Patent Number: 053496191
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be hereinafter described with reference to FIGS. 1 to 20. First, the principle of this invention will be explained with reference to FIGS. 1 to 3. FIG. 1 illustrates one embodiment for explaining the principle of the invention. In FIG. 1, an MOX fuel assembly of this embodiment comprises fuel rods of plutonium-uranium mixed oxide, i.e., MOX fuel rods 13 which are arranged in 8.times.8 lattice form, a large water rod 15 which is located in the center, four water rods 16 each of which is provided in one of the corners of the 8.times.8 fuel rod arrangement, and a channel box 17 which surrounds the fuel rods 13, and the water rods 15, 16. The water rods 15, 16 have a length equal to that of a fuel effective portion extending over substantially the entire length of the fuel rod 13. When the fuel assembly 1 is mounted on a reactor core, a position adjacent to one of the corner portions of the fuel assembly 1 is used as a space in which a cruciform control rod 11 is inserted. For the purpose of comparison, FIG. 2 shows, in cross section, an 8.times.8 type MOS fuel assembly according to the above-described conventional technique disclosed in Japanese Patent Unexamined Publication No. 63-293493. In this fuel assembly 2, MOX fuel rods 13 are employed instead of the uranium fuel rods 192 of the uranium fuel assembly shown in FIG. 22, and four water rods 26 are used in place of four of the fuel rods 192 in the vicinity of the central large water rod 206. The fuel assembly 2 has substantially the same structure as the MOX fuel assembly 1 shown in FIG. 1 except for the arrangement of the four water rods 16 and 26. FIG. 3 illustrates comparison of void reactivity coefficients of these two MOX fuel assemblies 1 and 2. The point A indicates a void reactivity coefficient of the MOX fuel assembly 1 shown in FIG. 1, and the point B indicates a void reactivity coefficient of the MOX fuel assembly 2 shown in FIG. 2. As shown in FIG. 3, the void reactivity coefficients have different values depending upon the locations of the water rods even if the total numbers of the water rods are the same. When the water rod 16 is located in each of the corner portions as shown in FIG. 1, the absolute value becomes small and the improvement effect becomes large when compared with a case in which the water rods are located in the vicinity of the central portion, for the following reason: When the water rods are provided in the corner portions away from the center, the water rods are located in saturated water regions in the vicinity of corner portions of a water gap outside of the channel box. As a result, neutron absorption effect of the water rods is increased, and a change in neutron absorption of the fuel rods in accordance with a change in the void fraction becomes relatively small, i.e., a change in the reactivity coefficient is decreased. Further, a neutron absorption cross section of plutonium which is fissile material is larger than that of uranium. Consequently, in the case of the MOX fuel assembly, when neutron absorption effect of the water rods is increased, neutron absorption effect of the fuel rods is decreased so that the effect of reducing the void reactivity change will be greater than that of the uranium fuel assembly. In the design of the conventional uranium fuel assembly, flattening of the local power distribution in the fuel assembly has been regarded as an important factor. Therefore, the water rods are located in the center of the arrangement of the fuel rods so as to flatten the thermal neutron distribution, thereby flattening the power distribution. In the MOX fuel assembly, however, reduction of the void reactivity coefficient is a primary problem, and in order to solve this problem, an increase in the linear heat generation ratio due to an increase in the number of water rods must be made as small as possible. In consequence, the arrangement of water rods which enhances the effect of reducing the void reactivity coefficient should preferably be selected, and the number of the required water rods should preferably be made as small as possible. In the present invention, since the four water rods 16 are respectively provided in the four corner portions of the arrangement of the fuel rods, the void reactivity coefficient per water rod is improved more effectively, as described above. As a result, the number of additional water rods required for improving the void reactivity coefficient is suppressed to the minimum, so that it is possible to provide a highly enriched MOX fuel assembly which is as excellent as the uranium fuel assembly in respect of the plutonium load and the linear heat generation ratio. In the fuel assembly shown in FIG. 1, the water rods 16 are located in the corner portions of the fuel rod arrangement. However, even if the water rods 16 are located in positions in the lattice form which are adjacent to the corner portions, it is possible to enhance the effect of increasing neutron absorption by the saturated water regions in the vicinity of the corner portions of the water gap, and to increase the effect