JP2002350579A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the allowance of the stop of a boiling water reactor without worsening the economic benefit of a fuel in a fuel assembly which is loaded on the boiling water reactor. SOLUTION: A region in a length of 4/24 to 13/24 of the effective length of the fuel is installed in the upper part in the axial direction as a low-reactivity region 12c having a fuel such as natural uranium. A region directly under it contains upper-part low-reactivity fuel rods U2 whose reactivity in the upper part region of a fuel rod is lowered as a high-reactivity region containing more fissile materials than that in the low-reactivity region. The fuel rods U2 are arranged in the outermost circumference of the fuel assembly. Alternatively, two fuel rods are arranged in positions facing two side faces into which a cross-shaped control rod 11 is not inserted from among the outermost circumference of the fuel assembly or in positions adjacent to the fuel rod in a corner part into which the control rod 11 is not inserted from among the outermost circumference of the fuel assembly, or one fuel rod is arranged in a corner part into which the control rod 11 is not inserted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly loaded in a boiling water reactor, and more particularly to a fuel assembly having an improved reactor shutdown margin without deteriorating fuel economy.

[0002]

2. Description of the Related Art The core of a nuclear reactor is composed of a large number of fuel assemblies and a cruciform control rod inserted and removed therebetween, and four fuel assemblies are provided around one cruciform control rod. Are arranged. A fuel assembly generally used in a boiling water reactor is composed of a plurality of fuel rods in which a fuel cladding tube is filled with a large number of fuel pellets such as uranium oxide.

[0003] One example of this fuel assembly is a longitudinal sectional view of FIG. 13 (a), and a sectional view taken along the line BB in FIG. 13 (a).
And (c) is a sectional view taken along the line CC in (a).

[0004] The fuel assembly 1 comprises a plurality of long fuel rods 2 and a plurality of short fuel rods 3 and one or two water rods 4 formed in a square lattice by a plurality of spacers 5 arranged in the axial direction. I'm bundling.

Further, among the bundled fuel rods,
The long fuel rods 2 are fixed to the upper tie plate 7 and the lower tie plate 8 together with the external spring 6, and the short fuel rods 3 are fixed to the lower tie plate 8. Surrounded by

The lengths and number of the long fuel rods 2 and short fuel rods 3 and the water rods 4 in the channel box 9 and the shapes and arrangements thereof differ depending on the nuclear reactor. It is not limited to the fuel assembly 1 shown.

[0007]

Incidentally, one of the design conditions in the design of the reactor fuel is a condition that a cold reactor shutdown margin of 1.0% Δk or more is secured throughout the operation cycle. The reactor shutdown margin indicates the subcriticality when the control rod with the greatest reactivity control effect is pulled out and all the other control rods are inserted.For reactor safety, the reactor shutdown margin is Must be secured.

Further, in the reactivity control in the BWR, a reactivity suppressing effect by mixing a burnable poison such as gadolinia into the fuel rod is used in addition to the reactivity suppressing effect by the control rod. Since this burnable poison has the ability to absorb neutrons, it has the effect of reducing the infinite multiplication factor at the beginning of combustion. As the burn proceeds, the burnable poison burns out and the effect of suppressing the infinite multiplication factor disappears. However, uranium-2, which is a fissile substance contained in the fuel accompanying the burn, is burned.
Since the content of 35 is also reduced, by appropriately adjusting the content of this burnable poison, the infinite multiplication factor of the control rod insertion state and the withdrawal state at the time of cold temperature from the beginning of combustion to the period when the burnable poison is burned out. And a necessary furnace stop margin can be secured throughout the cycle. Generally, the content of the burnable poison is adjusted so as to almost burn out at the end of the reactor cycle from the viewpoint of maximizing the reactivity of the core at the end of the operation cycle of the reactor.

On the other hand, there is a need to increase the burnup of the fuel in order to improve the economy of the fuel. If the uranium enrichment of the fuel increases with the increase of the burnup, the reactivity of the fuel increases. In order to suppress the reactivity, the number of fuel rods into which the burnable poison is mixed and the burnable poison concentration tend to increase. If the number of fuel rods containing burnable poisons or the concentration of burnable poisons is excessively increased, burnable poisons will remain at the end of the reactor operation cycle, resulting in loss of reactivity. The result is reduced fuel economy. Also, since the power of the reactor is lower in the periphery of the core than in the vicinity of the center of the core, the burning of the burnable poison contained does not proceed as compared with the center, and there is a margin for reactor shutdown near the center of the core. If the burnable poison is contained in an amount suitable for ensuring the fuel, the burnable poison remains in the periphery of the core at the end of the cycle, and the reactivity is lost.

In view of the foregoing, an object of the present invention is to provide a fuel assembly having an improved reactor shutdown margin without deteriorating fuel economy.

[0011]

The invention according to claim 1 is
In a fuel assembly for a boiling water reactor configured by bundling a plurality of fuel rods in a square lattice, a low reactivity region having a length of 4/24 or more and 13/24 or less of an active fuel length is provided at an upper portion in an axial direction. In addition to the low-reactivity fuel rods, a region immediately below the low-reactivity region is a high-reactivity region containing more fissile material than the low-reactivity region and the reactivity of the fuel rod upper region is reduced. .

By providing the upper low-reactivity fuel rod,
Through combustion, the infinite multiplication factor when the control rod is inserted at the time of cold temperature can be suppressed low, and the furnace stop margin can be improved. Further, since it is not necessary to increase the content of the burnable poison in order to improve the furnace shutdown margin, the furnace shutdown margin can be improved without impairing fuel economy.
Alternatively, when there is an excessive furnace shutdown margin, the content of the burnable poison can be reduced within a range that can satisfy the furnace shutdown margin, so that the reactivity loss at the end of the cycle due to the remaining burnable poison is eliminated. Can improve fuel economy.

