JPH102982A - Core for nuclear reactor and its operating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fuel economy by suitably controlling excess reactivity, assuring sufficient reactor shutdown margin and further increasing run-out burnup in a BWR core having enhanced mean enrichment of an initial loading fuel. SOLUTION: Out of two types of high enrichment fuel assemblies 2, the assembly having many number of combustible poison-filled fuel rods opposed to a water rod is represented by A, and the assembly having little number of rods is represented by B. The B is loaded in a core peripheral region 30. The many A is loaded and control cells 14 made of low enrichment fuel assembly L21 are discretely disposed in a core inner region 31. With this constitution, a distortion of radial power distribution of the core is flattened, and sufficient reactor shutdown margin is assured. When combustion of this disposition is called a first cycle, life of gadolinium can be prolonged by replacing positions of fuel assemblies 19 to 21 in a second cycle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core of a boiling water reactor and a method of operating the core, and more particularly, to a nuclear reactor comprising an initially loaded fuel assembly whose fuel economy is greatly improved by increasing the extraction burn-up. It relates to the core and its operation method.

[0002]

2. Description of the Related Art Fuel initially loaded in a reactor core after a reactor is constructed is referred to as initially loaded fuel. In the past, fuels with a uniform enrichment of fuel assemblies were used as such initially loaded fuels, but in recent years, in order to improve the take-out burnup, multiple types of fuel assemblies with different enrichments have been used. It came to be used in combination. However, in any case, the average enrichment of the initially loaded fuel is set to 2.1 to 2 so that the excess reactivity of the core becomes almost zero at the end of the first cycle.
It is set in the range of 2.5%.

[0003] As an example of such an initially loaded core, there is a fuel arrangement of a conventional boiling water reactor constituted by an initially loaded fuel composed of a combination of four types of fuel assemblies having different average enrichments of fuel uranium. FIG. 12 shows a configuration diagram of a 1/4 core showing an example. The core 1 shown in this figure is composed of 872 fuel assemblies 2, 3, 4, and 5. In this figure, the squares □ represent one fuel assembly. Here, four types of fuel assemblies are used. Fuel 2 indicated by the symbol H has an average enrichment of 3.7%,
It is equal to the average enrichment of the newly loaded replacement fuel assembly at the time of refueling. The fuel assemblies 3, 4, and 5 indicated by symbols M, L, and S have an average enrichment of 2.5%, respectively.
1.6% and 0.9% are shown. This fuel assembly 3,
For 4, 5 the replacement fuel assemblies are 1, 2,
The enrichment is determined so that the reactivity after staying in the furnace for three cycles is approximately the same as the reactivity. Further, in the core shown here, each of the fuel assemblies 2, 3, and 4 is loaded with 200 units, and the fuel assembly 5 is loaded with 272 units.
The overall average enrichment of the initially loaded fuel is 2.1%.

The four types of fuel assemblies 2, 3, 4, and 5 used here are all examples of high burnup fuel assemblies. FIG. 13A is a longitudinal sectional view of this high burn-up fuel assembly.
Shown in 13 (b) and 13 (c) show cross-sectional views taken along arrows bb and cc in FIG. 13 (a), respectively.

The fuel assembly shown here is a long fuel rod 6
And the short fuel rods 7 shorter than the long fuel rods 6 and the water rods 8 through which the coolant flows inside are bundled by a spacer 9 in a square grid of 9 rows and 9 columns and fixed to the upper tie plate 10 and the lower tie plate 11. The bundle of fuel rods is surrounded by a channel box 12. By employing the short fuel rods 7, it is possible to reduce the pressure loss by expanding the coolant flow path at the upper part of the fuel assembly and improve the furnace stop margin.

Further, in the initially loaded core 1 shown in FIG. 12, control rods are located at positions 13 indicated by circles in FIG.
Five are provided. One control rod and four fuel rods surrounding the control rod are collectively referred to as one cell. Fuel that does not constitute a cell exists in a part of the outermost periphery of the core.

By the way, in order to appropriately control the excess reactivity of the reactor core during the operation of the reactor, at least a part of the control rod arranged at the position 13 in the figure is moved. At this time, the power distribution of the fuel assembly adjacent to the control rod is distorted as the control rod moves. Therefore, by disposing discretely in the reactor core control cells in which a fuel assembly having a low enrichment or a fuel assembly having advanced combustion and a low reactivity is arranged in a group, the distortion of the power distribution is reduced. Can be eased. In FIG. 12, the control cell 14 composed of the low enrichment fuel assembly 5 is indicated by a thick frame. In this core, 21 control cells 14 are arranged.

When the reactor is operated with the initially loaded core shown in FIG. 12 and the first cycle is completed, about 200 fuel assemblies with reduced reactivity in the core are taken out of the core and replaced. The second cycle operation is performed with the fuel assemblies loaded. In the same manner, the operation is repeated in the third, fourth, and so on.

In recent years, developments have been made to increase the burn-out of such initially loaded fuel assemblies, thereby improving the fuel economy. For this purpose, it is necessary to increase the average enrichment of the initially loaded fuel. However, increasing the enrichment increases the excess reactivity particularly in the first cycle and the second cycle, and decreases the margin for shutting down the furnace. Issues arise.

In a boiling water reactor, in order to control the excess reactivity of the reactor core within an appropriate range of 1 to 2% Δk, burnable poison such as gadolinia is sometimes added to some fuel rods constituting a fuel assembly. Is contained. The number of fuel rods containing burnable poison per fuel assembly is set so that the excess reactivity at the beginning of the cycle is within an appropriate range, and the concentration of burnable poison is set in consideration of the cycle length. Is done.
In the initially loaded core having an average enrichment of 2.1% shown in FIG. 12, an appropriate number of fuel rods constituting the fuel assemblies 2, 3, and 4 are combined with a fuel rod mixed with gadolinia at a concentration of 7.5%. In this case, the excess reactivity in the first cycle is 1.5 to 2% Δ
It keeps almost constant in the range of k, and becomes almost zero at the end of the cycle.

However, if the average enrichment of the initially loaded fuel is increased, the excess reactivity after the middle stage of the cycle increases even if the operation period of the reactor is constant. For example, FIG.
In the first loading fuel at 2, each fuel assembly 2, 3,.
In the initial loading core in which the average enrichment is adjusted to 2.7% by adjusting the number of loaded bodies of 4 and 5, the excess reactivity exceeds 3% Δk in the latter half of the cycle.

When the average enrichment is increased, the difference between the reactivity when the reactor is in operation and the reactivity when the reactor is stopped is generally increased. Therefore, in an initially loaded core in which the average enrichment is significantly higher than in the past, it becomes difficult to secure a sufficient reactor shutdown margin even if the excess reactivity can be controlled within an appropriate range. Here, the reactor shutdown margin means that all control rods are inserted into the core during normal reactor shutdown,
This refers to the subcriticality when one control rod is not inserted into the core for some reason, particularly when the control rod having the highest reactivity value is not inserted. Even in such a case, it is required by design to keep the reactor subcritical.

