JPH08285977A - Fuel assembly for boiling water type reactor - Google Patents

Abstract

PURPOSE: To obtain a fuel assembly with a sufficient thermal margin by a constitution arranging a plurality of short fuel rods containing a combustible toxic substance at a plurality of corner positions at a second layer from the outermost periphery. CONSTITUTION: Concentration is in the order of a>-e>. The average concentration of a fuel assembly 1 is 4.43wt.%. Short fuel rods with a number 4 and a sign A are arranged at four locations of the corner of a group of fuel rods from the outermost periphery of the assembly 1 to a second layer and two out of them are short fuel rods with a sign A containing combustible toxic gadolinia. With this configuration, the local output around the fuel assembly adjacent to the short fuel rod with the sign A can be reduced due to the thermal neutron absorption effect of the combustible toxic substance and the concentration of four outermost periphery fuel rods adjacent to the short fuel rod with the sigh A can be insreased to a miximum concentratiom (a). Further, the number of combastible toxic substance at a region which is above the upper edge of the short fuel rod with a sign A can be reduced by two as compared with that of a lower region and the difference between upper and lower reaction degrees in axial direction at the sinitial stage of combustion can be adjusted, thus suppressing the output peak of the lower region at the first half of operation cycle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used in a boiling water reactor, and more particularly to a fuel assembly for a boiling water reactor suitable for high burnup and a long operating cycle.

[0002]

2. Description of the Related Art In recent years, a high burnup of fuel has been promoted in order to improve the economical efficiency of nuclear power generation. An example of such a high burnup fuel assembly is shown in FIG. FIG.
(A) is an elevational view showing a partial vertical cross section of the fuel assembly,
(B) is a sectional view taken along the line BB of (a), and (c) is a sectional view taken along the line C-C of (a). As shown in FIG. 15, the fuel assembly 1 includes a long fuel rod 2 filled with a nuclear fuel material such as enriched uranium, a short fuel rod 3 having an effective portion shorter than that of the long fuel rod 2, and a coolant inside. The large diameter water rods 6 that flow through are bundled by a spacer 8 in a square lattice shape and fixed to the upper tie plate 4 and the lower tie plate 5 to form a fuel rod bundle. The fuel rod bundle is surrounded by a channel box 7. It is configured. Further, an external spring 9 is interposed between the long fuel rod 2 and the upper type plate 4 to fix the fuel rod.

A fuel assembly for high burnup is disclosed in, for example, Japanese Unexamined Patent Publication No.
Compared with the conventional low burnup fuel assembly disclosed in Japanese Patent No. 296192, it has the following features.
That is, in order to achieve a high burnup, the fuel is enriched, so that (1) the axial power peaking due to the void distribution is further increased, and (2) various types of residence time in the furnace are different. Since the fuel is mixed in the core, the radial power peaking is also increased.

Therefore, the thermal margin such as the maximum line power density and the minimum limit power ratio is reduced. In order to improve these, in the fuel assembly 1 of FIG. 15, the number of fuel rods is increased by changing the fuel rod array from 8 rows 8 columns of the conventional fuel to 9 rows 9 columns. As a result, the maximum linear power density per fuel rod can be reduced.

However, as the number of fuel rods increases, the flow path between the fuel rods becomes narrower and the pressure loss of the coolant increases. In particular,
Since the coolant is a two-phase flow, the pressure loss above the fuel is large,
The stability of the reactor may be impaired. Therefore, FIG.
In the fuel assembly 1, the short fuel rods 3 in which some of the fuel rods are shorter than the long fuel rods 2 are used to cancel the increase in pressure loss due to the increase in the number of fuel rods. The length of the short fuel rod 3 is about 2/3 of that of the long fuel rod 2.

Further, when the output becomes excessive, the heat transfer from the fuel rods to the coolant makes a boiling transition from the efficient nucleate boiling to the inefficient film boiling, but the output of the fuel assembly at this time is limited. Is the output. Since this boiling transition is likely to occur in the upper part of the fuel rod, the position of the short length fuel rod 3 is selected to be a place where the cooling efficiency of the fuel rod is poor, thereby increasing the minimum limit output ratio.

In the fuel assembly 1 of FIG. 15, 2 from the outermost circumference.
A total of 8 short fuel rods 3 at four corners of the fuel rods located in the second layer and at corners and midpoints (4 places) of the corners.
The book is arranged. The fuel rods at these positions are surrounded by eight fuel rods, which are all heat sources, around themselves, so the fuel rods are considered to have poorer cooling efficiency than the fuel rods adjacent to the outermost fuel rods or water rods. Has become. Particularly, in the conventional fuel assembly having the short fuel rods 3,
In many cases, short fuel rods 3 are arranged at four corner positions of the second layer from the outermost periphery.

The short fuel rod 3 further has the effect of improving the reactor shutdown margin. When the nuclear reactor is shut down, the neutron flux forms a peak at a position 1/4 to 1/3 below the upper end of the core. When the reactor is shut down, the coolant acts as a neutron absorber because it has a low temperature and a high density. Therefore, it is possible to improve the reactor shutdown margin by reducing the number of fuel rods and increasing the amount of coolant in the axially upper part.

As described above, at least four of the short fuel rods 3 are arranged at the corner positions of the fuel rod group which is composed of the long fuel rods 2 and the short fuel rods 3 and is located in the second layer from the outermost periphery. 16 shows an example of the enrichment distribution of the burned fuel assembly.
Shown in In the figure, the same parts as those in FIG. 15 are designated by the same reference numerals, and overlapping description will be omitted.

FIG. 16 (a) is a cross-sectional view of the fuel assembly, and FIG. 16 (b) shows the axial length and enrichment of the fuel assembly of FIG. 16 (a). In FIG. 16A, the circles indicating the fuel rods, 1-4, G1 to G4, and no symbol indicate the types of fuel rods, and the enrichment and the effective length of the fuel rods corresponding to FIG. 16B are shown. .

[0011]

However, the above-described fuel assembly has the following problems in further increasing the burnup. In general, in a fuel assembly for a boiling water reactor, non-boiling water (water gap region) flows between channel boxes, and the neutron moderating effect of water causes the fuel assembly outer peripheral portion, especially fuel. The thermal neutron flux near the corner of the assembly is high, and the local power at that part tends to be high. Therefore, in the conventional fuel, in order to improve the thermal characteristics, the concentration of the fuel rods in the outer peripheral portion of the fuel assembly is reduced to flatten the local output distribution in the fuel assembly.