According to a second aspect of the present invention, in the first aspect of the invention, the upper low-reactivity fuel rod has a low-reactivity region including a position where the axial power distribution of the core at the time of cold temperature is the largest. It is characterized in that it is formed in a region. The low reactivity area is located in the area where the axial power distribution is greatest at low temperatures in the core, so the infinite multiplication factor when the control rod is inserted at cold temperatures is efficiently suppressed to improve the furnace stop margin. can do.

The invention according to claim 3 is the invention according to claim 1 or 2
In the invention according to the fifth aspect, the number of upper low-reactivity fuel rods is set to nine or less. That is, by minimizing the number of low-reactivity fuel rods in the upper part, the reactor shutdown margin can be improved without reducing the reactivity at the end of the cycle.

The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the upper low-reactivity fuel rod is:
It is characterized by being arranged at the outermost periphery of the fuel assembly.
That is, by arranging the upper low-reactivity fuel rods on the outermost periphery of the fuel assembly, the infinite multiplication factor when the control rods are inserted at the time of cold temperature can be efficiently suppressed, and the furnace stop margin can be improved.

The invention according to claim 5 is the invention according to any one of claims 1 to 3, wherein the upper low-reactivity fuel rod is
It is characterized in that it is disposed at a position facing two side surfaces of the outermost periphery of the fuel assembly where the cross control rod is not inserted. Thus, by arranging the upper low-reactivity fuel rods at positions on the outermost periphery of the fuel assembly facing the two side surfaces where the cross-shaped control rods are not inserted, the infinite multiplication factor when the control rods are inserted at the time of cold temperature is obtained. Can be efficiently suppressed and the furnace stop margin can be improved.

The invention according to claim 6 is the invention according to claim 1 or 2
In the invention according to the first aspect, two upper low-reactivity fuel rods are arranged at positions on the outermost periphery of the fuel assembly adjacent to the fuel rods at the corner positions where the cross control rods are not inserted. In this way, by arranging two upper low-reactivity fuel rods at positions on the outermost periphery of the fuel assembly adjacent to the fuel rods at the corner positions where the cross-shaped control rods are not inserted, when the control rods are inserted at the time of cold temperature, The infinite multiplication factor can be efficiently suppressed to improve the furnace stop margin.

The invention according to claim 7 is the invention according to claim 1 or 2
In the invention according to the above, one upper low-reactivity fuel rod is arranged at a corner position where the cross-shaped control rod is not inserted. In this way, by arranging one upper low-reactivity fuel rod at the corner position where the cross-shaped control rod is not inserted, the infinite multiplication factor when the control rod is inserted at the time of cold temperature is efficiently suppressed, and the furnace stop margin is provided. Can be improved.

The invention according to claim 8 is characterized in that, in the invention according to claims 1 to 7, the low-reactivity region of the upper low-reactivity fuel rod is made of natural uranium.

According to a ninth aspect of the present invention, in the first to seventh aspects, the enrichment of uranium-235 in the low-reactivity region of the upper low-reactivity fuel rod is set to about 1 wt% or less. I do. The low-reactivity region of the upper low-reactivity fuel rod is set to 1 wt% of natural uranium or uranium-235 enrichment.
% Or less to suppress the infinite multiplication factor when the control rod is inserted at the time of cold temperature through combustion,
Furnace shutdown margin can be improved.

The invention according to claim 10 is the invention according to claims 1 to 7
In the invention according to any one of the above, the low-reactivity region of the upper low-reactivity fuel rod is made of recovered uranium obtained by reprocessing spent fuel. By making the low-reactivity region of the upper low-reactivity fuel rod the recovered uranium,
The same effect as when the region is made of natural uranium can be obtained, and the infinite multiplication factor when the control rod is inserted at the time of cold temperature can be suppressed through combustion, and the furnace shutdown margin can be improved.

The invention according to claim 11 is the invention according to claims 1 to 7
In the invention according to any one of the above, the low-reactivity region of the upper low-reactivity fuel rod is made of depleted uranium obtained from waste material in the U-235 enrichment step. That is, even if the low-reactivity region of the upper low-reactivity fuel rod is made of depleted uranium, the same effect as when the region is made of natural uranium can be obtained. The multiplication factor can be suppressed, and the furnace stop margin can be improved.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. Note that the same components as those of the above-described related art are denoted by the same reference numerals, and detailed description thereof will be omitted.

(First Embodiment) FIG. 1 is a view showing a first embodiment of the present invention, and FIG. 1 (a) is a cross-sectional view showing a fuel rod arrangement in a high burnup fuel assembly. (B) shows the axial distribution of fuel and burnable poisons in the fuel rods.

In the fuel assembly, the distribution of the concentration of fissile material as a fuel and the concentration of burnable poisons are given in the axial direction and the radial direction, but detailed distribution is omitted here. Will be explained. In the first embodiment, the fuel rods are arranged in nine rows and nine columns as in the case of applying to the fuel assembly 1 shown in FIG. Further, in this fuel assembly, a short fuel rod 3 which is about ／ shorter than a general long fuel rod 2 is used as a fuel rod.

That is, the fuel assembly 1 shown in FIG.
Numeral 0 denotes a control rod 11 which can be inserted into and removed from the reactor core and has a cross-shaped cross section. The uranium fuel rods are arranged in a square lattice of 9 rows and 9 columns, and are 66 long fuel rods. Fuel rod U
1 and nine long fuel rods U2, and 16 long Gd fuel rods G containing gadolinia (hereinafter abbreviated as Gd) as a burnable poison.

Further, it is constituted by disposing eight short fuel rods U3 and two large diameter water rods 4 (W).

Generally, in a high burn-up fuel assembly,
Although a natural uranium blanket is provided at the upper and lower ends in the axial direction in order to reduce the leakage of neutrons in the axial direction and increase the economic efficiency, as shown in FIG.
, U2, and the long Gd fuel rod G, the lower natural uranium blanket portion 12a is provided at the lower end in a region of 1/24 of the active fuel length, and the long fuel rod U1 and the long Gd fuel rod G are provided. Has an upper natural uranium blanket 12b in a region of 2/24 length at the upper end.

