JPH10307196A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel assembly corresponding to D-grid core capable of flattening the power in the cross section vertical to the axis of a fuel assembly and simultaneously effectively improving core shutdown margin in cooled time. SOLUTION: By arranging a thick diameter water rod 3 in the center and a plurality of fuel rods 2 in a square grid, set are a region (a) 4 of fuel rod group facing to wide gap water region, a region (b) 5 of fuel rod group other than the region (a) 4 in the wide gap water region side, a region (d) 7 of fuel rod group facing to narrow gap water region and a region (c) 6 of fuel rod group other than the region (d) 7 in the narrow gap water region side. When the relative quantity of fissile material contained in each 4 fuel rod group to that in the whole regions and average enrichment are compared, the relative quantity of the fuel rod group and the average enrichment in the region (a) 4 are minimized and those in the region (b) 5 are maximized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly in a boiling water reactor (BWR), and more particularly to a fuel assembly compatible with a D lattice core suitable for high burn-up.

[0002]

2. Description of the Related Art FIG. 2 is a partial horizontal sectional view of a fuel assembly group of a conventional BWR currently in operation. That is, the fuel bundle formed by arranging the fuel rods 2 in a square lattice is set in the center of the unit cell 22, and the channel box 23 is placed so as to surround the fuel bundle, so that the light water of the coolant does not boil. To form a gap water region that becomes a flow path for flowing through. The gap water region has two types, a side 25 into which the control rod 24 is inserted and a side 26 where the control rod 24 is not taken in and out.

In order to reduce the power generation cost of a light water reactor,
It is effective to reduce the fuel cycle cost. As one of the methods, it is considered to increase the average removal burnup of the fuel assembly. In general, the average enrichment of the fuel assembly increases as the removal burn-up increases. As the fuel enrichment increases, the reactivity at the cold temperature (about 20 ° C.) increases, and the difference between the reactivity at the time of operation (about 280 ° C.) and the reactivity at the time of cold temperature increases (the reactivity at the time of cold temperature at the time of operation). Difference increases). When the reactivity is high, the control rod controls the neutron generation and extinction in a balanced manner.

As shown in FIG. 2, the control rod 24 is present on one side of the fuel assembly both during operation and at cold temperature. The neutron absorber B ₄ C of the control rod reduces the reactivity by absorbing thermal neutrons in particular. Even if the relative enrichment distribution of the fuel assemblies is the same, as the enrichment increases, the neutron absorption by the control rods decreases, and the control rod value at cold temperature decreases. Here, the control rod value is the magnitude of the reactivity of the control rod, and is simply a value proportional to the square of the neutron flux at the position of the control rod.

[0005] Therefore, in the case of a high burn-up core, the margin for stopping the furnace at a cold temperature tends to be small. This means
When the reactor is shut down, the criticality tends to reach, which is not preferable. In order to improve the furnace shutdown margin at the time of cold temperature, (1) a method of increasing the control rod value to increase the difference between the cold reactivity at the time of control rod insertion and the cold reactivity at the time of no control rod insertion, And (2) a method of reducing the difference in reactivity between operation and cold temperature is conceivable.

As an example of the above (1), see, for example,
The technique described in Japanese Patent Application Laid-Open No. 1-275696 is known. FIG. 3 is a horizontal cross-sectional view of a fuel assembly group according to prior art 1.
The fuel assembly shown in FIG. 3 is a method of increasing the control rod value by making the enrichment of the fuel rod 32 facing the cruciform control rod 31 higher than the average fuel enrichment of the fuel assembly.

Further, for example, a technique described in Japanese Patent Application Laid-Open No. 63-98590 is known. FIG. 4 is a horizontal cross-sectional view of a fuel assembly group according to prior art 2. In the fuel assembly shown in FIG. 4, a cross-shaped water channel 42 is arranged at the center of the fuel assembly 41 and the fuel assembly is divided into four sub-fuel assemblies 43 and 44.
Sub-fuel assembly 44 closest to the control rod 45 when divided into
In this method, the value of the control rod in the cold state is increased by making the average enrichment of the sub-fuel assembly 43 higher than the average enrichment of the other sub-fuel assemblies 43.

