Patent Number: 048511811
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention applied to a pressurized water type nuclear reactor which belongs to the light water reactor will be explained with reference to FIG. 1. and FIG. 2. This embodiment is the pressurized water type nuclear reactor which divides a reactor core into two areas. A pressurized water type nuclear reactor 1 includes a reactor pressure vessel 2 having a reactor core 12. The reactor pressure vessel 2 has on its wall face an inlet nozzle 3 and an outlet nozzle 4. The reactor pressure vessel 2 is provided with a sealed lid 5. A substantially cylindrical reactor core shroud 6 is hung from the shelf of the reactor pressure vessel 2 which is close to the sealed lid 5 at the top thereof. Baffles 7 are attached to the reactor core shroud 6 at its lower section. A lower reactor core support plate 8 which is provided with holes to receive the lower ends of a fuel assembly A and a fuel assembly B is attached to the lower end of the reactor core shroud 6. An upper reactor core support plate 9 which is provided with holes to receive the upper ends of the fuel assembly A and the fuel assembly B is supported on the upper support plate 11 with large beams by means of a plurality of support columns 10. A plurality of guide tubes 26 are provided between the upper support plate 11 and the upper reactor core support plate 9. Each guide tube 26 is provided with openings 33. The reactor core 12 is formed at the lower section and the inside of the reactor core shroud 6. The reactor core 12 comprises of many fuel assemblies A, many fuel assemblies B and a tubular partition member 13. The tubular partition member 13 is made of zircalloy (zirconium alloy). In this embodiment the reactor core 12 is divided by the tubular partition member 13 into two areas, namely an inside central area (a high conversion area) 27 and an outside peripheral area (a burner area) 28. The fuel assembly A is arranged in the central area 27 inside of the partition member 13. The fuel assembly B is located in the peripheral area 28 outside of the partition member 13. The fuel assembly A includes, as shown in FIG. 3 and FIG. 4, a plurality of fuel rods 15, a lower tie plate 16, an upper tie plate 17 and spacers 18. 21 denotes a handle. The shapes of the lower tie plate 16 and the upper tie plate 17 are regular hexagon form, respectively. Both ends of the fuel rods 15 are supported respectively by the lower tie plate 16 and the upper tie plate 17. The lower tie plate 16 has in its inside a cylindrical section 16A, and the inside cylindrical section 16A is constituted with a plurality of connecting plates 16B. The connecting plates 16B are arranged radially and connect the inside cylindrical section 16A to an outside cylindrical section 16C. The fuel rods 15 are arranged to form a regular hexagon as shown in FIG. 4. Some of the fuel rods 15 function as the tie rods 15A. Both ends of the tie rods 15A pierce the lower tie plate 16 and the upper tie plate 17 respectively. The lower end of the tie rods 15A is secured by a nut 19 and the uppers end of the tie rods 15A are secured by a tightening nut 20. The upper tie plate 16 and the lower tie plate 17 are connected respectively by those tie rods 15A. The fuel rods 15 (including the tie rods 15A) are filled with fuel pellets which are enclosed in the cladding tubes made of zilcalloy. The fuel pellets include uranium-235 as a fissile material. In the axial direction of the bundle of the fuel rods 15 a plurality of spacers 18 are provided. The spacers 18 prevent the adjacent fuel rods 15 from contacting mutually and secure flow the channels among the fuel rods 15 for the coolant (light water). The fuel assembly B includes, as shown in FIG. 5 and FIG. 6, a plurality of fuel rods 15 and a plurality of burnable poison rods 15B, a lower tie plate 23 and an upper tie plate 24 and spacers 25. In the fuel assembly B also its lower tie plate 23 and its upper tie plate 24 are connected respectively by the tie rods 15A which are a part of the fuel rods 15 as in the fuel assembly A. The lower tie plate 23 has a cylindrical section 23A in its inside, and the cylindrical section 23 is connected to an outside cylindrical section 23C by means of a plurality of connecting plates 23B. Both ends of the fuel rods 15 and the burnable poison rods 15B are supported on the lower tie plate 23 and the upper tie plate 24 respectively. The fuel rods 15 (including the tie rods 15A) have the same construction as in the fuel assembly A. The burnable poison rods 15B are filled with a mixture of the hydrogenerated zirconium as a moderator and gadolinium which is a burnable posion. The condensation distribution of the hydrogenerated zirconium and the gadolinium in the burnable poison rods 15B are axially uniform respectively. The spacers 25 formed with a regular hexagon shape and hold the fuel rods 15 which are supported by the lower tie plates 17 and the upper tie plates 24 to form a regular hexagon bundle. Several spacers 25 are provided in the axial direction and prevent mutual contact of the fuel rods 15. In place of the burnable poison rods 15B the fuel rods 15 in which urainium dioxide (UO.sub.2) pellets mixed with gadolinium are filled, may be provided in the fuel assembly B. The distance among the