Patent Number: 
Section: description

Embodiment of the present invention will be described below in detail, referring to the accompanied figures. Although the reactor cores of electric power of 1350 MW class are described in the following embodiments, the output power capacity is not limited to 1350 MW. It should be recognized that the present invention may be applied to the reactor cores having the other output power capacity by changing number of the fuel assemblies. (First Embodiment) A first embodiment of the present invention will be described, referring to FIG. 1 and FIG. 7 to FIG. 12. FIG. 1 is a cross-sectional plan view showing the first embodiment of a reactor core having an electric output power of 1356 MWe. FIG. 1 shows 504 fuel assemblies 1; and 157 control rod drive mechanisms 2 each of which operates three large-diameter control rods to be inserted into three fuel assemblies, respectively. FIG. 7 shows the cross section of the fuel assembly lattice. In a channel box 3, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 12 fuel rod rows. In the central portion of the fuel assembly, a guide tube 6 to insert the large-diameter control rod 5 thereinto is disposed in the region having an area equivalent to 3 fuel rod layers, that is, an area equivalent to 19 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. FIG. 8 shows an arrangement of fuel assemblies under the equilibrium core state. Each of the numerals written in each of the fuel assemblies 1 indicates a period staying in the reactor core by cycle numbers. The 5 cycle fuels staying in the reactor core for the longest period are loaded in the outermost periphery of the reactor core where the neutron importance is low. The fuels of 1 cycle staying period in the reactor core having the highest neutron infinite multiplication factor are loaded in the outer region of the reactor core in the inner side of the outermost periphery to flatten the power distribution in the radial direction of the reactor core. In the inner region of the reactor core, the fuels of 2 to 4 cycle staying periods in the reactor core are distributively loaded to flatten the power distribution in the radial direction of the reactor core. FIG. 9 shows an orifice distribution in the equilibrium reactor core state, and the numeral written in the fuels indicates difference in opening degree of an orifice placed in the fuel supporting portion, and there are two regions for the orifice opening degree. The orifice diameter in the reactor outermost peripheral region (number 1) where the fuel assembly power is small is smaller than the orifice diameter in the inner region. FIG. 10 shows the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core. Therein, the uranium to be added with plutonium is the depleted uranium. The height of the reactor core is 70 cm, and the reactor core is divided three regions at the levels of 20 cm and 49 cm from the bottom end of the reactor core, and the fissionable plutonium enrichments are 19 wt %, 0 wt % and 19 wt %, respectively, and the average fissionable plutonium enrichment is 11.1 wt %. Further, depleted uranium blankets having heights of 20 cm and 25 cm are attached to the top and the bottom of the reactor core portion, respectively. FIG. 11 is a horizontal cross-sectional view showing the 19 wt % fissionable plutonium enrichment region of the fuel assembly. The fissionable plutonium enrichments are three kinds of 19.1 wt %, 18.5 wt % and 17.5 wt %, and the average enrichment is 19 wt %. FIG. 12 shows the distributions of core-average output power and core-average void fraction in the axial direction of the reactor core. The core-average void fraction is 60%, and the mass steam quality at reactor core exit is 32 Wt %. Operation of the present embodiment will be described below. By the combination of the regular triangular lattice closed-compact hexagonal fuel assembly having a gap between rods of 1.3 mm, the core-average void fraction of 60% and the large-diameter control rod, an effective water-to-fuel volume ratio of 0.27 was attained, and an in-core breeding ratio of 0.87, a blanket breeding ratio of 0.14 and a total breeding ratio of 1.01 were realized. That is, in the present embodiment, by reducing the effective water-to-fuel volume ratio from nearly 2.0 in the existing reactor to 0.27, the light water reactor having the breeding ratio of 1.01 is realized. The output power of the present reactor core is 1350 MWe which is equal to that of the existing ABWR, and the circumscribed radius of the reactor core is 2.9 m which is nearly equal to that of the ABWR. The height of the reactor core is 70 cm, and the blankets having heights of 20 cm and 25 cm are attached to the top and the bottom of the reactor core to form a short-length fuel assembly. However, since the fuel rods are closely packed, the total length of the fuel