JPH10239472A - Radiation shield for nuclear fusion reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make thin the thickness of a shielding wall through improvement in a shielding performance, thereby reducing a construction cost by setting the thickness of metal layer of a shield at a specified value that is several times average free stroke of neutrons passing through the material of the shield. SOLUTION: A shielding wall of a vacuum vessel 4 is divided into a vacuum side mixture layer provided with a cooling flow path 9 and an atmospheric side metal layer having no path 9. The path 9 is formed so that it may dominate 10% of capacity of the whole capacity of a shield and the thickness (c) of the metal layer is set 0.8-1.2 times the average free stroke of neutrons in the material of the shield. Accordingly, nuclear heat generation of neutrons and gamma rays can be restrained to the minimum and the radiation shielding performance is made optimum. Furthermore, when several steps of flow paths 9 are arranged in the thickness direction of the shield without substantial overrupped conditions, the shielding performance is improved still more. Also, when a flow path of a horizontal duct wall 8 is formed to be rectangular in the main radius direction, streaming of radiation is widely reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation shield for a nuclear fusion reactor, and more particularly to a radiation shield having improved shielding performance in order to reduce the thickness of a shield constituting a vacuum vessel.

[0002]

2. Description of the Related Art An example of an international fusion reactor (ITER), which is currently being designed by international cooperation, will be described. I
FIG. 2 shows a bird's-eye sectional view of the TER.

In the ITER, a toroidal coil 7 for generating a magnetic field for confining plasma, a poloidal coil 6, and a center solenoid coil 1 for generating a plasma current are installed at positions close to the vacuum vessel 4. These coils are all superconducting coils. The horizontal duct 8 extends in the main radial direction from a position adjacent to the adjacent toroidal coil 7 and penetrates to the cryostat 5, and the horizontal duct wall functions as a radiation shield for protecting the superconducting coil from neutrons generated by the plasma 3. Fulfill. Since the horizontal duct is connected to the vacuum vessel via the bellows, it is considered to be a part of the vacuum vessel, and the structure thereof is the same as that of the vacuum vessel 4.

For a superconducting coil of a fusion reactor, the nucleation heat rate of a winding portion (hereinafter referred to as a winding portion) of the superconducting coil due to neutrons and secondary gamma rays generated by the plasma 3 is reduced to prevent quench. And to reduce radiation damage to the stabilized copper and electrical insulation of the windings. Therefore, it is required to install a radiation shield having a necessary and sufficient thickness for the superconducting coil.
On the other hand, for the fusion reactor as a whole, downsizing of the reactor is an important issue in order to reduce construction costs. Therefore, it is necessary to reduce the thickness of the vacuum vessel while maintaining the same shielding performance as before by improving the radiation shielding performance of the vacuum vessel of the fusion reactor. In particular,
It is necessary to make the vacuum vessel wall 2 on the inboard side thin.

Conventionally, the shield is made of a single stainless steel and has a structure as shown in FIG. When the cross section of the shield is divided into two layers by a virtual line a, a cooling channel is arranged in one layer, and at least one of the cooling channels is arranged so as to be in contact with a. There are no cooling channels in the other layer. At this time, if the former layer is a mixed layer and the latter is a metal layer, conventionally, the mixed layer thickness d of the shield is 45c.
m, and the metal layer thickness c was 5 cm. With this structure, the layout of the cooling channel is not optimized from the viewpoint of shielding performance, and the shielding performance is necessary and sufficient, and the thickness is not minimized, which results in high construction cost There is.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to improve the shielding performance of a conventional radiation shield, reduce the thickness of the shielding wall, and reduce the construction cost of a fusion reactor.

[0007]

SUMMARY OF THE INVENTION According to the present invention, there is provided a neutron passing through a metal layer of a shielding body through a material of the shielding body in order to reduce the thickness of the shielding wall by improving the shielding performance of the radiation shielding body. 0.8 to 1.2 times the mean free path of Therefore, if the shielding wall is stainless steel, the metal layer is 8-12cm
The above-mentioned problem can be achieved by setting to.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The function of a metal layer for improving radiation shielding performance in the present invention will be described below. Now, it is considered that the thickness of the radiation shielding wall is kept constant and the number of metal layers is increased from a state where the entire shielding wall is a mixed layer. Specifically, the thickness of the entire shield made of stainless steel is fixed at 50 cm, the horizontal axis is the thickness of the metal layer, and the vertical axis is the winding part of the superconducting coil when the entire shield wall is a mixed layer. FIG. 4 shows a graph drawn as the nuclear heat generation rate when the nuclear heat generation of Example 1 is set to 1. The vertical axis is a value standardized by the nuclear heating rate of the winding portion when there is no metal layer.

