JP2008145359A - Boiling water reactor core and method of operation - Google Patents

Abstract

An object of the present invention is to increase the average concentration of a core while ensuring the safety of a nuclear reactor.
In a core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is a vertical direction, a first fuel assembly is arranged. The outer peripheral region and the inner side in the radial direction of the outer peripheral region and the second fuel assemblies 1, 2, 4 containing less fissile material than the first fuel assembly 3 are arranged. A circumferential region. The second fuel assemblies 1, 2, 4 may be fuel assemblies having a smaller content of combustible poisons than the first fuel assembly 3.
[Selection] Figure 1

Description

  The present invention relates to a core and operating method of a boiling water reactor.

  In nuclear reactors, neutrons are absorbed by fissionable materials, causing fission, and neutrons released along with the energy continue to produce energy by a chain reaction. The state in which this chain reaction is in equilibrium is called criticality, and the reactor operated at a constant power continues to maintain this state. In addition, the state in which the chain reaction increases is called supercritical, and the state in which the chain reaction decreases is called subcritical.

  Since a nuclear reactor must be operated without refueling for a predetermined period of time, for example, about one year, more nuclear fissionable material is loaded in the core than is necessary for maintaining criticality. Therefore, in the period up to the middle of the predetermined operation period, the nuclear reactor becomes supercritical without the control material.

  This excess reactivity is referred to as excess reactivity, and it is important to appropriately control the excess reactivity throughout the operation period. As a technique for controlling the excess reactivity throughout the operation period, a technique in which a flammable poison is mixed into the fuel is well known. A flammable poison is a neutron absorber that gradually burns and decreases in the amount of material throughout the operation period. Gadolinia, which is used in combination with nuclear fuel materials, is known.

  The infinite multiplication factor of the fuel assembly containing the combustible poison increases once with combustion and then decreases. Generally, if the number of fuel rods containing a flammable poison increases, the infinite multiplication factor at the initial stage of combustion decreases. Further, if the concentration of the flammable poison is increased, the time when the flammable poison is burned out can be delayed, so that the maximum value of the infinite multiplication factor can be suppressed. For this reason, the excess reactivity can be appropriately controlled by the combination of the mixing concentration of the flammable poison and the number of fuel rods mixed with it.

  In the initial loading core, a part of the loaded fuel assembly is taken out after the operation of the first cycle is completed and replaced with a new replacement fuel assembly. The fuel assembly taken out in the first cycle has a lower burnup and generates less energy than other fuel assemblies. For this reason, the fuel economy increases as the number of fuel assemblies taken out in the first cycle decreases.

  In order to improve fuel economy, it is desired to further increase the core average enrichment of the first loaded core. In the highly enriched fuel assembly loaded in the initial loading core, pellets having an enrichment of 4.9 wt% are used for the fuel rods inside the fuel assembly. However, it is difficult to use pellets with a concentration of 5 wt% or more due to limitations in processing facilities and transportation. Therefore, in order to further increase the fuel assembly average enrichment, it is necessary to increase the enrichment of the fuel rods in the vicinity of the fuel assembly corner region where the relatively low enrichment fuel rods are conventionally arranged.

  In order to make effective use of fissile material, an initially loaded core using a plurality of fuel assemblies with different uranium enrichments according to the period of stay in the reactor is known.

For example, Patent Document 1 describes that the thermal characteristics can be improved by devising the arrangement of fuel rods containing gadolinia. Patent Document 2 describes that gadolinia burning can be suppressed by arranging fuel rods containing gadolinia adjacent to each other in the assembly. By using these techniques and increasing the core average enrichment, it is possible to operate the second cycle without removing the initially loaded fuel after the end of the first cycle. Furthermore, Patent Document 3 optimizes the excess reactivity by loading a fuel assembly containing a flammable poison that has not progressed in combustion in the first cycle into the inner region in the second cycle. It describes what you can do.
Japanese Patent No. 3186546 Japanese Patent No. 2577367 Japanese Patent No. 3779299

  In order to improve fuel economy, it is preferable to increase the core average enrichment. However, since there are many thermal neutrons in the vicinity of the fuel assembly corner region, thermal characteristics may not be satisfied if highly enriched fuel rods are used.

