Patent Number: 
Section: description

In the following, preferred embodiments of the present invention will be described. Embodiment p1 FIG. 1A is, in a first embodiment of a fuel assembly of the present invention, a transversal sectional view showing an arrangement of fuel rods 12 and 13 with a control rod 11 in an upper left position. The fuel rods 12 that contain no gadolinia are shown with reference marks 1, 2, 3, 4, and 5 for individual types, the gadolinia fuel rods 13 being shown with G1, G2, and G3, respectively. Furthermore, WR denotes a water rod 14. FIG. 1B, in the fuel assembly of the first embodiment, is a diagram showing enrichment distributions of fissile material and concentration distributions of gadolinia, in a vertical direction (axial direction) of the fuel rods (1, 2, 3, 4, 5 G1, G2, and G3), respectively. Furthermore, in the present embodiment, the isotopic composition of gadolinium constituting gadolinia is shown in Table 1. In this embodiment, the respective isotopic compositions of Gd-155 and Gd-157 are 30 wt % and 50 wt %, and both isotopes are enriched more than the natural isotopic abundance of the natural gadolinium. A particular isotope of gadolinium can be enriched by means of atomic vapor laser isotope separation (AVLIS). The fuel rods (1, 2, 3, 4, 5, G1, G2, and G3) have a plurality of segments of different enrichments of a fissile material (for instance, uranium). The uranium enrichments in the respective segments are in the decreasing order of A1 greater than B1 greater than C1 greater than D1 greater than E1 greater than F1 greater than G. In the case of uranium being used as the fissile material, an enrichment of uranium is that of uranium 235. G denotes the enrichment of the fissile material in uranium, that is 0.2 wt %. In addition, the enrichment D1 is approximately identical with the enrichment of the fissile material averaged over an entire fuel assembly. All fuel rods (1, 2 and 5) that contain no gadolinia, including the segments of the upper and lower ends that contain the natural uranium alone, are divided into three segments in the axial direction, the fuel rods (3 and 4) are divided into five segments. As shown in FIG. 1A, all the fuel rods of which uranium enrichment is the highest (1) and the fuel rods of which enrichment is the second highest (2) are not disposed on the outermost periphery of the fuel assembly. There are three kinds of G1, G2 and G3 of the gadolinia fuel rods 13 and the upper and lower ends thereof are provided with the natural uranium segments that contain the natural uranium alone. The intermediary portion excluding the natural uranium segments at the upper and lower ends has a plurality of segments of different gadolinia concentrations. For the respective segments, three kinds of the gadolinia concentrations of a1 (2.5 wt %), b1 (2.0 wt %) and c1 (1.5 wt %) are applied and a difference of the gadolinia concentrations of adjacent segments is set at 0.5 wt % or 1.0 wt %. The average gadolinia concentrations of the respective gadolinia fuel rods are 2.2 wt % for G1, 2.0 wt % for G2 and 1.8 wt % for G3. The uranium enrichments for the respective segments are all D1. The operation term M per one cycle assumed for the fuel assembly is 15 months and the power density P is 50 kw/l. In the present embodiment, the gadolinia concentration G0 averaged over the entire gadolinia fuel rod is approximately 2.06 wt %, being less than 2.3, the value obtained from 0.25xc2x7Pxc2x7M/W (=0.25xc3x9750xc3x9715/(50+30)). That is, the expression (G0 less than 0.25xc2x7Pxc2x7M/W) holds and the gadolinia concentration is set so that gadolinia almost burns out at the cycle end. FIG. 2 shows, in the first embodiment of the present invention, variations of the number densities of the gadolinium isotopes accompanying the burnup. At the specific burnup of 12 GWd/t corresponding to the cycle end, Gd-155 and Gd-157 reach their equilibrium concentrations, respectively. That is, the gadolinia concentration is found to be appropriate to be essentially burned out at the cycle end. When the gadolinia concentration averaged over the entire bundle is 2.3 wt % or less, the gadolinia burns out. However, when the gadolinia concentration is more than 2.3 wt %, the residual reactivity of gadolinia at the cycle end increases drastically. As shown in the above, in the fuel assembly of the first embodiment, gadolinium oxide (enriched gadolinia) in which Gd-155 and Gd-157 each are enriched more than the isotopic abundance of the natural gadolinium is used. As a result, the gadolinia concentration can be lowered to one half or less that of the fuel assembly using the natural gadolinia. Accordingly, the thermal conductivity of the gadolinia fuel rod can be remarkably improved. Furthermore, that Gd-155 and Gd-157 are enriched more than the isotopic abundance of the natural gadolinium accompanies a decrease of the isotopic abundance of Gd-156, resulting in a decrease of poison reactivity due to the residual gadolinium. As a result, in comparison with the fuel assembly in which the natural gadolinia is used, a necessary enrichment of the fissile material can be decreased. The specific burnup is