JPH0277687A - Fuel assembly, assembling method for fuel assembly and core - Google Patents

Abstract

PURPOSE:To contrive high reactivity by allowing a cross section ratio of moderator to fuel to be smaller in a center part than in a periphery part to lengthen the intervals between fuel rods facing the boundary of a periphery part subassembly so as to form a gap water region between both the subassemblies. CONSTITUTION:A fuel assembly 1 is formed of a periphery part subassembly 3 near a gap water region 2 and a center part subassembly 4 located at the center part of the assembly. The assembly 3 has fuel rods 6 of high enrichment including the fuel rods which face a channel box 5 and the fuel rods 6 are arranged in the form of a lattice at fuel rod pitches P1. The assembly 4 has fuel rods 7 of low enrichment including the fuel rods arranged at longer pitches P2 than the fuel pitches P1 of the fuel rods 6 arranged in the assembly 3 and the fuel rods 7 have the same outer diameter as the fuel rods 6 arranged in the assembly 3. The assembly 3 is surrounded by gap water regions 2, 8. Further, the fuel rods 7 are arranged in the form of a lattice at smaller fuel rod pitches P3 than the fuel rod pitches P1 of the fuel rods 4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a boiling water nuclear reactor, and particularly to a fuel assembly for a boiling water reactor suitable for improving fuel economy.

[Conventional technology]

A conventional fuel assembly typically has a configuration as shown in FIG. That is, the fuel assembly 100 is composed of a large number of fuel rods 102 assembled in a certain area surrounded by a channel box 101 in a core coolant, for example, water. A control rod 103 and a neutron detector instrumentation tube 104 are arranged outside the channel box 101. This distance between each fuel assembly 100 is determined by the control rod 103.
The area around each fuel assembly 100 is filled with cooling water. In this case, the fuel rods 102 located at the periphery of the fuel assembly 100 are surrounded by more water than the other fuel rods 102 located at the center of the fuel assembly 100. . As a result, the following nuclear non-homogeneity effects (i) and (ii) occur between the periphery and the center of the fuel assembly 100.

(i) Neutrons are effectively thermalized around the fuel assembly where water, which is a neutron moderator, is abundant, and the number of thermal neutrons in the periphery is larger than in the center.

(ii) As a result, a difference occurs in the average neutron energy and the infinite neutron multiplication factor between the periphery and the center of the fuel assembly.

The following three conventional techniques are available in view of such nuclear non-homogeneity effects.

Prior art 1 is a method described in Japanese Patent Application Laid-Open No. 60-144692, and as shown in FIG.
A large water rod 105 is inserted into the center of the fuel cell 4, and the hydrogen to uranium atomic ratio (hereinafter referred to as H7
(referred to as the U ratio). That is, by inserting the large water rod 105, the fuel assembly is homogenized,
The power distribution in the radial direction of the aggregate was flattened.

Conventional technology 2 is a method described in Japanese Patent Application Laid-Open No. 132192/1982, in which low enrichment fuel is loaded in the center of the assembly.
Plutonium-239 produced in the early stages of combustion is effectively used as nuclear fuel.

Conventional technology 3 is a method described in Japanese Patent Application Laid-Open No. 60-93990, and as shown in FIG.
By making the spacing between the fuel rods 198 located at the center of the assembly closer than the spacing between the fuel rods 107 located at the periphery of the This aims to improve fuel economy by converting the parent fuel material into fissile material.

[Problem to be solved by the invention]

In prior art 1, a large water rod 105 is inserted into the center of the fuel assembly to flatten the power distribution and burnup distribution. It was difficult to improve economic efficiency by increasing the amount of fuel installed.

In addition, in the prior art 2 and 3, among the fuel rods 107 arranged at the periphery of the fuel assembly, the fuel rods 107 adjacent to the low-enrichment fuel rods or the high-density arranged fuel rods 108 arranged in the center is the fuel rod 10 located on the outer peripheral side of the assembly
Since the H/U ratio was lower than that of No. 7, fuel economy was poor and it was difficult to increase burnup.