  Therefore, an object of the present invention is to increase the average concentration of the core while ensuring the safety of the nuclear reactor.

  In order to achieve the above-described object, the present invention provides a first reactor in a boiling water reactor core formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region having an axis as a vertical direction. An outer peripheral region in which the fuel assemblies are arranged, and a second fuel assembly that is more radially inward of the cylinder than the outer peripheral region and contains less fissile material than the first fuel assembly. And an inner peripheral region in which are arranged.

  Further, according to the present invention, the first fuel assembly is arranged in a core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is a vertical direction. And a fuel rod containing less fissionable material in the corner portion than the fuel rod in the corner portion of the first fuel assembly. And an inner peripheral region in which the arranged second fuel assemblies are arranged.

  Further, according to the present invention, the first fuel assembly is arranged in a core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is a vertical direction. And a second fuel assembly in which the content of the combustible poison is smaller than that of the first fuel assembly is arranged inside the outer peripheral region in the radial direction of the cylinder with respect to the outer peripheral region. And a peripheral region.

  The present invention also relates to a method for operating a boiling water reactor comprising a core formed by arranging fuel assemblies extending in an axial direction in a substantially cylindrical region having an axis in a vertical direction. A fuel assembly is arranged to form an outer peripheral region of the core, and a second portion containing a less fissile material than the first fuel assembly inside the outer peripheral region in the radial direction of the cylinder. And a first cycle step of maintaining the core in a critical period for a predetermined period after the core formation step. To do.

  According to the present invention, the core average enrichment can be increased while ensuring the safety of the nuclear reactor.

  Embodiments of the core of a boiling water reactor according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or similar structure, and the overlapping description is abbreviate | omitted.

[First Embodiment]
FIG. 2 is a transverse cross-sectional view (horizontal cross-sectional view) in the first embodiment of the core of the boiling water nuclear reactor according to the present invention. FIG. 3 is a cross-sectional view of the fuel assembly in the present embodiment. FIG. 2 is a diagram showing the upper left quarter of the cross section of the core, and the remaining portion is rotationally symmetric with respect to the illustrated portion with the central axis in the vertical direction of the core as the symmetry axis. Note that the entire core does not need to be rotationally symmetric, and may be mirror-symmetrical or a core without symmetry.

  The boiling water reactor has a region 10 in which spaces 22 in which square-tubular fuel assemblies 26 are arranged are arranged in a square lattice pattern. The region 10 is formed in a substantially cylindrical shape as a whole, and the axis of the fuel assembly 26 is directed in the same direction as the axis of the cylinder. Fuel assemblies 26 are arranged in these spaces 22 to form a core.

  The space 22 in which the fuel assembly 26 is arranged is slightly larger than the fuel assembly 26, and the control rod 21 can be inserted between the four adjacent fuel assemblies 26. A space 22 in which a fuel assembly 26 that is not adjacent to the control rod 21 is also present in a part of the region 10 on the outer side in the radial direction.

  In the core of the present embodiment, 872 fuel assemblies 26 are loaded, and 205 control rods 21 are arranged.

  The fuel assembly 26 includes cylindrical fuel rods 24 arranged in a square lattice in 9 rows and 9 columns, and a rectangular tube-shaped water channel 25 arranged in the center of the array of fuel rods 24. Yes. The water channel 25 occupies an area of nine fuel rods 24 in 3 rows and 3 columns. The fuel rod 24 and the water channel 25 are held by tie plates (not shown) provided at both ends in the axial direction and spacers (not shown) provided at several locations in the axial direction. The outer periphery of the fuel assembly 26 is surrounded by a rectangular tubular channel box 23.