adjacent fuel rods 15, that is, the rod pitch is larger in the fuel assembly B than that in the fuel assembly A. Accordingly the number of fuel rods 15 which constitute the fuel assembly B is smaller than the number of fuel rods 15 which constitute the fuel assembly A. All of the horizontal cross-sectional areas (the cross-sectional area perpendicular to the axis of the fuel assembly) of the lower tie plates 16 and 23 and the upper tie plates 17 and 24 are the same. The cylindrical section 16A of the fuel assembly A and the cylindrical section 23A of the fuel assembly B are inserted into the holes of the lower reactor core support plate 8, and the cylindrical section 16C of the fuel assembly A and the cylindrical section 23C of the fuel assembly B are installed respectively on the lower reactor core support plate 8. The upper section 17A of the upper tie plate 17 for the fuel assembly A and the upper section 24A of the upper tie plate 24 for the fuel assembly B are inserted respectively into the holes in the upper reactor core support plate 9. In this way the fuel assembly A and the fuel assembly B are supported respectively by the lower reactor core support plate 8 and the upper reactor core support plate 9. In the central area 27 the adjacent lower tie plate 16 and the upper tie plate 17 of the fuel assembly A are in contact with each other. The lower tie plate 16 and the upper tie plate 17 of the fuel assembly A which are located respectively at the utmost periphery of the central area 27 in contact with the inner face of the partition member 13. In the peripheral area 28 also the adjacent lower tie plate 23 and the upper tie plate 24 of the fuel assembly B are in contact with each other. The lower tie plate 23 and the upper tie plate 24 of the fuel assembly B which is located respectively at the utmost inside of the peripheral area 28 are in contact with the outer face of the partition member 13. The pressurized water type nuclear reactor is provided with a control rod driving mechanism 29 which is operated by hydraulic pressure. The construction of the control rod driving mechanism 29 is the same as disclosed in U.S. Pat. No. 3,607,629. The control rod drive mechanism 29 is mounted on an upper flange 32 of an adaptor tube 31. The adaptor tube 31 not only pierces the sealed lid 5, but also is attached to the sealed lid 5 by welding. A control rod 34 shown in FIG. 7 is removably installed to the lower end of the control rod driving mechanism 29. The control rod 34 moves up and down in a guide tube 26. The control rod 34 into the fuel assembly A and the fuel assembly B and also the operation to pull it out of them. The fuel assembly A and the fuel assembly B into which the control rod 34 is inserted are generally called a fuel assembly for control A.sub.o and a fuel assembly for control B.sub.o respectively. The fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o are arranged in the reactor core 12. The fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o include the fuel assembly A and the fuel assembly B by the proportion of three fuel assemblies A and one fuel assembly B, and are arranged coaxially with the guide tube 26 which is positioned right under the adaptor tube 31. The construction of the control rod 34 will be explained with reference to FIG. 7. The control rod 34 comprises a main body 35 connected to the control rod driving mechanism 29, support members 36 attached radially to the main body 35, and neutron absorption rods 37 which are installed to the support members 36 at their end portions. The neutron absorption rod 37 is a sealed cladding tube which is filled with a neutron absorption material, boron carbide (B.sub.4 C). Now the fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o into which the control rod 34 is inserted will be explained. The fuel assembly for control A.sub.o is different from the fuel assembly A in that a plurality of hollow control guide tubes 38 are, as shown in FIG. 8, used in place of the tie rods 15A. The control guide tubes 38 connect the lower tie plate 16 and the upper tie plate 17. The fuel assembly for control B.sub.o is different from the fuel assembly B in which the plurality of hollow control guide tubes 38 are, as shown in FIG. 9, used in place of the tie rods 15A and connect the lower tie plate 23 and the upper tie plate 24. The fuel rods 15 and the fuel assembly for control B.sub.o charged at the peripheral areas 28 are the fuel rods 15 which constitute the fuel assembly A and the fuel assembly for control A.sub.o and they had been burned for the specified period in the central area 27. The rod number of in the fuel assembly B (including the fuel assembly for control B.sub.o) which are charged in the peripheral area 28 is about twice as many as the rod number of in the fuel assembly A (including the fuel assembly for control A.sub.o) which are charged in the central area 27. Namely the cross-sectional area in the peripheral area 28 is about twice as large as the cross-sectional area of the central area 27. During the operation of the light water reactor the coolant (light water) which is sent from a steam generator (not shown) and also serves as a moderator enters the reactor pressure vessel 2 through the inlet nozzle 3. The coolant flows downwardly in an annular channel 39 which