Further, for the long fuel rod U2, a low-reactivity region 12c in which a region 5/24 from the upper end of the fuel rod is made of natural uranium is provided, and a low-reactivity region 12c is provided.
The region immediately above the lower natural uranium blanket 12a than immediately below is a high-reactivity region containing more fissile material than natural uranium, the reactivity of the upper region of the long fuel rod U2 is reduced, and the upper low-reactivity fuel rod U2 There is.

The average of the fissile material enrichment in the fuel assembly 10 is as follows:
The high enrichment of 4.2%, which is higher than that of normal fuel.

Further, the number of the low-reactivity fuel rods U2 in the fuel assembly 10 is set to nine,
The cross-shaped control rod is disposed at a position facing two side surfaces where the control rod is not inserted.

FIG. 2 is a characteristic diagram of a cold-temperature reactor stop margin in a core loaded with the fuel assembly 10 configured as described above in a typical cold-temperature reactor stop margin.

Here, the curve 13 (solid line) represents the fuel assembly 1 in which the low reactivity region 12c is provided above the long fuel rod U2.
In the case of 0, the curve 14a (two-dot chain line)
No low-reactivity region is provided above the long fuel rod U2, and the upper natural uranium blanket 12b is provided in a region having a length of 2/24 at the upper end, and the region immediately below the upper natural uranium blanket is enriched uranium. In the case of the same axial region configuration as the normal long fuel rod U1, a curve 14b (dotted line) is further obtained.
Represents a case where a fuel assembly of a conventional design having an average enrichment of about 3.7% is loaded.

As shown in FIG. 2, the design target of the reactor shutdown margin is generally a fuel assembly of the conventional design having a cold shutdown margin of 1.0% Δk or more and an average enrichment of about 3.7%. When loaded, the design goal can be achieved, as shown by curve 14b. However, when the enrichment is further increased in order to increase the burnup, if the enrichment is generally about 4% or more, it becomes difficult for the furnace stop margin to satisfy the design target. Curve 14a when increasing to about 4.2%
As shown in the figure, it is understood that the design target of 1.0% Δk, which is the design target of the cold shutdown time, is not achieved in the period other than the middle stage of the cycle.

On the other hand, in the curve 13 in the case of the fuel assembly 10 in which the low reactivity region 12c is provided above the long fuel rod U2, the long fuel is provided without providing the low reactivity region above the long fuel rod U2. In the case of the same axial region configuration as the rod U1 (curve 1
Compared with 4a), the furnace shutdown allowance at cold temperature is 0.3 to 0.4.
% Δk increases, and the design target is sufficiently achieved.

Next, in the case of the fuel assembly 10 in which the low reactivity region 12c is provided on the upper part of the long fuel rod U2, the reason why the cold room shutdown margin improves as shown by the curve 13 will be described.

FIG. 3 is a graph showing the infinite multiplication factor characteristic of the infinite multiplication factor at the time of cold temperature. That is, FIG. 3 shows the effect of suppressing the reactivity by the control rod at the time of cold temperature, and the larger the value of the curve of FIG. Means that the reactivity is suppressed when "."

Here, the curve 15 is a cross section of the fuel assembly 10 in which the low reactivity region 12c is provided above the long fuel rod U2 (where the fuel effective length is 2 from the lower end).
(Cross section from 0/24 to 22/24) shows the combustion change when the infinite multiplication factor when the control rod is inserted at the cold temperature is subtracted from the infinite multiplication factor when the control rod is pulled at the cold temperature. Does not provide a low-reactivity region above the long fuel rod U2 of the fuel assembly 10, provides the upper natural uranium blanket 12b in a region having a length of 2/24 at the upper end, and provides a region immediately below the upper natural uranium blanket. Is the effective fuel length from 20/24 to 2 from the lower end when the enriched uranium has the same axial region configuration as the normal long fuel rod U1.
A similar combustion change in a section up to 2/24 is shown.

The cross section of the fuel assembly 10 provided with the low reactivity region 12c (from the lower end, the active fuel length of 20/24 to 22 /
24, the neutron flux around the fuel rod on the side opposite to the control rod when the control rod is withdrawn is reduced to the low reactivity region 12.
c is suppressed by natural uranium, and the neutron flux around the two side surfaces (control rod side) where the cross-shaped control rod is inserted is relatively increased. As a result, even when the infinite multiplication factor in the control rod withdrawal state at the cold temperature is the same, the infinite multiplication factor when the control rod is inserted is lower through combustion than when no natural uranium is arranged on the non-control rod side. And as shown in FIG.
Curve 15 is 2% Δk at the beginning of combustion compared to curve 16.
And even at a burnup of 20 GWd / t, it is higher by about 1% Δk.

The cold reactor shutdown margin represents the subcriticality when one control rod having the highest reactivity control effect is pulled out with all the control rods of the core inserted. As shown by the curve 15 in FIG. 3, in the case of fuel having a large reactivity suppression effect by the control rods at the time of cold temperature, the effective multiplication factor of the core becomes lower when all the control rods of the core are inserted. One control rod with large degree control effect
Even if this is pulled out, the subcriticality increases,
Furnace shutdown margin is improved. The curves in FIG. 3 show the characteristics of the cross section provided with the low reactivity region 12c. In the cross sections other than the cross section provided with the low reactivity region 12c, the control rod withdrawn state and the control rod inserted state at the time of cold temperature are shown. Since the difference between the infinite multiplication factors is not improved, even if the curve 15 in FIG. 3 is higher than the curve 16 by 1 to 2% Δk, the core shutdown margin at the time of the cold temperature of the core should be improved to 1 to 2% Δk. Is not
As shown in FIG. 2, the improvement is 0.3 to 0.4% Δk.