  Inside the fuel rod 24, a fissile material such as uranium 235 is stored as a pellet baked into a cylindrical shape together with uranium 238, for example. The enrichment of uranium 235 in each fuel rod 24 is different for each fuel rod 24, and the amount of fissile material contained in each fuel rod 24 is also different for each fuel rod 24. In addition, you may provide the area | region where enrichment differs in the axial direction of the fuel rod 24. FIG.

  FIG. 1 is a ¼ cross-sectional view showing the arrangement of the fuel assemblies in the core of the first cycle in the present embodiment. Reference numerals 1, 2, 3, and 4 indicate the types of the fuel assemblies 26, respectively, and the same reference numerals indicate fuel assemblies in which the arrangement of the fuel rods 24 is the same.

  A fuel assembly indicated by reference numeral 3 is arranged on the outer periphery of the first cycle core 11, that is, the initial loading core 11. Further, fuel assemblies indicated by reference numerals 1, 2, and 4 are disposed on the inner peripheral portion of the initial loading core 11. Here, the fuel assembly 26 indicated by reference numeral 3 is referred to as a first fuel assembly, and the fuel assembly 26 indicated by reference numerals 1, 2, and 4 is referred to as a second fuel assembly.

  The amount of fissile material contained in the first fuel assembly 3 is greater than the amount of fissile material contained in the second fuel assemblies 1, 2, 4. In the case of the fuel assembly 24 having the same fuel type other than the enrichment such as the number of fuel rods, the assembly average enrichment of the first fuel assembly 3 is the second fuel assembly 1, 2, 4 It becomes higher than the average concentration of aggregates.

  The fuel assemblies indicated by reference numerals 1 and 2 in the second fuel assembly are highly enriched fuels having a relatively high assembly average enrichment, and the fuel assembly indicated by reference numeral 4 is the average assembly enrichment. It is a low enriched fuel with a relatively low degree.

  In general, since neutron leakage is large at the outer peripheral portion of the core, the output per unit fuel assembly tends to be smaller than that at the inner peripheral portion. For this reason, the first fuel assembly 3 loaded on the outer peripheral portion of the core 11 has a higher assembly average enrichment than the second fuel assemblies 1, 2, and 4 loaded on the inner peripheral portion. Also, the thermal limit value can be easily satisfied.

  FIG. 4 is a cross-sectional view showing an example of the enrichment of each fuel rod of the first fuel assembly in the present embodiment. FIG. 5 is a cross-sectional view showing an example of the enrichment of each fuel rod of the second fuel assembly in the present embodiment. FIG. 5 shows an example of the fuel assembly indicated by reference numeral 1 in the second fuel assembly. 4 and 5, the positions indicated by U1, U2, and U3 indicate that uranium fuel rods are disposed, and the positions indicated by G indicate that gadolinia-containing fuel rods are disposed. W indicates the position of the water channel.

  The uranium fuel rod is a fuel rod 26 that does not contain a flammable poison such as gadolinia inside, and the fuel rod containing gadolinia is a fuel rod 24 that contains a flammable poison such as gadolinia together with a fissile material. It is. The uranium fuel rods indicated by U1, U2, and U3 each have a different enrichment of uranium 235, the enrichment of the uranium fuel rod indicated by U1 is the highest, and the enrichment of the uranium fuel rod at the location indicated by U3 is the lowest. . The enrichment of the uranium fuel rod indicated by U1 is, for example, 4.9 wt%. The enrichment of the fuel rods arranged at the position indicated by U1 in FIGS. 4 and 5 may be different from each other.

  In general, since there are many thermal neutrons at the corner portion of the fuel assembly 26, the output tends to be higher than at other positions. Further, in order to operate the nuclear reactor safely, the fuel assembly 26 needs to satisfy a predetermined limit value (thermal limit value) in thermal characteristics such as the maximum linear power density and the minimum limit power ratio. . Therefore, in the second fuel assemblies 1, 2, and 4 loaded on the inner peripheral portion of the core 11, the fuel rods 24 disposed at other positions at the corner portions and the positions indicated by U2 and U3 in the vicinity thereof. A fuel rod 24 having a lower enrichment is disposed.