To cope with such a problem, Japanese Patent Laid-Open No. Hei 7-244184 discloses an invention which aims to control the excess reactivity in the first cycle and the second cycle within an appropriate range and to secure a sufficient furnace stoppage margin. No. 6,086,045. In the inventions described in claims 3 and 4 of the publication, arrangements of the low-enrichment fuel and the high-enrichment fuel in the furnace are defined as means for securing the reactor shutdown margin in the first cycle. . Further, in the invention described in claims 5 to 7 of the publication, as a means for reducing the excess reactivity in the second cycle and further improving the fuel combustion efficiency, when shifting from the first cycle to the second cycle, Regulates the movement of fuel. In the invention described in claims 9 to 12 of the publication, the means for controlling the excess reactivity in the first cycle and the second cycle to an appropriate range and flattening the distortion of the power distribution in the core radial direction. The arrangement of the fuels in the core is defined assuming that the fuel with the highest enrichment among the initially loaded fuels is composed of two types of fuels having different numbers of burnable poison-containing fuel rods. Claims 19 and 2 of the publication
In the invention described in No. 0, the arrangement of the burnable poison-containing fuel rods in the fuel assembly is specified in order to reduce the excess reactivity in the second cycle.

[0014]

In order to control the excess reactivity, there is a method of setting the concentration of the burnable poison high to extend the life of the burnable poison. However, if the concentration of the burnable poison is excessively increased, the thermal conductivity of the fuel rods decreases, and the fuel temperature tends to increase. desirable.

As a method of extending the life without changing the concentration of the burnable poison, there is a method of disposing the burnable poison-containing fuel rod at a position not facing the water rod in the fuel assembly. In other words, burnable poisons have the property of absorbing low-energy neutrons more slowly, so neutron absorption is suppressed by arranging them away from the water rod facing the water rod where there is more cooling water to slow down neutrons. The effect as a burnable poison can be prolonged.

On the other hand, if the burnable poison-containing fuel rod is arranged at a position facing the water rod, there is an advantage that the furnace stop margin is improved. Compared to the high-temperature reactor operating state, the reactor with the water density is large and the low-temperature
The neutron moderating effect is large. Therefore, when the fuel rod containing the burnable poison is arranged near the water rod, the neutron absorption effect of the burnable poison becomes larger in the reactor stopped state than in the reactor operation, and as a result, the reactor stop margin is improved.

In the fuel assembly according to claims 19 and 20 disclosed in Japanese Patent Application Laid-Open No. 7-244184, at least one burnable poison-containing fuel rod is provided in the X direction and the Y direction by the other fuel rods. It is arranged to face the burnable poisoned fuel rod. That is, five burnable poison-containing fuel rods are arranged in a cross shape. Since the neutron absorption effect of the burnable poison is suppressed particularly in the fuel rod located at the center among the fuel rods containing the burnable poison in the cross-shaped arrangement, the effect of the burnable poison can be maintained for a long time. . However, considering the advantages and problems arising from arranging the burnable poison-containing fuel rods at the position facing the water rod as described above, it is necessary to appropriately control the excess reactivity in the reactor core and secure the reactor shutdown margin. Hitherto, no consideration has been given as to where in the fuel assembly the burnable poisoned fuel rods should be located in order to improve the two points simultaneously.

The present invention has been made in view of the above circumstances, and in a reactor core in which the average enrichment of initially loaded fuel is increased, the excess reactivity is controlled to an appropriate range and a sufficient reactor shutdown margin is secured. It is another object of the present invention to provide a nuclear reactor core in which the fuel economy is greatly improved by increasing the take-up burnup of the initially loaded fuel.

[0019]

In order to achieve the above object, according to the present invention, there is provided a first group of fuel rods containing a nuclear fuel substance and containing no burnable poison, and a second group of fuel rods containing a nuclear fuel substance and a burnable poison. In a core of a nuclear reactor having a plurality of fuel assemblies each formed by bundling fuel rods and water rods through which cooling water flows in a grid, the plurality of fuel assemblies include the second group of fuels. A nuclear reactor core includes at least two types of fuel assemblies each having a different ratio of fuel rods disposed at positions facing the water rod among the rods.

Of the second group of fuel rods containing burnable poisons,
As an example of two types of fuel assemblies having different proportions of the fuel rods arranged at positions facing the water rod, a cross-sectional view showing the fuel rod arrangement of two types of fuel assemblies 15 and 16 is shown.
These are shown in FIGS. 8A and 8B, respectively. FIG. 9 is a graph showing the infinite multiplication factor of each of the fuel assemblies 15 and 16.

A second group of fuel rods 17 and 18 containing burnable poisons is designated by the symbol G in the figure. Each of the fuel assemblies 15 and 16 shown in FIG. 7 includes ten second group fuel rods. In the fuel rods G of the second group, the ratio of the fuel rods (indicated by reference numeral 18 in the drawing) arranged at positions facing the water rods is 20% in the fuel assembly 15 and is 16%. Is 0%.

FIG. 9 is a graph showing the combustion transition of these fuel assemblies 15 and 16 at infinite multiplication factor. here,
The infinite multiplication factors of the fuel assemblies 15 and 16 when the reactor is in operation are denoted by a and b, respectively, and the infinite multiplication factors when the reactor is stopped are denoted by c and d, respectively. In each case, the control rod is not inserted. Although all control rods are actually inserted when the reactor is stopped, it is necessary to assume that one control rod will not be inserted for some reason in consideration of the reactor shutdown margin. Consider below.

In the operation state of the nuclear reactor, the infinite multiplication factor b of the fuel assembly 16 is larger than that of the fuel assembly 15 in the initial stage of combustion. Becomes larger. Therefore, the fuel assembly 16
It can be seen that the neutron absorption effect of the burnable poison is smaller and the life of the burnable poison is longer. On the other hand, in the reactor shutdown state, the infinite multiplication factor c of the fuel assembly 15 is smaller than that of the fuel assembly 16 for a long period from the initial stage of combustion. That is, the fuel assembly 15 improves the furnace stop margin.

Thus, by effectively using the characteristics of these fuel assemblies, it is possible to simultaneously improve the reactor shutdown margin and reduce the excess reactivity over a long period of time. For example, in a location where the reactor shutdown margin is likely to be severe in the core, the fuel assembly 1 having a relatively large proportion of the fuel rods disposed at the position facing the water rods among the fuel rods of the second group is used.
It is effective to arrange a large number of fuel assemblies 16 at a position where the number of fuel assemblies 5 is not so severe in terms of reactor stop margin.

Further, according to the present invention, at least two types of fuel assemblies having different average enrichments are provided, and a fuel assembly having the highest enrichment among these fuel assemblies includes a first fuel assembly, The fuel rods of the second group include a second fuel assembly in which a proportion of the fuel rods arranged at a position facing the water rod is smaller than the first fuel assembly. A reactor core is provided.

