JP2002048886A - Fuel assembly for boiling water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly for a boiling water reactor capable of limiting a shutdown margin and the minimum critical power ratio within an allowable range, ensuring thermo-mechanical integrity of fuel rods containing burnable poison and increasing operational margin of maximum linear power density and proper for attaining prolongation of operational cycle and high burnup without increasing the load of pellet charging work in production process. SOLUTION: Fuel rods not including burnable poison among a multitude of fuel rods constituting fuel assemblies for a boiling water reactor having a matrix grid of 9×9 or more arranged in square grid of fuel rods and average enrichment of 4.0 wt.% or more contain first fuel rods charged with fuel pellets of lower enrichment in the upper region than those in the lower region of effective heating length, and a second fuel rods charged with fuel pellets of higher enrichment in the upper region than those in the lower region. The boundary between the upper and the lower regions of the first fuel rods is positioned upper than the middle and the boundary between the upper and the lower regions of the second fuel rods is positioned lower than the middle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor, and more particularly to a fuel pellet enrichment distribution in a fuel rod constituting a fuel assembly aimed at high burnup. It is about.

[0002]

2. Description of the Related Art A boiling water reactor (hereinafter referred to as BWR)
Then, in order to improve the economic efficiency, the operation cycle is extended with the aim of economic effects by improving the plant utilization rate (operating rate), and the energy taken out per fuel assembly is increased to improve the economic efficiency. Aiming to increase the average withdrawal burnup is known as an effective means.

[0003] For these two purposes, uranium 235
Increase the average enrichment of fissile material represented by
Increasing the concentration of burnable poisons represented by gadolinia has become an effective and indispensable means. At present, a fuel assembly having a 9 × 9 lattice is one of the candidates, and has a configuration in which water rods occupying an area equivalent to a plurality of fuel rods are arranged in a fuel rod arrangement of 9 rows and 9 columns. This fuel assembly is
In particular, it has the following features, and is suitable for the purpose of improving economic efficiency as described above.

(1) A 9 × 9 grid fuel assembly can increase the number of fuel rods by about 20% as compared with the preceding 8 × 8 grid fuel assembly, so that the average linear power density is reduced. Further, since the total surface area of the fuel rod can be increased, the output (critical output) until the fuel rod reaches the boiling transition is improved. The above two points are useful for securing operation margins against thermal limit values such as maximum linear power density and minimum limit power during operation.

(2) In general, when the average enrichment of a fuel assembly is increased, the spectrum becomes harder, and as a result, the void reactivity coefficient increases to the negative side. For this reason, during operation, the reactivity of the upper portion of the fuel having a high void ratio is lower than that of the lower portion of the fuel having a low void ratio, so that the axial power distribution tends to be distorted to the lower portion as the enrichment is increased. . An increase in the output below the fuel is accompanied by an increase in the maximum linear power density, and the thermal operating margin is reduced. A water rod occupying a region corresponding to a plurality of fuel rods used for 9 × 9 fuel forms a specific boiling region having a large area, so that the axial change of the water to uranium ratio in the axial direction is small, and the axial output is small. The distribution can be flattened, and as a result, an increase in the maximum linear output density can be suppressed.

As shown in the arrangement diagram of FIG. 8A, in such a fuel assembly 200 having a 9 × 9 lattice arrangement, a conventional one in which water rods are arranged in an area 201 corresponding to nine fuel rods is as follows. As a technology to achieve high enrichment under the limitation of the maximum pellet enrichment of 5 wt% or less, assembling aiming at a maximum operation cycle of up to 18 months as a long operation cycle and aiming at an average extraction burnup of about 50 GWd / t as a target of high burnup Body average concentration 4w
The fuel assemblies were more than t%.

In this prior art fuel assembly 200, as shown in FIG. 8 (b), a type 101 fuel rod in which the entire area excluding the low enrichment pellet blanket portions at the upper and lower ends is filled with the highest enrichment fuel pellets. In addition, a type 102 fuel rod in which most of the lower region is filled with the highest enrichment fuel pellets and most of the upper region is filled with the lower enrichment fuel pellets is used as a corner rod. Type 10 in which the entire area other than the blanket is filled with fuel pellets having a lower enrichment than the arranged maximum enrichment
3 The gadolinia is placed in the arrangement position adjacent to both the fuel rods and the water gap, and at the arrangement position adjacent to this type 102 fuel rod and not laterally adjacent to the water gap, at least in part or most of the lower region. A type G101 fuel rod filled with pellets is arranged.

The aim of this prior art is to secure high enrichment to ensure safety. One of its features is that in the lower region where the linear output density tends to be large, the type 102 is used.
By preventing the power peaking of the fuel rod from becoming excessive in the early stage of combustion and from becoming too low in the middle stage of combustion, the maximum linear power density during operation is kept within an allowable range, and the line of the fuel rod containing gadolinia is controlled. By reducing the power density relative to the linear power density of uranium fuel rods, FP gas is released due to a rise in temperature due to a decrease in the thermal conductivity of gadolinia-containing fuel pellets even when a high concentration of gadolinia is added. The purpose is to ensure thermo-mechanical integrity so that the rate does not increase and the fuel rod internal pressure does not increase. Further, in the case of the conventional example, in the fuel rod 102 disposed at a position having a high relative output, the enrichment in the upper region is lower than the maximum enrichment, and therefore, it is necessary to suppress a decrease in the critical output leading to a boiling transition. Can be. The details of the above prior art are shown in Japanese Patent Application No. 2000-058878.

[0009]

However, in the case of the above-described prior art for the purpose of increasing the burnup, the maximum linear power density during operation remains within an allowable range, but when the initial combustion is focused on, The linear power density tends to have a smaller operating margin than in the middle cycle. This means that high-concentration gadolinia can be added, and fuel pellets with high-concentration gadolinia added are arranged below the effective heat generating portion to suppress the reactivity of the lower fuel in the middle stage of combustion, This is because sufficient measures have not been taken against the initial stage of combustion in which the maximum linear power density increases at the beginning of the cycle. Further, it has been found that there is a disadvantage that the axial power distribution tends to be unstable during the operation because there is almost no concentration difference in the axial direction.

To determine the enrichment and gadolinia distribution in the fuel assembly, it is necessary to: Economic requirements (high burnup,
1. The operation cycle is prolonged. 2. Safety requirements (thermal operation margin, stop margin, etc.); 3. In addition to the demands on the thermo-mechanical properties of the fuel rods (such as suppressing internal pressure rise of fuel rods containing gadolinia). It is necessary to design to satisfy various requirements such as manufacturing requirements.

The maximum enrichment limit (5 wt% or less) of fuel pellets is currently determined from the viewpoint of criticality control in the manufacturing process, but other manufacturing requirements include the following factors. That is, in the fuel assembly manufacturing operation, there is an operation of filling fuel pellets with different enrichment and gadolinia addition amounts into the fuel cladding tube, but this operation is usually performed for each type of fuel, When there are many types of fuel pellets to be filled in one type of fuel rod, not only does the work efficiency decrease, but also the risk of erroneous insertion of the types of pellets increases. In order to avoid this danger, it is desirable that as the number of fuel rods increases, particularly in the case of a fuel assembly having a 9 × 9 grid arrangement or more, the number of types of fuel pellets to be filled in each fuel rod is reduced.