An object of the present invention is to provide a fuel assembly that improves fuel economy by increasing fuel loading and achieving high reactivity by utilizing the non-homogeneous structure of the fuel assembly. An object of the present invention is to provide a method for assembling a fuel assembly and a reactor core.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a fuel assembly loaded in the core of a nuclear reactor that has a large number of fuel rods arranged in a lattice shape in a channel box and uses light water as a coolant. The inside of the channel box is divided into a peripheral subassembly consisting of fuel rod groups including fuel rods facing the channel box, and a central subassembly consisting of other fuel rod groups. The moderator-to-fuel cross-sectional area ratio of the triangular or square region formed by connecting the centers of the matching fuel rods is smaller in the central subassembly than in the peripheral subassembly,
and the fuel rod spacing of both subassemblies facing the boundary between the peripheral subassembly and the central subassembly is longer than the fuel rod spacing of the peripheral subassembly, and a gap water region is formed between both subassemblies. To be,
A fuel assembly is provided, characterized in that fuel rods are arranged in each subassembly.

In this fuel assembly, preferably, the intervals between the fuel rods arranged in each of the subassemblies are larger in the peripheral subassembly than in the central subassembly.

Preferably, the average enrichment of the fuel rods arranged in each of the subassemblies is lower in the central subassembly than in the peripheral subassemblies.

In addition, in order to achieve the above object, the present invention provides the method for reconfiguring a fuel assembly in which each of the subassemblies has a structure in which the subassemblies can be attached and detached from the fuel F18 combination. Provided is a method for assembling a fuel assembly, characterized in that a peripheral subassembly or a central subassembly is replaced.

In this case, one of the different fuel assemblies may be a fuel assembly without fuel rods located in the central subassembly region.

In order to achieve the above object, the present invention also provides a reactor core comprising the above fuel assembly.

The fuel assembly can be reconfigured as one fuel assembly by collecting a plurality of its central subassemblies,
This reconfigured fuel fls combination can be installed at the outermost periphery of the core, or conversely, it can also be installed to surround the control rods in an area other than the outermost periphery of the core to form a control cell. can.

[Effect]

In the present invention configured in this way, the gap water region outside the channel box around the fuel assembly and the gap water region formed between the peripheral subassembly and the central subassembly, In the peripheral subassembly, compared to the conventional example in which the spacing between fuel rods is the same throughout the fuel assembly, the H/U ratio is also large in the area adjacent to the central subassembly, and the relative thermal neutron flux distribution is more The neutron spectrum becomes flat and the neutron spectrum becomes softer (see Figure 2). Therefore, the burner rods in the peripheral subassembly adjacent to the central subassembly can be burned more efficiently, improving fuel economy. improves.

On the other hand, by making the spacing between the fuel rods arranged in each subassembly larger in the peripheral subassembly than in the central subassembly, the spacing between the fuel rods in the central subassembly is narrower than before, which improves the neutron spectrum. becomes harder. ff (I, compared to the conventional method, it becomes possible to create soft and hard regions of the neutron spectrum more clearly.By the way, when using low enrichment fuel for boat fishing, H
The smaller the H/U ratio, the higher the reactivity, and the smaller the rate of reactivity deterioration due to combustion.For highly enriched fuels, the larger the H/U ratio, the higher the reactivity (see Figures 3 and 4). , Therefore, by making the average enrichment of the fuel rods arranged in each subassembly lower in the central subassembly than in the peripheral subassembly, the peripheral subassembly with a large H/U ratio is High enrichment fuel suitable for a large H/U ratio is loaded, while the central subassembly of the assembly with a small H/U ratio is loaded with low enrichment fuel suitable for a small H/U ratio. This makes it possible to increase the amount of uranium fuel loaded without significantly reducing the reactivity.

Furthermore, in the method for assembling a fuel assembly of the present invention, the burnup of the fuel assembly can be reduced by exchanging the peripheral subassembly or the central subassembly between different fuel assemblies at the time of reactor shutdown. can be increased, further promoting fuel economy.