is formed between the reactor pressure vessel 2 and the reactor core shroud 6, and then the coolant flows into a lower plenum 40 which is formed below the lower reactor core support plate 8. This coolant then passes through the cylindrical section 16A and the cylindrical section 23A of the lower tie plates 16 and the lower tie plates 23. The fuel assembly A and the fuel assembly for control A.sub.o, and the fuel assembly B and the fuel assembly for control B.sub.o are supported respectively by the lower reactor core support plate 8. The coolant flows into each of the fuel assembly A and the fuel assembly B. Since the number of the fuel rods 15 in the fuel assembly B and the fuel assembly for control B.sub.o is smaller than that of in the fuel assembly A and the fuel assembly for control A.sub.o, the pressue loss in the fuel assembly B and the fuel assembly for control B.sub.o is smaller than that of in the fuel assembly A and the fuel assembly for control A.sub.o. This condition makes it easier for the large quantity of the coolant to flow through the fuel assembly B and the fuel assembly for control B.sub.o. An orifice 41 is provided as shown in FIG. 5 in the cylindrical section 23A of the fuel assembly B and the fuel assembly for control B.sub.o respectively in order to make uniform the flow rate of the coolant that is supplied to both the central area 27 and the peripheral area 28. Since the upper reactor core support plate 9 is in contact with the upper end of the cylindrical section 24B of the upper tie plate 24, the fuel assembly B and the fuel assembly for control B.sub.o prevent respectively floating up. The coolant is heated to the high temperature water in the process of ascending through the fuel assembly A and the fuel assembly B. The high temperature coolant discharged from the fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o for control flows into the guide tube 26 which is located right above them and flows out from the opening 33 into the upper plenum 42 which is positioned above the upper reactor core support plate 9. The rest of the high temperature coolant discharged from both the fuel assembly A and the fuel assembly B reaches the upper plenum 42 through the upper reactor core support plate 9. The coolants which flow in the central area 27 and the peripheral area 28 do not mix in the reactor core 12 because of the existence of the partition member 13, but the coolants are mixed in the upper plenum 42. The high temperature coolant flows out of the reactor pressure vessel 2 from the upper plenum 42 through the outlet nozzle 4 and then the coolant is sent to the steam generator. The control of the reactor output power is carried out by putting the neutron absorption rods 37 of the control rod 34 into the hollow control guide tubes 38 of the fuel assembly for control A.sub.o and the fuel assembly B.sub.o for control or pulling the control rod 34 out of the hollow control guide tubes 38. In the embodiment explained above the average number of the fuel rods 15 (including the tie rods 15A) arranged in per unit area in the central area 27 is larger than the average number of the fuel rods 15 (including the tie rods 15A) arranged in per unit area in the peripheral area 28. Accordingly the average density of the fuel rods 15 in per unit area in the central area (the high conversion area) 27 is larger than the average density of the fuel rods 15 in per unit area in the peripheral area 28 (the burner area). In the embodiment the average density of the fuel rods 15 in each of the central area 27 and the peripheral area 28 is set up so that the ratio (averge value) of the number of hydrogen atoms to that of fuel material atoms is about 1.0 in the central area 27 and the ratio (average value) of the number of hydrogen atoms to that of fuel material atoms is about 5.0 in the peripheral area 28. Since the central area (the high conversion area) 27 has a small value, about 1.0 for the ratio of the number of hydrogen atoms to that of fuel material atoms, the central area (the high conversion area) 27 becomes a breeding area, making larger the conversion ratio from uranium 238 that is contained in uranium oxide (UO.sub.2) pellets of the fuel rods 15 placed in the central area 27 to plutonium-239 during the operation of the light water reactor. Since the peripheral area (the burner area) 28 has a large value, about 5.0 for the ratio of the number of hydrogen atoms to that of fuel material atoms, the peripheral area (the burner area) 28 becomes the burning area, making it possible to burn the fissile materials effectively by activated fission of fissile materials such as the uranium-235 or the plutonium-239 which are contained in uranium oxide (UO.sub.2) pellets of the fuel rods 15 placed in the peripheral area 27 during the operation of the light water reactor. The fuel rods 15 charged in the central area 27, that is, the fuel assembly A and the fuel assembly for control A.sub.o are charged in the central area 28 in the period from the burnup O to the burnup E.sub.a as shown in FIG. 10. The neutron multiplication factors of the fuel assembly A and the fuel assembly for control A.sub.o drop as shown by the curve P.sub.1 (solid line) of FIG. 10 as the burnup period elapses. The fuel rods 15 charged in the peripheral area 28, that