JP2004354319A - Fuel assembly for boiling water nuclear reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure thermal operation margin in high degree of burnup, and to prolong an operation period and to enhance extracted degree of burnup, without impairing high enrichment. <P>SOLUTION: This fuel assembly having water rods having 4.9 wt%-5 wt% of maximum uranium enrichment, and arranged with 9×9 lattice array in which a width of gap water outside a channel box is narrower in an anti-control rod side is provided with fuel rods wherein no enrichment boundary exists in an upper side of a lower limit position in a position lower than by a distance corresponding to substantial 30% of fuel effective length from an upper end of the fuel effective length, and fuel rods wherein an enrichment boundary getting low in the enrichment exists. A cross-sectional average enrichment in an upper part area U of the fuel assembly just above a boundary B is lower than that of in a lower part area L of the fuel assembly just below the boundary B, as to the enrichment boundary in the lowermost position, a lower part area of a fuel rod group 1 in 15 of the control rod side fuel rods out of the outermost circumferential fuel rods and a lower part area of a fuel rod group 2 in 15 of the anti-control rod side fuel rods include respectively the maximum enrichment of pellets, and e2(U)-e1(U)>e2(L)-e1(L) is satisfied, where ei(x) is the average enrichment of the fuel rod group i (i=1 or 2) in an axial-directional area x (x=U or L). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４３】
以下、本発明による改善の程度を定量的に示す。前述のとおり、高濃縮度燃料では、最外周燃料棒で沸騰遷移が起こりやすい。最外周燃料棒について図５は沸騰遷移に至る限界出力を相対的に比較して示したものである。破線は従来例１に相当し、このうち最も限界出力が小さい燃料棒位置８を基準（０％）にしている。一点鎖線は従来例に対しチャンネル曲がりを考慮したときの様子である。
【００４４】
チャンネル曲がりが生じた場合、図３に示した影響を受ける結果、本例の場合、特に燃料棒１は燃料棒８よりも２．８％限界出力が低下する。つまり、チャンネル曲がりが生じた場合、原子炉運転中の熱的運転余裕は２．８％低下することになる。本実施例での限界出力の様子を図５の実線で示す。
【００４５】
これは、曲がりが生じた場合の結果として図示している。本実施例では、チャンネル曲がりが生じて制御棒挿入側で限界出力の低下があった場合でも、従来例１に比べて限界出力を約２％改善できる。なお、集合体平均濃縮度は４．３９ｗｔ％で、従来例１と変わらず高濃縮度化を阻害することはない。
【００４６】
以上説明したとおり、本発明は、高燃焼度化に伴いチャンネルボックスに曲がりが生じた場合であっても熱的運転余裕を確保し、しかも燃料集合体の高濃縮度化を阻害することなく運転長期化と取出燃焼度の向上を実現する沸騰水型原子炉用燃料集合体を得ることができる。
【００４７】
【発明の効果】
本発明は以上説明した通り、高燃焼度化に伴うチャンネル曲がりの影響を考えて熱的運転余裕を確保し、しかも燃料集合体の高濃縮度化を阻害することなく運転長期化と取出燃焼度の向上を実現する沸騰水型原子炉用燃料集合体を得ることができるという効果がある。
【図面の簡単な説明】
【図１】本発明での軸方向における領域Ｕ、領域Ｌ及びその濃縮度境界の定義を示す説明図である。
【図２】燃料棒群１と燃料棒群２との定義を示す説明図である。
【図３】チャンネル曲がりによる限界出力の低下の様子を示す説明図である。
【図４】本発明の沸騰水原子炉用燃料集合体の一実施例の構成を示す説明図である。
【図５】本実施例での限界出力の様子を示す線図である。
【図６】燃料棒が９行９列に配列された９×９格子の燃料集合体の断面形状の一従来例を示す説明図である。
【図７】核設計（半径方向及び軸方向の濃縮度分布）の一従来例を示す説明図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a 9 × 9 fuel assembly loaded in a D lattice core in a boiling water reactor (hereinafter, referred to as BWR), and particularly to a 9 × 9 fuel assembly aimed at high enrichment.
[0002]
[Prior art]
(Core type)
BWRs are roughly classified into two types of lattice cores. One is a symmetrical core in which the width of the gap water located outside the channel box surrounding the fuel assembly is equal on the control rod insertion side and the counter control rod insertion side, and corresponds to a C lattice core as an example.