of improving the void reactivity coefficient per water rod. In this case, locating the four water rods in rotation-symmetry is an important factor in reduction of the maximum linear heat generation ratio. That is to say, if the water rods are located in non-rotation-symmetry, the power distribution viewed in horizontal cross section of the fuel assembly is in non-rotation-symmetry, which results in a general characteristic that the power peaking is increased. When the water rods are provided in rotation-symmetry, the power peaking can be decreased so as to suppress an increase in the average linear heat generation ratio owing to addition of the four water rods, thus reducing an increase in the maximum linear heat generation ratio. Moreover, when the water rods are located in the corner portions, water rods must not be further provided at positions b shown in FIG. 1, which are adjacent to the water rods and in the second layer from the outer periphery. In other words, when two water rods are provided in each corner portion and a position in the second layer which are adjacent to each other, the MOX fuel rod (a fuel rod a shown in FIG. 1) in the outermost peripheral portion which is adjacent to the water rod in the corner portion has two surfaces in contact with the water rods and one surface in contact with the saturated water region outside of the channel box. Therefore, neutron moderation around this MOX fuel rod is increased. Particularly, the power change of MOX due to a change in neutron moderation is larger than that of uranium. For these reasons, the power peaking of the MOX fuel rod a becomes much higher than that of the other fuel rods. Consequently, such an arrangement is unfavorable from the standpoint of suppressing the increase in the linear heat generation ratio. Furthermore, in order to realize the above-described improvement effect of the void reactivity coefficient, the water rods 16 must have a length equal to that of a fuel effective portion extending over substantially the entire length of the fuel rod 13. Next, a first embodiment of the present invention based on the above-described principle will be described with reference to FIGS. 4 to 12. This embodiment is a 9.times.9 type MOX fuel assembly to which the invention is applied. As shown in FIG. 4, a fuel assembly 3 according to this embodiment comprises a bundle of a large number of elongated cylindrical fuel rods 53. In this bundle, the fuel rods 53 are supported at equal intervals by spacers 45. In addition to the fuel rods 53, water rods 54, 56 are incorporated in the bundle. The outer periphery of this bundle is surrounded with a channel box 57. An upper portion of the channel box 57 is bonded to an upper tie plate 43 while a lower portion thereof is bonded to a lower tie plate 44. Each of the fuel rods 53 is a fuel cladding filled with a large number of cylindrical fuel pellets of plutonium-uranium mixed oxide, and both upper and lower ends of this fuel cladding are sealed with an upper plug 48 and a lower plug 49. The upper plug 48 includes an extension portion which can be inserted in a support opening in the upper tie plate 43, and the lower plug 49 includes a fitting portion which can be closely fitted in a support opening in the lower tie plate 44. A cooling water inlet hole 40 is formed in a lower portion of each of the water rods 54, 56, and a cooling water outlet hole 41 is formed in an upper portion thereof, so that cooling water flows inside of the water rod 54, 56 from the lower portion to the upper portion. The water rods 54, 56 have a length substantially equal to the entire length of the fuel rod 53. FIG. 5 is a horizontal cross-sectional view of the fuel assembly 3. The fuel rods 53 are regularly arranged in a lattice form of nine rows and nine lines (9.times.9). Two water rods 54, which are large water rods, are provided in an area from which seven fuel rods are removed. A primary object of provision of these large water rods 54 is to increase neutron moderation effect in the central portion of the fuel assembly 3 and to flatten the local power distribution. Four water rods 56 have substantially the same diameter as the fuel rods 53, and are located in corner portions of the 9.times.9 lattice arrangement of the fuel rods. Next, the function of this embodiment will be explained. As a first comparative example of the first embodiment, FIG. 6 shows a 9.times.9 type uranium fuel assembly according to the conventional technique. This fuel assembly 4 comprises uranium fuel rods 65 which are arranged in a 9.times.9 lattice form, two large water rods 61 which are provided in the central portion, and a channel box 67 which surrounds the fuel rods 65 and the water rods 61. As similarly to the above-mentioned large water rods 54, a main object of provision of the large water rods 61 is to uniform the local power distribution. The large water rods 61 are provided in an area of the 9.times.9 fuel rod arrangement from which seven central fuel rods are removed, and have a length equal to that of a fuel effective portion extending over substantially the entire length of the fuel rod 65. This fuel assembly 