In a core loaded with two or more types of fuel assemblies having different average enrichments, the reactor shutdown margin tends to be strict at the position of the cell loaded with the highest enrichment. Also, due to the phenomenon (neutron hardening) in which the neutron spectrum shifts to higher energy as the fuel enrichment increases, the effect of the neutron absorption by the burnable poison decreases with the fuel enrichment. Therefore, in order to suppress the excess reactivity of the core for a long time, it is most effective to extend the life of the burnable poison contained in the fuel assembly having the highest enrichment. Therefore, in the fuel assemblies of the highest enrichment classified into two or more types, by designing such that the ratio of the fuel rods arranged at the position facing the water rod among the fuel rods of the second group is different, It is possible to improve the furnace shutdown margin and further prolong the effect of suppressing excess reactivity.

Further, according to the present invention, the reactor core of the first fuel assembly is characterized in that at least one of the fuel rods of the second group faces the water rod in two directions. provide.

From the graph of FIG. 9, the fuel assembly 15 in which the second group of fuel rods is disposed at the position 18 facing the water rod in two directions, as compared with the fuel assembly 16, It can be seen that the difference in surplus reactivity from when the furnace was stopped was small. By using such a fuel assembly having a small difference in reactivity between operation and shutdown, it is possible to further improve the reactor shutdown margin.

Further, in the present invention, in the second fuel assembly, at least one of the second group of fuel rods is in surface contact with another second group of fuel rods in at least one of the X and Y directions. A nuclear reactor core is provided.

Since the fuel rods of the second group have a neutron absorption effect by the burnable poison, the number of neutrons generated by fission is smaller than that of the fuel rods of the first group. Therefore, when the second group of fuel rods are arranged facing each other, the amount of neutrons flowing from the fuel rods adjacent to the position of the second group of fuel rods decreases, so that the progress of burning of the burnable poison becomes slow. , The service life is prolonged.

Further, according to the present invention, there is provided a nuclear reactor core, wherein the first fuel assembly and the second fuel assembly have the same number of the second group of fuel rods. In the invention disclosed in claims 9 to 12 of Japanese Patent Application Laid-Open No. Hei 7-244184, the excess reactivity is controlled to an appropriate range and the distortion of the radial power distribution in the core is flattened. Therefore, two types of fuel assemblies having different numbers of fuel rods of the second group have been used. That is, a fuel assembly including many fuel rods of the second group is disposed at a position where there is a high demand for suppressing the output in the radial direction. In contrast, FIG.
In the fuel assembly 15 shown in (a), since the two burnable poison-containing fuel rods 18 are in contact with the water rods in two directions, the number of the second group of fuel rods is the same as that of the fuel assembly 16. Nevertheless, the neutron absorption effect of the burnable poison is greater than that of the fuel assembly 16 shown in FIG. Therefore, the fuel assembly 15 has substantially the same operation as the fuel assembly including a large number of the second group of fuel rods in the disclosed invention. Further, since the number of fuel rods of the second group is equal in each fuel assembly having the highest enrichment, it is necessary to consider the number of fuel rods of the second group for each type of fuel assembly having the highest enrichment during fuel production. And the time and labor required for fuel production can be reduced.

Further, according to the present invention, there is provided a nuclear reactor core, wherein the first fuel assembly and the second fuel assembly have the same type and number of fuel rods constituting the fuel assembly.

With the above configuration, at least two of the fuel assemblies having the highest enrichment have the same type and number of fuel rods, and only the arrangement of the fuel rods is different.
Fuel production becomes easier.

Further, in the present invention, a large number of cells each comprising one control rod and four fuel assemblies surrounding the control rod are arranged in the core, and the core is located at the outermost periphery of the core. A core peripheral region comprising a fuel assembly located therein and a fuel assembly constituting a cell including at least one fuel assembly located at the outermost periphery of the core, and a core central region not belonging to the core peripheral region. At this time, the ratio of the number of the first fuel assemblies and the number of the second fuel assemblies in the core inner region is equal to the number of the first fuel assemblies and the second fuel assemblies in the core peripheral region. Provided is a reactor core characterized by being greater than the ratio.

As described above, the first fuel assembly has an action mainly for improving the furnace stop margin, and the second fuel assembly has an action mainly for extending the life of the burnable poison.
FIG. 10 is a configuration diagram of a 1/4 core showing an example of a distribution of control rod values at the time of reactor shutdown in a reactor core consisting only of fuel assemblies having the same specifications. Here, the relative value is shown assuming that the maximum value of the control rod value in the radial direction is 1. From this figure, it is understood that the control rod value of the cells belonging to the core peripheral region is significantly lower than the cells belonging to the core inner region. For this reason, it is required to improve the effect of the burnable poison on the reactor shutdown margin in the area inside the core, while there is not so severe requirement in the area around the core, and even if the effect by the burnable poison is small, sufficient furnace A stop margin can be secured.

Therefore, in the core peripheral region, more second fuel assemblies having a smaller effect of improving the reactor shutdown margin than the first fuel assemblies can be arranged. In this core peripheral region, a part of the neutrons leak out of the core, so that the output of the fuel assembly loaded at this position is low and the progress of combustion is slow, so that the life of the burnable poison is relatively long. Therefore, the life of the burnable poison can be further extended for the second fuel assembly disposed in the core peripheral region.

FIG. 9 shows that the infinite multiplication factor during the operation of the second fuel assembly is larger than that of the first fuel assembly from the early stage of combustion to the first half of combustion. Therefore, by disposing a large number of the second fuel assemblies in the low-power core peripheral region, it is possible to flatten radial distortion of the power distribution in the core.

Further, the present invention provides a nuclear reactor core, wherein the number of the second fuel assemblies is larger than the number of the first fuel assemblies in the region around the core. Further, the present invention provides a nuclear reactor core, wherein the number of the first fuel assemblies is larger than the number of the second fuel assemblies in the core inner region. Further, the present invention provides a nuclear reactor core, wherein the fuel assembly having the highest enrichment loaded in the core peripheral region is a second fuel assembly.

By arranging a large number of the second fuel assemblies in the core peripheral region, it is possible to extend the life of the burnable poison as the whole core while maintaining a high reactor shutdown margin. Further, by arranging a large number of the first fuel assemblies in the core inner region, the life of the burnable poison can be maintained long,
Furnace shutdown margin can be further improved. at the same time,
By arranging the first fuel assembly, it is possible to lower the power in the core inner region where the power is originally large, and to further flatten the distortion of the power distribution in the radial direction.

Further, according to the present invention, a part of a number of cells arranged in the core is a control cell composed of four fuel assemblies having low reactivity, and a cell facing the control cell in the core or The ratio of the number of the first fuel assemblies to the number of the second fuel assemblies loaded in the cell including at least one fuel assembly having the lowest enrichment is smaller than the control cells other than the control cells in the core inner region. Claims: 1. A nuclear reactor characterized in that it is smaller than the ratio of the number of said first fuel assemblies and that of said second fuel assemblies loaded in cells not facing the cells and containing no fuel assemblies with the lowest enrichment. Provide a core.

According to the present invention, in the core inner region,
The second fuel assembly is provided in a nuclear reactor core, wherein the second fuel assembly is loaded only in a cell facing the control cell or a cell including at least one fuel assembly having the lowest enrichment.