For the above reasons, the long fuel rod U2
In the case of the fuel assembly 10 in which the low reactivity region 12c is provided on the upper part of the fuel cell, the cold-temperature furnace shutdown margin is improved as shown by the curve 13 in FIG. Note that the reason why the axial position where the low reactivity region 12c is provided is the upper portion of the long fuel rod U2 is that the power distribution is higher in the upper portion of the core at the time of cold temperature, and the reactivity suppression effect by the control rod is more significant. This is because it effectively contributes to improving the reactor shutdown margin of the core.

In the example shown in FIG. 1, the upper low-reactivity fuel rod U2 is disposed on the outermost periphery of the fuel assembly on the side opposite to the control rod. It is not necessarily the outermost circumference on the side opposite to the control rod. If the fuel rod is located near the outermost periphery of the non-control rod side, the neutron flux on the control rod side at the time of control rod withdrawal increases, and the same effect as the curve 15 in FIG. 3 can be obtained. In FIG. 1, as an example that the neutron flux at the control rod side at the time of control rod withdrawal can be increased most effectively and the reactor shutdown margin can be improved, the upper low-reactivity fuel rod U2 is arranged at the outermost peripheral position on the anti-control rod side. An example is shown.

Further, as the number of the upper low-reactivity fuel rods U2 in the fuel assembly 10 increases, the neutron flux on the control rod side at the time of control rod withdrawal increases, and the effect of improving the reactor shutdown margin increases. However, from the viewpoint of increasing the burnup, as the number of the upper low-reactivity fuel rods U2 increases, the average enrichment of the fuel assembly decreases, which is not preferable. Therefore, it is better not to increase the number of upper low-reactivity fuel rods U2 more than necessary. In the example of FIG. 1, the number of upper low-reactivity fuel rods U2 is nine.

As described above, the upper low-reactivity fuel rod U
In the core loaded with the fuel assembly 10 in which the fuel cells 2 are arranged, it is not necessary to increase the content of the burnable poison to improve the reactor shutdown margin, so that the reactor shutdown margin is improved without impairing the fuel economy. Is done. Alternatively, if an excessive furnace shutdown margin is obtained by arranging the upper low-reactivity fuel rod U2, the content of the burnable poison can be reduced within a range that can satisfy the furnace shutdown margin. Reactivity loss at the end of the cycle due to residual toxicants can be eliminated, and fuel economy is improved.

The fuel assembly 10 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The first embodiment is an example in which the fuel rod arrangement is 9 rows and 9 columns, there is a short fuel rod U3, and the fuel rod arrangement is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low reactivity fuel rod U2 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
FIGS. 3A and 3B are diagrams showing an embodiment of the present invention, in which FIG. 3A is a cross-sectional view showing a fuel rod arrangement in a high burn-up fuel assembly, and FIG. 3B is an axial distribution diagram of fuel and contained burnable poisons of the fuel rods. Show. The detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 17 is an example in which the fuel assembly 17 is applied to the fuel assembly 1 shown in FIG. 13. As the 66 long fuel rods, 41 long fuel rods U1 and 9 long fuel rods U4 are used. , And 16 long Gd fuel rods G containing the burnable poison Gd.

Further, it is constituted by eight short fuel rods U3 and two large diameter water rods 4 (W).

As shown in FIG. 4 (b), the long fuel rods U1, U4 and the long Gd fuel rod G have a lower natural portion at a lower end portion having a length of 1/24 of the active fuel length. A uranium blanket portion 12a is formed, and an upper natural uranium blanket 12b is formed in a region having a length of 1/24 at an upper end portion.

Further, for the long fuel rod U4, there is provided a low reactivity region 12c in which a region having a length of 4/24 from an effective fuel length of 22/24 to 19/24 from the lower end of the fuel rod is made of natural uranium. At the same time, the area immediately above the lower natural uranium blanket 12a than immediately below the low reactivity area 12c is defined as a high reactivity area containing more fissile material than natural uranium, and the reactivity of the upper area of the long fuel rod U4 is reduced. The upper low-reactivity fuel rod U4 is used.

The average fissile material enrichment in the fuel assembly 17 is as follows:
The high enrichment of 4.2%, which is higher than that of normal fuel. The upper low-reactivity fuel rods U in the fuel assembly 17
The number 4 is nine, and it is arranged at the outermost periphery of the fuel assembly at a position facing two side surfaces where the cross-shaped control rod is not inserted. Here, the low-reactivity region 12c of the upper low-reactivity fuel rod U4 in the fuel assembly 17 is formed in a region including a position where the axial power distribution of the core at the time of cold temperature is the largest.

FIG. 5 is an axial power distribution diagram, in which the curve 1
8 shows the axial power distribution of the core at the time of cold temperature. As shown by the curve 18, at the time of cold temperature, the power distribution is high in the upper part of the core, and in this example, it is maximum at the position of the active fuel length 22/24 from the lower end.
In a boiling water reactor, voids are formed in the water, which is a coolant, in the core during power operation, and the voids have a higher void fraction toward the top, and therefore, due to the negative reactivity effect due to the formed voids, The output of the lower part tends to be relatively larger than that of the upper part. Therefore, a design has been designed to optimize the axial enrichment and burnable poison distribution so as to flatten the axial power distribution during the power operation. In this case, the output distribution is increased.

In the fuel assembly 17 configured as described above,
Similarly to the fuel assembly 10, the reactivity suppression effect of the control rod at the time of cold temperature in the cross section provided with the low reactivity region 12c of the upper low reactivity fuel rod U4 is similar to that of the lower low reactivity fuel rod U4. Since it becomes larger than the case where 12c is not provided, the furnace stop margin is improved. Further, the longer the length of the low-reactivity region 12c of the low-reactivity fuel rod U4, the greater the effect of improving the furnace shutdown margin becomes. However, from the viewpoint of high burnup, the length of the low-reactivity region 12c is reduced. As the fuel cell length becomes longer, the average enrichment of the fuel assembly becomes lower, which is not preferable. For this purpose, the low-reactivity region 12c is provided only in the region where the power distribution is the largest at the time of cold temperature, as in the upper low-reactivity fuel rod U4 in FIG. 4B, without impairing the fuel economy. The furnace shutdown margin can be effectively improved.