However, when it is necessary to increase the average enrichment of the fuel assembly to a level higher than that of the conventional fuel, such as the fuel for increasing burnup and the fuel for long-term operation cycle, the maximum enrichment of pellets is 4.9 wt%. Since there is a limit, it is unavoidable to increase the concentration of the fuel rods in the outer peripheral portion, and the thermal characteristics are deteriorated accordingly. The maximum pellet concentration is 4.9 wt.
The limit of% is determined from the limit of criticality safety of fuel manufacturing facilities, and cannot be easily changed.

On the other hand, in the fuel assembly having the short fuel rods 3 as described above, the number of fuel rods is different between the upper region and the lower region of the effective portion of the short fuel rods 3,
The reactivity characteristics at the time of operating the fuel assembly are different. That is, neutrons are more likely to be decelerated in the upper part where the moderator-to-fuel volume ratio is larger, so that the infinite multiplication factor in the upper part is larger than that in the lower part.

On the contrary, in the early stage of combustion, on the contrary, the reactivity effect of the burnable poison is higher in the upper part where the number of thermal neutrons is higher. Therefore, when the number of the burnable poison in the upper and lower parts of the fuel assembly is the same, The infinite multiplication factor of is smaller than that of the lower part.
In a boiling water reactor, the output peak is likely to occur in the lower part of the reactor due to the axial void distribution during power operation.
This has the effect of further emphasizing the lower output peak in the fresh fuel or in the early stage of the operation cycle.

An object of the present invention is to provide a fuel assembly having a short fuel rod at the corner position of the second layer from the outer peripheral portion of the fuel assembly when the fuel assembly is used for high burnup or for a long operating cycle. Both of the two problems, namely, it is difficult to suppress the increase in the local power near the corners of the fuel assembly due to the increase in enrichment, and the increase in the lower power peak in the fresh fuel or in the early stage of the operation cycle. It is to solve and provide a fuel assembly for a boiling water reactor having a sufficient thermal margin.

[0016]

In order to achieve the above-mentioned object, in the invention of claim 1, a long fuel rod and a short fuel rod having a fuel effective portion shorter than that of the long fuel rod are arranged in a grid pattern. Enclose the fuel bundle that is bundled in
In a fuel assembly for a boiling water reactor in which short fuel rods are arranged at all corner positions of the fuel rod group located in the second layer from the outermost periphery of this fuel bundle, the fuel rods located in the second layer from the outermost periphery It is characterized in that a plurality of short fuel rods containing a burnable poison is arranged at a plurality of corner positions in the group.

Further, in the invention of claim 2, the region above the upper end of the fuel effective portion of the short fuel rod is the upper portion, the region corresponding to this fuel effective portion is the lower portion, and the blanket region of the upper and lower ends in the axial direction of the fuel rod. It is characterized in that it is provided with a plurality of long fuel rods which are divided into upper and lower regions except for the above and which contain more burnable poison in the upper part than in the lower part.

Further, in the invention of claim 3, among a plurality of long fuel rods containing a large amount of burnable poison in the upper portion rather than the lower portion, a plurality of long fuel rods contain the burnable poison only in the upper portion. And

Further, in the invention of claim 4, among a plurality of long fuel rods containing more burnable poison in the upper part than in the lower part, a plurality of the long fuel rods further divide the lower part into a central part and a lower part, It is characterized in that it contains a burnable poison except for the central part and contains more in the upper part than in the lower part.

Further, four short fuel rods containing a burnable poison are arranged at four corner positions of the fuel rod group located in the second layer from the outermost periphery, and the burnable poison is placed from the bottom. 1 to 4 long fuel rods containing much in the upper part
It is characterized in that the book is arranged.

In the invention of claim 6, A fuel bundle formed by bundling a long fuel rod and a short fuel rod having an effective portion shorter than the long fuel rod in a lattice shape is surrounded by a channel box, and a non-interval between the outer walls of the channel boxes of adjacent fuel assemblies is surrounded. The width of the boiling water region is made larger on the control rod insertion side than on the non-insertion side, and the short fuel rods are arranged at all corner positions of the fuel rod group located in the second layer from the outermost periphery. In the fuel assembly, a plurality of short fuel rods containing a burnable poison are arranged at a plurality of corner positions in the fuel rod group located in the second layer from the outermost periphery, excluding the corner positions on the non-control rod side. It is characterized by having done.

Further, in the invention of claim 7, the region above the upper end of the fuel effective portion of the short fuel rod is the upper portion, the region corresponding to this fuel effective portion is the lower portion, and the blanket regions of the upper and lower ends in the axial direction of the fuel rod are. Except for the two areas above and below,
It is characterized by having a plurality of long fuel rods containing more burnable poison in the upper portion than in the lower portion.

Further, in the invention of claim 8, among a plurality of long fuel rods containing more burnable poison in the upper portion than in the lower portion, a plurality of rods contain the burnable poison only in the upper portion. To do.

Further, in the ninth aspect of the present invention, among a plurality of long fuel rods containing more burnable poison in the upper portion than in the lower portion, a plurality of the lower fuel rods further divide the lower portion into a central portion and a lower portion,
It is characterized in that it contains a burnable poison except for the central part and contains more in the upper part than in the lower part.

Further, in the invention of claim 10, in the fuel rod group located in the second layer from the outermost periphery, three burnable poisons are contained at three corner positions except the corner position on the non-control rod side. The short fuel rods are arranged, and one to three long fuel rods containing more burnable poisons in the upper portion than in the lower portion are arranged.

In the eleventh aspect of the invention, a plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion are arranged at positions adjacent to the water rod.

According to the twelfth aspect of the invention, a plurality of long fuel rods containing more burnable poison in the upper portion than in the lower portion are not arranged adjacent to the water rod, and are arranged at positions other than the outermost periphery. Is characterized by.

Further, in the thirteenth aspect of the present invention, the concentration of fuel in a plurality of long fuel rods containing more burnable poison in the upper portion than in the lower portion has fuel enrichment in most regions except the blanket regions at the upper and lower ends. Characterized by the highest value used in the body.