From the above viewpoints, the design of the enrichment and gadolinia distribution in the fuel assembly is complicated, and even if the design sufficiently satisfies the above-mentioned first to third requirements, 4. Obviously, a well-balanced design cannot be achieved unless manufacturing requirements are met.

SUMMARY OF THE INVENTION In view of the above problems, the present invention keeps the stop margin and the minimum limit power ratio within an allowable range, secures the thermomechanical soundness of the burnable poison-containing fuel rod, and reduces the maximum linear power density. An object of the present invention is to provide a fuel assembly for a boiling water reactor suitable for achieving a long operation cycle and high burnup without increasing the burden of pellet filling work in the manufacturing process, while increasing the operating margin. I do.

[0014]

According to a first aspect of the present invention, there is provided a fuel assembly for a boiling water reactor, wherein fuel pellets having a predetermined nuclear fuel enrichment are contained in a cladding tube. A boiling water reactor having an average enrichment of 4 wt% or more, in which a predetermined number of fuel rods which are filled and sealed are supported at both upper and lower ends by an upper tie plate and a lower tie plate as a bundle in a predetermined square lattice arrangement. In the fuel assembly for fuel use, the fuel rods containing no burnable poison of the plurality of fuel rods are provided in the upper region and the lower region of the cladding except for the upper and lower end natural uranium blanket regions in two types of fuels having different enrichments. As a fuel rod filled with pellets, a first fuel rod filled with fuel pellets of lower enrichment than the fuel pellets filled in the lower region in the upper region,
A second fuel rod in which an upper region is filled with fuel pellets having a higher enrichment than a fuel pellet in a lower region, wherein the first fuel rod has a boundary between the upper region and the lower region. The second fuel rod is provided at a position above the center of the effective heat generating region in the rod axis direction, and a boundary between the upper region and the lower region is provided at a position below the center of the effective heat generating region. Things.

Further, a fuel assembly for a boiling water reactor according to the invention according to a second aspect of the present invention is the fuel assembly for a boiling water reactor according to the first aspect, wherein an upper portion of the first fuel rod is provided. At least one of the boundary between the region and the lower region or the boundary between the upper region and the lower region of the second fuel rod is moved from the end position of the effective heat generation region near each boundary to the effective heat generation region length. It is provided at a position closer to the end than the inner position by a distance equivalent to 30%.

A fuel assembly for a boiling water reactor according to a third aspect of the present invention is the fuel assembly for a nuclear reactor according to the first or second aspect. And / or the second fuel rod includes two or more types of fuel rods provided at positions where boundaries between the upper region and the lower region are different from each other.

The fuel assembly for a boiling water reactor according to the invention according to the fourth aspect is the fuel assembly for a boiling water reactor according to the first aspect of the present invention. A water rod occupying an area equivalent to a plurality of fuel rods is disposed at a substantially central position of the grid-like array of 9 × 9 grid or more, and a fuel pellet filled in a lower area of the first fuel rod; And / or wherein the fuel pellets filled in the upper region of the second fuel rod have the highest enrichment in the fuel assembly, wherein the highest enrichment is 4.9 to 5.0 wt%. Things.

Further, the fuel assembly for a boiling water reactor according to the invention of claim 5 is the fuel assembly containing burnable poisons among the plurality of fuel rods according to claim 4. It is characterized in that it contains fuel pellets with a higher enrichment than the average enrichment of each section of the body.

The fuel assembly for a boiling water reactor according to the invention according to claim 6 is the fuel assembly for a boiling water reactor according to claim 1, wherein one of the first fuel rods is provided. Part or all of the second fuel rods are arranged at the outermost peripheral array position of the square lattice array, and some or all of the second fuel rods are arranged at an array position other than the outermost peripheral array of the square lattice array. It is a feature.

Further, the fuel assembly for a boiling water reactor according to the invention of claim 7 is the fuel assembly for a boiling water reactor according to claim 1, wherein All the fuel rods which do not contain the burnable poison are only the first fuel rod and the second fuel rod.

The fuel assembly for a boiling water reactor according to the invention according to claim 8 is the fuel assembly for a boiling water reactor according to claim 1, wherein the second fuel rod is It is characterized in that it includes a partial length fuel rod having an effective heat generation area shorter than the effective heat generation area of another fuel rod in the axial direction.

Further, in the fuel assembly for a boiling water reactor according to the ninth aspect of the present invention, the clad tube is filled with a predetermined number of fuel pellets having a predetermined nuclear fuel enrichment and sealed with a large number of fuels. In the fuel assembly for a boiling water reactor having an average enrichment of 4 wt% or more, wherein the rods are supported at upper and lower ends by an upper tie plate and a lower tie plate as a bundle in a predetermined square lattice array, A water grid occupying an area equivalent to a plurality of fuel rods is arranged at a substantially central position of the square grid array of the rods of 9 × 9 grid or more, and the burnable poison of the plurality of fuel rods is disposed. The fuel rods not containing are filled in the lower region as fuel rods in which the upper region and the lower region in the cladding tube except the upper and lower end natural uranium blanket regions are filled with two types of fuel pellets having different enrichments. A first fuel rod in which an upper region is filled with fuel pellets having a lower enrichment than fuel pellets, and a second fuel rod in which an upper region is filled with fuel pellets having a higher enrichment than the fuel pellets which are filled in a lower region Wherein the first fuel rod has a boundary between an upper region and a lower region provided at a position above a center of an effective heat generation region in the fuel rod axial direction, and the second fuel rod has a second boundary. In the fuel rod, a boundary between the upper region and the lower region is provided at a position lower than a center of the effective heat generating region, and the first fuel rod or the second fuel rod has a boundary between the upper region and the lower region. It is characterized by including two or more types of fuel rods provided at mutually different positions.

A fuel assembly for a boiling water reactor according to the invention according to claim 10 is the fuel assembly for a boiling water reactor according to claim 9, wherein an upper portion of the first fuel rod is provided. The boundary between the region and the lower region, or the upper region and the lower region of the second fuel rod, is separated from the end position of the effective heat region close to the respective boundary by about 30 times the length of the effective heat region.
% In a position closer to the end than the inside position.

A fuel assembly for a boiling water reactor according to the invention of claim 11 is the fuel assembly for a boiling water reactor according to claim 9, wherein the lower part of the first fuel rod is provided. The fuel pellets filled in the region and / or the fuel pellets filled in the upper region of the second fuel rod are the highest enrichment in the fuel assembly, and the highest enrichment is between 4.9 and 5.0 wt. %.

A fuel assembly for a boiling water reactor according to a twelfth aspect of the present invention is the fuel assembly for a boiling water reactor according to the eleventh aspect of the present invention. The fuel rods containing burnable poisons are characterized by including fuel pellets having an enrichment higher than the average enrichment of each section of the fuel assembly.