rod is nearly equal to that of ABWR fuel and the MCPR is 1.31 which sufficiently satisfies the thermal design standard value of 1.24. Because of the short-length fuel rods having the 70 cm reactor core portion, the plutonium inventory converted to the amount of fissionable plutonium per 1000 MWe output power is as small as 6.0 tons though the fuel rods are closely packed. Even including the period of plutonium staying outside the reactor core such as fuel reprocessing, the plutonium inventory is less than 10 tons per 1000 MWe. From the above reason, in the present embodiment having the breeding ratio of 1.01, using fissionable plutonium of 15 thousands tons and depleted uranium of 15 million tons produced from uranium reserves of 15 million tons in the world, 1500 units of 1000 MWe reactors can be continued to be operated for 10 thousands years and accordingly the system of long-term stable energy supply can be established. In the present embodiment, in regard to the height direction of the fuel assembly, there are the portions having the fissionable plutonium enrichment of 19 wt % in the upper and the lower positions, and the middle portion between them is formed of depleted uranium not containing the fissionable plutonium. When the output power is increased or when the core coolant flow rate is decreased, the steam void fraction in the reactor core is increased. At that time, the power distribution in the upper portion of the reactor core is swung to the middle region of the reactor core where the fissionable plutonium enrichment is not contained. Thereby, the negative void coefficient is inserted. Further, in the present embodiment, since the mass steam quality at the reactor core exit is 32 wt %, and accordingly all the coolant does not become vapor and the coolant can be maintained in a two-phase state even when an abnormal transient event occurs. Therefore, similar to the existing BWR, the radioactive substances accumulated in the reactor core such as corrosion products are enclosed in the reactor core by evaporation operation of boiling and prevented from being transported to the turbine side. From the above reason, the BWR of the present embodiment is capable of cope with the long-term stable energy supply under the degree of safety comparative with that of the fuel burning-only light water reactor under operation now and using the pressure vessel having a size nearly equal to that of the ABWR under construction now. The BWR of the present embodiment outputs the amount of power equal to that of the ABWR, and can attain burn-up of 65 GWd/t. A void coefficient of the existing BWR under operation now (a value at present) is xe2x88x927.0xc3x9710xe2x88x924xcex94k/k/% void. The value for the present embodiment is designed to be xe2x88x920.5xc3x9710xe2x88x924xcex94k/k/% void of which the absolute value is smaller than the value at present. As the result, the thermal margin to an event of increasing pressure or to an event of decreasing coolant temperature is relatively large. From the above reason, the BWR reactor core of the present embodiment has safety margins for various kinds of transient events which are larger than those of the existing BWR under operation now. In the present embodiment, the large-diameter control rod having an outer diameter larger than that of the fuel rod is employed as the absorption rod. By employing the large-diameter control rod, the mechanical strength of the control rod can be increased and accordingly bending and buckling of the control rod can be suppressed when the control rod is inserted or withdrawn. Furthermore, by using the large-diameter control rod, number of absorption rods per fuel assembly can be reduced, and accordingly the control rod can be easily manufactured to reduce the manufacturing cost. According to the present embodiment, by combination of the closed-packed hexagonal fuel assembly, the large-diameter control rod and the core-average void fraction of 60%, the breeding ratio of 1.01 can be realized by the fuel enriched by adding the fissile PU of average 11.1 wt % to the depleted uranium, and 1500 units of 1000 MWe reactors can be operated for 10 thousands years using the uranium reserves of 15 million tons in the world, and accordingly the long-term stable energy supply can be established. Further, since the diameter of the pressure vessel, the operating conditions such as output power and the used materials are the same as those of the existing BWR under operation, the electric power generation cost can be suppressed to the same level as that of the existing BWR even though the performance is largely progressed. Further, by employing the large-diameter control rod, maintaining the negative void coefficient by the axial fuel distribution and suppressing the mass steam quality to nearly 32 wt %, the safety margin can be secured in the same