JPH10253789A - Fuel assembly and core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress maximum linear power density in control rod side region below a limit value after the withdrawal of control rods even in the case aiming at high burnup. SOLUTION: A diagonal (m) separating the inside of a fuel assembly 1L into the control rod 11 side region and the opposite of control rod 11 side region is drawn to divide into the third region 8 including the 6 fuel rods 4 and 2 water rod on this diagonal (m), the first region 6 in control rod 11 side of the third region 8 and the second region 7 of the opposite side of the control rod 11. In the whole cross section plane positioning lower axial part (1/24 to 10/24), the average enrichment of whole 34 fuel rods 4 existing in the second region 7 is larger by 14% than the average enrichment of whole 34 fuel rods 4 existing in the first region 6. Also in the whole cross section plane positioning upper axial part (11/24 to 22/24), the average enrichment of whole fuel rods 4 existing in the second region 7 is made larger by 7% than the average enrichment of whole fuel rods 4 existing in the first region 6.

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 a reactor core provided with the same.

[0002]

2. Description of the Related Art Generally, a core of a boiling water reactor is located at a position adjacent to a control rod inserted at the time of operation to control the reactor power. Are placed in groups of four. This is called a control cell, which reduces the influence on the core average axial power distribution when the control rod is pulled out. In this control cell, the control rod is inserted into the core for a certain period of time, and is withdrawn as the combustion proceeds. Here, each fuel assembly of the control cell burns while the control rod is inserted, with the cross-sectional power distribution burning in a state where the control rod side region is small and the anti-control rod side region is large, The combustion of the fuel rods in the rod side region is delayed (hereinafter, referred to as partial combustion). Then, after the control rod is pulled out, on the contrary, the combustion in the control rod side region, which has been delayed, becomes active, and the output in this region increases. That is, the linear output density of the fuel rod in the control rod side region (=
Thermal output per unit length), and especially near the axially lower portion where the maximum linear power density is normally reached, near its limit value, thus reducing the thermal margin.

In order to make the power distribution uniform, for example, in Japanese Patent Application Laid-Open No. Hei 8-233967, in a cross section perpendicular to the axial direction, the inside of the fuel assembly is divided into a control rod side region and a non-control rod side region. A structure is disclosed in which the average enrichment of the control rod side region is lower than the average enrichment of the anti-control rod side region at a diagonal line which is divided into two.

[0004]

On the other hand, in recent years, higher burnup of the core has been attempted from the viewpoint of improving fuel economy. For this purpose, it is necessary to increase the average enrichment of the core, but in this case, the excess reactivity becomes high, and in order to suppress this, the control rod is withdrawn after being inserted into the core for a longer period than before. . As described above, after the control rod is withdrawn, the output of the control rod side region tends to increase due to partial combustion while the control rod is inserted, but this becomes more significant as the control rod insertion period is longer. Therefore, this tendency is remarkable in a core aiming at high burnup.

[0005] Here, in the above-mentioned known art, although the configuration is made in consideration of high burn-up, no particular consideration is given to the remarkability of the above tendency due to partial combustion. That is, it is difficult to suppress the maximum linear output density of the fuel rods in the control rod side region to the limit value or less because the output of the control rod side region is not sufficiently prevented from increasing after the control rod is pulled out.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fuel assembly and a core capable of suppressing the maximum linear power density in a control rod side region to a value equal to or less than a limit value after control rod withdrawal even when achieving high burnup. It is in.

[0007]