[0008]

The currently operating BWR has a C lattice core having the same area of the gap water regions 25 and 26 shown in FIG. 2 and an area of the gap water region 25 into which the control rod 24 is inserted. Has a larger D lattice core. When the above-mentioned prior art 1 (FIG. 3) is applied to the D-lattice core, the local power peaking coefficient during operation tends to increase on the gap water region side where the control rod 24 is inserted. Here, the output peaking coefficient is a ratio between the maximum output value and the average value,
In a furnace where this value is small, the output is flattened.

In the prior art 2 (FIG. 4), the control rod 45
In order to make the average enrichment of the sub fuel assembly 44 closest to the sub fuel assembly 43 higher than the average enrichment of the other sub fuel assemblies 43, the fuel enrichment of the fuel rods located at the outermost periphery of the sub fuel assembly is increased. I was That is, since the highly enriched fuel rods are adjacent to the cruciform waterway 42, the local output peaking coefficient during operation is particularly high in a high burnup fuel assembly whose average enrichment should be higher than the current one. The fuel rods adjacent to the mold channel 42 tend to be large. That is, the prior arts 1 and 2 do not consider an increase in the local output peaking coefficient during operation, and have a problem in that the thermal margin is reduced accordingly.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to effectively improve the margin for stopping the furnace at a cold temperature while flattening the output in a cross section perpendicular to the axial direction of the fuel assembly. An object of the present invention is to provide a fuel assembly compatible with a D lattice core.

[0011]

In order to solve the above-mentioned problems, a fuel assembly according to the present invention has a structure in which a plurality of fuel rods containing nuclear fuel material and a large-diameter water rod are arranged in a square lattice. A fuel assembly in which the large-diameter water rod is disposed at a central portion, wherein the fuel rod faces a wide gap water region into which a control rod is inserted in a cross section perpendicular to the axial direction of the fuel assembly. A group region a, a region b of the fuel rod group other than the region a on the wide gap water region side, a region d of the fuel rod group facing the narrow gap water region, and a fuel rod other than the region d on the narrow gap water region side A case where a group c is set and the relative amount of the fissile material contained in the whole region of the fissile material contained in each of the four regions is compared with the average enrichment of the fuel rods in each of the four regions. , The relative amount of the fuel rod group in the area a, While minimizing the serial average enrichment,
The relative amount and the average enrichment of the fuel rod group in the region b are maximized.

FIG. 5 is a horizontal sectional view of a fuel assembly loaded in a general D-lattice core. As shown in FIG. 5, the enrichment distribution in which the fuel enrichment of the fuel rod on the side 54 facing the narrow gap water region 52 is high is adopted. That is, the local output peaking coefficient is suppressed by lowering the enrichment of the outermost fuel rods of the fuel assembly adjacent to the wide gap water region 51. This is the reference case.

Now, the fuel region of the fuel assembly is set so as to face the fuel rod group [region a] facing the wide gap water region, the fuel rod group [region b] other than the wide gap water region side, and the narrow gap water region. The fuel rod group [region d] and the fuel rod group [region c] other than the narrow gap water region side are divided into four. In the reference case, the enrichment of the region a is minimum in each region. In the above-mentioned prior art 1, the control rod value at the time of cold temperature is improved by increasing the degree of concentration in the region a.

FIG. 6 is an explanatory diagram of the principle of the present invention, and is a diagram showing a change in control rod value at the time of cold / hot. FIG. 6 shows the case where the relative amount of U-235 included in the region a is the same as that of the reference case so that the relative amount is the minimum in each region, and the U-2 included in the region b is the same.
It shows a change in the control rod value at the time of cold when the relative amount of the control bar 35 is increased. Here, the relative enrichment of U-235 contained in region b is increased by increasing the fuel enrichment of the fuel rods not adjacent to large diameter water rod 3 located at the center of fuel assembly 1 than in the reference case. Is increasing.