[0029]

According to the fuel assembly of the present invention, a plurality of short fuel rods containing a burnable poison are arranged at corner positions of a second layer from the outer periphery of the fuel assembly. By doing so, the local neutron output around the corner of the fuel assembly adjacent to the short fuel rod can be reduced by the thermal neutron absorption effect of the burnable poison, and the fuel rod enrichment around the corner can be increased accordingly. Furthermore, the number of burnable poisons in the area above the upper end of the short fuel rod can be reduced by more than the number in the area below, so if the difference in the number of burnable poisons above and below is set appropriately, the axial direction at the initial stage of combustion The difference in vertical reactivity can be adjusted, and the output peak in the lower region in the first half of the operation cycle can be suppressed.

The invention according to claim 1 can be applied to various types of fuel grids (C, D, S, N grids).
Here, C, D, S, N lattices represent the types of boiling water reactor fuel assembly lattices, and the size of the channel box and the width of the water gap region between the channel boxes differ depending on the fuel lattice.

In the invention described in claim 1, since the short fuel rods containing the burnable poison are arranged,
It was possible to reduce the local output around the corner of the fuel assembly adjacent to the short fuel rod and to increase the enrichment of the fuel rod around the corner. However, since the difference in the number of burnable poisons in the axial direction is the same as the number of short fuel rods containing burnable poisons, in order to control the axial power distribution,
The number of short fuel rods will be limited.

Therefore, it is possible to increase the number of short fuel rods, further reduce the local output around the corners, and increase the enrichment of the fuel rods around the corners, without further deteriorating the axial power distribution. The invention as defined in claims 2 to 6 is configured as described above.

That is, according to the second aspect of the invention, the long fuel rod is divided into upper and lower regions, and the burnable poison is contained more in the upper portion than in the lower portion. With this configuration, the output peak of the lower portion at the beginning of the cycle can be further suppressed by the effect of the burnable poison of the lower portion. In addition, as described in JP-A-6-118188, the concentration of the burnable poison added to the lower portion must be the minimum concentration required to suppress the output peak of the lower portion at the beginning of the cycle. If the concentration is higher than necessary, combustion in the lower part in the middle of the cycle will be suppressed, and conversely a lower output peak will occur in the latter half of the cycle.
Therefore, as in the configuration according to claim 2, it is necessary that at least the concentration of the burnable poison in the lower portion is lower than that in the upper portion.

According to the structure of claim 3, by disposing the long fuel rod containing the burnable poison only in the upper portion, the difference in the number of fuel rods containing the burnable poison in the axial direction is further increased. Since it becomes possible to freely adjust, it becomes possible to more appropriately adjust the output distribution in the axial direction according to the take-out burnup of the fuel assembly and the operating period.

According to the structure of claim 4, since the burnable poison is not contained from the central portion of the long fuel rod, the output of the central portion at the beginning of the cycle is increased, and the lower part of the lower portion at the beginning of the cycle is correspondingly increased. The output peak can be suppressed. Further, since the number and concentration of burnable poisons can be adjusted from the upper and lower two regions to the upper, middle and lower three regions, it is possible to more appropriately control the axial power distribution throughout the operation cycle.

According to the structure of claim 5, the number of short fuel rods containing the burnable poison is further limited to four, which is the minimum necessary for reducing the local output near all the corners of the fuel assembly. By doing so, it is possible to reduce the output in the vicinity of all the corners of the fuel assembly and further increase the average enrichment of the fuel assembly.

Further, according to the limitation of the number of short fuel rods to four, the number of long fuel rods containing more combustible poison in the upper portion than in the lower portion is set to one to four, and the burnable poison is contained. The difference between the number of upper and lower fuel rods in the axial direction is limited to the range of 0 to 3, and the number of fuel rods containing the burnable poison in the lower portion is larger than that in the upper portion. This is because, in a boiling water reactor fuel assembly having short fuel rods, the infinite multiplication factor of the upper part is originally smaller than that of the lower part in the initial stage of combustion, and the output peak in the lower axial direction tends to be higher in the first half of the operation cycle. This is because there is a difference between 0 and 3 in order to suppress this.

The structure of claim 6 is intended for a D-lattice type fuel assembly. In the D-lattice fuel, since the width of the non-boiling water region between the outer walls of the channel boxes of the adjacent fuel assemblies is larger on the control rod insertion side than on the non-insertion side, the control rod insertion On the side, the thermal neutron flux distribution is larger and the local output tends to be higher.

That is, a short fuel rod (one rod) containing a burnable poison disposed on the non-control rod side where the width of the non-boiling water region is small and the local output is relatively low is a fuel rod containing no burnable poison. This configuration increases the degree of freedom in the number and position of fuel rods containing burnable poison.

The configuration of claim 7 is claim 2, the configuration of claim 8 is claim 3, the configuration of claim 9 is claim 4, and the configuration of claim 10 is claim 5. It has the same effect.

According to the eleventh aspect, since the long fuel rod containing the burnable poison in the upper portion more than in the lower portion is arranged at the position adjacent to the non-boiling water rod,
The difference in the reactivity value of the burnable poison during operation and during cold temperature becomes smaller. As a result, during the period in which the burnable poison remains unburned, the difference in reactivity during operation during cold temperature becomes small, and the reactor shutdown margin can be increased.

On the contrary, according to the structure of claim 12, the long fuel rod containing the burnable poison in the upper portion more than in the lower portion is not adjacent to the water rod, and is arranged at a position excluding the outermost periphery. The fuel rods containing burnable poisons having poor thermal conductivity can be moved away from the non-boiling water region where the neutron moderating effect is high, and the local output in the latter half of combustion of these fuel rods can be reduced. This ensures the integrity of the fuel rod.

According to the thirteenth aspect, since the enrichment of the long fuel rod containing the burnable poison is the highest value among those used in the fuel assembly, the average enrichment of the fuel assembly. Can be increased.

[0044]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of a fuel assembly for a boiling water reactor according to the present invention, and relates to claim 1. FIG. 1 (a) is a schematic cross-sectional view of a fuel assembly,
(B) shows the enrichment and gadolinia concentration in the axial distribution for the fuel rods in this fuel assembly, the circled numbers or symbols of the upper fuel rods, and the numbers or symbols of the lower fuel rods. Is consistent with. The fuel assembly shown in FIG. 1 has the same structure as the fuel assembly shown in FIG.
The same components are designated by the same reference numerals, and redundant description will be omitted.

