Patent Number: 048511811
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light water moderation type nuclear reactor, more particularly to a pressurized water type nuclear reactor with a once-through method or a boiling water type nuclear reactor with a once-through method. 2. Description of the Prior Art The method of utilizing the fuel materials in a light water moderation type nuclear reactor (hereinafter referred to as a light water reactor) is largely classified into the once-through method and the reprocessing and recycling method. With the once-through method the light water reactor uses the enriched uranium and in this method none of the fuel materials contained in the used fuel rods which are taken out of the light water reactor is reused (recycled) in the light water reactor. This once-through method or system cycling cost when cost of the reprocessing fuel is higher than that of the enriching uranium. Additionally, the purpose of the reprocessing and recycling method or system is to make new fuel rods by reprocessing fuel materials in the used fuel rods and to charge those new fuel rods into the light water reactor to reuse the fuel materials. One method to effectively use the fuel materials by the once-through method is to greatly increase the take-out burnup from the fuel assembly, that is, to realize a high degree of the burnup. The fuel assembly includes many fuel rods. It is required to raise the enrichment of the enriched uranium so as to achieve the high degree of the burnup. However, to realize such a raised enrichment of the enriched uranium, the following problems occur. In the center of the reactor core of the light water reactor there are the fuel assemblies with large difference in the neutron infinite multiplication factor because of a high enrichment of new fuel assemblies and the large take-out burnup. A difference in the output power share proportions of the individual fuel assemblies, therefore, the output power mismatch grains larger and also the output power peaking becomes larger. Further, as the enrichment increases, the surplus reactivity which has to be controlled in the initial stage of the burning increases. Therefore, the conventional fuel assembly using the fuel rods containing gadolinia has to increase the number of the fuel rods which contain the gadolinia. The fuel rods of reactor core of the conventional pressurized water type nuclear reactor have uniformly the ratio (r.sub.H/U) of the number of hydrogen atoms to the number of fuel material atoms of about 2.0. The characteristics of the reactor core of the conventional pressurized water type nuclear reactor is represented by the curve P.sub.5 (a dashed line) as shown in FIG. 11. The initial enrichment of the fuel rods of the conventional light water reactor is raised until the take-out burnup E.sub.b represented by the curve P.sub.5 as shown in FIG. 11 is realized. With this reactor core the initial neutron multiplication factor is large, and in order to suppress this, a large amount of the burnable poison material such as gadolinium has to be put in the fuel assemblies at the expense of the neutron economy. Furthermore, the mingling of the fuel assemblies, which are much different in the neutron multiplication factor, into the reactor core makes it difficult to flatten the output power distribution. The maximum burnup is restricted by the fuel rods having the peak power with the result of the lowered average take-out burnup. The mismatch in the neutron multiplication factor of the conventional light water reactor is large, therefore the average take-out burnup from the fuel assembly can not be made high. The realization of the high degree of the burnup can not realize is impossible. The conventional light water reactor has a uniform ratio (r.sub.H/U) (about 2.0) of the number of hydrogen atoms to the number of fuel material atoms in the reactor core. The variation of the neutron multiplication factor in the conventional light water reactor is shown the curve P.sub.4 (a dashed line) shown in FIG. 10. In the conventional light water reactor the fuel assemblies are exchanged for the new fuel assemblies at the burnup E.sub.c. Namely the average take-out burnup of the fuel rods charged in the reactor core of the conventional light water reactor is the burnup E.sub.c. The amount of the charged fuel in the conventional light water reactor is the same throughout the reactor core. The average take-out burnup E.sub.c of the fuel rods in the reactor core is not made larger, therefore the uranium saving can not be achieved in the conventional light water reactor. From the standpoint of the effective use of the uranium resources, a light water reactor has been proposed in which the conversion from uranium-238 to a fissile product (plutonium-239) is improved. In the "General Features of Advanced Pressurized Water Reactors with Improved Fuel Utilization" by Werner Oldekop et al in the Nuclear Technology, vol. 