JP3596831B2 - Boiling water reactor core - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a boiling water reactor core using a fuel containing plutonium.
[0002]
[Prior art]
The configuration of the core of a general boiling water reactor will be described with reference to the drawings. FIG. 9A is a schematic plan view showing a part of a core in which the unit lattice is arranged, and FIG. 9B is a schematic plan view showing a part of the core in which the unit lattice is arranged. Thus, a unit lattice 3 is formed by arranging four fuel assemblies 2 around the control rod 1 for controlling the output of the nuclear reactor, and a plurality of the unit lattices 1 are arranged in a lattice to form a reactor core. 4 are configured. In the core configured as described above, after operating for a certain period (hereinafter referred to as one cycle), 1/3 to 1/4 of the fuel assemblies of all the fuel assemblies are taken out of the core and replaced with the replacement fuel. Replaced with a new fuel assembly.
[0003]
By the way, the fuel used in the nuclear power plant contains uranium 238 which accounts for 99.3% of natural uranium. This uranium 238 is hardly burnable in a light water reactor, but is converted into plutonium, which is a nuclear fuel substance, by being used as fuel in a reactor core. By removing this plutonium from spent fuel and using it as fuel for nuclear power plants, uranium resources can be used effectively. Therefore, it is planned to use plutonium / uranium mixed oxide fuel (hereinafter referred to as MOX fuel) also as fuel for the boiling water reactor. In recent years, the use of these MOX fuels as a part of replacement fuel for existing reactors has been studied. However, as the use of MOX fuel increases, not only replacement fuel but also the first cycle is used. It is conceivable to use it as fuel for initial loading in the core for initial loading.
[0004]
[Problems to be solved by the invention]
Conventionally, a uranium fuel assembly having the same uranium 235 average enrichment (hereinafter, referred to as an average enrichment) of a fuel assembly has been loaded in an initially loaded core of a boiling water reactor. This initially loaded core made of a uranium fuel assembly having the same average enrichment (hereinafter referred to as a uniformly enriched initially loaded core) has also been pointed out in "Problems of the Background Art" in JP-A-59-15888. As described above, fuel having a high average enrichment or plutonium enrichment (hereinafter referred to as enrichment) is taken out at the time of refueling at the end of the first or second cycle. , Neutrons leak out of the reactor core, so that the output is low at the periphery and high at the center, causing a problem of uneven power distribution in the horizontal direction.
[0005]
FIG. 10 shows an example in which a MOX fuel assembly is used as a part of the fuel in the initially loaded core having the uniform enrichment as described above. FIG. 10 is a schematic plan view showing a quarter of the core of the boiling water reactor. In FIG. 10, a reactor core 4 is configured by arranging a plurality of fuel assemblies 2, and in the figure, each cell indicates one fuel assembly 2, and the symbol U in the cell indicates the medium enrichment. The uranium fuel assembly, symbol M, represents the MOX fuel assembly. The portion surrounded by the thick frame is the control cell 5. Here, the MOX fuel assembly has an effective fission enrichment of 3.5 wt% and a burnable poison of 3.0 wt%, and is loaded with 220 bodies in the entire core. The medium enriched uranium fuel assembly has an effective fission enrichment of 652 bodies are loaded with 2.5% by weight and 7.0% by weight of burnable poisons.
[0006]
It is conceivable that the same problems as those not including the MOX fuel assembly described above also occur in the initially-loaded core with uniform enrichment including the MOX fuel assembly.
[0007]
Furthermore, from the viewpoint of fuel economy, since the MOX fuel assembly has the characteristic that the reactivity decreases little even if the combustion proceeds, it is advantageous to stay in the furnace as long as possible. However, comparing the neutron behaviors of the MOX fuel assembly and the uranium fuel assembly, the MOX fuel assembly tends to have a lower proportion of low-energy neutrons that are more susceptible to fission reactions than the uranium fuel assembly. Therefore, when the two fuel assemblies are adjacent to each other, neutrons having low energy flow from the uranium fuel assembly into the MOX fuel assembly, and the output of the MOX fuel assembly tends to increase.
[0008]
As a result, the neutron distribution in the core becomes more non-uniform than that of the initially loaded core without uniform enrichment without the MOX fuel assembly, and the operation limit value of thermal characteristics such as the minimum critical power ratio and the maximum linear power density Since the margin for the above is further reduced, a countermeasure is required.