increased in the case of the enrichment of the fissile material being set the same. Furthermore, in the fuel assembly of the first embodiment, in the fuel rods that contain no gadolinia, the average enrichment of the fissile material is larger in the upper or lower portion than in the central portion. In addition, the gadolinia concentration in the lower portion of the gadolinia fuel rod is larger than that in the upper portion. Accordingly, these conditions are well suited for a core of a boiling water nuclear reactor of which power shows the lower peak, resulting in a decrease of the power peaking. In the first embodiment, the isotopic compositions of Gd-155 and Gd-157 are 30 wt % and 50 wt %, respectively. However, when Gd-155 and Gd-157 each is enriched more than the isotopic abundance of the natural gadolinium, the identical effect can be obtained. In that case, the gadolinia concentration G0 may be set to satisfy G0 less than 0.25xc2x7Pxc2x7M/W. Embodiment 2 FIG. 3A is, in a second embodiment of a fuel assembly of the present invention, a sectional view in a transversal direction showing an arrangement of the fuel rods with the control rod 11 in the upper left position. Reference marks 1, 2, 3, 4, and 5 denote the fuel rods that contain no gadolinia and reference marks G1, G2 and G3 denotes the gadolinia fuel rods, respectively. WR denotes the water rod 14. FIG. 3B is, in the second embodiment, a diagram showing enrichment distributions of the fissile material (uranium) and gadolinia concentration distributions, in an axial direction of the fuel rods (1, 2, 3, 4, 5, G1, G2 and G3). In this embodiment, the isotopic composition of gadolinium constituting gadolinia is shown in Table 2. As shown in table 2, only Gd-157 is enriched more than in the natural gadolinium and the isotopic composition of Gd-155 is lowered less than that in the natural gadolinium. In FIG. 3B, A2 through F2 each denotes the enrichment of uranium that is the fissile material and is in the decreasing order of A2 greater than B2 greater than C2 greater than D2 greater than E2 greater than F2 greater than G. G denotes the enrichment of uranium in the natural uranium, that is, 0.2 wt %. Furthermore, a2 through c2 each shows the gadolinia concentration, and a2 is 2.7 wt %, b2 2.2 wt %, c2 1.7 wt %. The average gadolinia concentrations of the respective gadolinia fuel rods are 2.4 wt % for G1, 2.2 wt % for G2 and 2.0 wt % for G3. In this embodiment, when the operation cycle term is 15 month and the power density is 50 kw/l, the gadolinia concentration G0 averaged over the entire fuel rods is less than the value of 2.9 that is obtained from 0.25xc2x7Pxc2x7M/W (=0.25xc3x9750xc3x9715/(60+5)). That is, the expression G0 less than 0.25xc2x7Pxc2x7M/W holds. Furthermore, in the present embodiment, a ratio of the content of Gd-155 to that of Gd-157 is one twelfth, that is, less than 0.1. As described above, Gd-155 has the neutron absorption cross section of approximately one fourth that of Gd-157 to be delayed in burnup with respect to Gd-157. Accordingly, there occurs a problem that Gd-155 remains after the burnup to cause the residual reactivity. However, by controlling Gd-155 at lower concentrations than that of Gd-157, the specific reactivity at which the both burn out can be approached. The variation of the number densities of the respective gadolinium isotopes with the burnup in the second embodiment is shown in FIG. 4. The specific reactivity at which the gadolinium isotopes establish equilibrium concentrations is approximately 20 GWd/t for Gd-155 and approximately 18 GWd/t for Gd-157. Both are found to burn out at the substantially same specific reactivity. FIG. 5 shows the variation of the respective gadolinium isotopes with the burnup when the natural gadolinium is used. From the figure, when the natural gadolinium is used, Gd-155 is found to delay largely in the burnup. As mentioned above, in the fuel assembly of the second embodiment, by decreasing the content (isotopic composition) of Gd-155 to one tenth or less that of Gd-157, the specific reactivity at which Gd-155 and Gd-157 burn out is made approximately the same and the residual reactivity of Gd-155 is decreased. Next, the third to seventh embodiments of the present invention will be explained. In the third to seventh embodiments, the fuel rods (1, 2, 3, 4, 5, G1, G2 and G3) are arranged in the periphery of the water rod WR, as shown in FIG. 1A that is a transversal sectional view of the first embodiment. Embodiment 3 In the third embodiment, the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia and the gadolinia fuel rods (G1, G2 and G3) have the distributions in the axial direction of the enrichment of the fissile material (uranium) and the gadolinia concentration, as shown in FIG. 6. In the figure, A3 through F3 denote the enrichments of uranium and are in the decreasing order of A3 greater than B3 greater than C3 greater than D3 greater than E3 greater than F3 greater than G. G denotes the enrichment of uranium in the natural uranium, that is, 0.2 wt %. Furthermore, a3 through d3 each denotes the gadolinia concentrations, a3 being 2.5 wt %, b3 2.0 wt %, c3 1.5 wt % and d3 1.0 wt %. Gadolinium constituting gadolinia, as identical with the first embodiment, has the isotopic composition shown in Table 1. Thus, in the fuel assembly of the third embodiment, a segment having the lowest gadolinia concentration of d3 is added to the upper end portion (just below the natural uranium segment) of the gadolinia fuel rod having the uranium enrichment and the gadolinia concentration shown in FIG. 1B. The uranium enrichment of the added segment is also D3 common to the entire gadolinia fuel rod. However, in FIG. 6, D3 in the added segment is omitted and only the gadolinia concentration is shown. The average gadolinia concentrations of the gadolinia fuel rods are 2.1 wt % for G1, 1.9 wt % for G2 and 1.7 wt % for G3. In general, in the segment close to the upper end of the fuel rod, the power becomes lower to delay the burnup, resulting in the gadolinia residues to cause the residual reactivity. However, in the third embodiment, the segment of which gadolinia concentration is the lowest is added to the upper end portion. Accordingly, the gadolinia residues at the upper end portion of the fuel rod can be reduced. Whereas in the present embodiment both Gd-155 and Gd-157 are enriched in comparison with the natural gadolinia, even in the fuel assembly containing gadolinium enriched only in Gd-157, the identical effect can be obtained. Embodiment 4 In the fourth embodiment, the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia and the gadolinia fuel rods (G1, G2 and G3) have the distributions in the axial direction of the enrichment of the fissile material (uranium) and the gadolinia concentration, as shown in FIG. 7. That is, in the gadolinia fuel rods (G1, G2 and G3), a segment having the lowest gadolinia concentration of d4 is further added to the lower end portion of the gadolinia fuel rod having the uranium enrichment and the gadolinia concentration shown in FIG. 6. The uranium enrichment of the added segment is also D4 common to the entire gadolinia fuel rod. Gadolinium constituting gadolinia has the isotopic composition shown in Table 1 identical with the first embodiment. The average gadolinia concentrations of the gadolinia fuel rods are 2.0 wt % for G1, 1.8 wt % for G2 and 1.6 wt % for G3. The other portions of the fuel assembly are configured similarly with the third embodiment and the explanation is omitted. In the fuel assembly of the fourth embodiment thus configured, the reactivity of the residual gadolinia can be further reduced. Embodiment 5 In the fifth embodiment, the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia and the gadolinia fuel rods (G1, G2 and G3) have the distributions in the axial direction of the enrichment of the fissile material (uranium) and the gadolinia concentration, as shown in FIG. 8. That is, in the gadolinia fuel rods (G2 and G3), an intermediate segment having the lowest gadolinia concentration of e5 (1.0 wt %) is added between the respective segments of the gadolinia fuel rod having the distribution of the uranium enrichment and the gadolinia concentration shown in FIG. 1B. Gadolinium constituting gadolinia has the isotopic composition shown in Table 1 similarly with the first embodiment. The uranium enrichment of the added intermediate segment is also D5 common to the entire gadolinia fuel rod. Furthermore, the added intermediate segment has a length of one twenty-fourth or less the effective length of the gadolinia fuel rod and the difference of the gadolinia concentration between the segments adjacent at least on one side is 0.5 wt % or more. The average gadolinia concentrations of the respective gadolinia fuel rods are 2.2 wt % for G1, 2.0 wt % for G2 and 1.8 wt % for G3. The other portions of the fuel assembly are configured similarly with the first embodiment, the explanation being omitted. In the fuel assembly of the fifth embodiment thus configured, a boundary position between the gadolinia concentrations in the gadolinia fuel rod can be accurately set. That is, the gadolinia concentration in the gadolinia fuel rod is non-destructively detected after molding the fuel by taking advantage of properties as magnetic body of gadolinia. In the gadolinia fuel rods (G2 and G3), by making the difference of the gadolinia concentration between the added intermediate segment and the segment adjacent on one side 0.5 wt % or more preferable to 0.5 wt %xcx9c1 wt %, the boundary portion can be detected with higher sensitivity. Due to such high sensitivity detection, the boundary of the gadolinia concentrations in the axial direction of the gadolinia fuel rod can be accurately determined and thermal margin to safety target can be largely secured. Embodiment 6 In the sixth embodiment, the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia and the gadolinia fuel rods (G1, G2 and G3) have the distributions in the axial direction of the enrichment of the fissile material (uranium) and the gadolinia concentration such as shown in FIG. 9. That is, in the gadolinia fuel rods (G1, G2 and G3), the natural uranium segment at the upper end has a length of three twenty-fifths or more the effective length of the gadolinia fuel rod and is longer than the natural uranium