In FIG. 1, this fuel assembly is a C-lattice fuel in which fuel rods are arranged in 9 rows and 9 columns, aiming at an average take-out burnup of 55 GWd / t for an operation period of 13 months. The fuel rod bundle is composed of 66 long fuel rods 2, eight short fuel rods 3 and two large diameter water rods.

A long fuel rod has an effective fuel length of 1 from the upper and lower ends.
The natural uranium blanket part is provided at the / 24 part (hereinafter referred to as the upper and lower ends). Except for the upper and lower ends of each long fuel rod, number 1 is fuel enrichment e, number 2 is fuel enrichment d, number 3 is fuel enrichment c, and no number is fuel enrichment a.
Fuel pellets are uniformly packed in the axial direction.

The long fuel rod designated by G1 has a fuel enrichment a except for the upper and lower end portions, which is 8 / of the effective fuel length of the upper portion.
2.5 wt% of gadolinia which is a burnable poison in the part of 24
%, 3.5 wt in the lower part of the active fuel length 14/24
% Contained. Reference symbol G2 contains fuel enrichment b and gadolinia of 2.5 wt%. Number 4 is a short fuel rod that is shorter than the long fuel rod, and is 1 of the effective fuel length of the long fuel rod.
The fuel has an effective fuel length of 4/24, the fuel enrichment is a, the symbol A is a short fuel rod, the fuel enrichment is b, and gadolinia is contained at 2.5 wt%.

Here, the degree of enrichment increases in the order of a>b>c>d> e. Average enrichment of fuel assembly is 4.4
It is 3 wt%. In this fuel assembly, short fuel rods having the number 4 and the symbol A are arranged at four corner positions of the fuel rod group located in the second layer from the outermost periphery.
The book is a short fuel rod with a code A containing a burnable poison (gadolinia).

With this construction, the local neutron output around the fuel assembly adjacent to the short fuel rod indicated by symbol A can be reduced by the thermal neutron absorption effect of the burnable poison, and the four maximum fuel cells adjacent to the short fuel rod indicated by symbol A can be reduced. The enrichment of the peripheral fuel rods can be increased to the maximum enrichment a.

Furthermore, since the number of burnable poisons in the region above the upper end of the short fuel rod designated by symbol A can be reduced by two from that in the region below, it is possible to adjust the difference in the vertical reactivity in the axial direction at the initial stage of combustion, The output peak in the lower region in the first half of the operation cycle can be suppressed.

Although the C-lattice has been described in the first embodiment, other various types of fuel lattices (D,
S, N grid). Where C, D, S,
The N lattice represents the type of lattice of the boiling water reactor fuel assembly, and the size of the channel box and the width of the water gap region between the channel boxes differ depending on the type of the fuel lattice. Particularly, in the case of the D-lattice fuel, the width of the non-boiling water region between the outer walls of the channel boxes of the adjacent fuel assemblies is larger on the control rod insertion side than on the non-insertion side. The thermal neutron flux distribution is larger on the rod insertion side and the local output tends to be higher.

FIG. 2 shows a second embodiment of the fuel assembly for a boiling water reactor according to the present invention, and relates to claim 1. 2 (a) is a schematic cross-sectional view of a fuel assembly, (b).
Shows the concentration and gadolinia concentration in the axial distribution for the fuel rods in this fuel assembly, and the circled number or code of the upper fuel assembly and the number or code of the lower fuel rod are the same. I am doing it. In addition, the fuel assembly of FIG.
It has the same structure as that of the fuel assembly shown in FIG. 12, and the same components are designated by the same reference numerals and duplicate description will be omitted.

In FIG. 2, this fuel assembly is aimed at an average take-out burnup of 55 GWd / t during an operation period of 13 months, and fuel rods are arranged in 10 rows and 10 columns.
The fuel rod bundle is composed of 80 long fuel rods 2, 12 short fuel rods 3 and 2 large diameter water rods.

In the fuel assembly, the long fuel rods are filled with fuel pellets having a fuel enrichment f of number 2 and a fuel enrichment a of no number uniformly in the axial direction except for the upper and lower end portions.
Further, reference numeral G1 is the fuel enrichment level b, and gadolinia is contained at 3.5 wt%. Reference symbol G2 is the fuel enrichment b, 3.0% by weight of gadolinia, which is a combustible poison, in the upper 8/24 of the active fuel length, and 4.0 wt in the lower 14/24 of the active fuel length. % Contained.
Number 1 is a short fuel rod that is shorter than the long fuel rod, has a fuel effective length of 14/24 of the fuel effective length of the long fuel rod, and has a fuel enrichment a. Reference symbols A and G3 are short fuel rods having a fuel enrichment of b and containing gadolinia of 3.0 wt%.

Here, the degree of enrichment increases in the order of a>b> f. The average enrichment of the fuel assembly was 4.45.
wt%. In FIG. 2, four short fuel rods A each containing a burnable poison (gadolinia) are arranged at a plurality of corner positions of the second layer from the outer peripheral portion of the fuel assembly. Furthermore, in the second embodiment, the difference in the number of fuel rods containing burnable poisons in the vertical direction is four. First
Although the difference in the number of fuel rods is large as compared with the embodiment, 10 rows 10
Since the row fuel has a smaller fuel rod diameter and a smaller reactivity value per fuel rod containing a burnable poison, the reactivity difference between the upper and lower axial directions is set to the same level as in the first embodiment.

With this structure, the enrichment of the eight outermost peripheral fuel rods adjacent to the short fuel rod of A can be set to the maximum enrichment a, and the axial direction of the fuel rod containing the burnable poison can be set. Due to the vertical difference, the output peak in the lower region in the first half of the operation cycle can be suppressed.

FIG. 3 shows a second embodiment of the fuel assembly for a boiling water reactor according to the present invention, which relates to claim 3 or claim 5. 3 (a) is a schematic cross-sectional view of the fuel assembly, and FIG. 3 (b) is a fuel rod in this fuel assembly.
The enrichment ratio and the gadolinia concentration are indicated by the axial distribution, and the circled numbers or symbols of the upper fuel assemblies and the numbers or symbols of the lower fuel rods match. The fuel assembly shown in FIG. 3 has the same structure as that of the fuel assembly shown in FIG. 12, the same components are designated by the same reference numerals, and duplicate description will be omitted.