According to the present invention, a plurality of fuel rods, each of which is filled with a fuel substance, and at least one water rod are arranged in a square lattice. And, in a fuel assembly provided with a guide post for fixing a channel fastener on the control rod side, on a predetermined cross section perpendicular to the axial direction of the fuel rod, the inside of the fuel assembly is defined as a control rod side region. 2 on the anti-control rod side area
The average enrichment of all the fuel rods arranged on the control rod side with respect to the diagonal fuel rod and the water rod is 6 times higher than the average enrichment of all the fuel rods arranged on the non-control rod side. A fuel assembly is provided that is reduced by at least 3%. In a core equipped with a plurality of fuel assemblies and control rods, the power of the entire core is controlled by gradually pulling out some control rods during operation and adjusting the output of the fuel assembly adjacent to the control rods I do. At this time, the fuel assembly whose output is adjusted by the control rods burns while the control rods are inserted, with the cross-sectional power distribution having a smaller control rod side area and a larger anti-control rod side area. Therefore, the combustion of the fuel rods in the control rod side region is delayed. As a result, after the control rod is pulled out, the output of the unburned control rod side area increases, and the maximum linear output density particularly in the lower part of the fuel rod in the axial direction tends to be excessive. This tendency is particularly remarkable when increasing the burnup because the control rod insertion period becomes longer. In order to equalize the power distribution after the control rods are drawn out and to suppress the maximum linear power density to an appropriate value, the fuel rods and water rods on the diagonal lines that form the boundary between the control rod side area and the anti-control rod side area What is necessary is just to make the average enrichment on the control rod side smaller than the average enrichment on the non-control rod side.
As shown in FIG. 5, the enrichment difference and the maximum linear output density have a monotonically decreasing relationship when the enrichment difference is up to about 14%.
As this difference increases, the output increase on the control rod side after the control rod is pulled out can be suppressed, and the maximum linear output density can be reduced. At this time, if the enrichment difference is set to 6.3% in the entire lower part in the axial direction, the maximum linear power density becomes 13.4 kW / ft, which is exactly equal to the limit value. Therefore, by setting the enrichment difference at least 6.3% on the cross section including the position with the highest linear output density, the enrichment difference at other positions is set to an appropriate value that does not exceed it. By doing so, it is possible to sufficiently suppress an increase in output on the control rod side after the pull-out, and it is possible to reduce the maximum linear output density at that position to 13.4 kW / ft or less, which is the limit value.

Preferably, in the fuel assembly, on a predetermined cross section perpendicular to the axial direction of the fuel rod and having an average enrichment in cross section higher than that of natural uranium,
The average enrichment of all the fuel rods disposed on the control rod side from the diagonal fuel rods and water rods that divides the inside of the fuel assembly into a control rod side area and a non-control rod side area is defined as A fuel assembly is provided, wherein the average enrichment of all the fuel rods disposed on the control rod side is reduced by 6.3% or more.

Preferably, in the fuel assembly, the plurality of fuel rods include a partial length fuel rod having an axial length shorter than the other fuel rods, and is perpendicular to the axial direction of the fuel rod. On a predetermined cross section in which all the partial length fuel rods are present, the fuel rods and the water rods on the diagonal line which divides the inside of the fuel assembly into a control rod side region and a non-control rod side region are divided into two. The fuel characterized in that the average enrichment of all the fuel rods arranged on the control rod side is smaller than the average enrichment of all the fuel rods arranged on the non-control rod side by 6.3% or more. An aggregate is provided.

[0010] Preferably, in the fuel assembly, the plurality of fuel rods include a partial length fuel rod having an axial length shorter than the other fuel rods, and is perpendicular to the axial direction of the fuel rod. On all cross sections where all the partial length fuel rods are present, control rods are formed by diagonally dividing fuel rods and water rods that divide the inside of the fuel assembly into a control rod side region and a non-control rod side region. A fuel assembly wherein the average enrichment of all the fuel rods arranged on the side of the fuel rod is smaller than the average enrichment of all the fuel rods arranged on the side opposite to the control rod by 6.3% or more. Is provided.

Preferably, in the fuel assembly, on all cross sections perpendicular to the axial direction of the fuel rods and having an average enrichment in cross section higher than that of natural uranium,
The average enrichment of all the fuel rods disposed on the control rod side from the diagonal fuel rods and water rods that divides the inside of the fuel assembly into a control rod side area and a non-control rod side area is defined as A fuel assembly is provided, wherein the average enrichment of all the fuel rods disposed on the control rod side is reduced by 6.3% or more.

Preferably, in the fuel assembly, the plurality of fuel rods include a plurality of high enrichment fuel rods having enrichment higher than other fuel rods, and the plurality of high enrichment fuel rods are included. A fuel assembly is provided in which at least 1/2 of the enrichment fuel rods are arranged closer to the control rod side than the diagonal fuel rods and water rods.

Preferably, in the fuel assembly, the plurality of fuel rods include a plurality of low enrichment fuel rods having enrichment lower than other fuel rods, and the plurality of low enrichment fuel rods are included. A fuel assembly is provided in which at least 1/2 of the enrichment fuel rods are arranged closer to the control rods than the diagonal fuel rods and water rods.

Preferably, in the fuel assembly, the plurality of fuel rods and at least one water rod are arranged in a 9 × 9 square lattice. Provided.

Preferably, in the fuel assembly, the enrichment of the plurality of fuel rods is all equal to or higher than that of natural uranium.

Preferably, in the fuel assembly, among the plurality of fuel rods, four fuel rods arranged at four corners of the square lattice arrangement have the same enrichment. A fuel assembly is provided.

