. The fuel rods comprise fuel rods containing a burnable poison (such as gadolinia (Gd.sub.2 O.sub.3)) and fuel rods not containing it.
In a boiling water reactor, because the void fraction (ratio of steam volume to total volume) varies over the core height, the power distribution in the axial direction of the core is low at an upper portion and high at a lower portion. Therefore, a burnable poison is used not only to restrict an excessive reaction rate but also to control the power distribution in the axial direction of the core. Examples of such use of a burnable poison are represented by the fuel assemblies shown in FIGS. 2A and 2B and FIGS. 3A and 3B of U.S. Pat. No. 4,587,090. The first of these includes fuel rods whose gadolinia concentration (4.5 wt %) in the axial direction is uniform and fuel rods which are divided into the upper and lower regions at the position of 11/24 from the lower end of their effective fuel length and whose gadolinia concentration is higher in the lower region (4.5 wt %) than in the upper region (3.5 wt %). The second of these fuel assemblies includes fuel rods of the latter, divided type and fuel rods which contain gadolinia (4.5 wt %) in the upper region and do not contain any gadolinia in the lower region.
Furthermore, the fuel assembly shown in FIG. 1 of Japanese Laid-Open Patent Specification No. 149588/1988 includes two kinds of fuel rods containing burnable poison. These fuel rods comprise fuel rods having a low gadolinia concentration and fuel rods having a high gadolinia concentration. In each gadolinia-containing fuel rod, gadolinia is distributed uniformly in the axial direction. These fuel rods, including those which do not contain gadolinia, are disposed in a rectangular array. In other words, each fuel rod is positioned at the point of intersection of a rectangular grid. In such a structure, the fuel rods having a low gadolinia concentration and the fuel rods having a high gadolinia concentration are disposed adjacent to one another in the rows and columns of the array, which are parallel to the side surfaces of the lower tie plate (i.e. in an X direction or in a Y direction orthogonal to the X direction).
Mention is also made of Japanese Laid-Open Patent Specification No. 217186/1987 referred to below.
In the boiling water reactor having in its core the fuel assembly shown in FIG. 2A (using the fule rods shown in FIG. 2B) or the fuel assembly shown in its FIG. 3A (using the fuel rods shown in FIG. 3B) of U.S. Pat. No. 4,587,090, the axial power peaking is high at the initial stage of an operation cycle. Axial power peaking describes the peak value of power at one axial location of the core, i.e. deviation from a uniform power distribution. Particularly when the latter fuel assembly (FIGS. 3A and 3B), in which the number of gadolinia-containing fuel rods is smaller in the lower region than in the upper region, is employed, the axial power peaking is very high at the initial stage of the operation cycle. The operation of the core is conducted at a high average void fraction with axially skewed power distribution in the first half of the operation cycle, and the average void fraction becomes low since the power of the lower region, where burn-up has proceeded, becomes low at the final stage of the operation cycle. A spectral shift effect occurs by changing in this manner the average void fraction during the operation cycle.
In the fuel assembly described in Japanese Laid-Open Patent Specification No. 149588/1988 with fuel rods having a low gadolinia concentration and fuel rods having a high gadolinia concentration, the gadolinia concentration of the former is below 60% of that of the latter. As a result of studies, the present inventors have found that in such a fuel assembly, the axial power peaking at the initial stage of the operation cycle (0 to about 2 GWd/st) reduces more than in the two fuel assemblies disclosed in U.S. Pat. No. 4,587,090.