A fuel assembly for a boiling water reactor according to the invention of claim 13 is the fuel assembly for a boiling water reactor according to claim 9, wherein one of the first fuel rods is provided. Part or all of the second fuel rods are arranged at the outermost peripheral array position of the square lattice array, and some or all of the second fuel rods are arranged at an array position other than the outermost peripheral array of the square lattice array. It is a feature.

The fuel assembly for a boiling water reactor according to the invention according to claim 14 is the fuel assembly for a boiling water reactor according to claim 9, wherein the second fuel rod is It is characterized in that it includes a partial length fuel rod having an effective heat generation area shorter than the effective heat generation area of another fuel rod in the axial direction.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for removing burnable poisons from a large number of fuel rods constituting a fuel assembly for a boiling water reactor as a bundle having a predetermined square lattice arrangement of 9 × 9 lattices or more. The fuel rods that do not include the fuel rods in which the upper and lower regions are filled with two types of fuel pellets having different enrichments in a region corresponding to the so-called effective heat generation region in the cladding tube excluding the upper and lower end natural uranium blanket regions. As shown in FIG. 7, the first fuel rod U1 in which the upper region A is filled with fuel pellets having a lower enrichment than the fuel pellet in the lower region B, and the fuel pellet in which the lower region D is filled. A second fuel rod U2 in which the upper enriched area C is filled with high enrichment fuel pellets.

Generally, the boiling transition from the nucleate boiling state to the film boiling state becomes severe immediately upstream of the spacer disposed above the fuel rod. By preferentially arranging the first fuel rods filled with low-enrichment fuel pellets, it is possible to improve the thermal operation margin, that is, the minimum limit power ratio.

Further, according to the present invention, the boundary between the upper region and the lower region of the first fuel rod is provided above the center of the effective heat generation region in the axial direction of the fuel rod, and the upper region of the second fuel rod is provided. And the lower region is provided at a position lower than the center of the effective heat generating region. Therefore, when viewed in the cross section of the fuel assembly, first, the first region in which the lower enrichment fuel pellets are filled in the upper region. As the average enrichment in the upper cross section of the fuel assembly becomes lower than the average enrichment in the cross section in the center of the fuel assembly, the reactivity at the upper part of the fuel assembly decreases, In some cases, the neutron flux distribution does not excessively become the upper peak, and also acts to improve the stopping margin.

The upper region filled with low enrichment fuel pellets is too long for the purpose of sufficiently increasing the average enrichment for high burnup in addition to the improvement of the minimum limit power ratio and the stop margin. The boundary between the upper region and the lower region of the first combustion rod is set at about 30% of the length of the effective heat generation region from the upper end of the effective heat generation region of the fuel assembly so that a high enrichment region can be secured without much. Desirably, the distance is higher than the lower position.

Further, when viewed in the cross section of the fuel assembly, the average enrichment in the lower cross section of the fuel assembly is reduced by the presence of the second fuel rod filled with the low enrichment fuel pellet in the lower region. Since the enrichment becomes lower than the average enrichment in the cross section of the central part of the assembly, the reactivity at the lower part of the fuel assembly decreases, and the lower distortion of the axial output distribution of the fuel assembly, which is a characteristic of BWR, is reduced. The maximum linear output density can be reduced by the amount.

In addition to suppressing the increase in the linear power density at the lower portion of the fuel assembly, the second combustion rod is also used for the purpose of sufficiently increasing the average enrichment for higher burnup. The boundary between the upper region and the lower region is about 30% of the length of the effective heat generation region from the lower end of the effective heat generation region of the fuel assembly so that the high enrichment region can be secured without increasing the low enrichment region. It is desirable to be below the upper position by a considerable distance.

Further, the first fuel rod and the second fuel rod are not limited to those in which the boundaries of the upper and lower regions are aligned at the same position. Regarding the first fuel rods, by setting the boundaries of the upper and lower regions to be higher than the arrangement positions where the boiling transition is particularly likely to occur, the likelihood of the boiling transition of each fuel rod can be equalized. Further, the limit output can be further improved without excessively reducing the average enrichment of the aggregate.

If two or more types of fuel rods are provided as the first fuel rod and the second fuel rod at the positions where the boundaries of the upper and lower regions are different from each other, the enrichment in the axial direction of the fuel assembly can be improved. The distribution can be varied gradually along the profile of the typical axial average power distribution of the BWR (cosine or some lower peak), and the axial power distribution of the fuel assembly during power operation is stabilized; The "independence" of the fuel assembly is increased, the degree of freedom in core design is increased, and safety is improved.

In the present invention, since the fuel assembly has an array of 9 × 9 grids or more, and a water rod occupying a region corresponding to a plurality of fuel rods is disposed at substantially the center of the array, It is possible to improve the average enrichment of the entire fuel assembly by making the fuel pellets filled in the lower region of the first fuel rod have the highest enrichment in the fuel assembly.

This is because, in the case of a fuel assembly having a grid array having a large number of fuel rods of 9 × 9 or more, the average linear output density can be reduced by the number of fuel rods, and about 7 to 9 fuel rods can be obtained. In the case of a fuel assembly in which a water rod occupying the rod region is provided, the water rod functions as a neutron absorber at a cold temperature to improve a stop margin, and an excessive lower peak due to high enrichment is added. Since the increase can be avoided, the maximum linear power density has some allowance, so that the highly enriched fuel pellets filled in the lower region of the first fuel rod are further regarded as having the highest enrichment in the fuel assembly. This is because even if the enrichment is increased, the maximum linear output density has a certain margin throughout the operation cycle. Thus, the first
The presence of the fuel rods also contributes to the high enrichment of the fuel assembly and the improvement of the stop margin when the number of fuel rods is large and the arrangement includes large-diameter water rods.

In general, when the average enrichment in the cross section of the fuel assembly is increased along with the increase in the burnup, the relative output of the fuel rod is determined in the vicinity of the water gap surrounding the fuel assembly on the outer peripheral side, That is, since the fuel rods arranged at the outermost periphery of the square lattice arrangement tend to increase in size, the operating output with respect to the limit output that leads to the boiling transition of the fuel assembly is reduced to improve the thermal operating margin, that is, the minimum limit output ratio. From the perspective of
It is desirable that the first fuel rods be arranged at the outermost periphery of the square lattice arrangement.

On the other hand, the presence of the second fuel rods suppresses the lower distortion of the axial power distribution of the fuel assembly, and the margin for the reduction in the maximum linear power density is partly due to the increase in the operating margin. Needless to say,
By arranging the second fuel rods at positions other than the outermost periphery of the square lattice array, local peaking can be increased, and the soundness of the fuel rods containing burnable poisons such as gadolinia can be further improved. Can be secured.

