Patent Number: 
Section: description

Embodiments of the present invention are described in detail hereinafter with reference to the accompanying drawings. (Embodiment 1) A fuel assembly according to the first embodiment of the present invention is described with reference to FIGS. 1 and 2. The fuel assembly of this embodiment is loaded into a reactor core wherein a water gap width on a control rod side (control rod-side water gap width) and that on a side opposite to the control rod side (opposite-side water gap width) are almost equal to each other. FIG. 1 is a cross sectional view of the fuel assembly and FIG. 2 is a schematic longitudinal sectional view thereof. As shown in FIG. 2, the fuel assembly has a fuel bundle (without a symbol), an upper tie plate 14, a lower tie plate 15 and a channel box 13. The fuel bundle has a plurality of fuel rods 10, one water rod 21 (not shown in FIG. 1) and a plurality of spacers 16. The channel box 13 has a square pipe shape and covers the fuel bundle from the outside. The upper tie plate 14 and the lower tie plate 15 hold upper end portions and lower end portions of the fuel rods 10, respectively. The spacers 16 are disposed axially at predetermined certain intervals for holding spaces between the fuel rods 10. As shown in FIG. 1, ninety-one fuel rods 10 are arranged in a square lattice of 10 rows by 10 columns (10xc3x9710) and one water rod 21 of a square pipe shape is disposed in a central region of 3 rows by 3 columns (3xc3x973). Nine fuel rods 10 can be disposed in this central region. Each fuel rod 10 has a zircalloy clad tube packed with nuclear fuel pellets formed by a dioxide of enriched uranium. As shown in FIG. 1, if the fuel assembly is divided into a control rod side and a side (anti-control rod side) opposite to the control rod side by a diagonal line 13a, the water rod 21 is shifted toward the control rod side. In other words, a center of the water rod 21 is shifted toward the one where the control rod 24 is inserted, away from a cross sectional center of the fuel assembly. As shown in FIG. 2, the channel box 13 is fixed to the fuel bundle by fixing a channel fastener 17 to a corner post 18 that is attached to the upper tie plate 14 at the one corner where the control rod 24 is inserted. Thus, the aforementioned control rod side corresponds to the channel fastener side or the corner post side. In this embodiment, since the water rod 21 is shifted toward the control rod side (channel fastener side, corner post side), thermal neutron flux near the control rod 24 increases and hence it is possible to enhance the control rod worth. Therefore, in comparison with a case where the water rod is disposed at the center of the fuel assembly or shifted toward a side opposite the control rod side, the reactor shutdown margin can be increased while attaining higher burnup of the fuel assembly. As a result, it is possible to improve the fuel economy and decrease the amount of spent fuel. (Embodiment 2) A fuel assembly according to the second embodiment of the present invention is described with reference to FIG. 3. FIG. 3 is a cross sectional view of the fuel assembly. This second embodiment is different from the first embodiment in that two types of fuel rods are used that have different active fuel lengths. The active fuel length is the length of the portion of the fuel rod packed with nuclear fuel pellets. More specifically, one type of fuel rod is a long-length (full-length) fuel rod 11 having a relatively large active fuel length and the other type of fuel rod is a short-length (part-length) fuel rod 12 having an active fuel length about {fraction (15/24)} that of the long-length fuel rod 11. As shown in FIG. 3, eight short-length fuel rods 12 are disposed in the second layer from the outside of the fuel assembly. One of them is disposed on the control rod side and five are disposed on the side opposite to the anti-control rod side. In this embodiment, it is possible to increase the reactor shut-down margin as in the first embodiment. In addition, this embodiment attains the following effect. The short-length fuel rods generally contribute to a flattening of the moderator (water) distribution in an axial direction of the fuel assembly. In this embodiment, since the short-length fuel rods 12 are disposed in a larger number on the side opposite to the control rod side than on the control rod side, it is also possible to flatten the moderator distribution in a cross section of the fuel assembly. These effects contribute to flattening of the local power distribution and to a decrease in the rise of reactivity when the reactor is in a cold shut-down condition. As a result, the thermal margin can be increased. (Embodiment 3) A fuel assembly according to the third embodiment of the present invention is described with reference to FIG. 4. FIG. 4 is a cross sectional view of the fuel assembly. In this embodiment, short-length fuel rods 12, arranged separately in the second embodiment, are concentrated around a water rod 21 of a square pipe shape. More specifically, seven short-length fuel rods 12 are disposed at positions adjacent to the water rod 21 on the side opposite to the control rod side. Five of the short-length fuel rods 12 are disposed in a half area on the side opposite to the control rod side with respect to the diagonal line 13a.  