  On the other hand, in the first fuel assembly 3 loaded on the outer peripheral portion of the core 11, the output per unit fuel assembly tends to be lower than that of the fuel assembly loaded on the inner peripheral portion. Also in the vicinity thereof, a fuel rod 24 indicated by a symbol U1 having a high enrichment can be arranged.

  Further, the first fuel assembly 3 of the present embodiment is more at a position where the distance from the corner portion 31 near the center of the core 11 is shorter than the distance from the corner portion 30 far from the center of the core 11. The fuel rod with gadolinia is arranged. By arranging the fuel rods 24 in this way, it is possible to suppress an excessive increase in the output in the portion close to the center of the core 11 where the output tends to be higher among the fuel rods 24 in the outer peripheral portion, and thermal restriction is achieved. The value can be satisfied.

  At the outer periphery of the core 11, the effect of suppressing excess reactivity of the core by the gadolinia-containing fuel rods is small, and a number of gadolinia-containing fuel rods are used for the first fuel assembly 3 in order to satisfy the thermal limit value. However, the excess reactivity does not decrease excessively.

  Thus, by increasing the fuel assembly average enrichment of the fuel assemblies on the outer periphery of the core, the economic efficiency can be improved while keeping the reactor safe.

  FIG. 6 is a ¼ cross-sectional view showing the arrangement of the fuel assemblies in the core of the second cycle in the present embodiment.

  The first fuel assembly 3 disposed on the outer peripheral portion of the initially loaded core 11 is disposed on the inner peripheral portion of the core 12 of the second cycle. Note that not all of the first fuel assemblies 3 need be disposed on the inner periphery of the core 12 in the second cycle.

  In general, the fuel assembly loaded on the outer periphery of the core is slower than the fuel assembly loaded on the inner periphery of the core, but combustion proceeds to some extent. That is, the first fuel assembly 3 disposed on the outer peripheral portion of the initial loading core 11 is combusted to some extent by the end of the first cycle. For this reason, the first fuel assembly 3 has a high average concentration of the assembly, but even if it is arranged in the inner periphery of the core 12 in the second cycle, it satisfies the thermal limit value in the second cycle. be able to. In particular, since the combustion proceeds rapidly at the corner of the fuel assembly 26 and in the vicinity thereof, the local peaking is rapidly reduced, and the fuel assembly 26 once burned at the outer peripheral portion of the initially loaded core 11 satisfies the thermal limit value. It becomes easy to let you.

  In addition, since gadolinia combustion is delayed at the outer peripheral part of the core, depending on the number of gadolinia-containing fuel rods and the concentration of gadolinia, it is loaded on the inner peripheral part of the core in the next cycle to lower the excess reactivity.

  Thus, by arranging the fuel assembly 3 having a high fuel assembly average enrichment disposed on the outer peripheral portion of the core 11 of the first cycle on the inner peripheral portion of the core 12 of the second cycle, the nuclear reactor can be safely operated. Economic efficiency can be improved while keeping.

[Second Embodiment]
FIG. 7 is a schematic diagram showing the axial gadolinia distribution of the first fuel assembly in the second embodiment of the core of the boiling water reactor according to the present invention. FIG. 8 is a schematic diagram showing the axial gadolinia distribution of the second fuel assembly in the present embodiment. FIG. 8 shows the fuel assembly indicated by reference numeral 1 among the second fuel assemblies 1, 2 and 4, but the same applies to the other fuel assemblies 2 and 4.

  The fuel assembly of the present embodiment has a plurality of axial regions with different gadolinia concentrations.