That is, on the lower side of the fuel assembly, the output sharing of the fuel rods arranged at the outermost periphery of the square lattice arrangement becomes relatively large. When the first fuel rods filled with the enriched fuel pellets are arranged in the outermost circumferential array, local peaking often becomes maximum in the outermost circumferential fuel rods from the initial to middle stages of combustion, and the maximum linear power density increases. Becomes However, due to the presence of the second fuel rods filled with low enrichment fuel pellets in the lower region, the average enrichment in the cross section of the lower portion of the fuel assembly is reduced by the average enrichment in the central cross section of the fuel assembly. The effect of flattening the axial output distribution by an amount lower than the degree exceeds the effect of increasing local peaking and acts in a direction to reduce the maximum linear output. Further, since the linear power density of the burnable poison-containing fuel rods decreases substantially in inverse proportion to the increase in the relative output of the fuel rods disposed on the outer periphery, the thermomechanical soundness of such burnable poison-containing fuel rods is reduced. In addition to improving the performance, even in the case of highly concentrated pellets, a high concentration of burnable poison can be added.

As described above, according to the present invention, even when the above-described conventional technique is not used, the burnable poison can be added to the highly enriched pellets, so that the fuel assembly can be further enriched. Contributes to higher burnup.

In the fuel assembly of the present invention, the fuel rods containing no burnable poisons are all the first fuel rods and the second fuel rods, so that the fuel rods have low enrichment at the upper and lower ends of each fuel rod. It is possible to increase the average enrichment of the entire fuel assembly without providing a natural uranium bracket. This does not include fuel rods filled only with fuel pellets of the same enrichment over the effective heat generation area, and thus the number of first fuel rods can be increased to secure a further stoppage margin. Therefore, no blanket is provided. This is because it is possible to compensate for a decrease in the suspension margin, which is a concern in some cases. In addition, the fuel economy is expected to decrease if the blanket is not provided, because the increase in the average enrichment of the entire fuel assembly increases the removal burn-up, so the fuel economy improves rather. ,No problem.

In addition, when the number of fuel rods increases as the square arrangement becomes 9 × 9 grids or more, the friction pressure loss of the coolant flow increases accordingly. The pressure loss can be reduced by removing a portion of the fuel rod above the effective heat generating portion and arranging a plurality of fuel rods having a short effective heat generating portion (hereinafter, partially long fuel rods). If the partial length fuel rod is adjacent to the water rod,
A large water region can be formed together with the water rod, neutron absorption can be enhanced, and a stop margin can be improved. On the other hand, since the number of fuel rods on the upper side of the fuel assembly decreases with the use of the partial length fuel rods, the heat flux on the surface of the fuel rods tends to be small, and the limit power tends to be low. Also, the lower part of the fuel has a larger amount of uranium than the upper part, the lower distortion of the power distribution tends to be large, and the linear power density tends to increase.

According to the present invention, the first fuel rod can improve the limit output, and a part of the second fuel rod can be a fuel rod having such a partial length. Similarly, a reduction in the maximum linear output density in the lower part can be expected.

As described above, in the fuel assembly of the present invention including the first fuel rod and the second fuel rod, the average enrichment and the concentration of burnable poisons of the fuel assembly are reduced due to economical requirements. In the case of the enhancement, the characteristics of the fuel in the axial direction and the direction perpendicular to the axial direction can be simultaneously and flexibly optimized with respect to requirements such as safety and thermo-mechanical characteristics of the fuel rod.

In addition, these first and second fuel rods
In the case where a blanket is provided, there is a maximum of four regions per fuel rod, and the number of fuel pellet filling operations per fuel rod is unchanged. Therefore, in the fuel assembly of the present invention, from the viewpoint of nuclear properties, the enrichment distribution in the axial direction is complicated for optimization purposes, but the number of regions of the fuel pellets by enrichment is minimized. It can be suppressed, and there is no risk of lowering the work efficiency and increasing the risk of erroneous insertion of the pellet type. this is,
This is particularly effective in a fuel assembly having a large number of fuel rods in a 9 × 9 grid array or a 10 × 10 grid array.

As described above, by simultaneously using the first fuel rods and the second fuel rods for the fuel assemblies of 9 × 9 grid or more, the average enrichment of the fuel assemblies is reduced to 4 wt%. Even a fuel assembly with a high burnup exceeding this can be a fuel assembly excellent in economy and safety.

[0048]

Embodiment 1 As a first embodiment of the present invention, a water rod W is arranged in a central portion of 3 × 3 fuel rods in a 9 × 9 lattice arrangement, and a fuel rod containing gadolinia as a burnable poison is provided. A fuel assembly composed of a first fuel rod, a second fuel rod, and a fuel rod filled with only the same enrichment fuel pellets over the effective heat generation region as fuel rods containing no poison. 10 is shown in FIG. FIG. 1A is a schematic view of an arrangement of a fuel assembly 10, and FIG.
(B) is a diagram of the region of the fuel pellet enrichment and gadolinia concentration filled in the inside (all 24 nodes) of various fuel rods arranged in the present fuel assembly.

In the first embodiment, the maximum uranium enrichment is set to 4.9 wt.
%, And all the fuel rods were provided with a low enrichment (0.71 wt%) natural uranium bracket B at each of the upper and lower nodes.

Among the fuel rods constituting the fuel assembly 10, the fuel rods that do not include gadolinia have the same maximum uranium enrichment over the entire effective heat generation area except the bracket B.
A type 1 fuel rod filled with 9 wt% fuel pellets,
The lower region (for 16 nodes) of the effective heat generation region is filled with the highest uranium enrichment of 4.9 wt% fuel pellets, and the upper region (for 6 nodes) is enrichment of 4.4 wt% lower than the lower region.
% Fuel pellets, which are the first fuel rods of the present invention, and the upper region (1) of the effective heat generation region.
The second fuel rod of the present invention in which the highest uranium enrichment of 4.9 wt% fuel pellets is filled in the lower region (for 6 nodes) and the lower enrichment 4.4 wt% fuel pellet is filled in the lower region (for the 6 nodes). And the same low uranium enrichment of 3.4 wt% throughout the effective heat generation region.
And type 4 fuel rods filled with fuel pellets. The number of fuel rods of type 2 and type 3, which are the first and second fuel rods, was the same number of eight each.

The gadolinia-containing fuel rods are arranged such that the upper region (for 6 nodes) is filled with 5.0 wt% gadolinia-containing uranium enrichment 4.4 wt% fuel pellets, and the lower region (for 16 nodes) is filled with 8 wt%. Uranium enrichment containing 0.0 wt% gadolinia Type G1 fuel rod filled with 4.4 wt% fuel pellets, and upper uranium enrichment 4.9 containing 6.0 wt% gadolinia in upper region (for 6 nodes)
The central region (for 10 nodes) is filled with 8.0 wt% gadolinia and the highest uranium enrichment is 4.9 wt%. The lower region (for 6 nodes) is filled with 9.0 wt% gadolinia. And a type G2 fuel rod filled with 4.9 wt% uranium enrichment fuel pellets.