59, November 1982 P. 212-227, a high conversion reactor (HCR) was proposed to lower the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material in the reactor core of the light water reactor from conventional 2.0 to 0.5 and raise the average energy of neutrons to make the plutonium conversion rate higher than 0.9. As a construction to bring about this ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material of 0.5 a dense lattice construction is employed. Because of the dense lattice construction, the high conversion reactor (HCR) has following serious problems therein raised from the aspects of the heat transfer or the floating. Such problems are as follows, for example, the pressure drop in the reactor core becomes about four (4.0) times as much as that of the conventional light water reactor, or by the unexpected accident with coolant loss the emergency coolant hardly enters into the reactor core. The high conversion reactor (HCR) including this example aims at an effective utilization of the fuel material by reprocessing and recycling the fuel assemblies taken out of the reactor core. In the high conversion reactor (HCR) the fuel cycle including the steps of the fuel reprocessing, the fuel reworking, etc. must be completed. Furthermore, even if the above described problems in the high conversion reactor (HCR) were solved therein, the utilization quantity of uranium in the high coversion reactor (HCR) may be not reach more than about two-and-a-half (2.5) times as much as that of the conventional light water reactor. SUMMARY OF THE INVENTION One object of the present invention is to provide a light water moderation type nuclear reactor wherein an amount of the natural uranium required to develop unit energy in the light water reactor can be reduced without reusing the fuel materials. Another object of the present invention is to provide a light water moderation type nuclear reactor wherein uranium consumption in the reactor core can be reduced efficiently. Further, an object of the present invention is to provide a light water moderation type nuclear reactor wherein a take-out burnup can be increased efficiently. Still another object of the present invention is to provide a light water moderation type nuclear reactor wherein a production of the plutonium in the fuel rods can be increased during the first half of the life time of the fuel rods. Furthermore, an object of the present invention is to provide a light water moderation type nuclear reactor wherein fissile materials in the fuel rods can be burned effectively during the second half of the life time of the fuel rods. A further object of the present invention is to provide a light water moderation type nuclear reactor wherein a mismatch in the neutron multiplication factors among the fuel rods can be made relatively small. Still another object of the present invention is to provide a light water moderation type nuclear reactor wherein a ratio of the number of hydrogen atoms to that of fuel material atoms can be adjusted minutely. Still a further object of the present invention is to provide a light water moderation type nuclear reactor wherein thermal neutrons can be utilized effectively. The present invention has a feature that in a radial direction of the reactor core, areas having different average densities of the fuel rods per unit cross-sectional area are provided, and the fuel rods which are arranged in a first area with larger average density of the fuel rods per unit cross-sectional area are subsequently moved to a second area where the fuel rods are arranged in the area with smaller average density of the fuel rods per unit cross-sectional area, the fuel rods being arranged in the second area with smaller average density of the fuel rods per unit cross-sectional area having bean burned in the first area with larger average density of the fuel rods per unit cross-sectional area. For increasing the plutonium production in the reactor core it is only required to shift the neutron energy spectrum to the high energy side in order to raise the rate of capturing and absorbing neutrons by uranium-238. For this it is necessary to lower the ratio of the number of hydrogen atoms to that of uranium atoms which has the largest moderation power for neutron. With a once-through light water reactor it is necessary to burn up most effectively plutonium-239, plutonium-241 and the enriched uranium-235 that are produced as the fissile materials. This requires that methods for improving the moderation of neutrons and raising its rate of absorption of the fissile materials by increasing the proportion of the thermal neutrons. It can be achieved by increasing the ratio of the number of hydrogen atoms to that of uranium atoms. This means that, as far as the neutron moderation is concerned, namely the ratio of the number of hydrogen atoms to that of uranium atoms, the measure for increasing the production of plutonium and the measure