By making one of the different fuel assemblies a fuel assembly in which no fuel rods are arranged in the central subassembly region,
The fuel assembly can function as a fuel assembly with a water rod arranged in the center.

〔Example〕

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

(Example 1) First, a first example of the present invention will be described with reference to FIG. In FIG. 1, a fuel assembly 1 is composed of a peripheral subassembly 3 located at the periphery of the assembly and adjacent to the gap water region 2, and a central subassembly 4 located at the center of the assembly. 0 peripheral subassembly 3
has highly enriched fuel 466 including fuel rods facing channel box 5, and fuel rods 6 are arranged in a lattice pattern with fuel rods and holes P1. The central subassembly 4 is
It has low enrichment fuel rods 7 including fuel rods arranged at a distance P2 that is longer than the fuel rod pitch P1 of the fuel rods 6 arranged in the peripheral subassembly 3, and the fuel rods 7 are arranged in the peripheral part subassembly 3. It has the same outer diameter as the fuel rods 6 arranged in the subassembly 3. When the fuel assembly 1 is configured in this way, a small gap water region 8 is formed between the peripheral subassembly 3 and the central subassembly 4, and the peripheral subassembly 3 is connected to the gap water region 2 and the gap water region 8. It is surrounded by area 8. Further, the fuel rods 7 of the central subassembly 4 are arranged in a grid at a fuel rod pitch P3 smaller than the fuel rod pitch P1 of the fuel rods 4 arranged in the peripheral subassembly 3. Therefore, the H/U ratio of each subassembly is smaller in the central subassembly 4 than in the peripheral subassembly 3.

By configuring the fuel assembly 1 as described above, the first
Compared to the prior art 1 described above in which a large water rod 105 is installed in the center of the fuel assembly 104 as shown in Figure 4, the amount of uranium loaded is increased because the fuel m7 is also installed in the center. can be done.

On the other hand, in the present embodiment 1, unlike the prior art 2 and 3 described above, since a small gap water region 8 exists between the peripheral subassembly 3 and the central subassembly 4, the peripheral part It is possible to more efficiently burn the fuel adjacent to the center subassembly 4 of the subassembly 3, that is, the uranium fuel contained in the fuel rods 6 in the third layer from the center of the fuel assembly 1.

This will now be explained with reference to FIG. Due to the outer gap water region 3, in the peripheral subassembly 3, as shown in FIG. The ratio is large, the relative thermal neutron flux distribution is flatter, and the neutron spectrum is softer. Therefore, the central subassembly 4
The fuel rods 6 of the peripheral subassembly 3 adjacent to the fuel rods 6 can be burned efficiently, improving fuel economy.

On the other hand, in the fuel assembly 1 of this embodiment, the fuel rod pitch P3 is narrower in the central subassembly 4 than in the peripheral subassembly 3, so that the neutron spectrum becomes harder. Therefore, due to the existence of the gap water region 8 and the setting of the fuel rod pitch P3 of the central subassembly 4, it is possible to create soft regions and hard regions of the neutron spectrum more clearly than in conventional techniques 2 and 3. . FIG. 3 shows the relationship between the H/U ratio and the infinite neutron multiplication factor using the fuel enrichment as a parameter. Also,
FIG. 4 shows the relationship between the H/U ratio and reactivity deterioration due to combustion using fuel enrichment as a parameter. As can be seen from these figures, for low enrichment fuel, H/U
The smaller the ratio, the higher the reactivity and the smaller the rate of reactivity deterioration due to combustion.On the other hand, with high enrichment fuel, H/U
The larger the ratio, the greater the degree of reactivity. Therefore, the peripheral subassembly 3 with a large H/U ratio has a large H/U ratio.
By loading high-enrichment fuel rods 6 suitable for a small H/U ratio, and loading low-enrichment fuel rods 7 suitable for a small H/U ratio into the central subassembly 4 with a small H/U ratio, It becomes possible to increase the amount of uranium fuel loaded without significantly reducing the reactivity.