[0009]
The present invention has been made in view of such a conventional situation, and an object of the present invention is to improve the economy of an initially loaded core in which a MOX fuel assembly and a uranium fuel assembly are mixed, and to limit the operation of thermal characteristics. The purpose of the present invention is to obtain a boiling water reactor core with a reduced margin for the value.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, in the core of the boiling water reactor according to the present invention, according to the first aspect of the present invention, a plurality of uranium fuels having different average enrichments of uranium 235 and a plutonium / uranium mixed oxide fuel assembly are provided. In the core of a boiling water reactor loaded with an assembly, the effective fission enrichment of this plutonium-uranium mixed oxide fuel assembly is the effective fission of the uranium fuel assembly having the lowest uranium 235 average enrichment in the early stage of combustion. The enrichment of the uranium fuel assembly is higher than the effective fission enrichment of the uranium fuel assembly having the highest uranium 235 average enrichment. The enrichment of the plutonium / uranium mixed oxide fuel assembly is smaller than that of the high enrichment uranium fuel assembly. It is greater than that of Chijimido uranium fuel assemblies.
[0011]
According to a second aspect of the present invention, in the first aspect of the present invention, a uranium fuel assembly having the lowest uranium 235 average enrichment is loaded on a unit cell including a control rod used as a control rod during the power operation of the reactor. It is characterized by.
[0012]
According to a third aspect of the present invention, in the first or second aspect of the invention, two or more sides of the four sides of the plutonium / uranium mixed oxide fuel assembly have the lowest uranium 235 average enrichment. It does not touch.
[0015]
According to a fourth aspect of the present invention, in the first to third aspects, the uranium fuel assembly having the lowest uranium 235 average enrichment is not mixed with a burnable poison.
[0016]
According to a fifth aspect of the present invention, in the first to fourth aspects of the present invention, the uranium fuel assembly having the highest uranium 235 average enrichment is loaded on the outermost periphery of the reactor core.
[0017]
According to a sixth aspect of the present invention, in the first to fifth aspects, the number of fuel rods in the uranium fuel assembly is larger than the number of fuel rods in the plutonium / uranium mixed oxide fuel assembly.
[0018]
According to a ninth aspect of the present invention, in the first to eighth aspects, at least one of the uranium enrichment and the burnable poison concentration of the uranium fuel rod included in the plutonium-uranium mixed oxide fuel assembly is in the axial direction. It is characterized by having a distribution.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings. Note that the same components as those of the conventional technique are denoted by the same reference numerals, and detailed description thereof will be omitted.
The first embodiment is according to claims 1 to 6, and FIG. 1 is a schematic plan view of a quarter of the core of the boiling water reactor according to the first embodiment. In FIG. 1, a core 4 includes a MOX fuel assembly 2 and a uranium fuel assembly 2 having a plurality of average enrichments. The uranium fuel assemblies have three types of average enrichment (low enrichment uranium fuel assembly, medium enrichment Uranium fuel assemblies and highly enriched uranium fuel assemblies). The symbols in the squares in the figure are M for MOX fuel assemblies, U1 for high enriched uranium fuel assemblies, U2 for medium enriched uranium fuel assemblies, and U3 for low enriched uranium fuel assemblies. Show body. The bold line indicates the control cell 5.
[0020]
Here, the MOX fuel assembly has an effective fission enrichment of 3.5 wt% and a burnable poison of 3.0 wt%, and is loaded with 220 bodies in the entire core. The highly enriched uranium fuel assembly has an effective fission enrichment of 3 wt%. 0.5 wt%, burnable poison is 7.0 wt%, 208 bodies are loaded, and the medium enriched uranium fuel assembly has an effective fission enrichment of 2.5 wt%, burnable poison is 7.0 wt%, and 204 The body-loaded, low-enrichment uranium fuel assembly has an effective fission enrichment of 1.0 wt%, no burnable poisons, and is loaded with 240 bodies. The effective fission enrichment is obtained by converting the concentration of fission material into uranium 235 enrichment having the same reactivity, and the conversion factor is 1 for a uranium fuel assembly and about 1 for a MOX fuel assembly. 0.7.
