JPH0227638B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to boiling water nuclear reactors.

A fuel assembly for a boiling water reactor is generally constructed as follows. In other words, a large number of fuel rods, each consisting of a cladding tube filled with a large number of fuel pellets made of uranium dioxide, etc., are regularly arranged and housed in a channel box, and are supported by tie plates provided at the top and bottom of the channel box. It is configured as follows. Note that the fuel rods are spaced apart from each other by spacers provided at several locations in the middle in the axial direction.

A large number of fuel assemblies having the above configuration are regularly arranged to form a part of the reactor core.

The coolant removes heat from the fuel rods while rising in the channel box, the coolant itself rises in temperature, and voids are generated in the coolant due to the generation of steam due to the rise in temperature. As shown in FIG. 1, the distribution of these voids is small in the lower part of the fuel assembly and becomes larger continuously towards the upper part. Further, if voids exist, the apparent density of the coolant decreases, so the apparent density is large at the bottom of the fuel assembly and continuously decreases toward the top, as shown in FIG.
In Figures 1 and 2, the vertical axis is the axial position of the fuel assembly, the horizontal axis in Figure 1 is the void ratio, and the horizontal axis in Figure 2 is the relative value with the coolant density at the inlet of the fuel assembly being 1. The density is shown respectively.

Since the reactivity of the fuel is proportional to the density of the coolant, the reactivity of the fuel assembly is large at the bottom and continuously decreases toward the top. For this reason, a peak in the power distribution occurs at the lower part of the fuel assembly, and the integrity of the fuel assembly may be impaired.

Conventionally, in order to suppress the peak of the power distribution, it has been known to operate the control rods by inserting them to the position facing the bottom of the fuel assembly. Since the generation of voids is suppressed near the tip of the control rod, the density of the coolant in that area increases, which may cause a large peak in the power distribution when the control rod is pulled out.

Additionally, a method has been used to adjust the axial power distribution of the fuel assembly by adjusting the coolant flow rate, but the adjustment ability of this method is relatively small, and it is used in conjunction with the dangerous insertion and removal of control rods. is normal.

Therefore, as a means to improve the axial power distribution without inserting the control rods or adjusting the coolant flow rate, as shown in Figure 3A, the fissionable material contained in the fuel pellets filled and sealed in the fuel rods has been proposed. The amount of a substance such as uranium-235 is increased in the upper pellet and decreased in the lower pellet, or the amount of a burnable poison such as gadolinia contained in the fuel pellet is decreased in the upper pellet as shown in Figure B. The pellets at the bottom are made larger. In addition, in FIGS. 3A and 3B, the vertical axis indicates the axial position of the fuel assembly, the horizontal axis in FIG. 3A indicates the uranium 235 content, and the horizontal axis in FIG. 3B indicates the gadolinia content. By doing as described above, the axial power distribution can be improved without operating the control rods or adjusting the coolant flow rate.

However, when the axial power distribution is improved by the above means, the boundary C of the axial distribution of fissile material or burnable poison for each fuel assembly is
Since the positions of the boundaries are aligned, the reactivity of the fuel assembly differs greatly in steps above and below the boundary.

However, as shown in Figure 1, the void distribution is
Since the power distribution increases continuously from the bottom to the top of the fuel assembly, in the case of a fuel assembly configured as shown in FIGS. It is greatly distorted at the position shown in FIGS. 4 to 6. A large output peak occurs at the distorted portion during the fuel cycle, and there is a risk that the integrity of the fuel assembly may be damaged. This is because, in the conventional means, the reactivity loss that continuously changes in the axial direction due to voids is compensated for by discontinuous changes in reactivity at the boundary C of the fuel assembly. In Figures 4 to 6, the vertical axis shows the axial position of the fuel assembly, and the horizontal axis shows the power density. Figure 4 shows the early stage of the core combustion cycle, Figure 5 shows the middle stage, and Figure 6 shows the final stage. Each is shown below.

Now, conventional designs have been made in which a plurality of types of fuel assemblies with different configurations are loaded into a reactor core for the purpose of improving fuel efficiency, flattening the power distribution in the radial direction of the core, and the like. Although the above problem concerns a core loaded with a single type of fuel assembly, it also occurs in a core loaded with a plurality of types of fuel assemblies.

The present invention was made based on the above circumstances, and an object thereof is to obtain a boiling water nuclear reactor that does not cause distortion in the axial power distribution in a reactor core loaded with multiple types of fuel assemblies with different configurations. It is said that

In the present invention, the above object is achieved by varying the boundaries of the content of fissile material or burnable poison between fuel assemblies.