for the high efficiency burnup of the fissile materials (plutonium-239, plutonium-241 and uranium-235) are measures. The present invention aims to realize in the same reactor core the above contradicting measure at the same time to produce more of the fissile materials and, furthermore, to utilize the fissile materials more efficiently for the saving of uranium consumption and the increasing burnup which will be described below in reference to FIG. 12 and FIG. 13. A reactor core 45 is divided radially by partition members 49 into a first area 46, a second area 47, . . . , and a Nth area 48 as shown in FIG. 12. The ratio of the number of hydrogen atoms to that of uranium atoms, i.e., r.sub.H/U in the first area is a.sub.1, r.sub.H/U in the second area is a.sub.2, and r.sub.H/U in the Nth area is a.sub.N ; and the following relations hold therein: EQU a.sub.1 &lt;a.sub.2 &lt; . . . &lt;a.sub.N The value of a.sub.1 is made to be between 1.0 and 2.0 which is smaller than the ratio of the number of hydrogen atoms to that of uranium atoms of the conventional light water reactor, and the value of a.sub.N is made to be over 5.0 which is larger than the value of the conventional light water reactor. New fuel rods with the burnup of 0 are charged at first to the first area 46 and it is burned to the shift point E.sub.1 as shown in FIG. 13. Next, the fuel rods which have been burned to the shift point E.sub.1 are moved to the second area 47. When those fuel rods are burned to the shift point E.sub.2 in the second area 47, those fuel rods are moved to the third area. New fuel rods which were first charged to the first area 46 move successively from the first area 46, to the second area 47, . . . , to the Nth area 48 during their life time by carrying out the above mentioned moving at each fuel exchange, and at the time of the (N-1)th fuel exchange the fuel rods which have been burned to the point E.sub.N in the Nth area are taken out. Accordingly during the first half of the life time of the fuel rods the fuel rods stay in the areas where the ratio of the number of hydrogen atoms to that of uranium atoms is small and the increased production of plutonium in the fuel rods is attempted. Since during the second half of the life time of the fuel rods the fuel rods stay in the area where the ratio of the number of hydrogen atoms to that of uranium atoms is large, it is possible to effectively burn the fissile materials in the fuel rods. FIG. 14 shows the variation in the conversion ratio (the ratio of the atom number concentration of the initial uranium-235 to that of the fissile materials after burning for the fuel rod) of about 4% uranium enrichment with the ratio of the number of hydrogen atoms to that of uranium atoms as the parameter. When the ratio of the number of hydrogen atoms to that of uranium atoms in the first area 46 is about 1.1, about 95% of the fissile materials in the initial uranium-235 atoms number concentration remain after the burning 30 GWd/t. The partition members 49 shown in FIG. 12 are the boundary layers that divide areas 46, 47 and 48, and the partition members 49 are provided as separators so that between the areas there is no flow of the coolant (light water) through them. The material for those partition members 49 is selected from a low neutron absorption, for example, zircalloy, etc. FIG. 13 shows a variation in the neutron multiplication factor for the burnup during the life time of the fuel rods according to the present invention. Even if the average neutron multiplication factor does not meet the critical condition in the first area 46, it may be designed so as to make the average neutron multiplication factor for whole area meet the critical condition. After second area 47, if necessary, the gadolinium is added to each of the areas 47 and 48 to control the reactivity. FIG. 15A, FIG. 15B and FIG. 15C show the power densities for the areas 46, 47 and 48 in the reactor core according to the present invention and also the radial distribution of the flow rate of the coolant (light water). The power density is higher in the area nearer to the center because that area is charged with denser fuel rods. The flow rate of the coolant is adjusted by an orifice provided at the coolant inlet of the reactor core so that the temperature of fuel rods and the temperature of the fuel cladding tube in each area will not exceed the standard value in the design. Furthermore, at the reactor core where the coolant is boiled, it is also possible to adjust the ratio of the number of hydrogen atoms to that of uranium atoms by adjusting the coolant flow rate to change the void factor in each of the areas. According to the present invention during the time from charging the fuel rods to the reactor core to subjecting the fuel rods to discharging treatment, the fuel rods are taken out and the burnup of the fuel rods can be improved trememdously, allowing the effective utilization of the fuel materials.