JPS6275290A - Core for boiling water type reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a core of a boiling water nuclear reactor in which a large number of fuel assemblies are arranged.

[Technical background of the invention and its problems] In general, in a nuclear reactor, distortion of the power distribution occurs within the core, and when a power peak occurs, there is a greater possibility that the integrity of the fuel will be impaired, leading to a decrease in the overall power. Therefore, in order to extract more thermal energy from the reactor core, the power distribution within the reactor core is made as flat as possible. By appropriately arranging various types of fuel rods within a fuel assembly, the power distribution within the reactor core is made evener.

FIG. 6 shows a part of the core of such a conventional boiling water reactor, in which a large number of fuel rods 3 are placed in a channel box 2 surrounding a handleable cross-shaped control rod 1. A fuel assembly 5 containing two water rods 4 is arranged. In the core of such a boiling water reactor, the channel box 2 of the fuel assembly 5 divides the inside of the core into an inch channel section 6, which is a flow path for boiling water, and an out channel section 7, which is a flow path for non-boiling water. It is separated into. inch yannel part 6
Since the boiling water flowing through the channel has a lower atomic density than the non-boiling water flowing through the out channel section 7, it has a lower neutron moderation ability. Because of this arrangement, a distortion occurs in that the thermal neutron flux distribution is high in the out channel portion 7 and low in the inch channel portion 6. Therefore, fuel assembly 5
A distortion occurs in the output distribution in which the output is high in the outer part and low in the inner part.

Therefore, conventionally, as shown in Figure 7, four types of fuel rods are used, in which the concentration of fissile material decreases in the order of a, b, c, and d. A fuel assembly 5 in which fuel rods S, c, and d with a low concentration of fissile material are arranged at the outer periphery is placed in the center of the reactor core to flatten the power distribution.

However, in order to increase the reactivity of the fuel assembly 5, fuel rods with a high Q degree of fissile material must be placed in locations with a large thermal neutron flux distribution, and in order to limit the output peak, In the reactor core in which the configured fuel assemblies 5 are arranged, the fuel rods a with a high concentration of fissile material are arranged inside the fuel assemblies 5 with a low thermal neutron flux distribution, so the loss of reactivity is large. Therefore, there is a problem that a considerable amount of unburned fissile material remains inside the spent fuel assembly 5 that has been burned in the reactor core, resulting in poor fuel economy.

[Object of the Invention] The present invention has been made in response to such conventional circumstances, and by flattening the thermal neutron flux distribution in the reactor core, the power distribution is flattened and the reactivity is improved. The present invention aims to provide a boiling water reactor core that can improve fuel economy.

[Summary of the Invention] That is, in the boiling water reactor core of the present invention, fuel rods filled with fissile material and hollow water rods serving as moderator channels have a substantially square or hexagonal cross section. The fuel assemblies are arranged at regular intervals in a lattice pattern in a cylindrical channel box having By arranging a large number of fuel rods so that the center-to-center distance does not exceed twice the center-to-center distance of adjacent fuel rods in the same channel box, the thermal neutron flux distribution within the reactor core is flattened, and the power distribution is flattened.
This makes it possible to improve fuel economy by increasing reactivity.

[Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a part of the core of a boiling water reactor according to an embodiment of the present invention, in which a large number of fuel rods 9 and four water rods 10 are arranged in a channel box 8. aggregate 1
1, the distance between the side walls of the channel box 8 is lll
lT11, the distance P1 between adjacent fuel rods 9 across the channel box 8, and the center-to-center distance P between adjacent fuel rods 9 in the same channel box 8;
2, the ratio (PI/P2) is 1.3. Note that since the cruciform control rods cannot be arranged between the fuel assemblies 11, the cluster control rods 12 are arranged inside the channel box 8.

In addition, the part where it is necessary to arrange the instrumentation tube guide tube between the fuel assemblies 11 in the reactor core has a shape in which one corner of the channel box 8 is cut off diagonally, as shown in FIG. Four fuel assemblies 13 are arranged, and an instrumentation tube guide tube 14 is arranged in this gap 8iS.

In the boiling water reactor core configured in this way, the moderator ■ surrounding the nuclear fuel rods 9 is more uniform than in the conventional boiling water reactor core, and the thermal neutron flux distribution is flat. As a result, the output distribution is flattened and the reactivity increases.

The graph in Figure 3 shows combustion changes with infinite multiplication factor. The vertical axis shows infinite multiplication factor, the horizontal axis shows burnup, straight line A is the core of a conventional boiling water reactor, and straight line 8 is 2 shows the combustion change of the infinite multiplication factor of the core of the boiling water reactor of this embodiment shown in FIG. 1;

As can be seen from this graph, in the core of the boiling water reactor of this example, the reactivity increases by 1 to 2%, and the difference increases as combustion progresses. This is because the uniformity of the fuel rod arrangement increases the conversion rate and reduces the rate of decrease in reactivity.

In this example, the distance between the side walls of the channel box in the out channel portion was set to 1 mm, and P1/P2 was set to 1.3 for the following reasons.

The graph in Figure 4 shows the distance between the side walls of the channel box in the out channel section and changes in output distribution and fuel economy due to changes in P1/P2. In this graph, the vertical axis shows the distance between the side walls of the channel box. , the horizontal axis is P
I/P2, curves C1 to C8 are fuel economy contour lines, and dotted lines D1 to D6 are output peak contour lines. From this graph, the distance between the channel box side walls, PI/
It can be seen that reducing both P2 improves both fuel economy and output peak flatness, but the distance between the channel box side walls is 10! When llTl1 or less and PI/P2 become 2 or less, the fuel economy contour interval becomes wider and the rate of change in fuel economy decreases. Therefore, by setting the distance between the channel box side walls to 10 mm or less and PI/P2 to 2 or less, an effective effect of improving fuel economy can be obtained. In addition, the distance between the side walls of the channel box is 1! In fact, it is extremely difficult to make PI/P2 smaller than 11 due to problems such as processing accuracy of the channel box and thermal expansion, and this also makes it difficult to make PI/P2 smaller than 1. In addition, when the distance between the side walls of the channel box is 5 mm or less and PI/P2 is 1.5 or less,
It is preferable to set the distance between the side walls of the channel box to 1 mn+ to 5 yen and to set PI/P2 to 1 to 1.5, since it is possible to obtain a large effect on both the fuel economy and the flatness of the output peak.

In this way, the distance between the side walls of the channel box is set to 1mTrl.
, P'+/P2 of 1.3 in the boiling water reactor core of this example, the core of the boiling water reactor of this embodiment has a value of 5.8 without impairing the integrity of the fuel.
% improvement in fuel economy. In addition, compared to the core of a conventional boiling water reactor, the gap between the fuel rods can be widened, reducing the resistance of the cooling water flow path, increasing the cooling efficiency, and increasing the amount of water flowing through the out channel. Steam quality is improved because the amount of water is reduced.If the out channel section is also in the boiling water region by providing through holes in the channel box 8 or making it mesh-like, the output distribution can be further flattened. The reactivity can be improved including the effect of reducing neutron absorption by the channel box.

In this example, the fuel assembly was constructed with a 9x9 fuel rod arrangement, but the present invention is not limited to this example, and the fuel rod arrangement may be 10x10.11x11, etc. It's a must have.

FIG. 5 shows a part of the core of a boiling water reactor according to another embodiment. In this embodiment, a large number of fuel rods 9 and water rods are arranged in a channel box 15 having an approximately hexagonal cross section. A large number of fuel assemblies 16 in which fuel assemblies 10 are arranged are arranged such that the distance between the side walls of the channel box 15 is 1 mm and 5PI/P2 is 1.3. Furthermore, a cluster type control rod] 7 is arranged within the channel box 15.

Even if the channel box 15 having a hexagonal cross section is used as in this embodiment, it is possible to obtain the same effect as in the embodiment in (e) above.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a part of the core of a boiling water reactor according to an embodiment of the present invention, and FIG. 2 is a part of other parts of the core of the boiling water reactor shown in FIG. 1. The cross-sectional view, Figure 3, shows
FIG. 4 is a graph showing changes in burnup with infinite multiplication factor; FIG. 4 is a graph showing changes in fuel economy and output peak due to changes in channel box side wall distance and PI/P2; FIG. 5 is another embodiment of the present invention. FIG. 6 is a cross-sectional view showing a part of the core of a conventional boiling water reactor, and FIG. 7 is a cross-sectional view showing a part of the core of a conventional boiling water reactor. FIG. 2 is a cross-sectional view showing a fuel assembly arranged in the core of a water reactor. 8...Channel box 9...
...Fuel rod 10...Water rod 11...Fuel assembly P1...Fuel rods adjacent across the channel box Center-to-center distance P2....... Center-to-center distance of adjacent fuel rods in the same channel box - Applicant Japan Atomic Energy Corporation Applicant Toshiba Corporation Patent Attorney Satoshi Suyama - Figure 1 Figure 2 Figure 3

Claims (2)

[Claims] (1) Fuel rods filled with fissile material and hollow water rods that serve as moderator flow paths are arranged in a lattice pattern at equal intervals inside a cylindrical channel box with an approximately square or hexagonal cross section. The distance between the side walls of the channel box of each fuel assembly is 10 mm or less, and the distance between the centers of adjacent fuel rods across the channel box is the same between the centers of adjacent fuel rods in the channel box. A reactor core of a boiling water reactor characterized in that a large number of reactors are arranged at a distance not exceeding twice the distance. (2) The distance between the side walls of the channel boxes of the nuclear fuel assembly is preferably 1 mm to 5 mm, and the distance between the centers of adjacent fuel rods across each channel box is preferably the same as the distance between the centers of fuel rods in the same channel box. 1-1
．． A core of a boiling water reactor according to claim 1, which is 5 times as large.