In FIG. 3, this fuel assembly aims at an average take-out burnup of 45 GWd / t during an operation period of 21 months, and has fuel rods arranged in 9 rows and 9 columns. The fuel rod bundle is composed of 66 long fuel rods 2, eight short fuel rods 3 and two large diameter water rods.

In the fuel assembly, except for the upper and lower end portions of the long fuel rod, the fuel pellets of number 1 are fuel enrichment g, number 2 is fuel enrichment f, and no number is fuel enrichment a in the axial direction. It is filled uniformly. The symbol G1 is the fuel enrichment h, 6.5% by weight of gadolinia, which is a combustible poison, in the upper 15/24 of the active fuel length, and 7% in the lower 7/24 of the active fuel length. 0.5 wt% is contained.
Reference symbol G2 is the fuel enrichment f, and gadolinia, which is a burnable poison, is present in the upper portion of the active fuel length of 8/24 at 7.5.
wt%, 6.5 at 14/24 of the lower active fuel length
wt% is included.

Further, the symbol B is the fuel enrichment h,
Except for the upper and lower ends, only the portion of 8/24 of the effective fuel length in the upper portion contains 6.5 wt% of gadolinia which is a burnable poison. The number 3 is a short fuel rod shorter than the long fuel rod, has a fuel effective length of 14/24 of the fuel effective length of the long fuel rod, and has a fuel enrichment a. The short fuel rod indicated by the symbol A has a fuel concentration of i and contains 6.5 wt% of gadolinia.

Here, the degree of enrichment increases in the order of a>h>i>f> g. The average enrichment of the fuel assembly is 4.17 wt%. In FIG. 3, four short fuel rods A each containing a burnable poison (gadolinia) are arranged at the corner positions of the second layer from the outer peripheral portion of the fuel assembly, and only the upper portion is burnable poison The four contained long fuel rods having the symbol B are arranged adjacent to the large diameter water rod.

The third embodiment has the same effect as the first embodiment for the purpose of reducing the output around the corners. In addition to this effect, by providing the fuel rod with the symbol B containing the burnable poison only in the upper portion, it becomes possible to more freely adjust the difference between the upper and lower axial numbers of the fuel rod containing the burnable poison. Therefore, the output distribution in the axial direction can be adjusted more appropriately according to the take-out burnup of the fuel assembly and the operating period.

Further, in the third embodiment, in order to reduce the local output in the vicinity of all the corners of the fuel assembly, the minimum required four codes A are only in the corners of the second circumference from the outermost circumference.
By arranging the short length fuel assemblies, it is possible to reduce the output in the vicinity of all the corners of the fuel assembly and further increase the average enrichment of the fuel assembly by that amount.

Further, since there is no difference in the number of fuel rods containing a burnable poison in the upper and lower parts in the axial direction, the output peak in the lower region in the first half of the operation cycle is different from that in the first or second embodiment. Is emphasized. Therefore, JP-A-6
Fuel economy is improved by the reactivity gain at the end of the operation cycle due to the spectrum shift effect as described in Japanese Patent Laid-Open No. 118188.

In the case of the third embodiment, as compared with the case of the first embodiment or the second embodiment, the operating period is long and the average taken-out burnup is small, so the proportion of the number of replacement fuels becomes small and the core diameter The aggregate output peaking coefficient in the direction becomes small. Therefore, even if the output peak in the axial lower region in the first half of the operation cycle is emphasized, the maximum linear power density does not deteriorate much compared to the first embodiment or the second embodiment.

Now, the conventional fuel assembly shown in FIG. 16 is compared with the fuel assembly of the third embodiment shown in FIG. In FIG. 16, the radial arrangement of the fuel rods constituting the fuel assembly and the axial concentration distribution thereof are as shown in FIG. 16 except for the long fuel rods G3 containing the burnable poison except the upper and lower ends and the burnable poison. It is the same as that shown in FIG. 3 except for the short fuel rod of No. 4 which is not contained. In FIG. 3, the short fuel rod number 4 in FIG. 16 is a short fuel rod containing a burnable poison, and the reference numeral G3 long fuel rod in FIG. 16 is a long fuel rod containing only a burnable poison in the upper portion. It is a fuel assembly with the above configuration.

Therefore, the fuel assemblies shown in FIGS. 3 and 16 are designed to have the same number and concentration of burnable poisons when viewed on the average in each axial region. Local power distribution in the lower fuel region in FIGS. 3 and 16 (initial stage of combustion, 40%
The void state) is shown in FIGS. 4 and 17. Than this,
The local output reduction effect near the fuel assembly corner of the short fuel rod A containing the burnable poison is clear, and the maximum value (LPF) of the local output distribution is 1.350 to 1.2.
It was reduced by 96 to 4%.

In the third embodiment, the number of the long fuel assemblies containing the burnable poison only in the upper part is four and there is no difference in the number in the axial direction. However, the present invention is not limited to this. . That is, if the number of the long fuel rods containing the burnable poison only in the upper part is set to 1 to 4 in accordance with the number of the short fuel rods having the symbol A being four, the fuel containing the burnable poison is included. The difference between the upper and lower rods in the axial direction is 0 to 3. As a result, the number of fuel rods containing the burnable poison in the lower portion can be equal to or larger than that in the upper portion. As a result, in a boiling water reactor fuel assembly having short fuel rods, the infinite multiplication factor of the upper part was originally smaller than that of the lower part in the early stage of combustion.
It is possible to suppress the tendency that the output peak at the lower portion in the axial direction becomes high in the first half of the operation cycle.

FIG. 5 is a fourth embodiment of the fuel assembly for a boiling water reactor according to the present invention, and it relates to claim 2 or claim 5. 5 (a) is a schematic cross-sectional view of the fuel assembly, and FIG. 5 (b) is a fuel rod in the fuel assembly.
The enrichment ratio and the gadolinia concentration are indicated by the axial distribution, and the circled numbers or symbols of the upper fuel assemblies and the numbers or symbols of the lower fuel rods match. It should be noted that the same components as those in FIG. 3 are denoted by the same reference numerals, and redundant description will be omitted.

In FIG. 5, the fuel assembly has an operating period 2
Aiming at an average extraction burnup of 45 GWd / t for one month, fuel rods are arranged in 9 rows and 9 columns. The fuel rod bundle is composed of 66 long fuel rods 2, eight short fuel rods 3 and two large diameter water rods.