  The first fuel assembly 3 loaded on the outer peripheral portion in the initial loading core 11 includes a region 42 having a small gadolinia concentration and a region having a medium gadolinia concentration in order from the top, between regions 41 not including gadolinia at the upper and lower ends. 43, a region 44 having a high gadolinia density and a region 45 having a low gadolinia density. Note that the gadolinia density of the region 42 having a small gadolinia density near the upper end and the region 45 having a small gadolinia density near the lower end may be the same or different.

  The second fuel assemblies 1, 2, 4 loaded on the inner peripheral portion in the initial loading core 11 have a region 46 having a lower gadolinia concentration, a gadolinia concentration in order from the top, between regions 41 not including gadolinia at the upper and lower ends. Has an intermediate region 47 and a region 48 having a high gadolinia density.

  FIG. 9 is a graph showing an example of the axial thermal power distribution of the core.

  The heat output of the core is generally low at the upper and lower ends in the axial direction. For this reason, in the part near the upper and lower ends in the axial direction, combustion of gadolinium tends to be delayed.

  In a boiling water reactor, the heat output in the region near the lower end tends to be high (bottom peak). Therefore, the gadolinium content in the region near the lower ends of the second fuel assemblies 1, 2, 4 loaded on the inner periphery of the initial loading core 11 is increased.

  On the other hand, if the gadolinium content is increased in the region near the lower end of the first fuel assembly 3 loaded on the outer peripheral portion of the initially loaded core 11, as with the second fuel assemblies 1, 2, 4. In some cases, combustion does not proceed sufficiently by the end of the first cycle. If the combustion does not proceed sufficiently, a large amount of fissile material such as uranium 235 remains, and if the combustion of gadolinia in the region near the lower end proceeds after the second cycle, an extreme bottom peak state may occur.

  For this reason, among the fuel assemblies loaded on the inner periphery of the core 12 in the second cycle, it is more likely that the combustion in the vicinity of the lower ends of a certain number of fuel assemblies will advance the bottom peak in the second cycle. It is preferable in avoiding this. However, if the gadolinium content in the lower part of the second fuel assemblies 1, 2, 4 loaded on the inner periphery of the core 11 of the first cycle is reduced, the thermal limit value may not be satisfied in the first cycle. is there.

  Therefore, in the present embodiment, a region 45 with a low gadolinium content is provided at a position near the lower end of the first fuel assembly 3, and the first fuel assembly 3 loaded on the outer peripheral portion of the initial loading core 11 is provided. The gadolinium content in the portion close to the lower end is made lower than those of the second fuel assemblies 1, 2, and 4 loaded on the inner periphery. In this way, combustion in a region near the lower end of the first fuel assembly 3 is promoted during the first cycle.

  If the fuel assembly is located on the outer peripheral portion of the initially loaded core 11, even if it becomes an extreme lower output peak, the output of the fuel assembly 26 as a whole is low, so that the thermal limit value is satisfied and the gadolinia combustion is performed. Can be promoted. For this reason, even if the core 12 of the second cycle is loaded on the inner periphery, it can be suppressed that the heat output distribution of the core 12 becomes an extreme bottom output peak.

[Third Embodiment]
FIG. 10 is a cross-sectional view showing an example of the enrichment of each fuel rod of the first fuel assembly in the third embodiment of the core of the boiling water reactor according to the present invention. FIG. 11 is a cross-sectional view showing an example of the enrichment of each fuel rod of the second fuel assembly in the present embodiment. 10 and 11, the positions indicated by U1, U2, and U3 indicate that uranium fuel rods are disposed, and the positions indicated by G1 and G2 indicate that gadolinia-containing fuel rods are disposed. WR indicates the position of the water rod. The uranium fuel rods indicated by U1, U2, and U3 have different uranium 235 enrichments, the uranium fuel rods indicated by U1 have the highest enrichment, and the uranium fuel rods at the location indicated by U3 have the lowest enrichment. The fuel rods with gadolinia indicated by G1 and G2 have different gadolinia concentrations, and the gadolinia concentration of the fuel rods indicated by G1 is lower than the gadolinia concentration of the fuel rod indicated by G2.