[0003]
The other is an asymmetric core in which the width of the gap water on the control rod insertion side is wider than the width of the gap water on the anti-control rod insertion side, which is called a D lattice core. In this D-lattice core, the neutron deceleration is large on the control rod insertion side with a wide gap width and small on the non-control rod insertion side with a narrow gap width. Have.
[0004]
(Concentration distribution)
In the BWR, it is necessary to flatten the relative power distribution of the fuel rods in the radial direction of the fuel rods to some extent in order to secure a thermal margin such as a maximum linear power density and a minimum critical power ratio related to boiling transition during operation. In general, in a fuel assembly loaded in a D-lattice core, the control rod insertion side where the output tends to increase is designed to have a lower enrichment than the non-control rod insertion side.
[0005]
(9 × 9 fuel)
In the BWR, it is known as an effective means to increase the economic efficiency by extending the operation cycle and improving the average removal burnup. For these two purposes, increasing the average enrichment of nuclear fuel in the fuel assembly has become an effective and indispensable means.
[0006]
For example, FIG. 6 is an explanatory view showing a conventional example of a cross-sectional shape of a fuel assembly having a 9 × 9 grid in which fuel rods are arranged in 9 rows and 9 columns. In this fuel assembly 60, one rectangular water rod 61 formed by replacing the area of nine fuel rods faces the control rod 62 by one fuel rod pitch from the center of the cross section of the fuel assembly. The structure is arranged so as to be biased toward the position side, that is, the side opposite to the control rod insertion side.
[0007]
The water rod 61 is shifted from the center in the conventional example of FIG. 6 in order to reduce the influence of the non-uniformity of the gap water in the D lattice on the non-uniformity of the thermal neutron flux distribution in the assembly. It is a design. Of course, even in this case, since the degree of the inhomogeneity relaxation is not perfect, the enrichment distribution is actually asymmetric in the radial direction in the actual nuclear design. This fuel assembly is an example of a 9 × 9 fuel assembly for high burnup, and the fuel assembly of the present structure has an enrichment distribution such that it has an average removal burnup of 45 GWd / t after 13 months of operation. Designed and burning in the country.
[0008]
(Concentration distribution)
Generally, the boiling transition from the nucleate boiling state to the film boiling state becomes severe near the lower portion of the spacer disposed above the fuel rod. In many cases, the thermal operation margin is improved by preferentially arranging fuel rods filled with relatively low enrichment fuel pellets in the upper region at arrangement positions where boiling transition is likely to occur. That is, the minimum limit output ratio can be reduced by devising the concentration distribution.
[0009]
FIG. 7 shows this conventional nuclear design (enrichment distribution in the radial and axial directions). Hereinafter, this is referred to as Conventional Example 1. The configuration of the conventional example 1 has been previously proposed by the inventor of the present invention (see FIG. 5 of Patent Document 1). Here, the numerical value in the fuel rod indicates the enrichment (wt%), and the numerical value with the letter G indicates the concentration (wt%) of gadolinia acting as a burnable poison.
[0010]
From the viewpoint of criticality in the manufacturing process, the uranium enrichment that can be handled at present is 5.0 wt% or less. The average enrichment of the fuel assembly of Conventional Example 1 is 4.39 wt%, which is a sufficiently high enrichment as a fuel for a D lattice core. In the first conventional example, the enrichment in the upper region is reduced with respect to the outermost fuel rods whose output is likely to be high, thereby improving the limit output to the boiling transition while maintaining high enrichment.
[0011]
(Bend of channel box)
By the way, when loading the fuel assembly into the furnace, a channel box is mounted on the outer periphery thereof. The channel box is elongated by neutron irradiation. As a result, if there is a difference in the fast neutron fluence in the radial direction, the channel box will be distorted.
[0012]
In the D-lattice core, the elongation on the anti-control rod insertion side is larger than that on the control rod insertion side because the gap water is small and the fast neutrons are large. As a result, the channel box bends along the axial direction toward the side opposite to the control rod insertion side (hereinafter, this phenomenon is referred to as channel bend). As a result, the gap water area decreases on the side opposite to the control rod insertion side, and the gap water area increases on the control rod insertion side.