segment at the lower end. Furthermore, it is longer in length than that of the natural uranium segment at the upper and lower ends of the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia. The other portions are configured similarly with the first embodiment, the explanation being omitted. In general, in the neighborhood of the upper end portion of the fuel rod, due to the low power, gadolinia is likely to remain. However, in the sixth embodiment, the length of the natural uranium segment at the upper end of the gadolinia fuel rod is longer than that of the natural uranium segment at the lower end. Accordingly, the residual reactivity of gadolinia at the upper end portion can be effectively reduced. Embodiment 7 In the seventh embodiment, the fuel rods (1, 2, 3, 4, and 5) containing no gadolinia and the gadolinia fuel rods (G1, G2 and G3) have the axial direction distributions of the enrichment of the fissile material (uranium) and the gadolinia concentration such as shown in FIG. 10. In the figure, A7 through F7 denote the enrichments of uranium that is the fissile material, respectively. A7 is 4.9 wt % or less, B7 being 4.1 wt % and A7 through F7 are in the decreasing order of A7 greater than B7 greater than C7 greater than D7 greater than E7 greater than F7 greater than G. The uranium enrichment averaged over the fuel assembly is 3.7 wt %. Furthermore, in the present embodiment, gadolinium has the isotopic composition shown in Table 1 similarly with the first embodiment. The enrichment of uranium in the gadolinia fuel rods is 3.7 wt % or more, the enrichment averaged over the entire fuel assembly. Still further, the gadolinia concentration G0 averaged over the entire gadolinia fuel rods is set at a value (wt %) satisfying the inequality of G0 less than 0.25xc2x7Pxc2x7M/W. In general, when the natural gadolinia is used, the enrichment of uranium that is the fissile material in the gadolinia fuel rod is approximately the enrichment averaged over the entire fuel assembly. However, in the fuel assembly of the seventh embodiment, gadolinia (enriched gadolinia) in which Gd-155 and Gd-157 are enriched more than the isotopic abundance of the natural gadolinium is used. Accordingly, in comparison with the fuel assembly in which the natural gadolinia is used, the concentration of gadolinia can be set lower, the maximum value of the uranium enrichment being suppressed low. Furthermore, in the case of the highest uranium enrichment being not changed, the average enrichment can be made larger to result in an increase of the specific burnup and in an improvement of burnup efficiency of the fuel. In the above first to seventh embodiments, the fuel assembly of square grid pattern of eight-columns/eight-rows is employed to explain, however, it is similarly effective also in a fuel assembly having an arrangement of square grid pattern of nine-columns/nine-rows or more. Furthermore, the fuel assembly using only the fuel rod having gadolinium enriched in the isotopes of odd mass number more than the isotopic abundance of the natural gadolinium is used to explain. However, as a part of the fuel rods, the fuel rods having the natural gadolinia may be used. Next, other embodiments of the present invention will be described. Embodiment 8 FIG. 11A is, in an eighth embodiment of a fuel assembly of the present invention, a transversal sectional view showing an arrangement of fuel rods 12 and 13 with the control rod 11 in an upper left position. The fuel rods 12 that contain no gadolinia are shown with reference marks 1, 2, 3, 4, 5, 6, V1 and V2 for each types, gadolinia fuel rods 13 being shown with G1, G2, and G3, respectively. The fuel rods are consisted of the long-length fuel rods shown by the reference marks of 1, 2, 3, 4, 5, 6, G1, G2, and G3, and the short-length fuel rods shown by the reference marks of V1 and V2 of which fuel effective portion is shorter than that of the long-length fuel rod. In this embodiment, the fuel assembly is loaded with a different spacing from adjacent fuel assemblies in the Reactor core comprising D lattice. WR denotes a water rod 14. FIG. 11B is a diagram showing, in an axial direction of the fuel rods (1, 2, 3, 4, 5, 6, V1, V2, G1, G2, and G3), distributions of the enrichment of the fissile material (uranium) and concentration of gadolinia. As shown in this figure, A8 through F8 and G are used as the uranium enrichment, these being in the decreasing order of A8 greater than B8 greater than C8 greater than D8 greater than E8 greater than F8 greater than G. G denotes the enrichment of the natural uranium, approximately 0.2 wt %. The long-length fuel rod has the portions containing the natural uranium alone at the upper and lower ends, respectively. In addition, as the content of gadolinia, a8 through c8 and d8 are employed. Here, a8 through c8 are concentrations of the oxide of the natural gadolinium (natural gadolinia), d8 being the concentration of oxide of the enriched gadolinium (enriched gadolinia) having the isotopic composition shown in the Table 1. These gadolinia concentrations are in the decreasing order of a8 greater than b8 greater than c8 greater than d8, d8 being the value of one