level as that of the existing BWR by maintaining the evaporating function by boiling to enclose the radioactive substances in the pressure vessel. In the present embodiment, aiming at the long-term stable energy supply, the description has be made on the construction, the operation and the effects of the fuel which is enriched by adding plutonium to depleted uranium produced as the residue at manufacturing enriched uranium used for the existing light water reactors. However, the same or more effects can be obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. In this case, the fissile PU enrichment can be reduced by 0.5 wt % or more compared to the case of using the depleted uranium due to increase in an amount of uranium-235 contained in the fuel. As a result, the breeding ratio to the fissile PU can be increased by nearly 3% or more, and the void coefficient can be made negative. In addition, since the Pu inventory can be reduced, number of the RBWRs capable of being operated can be further increased. Although the void coefficient is negative in the present embodiment, the power coefficient including the Doppler coefficient can be made negative even if the void coefficient is 0 or slightly positive. According to the study of the inventors of the present invention, it has been shown from an evaluation result on the safety that the negative or positive void coefficient is essentially no problem if the power coefficient is negative. Therefore, the thermal margin can be increased by increasing the length of the reactor core portion. Further, the breeding ratio can be increased by narrowing the gap between the fuel rods. Although only the fuel enriched by adding Pu to uranium has been described in the present embodiment, the other actinides can be added together with Pu. In this case, since the RBWR is high in the average energy of neutrons, plutonium is hardly converted to actinides having higher mass numbers and at the same time the actinides can be eliminated by nuclear fission reaction. Furthermore, in the present embodiment, there are the portions having the same fissionable plutonium enrichment in the upper and the lower portions, and the middle portion between them is formed of depleted uranium not containing the fissionable plutonium. However, it is not always necessary that the fissionable plutonium enrichments in the upper and the lower portions is equal to each other. Further, the depleted uranium region is arranged in the position slightly higher than the middle position of the reactor core in the present embodiment, but it is not limited to that position. The values of axial power peaking can be made equal by combining the fissile PU enrichments in the upper and the lower portions and the position of the depleted uranium region. (Second Embodiment) A second embodiment of the present invention will be described below, referring to FIG. 13. The present embodiment is a reactor core of an electric power output of 1356 MWe, and has further shortened fuel assemblies. The horizontal cross section and the cross section of the fuel assembly lattice of the present embodiment are the same as FIG. 1 and FIG. 7 of Embodiment 1, respectively. FIG. 13 shows the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core. Therein, the uranium to be added with plutonium is the depleted uranium. The height of the reactor core is 45 cm, and the reactor core is divided two regions at the levels of 8/12 from the bottom end of the reactor core, and the fissionable plutonium enrichment in the upper region is 13 wt % and the fissionable plutonium enrichment in the upper region is 12 wt %. Further, depleted uranium blankets having heights of 25 cm and 20 cm are attached to the top and the bottom of the reactor core portion, respectively. Operation of the present embodiment will be described below. The construction of the fuel assembly is the same as that of Embodiment 1. By the combination of the regular triangular lattice closed-compact hexagonal fuel assembly having a gap between rods of 1.3 mm, the core-average void fraction of 60% and the large-diameter control rod, an effective water-to-fuel volume ratio of 0.27 was attained, and a breeding ratio of 1.01 was realized. Comparing with Embodiment 1, in regard to the axial direction of the fuel assembly, the present embodiment does not have the depleted uranium region not containing the fissile PU, and the reactor core portion is as short as 45 cm. Further, in regard to the axial direction of the fuel assembly, the fuel assembly is an upper-and-lower two region fuel having different fissile PU enrichments at the levels of 15 cm from the top end of the reactor core, and the fissionable plutonium enrichment in the upper region is 13 wt % and the fissionable plutonium enrichment in the upper region is 12 wt %. On the other hand, when the steam void fraction in the reactor core is increased, the relative increasing amount of void fraction is large in the lower portion of the reactor core having a lower void fraction than the upper portion of the reactor core already reaching the saturation state. As the result, swing of the neutron flux distribution occurs from the upper portion of the reactor core having a higher neutron importance to the lower portion of the reactor core having a lower neutron importance, and thereby the negative void coefficient is inserted. The RBWR of the present embodiment outputs the amount of power equal to that of the ABWR, and can attain burn-up of 45 GWd/t using the pressure vessel having a size nearly equal to that of the ABWR under construction now. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Third Embodiment) A third embodiment of the present invention will be described below, referring to FIG. 14 to FIG. 17. The present embodiment is a reactor core in which the electric output power is the same as that of Embodiment 1, and number of fuel assemblies and the structure of the fuel assembly are changed from Embodiment 1. FIG. 14 is a cross-sectional plan view showing the present embodiment of a reactor core having an electric output power of 1356 MWe. FIG. 14 shows 609 fuel assemblies 7; and 193 control rod drive mechanisms 8 each of which operates large-diameter control rods to be inserted into three fuel assemblies. FIG. 15 shows the cross section of the fuel assembly lattice. In a channel box 9, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 11 fuel rod rows. At two positions in the fuel assembly, two guide tubes 11 to insert the large-diameter control rods 10 thereinto are disposed in the regions having an area equivalent to 2 fuel rod rows, that is, an area equivalent to 7 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. FIG. 16 shows an arrangement of fuel assemblies under the equilibrium core state. Each of the numerals written in each of the fuel assemblies 7 indicates a period staying in the reactor core by cycle numbers. The 5 cycle fuels staying in the reactor core for the longest period are loaded in the outermost periphery of the reactor core where the neutron importance is low. The fuels of 1 cycle staying period in the reactor core having the highest neutron infinite multiplication factor are loaded in the outer region of the reactor core in the inner side of the outermost periphery to flatten the power distribution in the radial direction of the reactor core. In the inner region of the reactor core, the fuels of 2 to 4 cycle staying periods in the reactor core are distributively loaded to flatten the power distribution in the radial direction of the reactor core. FIG. 17 shows an orifice distribution in the equilibrium reactor core state, and the numeral written in the fuels indicates difference in opening degree of an orifice placed in the fuel supporting portion, and there are two regions for the orifice opening degree. The orifice diameter in the reactor outermost peripheral region (number 1) where the fuel assembly power is small is smaller than the orifice diameter in the inner region. The axial distribution of the fissile PU enrichment averaged with the horizontal cross section of the fuel assembly is the same as that of FIG. 10 of Embodiment 1. The area of the region occupied by the control rod in the present embodiment is decreased from one region of 19 fuel rod unit lattice cells to two regions of 7 fuel rod unit lattice cells, but the control rod value is nearly equal to that of Embodiment 1 because the absorption rods are distributively inserted into the fuel assembly. On the other hand, in the present embodiment, number of fuel rods loaded in the reactor core is increased compared to Embodiment 1, and accordingly the average linear power density is reduced to improve the thermal margin. In the present embodiment, by the combination of the regular triangular lattice closed-compact hexagonal fuel assembly, the large-diameter control rod and the core-average void fraction of 60%, an effective water-to-fuel volume ratio of 0.28 is also attained. As the result, the reactor core characteristics are the same as those of Embodiment 1 and the same effect can be obtained. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Fourth Embodiment) A fourth embodiment of the present invention will be described below, referring to FIG. 18. The present embodiment is a reactor core of which the reactor core performance is improved on the base of the structure of Embodiment 1. The present embodiment is of 1356 MWe electric output power, and the reactor core cross section is the same as that of FIG. 1 of Embodiment 1. FIG. 18 shows the cross section of the fuel assembly lattice. In a channel box 3, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 12 fuel rod rows. In the central portion of the fuel assembly, a guide tube 6 to insert the large-diameter control rod 5 thereinto is disposed in the region having an area equivalent to 3 fuel rod rows, that is, an area equivalent to 19 fuel rod unit lattice cells. Outside of the guide tube, water excluding rods 12 for excluding the moderator between the guide tube and the fuel rods adjacent to the guide tube are arranged. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. All of the configuration of fuel assemblies in the reactor core, the orifice state and the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core are the same as FIG. 8, FIG. 9 and FIG. 10 of Embodiment 1, respectively. In the present embodiment, comparing with Embodiment 1 the effective water-to-fuel volume ratio can be improved and the power peaking in the fuel assembly can be suppressed by excluding the moderator around the guide tube. The reactor core characteristics are the same as those of Embodiment 1 and the same effect can be obtained. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Fifth Embodiment) A fifth embodiment of the present invention will be described below, referring to FIG. 19. The present embodiment is a reactor core of which the reactor core performance is improved on the base of the structure of Embodiment 1. The present embodiment is of 1356 MWe electric output power, and the reactor core cross section is the same as that of FIG. 1 of Embodiment 1. FIG. 19 shows the cross section of the fuel assembly lattice. In a channel box 3, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 12 fuel rod rows. At three positions in the fuel assembly, three guide tubes 11 to insert the large-diameter control rods 10 thereinto are disposed in the regions having an area equivalent to 2 fuel rod rows, that is, an area equivalent to 7 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. All of the configuration of fuel assemblies in the reactor core, the orifice state and the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core are the same as FIG. 8, FIG. 9 and FIG. 10 of Embodiment 1, respectively. The area of the region occupied by the control rod in the present embodiment is decreased from one region of 19 fuel rod unit lattice cells of Embodiment 1 to three regions of 7 fuel rod unit lattice cells. Thereby, the absorption rods can be distributively inserted into the fuel assembly, and consequently the control rod value is improved compared to Embodiment 1. The other reactor core characteristics are the same as those of Embodiment 1 and the same effect can be obtained. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Sixth Embodiment) The present embodiment is a case where the present invention is applied to a squire fuel assembly. FIG. 20 shows the construction of the present embodiment of the fuel assembly. In a channel box 13, fuel rods 14 of 10.8 mm diameter are closely arranged in a regular triangular configuration with a 1.3 mm minimum gap between the rods. In the central portion of the fuel assembly, a guide tube 16 to insert the large-diameter control rod 15 thereinto is disposed in the region having an area equivalent to 2 fuel rod rows, that is, an area equivalent to 7 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C, and the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. The large-diameter control rods to be inserted into four of the fuel assemblies are operated by one control rod driving mechanism. In the present embodiment, in order to flatten the fuel rod power peaking in the fuel assembly, the fissile PU enrichment of fuel rods facing the channel box and fuel rods facing the guide tube is made lower than that of the other fuel rods. In the present embodiment, by the combination of the regular triangular lattice closed-compact square fuel assembly having the minimum gap between rods of 1.3 mm, the large-diameter control rod and the core-average void fraction of 60%, an effective water-to-fuel volume ratio of 0.34 was attained, and a breeding ratio of 1.01 was realized. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Seventh Embodiment) A seventh embodiment of the present invention will be described below, referring to FIG. 21 to FIG. 25. The present embodiment is a reactor core in which the electric output power is the same as that of Embodiment 1, and the number of fuel assemblies, the structure of the fuel assembly and the control rod drive mechanism are changed from Embodiment 1. FIG. 21 is a cross-sectional plan view showing the present embodiment of a reactor core having an electric output power of 1356 MWe. FIG. 21 shows 313 fuel assemblies 18; and 313 control rod drive mechanisms 18 each of which operates a large-diameter control rod to be inserted into one fuel assembly. FIG. 22 shows the cross section of the fuel assembly lattice. In a channel box 19, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 