As described above, the upper low-reactivity fuel rod U
In the core loaded with the fuel assembly 17 in which the fuel cells 4 are arranged, it is not necessary to increase the content of burnable poisons in order to improve the reactor shutdown margin, so that the reactor shutdown margin is improved without impairing fuel economy. I do. Alternatively, if an excessive furnace shutdown margin is obtained by arranging the upper low-reactivity fuel rod U4, the content of the burnable poison can be reduced within a range that can satisfy the furnace shutdown margin. Reactivity loss at the end of the cycle due to residual toxicants can be eliminated, and fuel economy is improved.

The fuel assembly 17 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G, and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The second embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low-reactivity fuel rod U4 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Third Embodiment) FIGS. 6A and 6B are views showing a third embodiment, in which FIG. 6A is a cross-sectional view showing a fuel rod arrangement in a high burn-up fuel assembly, and FIG. 2 shows an axial distribution diagram of the fuel and the burnable poison contained in the fuel rod. In addition,
Detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 19 includes 48 long fuel rods U1 and two long fuel rods U5 as 66 long fuel rods.
One long Gd fuel rod G containing Gd, which is a burnable poison,
Six, eight short fuel rods U3, and two large diameter water rods 4 (W) are arranged.

As shown in FIG. 6 (b), the long fuel rods U1, U5 and the long Gd fuel rod G have a lower end in an area of 1/24 of the active fuel length. An upper natural uranium blanket 12b is formed on the natural uranium blanket portion 12a, and on the long fuel rod U1 and the long Gd fuel rod G in a region of 1/24 at the upper end.

Further, as for the long fuel rod U5, a low-reactivity region 12c in which a region having a length of 13/24 from the upper end of the fuel rod is made of natural uranium is provided.
An upper low-reactivity fuel rod in which the area immediately above the lower natural uranium blanket 12a than immediately below c is a high-reactivity area containing more fissile material than natural uranium, and the upper area of the long fuel rod U5 has low reactivity. U5.

The assembly-average fissile material enrichment in the fuel assembly 19 is determined in order to achieve a high burnup.
The high enrichment of 4.2%, which is higher than that of normal fuel. Further, the upper low-reactivity fuel rod U in the fuel assembly 19 is
The number of the fuel rods 5 is two, and the fuel rods are arranged at the outermost periphery of the fuel assembly at a position adjacent to the fuel rod at the corner where the cross control rod is not inserted.

FIG. 7 shows a typical change in combustion of the cold shutdown margin in the core loaded with the fuel assembly 19 configured as described above in a cold shutdown margin. here,
Curve 20 shows a case where two upper low-reactivity fuel rods U5 are arranged at positions on the outermost periphery of the fuel assembly adjacent to the fuel rods at the corner positions where the cross-shaped control rods are not inserted.
Is the same as the curve 14a in FIG. 2 and does not include the upper low-reactivity fuel rod U5 of the fuel assembly 19;
It represents the case where it is set to 1.

As shown in FIG. 7, the enrichment is increased for high burnup without providing the upper low reactivity fuel rod U5,
In the case of the curve 14a raised to about 4.2%, the design target of 1.0%, which is the design target of the cold shutdown time, is not achieved, and the lower low reactivity fuel rod U5 is
In the curve 20 when two fuel rods are arranged adjacent to the fuel rod at the corner position where the cross control rod is not inserted in the outermost periphery of the fuel assembly, the cold / hot furnace stop margin is about 0 in comparison with the curve 14a. .4%, which is well within the design target.

In the fuel assembly 19, two low-reactivity fuel rods U5 at the positions adjacent to the fuel rods at the corners where the cross-shaped control rods are not inserted are used as upper low-reactivity fuel rods U5.
In the axial section provided with uranium, natural uranium is disposed on the two opposite control rod side fuel rods farthest from the control rod insertion position, and two neutron fluxes around the control rod side in the control rod withdrawn state. The upper low reactivity fuel rod U5 effectively increases the fuel rod. As a result, even if the infinite multiplication factor in the control rod withdrawal state at the time of cold temperature is the same, the infinite multiplication factor when the control rod is inserted is lower than when the natural uranium is not arranged on the anti-control rod side, Furnace shutdown margin is improved.

The length of the low-reactivity region 12c in the upper low-reactivity fuel rod U5 of the fuel assembly 19 is set to 13/24 of the active fuel length. Low reactivity area 12 at the low fuel rod U2 and the upper low reactivity fuel rod U4 of FIG. 4 in the second embodiment.
c is longer than the upper low-reactivity fuel rod U5.
Is two, which is less than nine in the first and second embodiments. The cross section of the fuel assembly 19 where the low reactivity region 12c is provided (effective fuel length 12/24 to 23/24 from the lower end).
FIG. 8 shows an infinite multiplication factor characteristic diagram of FIG. 8 showing the combustion change of the difference between the control rod withdrawal state and the control rod insertion state in the infinite multiplication factor at the cold temperature in the cross section up to FIG. 8, a curve 21 is the characteristic of the cross section, and a curve 16 is the same as the curve 16 in FIG. As shown in FIG. 8, in the fuel assembly 19, the number of upper low-reactivity fuel rods U5 is set to two,
The difference between the curves 21 and 16 is reduced. However, even if the number of upper low-reactivity fuel rods U5 is small, by increasing the length of the low-reactivity region 12c, the reactor stop margin of the reactor core can be reduced as compared with the first embodiment and the second embodiment. Similar effects can be obtained. Upper low reactivity fuel rod U
When the length of the low-reactivity region 12c of FIG. 5 is further increased, the effect of improving the reactor shutdown margin increases, but the power distribution at the time of cold temperature is low at the lower part of the core, so the low-reactivity region 12c extends to the lower part of the core. The effect of improving the furnace shutdown margin does not improve as the length of the region in the upper part is increased even if the length is increased, and from the viewpoint of increasing the burnup, the low reactivity region 1
As the length of 2c becomes longer, the average enrichment of the fuel assembly becomes lower, which is not preferable.