In the first embodiment having such a configuration, a plurality of uranium fuel assemblies having different average enrichments of uranium and uranium 235 from the plutonium / uranium mixed oxide fuel assembly are loaded. The low-enriched uranium fuel assembly that has been sufficiently reduced is taken out, and the medium-enriched and high-enriched uranium fuel assemblies and the MOX fuel assembly with high reactivity are left in the furnace, so the number of replacement bodies is reduced. This is economically effective as compared with the core shown in FIG. 10 composed of the uranium fuel assembly and the MOX fuel assembly having the same average enrichment.
[0021]
The control cell 5 is loaded with a low-enrichment uranium fuel assembly. The control cell 5 is a unit cell including control rods that are inserted into the reactor core even during operation and used as adjustment rods used for reactivity control. Since the control cell 5 is loaded with a low-enrichment uranium fuel assembly having a small output, even if the output of the control cell fluctuates during operation due to the operation of the control rod, the thermal conditions do not become severe. There is no adverse effect on fuel integrity.
[0022]
Further, on the four sides around the MOX fuel assembly, two or more low-enrichment uranium fuel assemblies are configured so as not to be adjacent to each other. Thereby, a thermal margin can be improved. Hereinafter, this point will be described.
[0023]
Generally, when the MOX fuel assembly is loaded together with the uranium fuel assembly, the output tends to be relatively higher than that of the uranium fuel assembly, so that the thermal characteristics tend to be severe. This is because the low-energy neutrons in the MOX fuel assembly that are prone to fission are relatively smaller than the uranium fuel assembly, so the low-energy neutrons in the uranium fuel assembly flow into the MOX fuel assembly and This is because the output increases and the output of the uranium fuel assembly decreases. In particular, when adjacent to a low-enriched uranium fuel assembly having many low-energy neutrons in each average-enriched uranium fuel assembly, low-energy neutrons that easily cause fission flow into the MOX fuel assembly greatly. The rate of increase in the output of the MOX fuel assembly increases.
[0024]
FIG. 2 shows the relative output of the MOX fuel assembly when 0 to 3 low-enrichment uranium fuel assemblies are adjacent to the four sides around the MOX fuel assembly. As shown in FIG. 2, when the output of the MOX fuel assembly is compared with the case where the low enrichment uranium fuel assemblies are not adjacent to each other and the case where the low enrichment uranium fuel assemblies are adjacent to each other, the increase is about 4%. The increase is about 10%. In the normal design stage, the thermal characteristics have a margin of 10% with respect to the thermal limit, but if the output of the MOX fuel assembly increases by 10%, the thermal margin with respect to the limit will be lost. Therefore, as a result of these studies, it is possible to improve the thermal margin by adopting a configuration in which two or more low-enrichment uranium fuel assemblies are not adjacent to the four sides around the MOX fuel assembly.
[0025]
Further, in the first embodiment, the uranium average enrichment is low, medium, and high, and the effective fission enrichment of the MOX fuel assembly is larger than that of the medium enrichment uranium fuel assembly. , The enrichment is smaller than that of the uranium fuel assembly.
[0026]
FIG. 3 is a characteristic diagram showing changes in the relative reactivity of the MOX fuel assembly and the uranium fuel assembly due to combustion. The relative output in the figure is based on the reactivity at the start of combustion of a MOX fuel assembly that does not contain gadolinia (Gd), which is a burnable poison, as a reference (1.0). The relationship between the burnup and the relative reactivity is indicated by a two-dot chain line a. The MOX fuel assembly is shown by a solid line b, the high enriched uranium fuel assembly is shown by a dotted line c, and the low enriched uranium fuel assembly is shown by a dashed line d.
[0027]
As shown in FIG. 3, the three types of fuel assemblies (b, c, d) containing Gd continue to increase in reactivity until Gd is burned out, but after Gd is burned out, the reactivity decreases. Soup At this time, since the MOX fuel assembly has a smaller decrease in reactivity due to combustion than the uranium fuel assembly, even if the effective fission enrichment is between the medium enriched uranium fuel assembly and the high enriched uranium fuel assembly. , It can be burned for a long time, and the discharge burnup can be increased.
[0028]
However, since the output of the MOX fuel assembly tends to be relatively higher than that of the uranium fuel assembly, it is difficult to suppress the output when the effective fission enrichment is set to be larger than that of the high-enrichment uranium fuel assembly. And the thermal properties become severe. Conversely, if the effective fission enrichment of the MOX fuel assembly is made smaller than that of the low-enrichment uranium fuel assembly, the reactivity will decrease and the fuel must be replaced with new fuel at the end of the first cycle. The property that the reactivity of the MOX fuel assembly is unlikely to decrease cannot be used, and is not economically effective.