half or less that of a8 through c8. Furthermore, the gadolinia fuel rod (G1), excluding the upper and lower end portions constituted of the natural uranium alone, are divided into three segments in the axial direction, the gadolinia contents each being different. in the respective segments. In the eighth embodiment, the transversal section of the fuel assembly is divided by a diagonal line into two regions of a control rod side and an opposite-control rod side. At that time, for the fuel rods disposed in the region on the opposite-control rod side (for instance, the outermost periphery), the uranium enrichment is set at the highest A8. Whereas, for the fuel rods disposed in the region on the control rod side, the uranium enrichments of B8 through F8 that are lower than A8 are set. Furthermore, the gadolinia fuel rods (G1) disposed in the region on the opposite-control rod side contain the natural gadolinium, the uranium enrichment being set at the highest A8. Whereas, enriched gadolinium having the isotopic composition of 30 wt % of Gd-155, 50 wt % of Gd-157 and 20 wt % of the rest is contained in the gadolinia fuel rods (G2 and G3) disposed in the region on the control rod side. Since Gd-155 and Gd-157 are remarkably large in the neutron absorption cross section, the gadolinia content necessary for maintaining the same reactivity controllability can be less than that in the case of the natural gadolinium being used. In the eighth embodiment, the concentration d8 of the enriched gadolinia is largely decreased to be one half a8 through c8, the natural gadolinia concentration, or less. Accordingly, the uranium enrichments in the gadolinia fuel rods (G2 and G3) having enriched gadolinium can be made A8 and B8 that are larger in comparison with that of the existing fuel assembly. The uranium enrichment averaged over the fuel assembly (bundle) also can be increased from the existing value of 3.96 wt % to 4.03 wt % by approximately 0.08 wt %. due to an increase of the uranium enrichment averaged over he bundle as mentioned above, the number of fuel exchange in he fuel assembly can be decreased by approximately 1%, resulting in an improvement of fuel economy. Embodiment 9 FIG. 12A is, in the ninth embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 when the control rod 11 is disposed in the upper left position. Reference marks 1, 2, 3, 4, 5, 6, G1, G2 and G3 denote the long-length fuel rods, reference marks V1 and V2 the short-length fuel rods. Furthermore, the gadolinia fuel rods 13 are shown by G1, G2, G3 and V2. The short-length fuel rod V2 is the gadolinia fuel rod containing gadolinia. WR denotes the water rod 14. FIG. 12B is a diagram showing the distributions in the axial direction of the fuel rods (1, 2, 3, 4, 5, V1, V2, G1, G2, and G3) of the enrichment of the fissile material (uranium) and the concentration of gadolinia. As shown in the figure, the uranium enrichments of A9 through E9 and G are employed, their relationship being A9 greater than B9 greater than C9 greater than D9 greater than E9 greater than F9 greater than G. G denotes the enrichment of the natural uranium. In the long-length fuel rod, the portions containing the natural uranium alone are disposed at the upper and lower ends thereof. Furthermore, the gadolinia concentrations of a9 through c9 and d9 are employed, these being in the decreasing order of a9 greater than b9 greater than c9 greater than d9, d9 being one half or less of a9 through c9. Here, a9 through c9 are concentrations of the natural gadolinia, d9 being the concentration of the oxide of enriched gadolinium (enriched gadolinia) having the isotopic composition shown in the aforementioned Table 1. The gadolinia fuel rods (G1 and G3), excluding the upper and lower end portions containing only of the natural uranium, are divided in a plurality of segments in an axial direction. The gadolinia concentrations are different in the respective segments. As shown in the figure, in the ninth embodiment, at the positions of four corners of the fuel bundle disposed in the second position inwardly from the outer most periphery, the short-length gadolinia fuel rods V2 to which enriched gadolinia is added are disposed. The isotopic composition of the enriched gadolinium, as shown in Table 1, is 30 wt % for Gd-155, 50 wt % for Gd-157, and 20 wt % for the other isotopes. The concentration d9 of the enriched gadolinia in the short-length gadolinia fuel rods (V2) is one half or less the gadolinia concentrations (a9 through c9) in the gadolinia fuel rods (G1, G2 and G3) in which the natural gadolinia is used. Accordingly, the uranium enrichment in the gadolinia fuel rod (V2) can be A9 that is larger in comparison with that of the existing fuel assembly. Furthermore, in the ninth embodiment, at the positions of four corners of the second position inwardly from the outermost periphery, the gadolinia fuel rods having the enriched gadolinia are disposed. Accordingly, the power at the positions of four corners where the power tends to be high can be reduced, and thereby the uranium enrichment of these fuel rods can be increased. In the short-length gadolinia fuel rods, the power suppression effect due to