The invention will be explained in detail below with reference to the drawings. In FIG. 7, a fuel assembly 1 has 62 fuel rods 3 arranged in a regular grid pattern in a channel box 2 with a square cross section, and two fuel rods 3 are arranged adjacent to each other in the center of one diagonal of the channel box 2. It has a water rod 4. Note that 5 indicates a cross-shaped control rod, one of which is arranged for each set of four adjacent fuel assemblies.

In the fuel assembly having the above structure, several types of fuel rods 3 having different structures are used depending on the composition of the fuel pellets. Figure 8 shows 3 loading into the initial loading core.
The different types of fuel assemblies are shown. The average enrichment is lowered in the order of fuel assemblies 1a, 1b, and 1c, and in order to suppress excess reactivity, fuel assemblies 1
Gadolinia rods g ₁ and g ₂ are arranged in a, gadolinia rods g ₃ and g ₄ are arranged in 1b, and no gadolinia rod is arranged in 1c. The enrichment distribution of each fuel rod is uniform in the axial direction, while the fuel assembly 1
The gadolinia rod g ₁ inserted into a has gadolinia G ₂ w/o in the lower 1/3, G ₁ w/o in the upper 2/3 (G ₂ > G ₁ ),
The gadolinia rod g ₂ has a portion corresponding to gadolinia G ₂ w/o as gadolinia G ₁ w/o (weight percent), and the gadolinia rod g ₃ inserted into the fuel assembly 1b has gadolinia G ₁ w/o over its entire length. The same G ₄ has Gadolinia G ₁ w/o up to the bottom 2/3.

As a result, the number of inserted gadolinia rods is different between the fuel assemblies 1a and 1b, and the boundary positions of their concentrations are also different.

FIG. 9 shows the power distribution in the axial direction of the core when these three types of fuel assemblies are loaded in the core in approximately equal ratios. In this figure, the solid line _P1 represents the conventional output distribution in which the gadolinia distribution boundaries are aligned, as shown in Figures 4 to 6.
The dashed line P 2 is the same as that shown in the figure, and the broken line P ₂ represents the fuel assemblies 1a and 1 whose boundaries are C ₁ and C ₂ according to the present invention.
This is the output distribution according to b. As is clear from this figure, according to the present invention, the peak of the output distribution can be almost completely suppressed.

In the above example, the boundary positions of the axial gadolinia distribution are different for two types of fuel assemblies, but the present invention can be applied to three or more types of fuel assemblies. In this case, the peak of the axial power distribution can be suppressed much better.
In addition, in the above example, the axial output distribution is adjusted only by the gadolinia distribution, but the uranium
The present invention can also be applied when using the 235 enrichment distribution and when using both of the above. FIG. 10 shows an example. Boundaries with different axial positions in fuel assemblies 1A to 1C
The contents of uranium-235 and gadolinia differ between the upper and lower portions of a ₁ , a ₂ , and a ₃ . In Figures 10A and 10B, fuel assemblies 1A to 1C are schematically shown, and the vertical axis is the axial position of the fuel assembly, the horizontal axis in Figure A is the uranium-235 content, and the horizontal axis in Figure B is indicates the gadolinia content.

Note that the present invention is not limited to the above embodiments. The present invention can be applied to fissile materials other than uranium-235, such as plutonium. Similarly, the burnable poison is not limited to gadolinia, and for example, boron or the like can be used.

As described above, according to the present invention, the axial distribution of the fuel assembly is flattened, and the distortion of the power distribution at the boundary of the content of fissile material and/or burnable poison, and therefore the generation of peaks, is suppressed. As a result, the integrity of the fuel assembly is greatly improved.

[Brief explanation of drawings]

Figure 1 is a diagram showing the axial void fraction distribution of the fuel assembly, Figure 2 is a diagram showing the axial coolant density distribution, and Figure 3A is the peak of the axial power distribution due to the coolant density distribution. Fig. 3B is a diagram showing the axial distribution of gadolinia content in a conventional fuel assembly for suppressing uranium-235, and Figs. Fig. 7 is a cross-sectional view showing an example of the fuel assembly according to the present invention, and Fig. 8A shows the flammability in one embodiment. A schematic cross-sectional view showing the arrangement of poisonous substances, FIG. 10A and 10B are schematic longitudinal sectional views of other embodiments of the present invention.

Claims (1)

[Claims] 1. In reactor cores that are loaded with at least two or more types of new fuel at the time of refueling, fuel with lower burnable poison content is used above the boundaries of different axial positions of each fuel assembly than below. A boiling water nuclear reactor characterized in that either or both of the rods and the fuel rods having a higher content of fissile material in the upper part than in the lower part are arranged in each fuel assembly.