According to another aspect of the present invention, there is provided a fuel cell system comprising: a plurality of fuel assemblies; and a plurality of control rods having a substantially cross-shaped cross section which can be inserted between the plurality of fuel assemblies.
In the core, wherein the plurality of fuel assemblies include a low-enrichment fuel assembly having an average enrichment lower than the others, the low-enrichment fuel assembly includes the fuel assembly. A core is provided.

In order to further achieve the above object, according to the present invention, a plurality of fuel assemblies including a low-enrichment fuel assembly having an average enrichment lower than others, and a plurality of fuel assemblies inserted between the plurality of fuel assemblies are provided. A plurality of control rods each having a substantially cross-shaped cross-section possible, and a plurality of control cells in which the four low-enrichment fuel assemblies are arranged in a substantially square shape so as to sandwich one control rod. In the reactor core, there is provided a reactor core using the above fuel assembly as each low enrichment fuel assembly disposed in the control cell.

Preferably, in the core, at least one of the plurality of fuel assemblies is replaced or replaced with a new fuel assembly every time a predetermined combustion period elapses. And, after the start of operation of the core, when the predetermined combustion period has elapsed for the first time, the low-enrichment fuel assembly disposed in the control cell is not disposed or replaced, and the low-enrichment fuel assembly disposed in the control cell is not replaced. A core is provided, which is configured to arrange or replace a fuel assembly other than the enrichment fuel assembly.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. This embodiment is an embodiment of a fuel assembly applied to an initially loaded core. FIG. 2 is a diagram schematically showing the arrangement of the fuel assemblies in a quarter cross section of the core. As shown in FIG. 2, the core includes 872 fuel assemblies 1 and control rods that can be inserted between them to control the power of the core (not shown, see FIG. 3 described later).
And is constituted by. The fuel assembly according to the present embodiment is a low-enrichment fuel assembly 1L occupying a part of the fuel assemblies 1 and having the lowest average enrichment, and the control rods 5 ( See below)
208 are arranged at and the outermost peripheral position of the core. The high-enrichment fuel assembly 1H having an average enrichment higher than that of the low-enrichment fuel assembly 1L has 664 units arranged in regions other than the region where the high-enrichment fuel assembly 1L is arranged.

The control cells 5 are formed at 29 places in the core. FIG. 3 shows a schematic cross-sectional structure of the control cell 5. As illustrated, the control cell 5 is formed by arranging four low enrichment fuel assemblies 1L in a substantially square shape so as to sandwich one control rod 11 having a substantially cross-shaped cross section. Here, at the start of operation of the core, the control rod 11 is inserted from below the control cell 5 to a predetermined axial position, and is gradually withdrawn as combustion proceeds. The assemblies 1L and 1H are adapted for high burn-up, and therefore, the control rod 11 is operated after a longer period than a normal core (for example, burn-up 20 GWd /
(at time t), the control cell 5 is completely pulled out finally.

FIG. 1 shows a detailed cross-sectional structure and a distribution of enrichment of each fuel rod of the low-enrichment fuel assembly 1L arranged in the control cell 5, and FIG. 1 and 4
In the fuel assembly 1L according to the present embodiment, 74 fuel rods 4 filled with a fuel substance and two large-diameter water rods 10 are arranged in a square grid of 9 rows and 9 columns. To form a fuel bundle, and the periphery thereof is surrounded by a channel box 9. The upper and lower portions of the fuel bundle are supported by an upper tie plate 12 and a lower tie plate 13, respectively, and positioning spacers 14 are provided at a plurality of positions in the axial direction of the fuel bundle. Also, guide posts 12a and 12a are provided at the corners of the upper tie plate 12 on the control rod side and the opposite control rod side.
b are integrally formed, and a channel fastener 15 is fixed to the guide post 12a on the control rod 11 side. This channel fastener 1
Reference numeral 5 serves to interconnect the four fuel assemblies 1L constituting the control cell 5 and is fixed to the guide post 12a on the control rod side of each fuel assembly 1L. The guide post 12b on the opposite side of the control rod is a dummy for balancing the weight with the guide post 12a on the side of the control rod.