FIG. 11 is a block diagram of a 1/4 reactor core showing an example of a distribution of control rod values when the reactor is stopped in a reactor core where 21 control cells 14 are arranged. In this core, a control cell composed of a low-enrichment fuel assembly was arranged at a position 14 indicated by a thick frame in the figure, and a high-enrichment fuel assembly of the same specification was arranged at all positions other than the control cell. Here, the relative value is shown with the maximum value of the control rod value in the radial direction being set to 1. It can be seen from this figure that the control rod value of the control cell is smaller than that of the cell composed of only the high enrichment fuel assembly. However, among the cells consisting only of the high enrichment fuel assemblies, the cells located at the position facing the control cell have a control rod value of about 10 to 20% smaller than the cells not facing the control cell. Further, a cell including at least one low-enrichment fuel assembly has a relatively small control rod value, and thus has a sufficient reactor shutdown margin. Therefore, by arranging a large number of second fuel assemblies having relatively small reactor stop margins in such cells having a small control rod value, the burnable poison as the whole core can be secured while ensuring a sufficient reactor stop margin. Can extend the life of the device.

Further, in the present invention, the ratio of the number of the first fuel assemblies to the number of the second fuel assemblies in the core inner region is determined in the first cycle by the ratio between the first fuel assemblies and the second fuel assemblies in the core peripheral region. 2 is set to be larger than the ratio of the number of fuel assemblies, and in the second cycle, is set to be smaller than the ratio of the number of the first fuel assemblies to the number of the second fuel assemblies in the core peripheral region. A method for operating a core of a nuclear reactor is provided.

According to this configuration, in the first cycle, a large amount of the second fuel assemblies is loaded in the core peripheral region having a small control rod value. The two problems of extending the life of the poison can be achieved at the same time, and the radial output distribution can be flattened. In the second cycle, a part of the second fuel assembly, which is loaded in the core peripheral region in the first cycle and whose combustion progresses slowly and still has a large amount of burnable poisons, is converted into an effective multiplication factor of the entire core. Is arranged in the core inner region where the contribution of the reaction is large, the excess reactivity can be sufficiently reduced. FIG. 9 also shows that in the latter half of the combustion, the infinite multiplication factor during the operation of the reactor of the second fuel assembly is larger than that of the first fuel assembly. Therefore, with the above configuration, the distortion of the radial output distribution can be flattened in the second cycle.

Further, in the present invention, the number of the first fuel assemblies in the core peripheral region is smaller than the number of the second fuel assemblies in the first cycle and the number of the second fuel assemblies in the second cycle. Provided is a method for operating a reactor core, wherein the number of reactor cores is larger than the number of reactors. Further, the number of the first fuel assemblies in the core inner region is larger than the number of the second fuel assemblies in the first cycle and smaller than the number of the second fuel assemblies in the second cycle. A method for operating a core of a nuclear reactor is provided.

With this configuration, it is possible to flatten the distortion of the radial output distribution while controlling the excess reactivity in the second cycle. Further, in the present invention, the outermost periphery of the core is provided with a fuel assembly having the highest enrichment, the second fuel assembly in the first cycle, and the first fuel assembly in the second cycle.
And a method for operating a reactor core of a nuclear reactor characterized by loading the fuel assembly.

The outermost periphery of the core in the core peripheral region where the control rod value is small is a region where the influence on the reactor stop margin is small and the power is the smallest in the radial direction in the core. Therefore, according to the above configuration, in the first cycle, the progress of the burning of the burnable poison can be delayed as much as possible without deteriorating the furnace stop margin, and at the same time, the distortion of the radial power distribution is flattened. Can be. In the second cycle, the excess reactivity can be further reduced, and the radial power distribution can be flattened.

Further, according to the present invention, the first cycle to the second cycle
At the time of transition to the cycle, the low-enrichment fuel assembly loaded in the control cell composed of four low-enrichment fuel assemblies in the core inner region is replaced with the low-enrichment fuel assembly loaded in other than the control cell. And a method of operating a reactor core of a nuclear reactor, wherein

According to the present invention, when shifting from the first cycle to the second cycle, the second cycle loaded in the core peripheral region is used.
The present invention also provides a method of operating a reactor core of a nuclear reactor, wherein the fuel assembly is replaced with a first fuel assembly loaded in a core inner region.

[0050]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1A is a fuel arrangement configuration diagram of a quarter core of the nuclear reactor according to the first embodiment. The basic configuration of the fuel assembly loaded in this core is shown in FIG.
This is the same as the fuel assembly of the conventional boiling water reactor shown in FIG. In FIG. 1A, symbols A and B represent high enrichment initially loaded fuel assemblies 19 and 2 having an average enrichment of 3.7%, respectively.
0 and the symbol L represents a low-enrichment initially loaded fuel assembly 21 having an average enrichment of 1.6%. The average enrichment of the high enrichment fuel assemblies 19 and 20 is equal to the enrichment of the replacement fuel assembly loaded at the time of refueling. The initially loaded core is loaded with 688 high enrichment fuel assemblies A and B in total and 184 low enrichment fuel assemblies L, and the average enrichment of the entire initially loaded core is 3.3. %.

The core includes a core peripheral region 30 composed of a fuel assembly located at the outermost periphery of the core and a fuel assembly constituting a cell including at least one fuel assembly located at the outermost periphery of the core. It is divided into a core central region 31 which does not belong to the core peripheral region 30. The double line in FIG. 1A indicates the boundary between the core peripheral region 30 and the core central region 31.

As shown by the thick frame in FIG. 1A, the control cell 1 composed of four low-enrichment fuel assemblies 21 is shown.
21 are disposed in the core. In addition to the control cell 14, the low enrichment fuel assemblies 21 are almost uniformly loaded in the core.

FIG. 2 shows a configuration diagram of the distribution of enrichment in the axial direction of each fuel assembly. Further, here, a gadolinia-containing fuel rod using gadolinia as a burnable poison is partially adopted, and the axial distribution configuration of the fuel rod is also shown.
FIG. 2A is a configuration diagram of the high-enrichment fuel assemblies 19 and 20;
FIG. 2B is a configuration diagram of the low-enrichment fuel assembly 21, and FIG.
(C) is a block diagram of a replacement fuel assembly. In this figure, for example, “3.9e, 10G7.0” indicates that “the enrichment is 3.9% and there are ten gadolinia-containing fuel rods with a concentration of 7.0%”. Each of these fuel assemblies is a high burn-up fuel assembly having the same shape as the fuel assembly shown in FIG. 13, and two nodes at the upper end and one node at the lower end of the total length of 24 nodes are indicated by oblique lines in the figure. A natural uranium region containing no gadolinia is provided. The arrow in the figure indicates the upper end of the short fuel rod 7 shown in FIG. 2 (b) and 2 (c) of these fuel assemblies
Fuel assembly 21 and replacement fuel assembly 2 shown in FIG.
2 is the same as the fuel assembly shown in FIG. 7 in the invention disclosed in Japanese Patent Application Laid-Open No. 7-244184.