gadolinia does not appear at the position other than the effective portion in the axial direction. However, since the power distribution in the axial direction is the highest at the lower portion where the effective portion of the short-length fuel rod exists, the aforementioned effect can be fully exhibited. By increasing the uranium enrichment of the fuel rods disposed at the four corners of the fuel bundle in the second position inwardly from the outermost periphery, the uranium enrichment averaged over the bundle can be increased from the existing value of 3.96 wt % to 3.98 wt % by approximately 0.02 wt %, thereby improving the fuel economy. Embodiment 10 FIG. 13 is, in the tenth embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 with the control rod 11 in the upper left position. In the figure, reference marks P1, P2 and P3 denote fuel rods containing plutonium, respectively, with the added 20 number increasing, plutonium enrichment becomes lower (P1 greater than P2 greater than P3). G1 denotes a gadolinia fuel rod that contain no plutonium and in which uranium is mixed with the oxide of the enriched gadolinium (enriched gadolinia) having the isotopic composition shown in the aforementioned Table 1. WR denotes a water rod 14. As shown in the figure, in the tenth embodiment, the gadolinia fuel rods G1 containing the enriched gadolinia are disposed in the positions of four corners of the fuel bundle of the outermost periphery. The concentration of the enriched gadolinia in the gadolinia fuel rods G1, in comparison with that of the fuel assembly of the existing design in which the natural gadolinia is used (cf. Japanese Patent Publication (KOKOKU) No. HEI 5-8398), can be largely decreased. Furthermore, the uranium enrichment in the gadolinia fuel rods G1 is 2 wt % far higher than the existing value (0.711 wt %), due to the use of the aforementioned enriched gadolinium the uranium enrichment being largely increased. Embodiment 11 FIG. 14A is, in the eleventh embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 with the control rod 11 in the upper left position. Reference marks 1 and V1 denote the fuel rod 12 containing no gadolinia, G1 and G2 denoting the gadolinia fuel rod 13. Here, the fuel rods denoted by reference marks G1 and G2 are the long-length fuel rods, the fuel rods denoted by V1 being the short-length fuel rods. WR denotes a water rod 14. FIG. 14B is a diagram showing the enrichment distributions of uranium in the axial direction of the fuel rods (1, V1, G1, and G2) and the gadolinia concentration distributions. As shown in the figure, the uranium enrichments of A11, B11 and G are employed and these are in the decreasing order of A11 greater than B11 greater than G. G denotes the enrichment of the natural uranium. In the long-length fuel rod, the portions containing the natural uranium alone are disposed at the upper and lower ends, respectively. The gadolinia concentrations of a11 through c11 and d11 are employed and these are in the decreasing order of a11 greater than b11 greater than c11 greater than d11, d11 being one half or less all through c11. Here, whereas a11 through c1 are the concentrations of the natural gadolinia, d11 is the concentration of the oxide of the enriched gadolinium having the isotopic composition shown in the aforementioned Table 1. The gadolinia fuel rod (G1), excluding the upper and lower end portions consisting of the natural uranium, is divided into three segments in the axial direction and the gadolinia concentration in each segment is different from each other. As shown in the figure, in the eleventh embodiment, all the fuel rods other than the gadolinia fuel rods (G2), the uranium enrichment is one kind of A11. Furthermore, the gadolinia fuel rods (G2) containing the enriched gadolinia are disposed in the positions of coordinates of (1, 2) and (2, 1) and in the positions symmetrical therewith. Thereby, the fuel rods in these positions and in the neighboring outermost periphery corner positions can be suppressed in their power. In the gadolinia fuel rods (G2), due to the enriched gadolinia, there is no necessity of excessively lowering the uranium enrichment, as a result the uranium enrichment being able to set at B11 next highest to A11. Accordingly, due to such design, while suppressing the cost and time in the course of manufacturing fuel pellets as low as possible, the uranium enrichment averaged over the bundle can be heightened. Embodiment 12 FIG. 15A is, in the twelfth embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 when the control rod 11 is located in the upper left position. The fuel rods 12 that contain no gadolinia are denoted by reference marks of 1, 2, 3, 4, 5, 6, and V1 and V2 for the respective types, the gadolinia fuel rods 13 being denoted by reference marks G1, G2 and G3. The fuel rods are consisted of the long-length fuel rods shown by reference marks of 1, 2, 3, 4, 5, 6, G1, G2 and G3 and the short-length fuel rods shown by reference marks of V1 and V2. WR denotes a water rod 14. FIG. 15B is a diagram showing the distributions in the axial direction of the fuel rods (1, 2, 3, 4, 5, 6, V1, V2, G1, G2, and G3) of the uranium