The 74 fuel rods 4 are composed of five types of fuel rods having different enrichment distributions, and are distinguished by fuel rod symbols 1, 2, 3, 4, and P. Among them, fuel rod 4 (fuel rod symbol 1) has 16
Books are arranged. Also, each enrichment distribution is
The portion near the lower end of the effective length in the axial direction (the effective length is 1 and the axial position is 0/24 to 1/24 when expressed with the lower end as reference, the same applies hereinafter) and the portion near the upper end (axial position 22/24 to 24/24) The uranium enrichment (weight ratio occupied by U-235) is 0.711% by weight, which is the same as the uranium enrichment in natural uranium, and the uranium enrichment in the middle part (axial position 1/24 to 22/24) is A weight% (described later). Further, 30 fuel rods 4 (fuel rod symbol 2) are arranged as shown in the figure. Each enrichment distribution is, like fuel rod 4 (fuel rod symbol 1), a portion near the lower end of the effective axial length (axial position 0/24 to 1/24) and a portion near the upper end (axial position 2).
2/24 to 24/24) has a uranium enrichment of 0.711% by weight and an intermediate portion (axial position 1/24 to 2/24).
2/24), the upper part (axial position 10 / 24-22)
/ 24), the uranium enrichment of the lower part (axial position 1/24 to 10/24) is B weight% (described later). Further, fuel rod 4 (fuel rod symbol 3)
Are arranged as shown. Each enrichment distribution is similar to the fuel rod 4 (fuel rod symbols 1 and 2), in the vicinity of the lower end of the effective length in the axial direction (axial position 0/24 to 1/2).
4) and a portion near the upper end (axial position 22/24 to 24/2)
The uranium enrichment of 4) is 0.711% by weight, and
The uranium enrichment of the intermediate portion (axial position 1/24 to 22/24) is C weight% (described later). Also fuel rod 4
Fourteen fuel rods are arranged as shown. In each enrichment distribution, the uranium enrichment is 0.711% by weight, which is the same as natural uranium, over the entire length of the effective length in the axial direction (axial position 0/24 to 24/24). Further, unlike the fuel rods 4 (fuel rod symbols 1 to 4) described above, the fuel rods 4 (fuel rod symbols P) are partial-length fuel rods whose effective lengths in the axial direction are shorter than those. Eight are arranged. Each enrichment distribution is
The uranium enrichment is D weight% (described later) over the entire length of the effective length in the axial direction (however, the position is 1/24 to 15/24 in the axial direction because of the partial length). In the concentration distribution, A to D have a relationship of A ≧ B ≧ C ≧ D ≧ 0.711. As a result, when the enrichment of the fuel rods 4 classified into the five types is represented by the fuel rod symbol in the order of higher enrichment, 1 ≧
2 ≧ 3 ≧ P ≧ 4. At this time, the enrichment of all the fuel rods 4 is 0.711% by weight or more of natural uranium.

Here, the fuel assembly 1L according to the present embodiment
The most significant feature of the present invention is that a large enrichment difference is provided between the control rod 11 side and the non-control rod 11 side in a cross section perpendicular to the axial direction by appropriately disposing the fuel rods 4 as described above. . That is, in FIG. 1, a diagonal line m that divides the inside of the fuel assembly 1L into a control rod 11 side region and a non-control rod 11 side region is drawn, and the six fuel rods 4 and 2 on this diagonal line m are drawn. , A first region 6 on the control rod 11 side of the third region 8 and a second region 7 on the opposite side of the control rod 11. Then, as shown in FIG.
Fourteen (1/2 or more) of (fuel rod symbol 1) are arranged in the second area 7, and the remaining two are arranged in the third area 8. Further, 20 of the 30 fuel rods 4 (fuel rod symbol 2) are arranged in the first area 6, and the remaining 10 are arranged in the second area 7. Further, of the eight fuel rods 4 (fuel rod symbol P), three are in the first area 6, three are in the second area 7, and the third is
Are arranged in the region 8 of the second layer from the outermost layer in the lattice arrangement. In addition, among the 14 fuel rods 4 (fuel rod symbols 4) having the lowest enrichment, the first area 6 has seven or more 1/2 fuel rods, the second area 7 has five fuel rods, and the third area 8 has Two of them are arranged on the outer layer in a grid-like arrangement, and at four corners and in the vicinity thereof. As a result, all four corners of the lattice arrangement are fuel rods with fuel rod symbol 4 and have the same enrichment.
Further, four of the six fuel rods 4 (fuel rod symbol 3) are arranged in the first area 6 and the remaining two are arranged in the second area 7. And, by adopting such an arrangement, the lower part in the axial direction (axial position 1/24 to 10/2) is removed except for both ends.
In all cross sections located in 4), the second region 7
The average enrichment of all 34 fuel rods 4 in the first region 6
14% higher than the average enrichment of all 34 fuel rods 4
And the upper part in the axial direction (axial position 11 / 24-
22/24) in all cross sections
The average enrichment of all the fuel rods 4 in the region 7 is about 7% larger than the average enrichment of all the fuel rods 4 in the first region 6.