[0013]
The increase in the gap water area on the control rod insertion side further heats the neutrons in the vicinity, so in particular, the output of the fuel rod located on the outermost periphery of the control rod insertion side increases, and the critical output that causes a boiling transition And the thermal operation margin is reduced. With the increase in burnup in recent years, channel bending increases, and there is a concern that the minimum limit output may decrease.
[0014]
[Patent Document 1]
JP 2002-48886 A
[Problems to be solved by the invention]
As described above, in consideration of the channel bending, it is necessary to further improve the limit output on the control rod insertion side. Even in the case of a 9 × 9 fuel assembly in which the water rods are displaced to the control rod insertion side in order to alleviate the bias of the power distribution, considering the deterioration of the limit output on the control rod insertion side due to the channel bending, it is considered. Again, design considerations are required.
[0016]
Channel bend is considered to occur with the progress of combustion.However, even for a new fuel that has no combustion experience, if a fuel assembly with a channel bend is adjacent to this, the control rod side The gap water will expand. As described above, since even a new fuel is affected by the channel bending, it is necessary to consider the influence of the critical output due to the channel bending from the initial stage of combustion in the design.
[0017]
The present invention secures thermal operation allowance by considering the effect of channel bending accompanying higher burnup, and realizes longer operation and improved removal burnup without hindering higher fuel assembly enrichment. It is an object to obtain a boiling water reactor fuel assembly.
[0018]
[Means for Solving the Problems]
The fuel assembly for BWR according to the first aspect of the present invention has a lattice-shaped core in which the width of the gap water located outside the channel box surrounding the fuel assembly is narrower on the control rod insertion side than on the control rod insertion side. A fuel assembly loaded into
The fuel assembly has fuel rods filled with nuclear fuel enriched fuel pellets arranged in a 9 × 9 grid, and has water rods occupying a plurality of fuel rod regions,
The uranium maximum enrichment of the nuclear fuel pellets is in the range of 4.9 wt% to 5 wt%,
When the lower position is set at a lower position from the upper end of the active fuel length by a distance corresponding to approximately 30% of the active fuel length, the fuel rod is provided with a blanket made of low-enrichment pellets at upper and lower ends. Excluding the blanket portion, a fuel rod having no enrichment boundary above the lower limit position, and a fuel rod having an enrichment boundary where the enrichment is lower on the upper side above the lower limit position. ,
When a lowermost enrichment boundary of the enrichment boundaries is defined as a boundary B, an upper region (U) of the fuel assembly immediately above the boundary B is the fuel assembly immediately below the boundary B. A lower cross-sectional average enrichment of the fuel assembly than the lower region (L) of the body,
In the radial direction of the fuel assembly, of the outermost fuel rods, fifteen fuel rods on the control rod insertion side are referred to as fuel rod group 1 and fifteen fuel rods on the opposite control rod insertion side are referred to as fuel rod group 2,
The lower region of the fuel rod group 1 and the lower region of the fuel rod group 2 each include the highest enriched fuel pellet in the fuel assembly,
Assuming that ei (x) is the average enrichment of the fuel rod group i (i = 1 or 2) in the axial region x (x = U or L), the following equation (1) is satisfied. It is a feature.
[0019]
(Equation 2)
e2 (U) -e1 (U)> e2 (L) -e1 (L)> 0 (1)
The fuel assembly for BWR according to the invention described in claim 2 is characterized in that all of the fuel rod groups 1 described in claim 1 have the boundary B at the same axial position of the active fuel length. Things.
[0021]
The fuel assembly for BWR according to the invention described in claim 3 is the fuel assembly for BWR according to claim 1 or 2,
One square water rod is provided by replacing the area of nine fuel rods which are arranged one fuel rod pitch from the center of the cross section of the fuel assembly toward the anti-control rod insertion side. It is characterized by the following.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an explanatory view showing the definition of the regions U and L in the axial direction and their enrichment boundaries in the present invention. Some of the fuel rods have a lower enrichment on the upper side and have enrichment boundaries. As shown in the figure, when a lower position is set at a distance corresponding to about 30% of the active fuel length from the upper end of the active fuel length, an actual enrichment boundary is determined above the lower limit position. Of the enrichment boundaries that have just been determined, the enrichment boundary on the lowermost side is defined as the boundary B, the region immediately above the boundary B is an upper region (U), and the region immediately below the boundary B is a lower region ( L).
