JP2000098074A - Nuclear reactor core, its operation method, and fuel assembly - Google Patents

Abstract

(57) [Abstract] [Problem] Without impairing fuel economy and operation controllability,
Power peaking is reduced to obtain a reactor core having good core thermal characteristics. SOLUTION: A fuel assembly constituting a nuclear reactor core includes a fuel assembly group A including a first fuel assembly set such that burnable poison is substantially burned out at the end of a first combustion cycle period. And a fuel assembly group B including a second fuel assembly set to such an extent that the burnable poison remains unburned until the next cycle after the first fuel cycle.
Each fuel assembly group includes at least a fuel assembly in a first cycle and a fuel assembly in a second cycle, and includes a fuel rod containing a burnable poison contained in one second fuel assembly. Is set to be smaller than that of the first fuel assembly, and the concentration of the burnable poison contained in the second fuel assembly at the time of initial loading is set to be 1.2 times or more that of the first fuel assembly. .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor and a reactor core loaded with the fuel assembly, and more particularly to a reactor core made of high burnup fuel.

[0002]

2. Description of the Related Art In a fuel assembly of a boiling water reactor, gadolinia (Gd ₂ O ₃ ) is used as a burnable poison in order to suppress excessive reactivity in the early stage of combustion. Since gadolinia burns out rapidly with combustion, the infinite multiplication factor of the fuel assembly increases in the early stage of combustion, reaches a peak when the gadolinia burns out, and thereafter decreases as the combustion progresses. However, even after gadolinia is almost burned out, some gadolinium (Gd) isotopes remain in equilibrium and act as a constant loss of reactivity during the subsequent combustion period. This is called residual gadolinia reactivity.

A fuel assembly is composed of fuel rods containing gadolinia (gadolinia fuel rods) and other fuel rods. For example, as shown in the invention described in Japanese Patent No. 2577367, gadolinia fuel rods are used. There is known a method of delaying the gadolinia burnout period depending on the arrangement of the gadolinia.

[0004] In a conventional fuel assembly, it is general to set an initial concentration of gadolinia so that gadolinia burns out at the end of a cycle. This is because if gadolinia remains at the end of the cycle, the reactivity will be lost. In addition, the number of gadolinia fuel rods is determined so that the excess reactivity is constant throughout the operation cycle.In this case, when the core is composed of fuel assemblies having different burnups, the excess reactivity is constant throughout the operation cycle. This is because it is easy to A method of preparing two types of fuel assemblies with different numbers of gadolinia fuel rods and adjusting the loading ratio according to the fluctuation of the operation period to keep the excess reactivity during the operation cycle constant. Are known. Also in this case, the gadolinia concentration of each fuel assembly is almost burned out at the end of the first cycle.

[0005] The fuel assemblies constituting the core of a boiling water reactor are generally divided into a plurality of groups having different periods of stay in the reactor (the number of cycles in the reactor). When the fuel assemblies of the same group are adjacent to each other, power peaking is likely to occur. Therefore, in a normal core, the fuel assemblies of the same group are dispersed and arranged so as not to be adjacent to each other as much as possible. However,
In the initially loaded core, all the fuel assemblies stay in the furnace for the same period, but are divided into groups having different initial uranium enrichments and are similarly dispersed. The four fuel assemblies around the control rod used during normal operation are composed of burned fuel assemblies, and are called control cells. In general, the power peaking becomes smaller at the outermost periphery of the core and around the control cell, and conversely, the power peaking becomes larger at other places.

[0006]

In recent years, a high burnup which increases energy extracted from a single fuel assembly for the purpose of improving fuel economy and reducing the number of spent fuel assemblies in a boiling water reactor. Therefore, the burn-up increment per operation cycle (referred to as cycle burn-up) tends to increase. When the cycle burn-up increases, the difference in the infinite multiplication factor between the fuel assembly groups having different numbers of in-furnace cycles is increased, so that the output peaking is increased. This leads to deterioration of the thermal characteristics, and is a hindrance factor for increasing the burnup.