As described above, the upper low-reactivity fuel rod U
In the core loaded with the fuel assembly 19 in which the fuel cell 5 is disposed, it is not necessary to increase the content of the burnable poison in order to improve the reactor shutdown margin, so that the reactor shutdown margin is improved without impairing the fuel economy. I do. Alternatively, if an excessive furnace shutdown margin is obtained by arranging the upper low-reactivity fuel rod U5, the content of the burnable poison can be reduced within a range that can satisfy the furnace shutdown margin. Reactivity loss at the end of the cycle due to residual toxicants can be eliminated, and fuel economy is improved.

The fuel assembly 19 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The third embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low-reactivity fuel rod U5 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Fourth Embodiment) FIGS. 9A and 9B are diagrams showing a fourth embodiment of the present invention. FIG. 9A is a cross-sectional view showing the arrangement of fuel rods in a high burnup fuel assembly. (B) shows an axial distribution diagram of the fuel and the burnable poison contained in the fuel rod. The detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 22 has 49 long fuel rods U1 and one long fuel rod U6 as 66 long fuel rods.
One long Gd fuel rod G containing Gd, which is a burnable poison,
Six, eight short fuel rods U3, and two large diameter water rods 4 (W) are arranged.

As shown in FIG. 9 (b), the long fuel rods U1, U6 and the long Gd fuel rod G have a lower natural portion at a lower end portion having a length of 1/24 of the active fuel length. An upper natural uranium blanket 12b is formed on the upper end of the uranium blanket portion 12a, and on the long fuel rod U1 and the long Gd fuel rod G in a region of length 1/24.

Further, as for the long fuel rod U6, a low reactivity region 12c in which a region having a length of 13/24 from the upper end of the fuel rod is made of natural uranium is provided.
An upper low-reactivity fuel rod, in which the area immediately above the lower natural uranium blanket 12a than immediately below c is a high-reactivity area containing more fissile material than natural uranium, the upper area of the long fuel rod U6 has low reactivity. U6.

The average of the fissile material enrichment of the fuel assembly 22 in the fuel assembly 22 is as follows.
The high enrichment of 4.2%, which is higher than that of normal fuel. The upper low-reactivity fuel rods U in the fuel assembly 22
The number 6 is one, and it is arranged at the corner position where the cross-shaped control rod is not inserted in the outermost periphery of the fuel assembly.

In the fuel assembly 22, the number of upper low-reactivity fuel rods U6 is further reduced from two in the third embodiment (FIG. 6) to one, but the position is cross-shaped. By setting one of the corner positions where the mold control rod is not inserted, in the axial cross section provided with the low reactivity region 12c, natural uranium is contained in one of the fuel rods on the opposite side to the control rod farthest from the control rod insertion position. The same effect as in the case where the number of upper low-reactivity fuel rods U6 is two is obtained. That is, the neutron flux around the control rod side in the control rod withdrawal state is most effectively increased by one upper low-reactivity fuel rod U6, and as a result, the infinite multiplication factor in the control rod withdrawal state at the cold temperature is the same. Even in this case, the infinite multiplication factor when the control rod is inserted is lower than when no natural uranium is arranged on the side opposite to the control rod, and the furnace stop margin is improved as in the third embodiment. Further, since the number of upper low-reactivity fuel rods U6 is a minimum of one, the average enrichment of the fuel assembly can be increased as much as possible from the aspect of high burnup, and the fuel economy is improved. It is valid.

As described above, the upper low-reactivity fuel rod U
In the core loaded with the fuel assembly 22 in which the fuel cell 6 is disposed, it is not necessary to increase the content of the burnable poison in order to improve the reactor shutdown margin, so that the reactor shutdown margin is improved without impairing the fuel economy. I do. Alternatively, if an excessive furnace shutdown margin is obtained by arranging the upper low-reactivity fuel rod U6, the content of the burnable poison can be reduced within a range that can satisfy the furnace shutdown margin. Reactivity loss at the end of the cycle due to residual toxicants can be eliminated, and fuel economy is improved.

The fuel assembly 21 is an example of a high-burnup fuel assembly designed to achieve a high burnup. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The fourth embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which the natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low-reactivity fuel rod U6 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Fifth Embodiment) FIGS. 10A and 10B show a fifth embodiment of the present invention. FIG. 10A is a cross-sectional view showing a fuel rod arrangement in a high burnup fuel assembly. (B) shows an axial distribution diagram of the fuel and the burnable poison contained in the fuel rod. The detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 23 includes 49 long fuel rods U1 and one long fuel rod U7 as 66 long fuel rods.
One long Gd fuel rod G containing Gd, which is a burnable poison,
Six, eight short fuel rods U3, and two large diameter water rods 4 (W) are arranged.

As shown in FIG. 10 (b), the long fuel rods U1, U7 and the long Gd fuel rod G have a lower natural portion at a lower end portion having a length of 1/24 of the active fuel length. An upper natural uranium blanket 12b is formed on the upper end of the uranium blanket portion 12a, and on the long fuel rod U1 and the long Gd fuel rod G in a region of length 1/24.

Further, as for the long fuel rod U7, a low reactivity region 12c in which the enrichment of uranium-235 is 1.0 wt% is provided in a region 13/24 of the length from the upper end of the fuel rod, and a low reaction region 12c is provided. The uranium-235 enrichment in the region immediately below the lower natural uranium blanket 12a than immediately below the concentration region 12c is higher than 1.0 wt%, and as a high reactivity region containing more fissile material, the upper portion of the long fuel rod U7 The upper low-reactivity fuel rod U7 has a lower reactivity in the region.