In this fuel assembly, in FIG. 3, four long fuel rods of the symbol B containing burnable poisons only in the upper region, and a symbol C of the lower region to which a thin gadolinia of 0.5 wt% concentration was added. It is configured by replacing the long fuel rod.

As a result, as compared with the third embodiment, the output peak in the lower part at the beginning of the cycle can be further suppressed by the effect of the thin burnable poison added to the lower part. That is,
The concentration of combustible poison added to the lower part must be the minimum concentration required to suppress the output peak of the lower part at the beginning of the cycle, and if the concentration is higher than necessary, the combustion of the lower part of the middle part of the cycle will be suppressed. Conversely, a lower output peak will occur in the latter half of the cycle.

Therefore, the concentration of the burnable poison in the lower portion needs to be lower than that in the upper portion, and at the beginning of the cycle when there is a low concentration of gadolinia, the number of fuel rods containing the burnable poison in the upper and lower axial directions is increased. Since the difference is four, the lower output peak is suppressed.

FIG. 6 shows a fifth embodiment of a fuel assembly for a boiling water reactor according to the present invention, which relates to claim 4, claim 5 or claim 11. FIG. 6 (a) is a schematic cross-sectional view of the fuel assembly, and FIG. 6 (b) shows the enrichment and gadolinia concentration in the axial distribution of the fuel rods in this fuel assembly, which are indicated by circles in the upper fuel assembly. The boxed number or code matches the fuel rod number or code below.

In FIG. 6, the fuel assembly has an operating period of 2
Aiming at an average extraction burnup of 45 GWd / t for one month, fuel rods are arranged in 9 rows and 9 columns. The fuel rod bundle is composed of 66 long fuel rods 2, eight short fuel rods 3 and two large diameter water rods.

This fuel assembly is constructed by replacing the four long fuel rods designated by the reference symbol B containing burnable poisons only in the upper region in FIG. 3 with the long fuel rods designated by the reference symbol D.
Here, the long fuel rod denoted by D is the fuel enrichment h,
Except for the upper and lower ends, 6.5% by weight of gadolinia in the upper 8/8 of the active fuel length and 7/24 of the lower active fuel length
0.5 wt% is contained in the portion of No. 1, and gadolinia is not contained in the portion of 7/24 of the active fuel length in the central portion.

As described above, by eliminating the gadolinia in the central portion of the long fuel rod designated by the symbol D, it is possible to obtain
The output in the central portion at the beginning of the cycle is increased, and the output peak at the lower portion at the beginning of the cycle can be suppressed accordingly. Further, since the number and concentration of burnable poisons can be adjusted from the upper and lower two regions to the upper, middle and lower three regions, it is possible to more appropriately control the axial power distribution throughout the operation cycle.

Further, in this fuel assembly, four symbols D
The long fuel rods are disposed adjacent to the two large diameter water rods 6. This makes it possible to reduce the difference in the reactivity value of the burnable poison during operation and during cold temperature, so during the period when the burnable poison remains unburned, the difference in reactivity during cold operation during operation will be small and the reactor shutdown margin will be increased. be able to.

FIG. 7 shows the sixth embodiment of the fuel assembly for a boiling water reactor according to the present invention, and it relates to claim 12 or claim 13. FIG. 7A is a schematic cross-sectional view of the fuel assembly, and FIG. 7B shows the enrichment and gadolinia concentration in the axial distribution of the fuel rods in the fuel assembly, which are indicated by circles in the upper fuel assembly. Enclosed number or code,
It matches the fuel rod number or code below.

In FIG. 7, the fuel assembly has an operating period of 21
Aiming for an average extraction burnup of 45 GWd / t per month, it is an N lattice fuel arranged in 9 rows and 9 columns. N-lattice fuel has a wider water gap than C-lattice fuel.
Used for R etc.

In this fuel assembly, the number of the long fuel rods designated by D is one less than that of the short fuel rods designated by A. That is, the number of gadolinia-containing fuel rods in the upper region is smaller than that in the central portion by one. As a result, the output in the upper part of the first half of the cycle can be increased and the output peak in the lower part can be reduced.

Further, in this fuel assembly, the three long fuel rods D are not adjacent to the water rods and are arranged at positions other than the outermost periphery, so that the average enrichment of the fuel assembly can be increased. . Here, the maximum enrichment of fuel rods is 4.9 wt.
%.

FIG. 8 shows a seventh embodiment of the fuel assembly for a boiling water reactor according to the present invention, and it relates to claim 12 or claim 13. FIG. 8A is a schematic cross-sectional view of the fuel assembly, and FIG. 8B shows the enrichment and gadolinia concentration in the axial distribution of the fuel rods in the fuel assembly, which are indicated by circles in the upper fuel assembly. Enclosed number or code,
It matches the fuel rod number or code below.

In FIG. 8, the fuel assembly has an operating period of 13
Aiming for an average extraction burnup of 50 GWd / t per month, it is a C-lattice fuel arranged in 9 rows and 9 columns. In this fuel assembly, the number of the long fuel rods designated by the symbol D is one less than that of the short fuel rods designated by the symbol A. That is, the number of gadolinia-containing fuel rods in the upper region is smaller than that in the central portion by one. As a result, the output in the upper part of the first half of the cycle can be increased and the output peak in the lower part can be reduced.

Further, since the maximum enrichment is used for the four long fuel rods D, the average enrichment of the fuel assembly can be increased by that much. FIG. 9 shows an eighth embodiment of the fuel assembly for a boiling water reactor according to the present invention, which relates to claim 3, claim 5, or claim 11. FIG.
(A) is a schematic cross-sectional view of the fuel assembly, and (b) shows the concentration and gadolinia concentration in the axial distribution of the fuel rods in this fuel assembly, which is circled in the upper fuel assembly. The number or code matches the fuel rod number or code below.

In FIG. 9, the fuel assembly has an operating period of 13
Aiming for an average extraction burnup of 55 GWd / t per month, it is a C-lattice fuel arranged in 9 rows and 9 columns. In this fuel assembly, there were four short fuel rods with a code A containing burnable poison, and the burnable poison contained only in the upper portion by 2
A long fuel rod indicated by reference sign B is arranged. This allows
Since the difference in the number of fuel rods containing burnable poisons in the axial direction is two, the output peak in the lower region in the first half of the operation cycle can be suppressed.