In the fuel assembly 10, type 4 fuel rods are arranged at the four corners of a 9 × 9 square lattice array, and the outermost array position adjacent to these corner rods and the water gap, that is, the relative output becomes large. By arranging the first fuel rod, the type 2 fuel rod, at the eight arrangement positions where the boiling transition is most likely to occur, a reduction in the critical output was avoided. In addition, by arranging the type 3 fuel rods as the second fuel rods at positions other than the outermost peripheral array having a high output, it is possible to increase the output sharing of the outermost peripheral fuel rods that do not include gadolinia, To this extent, the relative output of the gadolinia-containing fuel rods (types G1 and G2) arranged not adjacent to either the water gap or the water rod W is reduced, and the thermomechanical integrity of the gadolinia-containing fuel rods is increased. .

The fuel assembly 1 according to the first embodiment as described above
FIG. 2 is a graph showing the result of determining the combustion change of the maximum linear power density in the equilibrium core using 0. This is based on the assumption that the core is operated for 18 months, using the conventional fuel assembly shown in FIG. 7 as a control, and the average removal burnup of the replacement fuel is about 50 GWd / t. The maximum linear power density is indicated by a broken line for the conventional type, and is indicated by a solid line for the core using the fuel assembly 10 of the first embodiment. The limit value of the maximum linear power density during normal operation is 13.4 kW / ft. As can be seen from the results of FIG. 2, the maximum linear power density in the case of using the first embodiment is lower than that of the conventional type in the middle of the minute cycle in which the concentration of gadolinia is lower on the lower side of the fuel. Although the density is slightly higher, there is a reduction effect at the beginning of the cycle, so the maximum value over the cycle period is reduced by about 0.5 kW / ft compared to the conventional type.

Embodiment 2 As a second embodiment of the present invention, as in the first embodiment,
FIG. 3 shows a fuel assembly 20 in which a water rod W is arranged in a central portion of 3 × 3 fuel rods in a × 9 lattice arrangement and gadolinia is used as a burnable poison. FIG. 3A is a schematic view of the arrangement of the fuel assemblies 20, and FIG. 3B is a diagram showing the enrichment of the fuel pellets and the gadolinia of the fuel pellets filled in the various fuel rods (24 nodes in total) arranged in the present fuel assembly. FIG. 3 is a diagram illustrating a region configuration according to density. In the second embodiment, the maximum uranium enrichment is set to 4.95 wt%, and all the fuel rods are provided with a low uranium enrichment (0.71 wt%) natural uranium bracket B at each of the upper and lower nodes.

In the second embodiment, the first fuel rod, the second fuel rod, and the fuel rod filled with only the same enrichment fuel pellets over the effective heat generation region are used as the fuel rods containing no poison. The fuel assembly is basically the same as the fuel assembly of the first embodiment in that it is used. However, the first fuel rod and the second fuel rod are used for the purpose of increasing the average enrichment of the fuel assembly and improving the thermal operation margin as compared with the first embodiment. The configuration of the fuel rod is set.

That is, the fuel rods not including gadolinia among the fuel rods constituting the fuel assembly 20 are filled with the same maximum uranium enrichment of 4.95 wt% fuel pellets over the entire effective heat generating region except the bracket B. The present invention in which a Type 11 fuel rod to be filled, a lower region of the effective heat generation region is filled with a fuel pellet having a maximum uranium enrichment of 4.95 wt%, and an upper region is filled with a fuel pellet having a lower enrichment of 4.45 wt% than the lower region A fuel rod having a maximum uranium enrichment of 4.95 wt% in the upper region of the effective heat generating region and a lower fuel concentration of 4.45 wt% in the lower region than the upper region. And the type 13 fuel rod of the present invention, which is filled with a fuel cell, and the effective heat generation region arranged at the four corners of the square lattice array, and the low backlash as a whole. And type 14 fuel rods enrichment of the fuel pellets are filled, a four.

However, in the second embodiment, the boundary between the upper region and the lower region of the type 12 fuel rod as the first fuel rod is positioned higher than that in the first embodiment, and the type 13 fuel rod as the second fuel rod. By positioning the boundary between the upper region and the lower region below the case of the first embodiment, the filling region of the highest uranium enriched fuel pellets from the first fuel rod is reduced from 16 nodes to 19 nodes as compared with the first embodiment. The second fuel rods were expanded from 16 nodes to 18 nodes to increase the average enrichment of the fuel assembly.

Here, the type 14 fuel rod which is filled with low-enrichment fuel pellets as corner rods as a whole is also set to have a higher enrichment than the fuel rods filled with corner rods in Example 1 and contains gadolinia. The uranium enrichment of the fuel pellets of the type G11 fuel rod and the type G12 fuel rod was also set higher than that of the gadolinia-containing fuel rod of Example 1, and the average enrichment of the fuel assembly was further improved. However, in the type 14 fuel rod, the effective heat generation region was divided into an upper region and a lower region, and the fuel pellets filled in the upper region had a lower enrichment than the lower region. That is, this type 1
Four fuel rods belong to the configuration of the first fuel rod of the present invention.

In the second embodiment, the number of type 12 fuel rods, which are the first fuel rods, is increased from that in the first embodiment, and the eight arrangement positions adjacent to the corner rod and the water gap where the boiling transition is most likely to occur. In addition, by preferentially arranging the four outermost circumferential arrangement positions where the boiling transition is likely to occur next, the limit output characteristics are improved as compared with the first embodiment. On the other hand, the number of type 13 fuel rods as the second fuel rods is also 18
By greatly increasing the number of books in the first embodiment,
The thermomechanical integrity of gadolinia-containing fuel rods was increased, and the effect of reducing the maximum linear power density was expanded.

As described above, in the second embodiment, the boundaries of the upper and lower regions are set higher for the type 12 fuel rods, and the boundaries of the upper and lower regions are set lower for the type 13 fuel rods. The average enrichment of Example 1 was higher than that of Example 1, and the thermal operation margin was able to be increased.

In the second embodiment, the boundary position between the upper and lower regions of the type 14 fuel rod is different from the boundary position of the upper and lower regions of the type 12 fuel rod. This is because of the type 12 and type 14 fuel rods at positions where boiling transitions are likely to occur.
This is because, for a type 14 fuel rod in which a boiling transition is more likely to occur, the low enrichment region is increased to make the boiling transition of each fuel rod more likely to occur, thereby improving the limit output. Further, with this configuration, the axial enrichment distribution of the fuel assembly is gradually changed along the profile (cosine or a slightly lower peak) of the typical axial average power distribution of the BWR. As a result, the axial power distribution of the fuel assembly during power operation is stabilized, the "independence" of the fuel assembly is increased, the degree of freedom in core design is increased, and safety is improved. Even in this case, the number of enrichment regions of each fuel rod that does not include gadolinia is four or less, and a decrease in work efficiency and an increase in the risk of erroneous insertion of a pellet type can be avoided.

Embodiment 3 As a third embodiment of the present invention, a water rod W is arranged in a central portion of 3 × 3 fuel rods in a 9 × 9 grid arrangement, and a fuel rod containing gadolinia as a burnable poison, FIG. 4 shows a fuel assembly 30 in which only the first fuel rod and the second fuel rod are used as fuel rods that do not include uranium and no natural uranium bracket is provided at the upper and lower ends of all the fuel rods. FIG. 4A is a schematic view of the arrangement of the fuel assemblies 30, and FIG. 4B is a diagram showing the fuel pellet enrichment and gadolinia filling inside the various fuel rods (24 nodes in total) arranged in the present fuel assembly. FIG. 3 is a diagram illustrating a region configuration according to density.