[0023]
Generally, it is the upper region of the fuel assembly that undergoes the boiling transition. The fuel assembly is provided with about seven to eight spacers at predetermined intervals to bundle fuel rods. The presence of the spacer has an effect of promoting the adhesion of the droplet to the fuel rod, and has an effect of improving the limit output.
[0024]
In the case of a fuel assembly having an improved spacer structure and an improved critical output, the axial position at which boiling transition occurs tends to shift to a lower side. This is because the output to the boiling transition is increased and the steam quality is increased, so that the lower portion of the fuel becomes thermally severe. Considering the performance of current fuels, the axial position at which the boiling transition occurs is just below the spacer about 30% below the upper end of the active fuel length.
[0025]
Therefore, in the present invention, attention is paid to the upper region where the boiling transition is likely to occur in the axial direction, and the output in the upper region is relatively reduced to improve the limit output.
[0026]
More specifically, the fuel assembly of the present invention is loaded on a D-lattice core in which the gap water width is loaded on a narrow lattice-shaped core on the side opposite to the control rod insertion side on the side opposite to the control rod insertion side, and a plurality of fuel cells are internally provided. 9 x 9 fuel assembly with water rods occupying the same fuel rod area. The maximum uranium enrichment of the nuclear fuel pellets used for the fuel rods of this fuel assembly is in the range of 4.9 wt% to 5 wt%, which is sufficiently high as a fuel for the D lattice core.
[0027]
Further, the present fuel assembly has a fuel rod having no enrichment boundary above the lower limit position when the lower position is set to a lower position from the upper end of the active fuel length by a distance corresponding to approximately 30% of the active fuel length, Fuel rods with enrichment boundaries. In the fuel rod having the enrichment boundary, the fuel rod in which the fuel pellets are arranged so that the enrichment is low on the upper side is used, and the fuel rod having the high concentration on the upper side is not included. Further, the fuel rods of the present fuel assembly may or may not be provided with blankets made of low-enrichment pellets at the upper and lower ends. If a blanket is provided, the enrichment boundary is determined excluding the blanket portion.
[0028]
Therefore, in the fuel assembly of the present invention, when the enrichment boundary located at the lowermost side of the enrichment boundary is the boundary B, the upper region (U) of the fuel assembly immediately above the boundary B is The cross-sectional average enrichment of the fuel assembly is lower than that of the lower region (L) of the fuel assembly immediately below.
[0029]
Further, in the fuel assembly of the present invention, of the outermost fuel rods in the radial direction of the fuel assembly, the fifteen fuel rods on the control rod insertion side are defined as the fuel rod group 1, and the fifteen fuel rods on the opposite control rod insertion side. Is referred to as a fuel rod group 2. FIG. 2 shows the definitions of the fuel rod group 1 and the fuel rod group 2. The fifteen outermost fuel rods on the control rod insertion side indicated by reference numerals 1 to 8 are referred to as a fuel rod group 1, and the fifteen outermost fuel rods on the opposite control rod insertion side indicated by reference numerals 10 to 17 are referred to as a fuel rod group 2. I do. The fuel rod of No. 9 which is neither the control rod insertion side nor the anti-control rod insertion side does not belong to the fuel rod group 1 or the fuel rod group 2.
[0030]
When the fuel rod group 1 and the fuel rod group 2 are defined as described above, the lower region of the fuel rod group 1 and the lower region of the fuel rod group 2 include the fuel pellets having the highest enrichment in the fuel assembly. , Ei (x) as the average enrichment of the fuel rod group i (i = 1 or 2) in the axial region x (x = U or L), satisfying the relationship shown in the above equation (1). It is.
[0031]
Specifically, an object of the present invention is to mitigate the effect of a decrease in the limit output due to channel bending in the radial direction. Here, it is assumed that a cosine distribution is assumed as an axial profile of the channel bend, and that a maximum bend amount of about 1 mm occurs at the center of the active fuel length. This maximum bend amount is a typical value assumed for the average bend amount in the case of a fuel with a high burnup of 55 GWd / t at the highest burnup in the D lattice core.
[0032]
FIG. 3 is an explanatory diagram showing a state in which the limit output is reduced due to channel bending. FIG. 3 is an evaluation of the change in the critical output of the outermost fuel rods for the fuel assembly of Conventional Example 1 based on the change due to the bending of the fuel rod relative power distribution in the radial direction. The position of the fuel rod on the horizontal axis follows the definition in FIG.