[0029]
Therefore, from such considerations, the effective fission enrichment of the MOX fuel assembly is set between the uranium fuel assembly with the lowest average enrichment and the uranium fuel assembly with the highest average enrichment, and the uranium average enrichment of the three types is different. In such a case, the effect of improving the economic efficiency can be obtained by providing between the medium enriched uranium fuel assembly and the high enriched uranium fuel assembly.
[0030]
In the first embodiment, the low-enrichment uranium fuel assembly is configured to contain no burnable poison. As a result, the low-enrichment uranium fuel assembly is taken out after the end of the first cycle, so that it can be burned during the first cycle as much as possible to increase the taken-out burnup. However, by adding the burnable poison to the other fuel assemblies, the excess reactivity at the beginning of the first cycle can be suppressed.
[0031]
The second embodiment is according to claim 7, and FIG. 4 is a schematic plan view of a quarter of the boiling water reactor core according to the second embodiment. Note that the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted. In FIG. 1, a core 4 is composed of a MOX fuel assembly 2 and a uranium fuel assembly 2 having a plurality of average enrichments. The uranium fuel assembly has three average enrichments (low enrichment uranium fuel assembly, medium enrichment, Uranium fuel assemblies and highly enriched uranium fuel assemblies). The symbols in the squares in the figure are M for MOX fuel assemblies, U1 for high enriched uranium fuel assemblies, U2 for medium enriched uranium fuel assemblies, and U3 for low enriched uranium fuel assemblies. Body, and a bold frame indicates a control cell.
[0032]
Here, the MOX fuel assembly has an effective fission enrichment of 3.5 wt% and burnable poisons of 3.0 wt%, and is loaded with 220 bodies in the entire core. The highly enriched uranium fuel assembly has an effective fission enrichment of 3 wt%. 2.5 wt%, burnable poison is 7.0 wt%, 216 bodies are loaded, and the medium enriched uranium fuel assembly has an effective fission enrichment of 2.5 wt%, burnable poison is 7.0 wt%, and 220 bodies The loaded, low enriched uranium fuel assembly has an effective fission enrichment of 1.0 wt%, no burnable poisons, and has 216 loaded.
[0033]
With the above configuration, the same operation and effect as in the first embodiment can be obtained in the second embodiment.
Further, in the second embodiment, a highly enriched uranium fuel assembly is loaded on the outermost periphery. As a result, the decrease in the reactivity of the high-enrichment uranium fuel assembly during the first cycle operation can be reduced, and the low-enrichment uranium fuel assembly disposed in the center of the core burns well, and accordingly, The removal burnup of the low-enrichment uranium fuel assembly removed after the end of the first cycle increases. In addition, since the reactivity of the highly enriched uranium fuel assembly is less reduced, the number of fuel replacement bodies can be reduced.
[0034]
FIG. 5 shows the power distribution in the radial direction of the core when the high enrichment uranium fuel assembly is loaded on the outermost periphery and when the low enrichment uranium fuel assembly is loaded. The relative power in the figure is a solid line e, the relative output of the core loaded with the high enriched uranium fuel assembly on the outermost periphery, and the relative output of the core loaded with the low enriched uranium fuel assembly on the basis of the core average power. This is indicated by a dotted line f. As shown in this figure, by arranging the highly enriched uranium fuel assembly on the outermost periphery, the radial distribution of the neutron flux can be flattened, and the output of the MOX fuel assembly, which tends to increase the output, can be reduced. Can be.
[0035]
The third embodiment relates to claim 8 and is characterized by a fuel assembly constituting the core of the boiling water reactor shown in FIG. 1 or FIG. This fuel assembly is shown in FIG. FIG. 6A is a cross-sectional view of the uranium fuel assembly, and FIG. 6B is a cross-sectional view of the MOX fuel assembly. In FIG. 6A, the uranium fuel assembly 6 has fuel rods 8a arranged in 9 rows and 9 columns in a channel box 7, and two water rods 9 arranged in the center thereof. In the MOX fuel assembly 10, fuel rods 8 b are arranged in eight rows and eight columns in the channel box 7, and one water rod 9 is arranged at the center thereof. Accordingly, the uranium fuel assembly 6 has 74 fuel rods, the MOX fuel assembly 10 has 60 fuel rods, and the uranium fuel assembly has 14 fuel rods.