15 fuel rod rows. In the central portion of the fuel assembly, a guide tube 21 to insert the large-diameter control rod 20 thereinto is disposed in the region having an area equivalent to 4 fuel rod rows, that is, an area equivalent to 37 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. FIG. 23 shows an arrangement of fuel assemblies under the equilibrium core state. Each of the numerals written in each of the fuel assemblies 17 indicates a period staying in the reactor core by cycle numbers. The 5 cycle fuels staying in the reactor core for the longest period are loaded in the outermost periphery of the reactor core where the neutron importance is low. The fuels of 1 cycle staying period in the reactor core having the highest neutron infinite multiplication factor are loaded in the outer region of the reactor core in the inner side of the outermost periphery to flatten the power distribution in the radial direction of the reactor core. The fuels of 2 to 4 cycle staying periods in the reactor core are distributively loaded in the inner region of the reactor core, and one 5 cycle fuel staying in the reactor core is loaded at the center of the reactor core. By doing so, the power distribution in the inner region is flattened. FIG. 24 shows an orifice distribution in the equilibrium reactor core state. The numeral written in the fuels indicates difference in opening degree of an orifice placed in the fuel supporting portion. There are 6 regions for orifice opening degree in total, that is, 5 regions for individual cycles staying in the reactor core shown in FIG. 23 and 1 region for the center of the reactor core. The orifice diameter in the reactor outermost peripheral region (number 5) where the fuel assembly output power is small is smaller than the orifice diameters in the inner region. FIG. 25 shows the axial distribution of the fissile PU enrichment averaged with the horizontal cross section of the fuel assembly. The uranium to be added with Pu is depleted uranium. In the present embodiment, number of fuel assemblies to be loaded in the reactor core is reduced from 504 assemblies of Embodiment 1 to 313 assemblies by increasing number of fuel rods per one fuel assembly to make the fuel assembly large in size, and thereby the reactor core is made small in size. By making the fuel assembly large in size and at the same time by increasing the region occupied by the control rod from the area equivalent to 19 fuel rod unit lattice cells of Embodiment 1 to the area equivalent to 37 fuel rod unit lattice cells, the control rod value is made nearly equivalent to that of Embodiment 1. Further, One unit of the control rod drive mechanism is used for each of the control rods to be inserted into the fuel assembly. In the present embodiment, by the combination of the regular triangular lattice closed-compact hexagonal fuel assembly, the large-diameter control rod and the core-average void fraction of 60%, an effective water-to-fuel volume ratio of 0.27 is also attained. As the result, the reactor core characteristics are the same as those of Embodiment 1 and the same effect can be obtained. In the present embodiment, the reactor core is constructed so that the large-diameter control rod is also inserted into the fuel assembly loaded in the outermost periphery of the reactor core. However, a reactor core may be designed so that the control rod is not inserted into the fuel assembly in the outermost periphery which has a small effect on securing reactor shut-down margin. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Eighth Embodiment) An eighth embodiment of the present invention will be described below, referring to FIG. 26. The present embodiment is a reactor core in which the electric output power is the same as that of Embodiment 1, and number of fuel assemblies, the structure of the fuel assembly and the control rod drive mechanism are changed from Embodiment 1. The present embodiment has an electric output power of 1356 MWe, and the reactor core is the same as FIG. 21 of Embodiment 7. FIG. 26 shows the cross section of the fuel assembly lattice. In a channel box 19, fuel rods 4 of 10.1 mm diameter are arranged in a regular triangular configuration with a 1.3 mm gap between the rods to form an equilateral hexagonal assembly having 15 fuel rod rows. At two positions in the fuel assembly, two guide tubes 6 to insert the large-diameter control rods 5 thereinto are disposed in the regions having an area equivalent to 3 fuel rod rows, that is, an area equivalent to 19 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. All of the configuration of fuel assemblies in the reactor core, the orifice state and the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core are the same as FIG. 23, FIG. 24 and FIG. 25 of Embodiment 7, respectively. The area of the region occupied by the control rod in the present embodiment is decreased from one region of 37 fuel rod unit lattice cells of Embodiment 7 to two regions of 19 fuel rod unit lattice cells. Thereby, the absorption rods can be distributively inserted into the fuel assembly, and consequently the control rod value is improved compared to Embodiment 7. The other reactor core characteristics are the same as those of Embodiment 7 and the same effect can be obtained. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Ninth Embodiment) The present embodiment is a case where the present invention is applied to a squire fuel assembly. FIG. 27 shows the construction of the present embodiment of the fuel assembly. In a channel box 22, fuel rods 23 of 9.8 mm diameter are closely arranged in a regular triangular configuration with a 1.3 mm minimum gap between the rods. In the central portion of the fuel assembly, a guide tube 25 to insert the large-diameter control rod 24 thereinto is disposed in the region having an area equivalent to 4 fuel rod rows, that is, an area equivalent to 37 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C, and the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. The large-diameter control rods to be inserted into four of the fuel assemblies are operated by one control rod driving mechanism. In the present embodiment, in order to flatten the fuel rod power peaking in the fuel assembly, the fissile PU enrichment of fuel rods facing the channel box and fuel rods facing the guide tube is made lower than that of the other fuel rods. In the present embodiment, by the combination of the regular triangular lattice closed-compact square fuel assembly having the minimum gap between rods of 1.3 mm, the large-diameter control rod and the core-average void fraction of 60%, an effective water-to-fuel volume ratio of 0.34 was attained, and a breeding ratio of 1.01 was realized. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. (Tenth Embodiment) A tenth embodiment of the present invention is explained, referring to FIG. 28. The present embodiment is a modification of the eighth embodiment, and it is a core in which the number of fuel assemblies, the construction of each fuel assembly and each control rod drive mechanism are changed, for the same electric output as in the first embodiment. In the present embodiment, the electric output is 1356 MWe, a core cross-sectional view thereof is the same as FIG. 21 of the seventh embodiment. FIG. 28 shows a cross-section of a fuel assembly lattice. Inside a channel box 19, fuel rods 4 of diameter 10.1 mm are arranged in a regular triangular configulation with a gap of 1.3 mm between the fuel rods to form an equilateral hexagonal fuel assembly having 15 fuel rod rows. Inside the fuel assembly, guide tubes 6, in each of which a large-diameter control rod 5 is inserted, are arranged at six locations, and each guide tube 6 is disposed in a region having an area equivelent to two fuel rod rows, that is, an area equivalent to 7 fuel rod unit lattice cells. The large-diameter control rod is formed of an absorption rod of a stainless steel tube filled with B4C. Further, the large-diameter control rod has a follower portion in the top end portion, the follower portion being made of carbon which is a substance having a slowing-down power smaller than that of light water. All of the configuration of fuel assemblies in the reactor core, the orifice state and the axial distribution of fissionable plutonium enrichment averaged over the horizontal cross section of the fuel assembly for the equilibrium reactor core are the same as FIG. 23, FIG. 24 and FIG. 25 of Embodiment 7, respectively. The area of the region occupied by the control rod in the present embodiment is decreased from one region of 37 fuel rod unit lattice cells of Embodiment 7 to six regions of 7 fuel rod unit lattice cells. Thereby, the absorption rods can be distributively inserted into the fuel assembly, and consequently the control rod value is improved compared to Embodiment 7. The other reactor core characteristics are the same as those of Embodiment 7 and the same effect can be obtained. In the present embodiment, the same or more effects can be also obtained by the fuel enriched by adding plutonium to natural uranium or the degraded uranium recovered from used fuel or the low enriched uranium instead of the depleted uranium. Further, the other actinides can be added together with Pu. According to the present invention, by attaining the breeding ratio of near 1.0 or more than 1.0 using the fuel which is enriched by adding plutonium or plutonium to depleted uranium, natural uranium, degraded uranium or low enriched uranium, the depleted uranium, the natural uranium, the degraded uranium or the low enriched uranium can be burned using the plutonium like a catalyst, which can contribute to the long-term stable energy supply.