[0033]
As shown in FIG. 3, the limit output is reduced in the fuel rod group 1 on the control rod insertion side due to the channel bending, and the effect is the largest at the corner portion closest to the control rod insertion side. It can be seen that it has decreased by 4%. On the other hand, in the fuel rod group 2 on the side opposite to the control rod insertion side, it can be seen that the limit output is improved. There is no change for the fuel rods belonging to none (position 9). These trends are not significantly affected by differences in fuel assembly structure and nuclear design.
[0034]
In the present invention, the marginal output is improved by reducing the enrichment of the outermost fuel rod near the control rod insertion side where the marginal output is reduced in consideration of the tendency of the marginal output to change due to channel bending. In addition, in order to prevent the average enrichment of the assembly from being lowered, the enrichment is increased for the outermost fuel rods on the side opposite to the control rod where the limit power is increased. Further, the above configuration is applied to the upper region where boiling transition is likely to occur so as not to hinder the enrichment of the lower cross section.
[0035]
Further, in the D lattice core, as described above, the output tends to increase at the fuel rod on the control rod insertion side. In order to secure the operation margin of the maximum linear power density which is the thermal limit value during operation, the average enrichment of the fuel rod group 1 on the control rod insertion side is also required for the lower cross section where the linear power density tends to increase. It is desirable to make the enrichment lower than that of the fuel rod group 2.
[0036]
From the above, it was found that the average enrichment of the fuel rod group 1 on the control rod insertion side should be lower than that of the fuel rod group 2, and that the difference should be greater in the upper section. . Formula (1) formalizes this configuration. With this configuration, it is possible to configure a fuel assembly suitable for high burnup, which ensures a thermal operation margin of the minimum limit output ratio and does not hinder high enrichment.
[0037]
【Example】
FIG. 4 is an explanatory view showing the configuration of an embodiment of the fuel assembly for a boiling water reactor according to the present invention. FIG. 4 shows a configuration of the present invention in which the concentration distribution is changed from that of the conventional example 1. The natural uranium of each of the upper and lower 1/24 nodes (the active fuel length is divided into 24 nodes) is not changed from the conventional example 1, so that the description of the natural uranium is omitted in the following description.
[0038]
In the present embodiment, the following additional changes are made to the types of the fuel rods compared to the conventional example 1. First, fuel type 0 formed from only the highest enrichment was added. For fuel type 1, the enrichment boundary was lowered by 3/24 node to fuel type 1 '. In addition, the fuel type 3 is provided with an enrichment boundary at the same position as the fuel type 1 'so that the upper portion has a low enrichment. For fuel type 4, an enrichment boundary was provided such that the upper portion of the fuel cell above 21 nodes had low enrichment, and the fuel type was 4 '. Similarly, fuel type 6 ′ was provided with an enrichment boundary so that the upper portion had low enrichment.
[0039]
Regarding the fuel rod arrangement, fuel rod types 1 ', 3', 4 'and 6' whose upper portions had low enrichment were arranged at the outermost peripheral position on the control rod insertion side. Further, a fuel rod type 0 having the highest enrichment was disposed at the outermost peripheral position on the side opposite to the control rod insertion side. However, a fuel rod type 4 'having a relatively low enrichment is arranged at a corner position where the output is likely to be high even on the side opposite to the control rod insertion side.
[0040]
Although the state of improvement of the limit output by this configuration will be described later, the fuel rod at the corner position on the counter control rod insertion side does not necessarily need to reduce the enrichment at the upper portion because the limit output has a margin. Fuel rod type 4 may be used. Here, the number of types of fuel rods is increased, and the fuel rod type 4 'is arranged in order to suppress an increase in manufacturing complexity.
[0041]
In the present embodiment, the average enrichment of each area is as follows, and it can be seen that the equation (1) is clearly satisfied.
[0042]
[Table 1]

[0043]
Hereinafter, the degree of improvement according to the present invention will be quantitatively shown. As described above, in the case of the highly enriched fuel, boiling transition easily occurs in the outermost fuel rod. FIG. 5 shows the relative output of the outermost fuel rods which leads to the boiling transition in comparison. The broken line corresponds to Conventional Example 1, in which the fuel rod position 8 having the smallest limit output is set as a reference (0%). The alternate long and short dash line shows a state in which channel bending is taken into account in the conventional example.