In the fuel assembly 23, the average fissile material enrichment in the assembly is determined in order to achieve a high burnup.
The high enrichment of 4.2%, which is higher than that of normal fuel. The upper low-reactivity fuel rod U in the fuel assembly 23
The number 7 is one, and it is arranged at the corner position where the cross control rod is not inserted in the outermost periphery of the fuel assembly.

Fuel rod U7 at the upper part of the fuel assembly 23
In the low reactivity region 12c, the enrichment of uranium-235 is set to 1.0 wt%, and if the enrichment is about 1 wt% or less, the same effect as in the case where the uranium-235 is made of natural uranium is used to improve the furnace shutdown margin. it can.

The fuel assembly 23 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The fifth embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low-reactivity fuel rod U7 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Sixth Embodiment) FIG. 11 is a view showing a sixth embodiment of the present invention. FIG. 11A is a cross-sectional view showing the arrangement of fuel rods in a high burnup fuel assembly. (B) shows an axial distribution diagram of the fuel and the burnable poison contained in the fuel rod. The detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 24 has 49 long fuel rods U1 and one long fuel rod U8 as 66 long fuel rods.
One long Gd fuel rod G containing Gd, which is a burnable poison,
Six, eight short fuel rods U3, and two large diameter water rods 4 (W) are arranged.

As shown in FIG. 11 (b), the long fuel rods U1 and U8 and the long Gd fuel rod G have a lower natural portion at a lower end portion having a length of 1/24 of the active fuel length. An upper natural uranium blanket 12b is formed on the upper end of the uranium blanket portion 12a, and on the long fuel rod U1 and the long Gd fuel rod G in a region of length 1/24.

Further, as for the long fuel rod U8, a low reactivity region 12c is provided in which a region having a length of 13/24 from the upper end of the fuel rod is recovered uranium, and the low reactivity region 12c is provided.
The area immediately above the lower natural uranium blanket 12a than immediately below the area c is regarded as a normal enriched uranium area, and the long fuel rod U8 is used.
Low-reactivity fuel rods U having reduced reactivity in the upper region of
It is set to 8.

In the fuel assembly 24, the average fissile material enrichment in the assembly is determined in order to achieve a high burnup.
The high enrichment of 4.2%, which is higher than that of normal fuel. The upper low-reactivity fuel rods U in the fuel assembly 24
The number 8 is one, and it is arranged at the corner position where the cross control rod is not inserted in the outermost periphery of the fuel assembly.

The fuel rod U8 at the upper part of the fuel assembly 24 has a low enrichment.
Is a recovered uranium, and the reactivity of the recovered uranium is as low as that of natural uranium. Therefore, the same effect as when the region is made of natural uranium can improve the furnace shutdown margin.

The fuel assembly 24 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rod U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

Further, the sixth embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. Is not limited to this, and the same effect can be obtained by arranging the upper low-reactivity fuel rod U8 in other designs such as the fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns. Can be

(Seventh Embodiment) FIGS. 12A and 12B are diagrams showing a seventh embodiment of the present invention. FIG. 12A is a cross-sectional view showing a fuel rod arrangement in a high burn-up fuel assembly. (B) shows an axial distribution diagram of the fuel and the burnable poison contained in the fuel rod. The detailed description of the same components as those in the first embodiment will be omitted, and different components will be described.

The fuel assembly 25 includes 49 long fuel rods U1 and one long fuel rod U9 as 66 long fuel rods.
One long Gd fuel rod G containing Gd, which is a burnable poison,
Six, eight short fuel rods U3, and two large diameter water rods 4 (W) are arranged.

As shown in FIG. 12 (b), the long fuel rods U1, U9 and the long Gd fuel rod G have a lower end in an area of 1/24 of the active fuel length at the lower end. An upper natural uranium blanket 12b is formed on the natural uranium blanket portion 12a, and on the long fuel rod U1 and the long Gd fuel rod G in a region of 1/24 at the upper end.

Further, with respect to the long fuel rod U9, a low reactivity region 12c in which a region 13/24 from the upper end of the fuel rod is depleted uranium is provided, and the low reactivity region 12c is provided.
The area immediately above the lower natural uranium blanket 12a than immediately below the area c is regarded as a normal enriched uranium area, and the long fuel rod U9 is used.
Low-reactivity fuel rods U having reduced reactivity in the upper region of
Nine.

In the fuel assembly 25, the average fissile material enrichment in the assembly is determined in order to achieve a high burnup.
The high enrichment of 4.2%, which is higher than that of normal fuel. The upper low-reactivity fuel rods U in the fuel assembly 25
The number 9 is one, and it is arranged at the corner position where the cross control rod is not inserted in the outermost periphery of the fuel assembly.

Fuel rod U9 of the upper low enrichment of fuel assembly 25
The low-reactivity region 12c is made of depleted uranium, and the reactivity of depleted uranium is lower than that of natural uranium. Therefore, the furnace shutdown margin can be more effectively improved than when the region is made of natural uranium.

The fuel assembly 25 is an example of a high burn-up fuel assembly designed to achieve high burn-up. Therefore, the long fuel rod U1, the long Gd fuel rod G and the water The length, number, shape and arrangement of the rods 4, the short fuel rods U3 and the natural uranium blanket 12
The presence and length of a and 12b vary depending on the design.

The seventh embodiment is an example in which the fuel rod array has 9 rows and 9 columns, has short fuel rods U3, and is applied to a fuel in which natural uranium blanket portions 12a and 12b are installed. The characteristics are not limited to this,
In other designs such as a fuel rod arrangement of 8 rows and 8 columns or 10 rows and 10 columns, the same effect can be obtained by disposing the upper low reactivity fuel rod U9.