enrichment and the gadolinia concentration. As shown in the figure, the uranium enrichments of A12 through F12 and G are used and these are in the decreasing order of A12 greater than B12 greater than C12 greater than D12 greater than E12 greater than F12 greater than G. G denotes the enrichment of the natural uranium. In the long-length fuel rods, the portions containing the natural uranium alone are disposed at the upper and lower ends. Furthermore, the gadolinia concentrations of a12 through c12 and d12 are used. These are in the decreasing order of a12 greater than b12 greater than c12 greater than d12, d12 being one half or less of a12 through c12. Here, a12 through c12 are the concentrations of the natural gadolinia, d12 being the concentration of the oxide of the enriched gadolinium having the isotopic composition shown in the aforementioned Table 1. Furthermore, the gadolinia fuel rods (G1, G2 and G3), excluding the upper and lower ends having of the natural uranium, are divided into a plurality of segments in the axial direction. There is disposed the difference of the gadolinia concentrations in the respective segments. In the segment just below the upper end portion that has the natural uranium, the enriched gadolinia is added to uranium. Thus, in the twelfth embodiment, the uranium enrichment distribution is utterly same with that of the existing fuel assembly (cf. FIG. 22). Accordingly, the uranium enrichment averaged over the bundle is same with that of the existing one of 3.96 wt %. However, in the respective gadolinia fuel rods, the enriched gadolinium is used in the upper end portion of which power is low and where residues remain much when the natural gadolinium is used. Accordingly, the reactivity at the cycle end is increased, resulting in an improvement of fuel economy. Embodiment 13 FIG. 16 is, in the thirteenth embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 when the control rod 11 is placed in the upper left position. The fuel rods 12 that contain no gadolinia are denoted by reference marks 1, 2, 3 and 4, the gadolinia fuel rod 13 being denoted by reference mark G1. In the fuel rods 12 that contain no gadolinia, with the number decreasing, the uranium enrichment becomes higher (1 greater than 2 greater than 3 greater than 4). The gadolinia fuel rod G1 contains the oxide of the enriched gadolinium (enriched gadolinia) having the isotopic composition shown in Table 3. WR denotes a water rod 14. In the present embodiment, the fuel rods having the highest uranium enrichment are disposed at a part of the fuel bundle of the outermost periphery, for instance at the coordinate positions of (1, 4) and (1, 5). In order to suppress the power of the fuel rods from becoming excessive, in a part of the positions in the second position inwardly from the outermost periphery, for instance in the positions of (2, 3) and (2, 4), two or more of gadolinia fuel rods (G1) are disposed adjacent through a face. The gadolinia fuel rods (G1) contain enriched gadolinium having the isotopic composition of 20 wt % of Gd-155, 60 wt % of Gd-157 and 20 wt % of the other isotopes. In thus configured thirteenth embodiment, the number of pieces of the gadolinia fuel rod can be suppressed to result in securing thermal margin of the fuel rods. That is, in the existing fuel assembly, when the gadolinia fuel rods are disposed adjacent through a face, the reactivity controllability of gadolinium deteriorates. Accordingly, at the coordinate positions of for instance (4, 4) or the like, several pieces of the gadolinia fuel rod need to be disposed. As a result, the power of the fuel rods decreases, and the power of the fuel rods other than these rises on the contrary. Thereby, deterioration of power peaking factor of the fuel rods is caused. However, in the thirteenth embodiment, the enriched gadolinium is used in the gadolinia fuel rods disposed adjacent through a face. As a result, the reactivity controllability is high, and the gadolinia fuel rods need not to be further increased. Accordingly, without making the power peaking factor excessively large, thermal margin of the fuel rods can be secured. Embodiment 14 FIG. 17 is, in the fourteenth embodiment of the fuel assembly of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 with the control rod disposed in the upper left position. The fuel rods comprise two kinds of different diameters. In the figure, reference marks 1, 2, 3 and 4 denote the fuel rods 12 that contain no gadolinia and reference marks G1 and G2 denote the gadolinia fuel rods 13, respectively. The reference marks with circle denote the fuel rods of small-sized diameter. In the fuel rods 12 containing no gadolinia, with the number decreasing, the uranium enrichment becomes higher (1 greater than 2 greater than 3 greater than 4). For the uranium enrichment of the gadolinia fuel rods 13, it is preferable that the uranium enrichment is the same in G1 and G2, or the uranium enrichment of G1 is made larger than that of G2. The gadolinia fuel rods (G1) contain the natural gadolinia and the gadolinia fuel rods (G2) of small-sized diameter contain the oxide of the enriched gadolinium (enriched gadolinia) having