[0036]
With this configuration, the uranium fuel assembly 6 can increase the average enrichment and the removal burnup because the number of fuel rods 8 is large and the margin of linear output density is large. In addition, since the output of the uranium fuel assembly 6 is increased by increasing the average enrichment, the output of the MOX fuel assembly 10 can be relatively reduced, and the thermal characteristics of the MOX fuel assembly 10 can be improved.
[0037]
The fourth embodiment is according to claim 8, and is characterized by a MOX fuel assembly constituting the core of the boiling water reactor shown in FIG. 1 or FIG. This MOX fuel assembly is shown in FIG. 7A is a cross-sectional view of the MOX fuel assembly, and FIG. 7B is an axial distribution diagram of the burnable poison-containing fuel rod. In FIG. 7A, the MOX fuel assembly 10 includes four types of plutonium fuel rods and one type of uranium fuel rod. In the figure, symbols P1 to P4 indicate plutonium enrichment in numerical order with P1 being the highest, and symbol G indicates a burnable poison-containing fuel rod 11 containing gadolinia, which is a uranium fuel rod and a burnable poison. As shown in FIG. 7B, the enrichment and gadolinia concentration of the burnable poison-containing fuel rod 11 have a distribution in the axial direction.
[0038]
FIG. 8 is an axial power distribution diagram of the MOX fuel assembly in a state where the control rod is pulled out. In the figure, the relative output is obtained by standardizing the average output to 1.0, and the solid line h represents the MOX fuel assembly shown in FIG. 7 in which the uranium fuel rods are provided with the axial distributions of enrichment and gadolinia concentration. It is an axial power distribution, and a dotted line g is an axial power distribution map of the MOX fuel assembly in which the axial distribution of the enrichment and gadolinia concentration is not given to the uranium fuel rod.
[0039]
As shown in FIG. 8, the axial power distribution of the MOX fuel assembly becomes flat by giving the enrichment and the concentration of the burnable poison in the uranium fuel rod an axial distribution. Therefore, the axial power distribution of the entire core is flattened, and the thermal characteristics and the nuclear characteristics are improved. In addition, in the example of FIG. 7, both the concentration and the gadolinia concentration have the axial distribution, but the same effect can be obtained with either one.
[0040]
Further, in the above-described embodiment, three types of the average enrichment of the uranium fuel assembly have been described, but the types of the average enrichment of the uranium fuel assembly are not limited to the three types. The same effect can be obtained by using two or four or more types.
[0041]
【The invention's effect】
As described above, in the core of the boiling water reactor of the present invention, according to the first aspect of the invention, the core is constituted by the MOX fuel assembly and the uranium fuel assembly having several types of average enrichment. In addition, the economic efficiency and core characteristics are improved , and furthermore, by appropriately setting the initial combustion effective fission enrichment of the MOX fuel assembly and the uranium fuel assembly, the fuel economy is improved and the thermal characteristics are improved. It has the effect of relaxing .
[0042]
According to the second aspect of the present invention, the control cell is loaded with a uranium fuel assembly having the lowest average enrichment, so that the output of the assembly in the control cell fluctuates due to the operation of the control rod to improve the fuel integrity. This has the effect of not causing adverse effects.
[0043]
According to the third aspect of the present invention, the output of the MOX fuel assembly is suppressed by preventing the uranium fuel assembly having the lowest average enrichment from being adjacent to two or more sides among the four sides around the MOX fuel assembly, thereby reducing the thermal characteristics. This has the effect of reducing the pressure.
[0045]
According to the sixth aspect of the invention, there is an effect that the excess reactivity is suppressed, the removal burn-up of the uranium fuel assembly having the lowest average enrichment is increased, and the economic efficiency is improved.
According to the seventh aspect of the present invention, by loading the uranium fuel assembly having the highest average enrichment on the outermost periphery, the fuel economy is improved, the neutron flux distribution in the furnace is flattened, and the thermal characteristics are improved. It has the effect of relaxing.