is, the fuel assembly B and the fuel assembly for control B.sub.o are charged as shown in FIG. 10 in the peripheral area 28 during the period from the burnup E.sub.a to the burnup E.sub.b. The neutron multiplication factors of the fuel assembly B and the fuel assembly for control B.sub.o drop as shown by the curve P.sub.2 (solid line) of FIG. 10 as the burnup period elapses. At the initial stage of the curve P.sub.2, namely after passing the burnup E.sub.a the neutron multiplication factor increases towards a point X. It is because gadolinium in burnable poison rods 15B disappears gradually. The gadolinium disappears entirely at the point X. The fuel assembly A and the fuel assembly for control A.sub.o which are charged in the central area 27 and have reached the burnup E.sub.a will find it difficult to continue burning in the central area 27 where the ratio of the number of hydrogen atoms to that of fuel material atoms is about 1.0, because their neutron multiplication is low. For this reason the fuel rods 15 in the fuel assembly A and in the fuel assembly for control A.sub.o which have been charged in the central area 27 and reached the burnup E.sub.a are moved and charged in the peripheral area 28 while the light water reactor operation is stopped. After this their burning is continued up to the burnup E.sub.b in the peripheral area 28 where the ratio of the number of hydrogen atoms to that of fuel material atoms is about 5.0. The movement of the fuel rods 15 from the central area 27 to the peripheral area 28 is made under the condition that the fuel assembly A and the fuel assembly for control A.sub.o are disassembled and reassembled to become the fuel assembly B and the fuel assembly for control B.sub.o. This reassembly of the fuel assembly A and the fuel assembly for control A.sub.o to the fuel assembly B and the fuel assembly for control B.sub.o will be explained based on the fuel assembly A and the fuel assembly B. The fuel assembly A which possesses the fuel rods 15 that have been charged in the central area 27 and reached the end of their life (have reached the burnup E.sub.a) is taken out of from the reactor core 12 after the light water reactor stopped, and moved to a fuel pump (not shown ) outside of the reactor pressure vessel 2. Namely, after the light water reactor operation has stopped, the sealed lid 5, the upper support plate 11 and the guide tube 26 are removed from the reactor pressure vessel 2. The control rods 34 are separated from the control rod driving mechanism 29 and are inserted into the fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o. In this state the fuel assembly A which has reached the burnup E.sub.a is moved from the reactor core 12 to the fuel pool. The work to pull out the fuel assembly for control A.sub.o and the fuel assembly for control B.sub.o from the reactor core 12 is conducted after the control rod 34 in the reactor core is pulled out. Then the tightening nut 20 of the fuel assembly A that has been moved to the fuel pool is removed, and after the upper tie plate 17 has been removed from the tie rods 15A, each of the fuel rods 15 is pulled out upwardly to be removed from the lower tie plate 16. After the nut 19 is removed, the tie rods 15A are removed from the lower tie plate 16. The spacers 18 are also removed from the tie rods 15A. This disassembly work for the fuel assembly A is carried out in the water of the fuel pool by means of remote-controllable tools. The tie rods 15A which are removed in the disassembly work of the fuel assembly A are installed on the lower tie plate 23 which constitute the fuel assembly B and the nut 19 is installed to its lower end. The spacer 25 is already installed on the tie rods 15A. The fuel rods 15 removed from the fuel assembly A are inserted into the spacers 25 and their lower ends are installed to the lower tie plate 23. The burnable poison rods 15B which are provided separately are inserted into the spacers 25 like the fuel rods 15 and installed to the lower tie plate 23. The burnable poison rod 15B is effective to suppress the initial surplus reactivity of the fuel assembly B and the fuel assembly for control B.sub.o which are charged in the peripheral area 28 to the state before the point X on the curve P.sub.2 shown in FIG. 10. It is necessary because the fuel assembly B and the fuel assembly for control B.sub.o are charged in the area where the ratio of the number of hydrogen atoms to that of fuel material atoms is large. The upper tie plate 24 pinches the fuel rods 15 and installs it by the tightening nut 20 to the tie rod 15A which is positioned on the side opposite to the lower tie plate 23. When in place of the burnable poison rods 15B the fuel rods with pellets containing gadolinium are used, it is necessary that the composition of the fissile material in the fuel pellets of the fuel rod should be the same as the fissile material contained in the fuel rods 15 which constitute the fuel assembly at the burnup E.sub.a. Out of one fuel assembly A two fuel assemblies B can be obtained. The work of disassembling the fuel assembly for control A.sub.o and the disassembly work of the fuel assembly for control B.sub.o proceeds in the same way. The fuel rods 15 (including the tie rods 15A) of the fuel assembly for control A.sub.o and the control guide