FIGS. 3A and 3B respectively show FIGS.
It is sectional drawing which shows the arrangement | positioning of the fuel rod of the high-enrichment fuel assemblies 19 and 20 shown with the code | symbol A and B in (a). These cross-sectional views show c-c in the fuel assembly of FIG.
It is sectional drawing in the c arrow direction direction position. The fuel rod indicated by the symbol G in FIG. 3 is a fuel rod containing gadolinia. Also 1 to 4
The fuel rods numbered up to indicate fuel rods that do not include gadolinia and have higher enrichment in ascending order of number. The fuel assemblies 19 and 20 have the same four types of gadolinia-free fuel rods and the structure of gadolinia-containing fuel rods and the number of fuel rods in the fuel assemblies. However, as shown in FIG. 3, in the fuel assembly 19, five of the eleven gadolinia-containing fuel rods face the water rod, while in the fuel assembly 20, the eleven gadolinia-containing fuel rods of the fuel assembly 20 face the water rod. One is facing the water rod. That is, the ratio of the fuel rods arranged at positions in contact with the water rods among the gadolinia-containing fuel rods is determined by the fuel assembly 1
9 is 5/11 and fuel assembly 20 is 1/11,
The ratio is higher in the fuel assembly 19.

The fuel assemblies 19 and 2 shown in FIG.
The gadolinia-containing fuel rods G indicated by the reference numeral 18 in the figure each face one of the two water rods 8. This fuel rod 18 does not include gadolinia above the portion indicated by the arrow in the axial distribution configuration diagram shown in FIG. Further, gadolinia-containing fuel rods G denoted by reference numeral 23 in the drawings are present in each of the two fuel assemblies 19 and 20 shown in FIG.
It is in contact with four gadolinia-containing fuel rods in the X and Y directions. Here, the X direction and the Y direction refer to the directions indicated by arrows in FIG.

With this arrangement, the burnup when gadolinia in the fuel assembly burns out is about 25 GWd.
/ T. On the other hand, when the gadolinia-containing fuel rods having the same concentration are arranged in the fuel arrangement shown in FIG. 8, the burnup when the gadolinia in the fuel assembly burns out is about 20 GW.
d / t. Therefore, by arranging gadolinia-containing fuel rods as shown in FIG. 3, the life of the gadolinia can be extended by about 20% as compared with the case of FIG.

FIG. 4 is a graph showing the combustion transition of the high enrichment fuel assemblies 19 and 20 at infinite multiplication factor. Here, the infinite multiplication factors of the high enrichment fuel assemblies 19 and 20 during the operation of the reactor are a and b, respectively, and the infinite multiplication factors when the reactor is stopped are c and d, respectively. In each case, the control rod is not inserted. As can be seen from FIG.
In the early stage of combustion, the fuel assembly 19 has a smaller infinite multiplication factor at the time of reactor shutdown than the fuel assembly 20, so that the reactor shutdown margin is improved. On the other hand, combustion advances and burnup is 15 GW
At the stage exceeding d / t, the fuel assembly 20 has a smaller infinite multiplication factor during the operation of the reactor than the fuel assembly 19. Therefore, the life of gadolinia in the fuel is longer in the fuel assembly 20.

FIG. 1 (b) shows the core peripheral region 30 and the core inner region 31 in the arrangement shown in FIG. 1 (a).
Highly enriched fuel assemblies 19 and 2 indicated by symbols A and B
It is a table showing the body number of 0. As shown here, the number of the fuel assemblies 20 in the core peripheral region 30 is larger than that of the fuel assemblies 19, and the fuel assembly 20 in the core inner region 31.
The number of fuel assemblies 19 is larger than that of FIG. The ratio of the number of fuel assemblies 19 and the number of fuel assemblies 20 in the core inner region 31 is larger than the ratio of the number of fuel assemblies 19 and the number of fuel assemblies 20 in the core peripheral region 30. Here, the ratio between the numbers of the fuel assemblies 19 and the fuel assemblies 20 is a value obtained by dividing the number of the fuel assemblies 19 by the number of the fuel assemblies 20.

As a result of the above arrangement, the core according to the present embodiment has an initial enrichment of about 1.3% Δk despite the fact that the average enrichment of the initially loaded fuel is set to 3.3%, which is higher than the conventional one. Shows a substantially constant excess reactivity within the range of
The above-mentioned sufficient furnace stop margin can be secured.

Hereinafter, the operation of the present embodiment will be described with reference to the two types of high enrichment fuel assemblies 19 and 20 indicated by symbols A and B in the core shown in FIG.
This will be described in comparison with the case where the fuel assembly 20 is arranged or the case where all the fuel assemblies 20 are arranged. In the core shown in FIG. 1 in which the fuel assemblies 20 are entirely replaced by the fuel assemblies 19, the reactor stop margin is slightly improved as compared with the present embodiment, but the output in the core peripheral region is reduced, so that the diameter is reduced. Directional peaking increases. That is, the distortion of the power distribution in the radial direction increases. In the core shown in FIG.
Is replaced by the fuel assembly 20, the reactor shutdown margin is deteriorated as compared with the present embodiment, and the power in the core inner region is increased, so that the radial peaking is increased. Therefore, by disposing the two types of high enrichment fuels 19 and 20 at appropriate positions as in the present embodiment, it is possible to flatten the distortion of the radial power distribution and secure a sufficient reactor shutdown margin. it can.

Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. The same components as those in the first embodiment are denoted by the same reference numerals, and detailed description is omitted.
In the operating method of the reactor core of the nuclear reactor according to the second embodiment, since the fuel arrangement pattern is different between the first cycle and the second cycle of the combustion, the fuel arrangement is changed when shifting from the first cycle to the second cycle. Is partially changed. The core of the nuclear reactor in the first cycle of the present embodiment is the same as the core shown in FIG. 1 in the first embodiment. FIG. 5 shows the core in the second cycle. FIG. 5A is a fuel arrangement diagram of a 1/4 core of the nuclear reactor in the second cycle of the present embodiment, and FIG. 5B is a diagram showing the arrangement around the core periphery region 30 and the arrangement shown in FIG. In the core inner region 31,
Highly enriched fuel assemblies 19 and 2 designated A and B
It is a table showing the body number of 0.

When the first cycle is completed and the process proceeds to the second cycle, the replacement of the initially loaded fuel assembly with the replacement fuel assembly is not performed, but only the arrangement of the initially loaded fuel assembly is changed. First, in the first cycle, the low-enrichment fuel assemblies 21 loaded in the control cells 14 in the core inner region 31 are replaced with low-enrichment fuel assemblies 21 loaded in other than the control cells. In the second cycle, reference numeral 14 indicates that the excess reactivity increases in comparison with the first cycle.
Four control cells are newly arranged at the location indicated by a, to make a total of 25 control cells.