When the thickness of the metal layer is greater than the mean free path of gamma rays in the shield (about 2 cm) and the mean free path of neutrons (about 10 c
m) or less, the improvement in gamma-ray shielding performance due to an increase in metal layer thickness exceeds the effect of an increase in nuclear heating rate due to a decrease in neutron shielding performance. Decreases with increasing thickness.

However, when the thickness of the metal layer further increases than the mean free path of the neutrons, the neutrons arriving at the back of the metal layer generate gamma rays in situ. Nuclear heating rate changes from decreasing to increasing.

From the above, there is a thickness of the metal layer that minimizes the nuclear heat generation rate. When the thickness of the shield is about 50 cm, the thickness of the metal layer becomes the minimum when the thickness is 8 to 12 cm.

[0012]

An embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a horizontal sectional view of a part of the container and the horizontal duct when the radiation shield of the present invention is applied to the vacuum container 4 and the horizontal duct wall 8, and is a sectional view corresponding to the equatorial plane of FIG.

The thickness b of the entire shielding wall constituting the wall surface of the vacuum vessel 4 is 49.4 cm, and the material of the shielding body is stainless steel.
The cooling channel 9 occupies at least 10% or more of the entire volume of the radiation shield. The thickness c of the metal layer is set to a thickness about the mean free path of neutrons in the metal layer, that is,
Set to 8-12cm. That is, the thickness d of the mixed layer is 41.4-3.
It is 7.4cm.

Conventionally, a radiation shield provided with a cooling channel in stainless steel is known, and this shielding member has 80% to 90% of the entire volume of stainless steel and the remainder is a cooling channel. . Since the cooling channels were arranged so as to be uniformly distributed over the entire shield, the entire shield was a mixed layer. It is known that such a shield can minimize nuclear heating of the winding of the superconducting coil due to neutrons. However, gamma rays are generated when neutrons hit the cooling passage inside the shielding body, and the shielding passage of the gamma ray decreases because the cooling passage exists near the atmosphere side of the shield,
Overall, the shielding performance of both neutrons and gamma rays was not so good. On the other hand, for gamma rays, the shielding effect increases by increasing the thickness of the metal layer. However, if the thickness of the metal layer increases further than the mean free path of neutrons in stainless steel, neutrons arriving at the back of the metal layer will generate gamma rays in situ, so by increasing the thickness of the metal layer, On the contrary, nuclear heat generated by gamma rays increases. Therefore, by setting the thickness of the metal layer to about 8 to 12 cm, which is about the mean free path of neutrons in stainless steel, nuclear heating of neutrons and gamma rays can be minimized. Since nuclear heating is proportional to neutron flux and gamma ray flux, the thickness of the metal layer at which the nuclear heating rate changes from decreasing to increasing is determined by the ratio of the intensity of neutrons and gamma rays immediately before entering the metal layer.
If the volume ratio of stainless steel of the entire shield and water in the cooling channel is constant, the intensity ratio of neutrons and gamma rays immediately before entering the metal layer does not depend on the thickness of the mixed layer, It will be a constant value. If the strength ratio is constant, the optimum value of the metal layer thickness is 8 to 12 cm regardless of the thickness of the shielding wall.

Even if the shielding wall is made of a metal other than stainless steel, the ratio of the metal to the cooling channel volume is maintained at the above-mentioned value, and the thickness of the metal layer is set to 0.8 to the mean free path of neutrons in the shielding wall. 1.2
The radiation shielding performance can be optimized by making the length twice as long.

FIG. 5 shows a case where the radiation shield of the present invention is applied to a horizontal duct wall 8. Cooling channel 9 in horizontal duct
Is bent in the main radial direction into a rectangular shape as shown in FIG. The horizontal duct extends from the vacuum vessel 4 to the cryostat 5 as shown in FIG. Cryostat 5
Also serves as a biological shield, and it is necessary to reduce radiation (mainly neutrons and gamma rays) from plasma in order to realize a radiation environment in which humans can work outside the cryostat 5. For this reason, during operation, except for some ducts for measurement, plasma heating, and current driving, the inner space of the duct is filled with a radiation shield of sufficient thickness, and neutrons and gamma rays from the plasma are filled. Is shielded. In the duct wall, a cooling channel 9 for supplying a coolant such as water or helium gas to the duct wall and recovering the coolant from the duct wall penetrates in the main radial direction. If a hole is formed in the main radial direction of the radiation shielding wall in the direction in which the radiation flows, a phenomenon called radiation streaming, in which the radiation passes through the portion without attenuating, occurs. There is a high possibility that helium gas, which can be used at a high temperature, will be used as a coolant in a fusion reactor to be used for power generation in the future. When gas is used as the coolant, this streaming effect becomes remarkable. With the conventional horizontal duct wall structure, radiation can be streamed through the header, and an environment where humans can work outside the cryostat 5 can be realized. There is a problem that it becomes difficult. Therefore, by arranging such a cooling channel as shown in FIG. 5 in which the cooling channel is bent, scattering of radiation in the cooling channel 9 can be increased, and streaming of radiation can be greatly reduced. FIG. 6 shows another embodiment of FIG. The wall loss of the coolant can be reduced by smoothly bending the rectangular corner with the cooling pipe of FIG.