[0044]
When the channel is bent, the influence shown in FIG. 3 is obtained. As a result, in this example, the fuel rod 1 has a 2.8% lower limit power than the fuel rod 8 in particular. That is, when the channel is bent, the thermal operating margin during the operation of the reactor is reduced by 2.8%. The state of the limit output in this embodiment is shown by a solid line in FIG.
[0045]
This is illustrated as a result of the occurrence of a bend. In the present embodiment, even if the channel is bent and the limit output decreases on the control rod insertion side, the limit output can be improved by about 2% as compared with the conventional example 1. The aggregate average enrichment is 4.39 wt%, which does not hinder high enrichment as in Conventional Example 1.
[0046]
As described above, the present invention secures a thermal operation margin even when the channel box is bent due to high burnup, and furthermore, operates without hindering high enrichment of the fuel assembly. It is possible to obtain a boiling water reactor fuel assembly realizing prolonged operation and improved removal burnup.
[0047]
【The invention's effect】
As described above, the present invention secures a thermal operation margin by considering the influence of channel bending accompanying high burnup, and furthermore, prolongs operation and removes burnup without obstructing high enrichment of the fuel assembly. There is an effect that it is possible to obtain a fuel assembly for a boiling water reactor that realizes an improvement in fuel economy.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing definitions of a region U, a region L in the axial direction, and a boundary of enrichment in the present invention.
FIG. 2 is an explanatory diagram showing definitions of a fuel rod group 1 and a fuel rod group 2.
FIG. 3 is an explanatory diagram showing a state in which a limit output is reduced due to channel bending.
FIG. 4 is an explanatory view showing a configuration of an embodiment of a fuel assembly for a boiling water reactor according to the present invention.
FIG. 5 is a diagram showing a state of a limit output in the present embodiment.
FIG. 6 is an explanatory diagram showing a conventional example of a sectional shape of a fuel assembly having a 9 × 9 lattice in which fuel rods are arranged in 9 rows and 9 columns.
FIG. 7 is an explanatory diagram showing a conventional example of a nuclear design (enrichment distribution in radial and axial directions).

Claims (3)

A fuel assembly loaded in a narrow lattice-shaped core in which a gap water width located outside a channel box surrounding the fuel assembly is closer to a control rod insertion side than to a control rod insertion side,
The fuel assembly has fuel rods filled with nuclear fuel enriched fuel pellets arranged in a 9 × 9 grid, and has water rods occupying a plurality of fuel rod regions,
The uranium maximum enrichment of the nuclear fuel pellets is in the range of 4.9 wt% to 5 wt%,
When the lower position is set at a lower position from the upper end of the active fuel length by a distance corresponding to approximately 30% of the active fuel length, the fuel rod is provided with a blanket made of low-enrichment pellets at upper and lower ends. Excluding the blanket portion, a fuel rod having no enrichment boundary above the lower limit position, and a fuel rod having an enrichment boundary where the enrichment is lower on the upper side above the lower limit position. ,
When a lowermost enrichment boundary of the enrichment boundaries is defined as a boundary B, an upper region (U) of the fuel assembly immediately above the boundary B is the fuel assembly immediately below the boundary B. A lower cross-sectional average enrichment of the fuel assembly than the lower region (L) of the body,
In the radial direction of the fuel assembly, of the outermost fuel rods, the fifteen fuel rods on the control rod insertion side are referred to as a fuel rod group 1 and the fifteen fuel rods on the anti-control rod insertion side are referred to as a fuel rod group 2.
The lower region of the fuel rod group 1 and the lower region of the fuel rod group 2 each include the highest enriched fuel pellet in the fuel assembly,
Assuming that ei (x) is the average enrichment of the fuel rod group i (i = 1 or 2) in the axial region x (x = U or L), the following equation (1) is satisfied. A fuel assembly for a boiling water reactor.

2. The fuel assembly for a boiling water reactor according to claim 1, wherein all of the fuel rod groups 1 have the boundary B at an axial position having the same effective fuel length. 3. One square water rod is provided by replacing the area of nine fuel rods arranged one fuel rod pitch from the center of the cross-section of the fuel assembly toward the non-control rod insertion side. The fuel assembly for a boiling water reactor according to claim 1 or 2, wherein:

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