[0103]

As described above, the present invention relates to a fuel assembly for a boiling water reactor, which is constituted by bundling a plurality of fuel rods in a square lattice, and having a fuel active length of 4 /
A low-reactivity region having a length of 24 or more and 13/24 or less is provided, and a region immediately below the low-reactivity region is a high-reactivity region containing more fissile material than the low-reactivity region to lower the reactivity of the fuel rod upper region. The low-reactivity fuel rod has a high reactivity with the upper low-reactivity fuel rod. Improve the margin. Also,
If an excess reactor shutdown margin is obtained by arranging the upper low-reactivity fuel rods, the content of the burnable poison can be reduced within a range that can satisfy the reactor shutdown margin. As a result, the reactivity loss at the end of the cycle can be eliminated, and the fuel economy can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a fuel assembly according to a first embodiment of the present invention, in which (a) is a cross-sectional view of a fuel rod arrangement, and (b) is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

FIG. 2 is an explanatory diagram of a cold-time furnace stop margin characteristic with respect to a burnup.

FIG. 3 is an explanatory diagram of characteristics of a difference between a control rod withdrawn state and a control rod inserted state at an infinite multiplication factor at a cold temperature.

4A and 4B show a fuel assembly according to a second embodiment of the present invention, wherein FIG. 4A is a cross-sectional view of a fuel rod arrangement, and FIG. 4B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

FIG. 5 is an explanatory view of an axial output distribution in the second embodiment.

6A and 6B show a fuel assembly according to a third embodiment of the present invention, wherein FIG. 6A is a cross-sectional view of a fuel rod arrangement, and FIG. 6B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

FIG. 7 is a diagram showing a furnace stop margin characteristic at a cold temperature with respect to burnup in the third embodiment.

FIG. 8 is an explanatory diagram of a characteristic of a difference between a control rod pulled-out state and a control rod inserted state at an infinite multiplication factor at a cold temperature in the third embodiment.

9A and 9B show a fuel assembly according to a fourth embodiment of the present invention, wherein FIG. 9A is a cross-sectional view of a fuel rod arrangement, and FIG. 9B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

10A and 10B show a fuel assembly according to a fifth embodiment of the present invention, wherein FIG. 10A is a cross-sectional view of a fuel rod arrangement, and FIG. 10B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

11A and 11B show a fuel assembly according to a sixth embodiment of the present invention, wherein FIG. 11A is a cross-sectional view of a fuel rod arrangement, and FIG. 11B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

12A and 12B show a fuel assembly according to a seventh embodiment of the present invention, wherein FIG. 12A is a cross-sectional view of a fuel rod arrangement, and FIG. 12B is an axial distribution diagram of fuel and burnable poisons of the fuel rod.

13A and 13B are views showing a conventional fuel assembly, wherein FIG. 13A is a longitudinal sectional view, FIG. 13B is a sectional view taken along the line BB in FIG. 13A,
(C) is a sectional view taken along the line CC in (a).

[Explanation of symbols]

1,10,17,19,22,23,24,25 Fuel assembly 2, U1, U2, U4-U9 Long fuel rod 3, U3 Short fuel rod 4 Water rod (W) 5 Spacer 6 External spring 7 Upper part Tie plate 8 Lower tie plate 9 Channel box G Long Gd fuel rod 11 Control rod 12a Lower natural uranium blanket part 12b Upper natural uranium blanket part 12c Low reactivity region 13, 15, 16, 18, 20, 21 Curve (solid line) 14a curve (two-dot chain line) 14b curve (dotted line)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Jun Saeki 2-3-1 Kawa, Yokosuka City, Kanagawa Prefecture Inside Niuclear / Fuel Co., Ltd. (72) Inventor Ryoji Masumi 2-chome River, Yokosuka City, Kanagawa Prefecture No.3-1 Inside Niuclear Huyell Co., Ltd. (72) Inventor Yuko Haraguchi 2-3-1-1, Kawaguchi, Yokosuka City, Kanagawa Prefecture Inside Niuclear Huyell Japan Co., Ltd.

Claims (11)

[Claims] 1. A fuel assembly for a boiling water reactor, wherein a plurality of fuel rods are bundled in a square lattice to form a fuel assembly having a length not less than 4/24 and not more than 13/24 of an active fuel length at an upper portion in an axial direction. A low-reactivity fuel rod having an upper low-reactivity fuel rod having a low-reactivity region and having a region immediately below the lower-reactivity region as a high-reactivity region containing more fissile material than the low-reactivity region and having lower reactivity in the fuel rod upper region. A fuel assembly, characterized in that: 2. An upper low-reactivity fuel rod according to claim 1, wherein the low-reactivity region is formed in a region including a position at which the axial power distribution of the core at the time of cold temperature becomes maximum.
The fuel assembly according to claim 1. 3. The fuel assembly according to claim 1, wherein the number of the upper low-reactivity fuel rods is nine or less. 4. The fuel assembly according to claim 1, wherein the upper low-reactivity fuel rod is disposed at an outermost periphery of the fuel assembly. 5. The upper low-reactivity fuel rod is located at a position facing two side surfaces of the outermost periphery of the fuel assembly where the cross control rod is not inserted. 4. The fuel assembly according to any one of the above items 3. 6. The upper low-reactivity fuel rods are arranged in two positions on the outermost periphery of the fuel assembly adjacent to the fuel rods at the corner positions where the cross control rods are not inserted. The fuel assembly according to claim 1. 7. The fuel assembly according to claim 1, wherein one of the upper low-reactivity fuel rods is disposed at a corner position where a cross-shaped control rod is not inserted. 8. The fuel assembly according to claim 1, wherein the low-reactivity region of the upper low-reactivity fuel rod is made of natural uranium. 9. The method according to claim 1, wherein the low-reactivity region of the upper low-reactivity fuel rod has an enrichment of uranium-235 of about 1 wt% or less. Fuel assembly. 10. The method according to claim 1, wherein the low-reactivity region of the upper low-reactivity fuel rod is recovered uranium obtained by reprocessing spent fuel. Fuel assembly. 11. The low-reactivity region of the upper low-reactivity fuel rod is depleted uranium obtained from waste material in a U-235 enrichment step. A fuel assembly according to any of the above.