Further, as a measure for improving the fuel economy of the core, a low-leakage reactor in which a fuel assembly group having the smallest infinite multiplication factor is arranged at the outermost periphery of the core is known. However, in low-leakage reactors, the balance of the number of fuel assemblies in each group is disrupted in the center of the core, so that fuel assemblies in groups with a high infinite multiplication factor must be adjacent to each other, This results in radial output peaking. Therefore, conventionally, the arrangement of the fuel assemblies in the furnace cannot be the above-described low-leakage type, and this is a hindrance to improving fuel economy.

Next, the deterioration of the thermal characteristics will be described. Generally, the infinite multiplication factor of the fuel in the first cycle increases from the beginning to the end of the cycle. The infinite multiplication factor of the fuel in the second cycle decreases from the beginning to the end of the cycle. Therefore, at the beginning of the cycle, output peaking occurs in the fuel in the second cycle having a large infinite multiplication factor, and the thermal characteristics deteriorate. Conversely, in the latter half of the cycle, output peaking occurs in the fuel in the first cycle, resulting in poor thermal characteristics.

As for the linear output density, which is an example of thermal characteristics, the larger the value, the smaller the safety margin. Generally, the fuel having the highest linear power density is the fuel in the first cycle in the second half of the cycle. Also, the smaller the value of the limit power ratio, the smaller the safety margin. Generally, the minimum power ratio is usually the smallest in the early cycle of the second cycle fuel, which is about 20% of the entire cycle. Therefore, output peaking during this period is particularly problematic.

If the initial concentration of gadolinia is increased so that the gadolinia remains unburned until the next cycle after the end of the first cycle, the peak of the infinite multiplication factor shifts to the side where the burnup advances, and the maximum value of the peak decreases. Become. For this reason,
The difference in the infinite multiplication factor between the fuel assemblies having different numbers of cycles in the furnace becomes smaller, and the power peaking can be reduced. In particular, in order to improve the above-mentioned limit power ratio,
The concentration may be increased so that gadolinia remains unburned at least 1.2 times the cycle period. In order to improve the linear output density, the higher the gadolinia density, the better.

However, in this case, two problems occur together. One is that fuel economy is deteriorated due to reactivity loss due to gadolinia remaining at the end of the cycle. The other is that the excess reactivity is not constant throughout the operation cycle, and the operation controllability deteriorates. For example, in the fuel assembly described in Japanese Patent Application Laid-Open No. 62-106391, it is shown that the excess reactivity can be kept constant by increasing the gadolinia concentration in a part of the fuel assembly. However, even in this case, the occurrence of reactivity loss due to gadolinia remains as a problem.

The present invention has been made in view of these circumstances, and has been made to reduce the power peaking without impairing the fuel economy and operation controllability to increase the burnup of the fuel assembly, and to reduce the core thermal characteristics. The purpose of the present invention is to provide a good reactor core.

[0013]

In order to achieve the above object, the invention according to claim 1 of the present invention provides a fuel assembly including a fuel rod having a fuel element therein and a water rod through which a coolant flows. In a reactor core that is configured by loading a plurality of bodies and the operation period is a plurality of fuel cycle periods, a fuel assembly that constitutes the reactor core includes:
(A) a first fuel assembly group consisting of a first fuel assembly set to such an extent that the burnable poison is substantially burned out at the end of the first combustion cycle period, and (B) the first burnable poison is a first fuel assembly group. A second fuel assembly that is set to an extent that it remains unburned until the next cycle after the first fuel cycle
Each of the fuel assembly groups includes at least a first cycle fuel assembly and a second cycle fuel assembly, and a second fuel assembly. The number of burnable poison-containing fuel rods included in one fuel assembly is set smaller than the number of burnable poison-containing fuel rods included in one first fuel assembly, and the number of burnable poisonous fuel rods included in the second fuel assembly at the time of first loading is further reduced. The concentration of the burnable poison is set to be at least 1.2 times the concentration of the burnable poison contained in the first fuel assembly when initially loaded.