FIG. 5 compares the effects of prior art 1 and 2 and the present embodiment 1. In the present embodiment 1, the fuel assembly 1 is configured as shown in FIG. 1, and the peripheral subassembly 3 The average enrichment of the central subassembly 4 is 0.711410, and the overall average fuel enrichment is 3.6W10. Further, the fuel rod pitch P1 is 14.3 am, the fuel rod pitch P2 is 15.4 nna, and the fuel rod pitch P3 is 13.2 am. On the other hand, conventional technology! The fuel rod pitch was 14.3 m, and the average enrichment was 3°6°10, the same as in Example 1. The fuel rod pitch of Prior Art 2 is also 14.3 mm, and the average enrichment is also 3.61410, which is the same as in Example 1.

From FIG. 5, in Example 1, the reactivity improved by 0.5% koO compared to Conventional Technology 2, the fuel inventory (fuel loading amount) increased by 13% compared to Conventional Technology 1, and the uranium saving effect was lower than that of Conventional Technology 2. It can be seen that there is an improvement of 3% compared to Technology 1 and 2% compared to Conventional Example 2.

Furthermore, in the first embodiment, when the central subassembly 3 is configured to be detachable from the fuel assembly 1,
By reconfiguring the fuel assembly, it is possible to further improve fuel economy.

FIG. 6 shows a reassembly method for reassembling the subassemblies 3 and 4 of the fuel assembly 1 and loading them into the reactor core. The fuel assembly 1 stays in the reactor core for three cycles in operation A, and is temporarily taken out of the reactor. however,
The average extraction burnup of the central subassembly 4 is approximately half of the core average extraction burnup. Furthermore, as shown in Figure 4, the deterioration in reactivity due to combustion of natural uranium is small, so the central subassembly 4 is removed during periodic inspections and replaced with other fuel assemblies, such as those located around the core. After reassembling it in combination with the peripheral subassembly 10 loaded in the fuel assembly that has not been sufficiently burned or with the peripheral subassembly 10 made up of new fuel to form a new fuel assembly 11, it is reassembled. In operation B, it is loaded into the core.

When the fuel assembly 11 stays in the reactor for three cycles and is taken out of the reactor, the average burnup of the central subassembly 4 becomes equal to the fuel assembly average, so that the fuel assembly burnup can be increased. By taking advantage of the fact that zero or more fuel assemblies can be reassembled, the uranium saving effect can be further improved by 1%.

In addition, as another method for reassembling the fuel assembly, for example, in operation A in FIG. 6, the fuel assembly The central subassembly 4 in the body 1 is removed from the central subassembly area and the third
In the second cycle, it is also possible to construct a new fuel assembly using only the peripheral subassemblies. in this case,
The reassembled fuel assembly without the central subassembly 4 is
It can function in the same way as the fuel assembly in which the water rod 105 is arranged in the center of Prior Art 1 (see FIG. 14).

Further, the central subassembly 4 removed during reassembly can be reassembled as a new fuel assembly as described above.

Furthermore, in this embodiment, by arranging, for example, eight central subassemblies around the periphery of the fuel assembly 1, it is possible to reassemble a new fuel assembly using only the central subassembly 4. be. The fuel assembly reassembled in this way can be loaded, for example, to the outermost periphery of the reactor core to reduce leakage of neutrons from the core system and increase reactivity. It is also possible to construct a control cell by mounting this fuel assembly so as to surround a control rod.

In this example, the average concentration of the central subassembly 4 and the peripheral subassembly 3 is shown in Figure 5, but this is an example and is not limited to this.
Furthermore, the number of fuel rods loaded in the central subassembly 4 may be in a square matrix other than 3 rows and 3 columns.Furthermore, the number of fuel rods loaded in the central subassembly 4 may be a square matrix other than 3 rows and 3 columns. The present invention can be achieved even if a plurality of long fuel rods are inserted.

(Modification 1 of Example 1) A first modification of the first embodiment will be described with reference to FIG. 7.