4 is different from the MOX fuel assembly 3 shown in FIG. 5 with respect to an arrangement of uranium fuel rods and the provision of the uranium fuel rods 65 in the corner portions instead of the water rods. Further, as a second comparative example, FIG. 7 shows a 9.times.9 type MOX fuel assembly. In this fuel assembly 5, MOX fuel rods 73 are employed instead of the uranium fuel rods 65 of the uranium fuel assembly 4 shown in FIG. 6. The fuel assembly 5 has substantially the same structure as the MOX fuel assembly 3 shown in FIG. 5 except for the MOX fuel rods 73 provided in the corner portions in place of the water rods. FIG. 8 illustrates comparisons of void reactivity coefficients and maximum linear heat generation ratios of the MOX fuel assembly 3 shown in FIG. 5, the uranium fuel assembly 4 which is the first comparative example shown in FIG. 6 and the MOX fuel assembly 5 which is the second comparative example shown in FIG. 7. In FIG. 8, the point F indicates values of the MOX fuel assembly 3 of FIG. 5, the point A indicates values of the uranium fuel assembly 4 of FIG. 6, and the point B indicates values of the MOX fuel assembly 5 of FIG. 7. As shown in FIG. 8, a void reactivity coefficient of the MOX fuel assembly 3 according to the present invention which is indicated by the point F is about -8.5 [%K/K/%void] and substantially equal to a void reactivity coefficient of the uranium fuel assembly 4 indicated by the point A which is about -8.3 [%K/K/%void]. However, a void reactivity coefficient of the MOX fuel assembly 5 indicated by the point B is about -10.4 [%K/K/%void] and about 30% larger than the value indicated by the point F. On the other hand, a maximum linear heat generation ratio of the MOX fuel assembly 3 indicated by the point F is about 12.7 [kW/ft] and substantially equal to a maximum linear heat generation ratio of the uranium fuel assembly 4 indicated by the point A which is about 12.0 [kW/ft]. A maximum linear heat generation ratio of the MOX fuel assembly 5 indicated by the point B is equal to the maximum linear heat generation ratio of the uranium fuel assembly 4 which is about 12.0 [kW/ft]. According to the above-described comparison results, when the uranium fuel rods in the uranium fuel assembly are substituted by the MOX fuel rods, the maximum linear heat generation ratio is not changed because the water to fuel volume ratio is the same, but the absolute value of the void reactivity coefficient is about 30% larger. That is to say, in relation to the void reactivity coefficient alone, substitution of the uranium fuel by the MOX fuel is not favorable. In other words, in order to improve the void reactivity coefficient, the number of water rods must be increased, which results in an increase of the maximum linear heat generation ratio. However, in this embodiment in which the water rods are provided in the corner portions of the arrangement of the fuel rods according to the above-described principle, substantially the same void reactivity coefficient as in the case of the uranium fuel assembly can be obtained without increasing the linear heat generation ratio by a large extent. The effect of this embodiment of improving the void reactivity coefficient and the maximum linear heat generation ratio will now be described more specifically. As a third comparative example, FIG. 9 shows another 9.times.9 type MOX fuel assembly. This fuel assembly 6 has substantially the same structure as the MOX fuel assembly 5 shown in FIG. 7 except for four water rods 96 which are provided in four corners of a square defined by four sides each having five fuel rods in the vicinity of the two large water rods 61 located in the central portion, so as to decrease the absolute value of the void reactivity coefficient. In FIG. 8, the point G indicates a void reactivity coefficient and a maximum linear heat generation ratio of the fuel assembly 6. As compared with the MOX fuel assembly 5 indicated by the point B, the void reactivity coefficient is slightly improved and is about -10.2 [%K/K/%void], but the absolute value is still 20% larger than the value of the uranium fuel assembly 4 indicated by the point A. Moreover, the number of the fuel rods is decreased, and the average linear heat generation ratio is increased. As a result, the maximum linear heat generation ratio is increased and is about 12.7 [kW/ft]. Further, as a fourth comparative example, FIG. 10 shows another 9.times.9 type MOX fuel assembly. In this fuel assembly 7, the number of water rods 96 in the MOX fuel assembly 6 shown in FIG. 8 is increased to eight, and the water rods 96 are provided in four corners of a square defined by four sides each having seven fuel rods in the vicinity of the outer periphery of the fuel rod arrangement and provided on middle points of the four sides of the square. In FIG. 8, the point C indicates a void reactivity coefficient and a maximum linear heat generation ratio of the fuel assembly 7. When it is compared with the fuel assembly 6 indicated by the point G, the void reactivity coefficient is improved and is about -9.5 [%K/K/%void]. However, since the number of the water rods is