JP2001242272A - Radiation shielding body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shielding body improved in shielding performance by suppressing the upward directivity of leakage radiation while ensuring the necessary volume and shift path of gas in a GEM. SOLUTION: In a gas expansion mechanism installed around a core of a fast reactor, a gas shift path is formed occupying almost the whole area in a gas space in the mechanism.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas expansion mechanism for a fast reactor, and more particularly to a shield for shielding radiation leaking in the mechanism.

[0002]

2. Description of the Related Art FIG. 2 shows a schematic configuration of a conventionally planned fast reactor, and an example of the conventional plan will be described with reference to FIG.

In a fast reactor, a reactor core 3 that generates heat by nuclear fission is provided substantially at the center, and axial shields 13 are provided above and below the core in order to shield radiation generated from the core.
However, a radial shield 11 is provided in the side direction. These reactors are immersed in a coolant 10 for removing heat generated from the core, and the coolant is contained in a steel vessel 2. The coolant is circulated by the pump 6 for heat exchange outside the container. A slab 5 is provided as an upper lid on the upper part of the coolant, and a heat exchanger 4 for emergency core cooling is installed diagonally above the core through the slab.

[0004] In the core of a fast reactor, there is a plan to install a gas expansion module 12 (hereinafter abbreviated as GEM) in response to a core flow reduction type reactor shutdown failure event as a measure for improving core safety. Is inserted in contact with the core and surrounding the core.

FIG. 3 shows the principle of GEM.
The principle of GEM will be described with reference to FIG.

[0006] The principle of GEM is that during normal rated operation, the inert gas 8 inside is compressed by the pump pressure, and the gas amount is adjusted so that the liquid level of the coolant 10 is slightly above the upper end of the core. And the coolant in the GEM acts as a neutron reflector. On the other hand, when the function of the pump is reduced and the coolant flow rate is extremely reduced, the gas expands due to the decrease in the pressure and the coolant level is lowered, and neutrons leak from the gas region to produce a negative reactivity effect. As described above, a gas region through which radiation is easily transmitted is formed in a region above the upper end of the core due to the function of the GEM inside the GEM, and the core is surrounded by the axial shield and the radial shield as shown in FIG. Due to the enhanced shielding performance, GEM is the main leakage path of radiation generated in the core to the outside of the core.

FIG. 4 shows a schematic structure of a GEM that has been conventionally planned. An example of the conventional plan will be described with reference to FIG.

The GEM shield 1 is installed below the handling head 7 in the wrapper tube 9 and shields the radiation that has risen in the lower gas region here.

[0009]

In the prior art, GEM
Because there is a large gas space above the inner coolant level,
Radiation generated in the reactor core and flowing into the GEM has strong directivity in the upward direction due to the space, and leakage to the upper part is accelerated. Therefore, as shown in FIG. 2, the GEM is a main transmission path for radiation leaking to the outside of the reactor core. This radiation reaches the heat exchanger and activates the secondary coolant inside. In order to suppress activation, it is conceivable to install a shield around the heat exchanger. However, in that case, the diameter of the container increases and the cost increases. In addition, it is conceivable to shield the radiation by increasing the thickness of the GEM shield. The shaft length also needs to be increased to the same length, which can be a cost increase factor.

An object of the present invention is to provide a shielding body which improves shielding performance by suppressing the upward directivity of leaked radiation while securing a necessary volume and a transfer path of the gas in the GEM. .

[0011]

Means for Solving the Problems In order to achieve the above object, the total gas volume in the GEM is set to a required volume, and a shielding body having a shape that does not form a united gas space while securing a gas transfer path is provided. . Specifically, the range of the shielding body is set from almost directly above the coolant level during rated operation to the lower surface of the handling head in the axial direction, and to the vicinity of the inner surface of the wrapper pipe in the radial direction. Further, the shape of the shield is a shape capable of forming a continuous gas space, and the volume occupied by the shield is set so that the sum of the remaining gas space volumes can secure the gas volume required as a GEM.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG.

FIG. 1 shows a sectional shape of the GEM and an upper shield.

In this embodiment, the GEM shield 1 is a shield composed of a plurality of screw blades around the center rod, and a continuous gas space necessary for the function of the GEM is formed between the blades. Is done.

Therefore, since the necessary gas space can be present inside the shield, the occupation range of the entire shield is from almost directly above the coolant level during rated operation in the axial direction to the lower surface of the handling head. , And in the radial direction, up to near the inner surface of the wrapper tube.
Therefore, in the conventional embodiment, radiation
The large gas space above the inner coolant level is divided by screw-type blades, and the shield is
It is possible to occupy almost the entire area of the space above the coolant level in the GEM, so that the directivity of radiation in the GEM to the upper direction can be suppressed, and as a result, the shielding performance of the GEM shield improves. I do.

In the shield, a continuous gas space can be formed between the screw-type blades, so that a gas transfer path in the GEM can be secured, and the gas can expand when the pump is stopped. By adjusting the shape, the gas space volume can secure the required volume for the function, so that the function of the GEM is not impaired. The number, shape and dimensions of screw-type blades are based on the total
What is necessary is just to set so that the required gas volume can be secured as M.

[0017]

The effect of the present invention is that the shielding body can occupy almost the entire gas space in the GEM.
The directivity of the radiation in the upper part can be suppressed, and as a result, the shielding performance of the shielding body is improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the shape of a GEM shield according to one embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration around a nuclear reactor of a fast reactor.

FIG. 3 is a diagram showing a shape of a GEM shield of a conventional plan.

FIG. 4 is a diagram showing the principle of GEM.

[Explanation of symbols]

1 ... GEM shield, 2 ... container, 3 ... core, 4 ... heat exchanger,
5 slab, 6 pump, 7 handling head, 8
... gas, 9 ... wrapper tube, 10 ... coolant, 11 ... radial shield, 12 ... gas expansion mechanism (GEM), 13 ... axial shield.

Claims (2)

[Claims] In a gas expansion mechanism installed around a core of a fast reactor, a shield occupies substantially the entire area of a gas space in the mechanism and forms a gas transfer path. 2. The shield according to claim 1, wherein the shield has a screw shape.