In the modified fuel assembly IA in which the fuel rods loaded in the central subassembly in Embodiment 1 may have a shape other than a square matrix, the central subassembly 4A
It is constructed in the shape of a cross, consisting of a central fuel rod and four fuel rods located around it. Therefore, the gap water region 8A formed between the central subassembly 4A and the peripheral subassembly 3 also has a cross shape. As described above, the present invention can be achieved even if the central subassembly 4A is configured in a shape other than a square matrix.

(Modification 2 of Example 1) A second modification of the first embodiment will be explained with reference to FIG. 8.

In this fuel assembly IB, the outer diameter of the fuel rods 7B arranged in the central subassembly 6B of the assembly is larger than the outer diameter of the fuel rods 6 arranged in the peripheral subassembly 3. Central subassembly 68 H/
The U ratio is smaller than that of the peripheral subassembly 3.

This also provides the same effects as in the first embodiment.

(Modification 3 of Example 1) A third modification of the first embodiment will be described with reference to FIG. 9.

In this fuel assembly IC, the central subassembly 40
Although the outer diameter of the fuel rods 7C arranged in the peripheral subassembly 3 is smaller than that of the fuel rods 3 arranged in the peripheral subassembly 3, they are arranged more densely than the peripheral subassembly 3. There is.

That is, the fuel rod pitch P3 of the central subassembly 4C is narrower. By doing so as well, the same effects as in the first embodiment can be achieved.

(Modification 4 of Example 1) Still another modification of Example 1 will be described with reference to FIG. 10. In this fuel assembly ID, a central subassembly 4D is formed by arranging fuel rods 7 in a triangular lattice shape. Also in this case, the same effects as in the first embodiment can be achieved.

(Example 2) In the above example, the number of fuel rods loaded in the fuel assembly was 9 rows and 9 columns, but the number of fuel rods may be greater than this. Example 2 in the case of
In the fuel assembly 20 of this embodiment, which will be described with reference to the drawings, there is also a gap water region 22 in the peripheral sub-assembly 21 in which the fuel rods are widely spaced, and the peripheral sub-assembly 21 is connected to this gap. The water region 22 is divided into two parts 21A and 21B. However, the fuel rod pitch D1 that defines the gap water region 22 is narrower than the fuel rod pitch D2 that defines the gap water region 8 between the central subassembly 4 and the peripheral subassembly 21. In this example as well, the same effects as in Example 1 can be obtained.

(Embodiment 3) Still another embodiment of the present invention will be explained with reference to FIG. 12.6 As explained above, in the above embodiment, the H/U ratio in the central subassembly region is small and the neutron The hardness of the spectrum promotes conversion in the central subassembly, allowing it to accumulate more plutonium-239. Therefore, if the center subassembly is constructed to be removable, it is possible to collect mainly plutonium-239 by assembling a plurality of center subassemblies and reassembling the fuel assembly.
It is possible to construct a fuel assembly using this as fuel.

FIG. 12 shows such an embodiment, and a fuel assembly 30 is constructed by collecting, for example, nine central subassemblies 4 of the first embodiment shown in FIG. By loading this fuel assembly 30 into the reactor core, the plutonium 239 accumulated in the above embodiment can be used effectively, and fuel economy can be further improved.

〔Effect of the invention〕

As explained above, according to the present invention, since the water rod is not arranged in the center but the center subassembly is arranged,
In addition to increasing the amount of uranium loaded, a gap water region is formed between the peripheral subassembly and the central subassembly, so that the fuel placed in the peripheral subassembly can be used to fuel the central subassembly. Fuel adjacent to the lee can also be burned efficiently.

Additionally, the spacing between the fuel rods arranged in each subassembly is made larger in the peripheral subassembly than in the central subassembly, and the average enrichment of the fuel rods arranged in each subassembly is increased between the peripheral subassemblies and the central subassembly. Since it is lower in the central subassembly, it is possible to increase the uranium fuel loading without significantly reducing the reactivity.