increased, the maximum linear heat generation ratio is increased and is about 13.4 [kW/ft]. Taking into account the fact that an operational limit value of the maximum linear heat generation ratio in a light water reactor is about 13.4 [kW/ft], this means that there is no margin at all with respect to the operational limit value, and it is not favorable in respect of the design. Moreover, as a fifth comparative example, FIG. 11 shows another 9.times.9 type MOX fuel assembly. In this fuel assembly 8, the number of water rods 96 in the MOX fuel assembly 7 shown in FIG. 9 is eight and unchanged, and two of the water rods 96 are provided on each side of a square defined by the outermost periphery of the fuel rod arrangement. In FIG. 8, the point D indicates a void reactivity coefficient and a maximum linear heat generation ratio of this case. The void reactivity coefficient is improved and at substantially the same level as the uranium fuel assembly 4 indicated by the point A. However, since the number of the water rods is eight and unchanged, the maximum linear heat generation ratio is about 13.4 [kW/ft] and equal to that of the MOX fuel assembly 7 indicated by the point C. That is to say, there remains the problem that there is no margin with respect to the operational limit value. Furthermore, as a sixth comparative example, FIG. 12 shows another 9.times.9 type MOX fuel assembly. In this fuel assembly 9, two of the eight water rods 96 in the MOX fuel assembly 8 shown in FIG. 10 are provided at two positions in the lattice form which are adjacent to each corner of the fuel rod arrangement. In FIG. 8, the point E indicates a void reactivity coefficient and a maximum linear heat generation ratio of this case. Although the maximum linear heat generation ratio is unchanged, the void reactivity coefficient is further improved, and its absolute value is lower than the value of the uranium fuel assembly 4 indicated by the point A. However, since the number of the water rods is eight and unchanged, the maximum linear heat generation ratio is about 13.4 [kW/ft] and equal to that of the MOX fuel assembly 7 indicated by the point C. That is to say, there remains the problem that there is no margin with respect to the operational limit value. According to the above-described results of comparing the void reactivity coefficients and the maximum linear heat generation ratios of the fuel assemblies 5, 6, 7 respectively shown in FIGS. 7, 9, 10, the void reactivity coefficient can be improved by increasing the number of the water rods, but at the same time, the maximum linear heat generation ratio is increased, so that there will be no margin with respect to the operational limit value. Consequently, by merely increasing the number of the water rods, the maximum linear heat generation ratio reaches the designed limit before the void reactivity coefficient reaches the target value indicated by the point A, thereby failing to achieve the object of the present invention. Also, according to the results of comparing the void reactivity coefficients of the fuel assemblies 7, 8, 9 respectively shown in FIGS. 10, 11, 12, the absolute value of the void reactivity coefficient of the MOX fuel assembly is decreased as the water rods are located closer to the outer periphery of the arrangement of the fuel rods, and it is further decreased as the water rods are located closer to the corner portions of the outer periphery. Therefore, in the fuel assembly 3 of this embodiment, the number of the water rods 56 is made as small as four so that the maximum linear heat generation ratio will be suppressed to about 12.7 [kW/ft] which is lower than the operational limit value of about 13.4 [kW/ft]. On the other hand, one of the four water rods 56 is provided in each corner portion of the arrangement of the fuel rods so that the void reactivity coefficient will be about -8.5 [%K/K/%void]. Although the absolute value is slightly larger than that of the fuel assembly 9 of FIG. 12 indicated by the point E, it can be improved to have a value at substantially the same level as the uranium fuel assembly 4 of FIG. 5 indicated by the point A. According to this embodiment, the four water rods 56 are respectively located in the four corner portions of the arrangement of the fuel rods in the MOX fuel assembly 3, and consequently, it is possible to obtain the void reactivity coefficient and the maximum linear heat generation ratio which are at substantially the same level as the uranium fuel assembly 4. FIG. 13 shows a second embodiment of the present invention. In this fuel assembly 10, the four water rods 56 located in the respective corner portions of the arrangement of the fuel rods in the 9.times.9 type MOX fuel assembly 3 shown in FIG. 5 are provided in rotation-symmetry such that each water rod is located at one of the two positions in the lattice form which are adjacent to each corner of the fuel rod arrangement. That is to say, the four water rods 56 in the fuel assembly 3 are rotated in the same direction about the center of the arrangement of the fuel rods and moved to positions in the next lines. In FIG. 8, the point H indicates a