In the above configuration, the third region 8
The fuel rods 4 and the two water rods 10 constitute a diagonal fuel rod and a water rod that divide the inside of the fuel assembly into a control rod side region and an anti-control rod side region, and The 34 fuel rods 4 in the first region 6 constitute all the fuel rods arranged on the control rod side with respect to the diagonal, and the 34 fuel rods 4 in the second region 7 are opposite to the diagonal. All the fuel rods arranged on the control rod side are configured.

Next, the operation and effect of this embodiment will be described.
In the present embodiment, the maximum linear output density on the control rod side after the control rod is pulled out is increased by increasing the difference between the average enrichment on the control rod side and the average enrichment on the control rod side.

In the fuel assembly, the present inventors have determined that the enrichment difference between the average enrichment on the control rod side and the average enrichment on the control rod side and the maximum line on the control rod side after the control rod has been pulled out. The relationship with the power density was analyzed, and the results shown in FIG. 5 were obtained.

FIG. 5 shows a fuel assembly provided with a fuel rod arrangement similar to that of the fuel assembly 1L, showing an axial position 1/24 to
When a predetermined enrichment difference [%] is provided between the average enrichment of the second region 7 and the average enrichment of the first region 6 in all the cross sections of 10/24, the burnup is 20 GWd / FIG. 9 is a diagram showing the value of the maximum linear output density [GWd / t] of the first area 6 when the control rod is completely pulled out at t. The horizontal axis shows the concentration difference and the vertical axis shows the maximum linear output density.
Analyzed to about 14%. The reason that the axial position 1/24 to 10/24 is selected as the analysis object is that the maximum linear output density due to the partial combustion tends to be excessively large in the lower axial direction where the control rod 11 is inserted longer. Therefore, it is suitable as an example of the lower part, and the partial length fuel rod 4
(Fuel rod symbol P).
As shown in FIG. 5, the enrichment difference and the maximum linear output density have a monotonically decreasing relationship when the enrichment difference is up to about 14%.
When the difference in enrichment is 3.0%, the maximum linear output density is about 13.
Although it is 64 kW / ft, the maximum linear output density decreases to about 13.5 kW / ft when the enrichment difference reaches 5.0%, and the maximum linear output density increases when the enrichment difference further increases to 6.3%. Becomes 13.4 kW / ft, which is exactly equal to the limit value. When the enrichment difference is further increased, the maximum linear output density falls below the limit value. When the enrichment difference is 10%, the maximum linear output density is about 13.14 kW / ft, and the enrichment difference is 1
At 2.5%, the maximum linear power density is about 12.9 kW / ft. Therefore, according to FIG. 5, in the fuel assembly for increasing the burnup, in order to make the maximum linear power density on the control rod side after the control rod withdrawal equal to or less than the limit value, the axial position 1 / 24-10 It has been found that it is effective to make the enrichment difference between the control rod side and the control rod side in all cross sections of / 24 at 6.3% or more.

In the fuel assembly 1L of the present embodiment,
In all the cross sections at the axial positions 1/24 to 10/24, the average enrichment of the second region 7 is about 14% larger than that of the first region 6, and satisfies the above condition.
Thereby, the maximum linear output density after pulling out the control rod 11 can be made equal to or less than the limit value of 13.4 kW / ft.

The above operation and effect will be described more specifically by comparison of local peaking coefficients using a comparative example.
FIG. 6 is a diagram illustrating a detailed cross-sectional structure of a fuel assembly 101L according to a comparative example. Members equivalent to those of the fuel assembly 1L of the embodiment shown in FIG. 1 are denoted by the same reference numerals and fuel rod symbols. As shown in FIG. 6, the fuel assembly 101L according to this comparative example uses the same fuel rods 4 as the fuel assembly 1L of the embodiment, but has a different arrangement. That is, the number of the fuel rods 4 (fuel rod symbol 1) is increased by two to eighteen, and eight fuel rods are arranged in the first area 6 and the second area 7, and the remaining two are arranged in the third area 8. Have been placed. Also, the number of fuel rods 4 (fuel rod symbol 2) is reduced by two to 28,
14 in each of the area 6 and the second area 7. The arrangement of the 14 fuel rods 4 (fuel rod symbol 4) and the eight fuel rods (fuel rod symbol P) is the same as in FIG.
The six fuel rods 4 (fuel rod symbol 3) are arranged in other areas. And, by adopting such a fuel rod arrangement, a portion excluding both ends in the axial direction (1/24 to 22 /
In all the cross sections located at 24), the average enrichment of all 34 fuel rods 4 in the second region 7 is greater than the average enrichment of all 34 fuel rods 4 in the first region 6. About 3%
It is getting bigger. Other points are the same as those of the embodiment shown in FIG.

Here, the fuel assembly according to the comparative example and the fuel assembly according to the embodiment shown in FIG. 1 were operated with the control rod 11 inserted up to the burn-up of 20 Gwd / t, and then the control rod 11 was moved. Table 1 shows the local peaking coefficient obtained by analysis in the cross section perpendicular to the axial direction when the wire was pulled out.

[0032]

[Table 1]

Table 1 shows the analysis results at an arbitrary cross section in the lower region (axial position 1/24 to 10/24) of each fuel assembly, and these coefficients are shown in FIG.
As is clear from FIG. 6, the value is the value of the fuel rod 4 (fuel rod symbol 4) at the corner on the control rod 11 side where the output increases most after the control rod 11 is pulled out. The reason for selecting the axial positions 1/24 to 10/24 here is the same as in the case of FIG. As shown in Table 1, the local peaking coefficient is 1.7 in the comparative example, whereas it is 1.6 in the embodiment, which is about 6% lower than the comparative example. You can see that. This is because, in the fuel assembly 101L of the comparative example, the difference in the burnup between the first region 6 and the second region 7 where the combustion is delayed when the control rod 11 is inserted is not sufficient. This is because the combustion in the first region 6 that has been delayed in burning when pulled out becomes more active than in the second region 7. As a result, the output of each fuel rod 4 existing in the first region 6 is increased as a whole, the local peaking coefficient of the fuel rod 4 (fuel rod symbol 4) at the corner of the control rod 11 is increased, and the maximum linear output is obtained. Density increases. In contrast, the fuel assembly 1 of the embodiment shown in FIG.
In L, as described above, the first region 6 and the second region 7
Therefore, the first region 6 does not burn very actively even when the control rod 11 is pulled out. Therefore, the output of each fuel rod 4 (fuel rod symbol 4) existing in the first region 6 is not so high as a whole, and the local peaking coefficient of the fuel rod 4 (fuel rod symbol 4) at the corner on the control rod 11 side. And the maximum linear power density does not increase much.

As described above, according to the present embodiment, the average enrichment of the second region 7 is higher than that of the first region 6 in all the cross sections at the axial positions 1/24 to 10/24. Since the value is increased by 6.3% or more, the local peaking coefficient of the first region 6 on the control rod 11 side after the control rod 11 is pulled out can be reduced, and the maximum linear output density is suppressed, which is the limit value. It can be 4 kW / ft or less. As a result, continuous operation for a longer time than before can be performed while suppressing the maximum linear output density.

In the above embodiment, 6. in all the cross sections at the axial positions 1/24 to 10/24.
Although the enrichment difference of 3% or more is set, it is not limited to this.
That is, as described above, the excessive maximum linear output density due to the partial combustion is remarkably generated in the lower part in the axial direction. Therefore, the axial position range in which the enrichment difference of 6.3% or more is set is further expanded upward, for example, from 1/24 to the range in which all the partial length fuel rods 4 (fuel rod symbols P) are present. It is clear that the same effect can be obtained even if the enrichment difference of 6.3% or more is set on all the 15/24 cross sections. Moreover, it expands further upwards, except for both ends in the axial direction.
The same applies to the case where the enrichment difference of 6.3% or more is set on all the cross sections.

In addition, even if the enrichment difference of 6.3% or more is not necessarily set in all the cross sections at the axial positions 1/24 to 10/24 as in the above embodiment, the linear output density actually increases. One of the maximum axial positions (1/2
5. somewhere between 4-10 / 24)
By setting the enrichment difference of 3% or more, if an appropriate enrichment difference (for example, 6%) that does not exceed the value in other cross sections, the same output suppression effect as in the above embodiment can be obtained. it is conceivable that. Considering this further,
In the structure of a general fuel assembly, the linear power density is maximized somewhere in the lower part in the axial direction (axial position 1/24 to 10/24 in the example of the above embodiment), but depending on the structure of the fuel assembly, In some cases, the linear output density becomes maximum at other positions, and the concept of the above embodiment can be applied to such a case. For example, the axial position 1/2 corresponding to the range where all the partial length fuel rods 4 (fuel rod symbol P) exist.
In the fuel assembly where the linear power density becomes maximum somewhere between 4 and 15/24, it is sufficient if the enrichment difference is 6.3% or more in the cross section somewhere between this 1/24 and 15/24, Axial position 1/24 to 22 / other than both ends in the axial direction
In the fuel assembly where the linear power density becomes maximum somewhere at 24, an enrichment difference of 6.3% or more at this cross section somewhere between 1/24 to 22/24 is sufficient, and the shaft is even wider. Axial total length 0 / 24-24 including both ends
In a fuel assembly having a maximum linear power density somewhere at / 24, it is sufficient that the enrichment difference is 6.3% or more in the cross section somewhere between 0/24 and 24/24. In any case, an appropriate concentration difference may be set so as not to exceed the value in other cross sections. In these cases, the same effects as in the above embodiment can be obtained.

In the above embodiment, the case where the fuel rods 4 and the water rods 10 are arranged in 9 rows and 9 columns has been described as an example, but the invention is not limited to this. That is, when the number of grids is increased to 10 rows, 10 columns, 11 rows, 11 columns,..., The present inventors have confirmed that the straight line shown in FIG. did. This means that, as the number of lattices increases, the enrichment difference between the non-control rod side and the control rod side where the maximum linear power density on the control rod side = 13.4 kW / ft becomes smaller than 6.3%. means.
Therefore, even in such a case, the maximum linear output density can be set to 13.4 kW / ft or less by setting the enrichment difference to at least 6.3% or more, and the same effect as in the above embodiment can be obtained. Obtainable.

In general, in the core, every time a predetermined combustion period (= cycle) elapses, a part of a large number of fuel assemblies provided in the core is replaced with the same number of new fuel assemblies. Or moving within the core (rearrangement)
I do. However, in the core provided with the fuel assembly 1L according to the above embodiment, as described above, the fuel assembly 1L can perform continuous operation for a longer period of time than before while suppressing the maximum linear power density. Sometimes, the fuel assembly 1L of the control cell 5 is not replaced or moved, and the fuel assembly 1L is continuously used in the second cycle (that is, at the end of the first cycle, only the other fuel assemblies are replaced or moved). When fuel cell assembly 1L is completed, replacement or movement of fuel assembly 1L may be performed.

Further, in the above embodiment, the operation and effect of the case where the low enrichment fuel assembly 1L is disposed in the control cell 5 has been described as an example. However, from the viewpoint of reducing the production type of the fuel assembly, as described in FIG. 2, the low enrichment fuel assembly used for the control cell 5 and the low enrichment fuel assembly arranged other than the control cell 5 are used. In some cases, they have a common structure. Therefore,
The low-enrichment fuel assembly 1L of the control cell 5 may be disposed in a portion other than the control cell 5 of the core.

[0040]

According to the present invention, it is possible to sufficiently suppress an increase in the output on the control rod side after the drawing, and to reduce the maximum linear output density to the limit value of 13.4 kW / ft or less.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a detailed cross-sectional structure of a low-enrichment fuel assembly according to an embodiment of the present invention, and a diagram showing an enrichment distribution of each fuel rod.

FIG. 2 is a cross-sectional view schematically showing an arrangement of a fuel assembly in a quarter cross section of a core.

FIG. 3 is a cross-sectional view showing a schematic cross-sectional structure of a control cell.

FIG. 4 is a sectional view showing a detailed vertical sectional structure of the low enrichment fuel assembly shown in FIG. 1;

FIG. 5 is a diagram showing values of the maximum linear output density of the first area when a predetermined concentration difference is provided between the second area and the first area.

FIG. 6 is a cross-sectional view showing a detailed cross-sectional structure of a fuel assembly according to a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fuel assembly 1L Low enrichment fuel assembly 4 Fuel rod 5 Control cell 6 First area 7 Second area (opposite control rod side) 8 Third area 10 Water rod 11 Control rod 15 Channel fastener

──────────────────────────────────────────────────の Continuing on the front page (51) Int.Cl. ⁶ Identification code FIG21C 3/32 GDBE (72) Inventor Akiko Kanda 3-1-1, Kochimachi, Hitachi-shi, Ibaraki Pref. (72) Inventor Takaaki Mochida 3-1-1 Sachimachi, Hitachi-City, Ibaraki Prefecture Inside Hitachi, Ltd.Hitachi Plant Inside the Hitachi Works of Hitachi, Ltd.