  The first fuel assembly 5 and the second fuel assembly 6 of the present embodiment both correspond to the fuel rods 24 arranged in 9 rows and 9 columns and the fuel rods 24 for the central portion. It has two water rods arranged in position. The first fuel assembly 5 of the present embodiment is used instead of the first fuel assembly 3 of the first embodiment. The second fuel assembly 6 of the present embodiment is used in place of any of the second fuel assemblies 1, 2, and 4 in the first embodiment. Note that the other fuel assemblies loaded on the inner periphery of the initial loading core 11 may be fuel assemblies having two water rods as in the second fuel assembly 6 of the present embodiment, The fuel assembly having one water channel according to the first embodiment may be used.

  The second fuel assembly 6 of the present embodiment has 20 gadolinia-containing fuel rods, and these gadolinium-containing fuel rods are arranged so as not to be adjacent to each other. On the other hand, the first fuel assembly 5 has 22 gadolinia-containing fuel rods having a lower gadolinia content rate than the gadolinia-containing fuel rods used in the second fuel assembly 6, and Some are arranged next to each other.

  In order to suppress excess reactivity with gadolinia over a long period of the combustion period, it is necessary to increase the gadolinia concentration. However, the fuel pellets to which gadolinia is added tend to increase the temperature of the fuel pellets because the thermal conductivity decreases. For this reason, it is preferable that the gadolinia concentration is low from the viewpoint of fuel integrity.

  Thus, for example, Patent Document 2 discloses a method in which gadolinia-containing fuel rods are arranged adjacent to each other to suppress the combustion of gadolinia and maintain the neutron absorption effect over a long period of time even if the gadolinia concentration is not high. .

  Thus, the gadolinia containing fuel rods are arranged adjacent to each other and the number of the gadolinia containing fuel rods is increased as much as possible, so that the excess reactivity can be suppressed over a long period of time while reducing the gadolinia concentration. However, when such a fuel assembly is loaded on the inner periphery of the core, the number of fuel rods containing gadolinia is large, so that the excess reactivity at the initial stage of combustion may be excessively lowered.

  In the initial loading core 11 of the present embodiment, the first fuel assembly 5 in which the gadolinia-containing fuel rods are arranged adjacent to each other is loaded on the outer peripheral portion. Thereby, it is possible to suppress the excess reactivity over a long period of time while keeping the excess reactivity at the beginning of the first cycle appropriately and without excessively increasing the gadolinia concentration.

  In addition, this invention is not limited to each above-mentioned embodiment, It can implement with various forms. For example, the above description has been given by taking the first cycle and the second cycle of the boiling water reactor as an example, but the same applies to any two consecutive cycles. Moreover, although each above-mentioned embodiment demonstrated the core comprised by 872 bodies, it is applicable even if it is a smaller or larger core than this. Also, the fuel assembly has been described using 9 × 9 type fuel, but other types of fuel, for example, fuel in which fuel rods are arranged in 10 rows and 10 columns may be used. It can also be applied to uranium / plutonium mixed oxide fuel. These fuel assemblies may be mixed. As an example of the flammable poison, gadolinia is used as an example, but other flammable poisons such as elbia may be used.

  Furthermore, it can be implemented by combining the features of the embodiments.