the isotopic composition shown in the aforementioned Table 1. The gadolinia concentration in the gadolinia fuel rods (G2) having the enriched gadolinia is smaller (xc2xd or less) than that of the gadolinia fuel rods (G1) having the natural gadolinia. WR denotes a water rod 14. Since in the fourteenth embodiment thus configured two kinds of fuel rods of different diameter are disposed alternatively, uranium more than in the existing fuel assembly can be loaded. Furthermore, since the gadolinia fuel rods (G2) of small-sized diameter contain the enriched gadolinia, there is no need of increasing a gadolinia addition amount, and thermal margin of the gadolinia fuel rod is secured. Embodiment 15 FIG. 18 is, in the fifteenth embodiment of the fuel rods of the present invention, a transversal sectional view showing an arrangement of the fuel rods 12 and 13 with the control rod 11 disposed in the upper left position. In the figure, reference marks 1, 2, 3, 4 and 5 denote the fuel rods that contain no gadolinia and reference marks G1 and G2 denote the gadolinia fuel rods 13. The gadolinia fuel rods 13 comprise two kinds of different diameters. Ones with circled marks denote the fuel rods of small-sized diameter. In the fuel rods 12 that contain no gadolinia, with the number decreasing, the uranium enrichment is higher (1 greater than 2 greater than 3 greater than 4 greater than 5). Both the gadolinia fuel rods (G1 and G2) contain the oxide (enriched gadolinia) of the enriched gadolinium having the isotopic composition shown in the aforementioned Table 1. The concentration of the enriched gadolinia is same in all gadolinia fuel rods (G1 and G2) and is smaller (xc2xd or less) than that of an existing gadolinia fuel rod containing the natural gadolinia. For the uranium enrichment of the gadolinia fuel rods 13, it is preferable that the uranium enrichment is the same in G1 and G2, or the uranium enrichment of G1 is made larger than that of G2. WR denotes a water rod 14. In the fuel assembly of the present embodiment, due to the difference of the diameters of the gadolinia fuel rods, the specific burnup at which gadolinia burns out is different from rod to rod. In specific, the fuel rod of small-sized diameter burns out earlier. Furthermore, since the positions are high in power, the difference of the specific burnup at which the rods burn out becomes more conspicuous. Accordingly, in the fifteenth embodiment, the peak value of the infinite multiplication factor when considered over the entire fuel assembly becomes smaller and the infinite multiplication factor varies moderately. The problems such as deterioration of shut-down margin and an increase of channel peaking can be solved. In the aforementioned eighth through fifteenth embodiments, the explanations are given with the fuel assembly of square grid pattern of nine-columns/nine-rows as an example. However, the present invention is similarly effective in the arrangement other than the above, in particular in a fuel assembly having the number of arrangement larger than nine columns/nine rows. Whereas the explanations are given of the fuel assembly in which only the gadolinia fuel rods having the enriched gadolinium are used, a part of the gadolinia fuel rods may be replaced by ones that have only the natural gadolinium. Furthermore, a nuclear reactor can be configured by loading the fuel assembly having the enriched gadolinium set forth in the first through fifteenth embodiments and the fuel assembly having the natural gadolinium. In such nuclear reactor, the gadolinia concentration averaged over the fuel assembly having the natural gadolinium is preferable to be larger than that of the fuel assembly having the enriched gadolinium. When the fuel assembly having the enriched gadolinium and the fuel assembly having the natural gadolinium alone are mingled, the fuel assembly containing the enriched gadolinia has the reactivity of the residual gadolinia can be reduced more. After the specific burnup at which gadolinia burns out, namely at the point of time after the second cycle where the fuel is loaded, the infinite multiplication factor of the fuel assembly having the enriched gadolinium becomes larger than that of the fuel assembly having the natural gadolinium only. In general, in a nuclear reactor, due to leakage neutrons, the outer side tends to deteriorate in the power more than the inner side does. However, when the fuel assembly having the enriched gadolinium is disposed for instance at the outer side of the core, the power of the outer side of the core can be increased. Thereby, the output of the nuclear reactor can be flattened. Accordingly, thermal safety of the nuclear reactor can be improved. As obvious from the above explanations, according to the present invention, the residual reactivity of gadolinia can be reduced and at the same time thermal performance of the fuel can be improved. Furthermore, without damaging thermal margin of the gadolinia fuel rod, the enrichment of the fissile material averaged over the bundle can be increased and fuel economy can be improved. While in the foregoing specification, this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.