void reactivity coefficient and a maximum linear heat generation ratio of the fuel assembly 10, which are substantially the same values as the fuel assembly 3 of FIG. 5 according to the first embodiment which are indicated by the point F. That is to say, with the MOX fuel assembly 10 according to the second embodiment, it is possible to obtain the void reactivity coefficient and the maximum linear heat generation ratio which are at substantially the same level as the uranium fuel assembly 4. FIG. 14 shows one embodiment of a reactor core of a BWR according to the present invention. It is a diagram of the reactor core of the BWR, as viewed from above. This reactor core comprises 548 fuel assemblies each of which is indicated by a blank or shadowed square in the diagram. About 156 of the 548 fuel assemblies are MOX fuel assemblies 3 according to the present invention shown in FIG. 5, and about 392 of them are uranium fuel assemblies 4 shown in FIG. 6. FIG. 15 is an enlarged view of a portion P of the reactor core shown in FIG. 14, the portion P including four fuel assemblies indicated by squares blank and shadowed by oblique lines. As shown in FIG. 15, four fuel assemblies located to surround one control rod is a basic unit of the reactor core. In the embodiment shown in FIG. 15, one of the four fuel assemblies is an MOX fuel assembly 3 shown in FIG. 5, and the other three are uranium fuel assemblies 4 shown in FIG. 6. The MOX fuel-assembly 3 is characterized in that it includes water rods in corner portions of an arrangement of fuel rods, but the uranium fuel assembly 4 is characterized in that it does not include water rods in corner portions of an arrangement of fuel rods. As described with reference to FIG. 8, the void reactivity coefficient of the MOX fuel assembly 3 indicated by the point F is substantially the same as that of the uranium fuel assembly 4 indicated by the point A. Therefore, as shown in FIG. 15, even if one part of the uranium fuel assemblies which constitute the reactor core are substituted by the MOX fuel assemblies, the void reactivity coefficient of the whole reactor core is substantially the same as that of a reactor core comprising uranium fuel assemblies alone. All the transient behaviors of the reactor core such as a reactivity change when the void coefficient of the reactor core is abruptly changed due to an increase in the pressure or a sudden increase in the power are substantially the same as in the case of a reactor core comprising uranium fuel assemblies alone. This means that when safety of a reactor core comprising uranium fuel assemblies alone is confirmed, the same safety is surely obtained even if one part of the uranium fuel assemblies are substituted by MOX fuel assemblies, and it is an important factor in respect of safety of the reactor core. In the embodiment shown in FIG. 15, one of the four fuel assemblies is an MOX fuel assembly. However, since the void reactivity coefficients of the uranium fuel assembly and the MOX fuel assembly are substantially the same, as described above, two of the four fuel assemblies may be MOX fuel assemblies. Further, three or all of them may be MOX fuel assemblies. Even in such a case, the void reactivity coefficient of the whole reactor core will not be affected. In the above description, the 9.times.9 type arrangement of fuel rods in the fuel assembly is employed as an example. However, the 8.times.8 type MOX fuel assembly shown in FIG. 1 and the 8.times.8 type uranium fuel assembly shown in FIG. 22 have substantially the same void reactivity coefficients. Consequently, as shown in FIG. 16, even if one part of the fuel assemblies are constituted of 8.times.8 type MOX fuel assemblies, it is possible to obtain the same safety as a reactor core comprising uranium fuel assemblies alone, similarly to the embodiment shown in FIG. 15. In this manner, according to this embodiment, there can be provided a core of a light water reactor utilizing MOX fuel assemblies by which the void reactivity coefficient and the maximum linear heat generation ratio can be made substantially the same as uranium fuel assemblies. Although the embodiment of the present invention in relation to a boiling water reactor (BWR) has been described above, one embodiment of this invention in relation to a pressurized water reactor (PWR) will now be described with reference to FIGS. 17 to 20. In this embodiment, the invention is applied to a 17.times.17 type MOX fuel assembly. As shown in FIG. 17, a fuel assembly 150 of this embodiment comprises fuel rods 151 and 152 which are regularly arranged in 17 lines and 17 rows and supported by an upper nozzle 153, a lower nozzle 154 and a grid 158. In the grid 158, several control-rod guide tubes 155 in which control rods 156 supported by a control rod cluster 157 can be inserted are provided. These control-rod guide tubes 155 are designed in such a manner that coolant flows therein when the control rods 156 are not inserted, similarly to the above-described water rods. FIG. 18 is a cross-sectional view of the fuel assembly 150. In this fuel assembly 150, a neutron instrumentation tube 163 is provided in