According to this configuration, in the second fuel assembly group, the burnable poison remains unburned for at least 1.2 times the cycle period. Therefore, it is possible to improve the limit output ratio at the beginning of the cycle and the linear output density at the end of the cycle. Further, the output of the fuel assemblies belonging to the first fuel assembly group is dispersed by arranging the fuel assemblies in the furnace together with the fuel assemblies belonging to the second fuel assembly group having different infinite multiplication factors. Since the effect is obtained, the thermal characteristics can be improved. On the other hand, the second
The reactivity loss due to the burnable poison remaining by the fuel assembly group and the effect on the excess reactivity throughout the operation cycle period are alleviated by the presence of the first fuel assembly group and are reduced by the entire core. Become. Further, by setting the number of the fuel rods containing the burnable poison contained in the second fuel assembly to be smaller than that of the first fuel assembly, the total amount of the burnable poison is reduced, and the remaining burnable poison is reduced. Reactivity loss can be reduced.

Further, the invention according to claim 2 is the same as the invention according to claim 1.
In the nuclear reactor core described above, in the second fuel assembly, a fuel rod containing no burnable poison is disposed at a position adjacent to the outermost periphery of the fuel assembly and the water rod, and at least two burnable poisons are provided. The fuel rods are preferably arranged adjacent to each other. With this configuration, the burnable poison remains burnt effectively, so that the total amount of the burnable poison can be reduced, and the reactivity loss due to the remaining burnable poison can be reduced.

According to a third aspect of the present invention, in the reactor core of the first aspect, the uranium enrichment of the second fuel assembly is set higher than the uranium enrichment of the first fuel assembly. It is characterized by. Thereby, the reactivity loss due to the residual burnable poison caused by the second fuel assembly group can be compensated.

According to a fourth aspect of the present invention, in the reactor core of the first aspect, the fuel assemblies belonging to the second fuel assembly group are arranged at positions other than the outermost periphery of the core and the control cells, and The number of fuel assemblies belonging to the second fuel assembly group is set to 20% or less of the number of all fuel assemblies in the reactor core. Since the power peaking of the outermost periphery of the core and the control cell is originally low, a more effective effect can be obtained by arranging the burnable poison at a position where the power peaking is high excluding this position. Also, at a location other than the outermost periphery of the core or a location other than the control cell, a fuel assembly belonging to the second fuel assembly group is always added to a cell composed of four fuel assemblies.
When the bodies are arranged, the ratio of the number of bodies becomes about 20% of the total number of the cores, and at this time, an almost maximum power peaking reduction effect is obtained. It is assumed that loading more than this has a greater adverse effect of increasing reactivity loss due to unburned burnable poisons.

According to a fifth aspect of the present invention, there is provided the nuclear reactor core according to the first aspect, wherein the fuel cell includes four fuel assemblies centered on control rods for controlling the reaction of the reactor core. A cell which is not located at the outermost periphery and which is not adjacent to the control cell, wherein at least two fuel assemblies in the first cycle are arranged in the cell, or at least two fuel assemblies in the second cycle are arranged in the cell. In any of the cases, a fuel assembly belonging to the second fuel assembly group is arranged in the cell. With this configuration, in a cell adjacent to the outermost core cell or the control cell, the number of fuel assemblies in the first cycle is two or more, or the number of fuel assemblies in the second cycle is two or more. However, the reactivity value of the control rod is not so great. Therefore, more effective effects can be obtained by arranging the burnable poison in a cell other than the position. In addition, the control rod disposed at the center of the four fuel assemblies constituting the cell other than the position has a higher reactivity value. However, according to the fuel assemblies belonging to the second fuel assembly group, since the burnable poison remains unburned, the reactivity stop value of the control rod is reduced, so that the reactor shutdown margin can be improved. .

According to a sixth aspect of the present invention, in the reactor core of the first aspect, in addition to the first fuel assembly group and the second fuel assembly group, (C) a burnable poison is added to the first fuel assembly group and the second fuel assembly group. The third fuel assembly group includes at least three third fuel assembly groups each including a third fuel assembly that is set to the extent that it burns out in the middle of one combustion cycle period, and the third fuel assembly group serves as a control cell. It is characterized by being arranged adjacently. The maximum value of the infinite multiplication factor of the third fuel assembly is larger than that of the first fuel assembly group. On the other hand, since the power peaking around the control cell is low, by this configuration, if the power peaking is increased by the fuel assemblies belonging to the third fuel assembly group, the power balance of the whole core is changed, so that the power elsewhere is controlled. High output peaking can be reduced. Further, it is possible to compensate for the reactivity loss caused by the unburned burnable poison caused by the second fuel assembly group.

[0020] The invention according to claim 7 relates to the method of operating a nuclear reactor core according to claim 1, and when refueling is performed at the end of a fuel cycle period, first, the second fuel assembly group is used. And extracting fuel assemblies belonging to the first fuel assembly group. This method is effective because the fuel assemblies belonging to the second fuel assembly have a high reactivity due to the remaining burnable poison, and the excess reactivity increases when the fuel assemblies are taken out first.

In the first aspect of the present invention, it is preferable to implement the following configuration in addition to the configuration other than the above. First, the fuel assemblies belonging to the first fuel assembly group and the fuel assemblies belonging to the second fuel assembly group are arranged adjacent to each other such that their in-furnace stay periods are substantially the same. It is preferred that This is because, even if the two fuel assemblies have the same in-furnace stay period, the infinite multiplication factors are different, so that output peaking does not occur even if they are arranged adjacently. Second
At least two of the four fuel assemblies adjacent to the fuel assemblies belonging to the second fuel assembly group
Preferably, it is arranged to be a fuel assembly in a cycle or a second cycle. Since the output peaking is likely to occur in the arrangement in which the fuel assemblies in the first or second cycle are concentrated, if the fuel assemblies belonging to the fuel assembly group are arranged at such positions, the effect of reducing the output peaking is increased.

[0022] The invention according to claim 8 is a fuel assembly including a fuel element loaded in a reactor core having an operation period consisting of a plurality of fuel cycle periods and containing a burnable poison therein, wherein the burnable poison is At least two fuel rods containing no burnable poison are arranged at the outermost periphery of the fuel assembly and at a position adjacent to the water rod, so as to remain unburned until the next cycle after the first fuel cycle. The fuel rods containing burnable poisons are arranged adjacent to each other. With this configuration, the burnable poison remains unburned effectively, so that the loss of reactivity due to the remaining burnable poison can be reduced by reducing the total amount of the burnable poison.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a fuel arrangement of a nuclear reactor core according to the present embodiment. Although this figure shows a 1/4 core, in the present embodiment, it is assumed that the fuel is arranged in the core in a quarter rotational symmetry. The core shown in FIG. 1 is composed of two different fuel assembly groups described in detail below.

The fuel assembly group A is composed of a fuel assembly 2 set so that gadolinia burns out before the end of the first cycle, and is further classified into four types according to the number of cycles in the furnace. In FIG. 1, the fuel in the first cycle of the fuel assembly group A is represented by A1, and the fuels in the second, third, and fourth cycles are represented by A2, A3, and A4, respectively. Further, the fuel assembly group B is composed of the fuel assemblies 3 set so that gadolinia remains unburned even after the end of the first cycle until the next cycle, and is further classified into three types according to the number of cycles in the furnace. . That is, the gadolinia content of the fuel assembly 3 is set to be larger than that of the fuel assembly 2. First cycle, second cycle of fuel assemblies of fuel assembly group B
The fuel in the third and third cycles is B1, B2,
Expressed as B3.

In FIG. 1, a portion surrounded by a thick line indicated by reference numeral 10 is a control cell. It is composed of fuel assemblies A3 and A4 in the third and fourth cycles in which combustion has progressed. FIG. 2 is a graph showing a combustion change of the fuel assemblies 2 and 3 constituting the fuel assembly groups A and B at an infinite multiplication factor. Solid line 4A indicates fuel assembly 2
The broken line 4B indicates the fuel assembly 3 (the fuel assembly group A).
(Fuel assembly group B) is shown. Also, d1,
d2 indicates the approximate range of the first cycle and the second cycle, respectively.

In the fuel assembly group A, since the gadolinia of the fuel assembly 2 burns out at the end of the first cycle, the infinite multiplication factor becomes maximum at the end of the first cycle as shown by the solid line 4A. On the other hand, in fuel assembly group B, gadolinia remains unburned until the next cycle. Therefore, as shown by the broken line 4B, when compared with the fuel assembly group A, the first cycle fuel B1
The difference between the infinite multiplication factor between the second cycle fuel B2 and the second cycle fuel B2 is small.
That is, in the fuel assembly group B, the limit power ratio at the beginning of the cycle and the linear power density at the end of the cycle are improved. The fuel assembly group A has the effect of dispersing the power by forming the core in a mixed manner with the fuel assembly group B having different infinite multiplication factors, so that the thermal characteristics can be improved.

FIGS. 3 and 4 are sectional views showing an example of the fuel rod arrangement of the fuel assemblies 2 and 3 constituting the fuel assembly groups A and B, respectively. In each of the fuel assemblies 2 and 3 shown here, the fuel rods 7 are arranged in the channel box 9 in 10 rows and 10 columns, and the total number of the fuel rods 7 is 92. Also, two water rods 8 through which a coolant flows are arranged in the center. In FIG. 3, among the fuel rod numbers assigned to the fuel rods 7, 1, 2 and 3 indicate uranium fuel rods, and G1 and G2 indicate fuel rods containing gadolinia. The degree is large. The gadolinia concentration of the gadolinia-containing fuel rod G1, which holds 16 of the 72 fuel rods of the fuel assembly group A shown in FIG. 3, is 6 wt%, and the cycle burnup is 14 GWd /
It is set to almost burn out at time t.

On the other hand, in FIG.
A uranium fuel rod indicated by 3 and a gadolinia-containing fuel rod indicated by G1, G2, and G3 coexist. Among the gadolinia-containing fuel rods of the fuel assembly group B shown in FIG. 4, the gadolinia concentration of the fuel rod G1 having the highest gadolinia concentration is 10 wt%, and the gadolinia-containing fuel rod G1 of the fuel assembly group A is 10 wt%.
Is about 1.7 times the gadolinia concentration (6 wt%). The average gadolinia concentration in the fuel assembly of the fuel assembly group A is preferably set to be 1.2 times or more as compared with that of the fuel assembly group B.

The fuel assembly group B shown in FIG. 4 has 16 gadolinia-containing fuel rods, which is smaller than the fuel assembly group A shown in FIG. 3 having 20 gadolinia-containing fuel rods. The degree loss is reduced. Further, the 16 gadolinia-containing fuel rods of the fuel assembly group B shown in FIG. 4 are arranged other than the outermost periphery and the periphery of the water rod, and 12 of them are arranged adjacent to each other. . Thereby, gadolinia can be effectively left unburned, and residual gadolinia reactivity loss is reduced. Further, the average uranium enrichment of the fuel assembly group B shown in FIG. 4 is 4.45%, and the average uranium enrichment of the fuel assembly group A shown in FIG.
It is set higher than%. This makes it possible to compensate for the residual gadolinia reactivity loss.

The operation of the present embodiment will be described with reference to a graph as a verification result. FIG. 5 is a graph showing a change in surplus reactivity accompanying combustion. The broken line 5A indicates the case where the core is constituted only by the fuel assemblies 2 constituting the fuel assembly group A, that is, B1, B2, B2 in FIG.
It shows the excess reactivity when B3 is replaced with A1, A2, and A3, respectively. The surplus reactivity in this case is a substantially constant value during the cycle period, and the reactivity at the end of the cycle is sufficient.

The broken line 5B in FIG. 5 shows the excess reactivity when the core is constituted only by the fuel assemblies 3 constituting the fuel assembly group B. in this case,
The surplus reactivity greatly changes during the cycle period, and the reactivity is insufficient at the end of the cycle. A solid line 6 in FIG. 5 indicates the excess reactivity of the reactor core 1 according to the present embodiment shown in FIG. Out of 368 fuel assemblies in reactor core 1,
Twenty-four fuel assemblies belong to fuel assembly group B, accounting for 7% or less of the total. For this reason,
Fuel assembly group B for surplus reactivity change of reactor core 1
As shown by the solid line 6, the effect is reduced by the entire core and becomes small.

FIG. 1 shows the coordinates of the reactor core 1 (I, J).
It will be shown by. The fuel assembly at the position (I, J) = (11, 9) is the second cycle fuel A2 of the fuel assembly group A. Further, the fuel assembly located at the position (11, 10) adjacent thereto via the control rod 11 is the second cycle fuel B2 of the fuel assembly group B. These adjacent fuel assemblies A2 and B2 have substantially the same stay period in the furnace, but have different infinite multiplication factors, so that no output peaking occurs. Of the four fuel assemblies adjacent to the fuel assembly A2 at the (11, 9) position, three (A1, A1, B2) are the fuel assemblies in the first cycle or the second cycle. Since output peaking is likely to occur in such a position,
The output peaking reduction effect by the fuel assembly A2 at the position (11, 9) is also large.

Of the four fuel assemblies constituted around the control rod 11 shown in FIG. 1, three fuel assemblies (A2, A2, B2) in the second cycle are located. (11,
9) The fuel assembly A2 at the position is also included. This fuel assembly A2
, The reactivity value of the control rod 11 is reduced, and the furnace shutdown margin is improved. When operating the nuclear reactor, the fuel assembly group B is taken out of the fuel assembly group A at the end of each cycle in the core 1 in preference to the fuel assembly group A. In other words, after the fuel assembly group B is taken out, the fuel assembly group A is taken out, that is, taking out for refueling is selectively performed in two stages. Thereby, the residual gadolinia reactivity loss can be reduced. 6 and 7 are graphs showing the linear output density and the limit output ratio in the present embodiment, respectively. As shown by the solid line 12 in FIG. 6, it is understood that the linear power density of the core 1 according to the present embodiment is improved particularly in the middle stage of the cycle as compared with the case of the conventional core shown by the broken line 13. Similarly, as shown by the solid line 14 in FIG. 7, the limit power ratio of the core 1 according to the present embodiment is also improved as compared with the conventional case shown by the dashed line 15, and particularly in the operation cycle period in the initial operation. It can be seen that the improvement effect is large in the period of about 20%.

Hereinafter, a second embodiment of the present invention will be described. FIG. 8 is a quarter rotationally symmetric sectional view showing the fuel arrangement of the reactor core 16 according to the present embodiment. The difference from the first embodiment is that a fuel assembly group C indicated by C1 and C2 in FIG. 8 is introduced.

In the fuel assemblies constituting the fuel assembly group C, gadolinia is set to burn out during the cycle. FIG. 9 shows this fuel assembly group C
Fig. 2 shows the combustion change of the fuel assembly constituting
Are superimposed. According to this, as shown by the broken line 4C, the infinite multiplication factor becomes maximum during the first cycle. Since the fuel assembly group C is arranged adjacent to the control cell, the power peaking at this position increases, and the power balance of the entire core changes, so that high power peaking elsewhere can be reduced. The fuel assembly group C can compensate for the reactivity loss caused by the unburned gadolinia contained in the fuel assembly group B even at the end of the first cycle.

[0036]

As described above, according to the present invention, it is possible to obtain a reactor core having good core thermal characteristics by reducing output peaking without impairing fuel economy and operation controllability.

[Brief description of the drawings]

FIG. 1 is a 1/4 rotationally symmetric sectional view showing a fuel arrangement of a reactor core according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a combustion change of an infinite multiplication factor of the fuel assembly groups A and B shown in FIG.

FIG. 3 is a cross-sectional view showing a fuel rod arrangement of a fuel assembly 2 constituting a fuel assembly group A shown in FIG.

FIG. 4 is a cross-sectional view showing a fuel rod arrangement of a fuel assembly 3 constituting a fuel assembly group B shown in FIG.

FIG. 5 is a characteristic diagram showing a combustion change of the reactor core shown in FIG. 1 at an infinite multiplication factor.

FIG. 6 is a characteristic diagram showing a combustion change of a linear power density of the reactor core shown in FIG.

FIG. 7 is a characteristic diagram showing a combustion change of a critical power ratio of the reactor core shown in FIG.

FIG. 8 is a 1/4 rotationally symmetric sectional view showing a fuel arrangement of a nuclear reactor core according to a second embodiment of the present invention.

9 is a characteristic diagram showing a combustion change of the fuel assembly groups A, B, and C shown in FIG. 8 at an infinite multiplication factor.

[Explanation of symbols]

1, 16: Reactor core, 2, 3: Fuel assembly, 7: Fuel rod, 8: Water rod, 9: Channel box, 10
... control cell, 11 ... control rod.

Claims (8)

[Claims] 1. A reactor core comprising a plurality of fuel assemblies each including a fuel rod including a fuel element therein and a water rod through which a coolant flows, and having an operation period of a plurality of fuel cycle periods. In the fuel assembly constituting the reactor core, (A) the burnable poison is
A first fuel assembly group consisting of a first fuel assembly that is set to such a degree that it burns out at the end of the combustion cycle period of (B), in which the burnable poison burns to the next cycle after the first fuel cycle; Second set to the extent that it remains
And a second fuel assembly group comprising at least two fuel assemblies. Each fuel assembly group includes at least a first cycle fuel assembly and a second cycle fuel assembly. And the second
The number of burnable poison-containing fuel rods included in one fuel assembly is set to be smaller than the number of burnable poison-containing fuel rods included in the first fuel assembly, and the second fuel assembly is further provided. The concentration of burnable poisons in the body when first loaded,
A nuclear reactor core, wherein the concentration of the burnable poison contained in the first fuel assembly at the time of initial loading is set to 1.2 times or more. 2. In the second fuel assembly, a fuel rod not containing the burnable poison is disposed at an outermost peripheral portion of the fuel assembly and at a position adjacent to the water rod, and at least two of the burnable poisons are disposed. 2. The reactor core according to claim 1, wherein the poisoned fuel rods are arranged adjacent to each other. 3. The reactor core according to claim 1, wherein the uranium enrichment of the second fuel assembly is set higher than the uranium enrichment of the first fuel assembly. 4. The fuel assemblies belonging to the second fuel assembly group are arranged at positions other than the outermost periphery of the core and the control cells, and the number of fuel assemblies belonging to the second fuel assembly group is 2. The reactor core according to claim 1, wherein the number is set to 20% or less of the number of fuel assemblies in the reactor core. 5. A cell comprising four fuel assemblies centered on a control rod for controlling a reaction of a reactor core, wherein the cell is not located at the outermost periphery of the core and is not adjacent to the control cell. The fuel assembly of the first cycle is arranged in at least two fuel assemblies or the fuel assembly of the second cycle is arranged in at least two fuel cells in the second cycle. The reactor core according to claim 1, wherein fuel assemblies belonging to the fuel assembly group are arranged. 6. The reactor core according to claim 1, wherein, in addition to the first fuel assembly group and the second fuel assembly group, (C) a degree to which the burnable poison is burned out during the first combustion cycle period. And at least three groups of a third fuel assembly group configured by a third fuel assembly set as described above, wherein the third fuel assembly group is disposed adjacent to a control cell. The reactor core according to claim 1, wherein 7. A fuel assembly constituting a nuclear reactor core having an operation period including a plurality of fuel cycle periods, the fuel assembly including a fuel rod having a fuel element therein and a water rod having a coolant flowing therein. And (A) a first fuel assembly group comprising a first fuel assembly set such that the burnable poison is substantially burned out at the end of the first combustion cycle period.
(B) a second fuel assembly group consisting of a second fuel assembly set to such an extent that the burnable poison remains unburned until the next cycle after the first fuel cycle, Each fuel assembly group includes at least a fuel assembly of a first cycle and a fuel assembly of a second cycle, and the number of fuel rods containing burnable poison contained in the second fuel assembly. Is set to be smaller than the number of burnable poison-containing fuel rods included in the first fuel assembly, and the concentration of the burnable poison included in the second fuel assembly at the time of initial loading is equal to the first fuel assembly. Is set to be at least 1.2 times the concentration of the burnable poison contained in the fuel assembly at the time of initial loading, and when performing fuel exchange at the end of the fuel cycle period, first, the fuel belonging to the second fuel assembly group Take out the aggregate And then extracting a fuel assembly belonging to the first fuel assembly group. 8. A fuel assembly loaded in a reactor core having an operating period consisting of a plurality of fuel cycle periods and including a fuel element containing a burnable poison therein, wherein said burnable poison is contained in a fuel cycle after a first fuel cycle. Fuel rods containing no burnable poison are arranged at the outermost periphery of the fuel assembly and at a position adjacent to the water rod, and at least two gadolinia-containing fuel rods are set so as to remain unburned until the next cycle. A fuel assembly which is arranged adjacent to each other.