FIG. 3 is a ¼ cross-sectional view showing the arrangement of the fuel assemblies in the first cycle core in the first embodiment of the core of the boiling water reactor according to the present invention. 1 is a ¼ transverse cross-sectional view in a first embodiment of a core of a boiling water reactor according to the present invention. 1 is a cross-sectional view of a fuel assembly in a first embodiment of a core of a boiling water reactor according to the present invention. It is a cross-sectional view showing an example of the enrichment of each fuel rod of the first fuel assembly in the first embodiment of the core of the boiling water reactor according to the present invention. It is a cross-sectional view showing an example of the enrichment of each fuel rod of the second fuel assembly in the first embodiment of the core of the boiling water reactor according to the present invention. FIG. 3 is a ¼ cross-sectional view showing the arrangement of the fuel assemblies in the core of the second cycle in the first embodiment of the core of the boiling water reactor according to the present invention. It is a schematic diagram which shows the axial gadolinia distribution of the 1st fuel assembly in 2nd Embodiment of the core of the boiling water reactor which concerns on this invention. It is a schematic diagram which shows the axial gadolinia distribution of the 2nd fuel assembly in 2nd Embodiment of the core of the boiling water reactor which concerns on this invention. It is a graph which shows the example of the axial direction heat output distribution of a core. It is a cross-sectional view which shows the example of the enrichment of each fuel rod of the 1st fuel assembly in 3rd Embodiment of the core of the boiling water reactor which concerns on this invention. It is a transverse cross section showing an example of enrichment of each fuel rod of the 2nd fuel assembly in a 3rd embodiment of a core of a boiling water reactor concerning the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2, 3, 4, 5, 6 ... Fuel assembly, 11 ... Core of the first cycle (initial loading core), 12 ... Core of the second cycle, 21 ... Control rod, 23 ... Channel box, 24 ... Fuel Rod, 25 ... Water channel, 26 ... Fuel assembly

Claims (8)

In the core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is vertical,
An outer peripheral region in which the first fuel assemblies are arranged;
An inner circumferential region in which a second fuel assembly containing less fissionable material than the first fuel assembly is arranged inside the cylinder in the radial direction from the outer peripheral region;
A boiling water reactor core characterized by comprising: In the core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is vertical,
An outer peripheral region in which the first fuel assemblies are arranged;
A fuel rod that is located inside the cylinder in the radial direction from the outer peripheral region and contains less fissile material than the fuel rod in the corner portion of the first fuel assembly is disposed in the corner portion. An inner peripheral region in which fuel assemblies are arranged; and
A boiling water reactor core characterized by comprising: In the core of a boiling water reactor formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region whose axis is vertical,
An outer peripheral region in which the first fuel assemblies are arranged;
An inner peripheral region in which a second fuel assembly having a combustible poison content smaller than that of the first fuel assembly is arranged inside the cylinder in the radial direction from the outer peripheral region;
A boiling water reactor core characterized by comprising:   The maximum value of the concentration of the combustible poison contained in the first fuel assembly is smaller than the maximum value of the concentration of the combustible poison contained in the second fuel assembly, and the combustible contained in the first fuel assembly. 4. The core of a boiling water reactor according to claim 3, wherein the number of fuel rods containing toxic poisons is greater than the number of fuel rods containing flammable poisons contained in the second fuel assembly. .   The combustible poison content in the lower region of the predetermined axial length from the lower end of the first fuel assembly is larger than the combustible poison content in the lower region of the second fuel assembly. The core of the boiling water reactor according to any one of claims 1 to 4, wherein the core is small.   The core of a boiling water reactor according to any one of claims 1 to 5, wherein the core is an initial loading core. In a method for operating a boiling water reactor having a core formed by arranging fuel assemblies extending in the axial direction in a substantially cylindrical region having an axis in the vertical direction,
The first fuel assembly is arranged to form an outer peripheral region of the core, and is located inside the cylinder in the radial direction from the outer peripheral region and contains less fissile material than the first fuel assembly. A core forming step of arranging the second fuel assemblies to form an inner peripheral region of the core;
A first cycle step of keeping the core critical for a predetermined period after the core forming step;
A method for operating a boiling water reactor characterized by comprising: After the first cycle step, a fuel moving step of moving at least one of the first fuel assemblies to the inner peripheral region to form the core;
A second cycle step of keeping the core critical for a predetermined period after the fuel transfer step;
The method of operating a boiling water reactor according to claim 7, wherein:

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