In the third embodiment, all the fuel rods that do not include gadolinia are used as the first and second fuel rods, so that the fuel assembly can be performed without providing a low-enrichment natural uranium bracket at the upper and lower ends of each fuel rod. It is intended to increase the average enrichment of the whole body.

That is, the fuel rod not containing gadolinia has a maximum uranium enrichment of 4.95 in the lower area (21 nodes) of the entire effective heat generation area (24 nodes) without the bracket.
Top area filled with wt% fuel pellets (3 nodes)
The first fuel rod of the present invention, which is filled with fuel pellets having a lower enrichment of 4.45 wt% than the lower area, and a type 21 fuel rod, and an upper area (for 20 nodes) of the effective heat generation area
The second aspect of the present invention, in which a fuel pellet having a maximum uranium enrichment of 4.95 wt% is filled, and a lower region (for four nodes) is filled with a fuel pellet having a lower enrichment of 4.45 wt% than the upper region.
Type 22 fuel rods, which are fuel rods of the type, and an effective heat generation region arranged at four corners of a square lattice array are filled with fuel pellets of low uranium enrichment as a whole, and the effective heat generation region is divided into an upper region and a lower region. And a type 23 fuel rod belonging to the first fuel rod of the present invention in which the filled fuel pellets in the upper region are lower in enrichment than the lower region. In the third embodiment, the fuel rods containing no gadolinia are only the first and second fuel rods.
The number of type 21 fuel rods and the number of type 22 fuel rods could be set as many as 28 each.

As described above, by removing the blanket portion, the filling region of the highest enrichment fuel pellet occupied by the effective heat generation region over the entire fuel assembly 30 can be increased, and as a result, the average enrichment of the fuel assembly 30 can be increased. Degree is about 4.8
wt%.

In this case, there is a concern that the margin for stoppage and the fuel economy may be reduced because the blanket is not used. However, a large number of 32 first fuel rods of type 21 and type 23 are arranged. By doing so, a stop margin can be secured. Further, regarding the fuel economy, since the take-out burnup is increased by increasing the average enrichment of the fuel assembly 30, the fuel economy is rather improved.

Embodiment 4 Next, as a fourth embodiment of the present invention, application to a 9 × 9 lattice array, which is an asymmetric lattice core, that is, a core in which the water gap between the control rod side and the non-control rod side is different from each other. For fuel rod 3x
FIG. 5 shows a fuel assembly 40 in which a water rod W equivalent to 3 is eccentric to the non-control rod side. FIG. 5A is a schematic view of the arrangement of the fuel assemblies 40, and FIG. 5B is a diagram showing the enrichment of fuel pellets and the gadolinia of the fuel pellets filled inside (all 24 nodes) of various fuel rods arranged in the present fuel assemblies. FIG. 3 is a diagram illustrating a region configuration according to density.
Here, the maximum uranium enrichment is set to 4.95 wt%, and all the fuel rods are provided with a low uranium enrichment (0.71 wt%) natural uranium bracket B at each of the upper and lower nodes.

In general, in the case of an asymmetric lattice core, the enrichment distribution in the cross section of the fuel assembly is complicated. For example, 9 × 9
Even at the four corners of the grid array, different fuel rods filled with different low-enrichment fuel pellets are arranged in the effective heat generating area other than the bracket according to the distance from the control rod side.

That is, in FIG. 5A, the corner closest to the control rod side has the highest enrichment of 3.9 among all the corners.
A Type 34 fuel rod filled with 5 wt% fuel pellets is arranged, and then, at the two corners near the counter control rod, 3.45 wt of the next lower enrichment than the Type 34 fuel rod.
% Fuel pellets are filled in the upper region, and the lower region is 3.45 wt% in the lowermost region, at the corner closest to the control rod so as to have the lowest enrichment. Type 36 fuel rods belonging to the first fuel rods filled with% fuel pellets are arranged. Further, at the outermost circumferential array position adjacent to the control rod side corner rods, type 34 fuel rods sequentially filled with low enrichment fuel pellets, and then low enrichment 4.45
A type 33 fuel rod filled with wt% fuel pellets is arranged.

In this embodiment, all the remaining positions of the outermost peripheral array except for the fuel rods of the relatively low enrichment type are provided as brackets B as fuel rods not including gadolinia.
A long lower region (19% or more of the effective heat generation region excluding
Node) is filled with fuel pellets with a maximum uranium enrichment of 4.95 wt%, and the enrichment in the upper region is lower than that in the lower region.
A large number of 22 type 31 fuel rods, which are the first fuel rods of the present invention filled with 45 wt% fuel pellets, are arranged at 22 positions, and the effective heat generation region 7 is located at a position other than the outermost peripheral arrangement.
A second upper part of the present invention in which a long upper region (for 18 nodes) of 0% or more is filled with a maximum uranium enrichment of 4.95 wt% fuel pellets and a lower region is filled with a lower enrichment of 4.45 wt% fuel pellets than the upper region. The average enrichment of the fuel assembly 40 could be increased to 4.4 wt% by arranging a large number of 27 type 32 fuel rods, ie, the type 32 fuel rods.

As described above, according to the fourth embodiment, the asymmetric lattice, which has conventionally been considered to be more difficult to enrich at higher enrichment than the symmetric lattice core under the condition where the maximum enrichment limit (5 wt%) is imposed. By arranging a large number of type 31 fuel rods as the first fuel rods and type 32 fuel rods as the second fuel rods in spite of the fuel assemblies for the core, the average enrichment of the entire fuel assembly is achieved. The degree could be increased.

Embodiment 5 As a fifth embodiment of the present invention, in a 10 × 10 grid array,
A water rod W is arranged for 3 × 3 fuel rods, a plurality of partial-length fuel rods whose effective heat generation area is shorter than other fuel rods are arranged, and the partial-length fuel rods have a configuration of a second fuel rod. FIG. 6 shows a fuel assembly 50 having the above configuration. FIG. 6A shows the fuel assembly 50.
FIG. 2 (b) is a diagram of the region of the fuel pellet enrichment and gadolinia concentration filled inside various fuel rods (24 nodes in total) arranged in the present fuel assembly.

In the fifth embodiment, all the fuel rods not including gadolinia are used as the first and second fuel rods.
The arrangement of the partial length fuel rods compensates for the increase in friction pressure loss of the coolant flow accompanying the increase in the number of fuel rods.