the center of the 17.times.17 type arrangement of fuel rods, the control-rod guide tubes 155 and MOX fuel rods 151 enriched at high level are provided around the center, and MOX fuel rods 152 enriched at intermediate level are provided in the outer peripheral portion of the fuel rod arrangement. Also, a water rod 167 is provided in each of corner portions of the fuel rod arrangement. As a first comparative example of the present embodiment, FIG. 19 shows a 17.times.17 type uranium fuel assembly for a PWR according to the conventional technique. In this uranium fuel assembly 170, a neutron instrumentation tube 173 is provided in the center of the 17.times.17 arrangement of uranium fuel rods 171, and control-rod guide tubes 172 are provided around the center. The fuel assembly 170 is different from the MOX fuel assembly 150 shown in FIG. 18 in an arrangement of uranium fuel rods and a provision of the uranium fuel rods 171 in corner portions of the arrangement instead of the water rods. As a second comparative example of the present embodiment, FIG. 20 shows a 17.times.17 type MOX fuel assembly for a PWR. In this MOX fuel assembly 180, a neutron instrumentation tube 163 is provided in the center of the 17.times.17 arrangement of MOX fuel rods 151 enriched at high level, control-rod guide tubes 155 are provided around the center, MOX fuel rods 152 enriched at intermediate level are provided in the outer peripheral portion of the fuel rod arrangement, and MOX fuel rods 184 enriched at low level are provided in the respective corner portions of the fuel rod arrangement. The fuel assembly 180 is characterized in that it comprises three kinds of fuel rods which are enriched at different levels. The fuel assembly 180 is different from the MOX fuel assembly 150 shown in FIG. 18 in a provision of the low-level enriched MOX fuel rods 184 in the corner portions of the arrangement instead of the water rods. Since the reactor core of the above-mentioned PWR is not boiling during normal operation, the void reactivity coefficient in the case of the BWR is not a problem. However, there is a moderator temperature coefficient serving as an index for indicating a reactivity change with respect to a water density change. When MOX fuel is used, this moderator temperature coefficient has a property to decrease to have a negative value, as compared with the uranium fuel. In order to improve this property, it is effective to enhance neutron moderation effect by increasing the water to fuel volume ratio, similarly to the void reactivity coefficient in the case of the BWR. Therefore, even in the PWR fuel assembly, the moderator temperature coefficient can be improved by providing water rods in which a moderator (coolant) flows. However, similarly to the case of the BWR, in order to prevent an increase in the maximum linear heat generation ratio, it is important to make an increase in the number of water rods as small as possible for the following reason: In general, when an MOX fuel assembly is located adjacent to a uranium fuel assembly in a reactor core, power of fuel rods in the periphery of the MOX fuel assembly tends to increase owing to a neutron spectrum difference between uranium and MOX fuel assemblies, which results in one cause to increase the maximum linear heat generation ratio. This tendency is remarkable in the case of fuel rods in corner portions of the arrangement of fuel rods in particular. In consequence, in the above-described arrangement of water rods to improve the moderator temperature coefficient, locating the water rods in the respective corners of the fuel rod arrangement is especially effective to suppress the maximum linear heat generation ratio. Therefore, according to the present embodiment, the four water rods 167 are provided in the respective corner portions of the arrangement of fuel rods so as to improve the moderator temperature coefficient, while suppressing the maximum linear heat generation ratio. Further, two kinds of MOX fuel rods, i.e., high-level enriched fuel rods and intermediate-level enriched fuel rods, are enough, and low-level enriched MOX fuel rods are unnecessary. Consequently, the number of processing steps in the manufacture of MOX fuel is reduced. According to the present invention, there can be provided fuel assemblies for a light water reactor and a core of the light water reactor utilizing these fuel assemblies in which the void reactivity coefficient or the moderator temperature coefficient can be made substantially equal to-that of the uranium fuel assemblies without decreasing the plutonium load largely and without increasing the linear heat generation ratio largely even if the MOX fuel assemblies are used in place of the uranium fuel assemblies. Moreover, in the PWR, the number of kinds of enriched MOX fuel rods can be lessened, so that the number of processing steps in the manufacture of MOX fuel can be reduced. Incidentally, instead of the water rods, the solid moderator rods including carbon or the like as a solid moderator may be applied to the fuel assembly according to the invention so as to achieve the same meritorious results as the water rods.