Here, it is assumed that the relative amount of U-235 included in the region is represented by the number of U-235 included in the region / the number of U-235 included in the fuel assembly. As shown in FIG. 6, with respect to the reference case, U-
By increasing the relative amount of 235 by about 30% and making it larger than the relative amount of U-235 included in region c, the control rod value at the time of cold temperature is increased by about 0.7% Δk / k. According to the current design criteria for boiling water reactors, the reactor shutdown margin is 1% △ k
That is all. Therefore, it can be seen that the improvement effect of the present invention is great.

FIG. 7 is a diagram for explaining the principle of the present invention, and is a diagram showing a change in the reactivity difference at the time of cooling and at the time of operation. As shown in FIG. 7, when the relative amount of U-235 included in the region b is increased and is larger than the relative amount of U-235 included in the region c, the difference in the reactivity during operation at the cold temperature decreases.

FIG. 8 is a view for explaining the principle of the present invention.
FIG. 4 is a diagram illustrating a change in a local output peaking coefficient. FIG.
As shown in FIG. 7, increasing the amount of U-235 in region b has no significant effect on increasing the local output peaking coefficient. Even if the relative amount of U-235 included in the region b is increased by about 30% with respect to the reference case, the increase of the local output peaking coefficient is only about 3%. This is because, as described above, the relative amount of U-235 was minimized in the region a adjacent to the wide gap water region.

As described above, U included in region b
By increasing the relative amount of -235, it is possible to improve the value of the control rod at the time of cold and the reactivity difference at the time of cold and at the time of operation while suppressing the increase of the local output peaking coefficient, and to improve the margin for stopping the furnace at the time of cold. That is, by providing a large-diameter water rod in the center of the fuel assembly, while trying to flatten the output,
Further, by increasing the degree of enrichment in the region b, the furnace stop margin at the time of cold temperature can be effectively improved.

Specific methods for increasing the relative amount of U-235 contained in the region b include: (1) providing an enrichment distribution;
(2) Fuel inventory is changed between regions.
In the method (2), (a) the diameter of the fuel pellet is increased,
In addition to (b) increasing the number of fuel rods included in the region, (c) increasing the fuel pellet density, and preventing the thermal neutron flux distribution in the fuel assembly from becoming non-homogeneous in the center of the fuel assembly. By moving the inserted large-diameter water rod toward the region c, it can be realized by increasing the number of fuel rods included in the region b.

Moving the large-diameter water rod toward the narrow gap water region where there is little moderator can further improve the flattening of the thermal neutron flux distribution in the fuel assembly, and as a result, improve the reactivity. Can be planned. That is, the ratio of the thermal neutron flux in the narrow gap water region to the thermal neutron flux in the wide gap water region is 0.626 in the reference case.

On the other hand, when the relative amount of U-235 contained in the region b is excessively increased by the method (1), the local output peaking coefficient in the region b may increase. Therefore, in order to more effectively improve the furnace shutdown margin at the time of cold temperature while flattening the output, the above-mentioned method (1) "adding the fuel rod enrichment distribution" and the method (2) It is desirable to combine the "change of fissile material amount (relative amount) between regions".

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a horizontal sectional view of a fuel assembly showing one embodiment of the present invention. In the fuel assembly shown in FIG. 1, reference numeral 1 denotes a fuel assembly, in which fuel rods 2 are arranged in a square lattice, and a large-diameter water rod 3 is provided in the center of the fuel assembly 1. In the present embodiment, the enrichment (w / o) of the fuel rods 2 is set to decrease in the order of A, B, C..., And the enrichment of each fuel rod is as shown in Table 1.

[0023]

[Table 1] In the present embodiment, the amount of U-235 contained in each of the regions 4 (region a), 5 (region b), 6 (region c), and 7 (region d) is adjusted by the concentration distribution. In the present embodiment, the relative amount of U-235 in the region b is maximized by increasing the enrichment of the fuel not adjacent to the large diameter water rod 3. In the present embodiment, the relative amount in the region b is 30.0%, and the amount of U-235 is increased by about 20% as compared with the reference case shown in FIG. On the other hand, the relative amount of U-235 in region a is about 18%, which is the same as the reference case.

On the other hand, similar to the fuel enrichment distribution employed in the fuel assemblies loaded in the currently operating D-lattice core,
In the region d facing the narrow gap water region, the average fuel enrichment (wt%) is set to 3.75, which is higher than the average enrichment 3.67 in the region c, whereby the local output peaking coefficient can be suppressed. It is configured as follows.