That is, the fuel rods containing no gadolinia have the highest uranium enrichment of 4.95 wt% in the lower area (19 nodes) of the effective heat generation area (22 nodes) excluding the bracket B.
A first fuel rod of the present invention, which is a fuel rod filled with fuel pellets and filled in an upper region (for three nodes) with a lower enrichment of 4.45 wt% than the lower region; The upper region (for 15 nodes) is filled with the highest uranium enrichment 4.95 wt% fuel pellets, and the lower region (for 7 nodes) is enriched lower than the upper region.
The second fuel rod of the present invention, which is filled with 45 wt% fuel pellets, is the type 42 fuel rod, and the fuel pellets with low uranium enrichment throughout the effective heat generation area arranged at the four corners of the square lattice arrangement And the effective heat generation region is divided into an upper region (for 6 nodes) and a lower region (for 16 nodes), and the fuel pellets filled in the upper region have a lower enrichment than the lower region. Type 43 fuel rods belonging to the fuel rods and a partial length fuel rod. The upper region (for 9 nodes) of the effective heat generation region (for 13 nodes) is filled with fuel pellets having a maximum uranium enrichment of 4.95 wt% and the lower region (for (4 nodes) 4.4 lower enrichment than upper region
Type P fuel rods belonging to the second fuel rods of the present invention filled with 5 wt% fuel pellets.

In this fuel assembly 50, type 41 fuel rods are arranged at all outermost arrangement positions except for the four corners,
A total of 12 type P fuel rods, which are part length fuel rods, are arranged at eight positions in the arrangement position including the four corners of the second layer from the outermost periphery and four at the position adjacent to the water rod W.

Therefore, a large water region is formed by the water rod W and the type P fuel rod adjacent thereto, and at low temperatures, neutron absorption is increased and the stop margin is improved. In the fifth embodiment, the number of regions for each enrichment of each fuel rod is 4
To make the axial enrichment distribution follow the typical axial average power distribution profile (cosine or slightly lower peak) of a BWR, coupled with the presence of a partial length fuel rod, type P fuel rod Was completed.

In the embodiment of the present invention, as the nuclear fuel material,
Although uranium is shown as a representative, similar effects can be obtained by applying the structure of the present invention to other nuclear fuels such as plutonium. Further, the above-described embodiment has a 9 ×
The present invention is applied to a fuel structure in which a square water rod occupying an area of nine fuel rods is arranged in a 9 grid or 10 × 10 grid array. The same operation and effect can be obtained by applying the present invention to a high burnup fuel assembly.

Further, in this embodiment, the first fuel rod is arranged at the outermost periphery in view of the typical tendency of the high burnup fuel assembly, that is, the fact that the boiling transition is likely to occur at the outermost fuel rod. Although the second fuel rods are arranged at positions other than the outermost periphery, the arrangement of the fuel rods is not limited to this. For example, when the cooling capacity of the outermost fuel rods is increased by improving the spacers and the like, as a result, when the second-layer fuel rods are more likely to undergo a boiling transition, the first fuel rods are replaced with the second-layer fuel rods. It can be arranged at a position where boiling transition is likely to occur.

As described above, in the present invention, the number and arrangement of the first fuel rods and the second fuel rods are determined according to the degree of stop margin and thermal operation margin such as the limit output and the maximum linear power density. Thus, each margin can be determined to be optimized. At this time, in order to further simplify the pellet filling operation in the manufacturing process, one of the first fuel rod and the second fuel rod may be provided.

For example, if the maximum linear power density is acceptable due to the structure of the fuel assembly, but the margin for marginal stop or the limit output is small, the fuel assembly of the present invention uses the first fuel rod, A configuration without using the second fuel rod may be adopted. For example, in FIG. 6, the configuration is such that the enrichment pellets of 4.45 wt% disposed below the type 42 fuel rod and the type P fuel rod are replaced with the enrichment pellets of 4.95 wt%. In this case, the first fuel rod is made of two or more types of fuel rods provided at positions where the boundary between the upper region and the lower region is different from each other, thereby increasing the burnup of the fuel assembly and improving the stop margin. Thus, it is possible to equalize the likelihood of the boiling transition for each fuel rod and maximize the improvement in the critical output.

On the other hand, when the margin for stopping and the limit power are acceptable but the margin for the maximum linear power density is small, the fuel assembly of the present invention uses the second fuel rod and the first fuel rod. May not be used. For example, FIG.
, The 4.45 located above the type 41 fuel rods
wt% enrichment pellets to 4.95 wt% enrichment pellets and 3.95 placed on top of Type 43 fuel rods.
This is a configuration in which the wt% enrichment pellets are replaced with 4.45 wt% enrichment pellets. Again, the second fuel rod is
By using two or more types of fuel rods provided at positions where the boundaries between the upper region and the lower region are different from each other, the axial enrichment distribution is improved in a typical axial direction of the BWR with respect to the lower side where the power sharing is large. Along the profile of the power distribution (in this case, slightly lower peaks),
This contributes to the expansion of the degree of freedom in core design and, consequently, the improvement of safety.

[0082]

As described above, according to the fuel assembly for BWR of the present invention, the stop margin and the minimum limit power ratio are kept within the allowable range, and the thermomechanical integrity of the fuel rod containing burnable poison is improved. While maintaining the maximum linear power density, the operating margin can be expanded, the stable axial power distribution can be maintained, and a longer operation cycle and higher burnup can be achieved without increasing the burden of pellet filling work in the manufacturing process. Therefore, there is an effect that it can significantly contribute to improvement of safety and economy of the industry.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a BWR fuel assembly according to a first embodiment of the present invention, (a) is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and (b) is a fuel assembly. FIG. 4 is a diagram showing a region configuration of a fuel pellet enrichment and gadolinia concentration to be filled in various fuel rods arranged in the fuel cell.

FIG. 2 is a diagram showing a combustion change of a maximum linear power density in an equilibrium core using a fuel assembly according to Example 1 (vertical axis: maximum linear power density (kW / ft), horizontal axis: cycle fuel density (GWd) /
t)).

3A and 3B are schematic configuration diagrams of a fuel assembly for BWR according to a second embodiment of the present invention, wherein FIG. 3A is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and FIG. FIG. 4 is a diagram showing a region configuration of a fuel pellet enrichment and gadolinia concentration to be filled in various fuel rods arranged in the fuel cell.

FIG. 4 is a schematic configuration diagram of a fuel assembly for BWR according to a third embodiment of the present invention, where (a) is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and (b) is a fuel assembly. FIG. 4 is a diagram showing a region configuration of a fuel pellet enrichment and gadolinia concentration to be filled in various fuel rods arranged in the fuel cell.

FIG. 5 is a schematic configuration diagram of a fuel assembly for BWR according to a fourth embodiment of the present invention, where (a) is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and (b) is a fuel assembly. FIG. 4 is a diagram showing a region configuration of a fuel pellet enrichment and gadolinia concentration to be filled in various fuel rods arranged in the fuel cell.

FIG. 6 is a schematic configuration diagram of a fuel assembly for BWR according to a fifth embodiment of the present invention, where (a) is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and (b) is a fuel assembly. FIG. 4 is a diagram showing a region configuration of a fuel pellet enrichment and gadolinia concentration to be filled in various fuel rods arranged in the fuel cell.

FIG. 7 is a schematic region schematic diagram showing a configuration of a first fuel rod and a second fuel rod arranged in the fuel assembly of the present invention.