Further, since the number of short fuel rods designated by A is two, it is possible to increase the enrichment near the corners of the fuel rods. That is, the fuel assembly average enrichment can be increased by 0.04 wt% as compared with the fuel assembly shown in FIG.

FIG. 10 shows a ninth embodiment of the boiling water nuclear reactor fuel assembly according to the present invention, which relates to claim 3 or claim 11. FIG. 10A is a schematic cross-sectional view of the fuel assembly, and FIG. 10B shows the enrichment and gadolinia concentration in the axial distribution of the fuel rods in the fuel assembly. Enclosed number or code,
It matches the fuel rod number or code below.

In FIG. 10, the fuel assembly has an operating period 1
Aiming for an average extraction burnup of 55 GWd / t for 3 months, it is a C-lattice fuel arranged in 9 rows and 9 columns. In this fuel assembly, the difference from the fuel assembly shown in FIG. 9 is that four short fuel rods 1 other than the short fuel rod with the symbol A containing a burnable poison are arranged at the outermost peripheral position of the fuel assembly. I have just done it. The arrangement of the short fuel rods 1 has the effect of increasing the water gap region when the reactor is shut down, and the reactor shutdown margin can be improved by the neutron absorption effect of water.

11 and 12 are the 10th and 11th embodiments of the boiling water reactor fuel assembly according to the present invention. These fuel assemblies are arranged in 10 rows and 10 columns, and the shape of the fuel assembly itself has already been developed.
In both cases, the operation period was 13 months, and the average burnup was 5
It is designed for 5 GWd / t. FIG.
In FIG. 2, compared with FIG. 11, a water box 6 ′ is used instead of the large diameter water rod 6, and the total number of short fuel rods is changed from 14 to 8.

FIG. 13 shows a twelfth embodiment of a fuel assembly for a boiling water reactor according to the present invention, which relates to claim 6, claim 9 or claim 10. FIG. 13 (a)
Is a schematic cross-sectional view of the fuel assembly, (b) shows the concentration and gadolinia concentration in the axial distribution of the fuel rods in this fuel assembly, and the circled numbers or symbols of the fuel assembly above And the number or code of the lower fuel rod is the same.

In FIG. 13, the fuel assembly has an operating period of 1
Aiming for an average extraction burnup of 45 GWd / t for 3 months, it is a D-lattice fuel arranged in 9 rows and 9 columns. This fuel assembly is provided with three short fuel rods denoted by symbol A containing burnable poisons and two long fuel rods denoted by symbol D containing burnable poisons at the upper and lower portions. In the D-lattice fuel, the width of the water gap region on the non-control rod side is smaller than that on the control rod side, and the local output near the corner on the non-control rod side does not increase. Therefore, the short fuel rod indicated by E does not contain gadolinia and has the highest enrichment.

In this fuel assembly, the difference in the number of fuel rods containing burnable poison in the axial direction is one, and the output peak in the lower region in the first half of the operation cycle can be suppressed.

FIG. 14 shows a thirteenth embodiment of a fuel assembly for a boiling water reactor according to the present invention, which relates to claim 6, claim 9 or claim 11. FIG. 14 (a)
Is a schematic cross-sectional view of the fuel assembly, (b) shows the concentration and gadolinia concentration in the axial distribution of the fuel rods in this fuel assembly, and the circled numbers or symbols of the fuel assembly above And the number or code of the lower fuel rod is the same.

In FIG. 14, the fuel assembly is in the operation period 1
Aiming for an average extraction burnup of 45 GWd / t for 3 months, it is a D-lattice fuel arranged in 9 rows and 9 columns. This fuel assembly has three short fuel rods A1 and A2 containing burnable poisons and two long fuel rods D containing burnable poisons in the upper and lower portions. The difference in the number of fuel rods containing burnable poisons in the axial direction is one. Therefore, the output peak in the lower region in the first half of the operation cycle can be suppressed.

[0096]

As described above, according to the present invention, when a high burnup or a long operating cycle is aimed at, a short fuel rod is provided at the corner position of the second layer from the outer peripheral portion of the fuel assembly. Two problems of the fuel assembly that it has (increase of local output near the corner of fuel assembly due to increase of enrichment,
The new fuel or the increase in the lower output peak in the early stage of the operation cycle) can be solved at the same time, and a fuel assembly having a sufficient thermal margin can be provided.

[Brief description of drawings]

FIG. 1A is a transverse sectional view showing a first embodiment of a fuel assembly according to the present invention, and FIG. 1B is a longitudinal sectional view schematically showing the structure of a fuel rod in FIG. 1A.

2A is a transverse sectional view showing a second embodiment of the fuel assembly according to the present invention, and FIG. 2B is a longitudinal sectional view schematically showing the structure of the fuel rod in FIG. 2A.

3 (a) is a cross-sectional view showing a third embodiment of the fuel assembly according to the present invention, and FIG. 3 (b) is a vertical cross-sectional view schematically showing the structure of the fuel rod in (a).

FIG. 4 is a local power distribution in the cross section of the fuel assembly in the axially lower region when the burnup of the fuel assembly of the first embodiment of the present invention is 0.0 GWd / t and the void ratio is 40%. Fig.

5A is a cross-sectional view showing a fourth embodiment of the fuel assembly according to the present invention, and FIG. 5B is a vertical cross-sectional view schematically showing the structure of the fuel rod in FIG. 5A.

FIG. 6A is a transverse sectional view showing a fifth embodiment of the fuel assembly according to the present invention, and FIG. 6B is a longitudinal sectional view schematically showing the structure of the fuel rod in FIG. 6A.

FIG. 7A is a transverse sectional view showing a sixth embodiment of the fuel assembly according to the present invention, and FIG. 7B is a longitudinal sectional view schematically showing the structure of the fuel rod in FIG. 7A.

8A is a transverse sectional view showing a seventh embodiment of the fuel assembly according to the present invention, and FIG. 8B is a longitudinal sectional view schematically showing the structure of the fuel rod in FIG. 8A.

9 (a) is a transverse sectional view showing an eighth embodiment of the fuel assembly according to the present invention, and FIG. 9 (b) is a longitudinal sectional view schematically showing the structure of the fuel rod in FIG. 9 (a).

10 (a) is a cross-sectional view showing a ninth embodiment of the fuel assembly according to the present invention, and FIG. 10 (b) is a vertical cross-sectional view schematically showing the structure of the fuel rod in (a).