[0046]
In the invention according to claim 8, by increasing the number of fuel rods of the uranium fuel assembly to that of the MOX fuel assembly, the average enrichment of the uranium fuel assembly is increased, and the economic efficiency is improved by increasing the removal burnup. The effect of lowering the output of the MOX fuel assembly and relaxing the thermal characteristics is achieved while improving the performance.
[0047]
According to the ninth aspect of the present invention, the axial power distribution of the MOX fuel assembly is flattened by giving the axial distribution to the enrichment of the uranium fuel rods and the concentration of the burnable poison in the MOX combustion assembly, and This has the effect of alleviating the thermal characteristics of.
[Brief description of the drawings]
FIG. 1 is a schematic plan view showing a quarter of a core of a boiling water reactor according to a first embodiment of the present invention.
FIG. 2 is a characteristic diagram showing the adjacency effect of a low-enrichment uranium fuel assembly on a MOX fuel assembly.
FIG. 3 is a characteristic diagram showing a change in burnup of reactivity of a fuel assembly.
FIG. 4 is a schematic plan view showing a quarter of a core of a boiling water reactor according to a second embodiment of the present invention.
FIG. 5 is a diagram illustrating a neutron flux distribution in a radial direction of a core of a boiling water reactor according to a second embodiment of the present invention.
6A is a cross-sectional view of a uranium fuel assembly constituting a core of a boiling water reactor according to a third embodiment of the present invention, and FIG. 6B is a cross-sectional view of a MOX fuel assembly. FIG.
FIG. 7A is a cross-sectional view of a MOX fuel assembly constituting a core of a boiling water reactor according to a fourth embodiment of the present invention, and FIG. 7B is a sectional view of the MOX fuel assembly. FIG. 3 is an axial distribution diagram of a burnable poison-containing fuel rod.
FIG. 8 is an axial power distribution diagram of a MOX fuel assembly constituting a core of a boiling water reactor according to a fourth embodiment of the present invention.
9A is a schematic plan view showing a unit cell, and FIG. 9B is a schematic plan view showing a part of a core.
FIG. 10 is a schematic plan view showing a quarter of the core of a conventional boiling water reactor.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 control rod 2 fuel assembly 3 unit cell 4 core 5 control cell 6 uranium fuel assembly 7 channel boxes 8 a and 8 b fuel rod 9 water rod 10 MOX fuel assembly 11 flammability Poisoned fuel rod

Claims (7)

Effective nuclear fission of the plutonium / uranium mixed oxide fuel assembly in a boiling water reactor core loaded with a plutonium / uranium mixed oxide fuel assembly and a plurality of uranium fuel assemblies having different uranium 235 average enrichments The enrichment is greater than the effective fission enrichment of the uranium fuel assembly with the lowest uranium 235 average enrichment at the beginning of combustion, and less than the effective fission enrichment of the uranium fuel assembly with the highest uranium 235 average enrichment; Further, the uranium 235 average enrichment of the uranium fuel assembly is set to three types of high enrichment, medium enrichment, and low enrichment, and the effective fission enrichment of the plutonium-uranium mixed oxide fuel assembly is determined by the high enrichment uranium. A core for a boiling water reactor, which is smaller than that of a fuel assembly and larger than that of said medium enriched uranium fuel assembly . 2. The boiling water reactor according to claim 1, wherein a unit cell including a control rod used as a control rod during power operation of the reactor is loaded with a uranium fuel assembly having the lowest uranium 235 average enrichment. Core. 3. The boiling water type according to claim 1, wherein at least two sides out of four sides of the plutonium / uranium mixed oxide fuel assembly do not contact the uranium fuel assembly having the lowest uranium 235 average enrichment. Reactor core. Most uranium 235 average enrichment claims 1 to 3 boiling water nuclear reactor core according low uranium fuel assembly is characterized by not mixing the burnable poison. Most 235U claims 1 to 4 boiling water nuclear reactor core according to, characterized in that loading the average enrichment is higher uranium fuel assemblies in the core the outermost periphery of the reactor. Claims 1 to 5 boiling water nuclear reactor core according to the number of fuel rods of uranium fuel assembly, characterized in that more than the number of fuel rods of plutonium-uranium mixed oxide fuel assembly. At least one of the concentration of the uranium enrichment and burnable poison uranium fuel rods contained in the plutonium uranium mixed oxide fuel assemblies claims 1 and having an axial distribution boiling water for 6 wherein Reactor core.

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