For the highly enriched fuel assembly, the first
The fuel assemblies 20 loaded in the core peripheral region 30 in the cycle and the fuel assemblies 19 loaded in the core inner region 31 in the first cycle are exchanged. As a result, the fuel assembly 20 is arranged in the outermost periphery of the core as a highly enriched fuel assembly in the first cycle, but the fuel assembly 19 is arranged in the second cycle. In addition, the replacement of the fuel assembly allows the core peripheral region 30 and the core inner region 3
The number of the fuel assemblies 19 and 20 in 1 changes as shown in the table of FIG. 1B from FIG. That is, in the first cycle, the core inner region 3
The ratio of the number of the fuel assemblies 19 to the fuel assemblies 20 at 1 is larger than the ratio of the number of the fuel assemblies 19 to the fuel assemblies 20 in the core peripheral region 30. The relationship is reversed.

The operation of this embodiment will be described below.
In the first cycle, the same operation as in the first embodiment is obtained. In the first cycle, since the power in the core peripheral region 30 is low, the fuel assemblies 2 disposed in the core peripheral region 30
0 indicates that combustion does not progress relatively as compared with the core inner region 31.
As described above, the life of the gadolinia in the fuel assembly 20 is extended by the action of the fuel rod arrangement. Therefore,
The gadolinia in the fuel assembly 20 still remains sufficiently at the end of the first cycle. Therefore, in the second cycle, by arranging a large number of the fuel assemblies 20 in the core inner region 31, the surplus reactivity is sufficiently reduced mainly by the action of gadolinia located in the core inner region 31 so as to fall within an appropriate range. Can be controlled.

In FIG. 4 showing the transition of the infinite multiplication factor,
Symbols f1 and f2 are the first in the present embodiment, respectively.
It represents the combustion period of the cycle and the second cycle. As can be seen from FIG. 4, the infinite multiplication factor a of the fuel assembly 19 disposed in the core inner region 30 is smaller than the infinite multiplication factor b of the fuel assembly 20 in the first cycle, but the combustion proceeds and the second cycle Reverses to fuel assembly 2
It becomes larger than the infinite multiplication factor b of 0. Therefore, in the second cycle, the fuel assemblies 19 having a large infinite multiplication factor are arranged in the core peripheral region 30 and the fuel assemblies 20 that have much gadolinia remaining are arranged in the core inner region 31 so that the core assembly region has a large infinite multiplication factor. Distortion of the radial output distribution can be reduced.

Hereinafter, a third embodiment of the present invention will be described with reference to the drawings. Note that the same components as those in the first or second embodiment are denoted by the same reference numerals, and detailed description is omitted. FIG. 6A is a fuel arrangement configuration diagram of a 1/4 core of a nuclear reactor according to the third embodiment. In the core of the nuclear reactor according to the third embodiment, initially loaded fuel having three types of enrichment, low enrichment, medium enrichment, and high enrichment, is used. It should be noted that the fuel indicated by the symbols A, B, and L in the figure is the same as the fuel described in detail in the first embodiment, and the symbol M is the medium enriched fuel 24.
Represents

In this core, 296 low-enrichment fuel assemblies 21 having an average enrichment of 1.6%, indicated by symbol L,
Medium enrichment fuel assembly 24 having an average enrichment of 2.5%
Are loaded with 492 high-enrichment fuel assemblies 19 and 20 having an average enrichment of 3.7% indicated by symbols A and B in total, and the total average enrichment of the initially loaded fuel assembly is: 2.9%.

The low-enrichment fuel assembly 21 includes the outermost periphery of the core and the control cells 1 disposed discretely inside the core.
4 as well as a part of cells in the core inner region 31 near the outermost periphery of the core. The medium enrichment fuel assemblies 24 are loaded in cells including the low enrichment fuel assemblies 21 other than the control cells 14 in the core inner region 31.

FIG. 6 (b) shows the core peripheral region 30 and the core inner region 31 in the arrangement shown in FIG. 6 (a).
Highly enriched fuel assemblies 19 and 2 indicated by symbols A and B
It is a table showing the body number of 0. As shown here, the ratio of the numbers of the fuel assemblies 19 and the fuel assemblies 20 in the core inner region 31 is larger than the ratio of the numbers of the fuel assemblies 19 and the fuel assemblies 20 in the core peripheral region 30. The ratio of the number of fuel assemblies 19 to the number of fuel assemblies 20 loaded in the cells facing the control cells 14 or the cells including at least one low-enrichment fuel assembly 21 in the core inner region 31 is determined by the core number. It is smaller than the ratio of the number of the fuel assemblies 19 and the number of the fuel assemblies 20 loaded in the cells not facing the control cells 14 other than the control cells 14 in the inner region 31 and not including the low-enrichment fuel assemblies 21.

As shown in FIG. 6A, the core peripheral region 3
All the high enrichment fuel assemblies loaded at 0 are fuel assembly 2
0. In the core inner region 31, the fuel assemblies 20
Is loaded in a cell facing the control cell 14 or a cell near the core peripheral region 31 and including the low-enrichment fuel assembly 21. Since the control rod value is relatively small at such a loading position, the reactor shutdown margin is deteriorated even when the fuel assembly 20 having a large difference in reactivity between the reactor operation and the reactor shutdown is loaded. There is no. On the other hand, many fuel assemblies 19 having a smaller reactivity difference between the operation and the shutdown of the reactor than the fuel assemblies 20 are loaded at positions in the core inner region 31 where the control rod value is large. As a result, the life of the gadolinia can be prolonged while securing a furnace stop margin.

In this embodiment, the neutron leakage outside the reactor core is reduced by loading the low enrichment fuel assembly 21 on the outermost periphery of the reactor core, and the neutron utilization efficiency is improved. This arrangement may result in extremely low output at the outermost periphery of the core, but as a countermeasure, the second and third layers from the outermost periphery of the core are loaded with highly enriched fuel assemblies, and the radial output of the core is reduced. The distribution distortion is flattened. In addition, since most of the high enrichment fuel assemblies arranged in the second and third layers are the fuel assemblies 20 having a large infinite multiplication factor at the initial stage of combustion, the effect of reducing the distortion in the radial power distribution is further increased.

In the present embodiment, the average enrichment of the whole initially loaded fuel assembly is 2.9%, which is 3.3 in the second embodiment.
It is set lower than%. Therefore, after the combustion of the first cycle of the initially loaded fuel assembly is completed, 56 of the low enrichment fuel assemblies 21 that have undergone the most combustion are selected, and the replacement fuel assembly shown in FIG. In exchange for body 22,
Run the cycle. In the first cycle, the fuel assemblies 20 loaded in the core peripheral region 30 excluding the outermost periphery of the core
Is replaced with the fuel assembly 19 loaded in the core inner region 31. At this time, in the second cycle, the low enrichment fuel assembly 21 is arranged at the outermost periphery of the core, the fuel assembly 19 is arranged at the core peripheral region 30 excluding the outermost periphery of the core, Most of the enrichment fuel assembly will be occupied by the fuel assembly 20. Therefore, in the second cycle, the distortion of the power distribution in the radial direction of the core can be flattened, and the excess reactivity can be reduced by the action of the fuel assembly 20 in the core inner region 31.