FIG. 11 (a) is a tenth view of a fuel assembly according to the present invention.
2B is a horizontal cross-sectional view showing the embodiment of FIG. 1, and FIG. 6B is a vertical cross-sectional view schematically showing the structure of the fuel rod in FIG.

FIG. 12 (a) is an eleventh view of a fuel assembly according to the present invention.
2B is a horizontal cross-sectional view showing the embodiment of FIG. 1, and FIG. 6B is a vertical cross-sectional view schematically showing the structure of the fuel rod in FIG.

FIG. 13 (a) is a twelfth example of the fuel assembly according to the present invention.
2B is a horizontal cross-sectional view showing the embodiment of FIG. 1, and FIG. 6B is a vertical cross-sectional view schematically showing the structure of the fuel rod in FIG.

FIG. 14 (a) is a thirteenth fuel assembly according to the present invention.
2B is a horizontal cross-sectional view showing the embodiment of FIG. 1, and FIG. 6B is a vertical cross-sectional view schematically showing the structure of the fuel rod in FIG.

FIG. 15 (A) is an elevation view showing a high burnup fuel assembly in a partial cross section, (B) is a cross sectional view taken along the line BB of FIG. 15 (A),
(C) is a sectional view taken along the line CC of (A).

16A and 16B show a conventional example corresponding to FIG. 3, in which FIG. 16A is a cross-sectional view of a fuel assembly, and FIG. 16B is a vertical cross-sectional view schematically showing the structure of a fuel rod in FIG.

FIG. 17 is a local power distribution diagram in the cross section of the axially lower region when the burnup of the conventional fuel assembly shown in FIG. 16 is 0.0 GWd / t and the void ratio is 40%.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Fuel assembly 2 ... Long fuel rod 3 ... Short fuel rod 4 ... Upper tie plate 5 ... Lower tie plate 6 ... Large diameter water rod 6 '... Water box 7 ... Channel box 8 ... Spacer 9 ... External spring

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location G21C 3/32 GDBE

Claims (13)

[Claims] 1. A fuel bundle formed by bundling a long fuel rod and a short fuel rod having an effective fuel portion shorter than that of the long fuel rod in a lattice shape is surrounded by a channel box, and 2 from the outermost periphery of the fuel bundle. In a fuel assembly for a boiling water reactor in which the short fuel rods are arranged at all corner positions of the fuel rod group located in the first layer, a plurality of corners of the fuel rod group located in the second layer from the outermost periphery A fuel assembly for a boiling water reactor characterized in that a plurality of short fuel rods containing burnable poisons are arranged at positions. 2. A region above the upper end of the fuel effective portion of the short fuel rod is defined as an upper region, a region corresponding to this fuel effective region is defined as a lower region, and the axial direction of the fuel rod is divided into two upper and lower regions except for blanket regions of upper and lower ends. 2. A fuel assembly for a boiling water reactor according to claim 1, further comprising a plurality of long fuel rods containing more combustible poison in the upper part than in the lower part. 3. The plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion, a plurality of the long fuel rods contain the burnable poison only in the upper portion. Boiling water reactor fuel assembly. 4. Among a plurality of long fuel rods containing more of the burnable poison in the upper portion than in the lower portion, a plurality of fuel rods divide the lower portion into a central portion and a lower portion, and the burnable poison in the central portion. The fuel assembly for a boiling water reactor according to claim 2, characterized in that the fuel assembly is contained except for a larger amount in the upper portion than in the lower portion. 5. A fuel rod group located in the second layer from the outermost periphery, containing 4 burnable poisons at 4 corner positions.
5. The boiling water type according to claim 2, wherein the number of short fuel rods is arranged, and one to four long fuel rods containing more burnable poison in the upper part than in the lower part are arranged. Fuel assembly for nuclear reactors. 6. A fuel bundle formed by bundling a long fuel rod and a short fuel rod having an effective portion shorter than that of the long fuel rod in a lattice shape is surrounded by a channel box, and each channel of adjacent fuel assemblies is surrounded. The width of the non-boiling water region between the outer walls of the box is configured such that the control rod insertion side is larger than the non-insertion side, and the above-mentioned is provided at all corner positions of the fuel rod group located in the second layer from the outermost periphery. In a fuel assembly in which short fuel rods are arranged, a plurality of fuel rod-containing fuel rod groups located in the second layer from the outermost periphery, excluding the corner positions on the non-control rod side, are placed at a plurality of corner positions containing a burnable poison. A fuel assembly for a boiling water reactor characterized in that short fuel rods are arranged. 7. The region above the upper end of the fuel effective portion of the short fuel rod is the upper portion, the region corresponding to this fuel effective portion is the lower portion, and the axial direction of the fuel rod is divided into upper and lower two regions except the blanket regions at the upper and lower ends. 7. The fuel assembly for a boiling water reactor according to claim 6, further comprising a plurality of long fuel rods containing more combustible poison in the upper part than in the lower part. 8. The plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion, a plurality of the long fuel rods contain the burnable poison only in the upper portion. Boiling water reactor fuel assembly. 9. Among a plurality of long fuel rods containing more of the burnable poison in the upper portion than in the lower portion, a plurality of fuel rods divide the lower portion into a central portion and a lower portion, and the burnable poison in the central portion. The fuel assembly for a boiling water reactor according to claim 7, wherein the fuel assembly is contained in the boiling water reactor in a larger amount than in the lower portion. 10. Three short fuel rods containing a burnable poison are arranged at three corner positions of the fuel rod group located on the second layer from the outermost periphery, excluding the corner positions on the non-control rod side. The fuel assembly for a boiling water nuclear reactor according to any one of claims 7 to 9, wherein one to three long fuel rods containing more burnable poison in the upper portion than in the lower portion are arranged. 11. A plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion are arranged at a position adjacent to a water rod.
Alternatively, the fuel assembly for a boiling water nuclear reactor according to any one of 7 to 10. 12. A plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion are arranged not to be adjacent to the water rod but at positions other than the outermost periphery. A fuel assembly for a boiling water reactor according to any one of 2 to 5 or 7 to 10. 13. The enrichment of a plurality of long fuel rods containing the burnable poison in the upper portion more than in the lower portion is used in a fuel assembly in most of the regions except the blanket regions at the upper and lower ends. The fuel assembly for a boiling water reactor according to claim 12, wherein the fuel assembly has the highest value among all.