As described above, the U included in the area b
Due to the configuration of the fuel assembly in which the relative amount of -235 is increased, the control rod value at the time of cold is about 0.3% △ k / k larger than the reference case. In addition, the difference in reactivity between cold and hot during operation is about 0.02%
Δk / k becomes smaller. On the other hand, the increase in the local output peaking coefficient is suppressed to about 3%. That is, in the present invention, while suppressing the increase of the local output peaking coefficient, the relative amount of the U-235 on the control rod side is increased as compared with the reference case,
The control rod value at the time of cold temperature can be increased.

In the fuel assembly of the present invention, the value of the control rod at the time of cold operation and the reactivity difference at the time of operation at the time of cold operation can be improved, so that the reactor shutdown margin of the core loaded with the fuel assembly of the present invention can be improved. Further, as described in the operation, in the fuel assembly of the present invention, the neutron flux distribution in the assembly can be flattened. Therefore, the degree of irradiation damage to the channel box surrounding the fuel assembly adjacent to the fuel assembly according to the present invention can be reduced.

If the supply of plutonium becomes sufficient and MOX fuel is used in the future, for example, the relative amount of U-235 of U-235 in region b in the embodiment will be described.
By replacing the point corresponding to 35 with Pu-239, the effect of this embodiment can be enhanced.

[0028]

As described above in detail, according to the present invention, the output of the fuel assembly in a cross section perpendicular to the axial direction is flattened, and the reactor shutdown margin at the time of cold temperature is effectively improved. It is possible to provide a fuel assembly compatible with a D lattice core.

[Brief description of the drawings]

FIG. 1 is a horizontal sectional view of a fuel assembly showing one embodiment of the present invention.

FIG. 2 is a partial horizontal sectional view of a fuel assembly group of a conventional BWR.

FIG. 3 is a horizontal sectional view of a fuel assembly group according to Prior Art 1.

FIG. 4 is a horizontal cross-sectional view of a fuel assembly group according to prior art 2.

FIG. 5 is a horizontal sectional view of a fuel assembly loaded on a general D-lattice core.

FIG. 6 is a diagram illustrating the principle of the present invention, and is a diagram illustrating a change in control rod value at the time of cold temperature.

FIG. 7 is an explanatory diagram of the principle of the present invention, and is a diagram illustrating a change in the reactivity difference at the time of cooling and at the time of operation.

FIG. 8 is a diagram illustrating the principle of the present invention and is a diagram illustrating a change in a local output peaking coefficient.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Fuel rod, 3 ... Large diameter water rod, 4 ...
Area a, area 5 b, area 6 c, area 7 d.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Junichi Koyama 1168 Moriyama-cho, Hitachi City, Ibaraki Prefecture Inside the Energy Research Laboratory, Hitachi, Ltd. No. 1 Inside Hitachi, Ltd. Hitachi Plant (72) Inventor Junichi Yamashita 3-1-1 Kochicho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd. Hitachi Plant (72) Inventor Tatsuo Hayashi Kanda Surugadai, Chiyoda-ku, Tokyo 4-6, Inside Hitachi, Ltd.

Claims (1)

[Claims] 1. A fuel assembly comprising: a plurality of fuel rods enclosing nuclear fuel material and a large diameter water rod arranged in a square lattice; and the large diameter water rod arranged at a center. In the cross section perpendicular to the axial direction of the fuel assembly, a region a of the fuel rod group facing the wide gap water region into which the control rod is inserted, and a region b of the fuel rod group other than the region a on the wide gap water region side A region d of the fuel rod group facing the narrow gap water region, a region c of the fuel rod group other than the region d on the narrow gap water region side, and the entire region of the fissile material included in each of the four regions. When comparing the relative amount to fissile material contained in and the average enrichment of the fuel rods in each of the four regions, while minimizing the relative amount and the average enrichment of the fuel rod group in the region a, The relative amount of the fuel rod group in the region b; Fuel assembly, characterized in that of the maximum serial average enrichment.