As a modified example of this embodiment, there is an initially loaded core in which a medium enriched fuel 24 is loaded instead of the low enriched fuel 21 on the outermost periphery of the core. In this case, the neutron leakage from the core increases somewhat as compared with the third embodiment,
The effect of reducing the distortion of the radial power distribution is increased by the action of the medium enrichment fuel assembly 24.

Alternatively, as another modified example of the present embodiment, there is an initially loaded core composed of initially loaded fuel assemblies of four types of enrichment shown in FIG. In this case, in the third embodiment, the loading position of the low enrichment fuel assembly 21 in the core inner region 31 is located at the lowest enrichment fuel assembly or the second lowest enrichment fuel assembly of the four types. , The furnace stop margin can be improved. The fuel assemblies loaded on the outermost periphery of the core in the first cycle are not limited to the lowest enrichment fuel assemblies, but the second lowest enrichment fuel assemblies and the third lowest enrichment fuel assemblies Alternatively, it is conceivable to load a fuel assembly with the highest enrichment. Alternatively, an arrangement in which these four types of fuels are appropriately combined is also possible.

Hereinafter, a fourth embodiment of the present invention will be described with reference to the drawings. The same components as those in the first to third embodiments are denoted by the same reference numerals, and detailed description is omitted. The core of the nuclear reactor according to the present embodiment is similar to the high enrichment fuel assemblies 19 and 20 indicated by the symbols A and B in the first to third embodiments, respectively. It has been replaced by bodies 25 and 26. FIG. 7 is an arrangement configuration diagram of the highly enriched fuel assembly of the reactor core of the nuclear reactor according to the present embodiment. Here, FIG. 7 (a) relates to the fuel assembly 25, and FIG. 7 (b) relates to the fuel assembly 26. Symbol G represents a fuel rod containing gadolinia.

The high enrichment fuel assemblies 19 and 20 shown in FIG. 3 each have two gadolinia-containing fuel rods 23 in contact with the four gadolinia-containing fuel rods in the X and Y directions. That is, two sets of five gadolinia-containing fuel rods were arranged in a cross shape. On the other hand, in the present embodiment, in the fuel assemblies 25 and 26, the cross-shaped arrangement of the gadolinia-containing fuel rods is eliminated, and the gadolinia-containing fuel rods are adjacent to each other in at most two gadolinia-containing fuel rods.
It is arranged to be a book. Further, the ratio of the fuel rods arranged at the position facing the water rod 8 among the fuel rods containing gadolinia is 6/11 for the fuel assembly 25 and 0/10 for the fuel assembly 26. Have a higher ratio. Two gadolinia-containing fuel rods G, which are denoted by reference numeral 18 in the drawing, are present in the fuel assembly 25.
It faces two water rods 8.

In this embodiment, any one of the core arrangements of the first to third embodiments is adopted except that the fuel assemblies 19 and 20 are replaced with fuel assemblies 25 and 26, respectively.

Although the life of the gadolinia is shortened by changing the arrangement of the fuel rods in the high enrichment fuel assembly, the power distribution in the fuel assembly is flattened by uniformly disposing the gadolinia-containing fuel rods in the fuel assembly. As a result, the thermal margin is improved.

It is known that, in a fuel assembly, the position where the neutron absorption of gadolinia is generally maximum does not coincide with the position facing the water rod. Therefore,
Since the fuel rods containing gadolinia are concentrated at the position facing the water rod in the fuel assembly 25, the infinite multiplication factor in the initial stage of combustion may be larger than that of the fuel assembly 26 in some cases. However, in this embodiment, ten fuel rods containing gadolinia are arranged in the fuel assembly 26, whereas eleven fuel rods containing gadolinia are arranged in the fuel assembly 25. With this configuration,
The infinite multiplication factor in the initial stage of combustion of the fuel assembly 25 is suppressed lower than that of the fuel assembly 26.

As described above, in order to maintain the integrity of the fuel, the number of gadolinia-containing fuel rods disposed in the fuel may be reduced and the gadolinia concentration may be reduced. If the upper limit of the gadolinia concentration needs to be suppressed to 10% or less, the gadolinia concentration of all the gadolinia-containing fuel rods used in the fuel assemblies 25 and 26 is set to 10%, so that the surplus reaction in the second cycle is performed. The degree can be suppressed to the maximum. Further, depending on the setting conditions, it is conceivable to use 10% concentration gadolinia-containing fuel rods for the fuel assembly 25 and 11% concentration gadolinia-containing fuel rods for the fuel assembly 26. In this case, the effect of suppressing the excess reactivity in the second cycle is further enhanced by arranging many fuel assemblies 26 inside the core in the second cycle.

[0081]

As described above, according to the present invention, in the core of a nuclear reactor in which the average enrichment of the whole initially loaded fuel assembly is greatly increased, the range allowed from the viewpoint of maintaining the fuel integrity. Within this time, the concentration of burnable poisons is set higher than before, and from the beginning to the end of combustion, the excess reactivity can be controlled to an appropriate range, while at the same time ensuring a sufficient shutdown margin for the furnace. By increasing the take-up burn-up of the fuel assembly, the fuel economy can be greatly improved compared to the prior art. Further, according to the present invention, since the distortion of the radial power distribution can be flattened for a long period from the initial stage of combustion, the maximum linear power density can be controlled to be low and the pellet-
Preventing cladding tube interaction and controlling the minimum critical power ratio in an appropriate range can improve the thermal margin of the core.

[Brief description of the drawings]

FIG. 1 (a) is a fuel arrangement diagram of a quarter core of a nuclear reactor according to a first embodiment of the present invention, and FIG. 1 (b) is a core peripheral region and the inside of the core in the arrangement shown in FIG. It is a table | surface which shows the number of high-enrichment fuel assemblies 19 and 20 in an area | region.

2 (a) is a configuration diagram of the axial enrichment and gadolinia distribution of the high enrichment fuel assemblies 19 and 20, and FIG. 2 (b) is an axial enrichment and gadolinia distribution of the low enrichment fuel assembly 21. And (c) is a configuration diagram of the enrichment and gadolinia distribution of the replacement fuel assembly 24 in the axial direction.

3A is a sectional view showing a fuel rod arrangement of a high enrichment fuel assembly 19, and FIG. 3B is a sectional view showing a fuel rod arrangement of a fuel assembly of a high enrichment fuel assembly 20.

FIG. 4 is a graph showing the transition of the infinite multiplication factor of the highly enriched fuel assemblies 19 and 20 in the core of the nuclear reactor according to the first to third embodiments of the present invention.

FIG. 5 (a) shows a second embodiment according to the second embodiment of the present invention.
Fuel arrangement configuration diagram of the 1/4 core of the nuclear reactor related to the cycle,
(B) is a table showing the number of high enrichment fuel assemblies 19 and 20 in the core peripheral region and the core inner region in the arrangement shown in (a).

FIG. 6 (a) is a fuel arrangement diagram of a 1/4 core of a nuclear reactor according to a third embodiment of the present invention, and (b) is a core peripheral region and the inside of the core in the arrangement shown in (a). It is a table | surface which shows the number of high-enrichment fuel assemblies 19 and 20 in an area | region.