tubes 38 can be reused for the fuel assembly for control B.sub.o. The fuel rods 15 of the fuel assembly A can be used for the fuel assembly for control B.sub.o and the fuel rods 15 of the fuel assembly B can be used for the fuel assembly for control A.sub.o. When the fuel assembly A and the fuel assembly for control A.sub.o at the burnup E.sub.a are taken out of the central area 27, the fuel assembly B and the fuel assembly for control B.sub.o at the burnup E.sub.a are taken out of the peripheral area 28. As stated above, the assembled fuel assembly B and fuel assembly for control B.sub.o is moved to the reactor pressure vessel 2 from the fuel pool and charged at the specified position in the peripheral area 28. At the position in the central area 27 in which the fuel assembly A and the fuel assembly for control A.sub.o are taken out, the new fuel assembly A and the fuel assembly for control A.sub.o are charged. The fuel rods 15 which constitute those new fuel assembly A and fuel assembly for control A.sub.o are filled with the enriched uranium which has been produced by enriching natural uranium. The fuel assembly B and the fuel assembly for control B.sub.o which have reached the burnup E.sub.b and are taken out of reactor core 12 are stored as the used fuel assemblies, and are not subjected to reprocessing. The above described fuel assembly exchanges are conducted substantially once a year. Accordingly by a fuel exchange work once a year, one quarter of the fuel assembly A and the fuel assembly B are exchanged in the central area 27 and in the peripheral area 28, respectively. After the new fuel assembly A and the fuel assembly for control A.sub.o and the reassembled fuel assembly B and fuel assembly for control B.sub.o are charged in the reactor core 12, the sealed lid 5, the upper support plate 11 and the guide tubes 26 are installed in the reactor pressure vessel 2, and the control rod driving mechanism 29 and the control rod 34 which have been separated are connected by the operation of the control rod driving mechanism 29. After this, the control rod 34 is pulled out of the reactor core 12 to raise the light water reactor power, and the operation of the light water reactor starts. The fuel rods 15 once charged in the reactor core 12 are assembled into the fuel assembly A or the fuel assembly for control A.sub.o at first, then charged in the central area 27. The fuel rods 15 are burned as shwon by the curve P.sub.1 shown in FIG. 10 to reach the burnup E.sub.a. During this burning, the conversion ratio increases in the fuel rods 15 as explained in the foregoing and a large amount of plutonium is accumulated in the fuel rods 15. Then the fuel rods 15 are taken out of the central area 27 and installed into the fuel assembly B or the fuel assembly for control B.sub.o to be charged in the peripheral area 28. The fuel rods 15 are burned as shown by the curve P.sub.3 (two-dot chain line, and after the point X merging to the curve P.sub.2) of FIG. 10 to reach the burnup E.sub.b. The fuel rods 15 that have reached the burnup E.sub.b are taken out of the peripheral area 28 and then the fuel rods 15 are subjected to the discharging treatment. In the embodiment the fissile materials filled in the fuel rods 15 are burned effectively without reprocessing. The curve P.sub.4 shown in FIG. 10 shows the variation in the neutron multiplication factor in the conventional pressurized water type nuclear reactor which has the uniform ratio (about 2.0) of the number of hydrogen atoms to that of fuel material atoms in the reactor core. The enrichment of the fuel rods charged for the first charged for the first time in the reactor core is equal to the enrichment of the fuel rods 15 in the embodiment of the present invention which is charged in the central area 27 for the first time. In the conventional light water reactor the fuel assemblies are exchanged for new fuel assemblies at the burnup E.sub.c. According to the present invention by which the production of plutonium is increased in the central area (the high conversion area) 27 and the fissile materials in the peripheral area (the burner area) 28 are burned with a high efficiency, the average burnup of the fuel rods 15 at which the fuel rods 15 are taken out is the burnup E.sub.b, and the average burnup of the fuel rods 15 charged in the reactor core 12 of the conventional light water reactor is the burnup E.sub.c. If the amount of charged fuel in the embodiment which makes the average take-out burnup larger and the amount of charged fuel in the conventional light water reactor are the same, the reactor core 12 of the embodiment of the present invention is able to save uranium by (E.sub.b -E.sub.a)/E.sub.b in comparison with the reactor core of the conventional light water reactor. When a burnup ratio E.sub.a /E.sub.b is 2/3, the uranium saving by the reactor core 12 of the embodiment of the present invention is 33% of the uranium used in the reactor core of the conventional light water reactor. It is possible in the embodiment of the present invention to use the fuel effectively by the once-through method and, furthermore, the uranium saving is still achieved. FIG. 11 compares the characteristics of the reactor core 12 of the embodiment of the present invention in FIG. 1 with those of the conventional reactor core using the fuel rods which have uniformly the ratio of the number of hydrogen atoms to that of fuel material atoms of about 2.0 and of which the initial enrichment of the fuel rods is raised to be able to reach the take-out burnup E.sub.b which is the take-out burnup of the embodiment of the present invention. The characteristics of the conventional reactor core are represented by the curve P.sub.5. With this conventional reactor core the initial neutron multiplication factor is large, and in order to suppress this the large amount of burnable poison material such as the gadolinium has to be put in the fuel assemblies at the expense of the neutron economy. Furthermore, in the conventional light water reactor the mixing into the reactor core fuel assemblies which are much different in the neutron multiplication factor makes it difficult to flatten the output power distribution and the maximum burnup is, therefore, restricted by the fuel rods of peak output power with the result of the lowered average take-out burnup. In the reactor core 12 of the embodiment of the present invention the mismatch in the neutron multiplication factors among the fuels is relatively small and the average take-out burnup can be made high. Another embodiment of a boiling water type nuclear reactor which is a kind of light water reactor and to which the present invention is applied will be explained below. FIG. 16 is a cross-sectional view of the boiling water type nuclear reactor according to the present invention. The boiling water type nuclear reactor 50 is provided with a nuclear reactor pressure vessel 51 which is tightly sealed by a sealed lid 52 at its top. A reactor core shroud 53 is installed in the reactor pressure vessel 51. A steam separator 54 is mounted on the top end of the reactor core shroud 53, and a dryer 55 located above the steam separator 54. A lower reactor core support 56 and an upper reactor core support 57 are installed in the reactor core shroud 53. The upper section and lower section of a fuel assembly 61A and a fuel assembly 61B charged in the reactor core 58 are respectively supported by the lower reactor core support plate 56 and the upper reactor core support plate 57. A cylindrical partition member 62 is installed between the lower reactor core support plate 56 and the upper reactor core support plate 57. The partition member 62 divides the reactor core 58 radially into a central area (a high conversion area) 59 and a peripheral area (a burner area) 60. The fuel assembly 61A is charged in the central area 59 and the fuel assembly 61B is charged in the peripheral area 60. The fuel assembly 61A includes the fuel assembly A shown in FIG. 3 and a hexagonal and cylindrical channel box attached to the fuel assembly A. The channel box encloses the outside of a bundle of the fuel rods 15 that is bound by spacers 18, and the upper end of the channel box is removably attached to the upper tie plate 17. The fuel assembly 61B consists of the fuel assembly B shown in FIG. 5 and a hexagonal and cylindrical channel box which is attached removably to the fuel assembly 61B as the fuel assembly 61. This channel box encloses the outside of a bundle of the fuel rods 15 and the upper end of the channel box is removably attached to the upper tie plate 24. Part of the fuel assembly 61A charged in the central area 59 is a fuel assembly for control, and this fuel assembly for control consists of a fuel assembly for control A.sub.o shown in FIG. 8 which has control rod guide tubes 38 and to which the channel box is removably attached. Part of the fuel assembly 61B charged in the peripheral area 60 is a fuel assembly for control, and this fuel assembly for control consists of a fuel assembly for control B.sub.o shown in FIG. 9 to which the channel box is removably attached. The fuel assembly 61A, the fuel assembly for control and the fuel asembly 61B, the fuel assembly for control are arranged in the reactor core 58 as the reactor core 12 shown in FIG. 1. A jet pump 65 supplies to the reactor core 58 light water which is a coolant and also a moderator. The average density of the fuel rods 15 per unit area in the central area 59 and the average density of fuel rod 15 per unit area in the peripheral area 60 are arranged as in the reactor core 12 shown in FIG. 1. The average density of the fuel rods 15 per unit in the central area 59 is larger than that of in the peripheral area 60. The ratio of the number of hydrogen atoms to that of fuel material atoms in each of the central area 59 and the peripheral area 60 is the same as in the core reactor 12 shown in FIG. 1. A control rod driving mechanism (not shown) is attached to the cup-shaped bottom section of the reactor pressure vessel 51. A control rod guide tube 63 is provided in the reactor pressure vessel 51 and also is installed above the control rod driving mechanism and on the extension line through the control rod driving mechanism. The lower end of a control rod 64 is removably connected to the control rod driving mechanism. The control rod 64 moves up and down in the control rod guide tube 63, and also has a plurality of absorber rods as the control rod 34. Those absorber rods extend upwardly. The absorber rod of each control rod 64 gets in and exits from the control rod guide tube 38 from under. The control rod guide tube 38 belongs to each of the fuel asssembly for control and the fuel assembly for control which are charged in the central area 59 and in the peripheral area 60. When, as in the pressurized water type nuclear reactor described above as an embodiment of the present invention, the fuel assembly 61A charged in the central area 59 reaches the limit burnup E.sub.a, the fuel rods 15 assembled in the fuel assembly 61A are removed from the fuel assembly 61A and used as constituents in the fuel assembly 61B. The fuel rods 15 which constitute the fuel assembly 61B are, in turn, charged ih the peripheral area 60 where the ratio of the number of hydrogen atoms to that of fuel material atoms is large and the burning of the fuel rods 15 continues to reach the burnup E.sub.b. In the embodiment also fuel materials can be utilized effectively in the once-through method as in the above mentioned embodiment, and furthermore the uranium saving can be achieved with this embodiment. In the reactor core 58 of the embodiment shown in FIG. 16 the fuel assembly 61A and the fuel assembly 61B which are close to one another are arranged so that their channel boxes are in contact. As shown in FIG. 17, however, gaps 66 can be set up among the channel boxes which are adjacent to the fuel assembly 61B in the peripheral area 60. In the gap 66 a control rod 67 connected to the control rod driving mechanism is inserted from under in place of the control rod 64. The control rod 67 from which a blade containing the absorber rod extends in three directions is used only in the peripheral area 60. The control rod 64 is inserted into the central area 59 as the above described embodiment but not into the peripheral area 60. The average density of the fuel rods 15 per unit area in the peripheral area 60 is smaller than the average density of the fuel rods 15 in the peripheral area 60 of the embodiment shown in FIG. 16, because there are the gaps 66 among the fuel assemblies 61B. In this embodiment also the heat exchange in the above described embodiment is effected and achieved similarly. Another embodiment of reactor core in the pressurized water type nuclear reactor is shown in FIG. 18A and FIG. 18B. The reactor core of the embodiment has the central area 68 shown in FIG. 18A and the peripheral area 69 shown in FIG. 18B. Both the central area 68 and the peripheral area 69 correspond respectively to the central area 27 and to the peripheral area 28 shown in FIG. 1. The peripheral area 69 encloses the central area 68. In the central area 68 many fuel assemblies 70A are arranged side by side. In the peripheral area 69 many fuel assemblies 70B are arranged side by side with the light water area 71 in between. The fuel assembly 70A has the fuel rods 15 arranged as shown in FIG. 19, and the fuel rods 15 are retained in the upper tie plate and in the lower tie plate. The fuel assembly 70B is the burnable poison rods 15B replacing the fuel rods 15 in the central area 68. There is the partition member above explained between the central area 68 and the peripheral area 69. Into the control rod guide tube of the specified fuel assembly 70A in the central area 68 the absorber rod for the control rod is inserted. The control rods are inserted into the light water areas 71 in the peripheral area 69. FIG. 20A and FIG. 20B show another embodiment of the pressurized water type nuclear reactor. The reactor core of the embodiment has a central area 72 shown in FIG. 20A and a peripheral area 73 shown in FIG. 20B. The central area (the high conversion area) 72 and the peripheral area (the burner area) 73 correspond respectively to the central area 27 and to the peripheral area 28 shown in FIG. 1. The peripheral area 73 encloses the central area 72. The central area 72 and the peripheral area 73 are divided by the partition member as described above. In the central area 72 many fuel assemblies 73A are arranged side by side. In the peripheral area 73 there are light water areas 74 among the fuel assemblies 73B. The fuel assembly 73A has the fuel rods 15 arranged as shown in FIG. 21. Both ends of the fuel rods 15 are retained in an upper tie plate and an lower tie plate. The fuel assembly 73A is the burnable poison rod replacing the fuel rods 15. The absorber rod of the control rod is inserted into the control rod guide tube of the specified fuel assembly 73A in the central area 72. The control rod is inserted into the specified light water area 74 in the peripheral area 73. Each of the fuel assemblies 70A and 73A and the fuel assemblies 70B and 73B arranged in the recator core shown in FIG. 18A, FIG. 18B and FIG. 20A, FIG. 20B is exceedingly smaller than the fuel assembly A and fuel assembly B charged in the reactor core 12 of FIG. 1. In the reactor core of FIG. 18A, FIG. 18B, FIG. 20A and FIG. 20B, the average density of fuel rod per unit area in the peripheral areas 69 or 73 is smaller than the average density in the central area 68 or 72 because of the existence of the light water areas 71 or 74. The ratio of the number of hydrogen atoms to that of fuel material atoms is about 1.0 in the central area 68 or 72 and is about 5.0 in the peripheral area 69 or 73. In the pressurized water type nuclear reactor with the reactor core shown in FIG. 18A, FIG. 18B, FIG. 20A and FIG. 20B, the fuel assembly 70A and the fuel assembly 73A are disassembled when they reach the specified burnup E.sub.a as with the pressurized water type nuclear reactor shown in FIG. 1 and FIG. 2, and the fuel rods 15 are removed. Those fuel rods 15 and the burnable poison rods 15B are assembled to form the fuel assembly 70B and the fuel assembly 73A. The assembled fuel assembly 70B and the fuel assembly 73A are charged respectively in the peripheral areas 69 and 73. Accordingly the control rod once charged in the light water reactor can be burned up to the burnup E.sub.b as for the embodiment shown in FIG. 2. The take-out burnup for the embodiment is exceedingly increased as in the embodiment shown in FIG. 2 in comparison with the conventional light water reactor. This allows effective utilization of the fuel materials and the uranium saving. Furthermore, the effects (1)-(3) listed below are obtained by using the small size fuel assemblies as in the reactor core of FIG. 18A, FIG. 18B and FIG. 20A and FIG. 20B. (1) Minute adjustment of the ratio of the number of hydrogen atoms to that fuel material atoms can be made by adjusting the number of the fuel rods charged per unit cross-sectional area in the reactor core. (2) In the peripheral area, if the fuel assembly is removed, it leaves a light water gap area, but its width is not made any wider than necessary. (3) The fuel rods in the peripheral area can face the light water gap area except for the fuel assembly, and this makes it possible to utilize the thermal neutrons effectively. FIG. 22 shows another embodiment of the peripheral area 73 which employs the fuel assembly 73B. With the arrangement of the fuel assembly such as the fuel assembly 73B, the fuel rods are positioned with the equal distance among the fuel assemblies 73B. The reactor core construction in the pressurized water type nuclear reactor shown in FIG. 18A, FIG. 18B, FIG. 20A and FIG. 20B can be achieved the reactor core in the boiled water type nuclear reactor as well. FIG. 23 shows the mutual relationship between the change of the fissile material, quantity (W/O) and the change of the burnup; and also the mutual relationship between the change of the neutron multiplication (K .infin. ) and the change of the burnup in an embodiment of the present invention. The reactor core of the embodiment shown in FIG. 23 is divided into two areas, that is, one high coversion area (a central area) and one burner area (a peripheral area). The established conditions of the embodiment shown in FIG. 23 are as follows; (1) The initial concentration of the fissile material quantity (W/O) is 6%; (2) the reactor core is divided into the high conversion area and the burner area, and the burner area starts when the burnup reaches at the condition of the burnup 45 GWd/t or 45%; (3-a) the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fissile material in the high conversion area is about 0.9, and the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fissile material in the burner area is about 2.2; or (3-b) the ratio (r.sub.H/U) of the number of hydrogen atoms and the number of fuel material atoms in the high conversion area is about 2.0, and the ratio (r.sub.H/U) of the number of hydrogen atoms and the number of fuel material atoms in the burner area is about 5.0. In the high conversion area plutonium is generated and accumulated therein, and in the burner area the activated fissile material moved from the high conversion area is burned effectively. Thus the burnup attains a high value as about 100 GWd/t. Besides in the conventional light water reactor, the initial concntration of the fissile material quantity (W/O) is about 3%, and the burnup is about 30 GWd/t. Even if the initial concentration of the fissile material quantity (W/O) were about 6%, the burnup would reach not more than about 60 GWd/t in the conventional light water reactor. In comparison with to the burnup of the conventioanl light water, the burnup of the above embodiment of the present invention can be obtained highly about twice value. Furthermore, the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material in the high conversion area of the embodiment of the present invention is raised to about 0.9. In the comparison with to the value 0.5 for the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material in the high conversion reactor (HCR), the above value 0.9 for the ratio (V.sub.H /V.sub.F) of the volume light water and the volume of fuel material in the embodiment can reach about twice value. Therefore, the pressure drop in the reactor core of the present invention makes to lower and also the problems raised from the aspects of the heat transfer or the floating can be settled comparatively easily. Still further, in the above embodiment of the present invention, the fuel rods used in the burner area do not contain gadolinia. It is possible to use in the burners area the fuel rods not containing gadolinia, and to obtain the high burnup without the fuel rods not containing gadolium. In above embodiment, the burner area starts when the burnup reaches at the condition of the burnup 45%. Therefore the mismatch in the neutron indifinite multiplication factor can be to smaller. However the burner area may start at the range of the condition of the burnup about 40-50%.