FIG. 8 is a schematic configuration diagram showing an example of a BWR fuel assembly according to the related art, where (a) is a schematic diagram showing an arrangement of fuel rods of the fuel assembly, and (b) is arranged in the fuel assembly. FIG. 4 is a diagram showing a region configuration of a fuel pellet enriched in various fuel rods and a concentration of gadolinia according to the concentration of gadolinia.

[Explanation of symbols]

10, 20, 30, 200: 9 × 9 array fuel assembly 40: 9 × 9 array fuel assembly for asymmetric lattice core 50: 10 × 10 array fuel assembly 1, 4, 11, 33, 34, 35, 101 , 103: fuel rods (filled with fuel pellets of a certain enrichment over the entire effective heat generation region) U1, 2, 12, 14, 21, 23, 31, 31, 36, 4
1, 43, 102: first fuel rod U2, 3, 13, 22, 32, 42: second fuel rod P: partial length fuel rod (second fuel rod) G1, G2, G11, G12, G21 , G22, G31, G32, G
41, G42, G101, G102: Fuel rod with gadolinia W: Water rod B: Natural uranium bracket

Claims (14)

[Claims] 1. A number of fuel rods filled with a predetermined number of fuel pellets having a predetermined nuclear fuel enrichment in a cladding tube and sealed are bundled up and down by an upper tie plate and a lower tie plate as a bundle in a predetermined square lattice arrangement. In a fuel assembly for a boiling water reactor having an average enrichment of 4 wt% or more supported at both ends, a square lattice arrangement of the large number of fuel rods is 9 × 9 lattice or more, and substantially the center of the arrangement. A water rod occupying an area equivalent to a plurality of fuel rods is disposed at a position, and the fuel rods containing no burnable poison of the plurality of fuel rods are an upper area in the cladding tube excluding upper and lower end natural uranium blanket areas and As a fuel rod in which the lower region is filled with two types of fuel pellets having different enrichments, fuel pellets having a lower enrichment than the fuel pellets filled in the lower region are filled in the upper region. A first fuel rod, and a second fuel rod in which an upper region is filled with fuel pellets having a higher enrichment than a fuel pellet filled in a lower region, wherein the first fuel rod comprises an upper region. A boundary between the upper and lower regions is provided above the center of the effective heat generation region in the fuel rod axial direction, and the boundary between the upper region and the lower region is located at the center of the effective heat generation region. A fuel assembly for a boiling water reactor, provided at a lower position. 2. A method according to claim 1, wherein at least one of a boundary between an upper region and a lower region of the first fuel rod or a boundary between an upper region and a lower region of the second fuel rod is closer to the respective boundary. 2. The fuel assembly for a boiling water reactor according to claim 1, wherein the fuel assembly is provided at a position closer to the end than a position inside the region from an end position of the region by about 30% of the length of the effective heat generation region. body. 3. The fuel rod according to claim 1, wherein the first fuel rod and / or the second fuel rod include two or more types of fuel rods provided at positions where boundaries between an upper region and a lower region are different from each other. The fuel assembly for a boiling water reactor according to claim 1 or 2, wherein 4. A fuel pellet filled in a lower region of the first fuel rod and / or a fuel pellet filled in an upper region of the second fuel rod has a highest enrichment in a fuel assembly, The fuel assembly for a boiling water reactor according to claim 1, wherein the maximum enrichment is 4.9 to 5.0 wt%. 5. The fuel rod containing a burnable poison out of the plurality of fuel rods includes fuel pellets having an enrichment higher than the average enrichment of each cross section of the fuel assembly. 5. The fuel assembly for a boiling water reactor according to 4. 6. A part or all of said first fuel rods are arranged at the outermost peripheral arrangement position of said square lattice-like arrangement, and said second fuel rods
2. The fuel assembly for a boiling water reactor according to claim 1, wherein a part or all of the fuel rods are arranged at positions other than the outermost peripheral arrangement of the square lattice arrangement. 3. 7. The fuel rod according to claim 1, wherein all of the plurality of fuel rods that do not contain a burnable poison comprise only the first fuel rod and the second fuel rod. 4. The fuel assembly for a boiling water reactor according to item 1. 8. The fuel rod according to claim 1, wherein the second fuel rod includes a part-length fuel rod having an effective heat generation area shorter than the effective heat generation area of another fuel rod in the axial direction. Fuel assemblies for boiling water reactors. 9. A large number of fuel rods filled with a predetermined number of fuel pellets having a predetermined nuclear fuel enrichment in a cladding tube and sealed as a bundle in a predetermined square lattice-like arrangement are vertically moved by an upper tie plate and a lower tie plate. In a fuel assembly for a boiling water reactor having an average enrichment of 4 wt% or more supported at both ends, a square lattice arrangement of the large number of fuel rods is 9 × 9 lattice or more, and substantially the center of the arrangement. A water rod occupying an area equivalent to a plurality of fuel rods is disposed at a position, and the fuel rods containing no burnable poison of the plurality of fuel rods are an upper area in the cladding tube excluding upper and lower end natural uranium blanket areas and As a fuel rod in which the lower region is filled with two types of fuel pellets having different enrichments, the upper region is filled with fuel pellets having a lower enrichment than that in the lower region. A first fuel rod, and a second fuel rod filled with fuel pellets with a higher enrichment than the fuel pellets filled in the lower region in the upper region. The boundary between the upper region and the lower region is provided above the center of the effective heat generation region in the fuel rod axial direction, and the second fuel rod has a boundary between the upper region and the lower region. The first fuel rod or the second fuel rod is provided at a position lower than the center of the region, and the first fuel rod or the second fuel rod includes two or more types of fuel rods provided at positions where a boundary between an upper region and a lower region is different from each other. A fuel assembly for a boiling water reactor, comprising: 10. An end position of the effective heat generation region near a boundary between an upper region and a lower region of the first fuel rod or an upper region and a lower region of the second fuel rod close to each boundary. 10. The fuel assembly for a boiling water reactor according to claim 9, wherein the fuel assembly is provided at a position closer to the end than the inner side by a distance corresponding to about 30% of the effective heat generation area length from the fuel cell. 11. A fuel pellet filled in a lower region of the first fuel rod and / or a fuel pellet filled in an upper region of the second fuel rod has a highest enrichment in a fuel assembly, The fuel assembly for a boiling water reactor according to claim 9, wherein the maximum enrichment is 4.9 to 5.0 wt%. 12. The fuel rod containing a burnable poison of the plurality of fuel rods includes fuel pellets having a higher enrichment than the average enrichment of each cross section of the fuel assembly. 12. The fuel assembly for a boiling water reactor according to item 11. 13. A part or all of the first fuel rods are arranged at the outermost peripheral arrangement position of the square lattice arrangement, and a part or all of the second fuel rods are arranged at the outermost arrangement position of the square lattice arrangement. The fuel assembly for a boiling water reactor according to claim 9, wherein the fuel assembly is arranged at an arrangement position other than the outer periphery arrangement. 14. The fuel rod according to claim 9, wherein the second fuel rod includes a partial-length fuel rod having an effective heat generation area shorter than the effective heat generation area of another fuel rod in the axial direction. Fuel assemblies for boiling water reactors.