JPH02236497A - Neutron shielding body of fast breeder - Google Patents

Abstract

PURPOSE:To improve shielding performance and to reduce the weight and cost of the shielding body by segmenting the inside of a cladding pipe to plural parts axially by partition plates. CONSTITUTION:Neutron shielding materials 2 are packed into the cladding pipe 5 and both ends are hermetically sealed by end plugs 4. The inside of the cladding pipe 5 is segmented axially to the plural parts by the partition plates 3. The generation of nonuniform densities in the powdery shielding materials 2 during use is obviated in this way by planar sintered bodies and the density differences corresponding to the required shielding performance can be provided between the respective partitioned blocks. The weight and cost of the shielding body is thus reduced. The reduced weight and improved economy of not only the single shielding body but the reactor system as well are obtd. by using the shielding body which is provided with the density distribution in the axial direction to optimize the shielding performance.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a neutron shield for a fast breeder reactor, and particularly to a neutron shield suitable for reducing the size and weight of a reactor core sliding structure. It is.

[Prior art] As a material for the shield of a fast breeder reactor, Japanese Patent Application Laid-open No. 61-1
As described in No. 62790, the use of materials with high shielding performance, such as boron carbide, has been devised. These are often in the form of powder, and an example of using powder in a reactor is a control shaft for a boiling water type light water reactor, which is shown in Figure 4. In FIG. 4, powdered control material is vibrated and filled into a cladding tube 5, and both ends are sealed with end plugs 4. Since the packing density is about 70% of the theoretical density, in order to prevent the fine powder from moving downward during use and making the density uneven, a partition with a diameter of 3 mm was used as shown in Figure 4. Steel balls of about .5 cm are placed at strategic points.

[Problems to be Solved by the Invention] The conventional technology using the above-mentioned steel balls is not suitable for fast breeder reactors where the diameter of the cladding tube is set to a large diameter of about 150 mu to increase the volume fraction of the shielding material. is not suitable for the following reasons.

■The bulk of the steel increases and its shielding performance decreases.

■The temperature rises due to the heat generated by the reaction in the furnace, which may lead to melting of the steel.

An object of the present invention is to provide a lightweight shielding body that has excellent shielding performance and is partitioned with a heat-resistant material that does not deteriorate shielding performance and does not melt due to reactions in a furnace.

[Means for Solving the Problems] In order to solve the above problems, the structure of the neutron shielding body for a fast breeder reactor according to the present invention is such that a neutron shielding material is filled in a cladding tube and both ends are sealed with end plugs. The interior of the cladding tube is divided into a plurality of sections in the axial direction by partitions.

[Effects] Providing a partition plate in the powder-filled portion of the neutron shield has various effective effects as follows.

For example, if the powder shielding material is partitioned by a plate-shaped sintered body, non-uniform density will not occur during use.

By changing the packing density for each section, the number of shielding kites can be minimized and the weight can be reduced.

By changing the packing density for each section, it becomes possible to minimize the amount of helium l1 gas produced.

By changing the packing density for each section, it is possible to reduce the center temperature of the high temperature section or high heat generation section.

Furthermore, by using heat-resistant ceramics as the plate-shaped sintered body, the shield can be used at high temperatures.

Furthermore, by using a hot-pressed plate made of the same material as the shielding material for the plate-shaped sintered body, local deterioration of shielding performance can be prevented.

On the other hand, by providing slits in the plate-shaped sintered body itself, strong interaction with the cladding tube can be reduced.

[Example] Examples of the present invention will be described below with reference to FIGS. 1 to 3.

FIG. 1 is a schematic vertical cross-sectional view of a neutron shield showing one embodiment of the present invention.

The configuration of FIG. 1 is as follows: 2 is a powdered neutron shielding material, 3 is a
A partition plate for partitioning the shielding material, 5 is a cladding tube filled with the shielding material 2, 4 is an end plug for sealing the cladding tube 5, and 6 is for releasing helium gas generated inside the cladding tube. This is a vent mechanism.

Neutrons generated in the reactor core due to nuclear fission tend to reach the installed neutron shield and undergo attenuation, then reach the intermediate heat exchanger and activate the secondary Na coolant. In order to prevent activation of this secondary Na, it is necessary to install a neutron shield with excellent performance around the core.

The intensity of neutrons emitted from the reactor core differs depending on the direction in which they are emitted (neutron emission angle). Therefore, the packing density of the shielding material can be varied for each compartment within the neutron shielding body. In sections where the neutron radiation intensity is low, the packing density can be reduced, that is, the weight can be reduced.

As the shielding material to be filled, it is best to use a lightweight heat-resistant material, such as boron carbide (B4C), but in this case,
Helium gas is produced by neutron irradiation. Since the generation of a large amount of helium gas is disadvantageous for furnace control and heat removal, it is effective to limit the amount of boron carbide packed in each compartment and to change the packing density for each compartment. be.

Furthermore, the effect of the partition plate of this embodiment will be explained.

When boron carbide or the like is used as a shielding material, it generates heat due to reaction with neutrons. In the case of a fast breeder reactor, the coolant temperature is as high as 400 to 600°C, so the center temperature of the shield is also 1000'C or higher. In particular, near the coolant outlet of the reactor core, that is, on the upper end side, the temperature of the coolant is high, resulting in severe high temperatures.

In addition, the central region, where neutron intensity is high, also generates a large amount of heat, so
Temperatures rise and become harsher. Powdered shielding materials can lower the center temperature by taking advantage of the fact that thermal conductivity improves by increasing the packing density.

That is, by changing the packing density for each section, it is possible to change the thermal conductivity and make the temperature uniform for each section.

When irradiated in the furnace, the partition plate expands in volume (swells) and may mechanically interact with the cladding, including the shielding material. In this case, the concentration of local stress can be reduced by using the partition plate provided with slits.

FIG. 2 is a perspective view of a partition plate of the plate-shaped sintered body with slits. Slino 1 provided radially in the radial direction of the plate-shaped sintered body
The presence of ~ releases the energy stored in the plate-shaped sintered body and alleviates the effect of stress on the cladding tube.

FIG. 3 is a plan view of another embodiment of the partition plate with slits.

By increasing the curvature of the outer periphery of each partition plate, which is limited by the slit I, and making it convex, the inner surface of the cladding tube and the outer periphery of the plate-shaped partition material can easily cause slippage due to point contact. This prevents unnecessary stress from being generated.9 The plate-shaped partition material is preferably made of a heat-resistant material, and the cost can be reduced by using general-purpose ceramics. Furthermore, if a hot-pressed plate made of the same material as the shielding material is used, local deterioration of shielding performance can be prevented.

Usually boron carbide such as B4C is used.

In addition, the following is known about the packing density.

The ideal maximum density when packing powder with a single particle size is approximately 7
It is 4%. By mixing multiple types of powders with different particle sizes, even higher densities can be achieved. For example, as a result of carrying out a filling test using three types of powder, it was found that by changing the combination of particle size composition, the theoretical density ratio could be arbitrarily selected within the range of 70 to 90%. If this method is applied to this example, a more excellent shielding body can be obtained.

[Effects of the Invention] According to the present invention, the powder-like shielding material does not cause unevenness in density during use due to the plate-shaped sintered body, and the powder shielding material can be distributed between each partition section according to the required shielding performance. It is now possible to have a density difference, making it possible to reduce the weight and cost of the shielding body.

In addition, by using a shield that has a density distribution in the axial direction and optimizes shielding performance, it contributes to weight reduction and economic efficiency not only for the shield alone but also for the reactor system. can do.

In addition, by using ceramic for the partition plates, it can be used at high temperatures, and by using the partition plates made of the same material as the shielding material, shielding performance can be maintained at a constant level.

Furthermore, the slit-like partition plate can reduce mechanical interaction with the cladding tube.

[Brief explanation of drawings]

FIG. 1 is a schematic vertical cross-sectional view of a shield according to an embodiment of the present invention;
FIG. 2 is a perspective view of a partition plate according to one embodiment of the present invention, and FIG.
FIG. 4 is a plan view of a partition plate according to another embodiment of the present invention, and FIG. 4 is a schematic vertical cross-sectional view of a conventional control rod for a boiling water type light water reactor. Explanation of symbols> 1... steel ball, 2 shielding material, 3 partition plate, 4... end plug, 5... cladding tube, 6... vent machine dryer, 7 - slit, 8... Convex part.

Claims (1)

[Claims] 1. A cladding tube filled with a neutron shielding material and sealed at both ends with end plugs, in which the interior of the cladding tube is divided into a plurality of sections in the axial direction by partition plates. A neutron shield for fast breeder reactors characterized by: 2. The neutron shield for a fast breeder reactor according to claim 1, wherein the packing density of the neutron shielding material powder in each of the plurality of divided sections is set to an arbitrary value. 3. The neutron shield for a fast breeder reactor according to claim 2, wherein the packing density can be made arbitrary by mixing shielding material powders with different particle sizes. 4. The neutron shield for a fast breeder reactor according to claim 1, wherein the partition plate is made of ceramics. 5. The neutron shield for a fast breeder reactor according to claim 1, wherein the partition plate is made of the same material as the filler and thermally compressed. 6. A neutron shield for a fast breeder reactor according to any one of claims 1 to 5, characterized in that a radial slit is formed in the partition plate.