FIG. 7A is a cross-sectional view showing a fuel rod arrangement of a highly enriched fuel assembly 25 in a reactor core according to a fourth embodiment of the present invention, and FIG. 7B is a fourth embodiment of the present invention. It is sectional drawing which shows the fuel rod arrangement | positioning of the high enrichment fuel assembly 26 of the core of the nuclear reactor concerning a form.

FIGS. 8A and 8B are fuel rod arrangements of two types of fuel assemblies having different ratios of fuel rods arranged at positions facing water rods among fuel rods containing burnable poisons. FIG.

9 is a graph showing a transition of an infinite multiplication factor of the fuel assembly shown in FIG.

FIG. 10 is a configuration diagram of a 1/4 core showing an example of a distribution of control rod values at the time of reactor shutdown in a reactor core consisting only of fuel assemblies of the same specification.

FIG. 11 is a configuration diagram of a 1/4 reactor core showing an example of a distribution of control rod values when the reactor is stopped in a reactor core having 21 control cells.

FIG. 12 is a configuration diagram of a 1/4 core showing an example of a fuel arrangement of a conventional boiling water reactor constituted by four types of initially loaded fuel assemblies having different enrichments.

13A is a longitudinal sectional view of a high burn-up fuel assembly of a boiling water reactor, FIG. 13B is a sectional view taken along line bb of FIG. 13A, and FIG. 13C is a sectional view of FIG. It is sectional drawing in the cc arrow direction.

[Explanation of symbols]

 Reference Signs List 6 long fuel rod 7 short fuel rod 8 water rod 9 spacer 10 upper tie plate 11 lower tie plate 12 channel box 13 control rod 14 control cell 19, 20, 25, 26 high enrichment fuel assembly 21 low enrichment fuel assembly Body 22 Replacement fuel assembly 24 Medium enrichment fuel assembly 30 Peripheral core area 31 Inner core area

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location G21C 3/30 GDBP GDBY

Claims (18)

[Claims] 1. A grid comprising a first group of fuel rods containing a nuclear fuel substance and containing no burnable poison, a second group of fuel rods containing a nuclear fuel substance and a burnable poison, and a water rod through which cooling water flows. In a reactor core including a plurality of fuel assemblies configured in a bundle, the plurality of fuel assemblies include:
A nuclear reactor core comprising at least two types of fuel assemblies each having a different ratio of the fuel rods arranged at a position facing the water rod among the second group of fuel rods. 2. The fuel assembly having at least two types of fuel assemblies having different average enrichments, and the fuel assembly having the highest enrichment among the fuel assemblies includes a first fuel assembly and the second fuel assembly. 2. The fuel rod of the group includes a second fuel assembly in which a proportion of the fuel rod arranged at a position facing the water rod is smaller than the first fuel assembly. 3. The reactor core of the described reactor. 3. The nuclear reactor of claim 2, wherein at least one of the second group of fuel rods faces the water rod in two directions with respect to the first fuel assembly. Core. 4. In the second fuel assembly, at least one of the second group of fuel rods faces another second group of fuel rods in at least one of the X direction and the Y direction. 4. The reactor core of a nuclear reactor according to claim 2, wherein: 5. The reactor core according to claim 2, wherein the first fuel assembly and the second fuel assembly have the same number of fuel rods of the second group. 6. The reactor core according to claim 5, wherein the first fuel assembly and the second fuel assembly have the same type and number of fuel rods constituting the fuel assembly. 7. A plurality of cells each comprising one control rod and four fuel assemblies surrounding the control rod are arranged in the core, and the core is located at the outermost periphery of the core. When divided into a core peripheral region consisting of an assembly and a fuel assembly constituting a cell including at least one fuel assembly located at the outermost periphery of the core, and a core central region not belonging to this core peripheral region, The ratio of the number of the first fuel assemblies to the number of the second fuel assemblies in the core inner region is larger than the ratio of the number of the first fuel assemblies to the number of the second fuel assemblies in the core peripheral region. 7. The method according to claim 2, wherein:
The reactor core of the described reactor. 8. The reactor core according to claim 7, wherein the number of the second fuel assemblies is larger than the number of the first fuel assemblies in the core peripheral region. 9. The reactor core according to claim 7, wherein the number of the first fuel assemblies is larger than the number of the second fuel assemblies in the core inner region. 10. The nuclear reactor core according to claim 7, wherein the fuel assembly having the highest enrichment loaded in the core peripheral region is a second fuel assembly. 11. A part of a large number of cells arranged in a core is a control cell composed of four fuel assemblies having low reactivity, and a cell facing the control cell or a minimum enrichment in the core inner region. A first fuel assembly loaded in a cell containing at least one fuel assembly and a second fuel assembly
The first fuel assembly loaded in a cell that does not face the control cell other than the control cell in the core inner region and does not include the fuel assembly with the lowest enrichment, wherein the ratio of the number of fuel assemblies The reactor core according to claim 7 or 8, wherein the ratio is smaller than the ratio of the number of the bodies to the number of the second fuel assemblies. 12. The reactor according to claim 1, wherein in the core inner region, the second fuel assembly is loaded only to a cell facing the control cell or a cell containing at least one fuel assembly having the lowest enrichment. 12. The reactor core of claim 11. 13. The reactor core according to claim 2, wherein the first fuel assembly and the second fuel assembly in the core inner region are provided.
In the first cycle, the ratio of the numbers of the fuel assemblies is set to be larger than the ratio of the numbers of the first fuel assemblies and the second fuel assemblies in the core peripheral region, and in the second cycle, A method of operating a reactor core of a nuclear reactor, wherein the ratio is set to be smaller than the ratio of the number of the first fuel assemblies and the number of the second fuel assemblies in a core peripheral region. 14. The volume of the first fuel assembly in the core peripheral region is smaller than the volume of the second fuel assembly in the first cycle and smaller than the volume of the second fuel assembly in the second cycle. The method for operating a reactor core of a nuclear reactor according to claim 13, wherein the number is large. 15. The volume of the first fuel assembly in the core interior region is greater than the volume of the second fuel assembly in the first cycle and greater than the volume of the second fuel assembly in the second cycle. The method for operating a nuclear reactor core according to claim 13 or 14, wherein the number is small. 16. A fuel assembly having the highest enrichment on the outermost periphery of the core, wherein a second fuel assembly is loaded in a first cycle, and a first fuel assembly is loaded in a second cycle. The method for operating a reactor core of a nuclear reactor according to claim 13 or 15, wherein: 17. When shifting from the first cycle to the second cycle, the low-enrichment fuel assembly loaded in the control cell including four low-enrichment fuel assemblies in the core inner region is transferred to the control cell. 17. The method for operating a reactor core of a nuclear reactor according to claim 13, wherein the fuel cell is replaced with a low enrichment fuel assembly loaded other than the above. 18. A process for replacing a second fuel assembly loaded in a core peripheral region with a first fuel assembly loaded in a core inner region when shifting from the first cycle to the second cycle. The method for operating a reactor core of a nuclear reactor according to any one of claims 13 to 17, wherein: