JPH08129092A - Initial loading reactor core - Google Patents

Abstract

PURPOSE: To obtain an initial loading reactor core, in which thermal allowance can be improved and which corresponds to the increase of a burnup, by providing square- shaped cells constituted of four fuel assemblies having lowest mean enrichment and L-shaped cells constituted of three fuel assemblies having highest mean enrichment, arranging the L-shaped cells so as to be adjoined at the corners of the square shapes and setting the rate of the L-shaped cells at the specific rate or more of the whole. CONSTITUTION: 764 fuel assemblies in all reactor cores are constituted of 244 fuel assemblies 1 in low enrichment, 176 fuel assemblies 2 in intermediate enrichment and 344 fuel assemblies 3 in high enrichment, and control cells B are constituted of four fuel assemblies in low enrichment. The high enrichment fuel assemblies 3 are charged in regions, in which peaking coefficients in the radial direction of the outer circumference of a reactor core are reduced, and regions except the periphery of the reactor core. The three high-enrichment fuels configure L-shaped cells A at that time, and the 96 high-enrichment fuels of the L-shaped cells are arranged at the corners of the cells B, and occupy 12.6% (10% or more) of the total number of fuels, Peaking coefficients in the radial direction of the high-enrichment fuels of the L-shaped cells are reduced because of the low combustion outputs of control cells, thus lowering the maximum linear heat ratings of the high-enrichment fuels.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an initial loaded core of a boiling water reactor (BWR) installed in a nuclear power plant.

[0002]

2. Description of the Related Art Generally, in the BWR, the average enrichment of the fuel assemblies loaded in the core during the initial operation, that is, the so-called initial-loaded core is the same and one kind. By the way, the reactor is 1
Every cycle, 1/3 to 1/4 of the total number of fuel assemblies are taken out and replaced with fresh fuel. However, the average enrichment of the initially loaded fuel assembly for the core is set so that combustion can be performed in the core for a period of 2 to 3 cycles. In the fuel exchange at the end of the operation cycle using the initially loaded core fuel assembly (hereinafter referred to as the first cycle), combustion has not yet progressed sufficiently, and the fuel assembly with a large residual amount of uranium 235 is removed from the core. It is uneconomical to take it out.

The cycle in which the fuel is partially exchanged after the first cycle and the operation is continued is called the second cycle, the third cycle, or the like. The fresh fuel assembly loaded at the beginning of the operation cycle after the second cycle is called a replacement fuel assembly. The core in which the replacement fuel assemblies are continuously loaded for several cycles after the first cycle becomes a stable core. That is, the thermal characteristics of the previous cycle and the next cycle of the two continuous cycles are almost the same, and the thermal characteristics of the operation cycle are stable. This operation cycle is called an equilibrium cycle, and the core that has reached the equilibrium cycle is called an equilibrium core.

In such a nuclear reactor, an intermediate cycle (hereinafter,
It is preferable that the thermal characteristics and the incremental burnup in the cycle (referred to as a transition cycle) become similar to those in the equilibrium cycle or rapidly converge to those in the equilibrium cycle.
However, when the average enrichment of the fuel assembly is one as in the conventional initially loaded core, it takes a long time to transition to the equilibrium cycle, and the number of replacement fuel assemblies in the transition cycle also fluctuates. It was great.

Therefore, in the BWR, various types of fuel assemblies having different average enrichments are combined to form an initially loaded core, which is taken out from a fuel assembly having a low average enrichment for each cycle, and this is used as a new fuel. By exchanging with the aggregate,
Attempts have been made to increase the average extraction burnup from the fuel assemblies loaded in the initially loaded core and to speed up the transition to the next cycle. This technique is disclosed in, for example, Japanese Patent Laid-Open No.
It is described in Japanese Patent No. 8486.

On the other hand, in order to prolong the operating cycle and increase the burnup, it is necessary to increase the average enrichment even in the initially loaded core. As described above, when a plurality of types of fuel assemblies having different average enrichments are combined to form an initially loaded core, the enrichment difference between the fuel assemblies becomes large, and the high enrichment fuel assemblies (hereinafter, The difference in the nuclear characteristics between the enrichment fuel) and the low enrichment fuel assembly (hereinafter referred to as the low enrichment fuel) became large.

When fuel assemblies having greatly different nuclear characteristics are adjacent to each other, neutrons are exchanged because the neutron spectra are different from each other. As a result, in the early stage of combustion, the maximum line power density was large, the thermal margin of the reactor core was small, and improvement of the thermal margin had been a problem.

Further, if the average enrichment of the core is increased, the excess reactivity of the entire core tends to increase. Therefore, a large number of control rods need to be inserted into the core during operation. At this time, in the cell where the control rod is inserted,
When the control rod is burned while being inserted into the core for a long period of time, the fuel assembly forming this cell has a smaller output on the control rod insertion side and a larger output on the opposite side than the control rod insertion side. That is, since the combustion of the fuel rod on the control rod insertion side is delayed, there is a phenomenon that the output of the fuel rod on the control rod insertion side becomes large when the control rod is pulled out. Therefore, the fuel assembly that surrounds the control rod that is inserted into the core for a long time during operation is composed of the fuel assembly having the lowest average enrichment (low enrichment fuel).

As described above, in the initially loaded core aiming at high burnup, it is necessary to provide a large number of control cells surrounding the control rods with four low-concentration fuels. Therefore, the fuel assemblies loaded in the cells other than the control cells tend to have a large proportion of the highly enriched fuel, and it has been necessary to place the highly enriched fuels as close to each other as possible. Since the high enrichment fuel has a high enrichment, when these fuels are loaded adjacently, the output of these fuels tends to be high and the thermal margin of the reactor core tends to be severe.

[0010]

In the initially loaded core aiming at high burnup, since the core average enrichment is high, it is necessary to increase the number of control cells, and the excess reactivity becomes high. The control cell requires a low enrichment fuel. Therefore, it is necessary to increase the average core enrichment and load a large amount of highly enriched fuel into cells other than the control cell. However, arranging a large number of highly enriched fuels intensively means arranging fuels having high reactivity adjacent to each other, which inevitably results in a severe thermal margin.

It is an object of the present invention to provide an initial-loading core for high burnup, which can improve the thermal margin while arranging highly enriched fuels adjacent to each other.

[0012]

In order to achieve the above object, the present invention is directed to an initially loaded core formed by combining various types of fuel assemblies having different average enrichments of uranium 235. A square cell composed of four lowest fuel assemblies and an L-shaped cell composed of three fuel assemblies having the highest average enrichment are provided, and each fuel assembly of the L-shaped cell is the square. The L-shaped cells are arranged at the corners of the square-shaped cells so as to be adjacent to the cells having a rectangular shape.
It is characterized in that the ratio of the number of fuel assemblies of the L-type cells to the total fuel assemblies loaded in the core is 10% or more.

[0013]

The thermal limit value of the fuel assembly includes the maximum linear power density and the minimum limit power ratio. Of these, the maximum linear power density is determined by three peaking coefficients. The first peaking coefficient is a local peaking coefficient that represents the ratio of the output of one fuel rod to the average output of all the fuel rods in the fuel assembly in a cross section perpendicular to the axial direction of the fuel assembly. The second peaking coefficient is an axial peaking coefficient that represents a ratio of the maximum output to the average output over the entire axial direction among the outputs in the cross section perpendicular to the axial direction of the fuel assembly. The third peaking coefficient is a radial peaking coefficient that represents the ratio of the output of one fuel assembly to the average output of all fuel assemblies in the core.

The present invention reduces the radial peaking coefficient for high enrichment fuel among these three peaking coefficients.

FIG. 2 shows a loading pattern of the fuel, which is the basis of the present invention, in which three highly enriched fuels constitute an L-type cell. In FIG. 2, A is an L-shaped cell (shown by diagonal lines) and B is
Indicates a square cell (shown by a thick line) composed of four low-enrichment fuels. The radial peaking coefficient of a fuel assembly is influenced by various factors. Since the radial peaking coefficient represents the output of the fuel assembly, the higher the reactivity of the fuel assembly, the higher the output. Also,
If the output of the adjacent fuel assemblies is high, the influence thereof will increase the radial peaking coefficient. Further, when the control rod is inserted into the adjacent control cell, the output of the fuel assembly of that cell becomes low, and the radial peaking coefficient also becomes small due to the influence.

In the initially loaded core, the maximum linear power density is high for the highly enriched fuel. Therefore, if the radial peaking coefficient of the highly enriched fuel can be reduced, the maximum linear power density of the entire core is also low. It is possible to

Further, in the initially loaded core, in the case where the types of enrichment of the fuel assembly are three types in order to achieve high burnup,
At the beginning of the first cycle of the initially loaded core, the reactivity of the medium enrichment fuel assembly (hereinafter referred to as the medium enrichment fuel) is highest among the three enrichments. Therefore, if the medium enrichment fuel is adjacent to the high enrichment fuel in which the maximum linear power density is likely to occur in four directions, front, rear, left and right, the maximum linear power density tends to increase. FIG. 3 shows this example. The characteristic of this loading pattern is that the medium enrichment fuel 2 and the high enrichment fuel 3 are alternately arranged in a checkerboard shape. This arrangement has been well practiced in the past, and the fuel having a high reactivity and the fuel having a low reactivity are alternately arranged, so that the loading pattern has a small radial peaking coefficient.
However, regarding the maximum linear power density, which is one of the thermal limit values, since the local peaking coefficient is large at the position of I because of the high enrichment fuel, the maximum linear power density occurs at I.

On the other hand, in FIG. 2, the highly enriched fuel 3
Since the medium-enriched fuel 2 having high reactivity is loaded only in two directions of the front, rear, left, and right adjacent positions, the radial peaking coefficient of the high-enriched fuel 3 is smaller than I in FIG. However, the radial peaking coefficient of the medium enriched fuels in I and F of FIG. 2 is larger than the radial peaking coefficient of the medium enriched fuels in E and J of FIG. However, in the case of the medium enriched fuel, the local peaking coefficient is not so large as that of the highly enriched fuel, so that the larger the radial peaking coefficient does not affect the maximum linear power density of the entire core. The fuels D and L in FIG. 2 are adjacent to the control cell B into which the control rod is inserted during operation, and therefore are affected by the control cell. That is, since the fuel output of the control cell B is low, the radial peaking coefficient of the high enrichment fuels of D and L becomes small.

As described above, as shown in FIG.
By arranging the L-type highly enriched fuel in the corner of the control cell, the maximum linear power density of the highly enriched fuel can be suppressed to be small.

FIG. 4 shows the relationship between the maximum linear power density and the number ratio of the highly enriched fuel composing the L-type cell to the total fuel assemblies in the core. As can be seen from the figure, the maximum line power density can be made lower than the operation limit value by setting the above-mentioned number ratio to 10% or more.
Thus, the L-shaped loading pattern as shown in FIG. 2 reduces the maximum line power density. However, since there are many highly enriched fuels in the core, it is necessary to arrange the L-shaped cell shown in FIG. 2 at a position where the maximum linear power density is likely to occur in the highly enriched fuel and the radial peaking coefficient is large. . Therefore, it is effective to provide L-shaped cells in the core at a certain ratio or more to reduce the maximum linear power density.

As described above, in the present invention, by making the high enrichment fuel belonging to the L-type cell 10% or more of the total core fuel, the maximum linear power density is small and the initial loading capable of achieving high burnup is achieved. A core can be realized.

[0022]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 shows a first embodiment of the fuel loading pattern of the initially loaded core according to the present invention. In this loading pattern, the total number of fuel assemblies in the core is 764, of which 244 low enrichment fuel 1 and 17 intermediate enrichment fuel 2
6 bodies and high enrichment fuel 3 are composed of 344 bodies.
A control cell B (indicated by a bold line) for inserting a control rod used during operation is composed of four low-enrichment fuels, and the number thereof is 25 in the core. However, this does not mean that all of these 25 control cells are used during operation. The other low-enrichment fuels are mainly arranged in the outermost peripheral portion of the core, and the remaining low-enrichment fuels are arranged in other regions in a mixed manner with the high-enrichment fuels and the medium-enrichment fuels.

The high-enrichment fuel 3 is mainly loaded in two areas. The first region is a region where the radial peaking coefficients of the second layer and the third layer from the outermost periphery on the outer periphery of the core become smaller due to the leakage of neutrons. The second region is a region other than the periphery of the core. Here, three highly-enriched fuels constitute an L-shaped cell A (shown by diagonal lines), and many fuel cells are arranged at the corners of the control cell B. The number of L-type cells arranged at the corner of control cell B is 32
The number of high-concentration fuels that belong to this group is 96, and the ratio to the total number of fuel in the core is 12.6%.

In this embodiment, since the fuel output of the control cell is low, the radial peaking coefficient of the highly enriched fuel of the L-type cell is small. Therefore, it is possible to reduce the maximum linear power density of the highly enriched fuel and improve the thermal margin.

In the fuel assembly of this embodiment, the fuel rods are arranged in a square lattice of 8 rows and 8 columns, and one cylindrical water rod is arranged in the region of 4 lattices in the central portion. Fuel assemblies and fuel rods are arranged in a square grid with 9 rows and 9 columns,
A fuel assembly in which two cylindrical water rods are arranged in the area of 7 lattices in the center, or fuel rods are arranged in a square lattice of 9 rows and 9 columns, and are arranged in the area of 9 lattices in the center. A fuel assembly in which one prismatic water rod is arranged can be used.

(Embodiment 2) FIG. 5 shows a second embodiment of the fuel loading pattern of the initially loaded core according to the present invention. In this loading pattern, the total number of fuel assemblies in the core is 872, of which 272 are low enrichment fuels 1 and 20 are medium enrichment fuels 2.
4 units, high enrichment fuel 3 is composed of 396 units.
A control cell B (shown by a thick line) for inserting a control rod used during operation is composed of four low-enrichment fuels, and the number thereof is 29 in the core. However, these 2
It does not mean that all nine control cells are used during operation. The other low-enrichment fuels are mainly arranged in the outermost peripheral portion of the core, and the remaining low-enrichment fuels are arranged in other regions in a mixture with the high-enrichment fuels and the middle-enrichment fuels.

A large number of L-shaped cells A (indicated by diagonal lines) made of three highly enriched fuels are arranged at the corners of the control cell B. The number of L-type cells arranged in the corners of the control cell B is 40, and the number of highly enriched fuels belonging to this is 120, and the ratio to the total number of fuels in the core is 13.8%. is there.

Also in this embodiment, the same effect as that of the first embodiment can be obtained. Further, in this embodiment, since the number of L-shaped cells is larger than that in the first embodiment, the fuel power distribution in the core can be flattened more than in the first embodiment. Therefore, in this embodiment, the distribution of the radial peaking coefficient in the core can be made flatter than in the first embodiment. Further, the fact that the core size of this embodiment is larger than that of the first embodiment also contributes to flattening the distribution of the radial peaking coefficient. As the fuel assembly of this embodiment, the same fuel assembly as in the first embodiment can be used.

(Embodiment 3) FIG. 6 shows a third embodiment of the fuel loading pattern of the initially loaded core according to the present invention. In this loading pattern, the total number of fuel assemblies in the core is 764, of which 216 are low enrichment fuels 1 and 17 are intermediate enrichment fuels 2.
6 bodies and the highly enriched fuel 3 are composed of 372 bodies.
A control cell B (indicated by a bold line) for inserting a control rod used during operation is composed of four low-enrichment fuels, and the number thereof is 25 in the core. However, these 2
It does not mean that all five control cells are used during operation. The other low enrichment fuels are arranged in a mixture with the high enrichment fuel and the medium enrichment fuel.

Highly enriched fuel is loaded on the outermost periphery of the core. In addition, except for this region, three high-enrichment fuels mainly constitute the L-type cell A (shown by diagonal lines),
Many are arranged in the corners of the control cell B.
The number of L-type cells arranged in the corners of the control cell B is 32, and the number of highly enriched fuels belonging to this is 96, and the ratio to the total number of fuels in the core is 12.6%.
Is.

Also in this embodiment, the same effect as that of the first embodiment can be obtained. Further, in this embodiment, since the highly enriched fuel is loaded in the outermost peripheral portion of the core, the fuel output in the outer peripheral region of the core can be increased. Therefore, in this embodiment, the distribution of the radial peaking coefficient in the core is
It can be flattened more than in the above embodiment. As the fuel assembly of this embodiment, the same fuel assembly as in the first embodiment can be used.

(Embodiment 4) FIG. 7 shows a fourth embodiment of the fuel loading pattern of the initially loaded core according to the present invention. In this loading pattern, the number of fuel assemblies in the entire core is 764, of which 240 are low enrichment fuel 1 and 18 are intermediate enrichment fuel 2.
It consists of 4 units and 340 units of highly enriched fuel. There are two types of highly enriched fuels, and the difference between the two is the number of fuel rods containing gadolinia-containing pellets. In this embodiment, a fuel assembly having a small number of fuel rods containing gadolinia-containing pellets is used as the high enrichment fuel (low Gd fuel) 3
36 fuel-rich fuel assemblies with high enrichment (high Gd fuel) 4
There are 104 units, for a total of 340 units. The two types of the same highly enriched fuel are provided to reduce the radial peaking coefficient. A control cell B (indicated by a bold line) for inserting the control rod during operation is composed of four low-enrichment fuels, and the number thereof is 25 in the core. However, this does not mean that all of these 25 control cells are used during operation.
The other low-enrichment fuels are mainly arranged in the outermost peripheral portion of the core, and the remaining low-enrichment fuels are arranged in other regions in a mixed manner with the high-enrichment fuels and the medium-enrichment fuels.

A part of the highly enriched fuel consists of three L-type cells A.
(Indicated by diagonal lines), and many are arranged at the corners of the control cell B. There are two types of high-enrichment fuel forming the L-type cell, which are configured by combining a high-enrichment fuel (low Gd fuel) 3 and a high-enrichment fuel (high Gd fuel) 4. In this case, the number of L-type cells is 28, the number of highly enriched fuels belonging to this is 84, and the ratio to the total number of fuel bodies in the core is 11.0%.

Also in this embodiment, the same effect as that of the first embodiment can be obtained. Furthermore, in the present embodiment, by combining the high enrichment fuel (low Gd fuel) 3 and the high enrichment fuel (high Gd fuel) 4, the different output suppressing effects of these fuels are utilized, and It is possible to control the output distribution of the fuel in steps. Therefore, in this embodiment, the distribution of the radial peaking coefficient in the core can be made flatter than in the first embodiment. For this reason, in this embodiment,
Even if the number of L-type cells is smaller than that in the first embodiment, the first
It is possible to achieve the same effect as that of the embodiment. In this example, Gd is used as the high-enrichment fuel that constitutes the L-type cell.
Although two types of fuel assemblies having different numbers of fuel rods including is used, the same effect can be obtained by using three types of fuel assemblies. Further, as the fuel assembly of this embodiment, the same fuel assembly as that of the first embodiment can be used.

[0036]

According to the present invention, even if the high-enrichment fuels are arranged adjacent to each other, the radial peaking of the high-enrichment fuels can be reduced and the maximum linear power density can be kept low.
It is possible to obtain an initially loaded core capable of improving the thermal margin and increasing the burnup.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an initially loaded core according to the present invention.

FIG. 2 is a diagram showing an in-core fuel loading pattern of the present invention.

FIG. 3 is a diagram showing a fuel loading pattern in a core of a comparative example.

FIG. 4 is a relational diagram of the maximum linear power density and the number ratio of the highly enriched fuel forming the L-type cell.

FIG. 5 is a diagram showing a second embodiment of the initially loaded core according to the present invention.

FIG. 6 is a diagram showing a third example of the initially loaded core according to the present invention.

FIG. 7 is a diagram showing a fourth example of the initially loaded core according to the present invention.

[Explanation of symbols]

1 ... Low enrichment fuel, 2 ... Medium enrichment fuel, 3, 4 ... High enrichment fuel.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiro Yoshioka 3-2-1, Sachimachi, Hitachi City, Ibaraki Hitachi Engineering Co., Ltd. (72) Inventor Junichi Koyama 7-2, Omikacho, Hitachi City, Ibaraki Prefecture No. 1 Incorporated company Hitachi, Ltd. Power & Electric Machinery Development Headquarters (72) Inventor Akihiro Yamanaka 3-1-1 1-1 Saiwaicho, Hitachi City, Ibaraki Hitachi Ltd. Hitachi factory (72) Inventor Mitsuya Nakamura Ibaraki 3-1, 1-1 Sachimachi, Hitachi, Ltd. Hitachi, Ltd. Hitachi factory (72) Inventor Katsumasa Urikawa 3-1-1, Sachimachi, Hitachi, Ibaraki Hitachi Ltd., Hitachi, Ltd.

Claims (7)

[Claims] 1. An initially loaded core comprising a combination of various types of fuel assemblies having different average enrichments of uranium 235, wherein a square cell composed of four fuel assemblies having the lowest average enrichment, An L-shaped cell composed of three fuel assemblies having the highest average enrichment, and the L-shaped cells are arranged so that each fuel assembly of the L-shaped cells is adjacent to the square-shaped cell. An initially loaded core, which is arranged in a corner portion of a cell having a rectangular shape, and a ratio of the number of fuel assemblies of the L-shaped cells to all the fuel assemblies loaded in the core is 10% or more. 2. The initially loaded core according to claim 1, wherein the fuel assemblies forming the L-shaped cells are composed of a plurality of types of fuel assemblies having different numbers of fuel rods containing burnable poisons. Initially loaded core. 3. The initially loaded core according to claim 1, wherein the fuel assembly forming the square cells is adjacent to a control rod which is inserted into the core during power operation and used for power adjustment. Initially loaded core characterized by 4. The initially loaded core according to claim 1, wherein the fuel assembly having the lowest average enrichment does not contain burnable poisons. 5. The initially loaded core according to any one of claims 1 to 4, wherein the fuel rods forming the fuel assembly are arranged in a square lattice of 8 rows and 8 columns, and are arranged in a region corresponding to 4 lattices in the central portion. An initially loaded core characterized in that one cylindrical water rod is arranged. 6. The initially loaded core according to any one of claims 1 to 4, wherein the fuel rods constituting the fuel assembly are arranged in a square grid of 9 rows and 9 columns, and are arranged in a central area of 7 grids. An initially loaded core characterized in that two cylindrical water rods are arranged. 7. The initially loaded core according to any one of claims 1 to 4, wherein the fuel rods constituting the fuel assembly are arranged in a square lattice of 9 rows and 9 columns, and are arranged in an area corresponding to 9 lattices in the central portion. An initially loaded core characterized in that one prismatic water rod is arranged.