In this embodiment, an increase in the reactor shut-down margin and an increase in the thermal margin by flattening the local power distribution can also be attained as in the second embodiment. In addition, in this embodiment, a satisfactory moderation of neutrons is attained independently of the void fraction of the water (moderator) in the channel box 13 like a case that a cross sectional area of the water rod 21 increases effectively. Therefore, it is also possible to decrease an absolute value of the void coefficient. (Embodiment 4) A fuel assembly according to the fourth embodiment of the present invention is described with reference to FIG. 5. FIG. 5 is a cross sectional view of the fuel assembly. In this embodiment, which is a modification of the second embodiment, a certain improvement is made with respect to distribution of an average uranium enrichment (hereinafter referred to simply as xe2x80x9cenrichmentxe2x80x9d) of the fuel rods. More specifically, four types of long-length fuel rods with different enrichment are used, which include a fuel rod 1 of about 5 wt % (highest) enrichment, a fuel rod 2 of about 4 wt% enrichment, a fuel rod 3 of about 3 wt % enrichment, and a fuel rod 4 of about 2 wt % (lowest) enrichment. A fuel rod 1a can be the same short-length fuel rod as in the second embodiment and its enrichment is about 5 wt % (highest). Other constructional points are the same as in the second embodiment and therefore explanations thereof are omitted here. As shown in FIG. 5, the fuel rods 4 of the lowest enrichment are disposed at four corners of the outermost layer and the fuel rods 3 of the second lowest enrichment are disposed at positions close to the corners in the outermost layer. Fuel rods 2 of the second highest enrichment are disposed at positions adjacent to the water rod 21 in the row or column direction (vertical or transverse direction in FIG. 5). Further, fuel rods 1 of the highest enrichment are disposed at positions adjacent obliquely to the water rod 21. In a cross section perpendicular to an axis of the fuel assembly, the average enrichment of the fuel rods in one half area (hereinafter referred to as xe2x80x9copposite the control rod side areaxe2x80x9d) on the opposite the control rod side that is divided by the diagonal line 13a is higher than that of the fuel rods in the other half area (hereinafter referred to as the xe2x80x9ccontrol rod side areaxe2x80x9d) on the control rod side. In this embodiment, the same effect as in the second embodiment can be obtained. In addition, in this embodiment, since the average enrichment in the opposite to the control rod side area, where thermal neutron flux is relatively low, is set higher than that in said the control rod side area, the local power distribution can be flattened more effectively. (Embodiment 5) A fuel assembly according to the fifth embodiment of the present invention is described with reference to FIG. 6. FIG. 6 is a cross sectional view of the fuel assembly. In this embodiment, which is a modification of the fourth embodiment, a certain improvement is made with respect to an arrangement of gadolinia-filled fuel rods (hereinafter called xe2x80x9cGd fuel rodsxe2x80x9d). Gadolinia is one of burnable absorber. The Gd fuel rod 9 has an average uranium enrichment of about 4 wt % and an average gadolinia concentration of about 5 wt %. Sixteen Gd fuel rods 9 are arranged in the fuel assembly. Ten of them are disposed in the anti-control rod side area and six are disposed in the control rod side area. In the second layer from the outside of the fuel assembly, the Gd fuel rods 9 are disposed at eight positions adjacent to the fuel rods 1a (the short-length fuel rods of the highest enrichment) located at corner positions. Other constructional points are the same as in the second embodiment and therefore explanations thereof are omitted here. This embodiment also brings about the same effect as in the fourth embodiment. In addition, in this embodiment, since the Gd fuel rods are disposed in a larger number in the opposite the control rod side area, a larger number of neutrons are absorbed in the opposite the control rod side area than in the control rod side area. As a result, the thermal neutron flux in the control rod side area can be increased relatively and hence it is possible to enhance the control rod worth and increase the reactor shut-down margin in comparison with the fourth embodiment. (Embodiment 6) A fuel assembly according to the sixth embodiment of the present invention is described with reference to FIG. 7. FIG. 7 is a cross sectional view of the fuel assembly. In this embodiment, one cylindrical water rod 22 is disposed in the 33 central region instead of the water rod 21 in the third embodiment shown in FIG. 4. A cross sectional area of the water rod 22 is smaller than that of the water rod 21. Other constructional points are the same as in the third embodiment and therefore explanations thereof are omitted here. This embodiment also brings about the same effect as in the third embodiment. In addition, in this embodiment, since the cross sectional area of the water rod is set smaller than that in the third embodiment, wasteful absorption of neutrons by the water rod when the reactor is in a hot operating condition can be reduced. Therefore, it is possible to improve the neutron economy more than in the third embodiment. (Embodiment 7) A fuel assembly according to the seventh embodiment of the present invention is described with reference to FIG. 8. FIG. 8 is a cross sectional view of the fuel assembly. In this embodiment, which is a modification of the first embodiment shown in FIG. 1, the number of fuel rods is increased for the purpose of attaining higher burnup than in the first embodiment. That is, one hundred and five fuel rods 11 are arranged in a square lattice of 11 rows by 11 columns (11xc3x9711) and one water rod 23 of a square pipe shape is disposed in a central region of 4 rows by 4 columns (4xc3x974). Sixteen fuel rods can be disposed in this central region. As shown in FIG. 8, if the fuel assembly is divided into the control rod side and the opposite the control rod, the water rod 23 is shifted toward the control rod side. Therefore, this embodiment also brings about the same effect as in the first embodiment. Although one water rod is used in the above embodiments, there also may be used a plurality of water rods. In this case, if the water rods are shifted toward one corner where a control rod is inserted from a cross sectional center of the fuel assembly, the same effects as in the above embodiments can be obtained. Further, although enriched uranium is used as the nuclear fuel in the above embodiments, there also may be used a nuclear fuel obtained by replacing a portion or the whole of enriched uranium with plutonium-enriched uranium. In this case, the same effects as in the above embodiments can be obtained.