Additionally, if a fuel assembly assembly method is adopted in which peripheral subassemblies or central subassemblies are exchanged between different fuel assemblies during reactor shutdown, the fuel assembly burnup may be increased. , further improving fuel economy.

When one of the different fuel assemblies is a fuel assembly in which no fuel rod is arranged in the central subassembly region, the fuel assembly can function as a fuel assembly in which a water rod is arranged in the center. .

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention, FIG. 2 is a diagram showing the thermal neutron flux distribution within the fuel assembly, and FIG. 3 is a diagram showing the neutron infinite multiplication factor and 11/U. This is a diagram showing the relationship between the ratios, and Figure 4 shows the neutron-1! due to combustion. ! FIG. 5 is a diagram showing the relationship between the deterioration rate of the limiting multiplication factor and the H/U ratio, FIG. 5 is a diagram showing the uranium saving effect of this embodiment over the conventional technology, and FIG. FIG. 7 is a sectional view of a fuel assembly according to the first modification-1 of the same embodiment, and FIG. FIG. 9 is a sectional view of a fuel assembly according to a third modification of the same embodiment, and FIG. 10 is a fourth modification of the same embodiment. 11 is a sectional view of a fuel assembly according to another embodiment of the present invention;
FIG. 12 is a sectional view of a fuel assembly according to still another embodiment of the present invention, FIG. 13 is a sectional view of a conventional fuel assembly, and FIG. 14 is a sectional view of another conventional fuel assembly. FIG. 15 is a sectional view of yet another conventional fuel assembly. Explanation of symbols 1; IA~ID; 20; 30...Fuel assembly 3; 21
...Peripheral subassembly 4; 4A to 4D...Central subassembly 6...Peripheral subassembly fuel rod 7; 7B; 7C...Central subassembly fuel rod 8; 8A ... Gap water area applicant Hitachi, Ltd. Figure 1 H/U H/U Figure 6 Operation A Gate operation B

Claims (8)

[Claims] (1) In a fuel assembly loaded into the core of a nuclear reactor that has a large number of fuel rods arranged in a grid in a channel box and uses light water as a coolant, the inside of the channel box is It is divided into a peripheral subassembly consisting of fuel rod groups including facing fuel rods, and a central subassembly consisting of other fuel rod groups, and the centers of adjacent fuel rods in these subassemblies are connected. The moderator-to-fuel cross-sectional area ratio of the triangular or quadrangular region formed by the central subassembly is smaller than that of the peripheral subassembly, and the boundary between the peripheral subassembly and the central subassembly has a surface area. The fuel rods in each subassembly are arranged such that the fuel rod spacing in both subassemblies is longer than the fuel rod spacing in the peripheral subassembly, creating a gap water region between both subassemblies. A fuel assembly featuring: (2) The fuel assembly according to claim 1, wherein the intervals between the fuel rods arranged in each of the subassemblies are larger in the peripheral subassembly than in the central subassembly. (3) Claim 1 or 2, wherein the average enrichment of the fuel rods arranged in each of the subassemblies is lower in the central subassembly than in the peripheral subassemblies.
Fuel assembly as described. (4) In the method for assembling a fuel assembly according to claim 1, wherein each of the subassemblies has a structure that can be attached to and detached from the fuel assembly, peripheral subassemblies are separated between different fuel assemblies at the time of reactor shutdown. Or, a method for assembling a fuel assembly, characterized in that the central subassembly is replaced. (5) The method for assembling a fuel assembly according to claim 4, wherein one of the different fuel assemblies is a fuel assembly in which no fuel rods are arranged in the central subassembly region. (6) A reactor core comprising the fuel assembly according to claim 1. (7) A reactor core characterized in that a fuel assembly which is reconfigured by collecting a plurality of central sub-assemblies of the fuel assembly according to claim 1 is mounted on the outermost periphery of the reactor core. (8) A fuel assembly reconfigured using a plurality of central subassemblies of the fuel assembly according to claim 1 is installed in a region other than the outermost periphery of the reactor core so as to surround the control rods. Characteristic reactor core.