Next, FIG. 7 shows an embodiment in which two or more cooling channels in the radiation shielding wall of the present invention are arranged so as not to substantially overlap in the thickness direction of the shielding wall.

In the example of FIG. 7, the cooling channels 9 are arranged in two stages so as not to overlap in the thickness direction of the shield. Since neutrons have a longer mean free path in the radiation shield than charged particles and electromagnetic radiation, they generate heat in the entire thickness direction of the radiation shield in proportion to the neutron flux distribution in the radiation shield. In contrast, charged particles and electromagnetic waves have a shorter mean free path in the radiation shield than neutrons, and thus generate heat on the vacuum side of the radiation shield. From this, by disposing the cooling channel 9 near the vacuum side of the radiation shield having a high heat generation rate, the temperature gradient in the thickness direction of the radiation shield can be reduced,
Thermal stress of the radiation shield can be reduced. On the other hand, when a plurality of arrangements of the cooling channels 9 are provided in the thickness direction of the radiation shield, high energy neutrons are not decelerated by the inelastic scattering reaction with the stainless steel in the portion where there is no stainless steel, that is, in the portion of the cooling channel 9. Therefore, the shielding performance for high energy neutrons is particularly low. For this reason, if the cooling channels 9 overlap in the thickness direction of the radiation shield, high energy neutrons may penetrate the portion and the shielding performance may be reduced. In this case, as shown in FIG. 8, if the cooling channels 9 are arranged in three stages so that they do not substantially overlap in the thickness direction of the shield, high-energy neutrons can be easily decelerated by inelastic scattering reaction with stainless steel. In addition, the shielding performance of the radiation shielding body can be further improved.

[0019]

According to the present invention, the shielding effect is improved as compared with the conventional structure in which the thickness of the radiation shielding wall is the same. If the shielding effect is the same as before, the wall thickness can be reduced,
Furnace construction costs can be reduced.

Specifically, when the present invention is applied to the vacuum vessel shielding wall on the inboard side, the thickness of the mixed layer can be reduced by 0.6 cm, so that the cost can be reduced by 1.2 billion yen.

[Brief description of the drawings]

FIG. 1 is a horizontal sectional view of an equatorial plane of a thick-walled horizontal duct wall.

Figure 2 Bird's-eye view of the International Thermonuclear Experimental Reactor (ITER)

FIG. 3 is a schematic sectional view of a shielding wall.

Fig. 4 Results of radiation transport calculation

FIG. 5 is a horizontal duct wall in which a cooling channel is bent.

FIG. 6 is a horizontal duct wall in which a cooling channel is bent.

FIG. 7 is a horizontal duct wall in which cooling channels are arranged in two layers so as not to substantially overlap in the thickness direction.

FIG. 8 is a horizontal duct wall in which cooling channels are arranged in three layers so as not to substantially overlap in the thickness direction.

[Explanation of symbols]

 1: Solenoid coil 2: Vacuum vessel wall on inboard side 3: Plasma 4: Vacuum vessel 5: Cryostat 6: Poloidal coil 7: Toroidal coil 8: Horizontal duct wall 9: Cooling flow path of radiation shield a: Mixing layer and metal Layer boundary line b: Thickness of vacuum vessel and entire horizontal duct wall c: Thickness of metal layer d: Thickness of mixed layer

Claims (4)

[Claims] 1. A radiation shield for a fusion reactor for shielding radiation from plasma generated in a vacuum vessel, wherein the shield has a thickness of 30 to 100 cm. The body is composed of a metal layer and a mixed layer, wherein the metal layer is located on the atmosphere side, and its thickness is set to 0.8 to 1.2 times the mean free path of neutrons in the material constituting the metal layer. Radiation shield. 2. A radiation shield of a fusion reactor for shielding radiation from plasma generated in a vacuum vessel, wherein the shield is made of stainless steel having a thickness of 30 to 100 cm. The radiation shield, wherein the shield comprises a metal layer and a mixed layer, the metal layer is located on the atmosphere side, and the thickness thereof is set to 8 to 12 cm. 3. The radiation shield according to claim 1, wherein the radiation shield further has a horizontal duct part, and a cooling channel penetrates a wall part constituting the duct part in a main radial direction. A radiation shield characterized by bending a cooling flow path so as not to be bent. 4. The radiation shield according to claim 1, wherein a cooling medium passage forming a mixed layer of the radiation shield is formed in at least two layers in the thickness direction of the shield. A radiation shield characterized by being arranged so as not to overlap.