Claims (13)

[Claims] A plurality of fuel rods, each of which is filled with a fuel substance, and at least one water rod are arranged in a square lattice, and a guide post for fixing a channel fastener is provided on a control rod side. In the fuel assembly, on a predetermined cross section perpendicular to the axial direction of the fuel rod,
The average enrichment of all the fuel rods disposed on the control rod side from the diagonal fuel rods and water rods that divides the inside of the fuel assembly into a control rod side area and a non-control rod side area is defined as A fuel assembly, wherein the average enrichment of all the fuel rods arranged on the control rod side is reduced by 6.3% or more. 2. The fuel assembly according to claim 1, wherein the fuel assembly is on a predetermined cross section perpendicular to the axial direction of the fuel rod and having a cross section average enrichment higher than that of natural uranium. The average enrichment of all the fuel rods arranged on the control rod side from the diagonal fuel rod and water rod that divides the inside into the control rod side area and the anti-control rod side area is divided into the anti-control rod side. A fuel assembly wherein the average enrichment of all the arranged fuel rods is 6.3% or less. 3. The fuel assembly according to claim 1, wherein the plurality of fuel rods include partial length fuel rods whose axial length is shorter than others, and which is perpendicular to the axial direction of the fuel rods. And a fuel rod and water on a diagonal line that divides the inside of the fuel assembly into a control rod side region and a non-control rod side region on a predetermined cross section where all the partial length fuel rods are present. The average enrichment of all the fuel rods arranged on the control rod side of the rod is made smaller than the average enrichment of all the fuel rods arranged on the non-control rod side by 6.3% or more. Fuel assembly. 4. The fuel assembly according to claim 1, wherein the plurality of fuel rods include a partial length fuel rod having an axial length shorter than the other fuel rods, and being perpendicular to the axial direction of the fuel rod. And on all cross-sections where all the partial length fuel rods are present, the fuel rods and water rods on the diagonal line that divide the inside of the fuel assembly into a control rod side region and a non-control rod side region The fuel characterized in that the average enrichment of all the fuel rods arranged on the control rod side is smaller than the average enrichment of all the fuel rods arranged on the non-control rod side by 6.3% or more. Aggregation. 5. The fuel assembly according to claim 1, wherein the inside of the fuel assembly is located on all cross sections perpendicular to the axial direction of the fuel rods and having an average enrichment in cross section higher than that of natural uranium. The average enrichment of all the fuel rods disposed on the control rod side from the fuel rods and water rods that are diagonally divided into two parts, the control rod side area and the anti-control rod side area, is disposed on the anti-control rod side. 5. above the average enrichment of all the fuel rods.
A fuel assembly characterized by being reduced by 3% or more. 6. The fuel assembly according to claim 1, wherein the plurality of fuel rods include a plurality of high-enrichment fuel rods whose enrichment is higher than other fuel rods. A fuel assembly, wherein at least one-half of the high enrichment fuel rods is disposed closer to the control rod than the diagonal fuel rods and water rods. 7. The fuel assembly according to claim 1, wherein the plurality of fuel rods include a plurality of low-enrichment fuel rods whose enrichment is lower than other fuel rods. A fuel assembly, wherein at least one-half of the low-enrichment fuel rods are disposed closer to the control rod than the diagonal fuel rods and water rods. 8. The fuel assembly according to claim 1, wherein said plurality of fuel rods and at least one water rod are arranged in a square grid of 9 rows and 9 columns. body. 9. The fuel assembly according to claim 1, wherein the enrichment of the plurality of fuel rods is all equal to or higher than that of natural uranium. 10. The fuel assembly according to claim 1, wherein, of the plurality of fuel rods, four fuel rods arranged at four corners of the square lattice arrangement have the same enrichment. A fuel assembly, characterized in that: 11. A fuel assembly comprising: a plurality of fuel assemblies; and a plurality of control rods having a substantially cross-shaped cross section insertable between the plurality of fuel assemblies, wherein the plurality of fuel assemblies have an average enrichment higher than others. A core including a low-enrichment fuel assembly, wherein the low-enrichment fuel assembly includes the fuel assembly according to claim 1. 12. A plurality of fuel assemblies including a low-enrichment fuel assembly having an average enrichment lower than the others, and a plurality of control rods having a substantially cross-shaped cross section insertable between the plurality of fuel assemblies. A plurality of control cells in which four low-enrichment fuel assemblies are arranged in a substantially square shape so as to sandwich one of the control rods. A core, wherein the fuel assembly according to claim 1 is used as a fuel assembly. 13. A core according to claim 12, wherein at least one of said plurality of fuel assemblies is replaced or replaced with a new fuel assembly every time a predetermined combustion period elapses. And, after the start of operation of the core, when the predetermined combustion period has elapsed for the first time,
A configuration in which the arrangement side or replacement of the low enrichment fuel assembly arranged in the control cell is not performed, and the arrangement side or replacement of a fuel assembly other than the low enrichment fuel assembly arranged in the control cell is performed. A core characterized in that: